PD Dr. med. Armin JustInstitut für Physiologie
79104 Freiburg i. Br.
Tel. +49 761 203-5198+49 761 203-5198
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Renal blood flow autoregulation
Dynamic analysis of blood flow autoregulation of the kidney by transfer function analysis confirmed that the two known underlying mechanisms, the myogenic response (MR) and the tubuloglomerular feedback (TGF) are both active under physiological conditions in the conscious resting animal [O4]. To more directly and more quantitatively assess the contribution of these two regulating mechanisms, a method was developed investigating the time course of the response of RBF autoregulation to a rapid step change in renal artery pressure. Because both mechanisms comprise different response times, their actions can be dissociated in time by this approach. The studies revealed that MR and TGF both contribute about 30%, each, to the overall autoregulatory response in dogs under these conditions, i.e. in response to a pressure step from 50 mmHg to the resting level of arterial pressure of ~100 mmHg. In addition, a novel third autoregulatory mechanism was discovered with a time course slower than that of TGF and contributing another third [O11]. Later studies revealed the same mechanisms also in rats and mice, albeit with a slightly faster time-course than in dogs [[O14] O17] [O19]. A direct comparison of the results from rats, mice, and dogs is provided in a review article [R1]. Quantitative analysis indicated that in the conscious resting dog and for large pressure changes MR and TGF contribute ~30% each to the total response, while the remaining 30% seem to be brought about by the novel third mechanism [O11].
In subsequent work, the method was further refined in rats and mice to allow for assessment of the response to very small perturbations of arterial pressure within the physiological pressure range. These studies revealed contributions of 50% for MR, 35-50% for TGF, and up to 15% for the third mechanism [O14] [O17]. The same studies also demonstrated significant interactions between TGF and MR [O14].
Other studies assessed whether this balance among the regulating mechanisms is fixed or can be modulated and what the modulating factors are. Despite the well known augmenting effects of angiotensin II on baseline resistance, as well as on both MR and TGF alone, the relative contribution of the regulating mechanisms is not modified by varying levels of plasma angiotensin II in either dogs [O12] or rats [O17]. In contrast, the balance of contributions is strongly modulated by nitric oxide: nitric oxide mitigates the speed, strength, and contribution of MR to autoregulation [O17][R1]. All three of these effects reduce the speed of the overall autoregulation [O17]. This influence of nitric oxide depends on the integrity of TGF as it is prevented or reversed by furosemide, an inhibitor of TGF. In addition, nitric oxide fails affect the strength of MR in the skeletal muscle circulation [O17]. Transfer function analysis demonstrated a buffering role of nitric oxide on feedback oscillations of MR in the renal circulation and also revealed a novel modulating influence of nitric oxide in a confined frequency range below 0.01 Hz [O6]. In contrast to nitric oxide and similar to angiotensin II, sympathoadrenergic stimulation by phenylephrine or by baroreflex activation only slightly enhanced MR [A3]. Reduction of arterial pressure to hypotensive levels close to the lower limit of autoregulation (70-90 mmHg) reduced MR, abolished TGF and enhanced the contribution of the third autoregulatory mechanism [R1].
To characterize the third regulatory mechanism in more detail, mice were studied that are genetically deficient in TGF due to targeted deletion of the gene coding for A1 adenosine receptors (A1AR), the major signaling pathway of TGF [O19]. In these animals, the TGF-component of autoregulation was eliminated as expected associated with impaired overall autoregulation. However, myogenic response and the third mechanism were not different from those in wild-type mice. When TGF was pharmacologically inhibited by furosemide in these mice, the third mechanism was also eliminated and MR became stronger as in wildtype mice given furosemide [O19]. These results indicate that the third mechanism is independent of A1AR and substantially slower than classical TGF. The findings also suggest that the influence of TGF upon MR is unexpectedly not mediated via A1AR.
