Impaired renal blood flow autoregulation in two-kidney, one-clip hypertensive rats is caused by enhanced activity of nitric oxide

Endothelium-Derived Dopamine and 6-Nitrodopamine in the Cardiovascular System

The review deals with the release of endothelium-derived dopamine and 6-nitrodopamine (6-ND) and its effects on isolated vascular tissues and isolated hearts. Basal release of both dopamine and 6-ND is present in human isolated umbilical cord vessels, human popliteal vessels, nonhuman primate vessels, and reptilia aortas. The 6-ND basal release was significantly reduced when the tissues were treated with Nω-nitro-l-arginine methyl ester and virtually abolished when the endothelium was mechanically removed. 6-Nitrodopamine is a potent vasodilator, and the mechanism of action responsible for this effect is the antagonism of dopamine D2-like receptors. As a vasodilator, 6-ND constitutes a novel mechanism by which nitric oxide modulates vascular tone. The basal release of 6-ND was substantially decreased in endothelial nitric oxide synthase knockout (eNOS-/-) mice and not altered in neuronal nitric oxide synthase knockout (nNOS-/-) mice, indicating a nonneurogenic source for 6-ND in the heart. Indeed, in rat isolated right atrium, the release of 6-ND was not affected when the atria were treated with tetrodotoxin. In the rat isolated right atrium, 6-ND is the most potent endogenous positive chronotropic agent, and in Langendorff's heart preparation, it is the most potent endogenous positive inotropic agent. The positive chronotropic and inotropic effects of 6-ND are antagonized by β1-adrenoceptor antagonists at concentrations that do not affect the effects induced by noradrenaline, adrenaline, and dopamine, indicating that blockade of the 6-ND receptor is the major modulator of heart chronotropism and inotropism. The review proposes that endothelium-derived catecholamines may constitute a major mechanism for control of vascular tone and heart functions, in contrast to the overrated role attributed to the autonomic nervous system.

Sildenafil and Kidney Function in Heart Failure with Preserved Ejection Fraction

Key Points

Sildenafil induced an acute effect on eGFR without change in the overall eGFR slope after 24 weeks in a heart failure with preserved ejection fraction (HFpEF) cohort. N -terminal pro–brain natriuretic peptide levels and baseline diuretic use were most strongly associated with eGFR decline in this HFpEF cohort. Long-term studies are required to determine sildenafil's influence on kidney function and outcomes in HFpEF.

Background

CKD worsens the prognosis for people with heart failure with preserved ejection fraction (HFpEF). In the Phosphodiesterase-5 Inhibition to Improve Clinical Status and Exercise Capacity in Diastolic HFpEF (RELAX) trial, sildenafil decreased eGFR compared with placebo despite favorable kidney effects in preclinical models. Since acute eGFR decline precedes long-term kidney benefits for select medications, we assessed the influence of sildenafil on acute and chronic eGFR slopes.

Methods

The RELAX trial randomized 216 participants to placebo or sildenafil and assessed 24-week changes in cardiopulmonary exercise testing, cardiovascular imaging, and laboratory data. We applied linear mixed modeling to calculate the total, acute (0–12 weeks), and chronic (3–24 weeks) eGFR slopes by treatment. Using regression modeling, we assessed respective associations between eGFR slope and baseline data and clinical end points. We repeated the analyses using a binary outcome on the basis of a substantial (≥20%) decline in eGFR.

Results

The mean baseline eGFR was 60.8 ml/min per 1.73 m2, and the mean eGFR slope during follow-up was −3.21 ml/min per 1.73 m2 per year. Sildenafil did not alter total eGFR slope compared with placebo (difference +0.47 ml/min per 1.73 m2 per year, 95% confidence interval [CI], −6.63 to 7.57 ml/min per 1.73 m2 per year). Sildenafil users tended to experience a more negative acute eGFR slope (difference −3.15 ml/min per 1.73 m2 per year) and more positive chronic slope (+2.06 ml/min per 1.73 m2 per year) compared with placebo, but neither difference reached statistical significance. Baseline N -terminal pro–B-type natriuretic peptide and loop diuretic use were associated with worse eGFR trajectory regardless of treatment. Substantial eGFR decline was associated with increase in endothelin-1 and a greater risk of hospitalization or death (HR, 2.34, 95% CI, 1.21 to 4.53, P =0.01).

Conclusions

Sildenafil induced an acute effect on eGFR without change in the overall eGFR slope after 24 weeks in an HFpEF cohort, suggesting lack of long-term risk related to early reduction in eGFR after initiating treatment. Long-term studies are needed to determine the effect of sildenafil on kidney function in HFpEF.

Renin-Angiotensin System Induced Secondary Hypertension: The Alteration of Kidney Function and Structure

Long-term hypertension is known as a major risk factor for cardiovascular and chronic kidney disease (CKD). The Renin-angiotensin system (RAS) plays a key role in hypertension pathogenesis. Angiotensin II (Ang II) enhancement in Ang II-dependent hypertension leads to progressive CKD and kidney fibrosis. In the two-kidney one-clip model (2K1C), more renin is synthesized in the principal cells of the collecting duct than juxtaglomerular cells (JGCs). An increase of renal Ang I and Ang II levels and a decrease of renal cortical and medullary Ang 1–7 occur in both kidneys of the 2K1C hypertensive rat model. In addition, the activity of the angiotensin-converting enzyme (ACE) increases, while ACE2's activity decreases in the medullary region of both kidneys in the 2K1C hypertensive model. Also, the renal prolyl carboxypeptidase (PrCP) expression and its activity reduce in the clipped kidneys. The imbalance in the production of renal ACE, ACE2, and PrCP expression causes the progression of renal injury. Intrarenal angiotensinogen (AGT) expression and urine AGT (uAGT) excretion rates in the unclipped kidney are greater than the clipped kidney in the 2K1C hypertensive rat model. The enhancement of Ang II in the clipped kidney is related to renin secretion, while the elevation of intrarenal Ang II in the unclipped kidney is related to stimulation of AGT mRNA and protein in proximal tubule cells by a direct effect of systemic Ang II level. Ang II-dependent hypertension enhances macrophages and T-cell infiltration into the kidney which increases cytokines, and AGT synthesis in proximal tubules is stimulated via cytokines. Accumulation of inflammatory cells in the kidney aggravates hypertension and renal damage. Moreover, Ang II-dependent hypertension alters renal Ang II type 1 & 2 receptors (AT₁R & AT₂R) and Mas receptor (MasR) expression, and the renal interstitial fluid bradykinin, nitric oxide, and cGMP response to AT₁R, AT₂R, or BK B₂-receptor antagonists. Based on a variety of sources including PubMed, Google Scholar, Scopus, and Science-Direct, in the current review, we will discuss the role of RAS-induced secondary hypertension on the alteration of renal function.

Angiotensin II and the Renal Hemodynamic Response to an Isolated Increased Renal Venous Pressure in Rats

Elevated central venous pressure increases renal venous pressure (RVP) which can affect kidney function. We previously demonstrated that increased RVP reduces renal blood flow (RBF), glomerular filtration rate (GFR), and renal vascular conductance (RVC). We now investigate whether the RAS and RBF autoregulation are involved in the renal hemodynamic response to increased RVP. Angiotensin II (ANG II) levels were clamped by infusion of ANG II after administration of an angiotensin-converting enzyme (ACE) inhibitor in male Lewis rats. This did not prevent the decrease in ipsilateral RBF (-1.9±0.4ml/min, p<0.05) and GFR (-0.77±0.18ml/min, p<0.05) upon increased RVP; however, it prevented the reduction in RVC entirely. Systemically, the RVP-induced decline in mean arterial pressure (MAP) was more pronounced in ANG II clamped animals vs. controls (-22.4±4.1 vs. -9.9±2.3mmHg, p<0.05), whereas the decrease in heart rate (HR) was less (-5±6bpm vs. -23±4bpm, p<0.05). In animals given vasopressin to maintain a comparable MAP after ACE inhibition (ACEi), increased RVP did not impact MAP and HR. RVC also did not change (0.018±0.008ml/minˑmmHg), and the reduction of GFR was no longer significant (-0.54±0.15ml/min). Furthermore, RBF autoregulation remained intact and was reset to a lower level when RVP was increased. In conclusion, RVP-induced renal vasoconstriction is attenuated when ANG II is clamped or inhibited. The systemic effect of increased RVP, a decrease in HR related to a mild decrease in blood pressure, is attenuated also during ANG II clamp. Last, RBF autoregulation remains intact when RVP is elevated and is reduced to lower levels of RBF. This suggests that in venous congestion, the intact RBF autoregulation could be partially responsible for the vasoconstriction.

β-Blockade attenuates renal blood flow in experimental endotoxic shock by reducing perfusion pressure

Clinical data suggests that heart rate (HR) control with selective β1-blockers may improve cardiac function during septic shock. However, it seems counterintuitive to start β-blocker infusion in a shock state when organ blood flow is already low or insufficient. Therefore, we studied the effects of HR control with esmolol, an ultrashort- acting β1-selective adrenoceptor antagonist, on renal blood flow (RBF) and renal autoregulation during early septic shock. In 10 healthy sheep, sepsis was induced by continuous i.v. administration of lipopolysaccharide, while maintained under anesthesia and mechanically ventilated. After successful resuscitation of the septic shock with fluids and vasoactive drugs, esmolol was infused to reduce HR with 30% and was stopped 30-min after reaching this target. Arterial and venous pressures, and RBF were recorded continuously. Renal autoregulation was evaluated by the response in RBF to renal perfusion pressure (RPP) in both the time domain and frequency domain. During septic shock, β-blockade with esmolol significantly increased the pressure dependency of RBF to RPP. Stopping esmolol showed the reversibility of the impaired renal autoregulation. Showing that clinical diligence and caution are necessary when treating septic shock with esmolol in the acute phase since esmolol reduced RPP to critical values thereby significantly reducing RBF.

Short- and long-term administration of the non-steroidal mineralocorticoid receptor antagonist finerenone opposes metabolic syndrome-related cardio-renal dysfunction

AIM

To determine whether non-steroidal mineralocorticoid receptor (MR) antagonists oppose metabolic syndrome-related end-organ, i.e. cardiac, damage.

MATERIALS AND METHODS

In Zucker fa/fa rats, a rat model of metabolic syndrome, we assessed the effects of the non-steroidal MR antagonist finerenone (oral 2 mg/kg/day) on left ventricular (LV) function, haemodynamics and remodelling (using echocardiography, magnetic resonance imaging and biochemical methods).

RESULTS

Long-term (90 days) finerenone modified neither systolic blood pressure nor heart rate, but reduced LV end-diastolic pressure and LV end-diastolic pressure-volume relationship, without modifying LV end-systolic pressure and LV end-systolic pressure-volume relationship. Simultaneously, long-term finerenone reduced both LV systolic and diastolic diameters, associated with reductions in LV weight and LV collagen density, while proteinuria and renal nGAL expression were reduced. Short-term (7 days) finerenone improved LV haemodynamics and reduced LV systolic diameter, without modifying LV diastolic diameter. Moreover, short-term finerenone increased myocardial tissue perfusion and reduced myocardial reactive oxygen species, while plasma nitrite levels, an indicator of nitric oxide (NO) bio-availability, were increased.

CONCLUSIONS

In rats with metabolic syndrome, the non-steroidal MR antagonist finerenone opposed metabolic syndrome-related diastolic cardiac dysfunction and nephropathy. This involved acute effects, such as improved myocardial perfusion, reduced oxidative stress/increased NO bioavailability, as well as long-term effects, such as modifications in the myocardial structure.

Renal autoregulation and blood pressure management in circulatory shock

The importance of personalized blood pressure management is well recognized. Because renal pressure–flow relationships may vary among patients, understanding how renal autoregulation may influence blood pressure control is essential. However, much remains uncertain regarding the determinants of renal autoregulation in circulatory shock, including the influence of comorbidities and the effects of vasopressor treatment. We review published studies on renal autoregulation relevant to the management of acutely ill patients with shock. We delineate the main signaling pathways of renal autoregulation, discuss how it can be assessed, and describe the renal autoregulatory alterations associated with chronic disease and with shock.

Transient impairment of dynamic renal autoregulation in early diabetes mellitus in rats

Renal autoregulation is impaired in early (1 wk) diabetes mellitus (DM) induced by streptozotocin, but effective in established DM (4 wk). Furthermore nitric oxide synthesis (NOS) inhibition with NG-nitro-l-arginine methyl ester (l-NAME) significantly improved autoregulation in early DM but not in established DM. We hypothesized that autoregulation is transiently impaired in early DM because of increased NO availability in the kidney. Because of the conflicting evidence available for a role of NO in DM, we tested the hypothesis that DM reduces autoregulation effectiveness by reducing the spatial similarity of autoregulation. Male Long-Evans rats were divided into control (CON) and diabetic (DM; streptozotocin) groups and followed for either 1 wk (CON1, n = 6; DM1, n = 5) or 4 wk (CON4, n = 7; DM4, n = 7). At the end of the experiment, dynamic autoregulation was assessed in isoflurane-anesthetized rats by whole kidney RBF during baseline, NOS1 inhibition, and nonselective NOS inhibition. Kidney surface perfusion, monitored with laser speckle contrast imaging, was used to assess spatial heterogeneity of autoregulation. Autoregulation was significantly impaired in DM1 rats and not impaired in DM4 rats. l-NAME caused strong renal vasoconstriction in all rats, but did not significantly affect autoregulation dynamics. Autoregulation was more spatially heterogeneous in DM1, but not DM4. Therefore, our results, which are consistent with transient impairment of autoregulation in DM, argue against the hypothesis that this impairment is NO-dependent, and suggest that spatial properties of autoregulation may also contribute to reduced autoregulatory effectiveness in DM1.

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

Benefical therapeutic effect of Chinese Herbal Xinji'erkang formula on hypertension-induced renal injury in the 2-kidney-1-clip hypertensive rats

BACKGROUND

Increase in evidence shows that the role of kidney injury in hypertension is important. Xinji'erkang (XJEK), a Chinese herbal formula, has been identified as an effective preparation in the treatment of coronary heart disease and myocarditis. We have previously demonstrated that XJEK attenuate oxidative stress and hypertension target organ damage. The aim of this study was to assess the renal protective function of XJEK.

MATERIALS AND METHODS

Two Kidney One Clip (2K1C) model was adopted to induce hypertension in rats. We submitted male Sprague Dawley (150-180) g rats to either renal artery clipping or sham operation. Renal hypertension was established after four weeks of surgery. Rats were randomized divided into the four groups: sham-operated group (Sh-Op) (n=10), two-kidney, one-clip hypertension group (2K1C) (n=10), Xinji'erkang treatment group (XJEK) (n=10) and Fosinopril (n=10) treatment group. Drugs were administered orally daily for four weeks. Systolic pressures were measured every week using the tail-cuff apparatus. 24h before death, urine samples were collected for detect of urinary proteins. The kidney weight (KW) index was expressed as kidney weight/body weight (KW/BW). The histological changes were investigated by hematoxylin and eosin and Van Gieson staining. Immunohistochemical assay was employed to observe the intra-renal transforming growth factor-β1 (TGF-β1) protein expression. Serum creatinine (SCR) and blood urea nitrogen (BUN) were assayed by automatic biochemical analyzer. ELISA kit was used to assay Angiotensin II (Ang II) and TGF-β1 content in serum.

RESULTS

Administration of XJEK markedly alleviated the rise in blood pressure and declined LKW/BW ratio. Histo-pathological injuries including hypertrophic glomerular, glomerular sclerosis, glomerular and interstitial fibrosis were attenuated. XJEK also decreased SCR, BUN, urinary proteins in 24h urine, serum Ang II and TGF-β1 concentrations and the intra-renal TGF-β1 protein expression.

CONCLUSION

XJEK therapy in the 2K1C hypertensive rats affects the rise in blood pressure and ameliorates the severity of kidney injury. The protective effect is most likely due to the ability of XJEK to affect the Renin-Angiotensin-Aldosterone System (RAAS) and the TGF-β systems.

Cardiorenal syndrome--current understanding and future perspectives

Combined cardiac and renal dysfunction has gained considerable attention. Hypotheses about its pathogenesis have been formulated, albeit based on a relatively small body of experimental studies, and a clinical classification system has been proposed. Cardiorenal syndrome, as presently defined, comprises a heterogeneous group of acute and chronic clinical conditions, in which the failure of one organ (heart or kidney) initiates or aggravates failure of the other. This conceptual framework, however, has two major drawbacks: the first is that, despite worldwide interest, universally accepted definitions of cardiorenal syndrome are lacking and characterization of heart and kidney failure is not uniform. This lack of consistency hampers experimental studies on mechanisms of the disease. The second is that, although progress has been made in developing hypotheses for the pathogenesis of cardiorenal syndrome, these initiatives are at an impasse. No hierarchy has been identified in the myriad of haemodynamic and non-haemodynamic factors mediating cardiorenal syndrome. This Review discusses current understanding of cardiorenal syndrome and provides a roadmap for further studies in this field. Ultimately, discussion of the definition and characterization issues and of the lack of organization among pathogenetic factors is hoped to contribute to further advancement of this complex field.

Phosphodiesterase 5 attenuates the vasodilatory response in renovascular hypertension

NO/cGMP signaling plays an important role in vascular relaxation and regulation of blood pressure. The key enzyme in the cascade, the NO-stimulated cGMP-forming guanylyl cyclase exists in two enzymatically indistinguishable isoforms (NO-GC1, NO-GC2) with NO-GC1 being the major NO-GC in the vasculature. Here, we studied the NO/cGMP pathway in renal resistance arteries of NO-GC1 KO mice and its role in renovascular hypertension induced by the 2-kidney-1-clip-operation (2K1C). In the NO-GC1 KOs, relaxation of renal vasculature as determined in isolated perfused kidneys was reduced in accordance with the marked reduction of cGMP-forming activity (80%). Noteworthy, increased eNOS-catalyzed NO formation was detected in kidneys of NO-GC1 KOs. Upon the 2K1C operation, NO-GC1 KO mice developed hypertension but the increase in blood pressures was not any higher than in WT. Conversely, operated WT mice showed a reduction of cGMP-dependent relaxation of renal vessels, which was not found in the NO-GC1 KOs. The reduced relaxation in operated WT mice was restored by sildenafil indicating that enhanced PDE5-catalyzed cGMP degradation most likely accounts for the attenuated vascular responsiveness. PDE5 activation depends on allosteric binding of cGMP. Because cGMP levels are lower, the 2K1C-induced vascular changes do not occur in the NO-GC1 KOs. In support of a higher PDE5 activity, sildenafil reduced blood pressure more efficiently in operated WT than NO-GC1 KO mice. All together our data suggest that within renovascular hypertension, cGMP-based PDE5 activation terminates NO/cGMP signaling thereby providing a new molecular basis for further pharmacological interventions.

Systemic arterial and venous determinants of renal hemodynamics in congestive heart failure

Heart and kidney interactions are fascinating, in the sense that failure of the one organ strongly affects the function of the other. In this review paper, we analyze how principal driving forces for glomerular filtration and renal blood flow are changed in heart failure. Moreover, renal autoregulation and modulation of neurohumoral factors, which can both have repercussions on renal function, are analyzed. Two paradigms seem to apply. One is that the renin-angiotensin system (RAS), the sympathetic nervous system (SNS), and extracellular volume control are the three main determinants of renal function in heart failure. The other is that the classical paradigm to analyze renal dysfunction that is widely applied in nephrology also applies to the pathophysiology of heart failure: pre-renal, intra-renal, and post-renal alterations together determine glomerular filtration. At variance with the classical paradigm is that the most important post-renal factor in heart failure seems renal venous hypertension that, by increasing renal tubular pressure, decreases GFR. When different pharmacological strategies to inhibit the RAS and SNS and to assist renal volume control are considered, there is a painful lack in knowledge about how widely applied drugs affect primary driving forces for ultrafiltration, renal autoregulation, and neurohumoral control. We call for more clinical physiological studies.

Blood cleaner on-chip design for artificial human kidney manipulation

A novel design of a blood cleaner on-chip using an optical waveguide known as a PANDA ring resonator is proposed. By controlling some suitable parameters, the optical vortices (gradient optical fields/wells) can be generated and used to form the trapping tools in the same way as optical tweezers. In operation, the trapping force is formed by the combination between the gradient field and scattering photons by using the intense optical vortices generated within the PANDA ring resonator. This can be used for blood waste trapping and moves dynamically within the blood cleaner on-chip system (artificial kidney), and is performed within the wavelength routers. Finally, the blood quality test is exploited by the external probe before sending to the destination. The advantage of the proposed kidney on-chip system is that the unwanted substances can be trapped and filtered from the artificial kidney, which can be available for blood cleaning applications.

The renin-angiotensin system and the third mechanism of renal blood flow autoregulation

Autoregulation of renal blood flow comprises three mechanisms: the myogenic response (MR), the tubuloglomerular feedback (TGF), and a third mechanism (3M). The nature of 3M is unknown; it may be related to hypotensive resetting of autoregulation that probably relies on pressure-dependent stimulation of the renin-angiotensin system (RAS). Thus we used a normotensive angiotensin II clamp in anesthetized rats and studied autoregulation 1) by slow ramp-shaped reductions in renal perfusion pressure (RPP) followed by ramp-shaped RPP restorations and 2) by means of the step response technique: after 30 s of either total or partial suprarenal aortic occlusion, a step increase in RPP was made and the response of renal vascular conductance analyzed to assess the mechanisms' strength and initial direction (vasodilation or constriction). The angiotensin clamp abolished the resetting of autoregulation during ramp-shaped RPP changes. Under control conditions, the initial TGF response was dilatory after total occlusions but constrictive after partial occlusions. The initial 3M response presented a mirror image to the TGF: it was constrictive after total but dilatory after partial occlusions. The angiotensin clamp suppressed the TGF and turned the initial 3M response following total occlusions into dilation. We conclude that 1) pressure-dependent RAS stimulation is a major cause behind hypotensive resetting of autoregulation, 2) TGF sensitivity strongly depends on pressure-dependent changes in RAS activity, 3) the 3M is modulated, but not mediated, by the RAS, and 4) the 3M acts as a counterbalance to the TGF and might possibly be related to the recently described connecting tubule glomerular feedback.

Salt-resistant blood pressure and salt-sensitive renal autoregulation in chronic streptozotocin diabetes

Hyperfiltration occurs in early type 1 diabetes mellitus in both rats and humans. It results from afferent vasodilation and thus may impair stabilization of glomerular capillary pressure by autoregulation. It is inversely related to dietary salt intake, the "salt paradox." Restoration of normal glomerular filtration rate (GFR) involves increased preglomerular resistance, probably mediated by tubuloglomerular feedback (TGF). To begin to test whether the salt paradox has pathogenic significance, we compared intact vs. diabetic (streptozotocin) Long-Evans rats with normal and increased salt intake, 1 and approximately 3% by weight of food eaten, respectively. Weekly 24-h blood pressure records were acquired by telemetry before and during diabetes. Blood glucose was maintained at approximately 20 mmol/l by insulin implants. GFR was significantly elevated only in diabetic rats on normal salt intake, confirming diabetic hyperfiltration and the salt paradox. Renal blood flow dynamics show strong contributions to autoregulation by both TGF and the myogenic mechanism and were not impaired by diabetes or by increased salt intake. Separately, systolic pressure was not elevated in diabetic rats at any time during 12 wk with normal or high salt intake. Autoregulation was effective in all groups, and the diabetic-normal salt group showed significantly improved autoregulation at low perfusion pressures. Histological examination revealed very minor glomerulosclerosis and modest mesangial expansion, although neither was diagnostic of diabetes. Periodic acid-Schiff-positive droplets found in distal tubules and collecting duct segments were diagnostic of diabetic kidneys. Biologically significant effects attributable to increased salt intake were abrogation of hyperfiltration and of the left shift in autoregulation in diabetic rats.

Brown Norway rats show impaired nNOS-mediated information transfer in renal autoregulationThis article is part of a Special Issue on Information Transfer in the Microcirculation

Nonselective inhibition of NO synthase (NOS) augments myogenic autoregulation of renal blood flow (RBF) and profoundly reduces RBF. Previously in Wistar rats, we showed that augmented autoregulation, but not vasoconstriction, is duplicated by intrarenal inhibition of neuronal NOS (nNOS), whereas intrarenal inhibition of inducible NOS (iNOS) has no effect on RBF or on RBF dynamics. Thus macula densa nNOS transfers information from tubuloglomerular feedback to the afferent arteriole. This information flow requires that macula densa nNOS can sufficiently alter ambient NO concentration, that is, that endothelial NOS (eNOS) and iNOS do not alter local NO concentration. Because the Brown Norway rat often shows exaggerated responses to NOS inhibition and has peculiarities of renal autoregulation that are related to NO, we used this strain to study systemic and renal vascular responses to NOS inhibition. The first experiment showed transient blood pressure reduction by bolus i.v. acetylcholine that was dose-dependent in both strains and substantially prolonged in Brown Norway rats. The depressor response decayed more rapidly after nonselective NOS inhibition and the difference between strains was lost, indicating a greater activity of eNOS in Brown Norway rats. In Brown Norway rats, selective inhibition of iNOS reduced RBF (–16% ± 7%) and augmented myogenic autoregulation, whereas nNOS inhibition reduced RBF (–25% ± 4%) and did not augment myogenic autoregulation. The significant responses to intrarenal iNOS inhibition, the reduced modulation of autoregulation by nNOS inhibition, and the enhanced endothelial depressor response suggest that physiological signalling by NO within the kidney is impaired in Brown Norway rats because of irrelevant or inappropriate input of NO by eNOS and iNOS.

A perinatal nitric oxide donor increases renal vascular resistance and ameliorates hypertension and glomerular injury in adult fawn-hooded hypertensive rats

Enhancing perinatal nitric oxide (NO) availability persistently reduces blood pressure in spontaneously hypertensive rats. We hypothesize that this approach can be generalized to other models of genetic hypertension, for instance those associated with renal injury. Perinatal exposure to the NO donor molsidomine was studied in fawn-hooded hypertensive (FHH) rats, a model of mild hypertension, impaired preglomerular resistance, and progressive renal injury. Perinatal molsidomine increased urinary NO metabolite excretion at 8 wk of age, i.e., 4 wk after treatment was stopped (P < 0.05). Systolic blood pressure was persistently reduced after molsidomine (42-wk females: 118 +/- 3 vs. 141 +/- 5 and 36-wk males: 139 +/- 4 vs. 158 +/- 4 mmHg; both P < 0.001). Perinatal treatment decreased glomerular filtration rate (P < 0.05) and renal blood flow (P < 0.01) and increased renal vascular resistance (P < 0.05), without affecting filtration fraction, suggesting persistently increased preglomerular resistance. At 4 wk of age natriuresis was transiently increased by molsidomine (P < 0.05). Molsidomine decreased glomerulosclerosis (P < 0.05). Renal blood flow correlated positively with glomerulosclerosis in control (P < 0.001) but not in perinatally treated FHH rats. NO dependency of renal vascular resistance was increased by perinatal molsidomine. Perinatal enhancement of NO availability can ameliorate development of hypertension and renal injury in FHH rats. Paradoxically, glomerular protection by perinatal exposure to the NO donor molsidomine may be due to persistently increased preglomerular resistance. The mechanisms by which increased perinatal NO availability can persistently reprogram kidney function and ameliorate hypertension deserve further study.

Frequency modulation of renal myogenic autoregulation by perfusion pressure

Recent studies of renal autoregulation have shown modulation of the faster myogenic mechanism by the slower tubuloglomerular feedback and that the modulation can be detected in the dynamics of the myogenic mechanism. Conceptual and empirical considerations suggest that perfusion pressure may modulate the myogenic mechanism, although this has not been tested to date. Here we present data showing that the myogenic operating frequency, assessed by transfer-function analysis, varied directly as a function of perfusion pressure in the hydronephrotic kidney perfused in vitro over the range from 80 to 140 mmHg. A similar result was obtained in intact kidneys in vivo when renal perfusion pressure was altered by systemic injection of NG-nitro-l-arginine methyl ester (l-NAME). When perfusion pressure was not allowed to increase, l-NAME did not affect the myogenic operating frequency despite equivalent reduction of renal vascular conductance. Blood-flow dynamics were assessed in the superior mesenteric artery before and after l-NAME. In this vascular bed, the operating frequency of the myogenic mechanism was not affected by perfusion pressure. Thus the operating frequency of the renal myogenic mechanism is modulated by perfusion pressure independently of tubuloglomerular feedback, and the data suggest some degree of renal specificity of this response.

Nitric oxide, superoxide and renal blood flow autoregulation in SHR after perinatal L-arginine and antioxidants

AIM

Nitric oxide (NO) and superoxide are considered to be regulatory in renal blood flow (RBF) autoregulation, and hence may contribute to development of hypertension. To extend our previous observations that dynamic NO release is impaired in the spontaneously hypertensive rat (SHR) we investigated, firstly, if superoxide dependency of RBF autoregulation is increased in SHR and, secondly, if the beneficial effect of perinatal supplementation in SHR is partly as a result of early correction of RBF autoregulation. We hypothesized that perinatal supplementation by restoring dynamic NO release and/or decreasing superoxide dependency and would improve life-long blood pressure regulation.

METHODS

Autoregulation was studied using stepwise reductions in renal perfusion pressure in anaesthetized male SHR, SHR perinatally supplemented with arginine and antioxidants (SHRsuppl) and Wistar-Kyoto (WKY), prior to and during i.v. Nomega-nitro-l-arginine (NO synthase inhibitor) or tempol (superoxide dismutase mimetic).

RESULTS

Spontaneously hypertensive rat displayed a wider operating range of RBF autoregulation as compared with WKY (59 +/- 4 vs. 33 +/- 2 mmHg, respectively; P < 0.01). Perinatal supplementation in SHR decreased mean arterial pressure, renal vascular resistance and the operating range of RBF autoregulation (43 +/- 3 mmHg; P < 0.01). In addition autoregulation efficiency decreased. RBF autoregulation characteristics shifted towards those of normotensive WKY. However, dynamic NO release was still impaired and no clear differences in superoxide dependency in RBF autoregulation between groups was observed.

CONCLUSION

Perinatal supplements shifted RBF autoregulation characteristics of SHR towards WKY, although capacity of the SHRsuppl kidney to modulate NO production to shear stress still seems impaired. The less strictly controlled RBF as observed in perinatally supplemented SHR could result in an improved long-term blood pressure control. This might partly underlie the beneficial effects of perinatal supplementation.

Interactions contributing to kidney blood flow autoregulation

PURPOSE OF REVIEW

Autoregulation of renal blood flow has traditionally been considered to stabilize glomerular filtration, and thus tubular load, in the face of blood pressure fluctuations. This view arose because of the contribution of tubuloglomerular feedback, which senses distal tubular fluid composition, to regulation and autoregulation of renal blood flow. Studies have indicated a more important role for the myogenic mechanism. It has been proposed that the 'purpose' of autoregulation is to defend glomerular structure. Both these views may be incomplete because neither takes into consideration the complex interactions between tubuloglomerular feedback and the myogenic mechanism and among nephrons whose afferent arterioles derived from a common interlobular artery.

RECENT FINDINGS

Recent findings indicate that it is now indisputable that effective autoregulation is necessary for defense of glomerular structure. Extensive modulation of the myogenic mechanism by tubuloglomerular feedback has been shown using a variety of experimental designs that have illuminated one pathway (neuronal nitric oxide synthase at the macula densa) by which this occurs.

SUMMARY

These findings indicate that the myogenic mechanism can no longer be considered as a purely vascular mechanism in the kidney and instead receives information via tubuloglomerular feedback about the status of renal function.

IDENTIFICATION OF BLOOD PRESSURE CONTROL MECHANISMS BY POWER SPECTRAL ANALYSIS

1. Blood pressure and organ perfusion are controlled by a variety of cardiovascular control systems, such as the baroreceptor reflex and the renin-angiotensin system (RAS), and by local vascular mechanisms, such as shear stress-induced release of nitric oxide (NO) from the endothelium and the myogenic vascular response. Deviations in arterial blood pressure from its set point activate these mechanisms in an attempt to restore blood pressure and/or secure organ perfusion. However, the response times at which different cardiovascular mechanisms operate differ considerably (e.g. blood pressure control by the RAS is slower than blood pressure control via the baroreceptor reflex). 2. Owing to these different response times, some cardiovascular control systems affect blood pressure more rapidly and others more slowly. Thus, identifying the frequency components of blood pressure variability (BPV) by power spectral analysis can potentially provide important information on individual blood pressure control mechanisms. 3. Evidence is presented that the RAS, catecholamines, endothelial-derived NO and myogenic vascular function affect BPV at very low frequencies (0.02-0.2 Hz) and that low-frequency (LF) BPV (0.2-0.6 Hz) is affected by sympathetic modulation of vascular tone and endothelial-derived NO in rats. In humans, LF BPV (0.075-0.15 Hz) is affected by sympathetic modulation of vascular tone and myogenic vascular function. The impact of the RAS and endothelial-derived NO on BPV in humans requires further investigation. 4. In conclusion, power spectral analysis is a powerful diagnostic tool that allows identification of pathophysiological mechanisms contributing to cardiovascular diseases, such as hypertension, heart failure and stroke, because it can separate slow from fast cardiovascular control mechanisms. The limitation that some cardiovascular control mechanisms affect the same frequency components of BPV requires the combination of blood pressure spectral analysis with other techniques.

Assessment of renal autoregulation

The kidney displays highly efficient autoregulation so that under steady-state conditions renal blood flow (RBF) is independent of blood pressure over a wide range of pressure. Autoregulation occurs in the preglomerular microcirculation and is mediated by two, perhaps three, mechanisms. The faster myogenic mechanism and the slower tubuloglomerular feedback contribute both directly and interactively to autoregulation of RBF and of glomerular capillary pressure. Multiple experiments have been used to study autoregulation and can be considered as variants of two basic designs. The first measures RBF after multiple stepwise changes in renal perfusion pressure to assess how a biological condition or experimental maneuver affects the overall pressure-flow relationship. The second uses time-series analysis to better understand the operation of multiple controllers operating in parallel on the same vascular smooth muscle. There are conceptual and experimental limitations to all current experimental designs so that no one design adequately describes autoregulation. In particular, it is clear that the efficiency of autoregulation varies with time and that most current techniques do not adequately address this issue. Also, the time-varying and nonadditive interaction between the myogenic mechanism and tubuloglomerular feedback underscores the difficulty of dissecting their contributions to autoregulation. We consider the modulation of autoregulation by nitric oxide and use it to illustrate the necessity for multiple experimental designs, often applied iteratively.

Interactive modulation of renal myogenic autoregulation by nitric oxide and endothelin acting through ET-B receptors

In rats, nitric oxide modulates renal autoregulation in steady-state experiments and the myogenic mechanism in dynamic studies. Interactive modulation of autoregulation by nitric oxide and endothelin-1, predominantly involving endothelin B receptors, has been reported although it remains unclear whether the interaction is synergistic or obligatory or whether it affects the myogenic component of autoregulation. Nonselective inhibition of nitric oxide synthase (Lω-nitro-l-arginine methyl-ester; l-NAME) with endothelin A and B selective receptor antagonists BQ-123 and BQ-788, all infused into the renal artery, plus time series analysis were used to test the interactive actions of nitric oxide and endothelin on renal vascular conductance and on autoregulation. Nonselective endothelin receptor antagonism blunted the constrictor response to subsequent l-NAME but had no effect on previously established l-NAME-induced vasoconstriction. BQ-123 did not affect conductance and caused only minor reduction in myogenic autoregulatory efficiency. Responses to BQ-123 and l-NAME were additive and not interactive. BQ-788 and l-NAME each caused strong vasoconstriction alone and in the presence of the other, indicating that coupling between nitric oxide- and endothelin B-mediated events is not obligatory. l-NAME augmented myogenic autoregulation, and subsequent BQ-788 did not alter this response. However, BQ-788 infused alone also enhanced myogenic autoregulation but resulted in significant impairment of myogenic autoregulation by subsequent l-NAME. Thus the interaction between nitric oxide and endothelin is clearly nonadditive and, because it is asymmetrical, cannot be explained simply by convergence on a common signal pathway. Instead one must postulate some degree of hierarchical organization and that nitric oxide acts downstream to endothelin B activation.

Angiotensin II-induced calcium signaling in the afferent arteriole from rats with two-kidney, one-clip hypertension

The aim of this study was to investigate ANG II-induced Ca2+signaling in freshly isolated afferent arterioles (AA) from two-kidney, one-clip hypertensive (2K1C) rats, which have an elevated plasma and renal ANG II level, and different perfusion pressure and vascular tone in the clipped and nonclipped kidney. The Ca2+responses in vessels from 2K1C and control rats were similar in all groups ( P > 0.1). The intracellular Ca2+(Cai2+) response in the afferent arteriole after 10−8M ANG II stimulation was 0.57 ± 0.10, 0.50 ± 0.07, 0.48 ± 0.04, and 0.36 ± 0.05 in the control, sham, nonclipped, and clipped kidney, respectively. These data were consistent with the finding of unchanged AT1aR mRNA levels in AAs from all groups. Although the absolute values were similar, the dose-response curves to ANG II were different. In the control, sham, and nonclipped kidney from 2K1C, the dose-response curve leveled off between 10−8and 10−6M ANG II. In the clipped kidney, the dose-response curve was linear, with a significantly increased response at 10−6M compared with 10−8M ANG II ( P < 0.05). Inhibition of cyclooxygenase-1 (COX-1) with indomethacin enhanced the ANG II response in the nonclipped (Δ0.30 ± 0.09) and clipped (Δ0.30 ± 0.09) kidneys from 2K1C ( P < 0.005), but not in control rats (Δ−0.02 ± 0.11, P > 0.8). Conclusively, the ANG II-induced Cai2+response was reduced by COX-1-derived prostaglandins in 2K1C, in contrast to control animals, where the COX-1 inhibition had no effect. COX-2 inhibition with NS-398 did not increase the ANG II-mediated Cai2+response in any of the groups.

Correlation between noninvasive and endovascular Doppler in patients with atherosclerotic renal artery stenosis: A pilot study

Considerable interest exists in using endovascular techniques for assessing the renal circulation. The vasomotor response of the renal circulation in patients with hypertension and severe atherosclerotic renal artery stenosis was evaluated. Sixteen patients (5 men and 11 women) with a mean age of 73.2+/-7.1 years (range, 59-82 years) underwent noninvasive renal duplex ultrasonography and endovascular flow wire evaluation. The flow wire was positioned in the main renal artery distal to the stenosis and used to examine the reactivity of the renal circulation in response to intra-arterial papaverine before and after percutaneous revascularization. Resistive index was computed as (MPV - EDV)/MPV, where MPV is the maximum peak velocity and EDV is the end-diastolic velocity. Renal flow reserve was defined as the ratio of peak hyperemic response to baseline flow. Intervention was technically successful in 18/18 arteries. Heart rate (69+/-11 vs. 65+/-10 beats/min; P=NS) and systolic blood pressure (168+/-19 vs. 163+/-24 mm Hg; P=NS) remained constant following injection of intrarenal papaverine, while diastolic blood pressure decreased (77+/-13 vs. 65+/-11 mm Hg; P<0.006). Papaverine injection resulted in significant increases in average peak velocity (APV; 148% of baseline; P=0.0003), MPV (146% of baseline; P=0.001), and EDV (161% of baseline; P=0.0026), but had no effect on the endovascular resistive index (0.81+/-0.12 vs. 0.84+/-0.08; P=NS). Renal artery stenting produced no effect on APV or EDV, but a small increase in MPV (123% of baseline; P<0.05). In contrast, renal stenting resulted in a significant increase in the endovascular resistive index (0.77+/-012 vs. 0.84+/-0.08; P=0.003), pulsatility index (1.84+/-0.61 vs. 2.23+/-0.70; P<0.014), and renal flow reserve (1.49+/-0.44 vs. 1.86+/-0.73; P<0.0071). A correlation was found between the noninvasive and endovascular measures diastolic/systolic ratio (r=0.73; P<0.0009) and resistive index (r=0.63; P<0.0053). Patients with atherosclerotic renal artery stenosis have preserved vasomotor response to papaverine, as well as marked increases in renal flow and resistive index following successful renal artery stenting. Endovascular flow wire measurements are safe, easy to perform, and correlate well with noninvasive Doppler measurements.

Tubuloglomerular feedback-dependent modulation of renal myogenic autoregulation by nitric oxide

Nonselective inhibition of nitric oxide (NO) synthase (NOS) augments myogenic autoregulation, an action that implies enhancement of pressure-induced constriction and dilatation. This pattern is not explained solely by interaction with a vasoconstrictor pathway. To test involvement of the Rho-Rho kinase pathway in modulation of autoregulation by NO, the selective Rho kinase inhibitor Y-27632 and/or the NOS inhibitor N(omega)-nitro-l-arginine methyl ester (l-NAME) were infused into the left renal artery of anesthetized rats. Y-27632 and l-NAME were also infused into isolated, perfused hydronephrotic kidneys to assess myogenic autoregulation over a wide range of perfusion pressure. In vivo, l-NAME reduced renal vascular conductance and augmented myogenic autoregulation, as shown by increased slope of gain reduction and associated phase peak in the pressure-flow transfer function. Y-27632 (10 mumol/l) strongly dilated the renal vasculature and profoundly inhibited autoregulation in the absence or presence of l-NAME in vivo and in vitro. Afferent arteriolar constriction induced by 30 mmol/l KCl was reversed (-92 +/- 3%) by Y-27632. Phenylephrine caused strong renal vasoconstriction but did not affect autoregulation. Inhibition of neuronal NOS by N(5)-(1-imino-3-butenyl)-l-ornithine (l-VNIO) did not cause significant vasoconstriction but did augment myogenic autoregulation. Thus vasoconstriction is neither necessary (l-VNIO) nor sufficient (phenylephrine) to explain the augmented myogenic autoregulation induced by l-NAME. The effect of l-VNIO implicates tubuloglomerular feedback (TGF) and neuronal NOS at the macula densa in regulation of the myogenic mechanism. This conclusion was confirmed by the demonstration that systemic furosemide removed the TGF signature from the pressure-flow transfer function and significantly inhibited myogenic autoregulation. In the presence of furosemide, augmentation of myogenic autoregulation by l-NAME was significantly reduced. These results provide a potential mechanism to explain interaction between myogenic and TGF-mediated autoregulation.

Nitric oxide blunts myogenic autoregulation in rat renal but not skeletal muscle circulation via tubuloglomerular feedback

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 an approximately 5 s time constant and providing approximately 36% of perfect autoregulation. A secondary rise between 10 and 40 s brought RVR to 95% total autoregulatory efficiency, reflecting tubuloglomerular feedback (TGF) and possibly one or two additional mechanisms. The kinetics were similar after the RAP decrease. Inhibition of NOS (by l-NAME) increased RAP, enhanced the strength (79% autoregulation) and doubled the speed of the myogenic response, and promoted the emergence of RVR oscillations ( approximately 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 TGF (by furosemide) abolished the l-NAME effect on the myogenic response. RVR responses during furosemide treatment, assuming complete inhibition of TGF, suggest a third mechanism that contributes 10-20% and is independent of TGF, slower than the myogenic response, 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.

Impaired Ca2+ signaling attenuates P2X receptor-mediated vasoconstriction of afferent arterioles in angiotensin II hypertension

This study tested the hypothesis that afferent arteriolar responses to purinoceptor activation are attenuated, and Ca2+ signaling mechanisms are responsible for the blunted preglomerular vascular reactivity in angiotensin II (Ang II) hypertension. Experiments determined the effects of ATP, the P2X1 agonist beta,gamma-methylene ATP or the P2Y agonist UTP on arteriolar diameter using the juxtamedullary nephron technique and on renal myocyte intracellular Ca2+ concentration ([Ca2+]i) using single cell fluorescence microscopy. Six or 13 days of Ang II infusion significantly attenuated the vasoconstrictor responses to ATP and beta,gamma-methylene ATP (P<0.05). During exposure to ATP (1, 10, and 100 micromol/L), afferent diameter declined by 17+/-2%, 29+/-3%, and 30+/-2% in normal control rats and 8+/-3%, 7+/-3%, and 22+/-3% in kidneys of Ang II-infused rats (13 days). Renal myocyte intracellular calcium responses to ATP or beta,gamma-methylene ATP were also decreased in Ang II hypertensive rats. In myocytes of control rats, peak increases in [Ca2+]i averaged 107+/-21, 170+/-38, and 478+/-79 nmol/L at ATP concentrations of 1, 10, and 100 micromol/L, respectively. Ang II infusion for 13 days decreased the peak responses to ATP (1, 10, and 100 micromol/L) to 65+/-13, 102+/-20, and 367+/-73 nmol/L, respectively. The peak increases in [Ca2+]i in response to beta,gamma-methylene ATP were also reduced in Ang II hypertensive rats. However, angiotensin hypertension did not change the UTP-mediated vasoconstrictor responses or the myocyte calcium responses to UTP. These results indicate that the impaired autoregulatory response observed in Ang II-dependent hypertension can be attributed to impairment of P2X1 receptor-mediated signal transduction.

Superoxide scavenging attenuates renal responses to ANG II during nitric oxide synthase inhibition in anesthetized dogs

To assess the role of superoxide (O2−) and nitric oxide (NO) interaction in mediating the renal actions of ANG II, we examined the renal responses to intra-arterial infusion of ANG II (0.5 ng·kg−1·min−1) before and during administration of a superoxide dismutase mimetic, tempol (0.5 mg·kg−1·min−1), in the presence or absence of NO synthase inhibitor, nitro-l-arginine (NLA; 50 μg·kg−1·min−1), in anesthetized dogs pretreated with enalaprilat (33 μg·kg−1·min−1). In one group of dogs ( n = 7), ANG II infusion before tempol infusion caused decreases of 24 ± 4% in renal blood flow (RBF), 55 ± 7% in urine flow (V), and 53 ± 8% in urinary sodium excretion (UNaV) with a slight decrease in glomerular filtration rate (GFR; −7.8 ± 3.4%). Tempol infusion alone did not cause significant alterations in RBF, GFR, V, or UNaV; however, ANG II in the presence of tempol caused a smaller degree of decreases in RBF (−12 ± 2%), in V (−16 ± 5%), and in UNaV (−27 ± 10%) with a slight increase in GFR (6.6 ± 2.8%) than the responses observed before tempol. In another group of NLA-treated dogs ( n = 6), tempol infusion also caused significant attenuation in the ANG II-induced responses on RBF (−13 ± 3% vs. −22 ± 7%), GFR (−19 ± 5% vs. −33 ± 3), V (−15 ± 12% vs. −28 ± 4%), and UNaV (−11 ± 14% vs. −32 ± 7%). These data demonstrate that renal responses to ANG II are partly mediated by O2−generation and its interaction with NO. The sodium-retaining effect of ANG II is greatly influenced by O2−generation, particularly in the condition of NO deficiency.

Elevated arterial pressure impairs autoregulation independently of AT(1) receptor activation

OBJECTIVE

These studies determined the ability of AT1 receptor blockade or 'triple therapy', to reverse angiotensin II-induced hypertension and improve autoregulatory behavior.

DESIGN

Experiments to determine if regulation of systolic blood pressure, in the normotensive range, would improve renal microvascular autoregulatory behavior in angiotensin II-infused rats.

METHODS

Hypertension was induced by chronic angiotensin II infusion (60 ng/min) for 10-14 days. Two groups of angiotensin II-infused rats received either AT1 receptor blockade, with candesartan cilexetil, or triple therapy, with hydralazine, hydrochlorothiazide and reserpine, beginning on day 6 or day 0 of angiotensin II infusion, respectively. Sham animals were studied as normotensive controls. Systolic blood pressure was measured by tail cuff. Autoregulatory behavior was assessed using the juxtamedullary nephron technique in response to step (15 mmHg) increases in perfusion pressure from 65 to 170 mmHg.

RESULTS

Angiotensin II infusion increased systolic blood pressure from a baseline of 125 mmHg to 162 and 182 mmHg after 10 and 14 days, respectively. Candesartan cilexetil and triple therapy normalized the blood pressure to between 119 and 126 mmHg. Increasing perfusion pressure, from 65 to 170 mmHg, reduced afferent arteriolar diameter by 30% in sham-treated kidneys. Autoregulation was significantly blunted in angiotensin II-infused rats, resulting in a pressure-mediated vasoconstriction of only 10%. