Publications A. Just
Physiological concentrations of ANP exert a dual regulatory influence on renin release in conscious dogs.
Am J Physiol. 1992 Sep; 263(3 Pt 2): R529-36
The influence of physiological increments in circulating atrial natriuretic peptide (ANP) on renin release was determined in conscious dogs. Renin stimulus-response curves (RSRCs) were obtained by controlled reductions of renal perfusion pressure (RPP) under control conditions and during intrarenal or intravenous ANP infusions. Under all experimental conditions, the RSRCs were characterized by a plateau, a threshold pressure (Pth), and a steep slope below Pth. Intrarenal ANP infusion (0.9 ng.kg-1.min-1), which induced a calculated threefold elevation of renal arterial ANP concentration (but did not change systemic arterial ANP levels), increased the slope of the RSRC by 81% (P less than 0.05) with no effect on Pth. A quantitatively similar effect on the slope of the RSRC (+90%; P less than 0.05) was observed when systemic ANP levels were raised (from 37 +/- 2 to 71 +/- 9 pg/ml; P less than 0.05) by intravenous infusions (3.6 ng.kg-1.min-1). In addition, however, intravenously infused ANP reduced Pth from 91 to 85 mmHg (P less than 0.05), which caused a complete suppression of the renin response to a reduction of RPP down to 85 mmHg. These findings indicate that ANP can inhibit renin release at physiological plasma concentrations by shifting the RSRC to a lower pressure level; this shift is mediated by a modulation of extrarenal renin control mechanisms. The direct effect of ANP on renin release is one of stimulation.
Autoregulation and non-homeostatic behaviour of renal blood flow in conscious dogs.
J Physiol (London) 1993 Mar; 462: 261-73
1. Spontaneously occurring haemodynamic variations within 4 h affecting renal blood flow (RBF) were compared with externally induced short changes of renal artery pressure (RAP) in conscious resting dogs. 2. In all animals in which RAP was servo-controlled (n = 6), perfect autoregulation of RBF was observed. 3. In all 4 h recordings of spontaneous renal blood flow (n = 9), certain combinations of blood pressure and blood flow occurred remarkably frequently as indicated by three-dimensional frequency distributions. 4. Cluster analysis demonstrated significant differences between these areas of accumulation (P < 0.001). The average number of 'set points' per 4 h session was 3.1 +/- 0.3. 5. The shift from one set point to another is probably mediated by multiple control systems impinging on renal haemodynamics as suggested by 1/f fluctuations. 6. In seven dogs, an additional renal venous catheter allowed measurements of the arterial-venous (A-V) oxygen partial pressure (PO2) difference as an indicator of the renal metabolic demand. An inverse relationship between A-V PO2 difference and RBF (Y = X(-0.034) + 40.9, r = -0.9, P < 0.001) was found, indicating that the metabolic demands vary little (if at all) between the different set points. 7. The presented data suggest a modified view of renal homeostasis. There exist distinct combinations between RBF and RAP, which are very stable. Autoregulation merely buffers the fluctuations around these set points.
The blood pressure buffering capacity of nitric oxide by comparison to the baroreceptor reflex.
Am J Physiol. 1994 Aug; 267(2 Pt 2): H521-7
To compare the contribution of nitric oxide (NO) to the buffering of short-term and circadian fluctuations of arterial blood pressure with that of the baroreceptor reflex, conscious foxhounds were subjected to continuous 24-h blood pressure recordings. A pressure transducer was placed into the lumen of the abdominal aorta. Telemetry recordings were done under control conditions, following blockade of NO formation by intravenous bolus injection of NG-nitro-L-arginine (L-NNA; 16.5 +/- 2 mg/kg body wt) and after total sinoaortic and cardiopulmonary denervation in five dogs each. L-NNA produced a sustained elevation of mean arterial pressure (MAP; 137.2 +/- 6.4 mmHg vs. control, 112.9 +/- 3.7 mmHg). After denervation, no significant increase of MAP was found (113.5 +/- 4.1 mmHg), but the standard deviation of the MAP histogram was significantly greater (22.5 +/- 3.1 vs. 10.6 +/- 0.9 mmHg, P < 0.05). Sequential spectral analysis showed that total power between 0 and 0.5 Hz was elevated more than twofold after L-NNA (P < 0.05). This was due primarily to increased power in the range above 0.1 Hz. After denervation, total power increased about three-fold (P < 0.05), almost exclusively occurring below 0.04 Hz. Power in the range above 0.2 Hz was diminished, although not significantly. It is concluded that in the conscious dog, NO, as well as the baroreceptor reflex, is an effective blood pressure buffer. NO is most effective above 0.1 Hz, whereas the baroreceptors primarily buffer fluctuations slower than 0.04 Hz.
Modulation of erythropoietin formation by changes in blood volume in conscious dogs.
J Physiol (London) 1995 Oct 1; 488 ( Pt 1): 181-91
1. A possible influence of the filling of the circulatory system on the plasma concentration of erythropoietin, which is the major regulator of erythrocyte formation, was investigated in conscious dogs. 2. Over an experimental period of 5 h, the animals were subjected to either haemorrhage (hypovolaemia), blood volume expansion (hypervolaemia), or exchange transfusion of blood with dextran (isovolaemic anaemia). 3. A reduction of blood volume by 20% induced by haemorrhage increased plasma erythropoietin levels approximately 1.5-fold in the absence of significant changes in haematocrit. 4. An expansion of blood volume by 12% induced by an intravenous infusion of dextran did not change plasma erythropoietin levels, although the haematocrit decreased by 0.04. 5. A reduction of the haematocrit by 0.12 in the absence of changes in blood volume induced by an isovolaemic exchange transfusion (dextran vs. blood) increased plasma erythropoietin levels approximately 3-fold. 6. Total renal oxygen supply did not change in any of the three experimental protocols. 7. These data indicate that in dogs the erythropoietin production rate is modulated by changes in blood volume, and suggest a possible role of erythropoietin in the regulation of blood volume.
On the origin of low-frequency blood pressure variability in the conscious dog.
J Physiol (London) 1995 Nov 15; 489 ( Pt 1): 215-23
1. Baroreceptor denervation increases blood pressure variability below 0.1 Hz. This study was undertaken to determine to what extent these fluctuations originate from the central nervous system or from cardiovascular sources. 2. Blood pressure was recorded at a rate of 10 Hz for approximately 3.5 h in conscious, resting dogs. Power density spectra were calculated from all 2(17) points of each recording session and integrated between 0.0002 and 0.1 Hz. 3. Blockade of the afferent limb of the baroreceptor reflex by surgical denervation of sinoaortic and cardiopulmonary afferents (Den; n = 6) significantly increased integrated power more than sixfold compared with a control group (n = 11). 4. Impairment of the efferent limb in non-deafferented dogs by either alpha 1-adrenergic blockade with prazosin (Praz; n = 7) or ganglionic blockade with hexamethonium (Hex; n = 6) failed to raise variability. 5. Both prazosin (n = 6) and hexamethonium (n = 3) reduced the increased variability in denervated dogs. 6. In non-deafferented dogs receiving hexamethonium, elevation of mean blood pressure to the hypertensive level of the Den group, by a continuous infusion of noradrenaline (n = 4), did not change the variability. 7. It is concluded that in the absence of changes in posture, most of the increased blood pressure variability after baroreceptor denervation is derived from the central nervous system. 8. Direct comparison of power spectra of the Den (total variability) and Hex groups (variability derived from the cardiovascular system only) suggests that the central nervous system is also the prevalent source of low-frequency blood pressure variability in intact animals.
It was the aim of this study to quantify the measurement of pulsatile flow in the renal artery with the noninvasive magnetic resonance cine phase-contrast (MRCPC) method and combine it with the simultaneous assessment of pulsatile flow with a transit-time ultrasound (TTUS) flowmeter. In seven foxhounds with a chronically implanted precalibrated TTUS flow probe, MRCPC flow measurements were made in the renal artery with a temporal resolution of 32 ms. Mean and pulsatile flow signal were compared by the simultaneous ipsi- or contralateral measurement of the renal blood flow signal by both methods (TTUS and MRCPC). In addition, comparative MRCPC and TTUS flow measurements were made with artificial renal artery stenosis and after the administration of angiotensin II. The mean flow data assessed by the noninvasive MRCPC flow measurements showed an excellent correlation with corresponding TTUS recording (r = 0.99). The MRCPC flow signal displayed a waveform of the renal artery flow profile that was very similar to the TTUS flow pulse. The hemodynamic changes induced by angiotensin II or due to renal artery stenosis were also reliably detected by MRCPC. MRCPC provides a reliable noninvasive method for the quantification of mean blood flow and the assessment of the pulsatile flow signal in the renal artery and proves to be sensitive to hemodynamic changes of pathophysiological importance. Alternatively, the method may be used for studies in physiology that demand a noninvasive approach.
PURPOSE: To analyze the blood flow dynamics in renal artery stenosis with high-temporal-resolution cine phase-contrast magnetic resonance (MR) flow measurements. MATERIALS AND METHODS: Cine phase-contrast MR flow measurements were invasively validated with real-time intraoperative transit-time ultrasound (US). In 23 patients, 48 renal artery stenoses were confirmed at digital subtraction angiography. Cardiac-gated cine phase-contrast MR flow measurements were obtained in 32-msec intervals, and flow curves were calculated for the whole cardiac cycle. Hemodynamic parameters evaluated included the decrease in mean flow and the delay and reduction in the systolic velocity maximum due to decrease in or absence of the early systolic peak. RESULTS: Overall differentiation between renal artery stenosis (n = 31) and nonstenosed vessels (n = 17) with cine phase-contrast MR revealed a sensitivity of 90% and specificity of 94% compared with findings at digital subtraction angiography. High-grade stenoses (>50%, n = 19) were detected with cine phase-contrast MR with sensitivity of 100% and specificity of 93%. CONCLUSION: Quantitative and qualitative analysis of cardiac-gated cine phase-contrast MR flow velocity curves provided a highly accurate method to detect hemodynamic abnormalities in patients with suspected renal artery stenosis.
Very low frequency oscillations in arterial blood pressure after autonomic blockade in conscious dogs.
Am J Physiol. 1997 Jun; 272(6 Pt 2): R2034-9
The aim of this study was to investigate spontaneous variability of arterial blood pressure in conscious foxhounds in the absence of direct sympathetic and parasympathetic influences. Autonomic blockade was achieved by administration of the ganglionic blocking agent hexamethonium (n = 7). In contrast to the control group (n = 7), marked oscillations with a cycle length of 100 s (0.01 Hz) were observed. The relationship of the power densities of the oscillation band (0.01 +/- 0.005 Hz) to the total power increased threefold (0.213 +/- 0.007 vs. 0.057 +/- 0.005; P < 0.01). The 0.01-Hz oscillations typically commenced after some delay. To test whether the absence of the mechanoreceptor afferents was responsible for these fluctuations, we investigated an additional group of foxhounds that were subjected to total baroreceptor and cardiopulmonary receptor denervation (n = 7). Neither in this protocol, nor in a group subjected to denervation and ganglionic blockade (n = 6), did we observe sustained oscillations in this frequency range. Since the oscillations were not seen after combined afferent (mechanoreceptor denervation) and efferent (ganglionic) blockade, central oscillators as a source of the oscillations can be ruled out. A simple model of a circulating pressoric factor may explain the fluctuations, provided that there is a time delay between the stimulus and the release or action of the factor. The findings suggest that a circulating factor accounts for the 0.01-Hz oscillations, which is dependent on intact pathways from the cardiac receptors or baroreceptors to the central nervous system. This hypothesis is put forward since cardiopulmonary and baroreceptor denervation blocked the oscillations seen after ganglionic blockade.
Schoenberg-SO; Knopp-MV; Bock-M; Kallinowski-F; Just-A; Essig-M; Hawighorst-H; Zuna-I; Schad-L; Allenberg-JR; van-Kaick-G Einstufung hämodynamischer Veränderungen bei Nierenarterienstenosen mittels MR-Cine-Phasenkontrastfluss-Messungen[Classification of hemodynamic changes in renal artery stenosis using cine magnetic resonance phase contrast flow measurements]Radiologe. 1997 Aug; 37(8): 651-62
PURPOSE: To evaluate the use of high-temporal resolution cine MR phase-contrast flow measurements for assessment of flow dynamics in renal artery stenosis (RAS). MATERIAL AND METHODS: In a dog model, cine MR flow measurements were validated by comparing the MR flow data to an invasive transit-time ultrasound reference technique for different degrees of RAS. Cardiac-gated MR flow curves were recorded in 56 renal arteries of 28 patients with a temporal resolution of at least 32 ms. In all cases RAS was confirmed by digital subtraction angiography (DSA). Abnormalities of flow dynamics were assessed in the calculated flow curves using the MR parameters mean flow, maximum velocity, and time to systolic maximum. RESULTS: By means of the MR blood flow parameters high-grade stenoses (> 50%, n = 23) were detected with sensitivity of 100% and specificity of 94% with reference to DSA. The overall differentiation between stenoses (n = 37) and non-stenosed vessels (n = 19) revealed a sensitivity of 87% and a specificity of 100%. CONCLUSION: Analysis of cardiac-gated MR flow curves provides a non-invasive method to assess the hemodynamic significance of RAS and thus allows a functional evaluation in relation to the morphologic characteristics of the stenosis.
Autoregulation of renal blood flow in the conscious dog and the contribution of the tubuloglomerular feedback.
J Physiol (London) 1998 Jan 1; 506 ( Pt 1): 275-90
1. The aim of this study was to investigate the autoregulation of renal blood flow under physiological conditions, when challenged by the normal pressure fluctuations, and the contribution of the tubuloglomerular feedback (TGF). 2. The transfer function between 0.0018 and 0.5 Hz was calculated from the spontaneous fluctuations in renal arterial blood pressure (RABP) and renal blood flow (RBF) in conscious resting dogs. The response of RBF to stepwise artificially induced reductions in RABP was also studied (stepwise autoregulation). 3. Under control conditions (n = 12 dogs), the gain of the transfer function started to decrease, indicating improving autoregulation, below 0.06-0.15 Hz (t = 7-17 s). At 0.027 Hz a prominent peak of high gain was found. Below 0.01 Hz (t > 100 s), the gain reached a minimum (maximal autoregulation) of -6.3 +/- 0.6 dB. The stepwise autoregulation (n = 4) was much stronger (-19.5 dB). The time delay of the transfer function was remarkably constant from 0.03 to 0.08 Hz (high frequency (HF) range) at 1.7s and from 0.0034 to 0.01 Hz (low frequency) (LF) range) at 14.3 s, respectively. 4. Nifedipine, infused into the renal artery, abolished the stepwise autoregulation (-2.0 +/- 1.1 dB, n = 3). The gain of the transfer function (n = 4) remained high down to 0.0034 Hz; in the LF range it was higher than in the control (0.3 +/- 1.0 dB, P < 0.05). The time delay in the HF range was reduced to 0.5 s (P < 0.05). 5. After ganglionic blockade (n = 7) no major changes in the transfer function were observed. 6. Under furosemide (frusemide) (40 mg + 10 MG h-1 or 300 mg + 300 mg h-1 i.v..) the stepwise autoregulation was impaired to -7.8 +/- 0.3 or 6.7 +/- 1.9 dB, respectively (n = 4). In the transfer function (n = 7 or n = 4) the peak at 0.027 Hz was abolished. The delay in the LF range was reduced to -1.1 or -1.6 s, respectively. The transfer gain in the LF range (-5.5 +/- 1.2 or -3.8 +/- 0.8 dB, respectively) did not differ from the control but was smaller than that under nifedipine (P < 0.05). 7. It is concluded that the ample capacity for regulation of RBF is only partially employed under physiological conditions. The abolition by nifedipine and the negligible effect of ganglionic blockade show that above 0.0034 Hz it is almost exclusively due to autoregulation by the kidney itself. TGF contributes to the maximum autoregulatory capacity, but it is not required for the level of autoregulation expended under physiological conditions. Around 0.027 Hz, TGF even reduces the degree of autoregulation.
1. Adenosine has been suggested to be the mediator of a metabolic feedback mechanism which transfers acute changes in the tubular load into opposite changes in renal blood flow (RBF). The goal of the present experiments was to assess the importance of endogenously formed adenosine as a 'homeostatic metabolite' during short-term changes in metabolic demand. 2. In nine chronically instrumented conscious foxhounds, both the direct effects of adenosine injections (10, 30 and 100 nmol) into the renal artery and the temporal changes of RBF after short renal artery occlusions (15, 30 and 60 s duration), the most widely used experimental model to study the metabolic feedback mechanism in vivo, were studied. 3. Intrarenal bolus injections of adenosine (10, 30 and 100 nmol) induced dose-dependent decreases of RBF (RBF: -34 +/- 5, -59 +/- 4 and -74 +/- 4 %, respectively). This vasoconstrictor effect of adenosine was significantly larger (RBF: -51 +/- 4, -68 +/- 4 and -83 +/- 3 %, respectively) when the dogs received a low salt diet. 4. The post-occlusive responses were characterized by a transient hyperaemia with no detectable drop of RBF below the preocclusion level. The post-occlusive responses were affected neither by changes in local angiotensin II levels, nor by intrarenal infusions of hypertonic NaCl or blockade of A1 adenosine receptors. 5. When intrarenal adenosine levels were elevated by infusion of the adenosine uptake inhibitor dipyridamole, a transient, although weak, post-occlusive vasoconstriction was detected. 6. In summary, the present data demonstrate that adenosine acts as a potent renal vasoconstrictor in the conscious dog. The endogenous production of adenosine during short-lasting occlusions of the renal artery, however, appears to be too small to induce a post-occlusive vasoconstrictor response of RBF. These results suggest that a metabolic feedback with adenosine as 'homeostatic metabolite' is of minor importance in the short-term regulation of RBF in the conscious, unstressed animal.
Buffering of blood pressure variability by the renin-angiotensin system in the conscious dog.
J Physiol (London) 1998 Oct 15; 512 ( Pt 2): 583-93
1. The renin-angiotensin system (RAS) participates in the compensation of major blood pressure disturbances such as haemorrhage and is involved in the tonic long-term (> 1 day) maintenance of mean arterial blood pressure (MABP). Since its contribution to the short-term (< 1 h) buffering of normal blood pressure variability is not known, this was investigated in resting conscious dogs. 2. The regulatory efficiency and the response time of the RAS were studied by an acute step reduction of renal artery pressure to 70 mmHg for 1 h using a suprarenal aortic cuff. After a delay of at least 100 s, MABP rose exponentially by 22 +/- 5 mmHg in normal dogs (n = 4), by 6 +/- 3 mmHg after angiotensin converting enzyme (ACE) inhibition (n = 4), and by 25 +/- 5 mmHg after ganglionic blockade (n = 4). MABP returned to control after release of the cuff with similar time courses. The time constants of the MABP responses were in the range of 20 min. Thus, possible feedback oscillations of the RAS would be expected around 0.0025 Hz (1/(4 x 100 s)); a buffering effect would be possible below this frequency. 3. Blood pressure variability was investigated by spectral analysis of MABP from 3.75 h recordings in the frequency ranges of 0.002-0.003 Hz (feedback oscillations) and below 0.002 Hz (buffering effect). 4. ACE inhibition (n = 7) decreased MABP by 11 +/- 2 mmHg (P < 0.05), but in both frequency ranges integrated spectral density was not affected. ACE inhibition also failed to significantly change spectral density in either of the two frequency ranges under the following conditions: (1) during ganglionic blockade (n = 7), (2) during a low-sodium diet (except for a very slight elevation below 0.002 Hz) (n = 7), and (3) when the fall of MABP induced by ACE inhibition was compensated by an angiotensin II infusion (n = 7). 5. It is concluded that in spite of its high regulatory efficiency with an adequate response time the RAS does not directly contribute to the short-term buffering of blood pressure variability, nor does it give rise to feedback oscillations under normal resting conditions. Even if the RAS is stimulated by sodium restriction its contribution to short-term blood pressure buffering is only marginal.
BACKGROUND: Intravascular catheters are associated with severe infections in patients, but only few reports on this problem in animal research exist. OBJECTIVE: We report on catheter-related bacterial colonization and its consequences in long-term catheterized animals. MATERIAL AND METHOD: Foxhounds were instrumented with intravascular catheters and flow probes to study the regulation of renal blood flow and pressures. RESULTS: After flushing the catheters, alterations in renal blood flow were observed and these could be related to bacterial colonization of intravascular catheters with Pseudomonas species. After attention had been focused on aseptic technique in all experimental phases and prophylactic antibiotic lock instituted, the occurrence of Pseudomonas bacteremia ceased, and the magnitude and incidence of catheter-related colonization and infection by Pseudomonas species dropped considerably. CONCLUSION: The catheter-related colonization that occurred spontaneously in these animals resembled findings in animal experiments in which catheter-related infections were deliberately induced as well as observations made with regard to catheter-related infections in patients. This report emphasizes the importance of asepsis when working with animals with long-term intravascular catheters. We suggest that monitoring for this complication, e.g., by means of catheter cultures at the time of removal, should routinely be part of protocols for animal experiments using long-term intravascular catheters.
Exogenous endothelin-1 (ET-1) is a strong vasoconstrictor in the canine kidney and causes a decrease in renal blood flow (RBF) by stimulating the ETA receptor subtype. The aim of the present study was to investigate the role of endogenously generated ET-1 in renal hemodynamics under physiological conditions. In six conscious foxhounds, the time course of the effects of the selective ETA receptor antagonist LU-135252 (10 mg/kg iv) on mean arterial blood pressure (MAP), heart rate (HR), RBF, and glomerular filtration rate (GFR), as well as its effects on renal autoregulation, were examined. LU-135252 increased RBF by 20% (from 270 +/- 21 to 323 +/- 41 ml/min, P < 0.05) and HR from 76 +/- 5 to 97 +/- 8 beats/min (P < 0. 05), but did not alter MAP, GFR, or autoregulation of RBF and GFR. Since a number of interactions between ET-1 and the renin-angiotensin system have been reported previously, experiments were repeated during angiotensin converting enzyme (ACE) inhibition by trandolaprilat (2 mg/kg iv). When ETA receptor blockade was combined with ACE inhibition, which by itself had no effects on renal hemodynamics, marked changes were observed: MAP decreased from 91 +/- 4 to 80 +/- 5 mmHg (P < 0.05), HR increased from 85 +/- 5 to 102 +/- 11 beats/min (P < 0.05), and RBF increased from 278 +/- 23 to 412 +/- 45 ml/min (P < 0.05). Despite a pronounced decrease in renal vascular resistance over the entire pressure range investigated (40-100 mmHg), the capacity of the kidneys to autoregulate RBF was not impaired. The GFR remained completely unaffected at all pressure levels. These results demonstrate that endogenously generated ET-1 contributes significantly to renal vascular tone but does not interfere with the mechanisms of renal autoregulation. If ETA receptors are blocked, then the vasoconstrictor effects of ET-1 in the kidney are compensated for to a large extent by an augmented influence of ANG II. Thus ET-1 and ANG II appear to constitute a major interrelated vasoconstrictor system in the control of RBF.
Tonic and phasic influences of nitric oxide on renal blood flow autoregulation in conscious dogs.
Am J Physiol. 1999 Mar; 276(3 Pt 2): F442-9
The aim of this study was to investigate the influence of the mean level and phasic modulation of NO on the dynamic autoregulation of renal blood flow (RBF). Transfer functions were calculated from spontaneous fluctuations of RBF and arterial pressure (AP) in conscious resting dogs for 2 h under control conditions, after NO synthase (NOS) inhibition [NG-nitro-L-arginine methyl ester hydrochloride (L-NAME)] and after L-NAME followed by a continuous infusion of an NO donor [S-nitroso-N-acetyl-DL-penicillamine (SNAP)]. After L-NAME (n = 7) AP was elevated, heart rate (HR) and RBF were reduced. The gain of the transfer function above 0.08 Hz was increased, compatible with enhanced resonance of the myogenic response. A peak of high gain around 0.03 Hz, reflecting oscillations of the tubuloglomerular feedback (TGF), was not affected. The gain below 0.01 Hz, was elevated, but still less than 0 dB, indicating diminished but not abolished autoregulation. After L-NAME and SNAP (n = 5), mean AP and RBF were not changed, but HR was slightly elevated. The gain above 0.08 Hz and the peak of high gain at 0.03 Hz were not affected. The gain below 0.01 Hz was elevated, but smaller than 0 dB. It is concluded that NO may help to prevent resonance of the myogenic response depending on the mean level of NO. The feedback oscillations of the TGF are not affected by NO. NO contributes to the autoregulation below 0.01 Hz due to phasic modulation independent of its mean level.
Aortic pressure-diameter relationship assessed by intravascular ultrasound: experimental validation in dogs.
Am J Physiol. 1999 Mar; 276(3 Pt 2): H1078-85
Intravascular ultrasound (IVUS) has emerged as an important diagnostic method for evaluating vessel diameter and vessel wall motion. To evaluate the validity of IVUS in assessing changes in the pressure-diameter relationship we compared measurements of abdominal aortic diameters derived from IVUS with those simultaneously obtained at the same site using implanted sonomicrometers in five chronically instrumented conscious dogs and in seven acutely instrumented anesthetized dogs. Five hundred eighty beats were analyzed to obtain peak systolic and end-diastolic diameters and to calculate aortic compliance at different blood pressure levels induced either by an aortic pneumatic cuff or by intravenous injections of nitroglycerin or norepinephrine. IVUS agreed closely with sonomicrometer measurements at different blood pressure levels. However, IVUS slightly but significantly underestimated aortic diameters by 0.6 +/- 0.7 mm for systolic diameters (P < 0.001) and by 0.7 +/- 0.6 mm for diastolic diameters (P < 0.001) compared with the sonomicrometer measurements. We conclude that IVUS is a feasible and reliable method to measure dynamic changes in aortic dimensions and has the potential to provide ready access to assess aortic compliance in humans.
Previous studies in dogs have shown additive or even synergistic effects of combined angiotensin-converting enzyme inhibition and either nonselective endothelin subtype A/B (ETA/B) or selective endothelin subtype A (ETA) receptor blockade on renal vascular resistance and mean arterial blood pressure. A possible mechanism underlying this interaction may be a stimulation of the renin-angiotensin system during endothelin (ET) receptor blockade. We therefore investigated whether plasma renin activity and renin release are regulated by the ETA receptor. Experiments were made in conscious, chronically instrumented dogs receiving either saline or the selective ETA receptor antagonist LU 135252 (10 mg/kg IV). Eighty to 100 minutes after the administration of LU 135252 (n=5), heart rate (99+/-7 versus 81+/-6 bpm; P<0.05) and renal blood flow (327+/-40 versus 278+/-36 mL/min; P<0.05) were increased significantly, whereas mean arterial blood pressure tended to be lower (93+/-5 versus 105+/-7 mm Hg). These changes were associated with a 2-fold increase in plasma renin activity (0.74+/-0.12 versus 0.37+/-0.10 ng angiotensin I per milliliter per hour; P<0.05). Measurements of renin release at various renal perfusion pressures (n=5) with the use of a vascular occluder implanted around the left renal artery revealed that ETA receptor blockade did not alter renin release at resting renal perfusion pressure (78+/-25 versus 71+/-39 U/min) but strongly enhanced the sensitivity of pressure-dependent renin release <80 mm Hg approximately 2.2-fold. In conclusion, selective ETA receptor blockade is associated with a stimulation of the circulating renin-angiotensin system, which results from both a sensitization of pressure-dependent renin release and a larger proportion of blood pressure values falling into the low pressure range, where renin release is stimulated. These find-ings strengthen the view that ET and the renin-angiotensin system closely interact to regulate vascular resistance and provide a physiological basis for synergistic hypotensive effects of a combined blockade of both pressor systems.
Background. Intravascular ultrasound (IVUS) is frequently used as an adjunct to coronary angiography to guide revascularization procedures and, more recently, to estimate atherosclerotic plaque volumes. Although accuracy of IVUS imaging and analysis is crucial for these measurements, available data are scare. The purpose of this in vitro study is to determine the extent to which transducer position and equipment-related factors influence measurements accuracy. Methods. Cross-sectional views of tubular vessel phantoms (diameter 2-14 mm) were acquired 3,2 French catheters in coaxially centered, eccentric and oblique positions. Catheters were sequentially connected totwo different ultrasound systems (A and B) to estimate equipment-related variability. In system B, two software versions were used to analyze ultrasound images. Longitudinal views of phantom segments were reconstructed to document transducer misplacement. Results. Oblique transducer positioning resulted in a non-linear overestimation of phantom areas that was independent of lumen size and also resulted in dramatic distortions of theree-dimensionally reconstructed phantom geometry. Eccentric positioning did not significantly influence measurement accuracy. In coaxial positioning, differences between measured and true areas increased non-linearly from 0,36 to 4,5 mm 2 in system B and in a linear fashion from -0.01 to 2.68 mm2 in system A with increasing phantom diameters. Relative differences decreased form 11.4% to 2.9% with increasing reference areas in system B (positive off-set error). When using updated software in system B, the off-set error was negative and relative error dimininshed from-1.34% to 0.44% with increasing phantom size. Conclusion. Transducer position and equipment-related factors influence the accuracy of intravascular ultrasound, which may lead to misinterpratation of vessel size and geometry even in straight vessel segments. Transducer position may be controlled by the reconstruction of longitudinal images. Ultrasound equipment should be calibrated before using it for quantative measurements.
Interaction between nitric oxide and endogenous vasoconstrictors in the control of renal blood flow
Hypertension, 34: 1254-1258, 1999
The level of renal blood flow (RBF) is controlled by opposing vasoconstrictor and vasodilator influences. In a recent investigation in normotensive dogs, we found that combined blockade of endothelin type A (ETA) receptors and angiotensin II formation induces marked increases in RBF that were much larger than the effects of blocking either system alone. The aim of the present study was to determine the contribution of nitric oxide (NO) to this vasodilator response. Experiments were made in 6 conscious, chronically instrumented dogs subjected to 5 different experimental treatments on separate days. Blockade of ETA receptors alone by the selective antagonist LU 135252 had only minor effects on RBF compared with time-control experiments. Additional blockade of angiotensin II formation by angiotensin-converting enzyme inhibition with trandolaprilat caused a substantial increase of RBF by 50%. This vasodilation was entirely suppressed when NO formation was prevented by inhibition of NO synthase with NG-nitro-L-arginine methyl ester HCl. However, when during NO synthase inhibition renal vascular NO concentrations were clamped at control levels by infusing the NO donor S-nitroso-N-acetyl-D,L-penicillamine, the vasodilator response to combined blockade of ETA receptors and angiotensin II formation was completely restored (RBF 60%). These results indicate that the vasodilation after combined ETA receptor blockade and angiotensin-converting enzyme inhibition is not mediated by an increase in NO release but results from the unmasking of the tonic influence that is normally exerted by constitutively released NO. Accordingly, the tonic activity of endothelial NO synthase appears to be of major importance in the physiological regulation of renal vascular resistance by determining the vasomotor responses to endothelin and angiotensin II.
Correlation of hemodynamic impact and morphologic degree of renal artery stenosis in a canine model
J Am Soc Nephrol 11: 2190-2198, 2000
In a non-invasive comprehensive magnetic resonance (MR) exam, the morphologic degree of renal artery stenosis was correlated to corresponding changes in renal artery flow dynamics. Different degrees of stenosis were created using a chronically implanted inflatable arterial cuff in 7 dogs. For each degree of stenosis an ultra fast 3D gadolinium MR angiography with high spatial resolution was performed, followed by cardiac-gated MR flow measurements with high temporal resolution for determination of pulsatile flow profiles and mean flow. Flow was also measured by a chronically-implanted flow probe. In 3 of the dogs also transstenotic pressure gradients (?P) were measured via implanted catheters. Five different degrees of stenosis could be differentiated in the MR angiograms (0%, 30%, 50%, 80%, ?90%). The MR flow data agreed with the flowprobe within ± 20%. Stenoses between 30% and 80% gradually reduced the early systolic peak (Max1) of the flow profile, but only minimally affected the midsystolic peak (Max2) or mean flow. Stenoses ?90% significantly depressed mean flow by >50%. The ratio between Max1 and Max2 (Rmax1/2) gradually fell with the degree of stenosis. The onset of significant mean flow reduction and ?P were indicated by a drop of Rmax1/2 below 1–1.2. Thus, the analysis of high-resolution flow profiles allows detection of early hemodynamic changes even at degrees of stenoses not associated with a reduction of mean flow. Rmax1/2 allows differentiation of the grade of hemodynamic compromise for a given morphologic stenosis independent of mean flow in a single comprehensive MR exam.
Large vasodilatations in skeletal muscle in resting conscious dogs and their contribution to blood pressure variability
J Physiol (London) 527: 611-622, 2000
1. Large (up to +400%) transient (~20 s) increases of blood flow were observed in the external iliac arteries of resting conscious dogs (n=10) in the absence of major alerting or muscular activity. At the same time arterial pressure ( AP ) slightly fell while heart rate ( HR ) rose. 2. The vasodilatations were resistant to atropine, ganglionic, ß-adrenergic, and NO-synthase inhibition, but were suppressed by spinal or general anaesthesia. 3. Vasodilatations of similar appearance were elicited by an alerting sound; these were abolished by atropine. 4. The spontaneous vasodilatations occurred simultaneously and their magnitudes were well correlated between both legs, but were not correlated to the amount of concomitant activation of the surface electromyogram. The duration of this activation almost never outlasted 10 s. 5. The reactive hyperaemia observed after a total occlusion of the artery even for 16 s was not large enough to explain the size of the spontaneous vasodilatations. Occlusion during peak flow of the vasodilatations did not affect the size of the reactive hyperaemia. 6. Spectral analysis made separately for data segments with vasodilatation and those without revealed, that the vasodilatations substantially enhanced the variability of AP and HR at frequencies below ~ 0.1 Hz. 7. In conclusion, large coordinated skeletal muscle vasodilatations were identified in resting conscious dogs, which are initiated neurally, but not by sympathetic-cholinergic or nitroxidergic fibres and which do not show any clear correlation to muscular contraction. The vasodilatations substantially affect the regulation of skeletal muscle blood flow and explain a significant portion of AP and HR variability.
Autonomic cardiovascular control in conscious mice
Am J Physiol Regulatory Integrative Comp Physiol 279: R2214-R2221, 2000
Autonomic cardiovascular control was characterized in conscious, chronically catheterized mice by spectral analysis of arterial pressure (AP) and heart rate (HR) during autonomic blockade or baroreflex modulation of autonomic tone. Both spectra were similar to those obtained in humans, but at ~10x higher frequencies. The 1/f-relation of the AP spectrum changed to a more shallow slope below 0.1 - 0.2 Hz. Coherence between AP and HR reached 0.5 or higher below 0.3 - 0.4 Hz and also above 2.5 Hz. Muscarinic blockade (atropine) or beta-adrenergic blockade (atenolol) did not significantly affect the AP spectrum. Atropine reduced HR variability at all frequencies, but this effect waned above 1 Hz. Beta-adrenergic blockade (atenolol) slightly enhanced the HR variability only above 1 Hz. Alpha-adrenergic blockade (prazosin) reduced AP variability between 0.05 and 3 Hz, most prominently at 0.15 - 0.7 Hz. A shift of the autonomic nervous tone by a hypertensive stimulus (phenylephrine) enhanced, while a hypotensive stimulus (nitroprusside) depressed AP variability at 1 - 3 Hz; other frequency ranges of the AP spectrum were not affected except for a reduction below 0.4 Hz after nitroprusside. Variability of HR was enhanced after phenylephrine at all frequencies and reduced after nitroprusside. As with atropine, the reduction with nitroprusside waned above 1 Hz. In conclusion, in mice HR variability is dominated by parasympathetic tone at all frequencies, during both blockade and physiological modulation of autonomic tone. There is a limitation for further reduction but not for augmentation of HR variability from the resting state above 1 Hz. The impact of HR on AP variability in mice is confined to frequencies higher than 1 Hz. Limits between frequency ranges are proposed as 0.15 Hz between VLF and LF and 1.5 Hz between LF and HF.
[Experimental flow and perfusion measurements in the animal model with magnetic resonance tomography]Radiologe 41: 146-153, 2001
AIM: Validation of non-invasive methods for morphologic and functional imaging of the kidney under physiologic and pathophysiologic conditions. MATERIAL AND METHODS: In chronically instrumented animals (foxhounds) comparative measurements of renal flow and perfusion were performed. Magnetic resonance imaging techniques were compared to data obtained from implanted flow probes and total kidney weight post mortem. In the MR system, different degrees of renal artery stenosis could be induced by means of an implanted inflatable cuff. The degree of stenosis was verified with high-resolution 3D contrast-enhanced MR angiography (3D-CE-MRA) using an intravascular contrast agent. RESULTS: The MR-data agreed well with the invasively obtained results. Artifacts resulting from the implanted flow probes and other devices could be kept to a minimum due to appropriate selection of the probe materials and measurement strategies. Stenoses could be reproduced reliably and quantified from the induced morphologic and functional changes. CONCLUSION: Morphologic and functional MR techniques are well suited for non-invasive in vivo assessment of renal blood flow physiology
Dynamic characteristics and underlying mechanisms of renal blood flow autoregulation in the conscious dog
Am J Physiol Renal Physiol 280: F1062-F2071, 2001
The time course of the autoregulatory response of renal blood flow (RBF) to a step increase of renal artery pressure (RAP) was studied in conscious dogs. After reducing RAP to 50 mmHg for 60s renal vascular resistance (RVR) decreased to 50%. When RAP was suddenly increased again RVR returned to baseline with a characteristic time course (control; n=15): Within the first 10s it rose rapidly to 70% of baseline (first response), thus comprising 40% already of the total RVR-response. Thereafter, it increased at a much slower rate until it started to rise rapidly again at 2030s after the pressure step (second response). After passing an overshoot of 117% at 43s, RVR returned to baseline values. Similar responses were observed after RAP reduction for 5min or following complete occlusions for 60s. When the tubuloglomerular feedback (TGF) was inhibited by furosemide (40mgi.v., n=12), the first response was enhanced, now providing 60% of the total response, while the second response was completely abolished. Instead, RVR slowly rose to reach the baseline at 60s (third response). The same pattern was observed, when furosemide was given at a much higher dose (>600mg i.v.; n=6) or in combination with clamping of the plasma levels of nitric oxide (n=6). In contrast to RVR, vascular resistance in the external iliac artery after a 60s complete occlusion started to rise with a delay of 4s and returned to baseline within 30s. It is concluded, that in addition to the myogenic response and the TGF, a third regulatory mechanism significantly contributes to RBF autoregulation. This mechanism is independent of nitric oxide. The three mechanisms contribute about equally to resting RVR. The myogenic response is faster in the kidney than in the hindlimb.
Central hypercapnic chemoreflex modulation of renal sympathetic nerve activity in experimental heart failure
Bas Res Cardiol, 97: 177-186, 2002
Activation of the sympathetic nervous system plays an important role in the pathophysiology and progression of congestive heart failure (CHF). The precise mechanisms responsible for sympathetic activation in CHF are not yet clearly established. An altered central hypercapnic chemoreflex modulation of sympathetic nerve activity (SNA) might be an explanation. Therefore, the response of postganglionic renal SNA to elevation of CO2 concentration in the inspiratory air to 2, 4, and 6% was determined in anesthetized, artificially ventilated rats after denervation of peripheral baro- and chemoreceptors 2 weeks (group A; n=8) or 6 weeks (group B; n=11) after induction of an aorto-caval shunt, or 4 weeks after aortic banding (group C; n=7). In all CHF models, left ventricular enddiastolic pressure was increased (A 8 +/- 1, B 8 +/- 1, C 10 +/- 2 mmHg) as compared to sham operated controls (A 3 +/- 1, B 4 +/- 1, C 5 +/- 1 mmHg). Indicative of left ventricular hypertrophy and pulmonary congestion, wet weight of heart (A + 60%, B + 93%, C + 49%) and lungs (A + 15%, B + 36%, C + 12%) were also enhanced as compared to controls. Elevation of inspiratory CO2 concentration to 2,4, and 6% increased renal SNA by approximately 10, 20, and 30% from resting activity in all groups. The maximum SNA responses at 6% CO2 in the groups with CHF (A + 390 +/- 95, B + 425 +/- 133, C + 368 +/- 158 microVs) did not differ from those in the respective controls (A + 510 +/- 130, B + 570 +/- 180, C + 275 +/- 25 microVs). It is concluded that under these experimental conditions the central hypercapnic chemoreflex sensitivity is not altered in either of the employed models of CHF and therefore may not play a major role for the well-known elevation of SNA in CHF.
Role of angiotensin II in dynamic renal blood flow autoregulation of the conscious dog
J Physiol 538: 167-177, 2002
The influence of angiotensin II (ANGII) on the dynamic characteristics of renal blood flow (RBF) was studied in conscious dogs by testing the response to a step increase in renal artery pressure (RAP) after a 60 s period of pressure reduction (to 50 mmHg) and by calculating the transfer function between physiological fluctuations in RAP and RBF. During the RAP reduction, renal vascular resistance (RVR) decreased and upon rapid restoration of RAP, RVR returned to baseline with a characteristic time course: within the first 10 s, RVR rose rapidly by 40 % of the initial change (first response, myogenic response). A second rise began after 20-30 s and reached baseline after an overshoot at 40 s (second response, tubuloglomerular feedback (TGF)). Between both responses, RVR rose very slowly (plateau). The transfer function had a low gain below 0.01 Hz (high autoregulatory efficiency) and two corner frequencies at 0.026 Hz (TGF) and at 0.12 Hz (myogenic response). Inhibition of angiotensin converting enzyme (ACE) lowered baseline RVR, but not the minimum RVR at the end of the RAP reduction (autoregulation-independent RVR). Both the first and second response were reduced, but the normalised level of the plateau (balance between myogenic response, TGF and possible slower mechanisms) and the transfer gain below 0.01 Hz were not affected. Infusion of ANGII after ramipril raised baseline RVR above the control condition. The first and second response and the transfer gain at both corner frequencies were slightly augmented, but the normalised level of the plateau was not affected. It is concluded that alterations of plasma ANGII within a physiological range do not modulate the relative contribution of the myogenic response to the overall short-term autoregulation of RBF. Consequently, it appears that ANGII augments not only TGF, but also the myogenic response.
In this work absolute values of regional renal blood volume (rRBV) and flow (rRBF) are assessed by means of contrast-enhanced (CE) MRI using an intravascular superparamagnetic contrast agent. In an animal study, eight foxhounds underwent dynamic susceptibility- weighted MRI upon injection of contrast agent. Using principles of indicator dilution theory and deconvolution analysis, parametric images of rRBV, rRBF, and mean transit time (MTT) were computed. For comparison, whole-organ blood flow was determined invasively by means of an implanted flow probe, and the weight of the kidneys was evaluated postmortem. A mean rBV value of 28 ml/100 g was found in the renal cortex, with a corresponding mean rBF value of 524 ml/100 g/min and an average MTT of about 3.4 s. Although there was a systematic difference between the absolute blood flow values determined by MRI and the ultrasonic probe, a significant correlation (r(s) = 0.72, P < 0.05) was established. The influence of the arterial input function (AIF), T(1) relaxation effects, and repeated measurements on the precision of the perfusion quantitation is discussed.
The interrelation between the morphologic degree of renal artery stenosis and changes in parenchymal perfusion is assessed using an intravascular contrast agent. In seven adult foxhounds, different degrees of renal artery stenosis were created with an inflatable clamp implanted around the renal artery. Dynamic susceptibility-weighted gradient-echo imaging was used to measure signal-time curves in the renal artery and the renal parenchyma during administration of 1.5 mg/kg BW of an intravascular ultrasmall particle iron oxide (USPIO) contrast agent. From the dynamic series, regional renal blood volume (rRBV), regional renal blood flow (rRBF), and mean transit time (MTT) were calculated. The morphologic degree of stenosis was measured in the steady state using a high-resolution 3D contrast-enhanced (CE) MR angiography (MRA) sequence (voxel size = 0.7 x 0.7 x 1 mm(3)). Five patients with renoparenchymal damage due to long-standing renal artery stenosis were evaluated. In the animal stenosis model, cortical perfusion remained unchanged for degrees of renal artery stenosis up to 80%. With degrees of stenoses > 80%, cortical perfusion dropped to 151 +/- 54 ml/100 g of tissue per minute as compared to a baseline of 513 +/- 76 ml/100 g/min. In the patients, a substantial difference in the cortical perfusion of more than 200 +/- 40 ml/100 g/min between the normal and the ischemic kidneys was found. The results show that quantitative renal perfusion measurements in combination with 3D-CE-MRA allow the functional significance of a renal artery stenosis to be determined in a single MR exam. Differentiation between renovascular and renoparenchymal disease thus becomes feasible.
Dynamics and contribution of mechanisms mediating renal blood flow autoregulation
Am J Physiol Regul Integr Comp Physiol, 285: 619-631, 2003
We investigated dynamic characteristics of renal blood flow (RBF) autoregulation and the relative contribution of the underlying mechanisms within the autoregulatory pressure range in Sprague-Dawley rats. Renal arterial pressure (RAP) was reduced by suprarenal aortic constriction for 60 s, and then rapidly released. Changes in renal vascular resistance (RVR) were assessed following the rapid step reduction and rise in RAP. In response to the rise, RVR initially fell 5-10% and subsequently increased ~20%, reflecting autoregulatory efficiency (AE) of 93%. Within the initial 7-9 s, RVR rose to 55% of the total response providing AE of 37%, reaching maximum speed at 2.2 s. A secondary RVR increase began at 7-9 s and reached maximum speed at 10-15 s. The response times suggest that the initial RVR reflects the myogenic response and the secondary tubuloglomerular feedback (TGF). During inhibition of TGF by furosemide, AE was 64%. The initial rise in RVR was accelerated (0.29 vs 0.20 mmHg/(ml/min/g)/s, p<0.05) and enhanced, providing AE of 49% (p=0.005 vs 37%), but it represented only 88% of the total response. The remaining 12% indicates participation of a third regulatory component. The latter contributed up to 50% when the step increase in RAP began below the autoregulatory range. Augmentation of TGF by acetazolamide affected neither AE nor the relative myogenic contribution. Infusion of the Ca2+-channel blocker diltiazem markedly inhibited AE and the primary and secondary increases of RVR but left a slow component. In response to reduction of RAP the initial vasodilation constituted 73% of the total response, but was not affected by furosemide. Contribution of the third component was 9%. In conclusion, RBF autoregulation is primarily due to myogenic response and TGF, contributing 55% and 33-45% in response to a rise and 73 % and 18-27 % to reduction of RAP. The data imply interaction between TGF and myogenic response affecting strength and speed of the myogenic response during rises of RAP. The data suggest a third regulatory system contributing <12% normally, but up to 50% at low RAP; its nature awaits further investigation.
Dual constrictor and dilator actions of ETB receptors in the rat renal microcirculation: interactions with ETA receptors.
Am J Physiol Renal Physiol 286: F660-F668, 2004
The vascular actions of endothelin-1 (ET-1) reflect the combination of vasoconstrictor ETA and ETB receptors on smooth muscle cells and vasodilator ETB receptors on endothelial cells. The present study investigated the contribution of ET receptor subtypes using a comprehensive battery of agonists and antagonists infused directly into the renal artery of anesthetized rats to evaluate the actions of each receptor class alone and their interactions. ET-1 (5 pmol) reduced renal blood flow (RBF) 25±1 %. ETA antagonist BQ-123 attenuated this response to a 15±1 % decrease in RBF (p<0.01), indicating net constriction by ETB receptors. Combined receptor blockade (BQ-123 + BQ788) resulted in a renal vasoconstriction of 7±1 % (p=0.001 vs. BQ-123), supporting constrictor action of ETB receptors. In marked contrast, the ETB antagonist BQ-788 enhanced the ET-1 RBF response to 60±5 % (p<0.001), suggesting ETB-mediated net dilation. Consistent with ETA blockade, the ETB agonist sarafotoxin 6C (S6C) produced vasoconstriction, reducing RBF by 23±5 %. Dose-response curves for ET-1 and S6C showed similar degrees of constriction between 0.2-100 pmol. Both antagonists (BQ-123, BQ-788) were equally effective at 3-fold lower than the standard doses suggesting complete inhibition. We conclude that ETB receptors alone exert a net constrictor effect, but cause a net dilator influence when co-stimulated with ETA receptors. Such opposing actions indicate more complex than additive interaction between receptor subtypes. Model analysis suggests ETA-mediated constriction is appreciably greater without than with co-stimulation of ETB receptors. Possible explanations include ET-1 clearance by ETB receptors and/or a dilator ETB receptor function that counteracts constriction.
Thromboxane receptor mediates renal vasoconstriction and contributes to acute renal failure in endotoxemic mice.
J Am Soc Nephrol, 15: 2358-2365, 2004
Sepsis is a major cause of acute renal failure (ARF) and death. Thromboxane A2 (TxA(2)) may mediate decreases of renal blood flow (RBF) and/or GFR associated with LPS-induced sepsis. This study tested whether TxA(2) receptor blockade, with the use of TxA(2) receptor knockout (TP-KO) mice or a selective TP receptor antagonist (SQ29,548), would alleviate LPS-induced renal vasoconstriction and ARF. Under basal conditions, anesthetized TP-KO mice displayed a lower mean arterial pressure than wild-type (WT) mice (102 versus 94 mmHg; P < 0.05). RBF, renal vascular resistance (RVR), GFR, and urine flow did not differ among groups under basal conditions, suggesting little tonic influence of TxA(2) on renal TP receptors in health. In endotoxemic WT mice, 14 h after LPS (Escherichia coli LPS 8.5 mg/kg intraperitoneally), mean arterial pressure was reduced to 85 mmHg (P < 0.001), as were RBF (5.0 versus 9.3 ml/min per g kidney wt; P < 0.001) and GFR (0.38 versus 1.03 ml/min per g kidney wt; P < 0.001). Heart rate and RVR (71 versus 47 mmHg/ml per min; P < 0.05) increased. The decreases in RBF and GFR after LPS were attenuated in TP-KO mice versus WT mice (both P < 0.05). In both TP-KO and TP antagonist-treated mice, RVR remained stable in response to LPS versus WT mice that did not receive LPS. Delayed TP-antagonist treatment (12 h after LPS injection) ameliorated RBF and RVR but did not restore GFR. In other WT animals, TP-antagonist treatment for 2 h before intravenous LPS abolished the early renal vasoconstriction and alleviated the decrease in GFR. These results demonstrate that renal vasoconstriction during endotoxemic shock induced by LPS is mediated by TP receptors as indicated by pharmacologic blockade and genetic disruption of TP receptors.
Nitric oxide and NO-independent mechanisms mediate ETB receptor buffering of ET-1-induced renal vasoconstriction in the rat.
Am J Physiol Regul Integr Comp Physiol 288: R1168-R1177, 2005
Vascular ETB receptors exert both dilator and constrictor actions in a complex interaction with ETA receptors. The aim of this study was to clarify the presence and relative importance of nitric oxide and other possible mechanisms underlying the dilator effects of ETB receptors in the rat kidney. Complete inhibition of NO production (L-NAME, 25 mg/kg, iv) enhanced the renal vasoconstriction elicited by entothelin-1 (ET-1) injected into the renal artery from -15 to -30%. Counteraction of the L-NAME-induced vasoconstriction by infusion of the NO-donor nitroprusside (NP) into the renal artery did not reverse this effect (+NP=-29%), but nevertheless effectively buffered Ang II-mediated renal vasoconstriction. Similarly, renal vasoconstrictor responses to ET-1 were enhanced after a smaller dose of L-NAME administered into the renal artery (-22 vs. -15%) and unaffected by subsequent infusion of a vasodilator dose of NP (-21%). These results indicate that the responsiveness to ET-1 is buffered by endothelial ETB receptor stimulated phasic release of NO rather than the static mean ambient NO level. In other experiments, intrarenal infusion of ETB-receptor antagonist BQ788 further enhanced the constrictor response to ET-1 seen during NP + L-NAME (-92 vs. -49%), revealing a NO-independent dilator component. In controls, the vasoconstriction to ET-1 was unaffected by vehicle (-27 vs. -20%) and markedly enhanced during ETB receptor antagonism (-70%). The same pattern of ET-1 responses was observed when indomethacin was given to inhibit cyclooxygenase (control=-20%, indo=-22%, +ETB-antagonist=-56%) or MS-PPOH or Miconazole+indomethacin to inhibit epoxygenase alone (control=-10%, MSPPOH=-11%, +ETB-antag.=-35%) or in combination (control=-14%, indo+mico=-20%, +ETB-antag.=-43%). We conclude that phasic release of endogenous NO, but not the static ambient level, mediates part of the dilator effect of ETB receptors. In addition, playing a major buffering role is a NO-independent mechanism, perhaps reflecting clearance of ET by ETB receptors, that is distinct from prostanoids and epoxyeicosatrienoic acids.
Nitric oxide blunts myogenic autoregulation in rat renal but not skeletal muscle circulation via tubuloglomerular feedback.
J Physiol (London) 569: 959-974, 2005
This rat renal blood flow (RBF) study quantified the impact of nitric oxide synthase (NOS) inhibition on the myogenic response and the balance of autoregulatory mechanisms in the time domain following a 20 mmHg-step increase or decrease in renal arterial pressure (RAP). When RAP was increased, the myogenic component of renal vascular resistance (RVR) rapidly rose within the initial 7-10 s, exhibiting a ~ 5 s time constant and providing ~ 36% of perfect autoregulation. A secondary rise between 10 - 40 s brought RVR to 95% total autoregulatory efficiency; reflecting TGF and possibly one or two additional mechanisms. The kinetics were similar after the RAP decrease. Inhibition of NOS (L-NAME) increased RAP, enhanced the strength (79% autoregulation) and doubled the speed of the myogenic response, and promoted the emergence of RVR oscillations (~ 0.2 Hz); the strength (52%) was lower at control RAP. An equi-pressor dose of angiotensin II had no effect on myogenic or total autoregulation. Inhibition of tubuloglomerular feedback (TGF) (furosemide) abolished the L-NAME effect on the myogenic response. RVR responses during furosemide, assuming complete inhibition of TGF, suggest a third mechanism that contributes 10-20% and is independent of TGF, slower than myogenic, and abolished by NOS inhibition. The hindlimb circulation displayed a solitary myogenic response similar to the kidney (35% autoregulation) that was not enhanced by L-NAME. We conclude that NO normally restrains the strength and speed of the myogenic response in RBF but not hindlimb autoregulation, an action dependent on TGF, thereby allowing more and slow RAP fluctuations to reach glomerular capillaries.
Superoxide mediates acute renal vasoconstriction produced by angiotensin II and catecholamines by a mechanism independent of nitric oxide.
Am J Physiol Heart Circ Physiol. 292: H83-H92, 2007
NAD(P)H oxidases (NOX) and reactive oxygen species (ROS) are involved in vasoconstriction and vascular remodeling during hypertension produced by chronic angiotensin II (Ang II) infusion. These effects are thought to be mediated largely through superoxide anion (O2-) scavenging of nitric oxide (NO). Little is known about the role of ROS in acute vasoconstrictor responses to agonists. We investigated renal blood flow (RBF) reactivity to Ang II (4 ng), norepinephrine (NE, 20 ng), and α1-adrenergic agonist phenylephrine (PE, 200 ng) injected into the renal artery (ira) of anesthetized Sprague-Dawley rats. The NOX inhibitor apocynin (1-4 mg/kg/min ira, 2 min) or the superoxide dismutase mimetic tempol (1.5-5 mg/kg/min ira, 2 min) rapidly increased resting RBF by 8±1% (p<0.001) or 3±1% (p<0.05), respectively. During NO-synthase (NOS)-inhibition (L-NAME, 25 mg/kg iv), the vasodilation tended to increase (apocynin 13±4%, tempol 10±1%). During control conditions, both Ang II and NE reduced RBF by 24±4%. Apocynin dose-dependently reduced the constriction by up to 44% (p<0.05). Similarly, tempol blocked the acute actions of Ang II- and NE by up to 48-49% (p<0.05). In other animals, apocynin (4 mg/kg/min ira) attenuated vasoconstriction to Ang II, NE, and PE by 46-62% (p<0.01). During NOS-inhibition, apocynin reduced the reactivity to Ang II and NE by 60-72% (p<0.01), and tempol reduced it by 58-66% (p<0.001). We conclude that NOX-derived ROS substantially contribute to basal RBF as well as to signaling of acute renal vasoconstrictor responses to Ang II, NE, and PE in normal rats. These effects are due to (O2-) rather than H2O2, occur rapidly, and are independent of scavenging of NO.
A novel mechanism in renal blood flow autoregulation and the autoregulatory role of A1 adenosine receptors in mice.
Am J Physiol Renal Physiol 293: F1489-F1500, 2007
Autoregulation of renal blood flow (RBF) is mediated by a fast myogenic response (MR; approximately 5 s), a slower tubuloglomerular feedback (TGF; approximately 25 s), and potentially additional mechanisms. A1 adenosine receptors (A1AR) mediate TGF in superficial nephrons and contribute to overall autoregulation, but the impact on the other autoregulatory mechanisms is unknown. We studied dynamic autoregulatory responses of RBF to rapid step increases of renal artery pressure in mice. MR was estimated from autoregulation within the first 5 s, TGF from that at 5-25 s, and a third mechanism from 25-100 s. Genetic deficiency of A1AR (A1AR-/-) reduced autoregulation at 5-25 s by 50%, indicating a residual fourth mechanism resembling TGF kinetics but independent of A1AR. MR and third mechanism were unaltered in A1AR-/-. Autoregulation in A1AR-/- was faster at 5-25 than at 25-100 s suggesting two separate mechanisms. Furosemide in wild-type mice (WT) eliminated the third mechanism and enhanced MR, indicating TGF-MR interaction. In A1AR-/-, furosemide did not further impair autoregulation at 5-25 s, but eliminated the third mechanism and enhanced MR. The resulting time course was the same as during furosemide in WT, indicating that A1AR do not affect autoregulation during furosemide inhibition of TGF. We conclude that at least one novel mechanism complements MR and TGF in RBF autoregulation, that is slower than MR and TGF and sensitive to furosemide, but not mediated by A1AR. A fourth mechanism with kinetics similar to TGF but independent of A1AR and furosemide might also contribute. A1AR mediate classical TGF but not TGF-MR interaction.
Reactive oxygen species participate in acute renal vasoconstrictor responses induced by ETA and ETB receptors.
Am J Physiol Renal Physiol 294: F719-28, 2008
Reactive oxygen species (ROS) play important roles in renal vasoconstrictor responses to acute and chronic stimulation by angiotension II and norepinephrine, as well as in long-term effects of endothelin-1 (ET-1). Little is known about participation of ROS in acute vasoconstriction produced by ET-1. We tested the influence of NAD(P)H oxidase inhibition by apocynin (4 mg/kg/min, infused into the renal artery (ira)) on ETA and ETB receptor signaling in the renal microcirculation. Both receptors were stimulated by ET-1, ETA receptors by ET-1 during ETB antagonist BQ-788, and ETB by ETB agonist sarafotoxin 6C. ET-1 (1.5 pmol injected ira) reduced renal blood flow (RBF) 17+/-4%. Apocynin raised baseline RBF (+10+/-1%, p<0.001) and attenuated the ET-1 response to 10+/-2%, i.e., 35+/-9% inhibition (p<0.05). Apocynin reduced ETA-induced vasoconstriction by 42+/-12% (p<0.05) and that of ETB-stimulation by 50+/-8% (p<0.001). During nitric oxide (NO) synthase inhibition (LNAME), apocynin blunted ETA-mediated vasoconstriction by 60+/-8% (p<0.01), whereas its effect on the ETB-response (by 87+/-8%, p<0.001) was even larger without than with NO present (p<0.05). The cell-permeable superoxide dismutase mimetic tempol (5 mg/kg/min ira), which reduces O2(-) and may elevate H2O2, attenuated ET-1 responses similar to apocynin (by 38+/-6%, p<0.01). We conclude that ROS, O2(-) rather than H2O2, contribute substantially to acute renal vasoconstriction elicited by both ETA and ETB receptors and to basal renal vasomotor tone in vivo. This physiological constrictor action of ROS does not depend on scavenging of NO. In contrast, scavenging of O2(-) by NO seems to be more important during ETB stimulation. Key words: renal hemodynamics, vascular smooth muscle, afferent arteriole, reactive oxygen species, nitric oxide.
We studied the role of connexin 40 (Cx40) in autoregulation of renal blood flow (RBF), autoregulatory mechanisms, and agonist-induced vasomotor responses. Mice lacking Cx40 (Cx40-ko) had impaired steady-state autoregulation (7±22% of perfect vs. 102±11% in wild-type (wt), p<0.01) to a sudden step increase in renal perfusion pressure. Dynamic analysis of the three main mechanisms revealed markedly reduced tubuloglomerular feedback (TGF) in Cx40-ko (18±8 vs. 75±10%, p<0.01), while the most-rapid myogenic response (MR) and slowest third component were not consistently altered. In mice with Cx40 replaced by Cx45 (Cx40KI45) steady-state autoregulation (66±17%) and TGF (40±11%) were weaker than in wt and tended to be stronger than in Cx40-ko. L-NAME augmented MR similarly in all genotypes, maintaining impaired overall autoregulation (29±30 vs. 102±5%, Cx40-ko vs. wt, p<0.05). Responses of renovascular resistance and arterial pressure to norepinephrine (NE, 75 μg iv) and acetylcholine (ACh, 25 μg) were similar in all groups before or after L-NAME. Systemic and renal vasoconstrictor responses to L-NAME were also similar in all genotypes. Immunhistochemistry showed Cx37, Cx40, and Cx43 in preglomerular endothelial, renin-producing, and mesangial cells. In Cx40-ko and Cx40KI45, expression of Cx40 was absent. We conclude that Cx40 contributes to RBF autoregulation and is critical for TGF-mediated signal transduction to the afferent arteriole. This Cx40 function can partly be substituted by Cx45 and is independent of NO. The modulation of the renal MR by NO does not require Cx40, nor does acute renal vasoconstriction elicited by NE or vasodilation by ACh or NO.
Nitric oxide (NO) blunts the myogenic response (MR) in renal blood flow (RBF) autoregulation. We sought to clarify the roles of NO-synthase (NOS) isoforms, i.e. nNOS from macula densa, eNOS from the endothelium, and iNOS from smooth muscle or mesangium. RBF autoregulation was studied in rats and knockout (ko) mice in response to a rapid rise in renal artery pressure (RAP). The autoregulatory rise in renal vascular resistance within the first 6 s was interpreted as MR, from ~6 to ~30 s as tubuloglomerular feedback (TGF), and ~30 to ~100 s as third regulatory mechanism. In rats, nNOS inhibitor SMTC did not significantly affect MR (67±4 vs. 57±4 units). Inhibition of all NOS-isoforms by L-NAME in the same animals markedly augmented MR to 78±4 units. The same was found when SMTC was combined with Angiotensin II to reproduce the hypertension and vasoconstriction seen with L-NAME (58±3 vs. 54±7, L-NAME 81±2 units), or when SMTC was replaced by the nNOS-inhibitor NPA (57±5 vs. 56±7, L-NAME 79±4 units) or by the iNOS-inhibitor 1400W (50±1 vs. 55±4, LNAME 81±3 units). nNOS-ko mice showed the same autoregulation as wild-types (MR 36±4 vs. 38±3 units) and the same response to L-NAME (111±9 vs. 114±10 units). eNOS-ko had similar autoregulation as wild-types (44±8 vs. 33±4 units), but failed to respond to L-NAME (37±7 vs. 78±16 units). We conclude that the attenuating effect of NO on MR depends on eNOS, but not on nNOS or iNOS. In eNOS-ko mice MR is depressed by NO-independent means.
Endothelium-dependent vasodilation is mediated by nitric oxide (NO), prostaglandins (PG), and endothelium-derived hyperpolarizing factor (EDHF). We studied the contributions and temporal characteristics of these components in the renal vasodilator responses to acetylcholine (ACh) and bradykinin (BK) and in the buffering of vasoconstrictor responses to norepinephrine (NE) and angiotensin II (ANG II). Renal blood flow (RBF) and vascular conductance (RVC) were studied in anesthetized rats in response to renal arterial bolus injections before and after inhibition of NO-synthase (N(G)-nitro-L-arginine methyl ester, L-NAME), cyclooxygenase (indomethacin, INDO), or both. ACh increased RVC peaking at maximal time (tmax) = 29 s. L-NAME (n = 8) diminished the integrated response and made it substantially faster (tmax = 18 s). The point-by-point difference caused by L-NAME (= NO component) integrated to 74% of control and was much slower (tmax = 38 s). INDO (n = 9) reduced the response without affecting tmax (36 vs. 30 s). The difference (= PG) reached 21% of the control with tmax = 25 s. L-NAME+INDO (n = 17) reduced the response to 18% and markedly accelerated tmax to 16s (= EDHF). Results were similar for BK with slightly more PG and less NO contribution than for ACh. Constrictor responses to NE and ANG II were augmented and decelerated by L-NAME and L-NAME+INDO. The calculated difference (= buffering by NO or NO+PG) was slower than the constriction. It is concluded that NO, PG, and EDHF contribute >50%, 20-40%, and <20% to the renal vasodilator effect of ACh and BK, respectively. EDHF acts substantially faster and less sustained (tmax = 16 s) than NO and PG (tmax = 30 s). Constrictor buffering by NO and PG is not constant over time, but renders the constriction less sustained.
We studied the influence of soluble guanylate (sGC) on renal blood flow (RBF), glomerular filtration rate (GFR), and RBF autoregulation and its role in mediating the hemodynamic effects of endogenous nitric oxide (NO). Arterial pressure (AP), heart rate (HR), RBF, GFR, urine flow (UV), and the efficiency and mechanisms of RBF autoregulation were studied in anesthetized rats during intravenous infusion of sGC activator cinaciguat before and (except GFR) also after inhibition of NO synthase (NOS) by N(ω)-nitro-l-arginine methyl ester. Cinaciguat (0.1, 0.3, 1, 3, 10 μg·kg(-1)·min(-1), n = 7) reduced AP and increased HR, but did not significantly alter RBF. In clearance experiments (FITC-sinistrin, n = 7) GFR was not significantly altered by cinaciguat (0.1 and 1 μg·kg(-1)·min(-1)), but RBF slightly rose (+12%) and filtration fraction (FF) fell (-23%). RBF autoregulatory efficiency (67 vs. 104%) and myogenic response (33 vs. 44 units) were slightly depressed (n = 9). NOS inhibition (n = 7) increased AP (+38 mmHg), reduced RBF (-53%), and greatly augmented the myogenic response in RBF autoregulation (97 vs. 35 units), attenuating the other regulatory mechanisms. These changes were reversed by 77, 78, and 90% by 1 μg·kg(-1)·min(-1) cinaciguat. In vehicle controls (n = 3), in which cinaciguat-induced hypotension was mimicked by aortic compression, the NOS inhibition-induced changes were not affected. We conclude that sGC activation leaves RBF and GFR well maintained despite hypotension and only slightly impairs autoregulation. The ability to largely normalize AP, RBF, RBF autoregulation, and renovascular myogenic response after NOS inhibition indicates that these hemodynamic effects of NO are predominantly mediated via sGC.
Blockade of NO synthesis reduces mean RBF by about 30%. The GFR is slightly reduced or not changed at all. The autoregulatory capacity for both RBF and GFR in response to static or dynamic pressure changes is not affected. This suggests that under normal conditions, the renal vasculature is subject to a strong continuous influence by endogenous NO, which determines the mean RBF. However, although the tubuloglomerular feedback and myogenic response may be modulated by NO, the accuracy of the renal haemodynamic autoregulation as well as the mean GFR seem to be kept independent of the influence on mean RBF.
Physiology and pathophysiology of baroreceptor function and neuro-hormonal abnormalities in heart failure.
Basic Res Cardiol. 1998; 93 Suppl 1: 1-22
This review deals with the neuro-hormonal changes in congestive heart failure, a syndrome that is usually initiated by a reduction of cardiac output. Inorder to do this, we should like to 1) summarise previous and more recent evidence for a number of these neuro-humoral derangement's, 2) review the experimental evidence for an abnormality in the function of the arterial- and cardiopulmonary baroreceptor reflexes and 3) discuss, whether this abnormality or an interaction with renal mechanisms might cause the neuro-hormonal derangements in congestive heart failure.
AIM: New diagnostic strategies for evaluation of the kidney by fast MR imaging techniques. MATERIAL AND METHODS: A comprehensive morphologic and functional evaluation of the kidney is proposed using fast MR imaging of renal morphology, multiphase 3D gadolinium MR angiography, MR urography and MR flow measurements. A single MR examination is designed to grade renovascular disease and assess the hemodynamic and functional significance, detect and characterize renal lesions and evaluate the urinary tract. RESULTS: The combined analysis of morphologic and functional data allows reliable assessment of renal artery stenosis, benign and malignant renal masses and diseases of the renal collecting system and ureters, as well as congenital abnormalities in good agreement to the results of conventional imaging modalities. The improved tissue contrast and additional functional information compensates for the disadvantage of a lower spatial resolution. CONCLUSION: Combined morphologic and functional MR examination represents a reliable, non-invasive and cost-effective alternative imaging modality for comprehensive diagnostic evaluation of renal disease.
Quantitative Erfassung der renalen Funktion mit der Magnetresonanztomographie
[Quantitative recording of renal function with magnetic resonance tomography]
Radiologe 40: 925-937, 2000
AIM: To show the potential of various methods in magnetic resonance imaging for the evaluation of renal function. MATERIAL AND METHODS: A combined assessment of renal morphology, renal hemodynamics and function is proposed. Various techniques are explained, including multiphasic 3D gadolinium MR angiography, MR phase-contrast flow measurements, quantitative perfusion measurements with intravascular contrast agents, and MR renography and MR urography. The use of these techniques is demonstrated for renovascular diseases. RESULTS: The combined use of these techniques allows renal artery stenosis to be accurately detected and evaluation of renal blood flow, perfusion, glomerular filtration rate, and renal excretion. Based on true quantitative parameters, the hemodynamic and functional significance of the stenosis can be assessed. Renovascular diseases can be differentiated from renoparenchymal disease. CONCLUSION: For the assessment of renal function, functional magnetic resonance imaging techniques are an important alternative to nuclear medicine. The predictive value regarding the effect of revascularization is currently under investigation.
AIM: Validation of non-invasive methods for morphologic and functional imaging of the kidney under physiologic and pathophysiologic conditions. MATERIAL AND METHODS: In chronically instrumented animals (foxhounds) comparative measurements of renal flow and perfusion were performed. Magnetic resonance imaging techniques were compared to data obtained from implanted flow probes and total kidney weight post mortem. In the MR system, different degrees of renal artery stenosis could be induced by means of an implanted inflatable cuff. The degree of stenosis was verified with high-resolution 3D contrast-enhanced MR angiography (3D-CE-MRA) using an intravascular contrast agent. RESULTS: The MR-data agreed well with the invasively obtained results. Artifacts resulting from the implanted flow probes and other devices could be kept to a minimum due to appropriate selection of the probe materials and measurement strategies. Stenoses could be reproduced reliably and quantified from the induced morphologic and functional changes. CONCLUSION: Morphologic and functional MR techniques are well suited for non-invasive in vivo assessment of renal blood flow physiology.
The Mechanisms of Renal Blood Flow Autoregulation. Dynamics and Contributions. (Invited review)
Am J Physiol Regul Integr Comp Physiol 292: R1-R17, 2007
Autoregulation of renal blood flow (RBF) is caused by the myogenic response (MR), tubuloglomerular feedback (TGF), and a third regulatory mechanism that is independent of TGF but slower than MR. The underlying cause of the third regulatory mechanism remains unclear; possibilities include ATP, Ang II or a slow component of MR. Other mechanisms, which, however, exert their action through modulation of MR and TGF are pressure-dependent change of proximal tubular reabsorption, resetting of RBF and TGF, as well as modulating influences of Angiotensin II (Ang II) and nitric oxide (NO). MR requires <10 s for completion in the kidney and normally follows first-order kinetics without rate-sensitive components. TGF takes 30-60 s and shows spontaneous oscillations at 0.025-0.033 Hz. The third regulatory component requires 30-60 s; changes in proximal tubular reabsorption develop over 5 min and more slowly for up to 30 min, while RBF and TGF resetting stretch out over 20-60 min. Due to these kinetic differences, the relative contribution of the autoregulatory mechanisms determines the amount and spectrum of pressure fluctuations reaching glomerular and postglomerular capillaries and thereby potentially impinge on filtration, reabsorption, medullary perfusion, and hypertensive renal damage. Under resting conditions, MR contributes ~50% to overall RBF autoregulation, TGF 35-50%, and the third mechanism less than 15%. NO attenuates the strength, speed and contribution of MR, whereas Ang II does not modify the balance of the autoregulatory mechanisms.
- Evaluation of aortic compliance in humans (Editorial) Am J Physiol, 278: H1411-H1413, 2000
- The Orexins: Linking Circulatory Control with Behavior (Editorial) Am J Physiol Regul Integr Comp Physiol 285: 519-521, 2003
- Going with the Wnt? - Editorial Focus: Hyperaldosteronism, hypervolemia and increased blood pressure in mice expressing defective APC. (Invited editorial) Am J Physiol Regul Integr Comp Physiol 297: R568-R570, 2009
- Salt control. Focus on "High salt induces autocrine actions of ET-1 on inner medullary collecting duct NO production via upregulated ETB receptor expression". (Invited editorial) Am J Physiol Regul Integr Comp Physiol 311: R374-R376, 2016
Praktikum der Kreislaufphysiologie am Menschen (Practical course for cardiovascular physiology in humans)
Physiologie Heft 12, 1999
(Bericht über Unterrichtsmethoden, Report on teaching methods) Author's translation: We would like to present here a practical course, based on which we have been teaching the physiology of circulatory regulation in Heidelberg since two years. All experiments included herein are made on human subjects. By the use of non-invasive measurements of arterial pressure (Finapres), heart rate, and stroke volume (cardioimpedance), the arterial pressure pulse curve, hydrostatic pressure differences, as well as peripheral resistance are demonstrated. The student is led to the understanding of integrative regulatory functions by observation of the influences of respiration on the circulation as well as by investigation of the function and the time course of the baroreceptor reflex response (neck chamber), the hemodynamic responses to a reduction of the central blood volume (lower body negative pressure), as well as the consequences of the Valsalva-manoever. With the same or slightly expanded equipment, also many more interesting experiments are feasable.
A broader view of animal research.(comment)
BMJ. 334(7588):274, 2007
Perel et al examined only immediate preclinical testing of new drug therapies,1 but animal research aids medical science in many more ways Animal studies play a part in the initial development of candidate drugs, and the development and testing of medical devices and surgical procedures. Even more crucial, animal research informs clinical research by building the foundation of biological knowledge. Basic research that expands our understanding of how life systems function indicates to clinicians not only what direction to pursue but what directions are possible. Although animal research informs clinical research, its circumstances and experimental goals differ from those of clinical research. Thus their protocols and experimental designs necessarily differ. Animal studies generally seek a mechanism of action for treatment, rather than treatment efficacy. They are usually conducted on defined, genetically homogenous subjects with near perfect compliance, as opposed to the large scale diversity of genetics and behaviour of a clinical population. Some clinically necessary procedures, such as double blinding, serve little purpose in an animal study, since rats are not susceptible to the placebo effect. Furthermore, accepted standards for animal welfare as well as many national and institutional protocols insist that sample sizes of animal studies be small. Despite these differences, the protocol used by Perel et al to determine that the animal studies were of "poor" quality was based, for the most part, on standards meant for large clinical trials.
Recent abstracts, not yet published as original publication
Calcium signaling at different sites along the interlobular arteriole and in cremaster muscle arterioles
FASEB J 16 (4): A473, abstract 397.2, 2002
Responses of intracellular calcium ([Ca2+]i) were investigated in response to increased external KCl (50 mM) and norepinephrine (NE 1µM) in microdissected interlobular ILA, n=6) and cremaster muscle arterioles (CMA, n=5) from rats using FURA-2. In response to NE [Ca2+]i reached a transient peak after 5-15 sec and then declined to a constant elevated plateau level. High KCl evoked a more square-shaped response. The responses did not differ between proximal, middle and distal segments of the ILA. In CMA the peak after high KCl was larger than in ILA (+53±5 vs +32±3 nM, p=0.004), while the plateau level and the response to NE were similar. In dose response curves for KCl (10 - 100 mM) and NE (10 nM - 3o µM) CMA showed a higher maximum peak response to KCl (117±36 vs 44±8 nM), while sensitivity (ED50 47±3 vs 53±5 mM) and plateau response were not different from ILA. The peak response to KCl was unaffected by ?-adrenergic blockade indicating it was not due to perivascular nerves. Responses to NE were the same in both vessels. The time to peak after NE was not significantly different between CMA and ILA (13±2 vs 9±2 s for NE and 10±2 vs 11±1 s for KCl). In conclusion, there are only subtle variations in the pattern of the [Ca2+]i response along the ILA. The response to NE was surprisingly similar between both vessel types, while in response to high KCl CMA showed an exaggerated peak increase of [Ca2+]i.
Role of prostaglandins in the modulation of renal vascular responsiveness to angiotensin II with chronic changes in sodium intake.
FASEB J 17 (5-I): A95, abstract 90.11, 2003
The responsiveness of vascular beds to the constrictor effects of angiotensin II (AngII) decreases during chronic sodium restriction. AngII is known to downregulate vascular AT1 receptors and to stimulate local production of vasodilator prostaglandins (PGE2, PGI2). During sodium restriction the expression of cyclooxygenase-2 (COX-2) is upregulated in the renal cortex. Thus, we investigated the role of COX metabolites on the modulation of the renal blood flow (RBF) responses to AngII (4 ng) injected into the renal artery in rats kept on low or high salt diet for 10 days. Resting RBF and urine output were significantly lower in the low salt animals. The AngII-induced constriction was smaller after sodium restriction (13±1% vs 25±3% reduction of basal RBF, p<0.001). COX-inhibition by indomethacin enhanced the AngII response in both groups by a similar amount (D=+6±2% vs +7±2%), but the response to a larger dose of AngII (8 ng) in low salt rats was more enhanced by indomethacin (D=+20±5%, p<0.05). Exogenous PGE2 blunted the AngII response more in the low salt group (57±13% vs 13±9%, p<0.05), whereas the effect of PGI2 (iloprost) did not differ between groups (36±8% vs 33±6%). We conclude that the buffering efficiency of PGE2 but not PGI2 is enhanced during sodium restriction, possibly due to upregulation of EP4 receptors. This effect contributes in part to the attenuated constrictor response to AngII during salt deprivation. Supported by NIH grant RO1-HL02334.
Pressure-dependent variation of the contribution of myogenic response and tubuloglomerular feedback to renal blood flow autoregulation.
FASEB J 18 (4): A286, abstract 205.4, 2004
Renal blood flow (RBF) autoregulation is mediated by an intrinsic myogenic response (MR), tubuloglomerular feedback (TGF) and possibly a third regulatory mechanism. Micropuncture studies suggest a reduction of TGF at lower renal artery pressure (RAP). We investigated the relative participation of the regulating mechanisms at varying RAP in anesthetized euvolemic rats. The contribution of MR was derived from the autoregulatory changes in renal vascular resistance occurring within the first 7-9 s after a 20 mmHg step perturbation of RAP within the autoregulatory pressure range. Elevation of baseline RAP by iv infusion of angiotensin II enhanced the contribution of MR (51±3 vs 42±2 %, p<0.05, n=8). A similar change was seen during increased RAP with phenylephrine (62±5 vs 39±5 %, p<0.01, n=6) or baroreflex stimulation by carotid occlusion (63±4 vs 48±4 %, p<0.05, n=9). The pressure-dependency was reversed by mechanical restoration of RAP to basal levels. Reduction of RAP below resting levels to 90 mmHg diminished autoregulation, but the fraction of MR did not change (51±3 vs 51±2 %, n=10). Marked inhibition of TGF by furosemide enhanced the contribution of MR more at resting (78±3 vs 48±3 %, p<0.001, n=6) than at reduced RAP (61±4 % vs 54±5 %, p>0.5) indicating contribution of TGF is reduced and that of a third mechanism enhanced at lower RAP. We conclude that elevation of RAP (or vasoconstriction) enhances the participation of MR in autoregulation of RBF. Reduction of RAP reduces TGF and augments a third mechanism. The mechanisms mediating changes in the efficiency of MR as a function of RAP or initial vascular tone requires further investigation as does the nature of the third mechanism. Supported by NIH, R01-HL02334
Frequency response characteristics of sympathetic and autoregulatory vasomotor responses in the kidney and hindlimb.
FASEB J 20 (4): A759, abstract 472.4, 2006
The literature suggests vasomotor responses might be faster in the kidney than in other vascular beds. We compared vasoconstrictor response times to sympathetic nerve stimulation (SNS) as well as autoregulatory responses (AR) to a 20 mmHg step increase of perfusion pressure in the kidney and hindlimb of anesthetized rats. SNS of renal and lumbar nerves (1-8 Hz for 60 s) led to similar reductions in renal (RBF, -59% at 8 Hz) and iliac blood flow (IBF, -64% at 8 Hz). RBF fell biphasically with a rapid component within 3-5 s (τ=3.0 s) and a slower one over 30-50 s (τ=11 s), both contributing equally. In contrast, IBF fell monophasically within 3-5 s (τ =1.5 s). Dynamic modulation of SNS (~5 Hz, Δt on, Δt off, Δt=1, 2, 3, 4, 8, 15, 60 s) showed similar corner frequencies for RBF and IBF at ~0.15 Hz and an additional one for RBF at ~0.05 Hz. AR responses displayed a bimodal adaptation of RBF with a fast initial response within 9 s (τ=3.3 s) providing 38% AR efficiency, and a subsequent slower response within 30-120 s bringing total AR to 100%. Previous studies support the fast response of AR reflecting the myogenic response and the secondary tubuloglomerular feedback (TGF). AR in IBF displayed only one component (τ =2.5 s) providing 34% AR efficiency. It is concluded that the fast components of SNS and AR vasoconstrictor responses are of similar speed in kidney and skeletal muscle. In addition, secondary slower mechanisms operate in the kidney reducing the speed of overall RBF adaptation. Whether the secondary component in the SNS response is the same as TGF in AR requires further study. Supported by NIH (HL02334) and Guyton Award for Excell. in Integr. Physiol..
The role of reactive oxygen species in renal blood flow autoregulation.
FASEB J 22: 761.19, 2008
Autoregulation of renal blood flow (RBF) is mediated by a myogenic response (MR), tubuloglomerular feedback (TGF) and by a third mechanism that is slower than MR and TGF. Reactive oxygen species (ROS) are known to contribute to acute agonist-induced renal vasoconstriction and to enhance TGF. Little is known about the role of ROS in RBF autoregulation and underlying mechanisms. Autoregulatory mechanisms were assessed from the response of RBF to a rapid step-increase in renal artery pressure in rats. MR was derived from resistance changes within the first 5 s, TGF from those between 5 and 25s, and the third mechanism from 25-100s. During control, overall autoregulation was excellent (85±6%). MR provided autoregulatory efficiency of 63±6%, TGF 41±5% and the 3rd mechanism 5±6%. Inhibition of NAD(P)H oxidase by apocynin attenuated overall autoregulation to 62±7% (p<0.05) and MR to 39±4% (p<0.001) but barely affected TGF and 3rd mechanism (32±3 and 14±4%, p>0.06). Inhibition of nitric oxide synthase by LNAME markedly augmented MR (130±16%, p<0.001) leaving little room for TGF and 3rd mechanism. During LNAME, apocynin strongly blunted MR (to 56±11%, p<0.01), reversing the effect of LNAME. The superoxide dismutase (SOD) mimetic tempol tended to diminish MR similar to apocynin (38 vs. 52%, p>0.08). We conclude that ROS contribute to RBF autoregulation by strengthening MR in the normal kidney. This effect is mainly due to superoxide rather than H2O2 and does not require NO but instead is blunted by NO. Superoxide plays a major role in facilitating the modulation of MR by NO in the renal microcirculation. Supported by NIH (HL-02334) and Arthur C Guyton Award.
Connexin 37 contributes to resting arterial
pressure but is not essential for renal and systemic agonist-induced
Acta Physiologica 198, Suppl. 677 :P-SUN-52, 2010
OBJECTIVE: Connexins 37, 40, and 43 are expressed in mesangial, smooth muscle, renin-producing, and endothelial cells of the juxtaglomerular apparatus (JGA) in the kidney. The most abundant connexin in the JGA, Cx40, contributes to pressuredependent renin regulation and tubuloglomerular feedback, but not to agonist-induced renal vasomotor responses. The present work investigated the role of Cx37. METHODS: Arterial pressure (AP) and renal blood flow (RBF) were measured in anesthetized Cx37-deficient knockout mice (Cx37-ko, n=4) and wild-types from the same colony (wt, n=4) during baseline and in response to iv. bolus injections of norepinephrine (NE, 25 ng), angiotensin II (ANGII, 0.6 ng), and acetylcholine (ACH, 25 ng). RESULTS: AP was reduced in Cx37-ko (77±2 vs. 88±2 mmHg (ko vs. wt), p<0.05). Heart rate (534±27 vs. 544±51 bpm), RBF (1.7±0.3 vs. 1.8±0.1 ml/min), body weight (28±1 vs. 26±1 g) and kidney weight (170±12 vs. 177±12 mg) were not different. Pressor responses to NE (+31±8% vs. +28±7%), ANGII (+17±3 vs. +12±1%), and ACH (-43±1 vs. -29±9%) did not differ between Cx37-ko and wt. Responses of renal vascular resistance (RVR) were about half as strong in Cx37-ko as in wt (NE: +33±10 vs. +76±30, ANGII: +43±9 vs. +86±20, ACH: -17±5 vs. -35±3%). However, pressor and constrictor effects of nitric oxide synthase (NOS) inhibition (L-NAME 25 mg/kg, ko n=3, wt n=2) were similar in Cx37-ko and wt (AP +34±7% vs. +33±10%, RBF -52±1 vs. -45±3%) and during NOS-inhibition agonist-responses did not differ between genotypes e.g. ANGII: RVR +254% vs. +198%, n=2 each). CONCLUSION: Cx37-ko animals are hypotensive but show normal agonist-induced systemic vasomotor responses to NE, ANGII, and ACH. The attenuation of vasomotor responses in the kidney is likely due to the hypotension below the level of RBF autoregulation, as it was absent at the elevated AP during NOS-inhibition.
The Role of Prostaglandins in Renal Blood Flow Autoregulation.
Acta Physiologica 216, Suppl. 707: P07-06, 2016
Background: Autoregulation of renal blood flow (RBF) is mediated by three mechanisms, i.e. myogenic response (MR), tubuloglomerular feedback (TGF), and a third mechanism of unknown origin (3rdM). The relative contributions of these mechanisms are not static but subject to modulation. An important modulator is nitric oxide (NO), which mitigates MR and augments TGF and 3rdM, so that total autoregulation is maintained. Because NO is signaling via cGMP and prostaglandins PGE2 and PGI2 via cAMP, PGE2 and PGI2 might mitigate MR similarly to NO. Furthermore, Thromboxane A2 (TxA2) is known to enhance TGF. We therefore hypothesized that inhibition of cyclooxygenase (COX) will enhance MR and diminish TGF.
Methods: RBF autoregulation was tested in response to a rapid rise in renal artery pressure induced by release after a brief pressure reduction (by 20 mmHg for 60 s) in anesthetized rats. This allows distinguishing MR (0.5-6s after the pressure rise), TGF (5-30 s), and 3rdM (30-120 s). COX was inhibited by indomethacin. In separate experiments selective COX1- and COX2-inhibitors SC560 and nimesulide were used, followed each by indomethacin (5 mg/kg iv, each).
Results: Indomethacin slightly, but significantly attenuated total autoregulation (73 vs. 98% of perfect) and the 3rdM (13 vs. 24 units). TGF tended to be reduced on average (24 vs. 29 units), while MR was not affected (60 vs. 68 units). When all indomethacin data were pooled, the same result was found, with the TGF-reduction reaching significance. Nimesulide and SC560 each induced the same pattern, and subsequent Indomethacin had no additional effect. However, the influences of all 3 drugs on TGF were highly variable from animal to animal, ranging from no effect to complete abolition. Vehicles (Na2CO3 or DMF) had no effect. The determinants of the variability are not clear. The largest effect was mostly found in those animals with the lowest resting arterial pressure, but otherwise all rats were closely related, of similar age, only male, and had free access to the same food and water. However, even with complete abolition of TGF there was no compensatory enhancement of MR, so that total autoregulation was impaired, accordingly.
Conclusions: Endogenous prostaglandins do NOT modulate MR in RBF autoregulation under resting conditions. In contrast, TGF is augmented by COX-metabolites derived from COX1 or COX2, possibly TxA2. The influence, however, is highly variable for unknown reasons, but essential to RBF autoregulation.
- Hofmann K., Just A.
The role of prostaglandins in renal blood flow autoregulation during low and high sodium intake.
To be presented as poster at the Meeting of the German Physiological Society, Greifswald, Germany, March 17, 2017
to be published in Acta Physiologica Scandinavica, Supplement., 2017
BACKGROUND: Autoregulation of renal blood flow (RBF) is mediated by the myogenic response (MR), tubuloglomerular feedback (TGF) and a third regulatory mechanism (3rdM). Prostaglandins PGE2, PGI2 and thromboxane (TXA2) are produced in the kidney by cyclooxygenases COX-1 in endothelial cells and COX-2 in macula densa cells. PGE2 and PGI2, signaling via cAMP might attenuate MR similar to the effect of nitric via cGMP. Dietary sodium restriction is known to enhance TGF and macula densa COX-2 expression. TXA2 is known to enhance TGF during high, but not during low sodium diet.
METHODS: We therefore investigated the contribution of MR, TGF and 3rdM in RBF autoregulation in rats fed a diet with low (<0.03%) or high (2%) sodium content for 9 days, before and after COX-1- (SC560, 5mg/kg iv) or COX-2-inhibition (Nimesulide, 5 mg/kg iv). Autoregulation was challenged by a small rapid step increase in arterial pressure, induced by release after a 20mmHg reduction for 60s by an aortic occluder. MR was estimated from the rise in renal vascular resistance (RVR) during the first 6s, TGF from 6 to 30s, and 3rdM from 30 to 120s after the pressure step. The volatility of TGF was estimated from the area under the RVR curve above the linear connection between RVR at 6 and 30s.
RESULTS: Sodium intake caused surprisingly little alteration of the autoregulatory response. TGF volatility was slightly depressed in high versus low sodium diet, but the contributions of MR, TGF, and 3rdM did not differ between diets. COX-1 inhibition markedly depressed volatility and contribution of TGF similarly in both diets. COX-2-inhibition showed similar effects, but was not quite reaching significance. The 3rdM was reduced by COX-1 and COX-2-inhibition in low, but not in high sodium animals. COX-1 inhibition slightly depressed total autoregulation in both diets, COX-2 inhibition only during high sodium intake.
CONCLUSIONS: Sodium intake barely affects RBF autoregulation except for a slight depression of TGF-volatility with high sodium. COX-1 metabolites contribute to RBF autoregulation via TGF during both low and high sodium intake. COX-2 shows a similar trend but smaller effects. Both COX-1 and COX-2 support the 3rdM during high but not low sodium intake.