A newer work investigated the role of connexin 40 (Cx40) in TGF, myogenic response, autoregulation and agonist-induced renal vasomotor responses [O21]. The gap junction protein Cx40 is known to be expressed in the juxtaglomerular apparatus, the anatomical region mediating the signal for TGF and renin regulation. We found that mice, genetically deficient for Cx40, had greatly diminished TGF and impaired overall autoregulation. These changes were only partially ameliorated in mice in which the coding region for Cx40 had been replaced by another connexin, Cx45. In contrast, vasomotor responses to noradrenalin, acetylcholin, and nitric oxide were not affected by either the absence of Cx40 or by replacement with Cx45 showing that general vascular responsiveness was not disturbed. Thus Cx40 plays a decisive role in the signal transmission of TGF. In contrast, preliminary studies indicate connexin Cx37 (which is also expressed in the juxtaglomerular apparatus) does not seem to affect RBF autoregulation or agonist-induced vasomotor responses [A6].Recent work from Chapel Hill had studied the role of superoxide and other reactive oxygen species (ROS) in RBF autoregulation the regulating mechanisms. Results indicated that superoxide contributes slightly to RBF autoregulation in the presence of nitric oxide, mildly strenghtening MR with little influence on TGF or on the third mechanism [A5]. However, in the absence of nitric oxide, superoxide seems to profoundly augment MR. This not only confirms our previous findings of nitric oxide-independent effects of ROS, but may also seems to represent the mechanism responsible for the strong enhancement of MR in the kidney induced by elimination of nitric oxide [O17].
Recent studies in Freiburg aimed to define the roles of nitric oxide synthase (NOS) isoforms nNOS (in the macula densa cells), eNOS (in the endothelial cells), and iNOS (in the glomerular mesangium) for the mentioned modulatory influence of nitric oxide. Using pharmacological inhibitors in rats and gene-knockout for eNOS and nNOS in mice, both approaches demonstrated that only eNOS, but not nNOS or iNOS are responsible for this effect [O22]. Using a pharmacological activator of soluble guanylate cyclase (sGC), the key enzyme of the cGMP-signaling pathway, we could show that most if not all of the modulatory effects of NO on autoregulation are mediated through this signaling pathway [O24].
Other renal hemodynamic studies
One of the projects under the direction of Dr. William J. Arendshorst at the University of North Carolina in Chapel Hill investigated the involvement of prostanoids in the known reduction during sodium restriction of the responsiveness of the renal circulation to angiotensin II [R1]. We found that the efficiency of PGE2 but not PGI2 is augmented during sodium restriction, contributing to the blunted angiotensin II responsiveness in this condition.
Subsequent projects with Dr. Arendshorst investigated the interactions between the receptor subtypes for endothelin. Initially, we identified and quantified the dual vasodilator and constrictor influences exerted by the ETB-receptor subtype and their dependence on whether ETA receptors are co-stimulated or not [O15]. In a follup work we found that the buffering effect of ETB receptors is due to release of nitric oxide, but also to another, nitric oxide-independent, mechanism. The latter was also found to be separate from cyclooxygenase and epoxygenase metabolites and may thus reflect clearance of ET by ETB receptors [O16].
Other studies investigated the role of reactive oxygen species (ROS) in acute renal vasoconstrictor responses to various constrictor agents. The results indicate that ROS, most likely superoxide, contribute about half of the acute constrictor effect induced by angiotensin II and norepinephrine in the kidney [O18]. A subsequent study found the same degree of ROS involvement in acute renal vasoconstrictor responses to endothelin and to selective activation of ETA or ETB receptors [O20]. The similarity across various agonists suggests that the vasoconstrictor signaling of ROS may be a general feature of G-protein coupled receptors in the kidney that may also extend to other vascular beds. Furthermore, the constrictor contribution of ROS to either of these agonists was found to be independent of nitric oxide and therefore cannot be explained by scavenging of the dilator effect of nitric oxide by superoxide [O18], [O20]. In fact, in the case of ETB-receptor stimulation, the contribution of ROS was even larger in the absence than in the presence of nitric oxide [O20], indicating that scavenging in the opposite direction, i.e. blunting of superoxide by nitric oxide, may occur under certain physiological conditions. The subsequent autoregulation studies mentioned above [O17] discovered even stronger augmentation of the influence of ROS in the absence of nitric oxide. This has important implications on the interactions between superoxide and nitric oxide in vascular regulation as discussed in [O20].
In more recent work we studied the contributions and the dynamics of the three known components of endothelium-dependent vasodilation, i.e. nitric oxide, prostaglandins, and endothelium-derived hyperpolarizing factor [O23]. The dilator responses to the endothelium-dependent vasodilators acetylcholine (ACh) and bradykinin (BK) were found to be mediated >50% by nitric oxide, 20-40% by prostaglandins, and <20% by EDHF. However, the response times of these components differ considerably with EDHF being substantially faster (maximum effect after 16s) than nitric oxide and prostaglandins (max after 30s). The role of nitric oxide and prostaglandins in the endothelium-dependent attenuation of constrictor responses to norepinephrine and angiotensin II is also not constant over time, but reaches its most prominent effect later than the underlying constrictor response. Thereby, the attenuation not only reduces the magnitude but also the duration of the constriction.
Our latest work [O24] investigated the role of soluble guanylate cyclase (sGC), a key enzyme in the cGMP signaling pathway and main target of nitric oxide, in the control of renal blood flow and glomerular filtration rate. Infusion of cinaciguat, a pharmacological activator of sGC, reduced arterial pressure. Despite this well known hypotensive effect, blood flow and filtration were well preserved, with only a slight reduction of filtration fraction and slight impairment of autoregulation. This indicates safety of pharmacological sGC-activation with regard to renal function.
Previous studies on blood pressure regulation
Early studies concentrated on blood pressure variability. We foundthat the majority of resting blood pressure variability in thefrequency range below 0.01 Hz (~2 min cycle length) derives fromthe central nervous system and reaches the circulation by sympathetic vasomotor nerves (80%), rather than originating from within the circulation (20%) [O2]. Another source of central nervously derived blood pressure and heart rate variability originates from large but brief (10 s) vasodilator events in the skeletal muscle circulation [O8]. These vasodilations seem to be initiated via a neural pathway, which is, however, distinct from sympathetic cholinergic or nitroxidergic nerves. The phenomena also seem to be independent of metabolic factors and do not correlate with the magnitude of muscular contractions, in most cases being associated with negligible movements. We propose that these vasodilator responses might underlie the well-known but poorly understood vasodilation occurring at the onset of exercise.
Other studies investigated the regulatory mechanisms helping to keep these blood pressure fluctuations small. In the frequency range below 0.04 Hz such buffering is predominantly provided by the baroreceptor reflex [O1], while endogenous nitric oxide [O1] and renin-angiotensin system [O5] only marginally contribute in this frequency range. In conscious mice, spectra of cardiovascular variability were found to be very similar to those in dogs and humans, albeit shifted to 8-10-times higher frequencies [O9]. Furthermore, in contrast to larger mammals, the capacity for reduction of heart rate variability from the resting level seems to be limited in the HF-range in mice [O9]. This means that the LF/HF-ratio, which is used in humans as indicator of the sympathovagal balance, is not a useful parameter in mice.
In the laboratory of Prof. Horst Seller from the Department of Physiology and in a translational collaboration with cardiologists from the Department of Internal Medicine in Heidelberg (Prof. Markus Haass and Dr. Arnt Kristen) we studied the reflex stimulation of renal sympathetic nerve activity in response to an increase in inspiratory pCO2 in anesthetized rats [O13]. It was found that the central component of this hypercapnic reflex is functioning normally in various models of congestive heart failure in these rats.
A later work compared the kinetics of vasomotor responses in the renal and skeletal muscle circulation using both in in vivo blood flow studies in response to sympathetic nerve stimulation and in in vitro experiments measuring calcium transients in isolated microvessels during stimulation with norepinephrine. In contrast to the monophasic blood flow response in skeletal muscle, the time course in the kidney seems to be biphasic with the faster component resembling the response in skeletal muscle [A4]. However, dynamics and magnitude of calcium transients appear to be similar in isolated vessels from both vascular beds [A3].
Previous collaborative work with clinical partners
Analysis of the flow pulse curve was used to further develop a noninvasive diagnostic tool for the comprehensive assessment of renal artery stenosis by nuclear magnetic resonance (NMR) in cooperation with colleagues at the German Cancer Research Center (DKFZ, Drs. Stefan O. Schoenberg and Michael Bock) and the University Department of Surgery in Heidelberg (Prof. Ulrich Kallinowsky) [O3]. A later study brought the changes of the flow pulse in direct correlation with the angiographic degree of stenosis [O10]. These studies also led to further collaborations regarding the development of new NMR techniques for the assessment of tissue perfusion in the kidney and other studies applying both techniques to patients.
In the translational collaboration mentioned above between the Departments of Physiology (Prof. H. Seller) and Cardiology (Prof. Markus Haass and Dr. Arnt Kristen) at Heidelberg University we studied the hypercapnic sympathetic reflex in various rat models of congestive heart failure to confirm that this reflex is not affected [O13]. For the purpose of this study, we adapted to rats some of the methods that Prof. Seller had established in his laboratory over many years for cats.
Another collaboration with the Department of Internal Medicine (Drs. Stefan Hardt, Raffi Bekeredjian, and Prof. Helmut Kuecherer) assessed the applicability of intravascular ultrasound measurements for the evaluation of arterial compliance [O7].A collaboration with the Mouse Cardiovascular Pathophysiology Core Facility in the Carolina Cardiovascular Biology Center at the UNC Chapel Hill (Profs. Susan S. Smyth, Mauricio Rojas, and David Clemmons) investigated vascular compliance and pulse wave velocity in anesthetized and conscious mice.
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.
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.
Noninvasive analysis of renal artery blood flow dynamics with MR cine phase-contrast flow measurements.
Am J Physiol. 1997 May; 272(5 Pt 2): H2477-84
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
[Perspectives article by W.A. Cupples]
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.
Connexin 40 mediates tubuloglomerular feedback contribution to renal blood flow autoregulation.
J Am Soc Nephrol 20: 1577-1585, 2009
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.
Modulation of the Myogenic Response in Renal Blood Flow Autoregulation by NO Depends on eNOS, but not nNOS or iNOS.
J Physiol (London) 589: 4731–4744, 2011
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.
Temporal characteristics of nitric oxide-, prostaglandin-, and EDHF-mediated components of endothelium-dependent vasodilation in the kidney..
Am J Physiol Regul Integr Comp Physiol. 305(9):R987-R998, 2013
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.
Role of soluble guanylate cyclase in renal hemodynamics and autoregulation in the rat. Am J Physiol Renal Physiol. 307(9):F1003-1012, 2014
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.
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.
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.
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.
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 vasomotor responses.
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.
|Institution||Degree||Years||Field of Study|
|Albert-Ludwigs-Universität, Freiburg i.Br., Germany||1985–1988||Medicine|
|Ruprecht-Karls-Universität, Heidelberg, Germany||MD||1988–1992||Medicine|
|Ruprecht-Karls-Universität, Heidelberg, Germany||Dr.med.||1988–1992||Physiology|
|Ruprecht-Karls-Universität, Heidelberg, Germany||habil.||1994–2000||Physiology|
Interim Professorship (Vertretungsprofessur)
Physiologisches Institut I
Albert-Ludwigs-Universität Freiburg, Freiburg i. Br., Germany
Dept. of Cell & Molecular Physiology, Prof. W.J. Arendshorst
University of North Carolina at Chapel Hill, Chapel Hill, NC, USA
Visiting Research Instructor
Dept. of Cell & Molecular Physiology, Prof. W.J. Arendshorst
University of North Carolina at Chapel Hill, Chapel Hill, NC, USA
Institut für Physiologie und Pathophysiologie,
Abt. Biophysik des Kreislaufs and Abt. Autonomes Nervensystem, Prof. H.R. Kirchheim and Prof. H.Seller
Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany
Postdoctoral Research Associate (Stipendiate of the German Research Foundation)
Institut für Physiologie und Pathophysiologie
Abt. Biophysik des Kreislaufs, Prof. H.R. Kirchheim
Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany
Klinikum der Universität, Abt. Innere Medizin III (Cardiology, Angiology, Pulmonology)
Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany
Klinikum der Universität, Abt. Innere Medizin III (Cardiology, Angiology, Pulmonology)
Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany
Institut für Physiologie und Pathophysiologie
Abt. Biophysik des Kreislaufs
Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany
- Postdoctoral Stipend, German Research Foundation, Graduiertenkolleg für Experimentelle Nieren- und Kreislaufforschung.
- Arthur C. Guyton Award for Excellence in Integrative Physiology of the American Physiological Society
- New Investigator Award of the Water and Electrolyte Homeostasis Section of the American Physiological Society
- Chief instructor for the subcourse Cardiovascular Regulation within the 12-subcourse practical course for physiology for medical students at the University of Heidelberg
Invited lecturer for physiology
Dept. of Physiology, Kyrgyz Medical Academy, Bishkek, Kyrgyzstan, Central Asia supported by the German Academic Exchange Service (DAAD)
Editorial Board member
American Journal of Physiology, Regulatory, Integrative and Comparative Physiology
- Committee member of the Animal Care and Experimentation Committee of The American Physiological Society
Editorial Board member
American Journal of Physiology, Renal Physiology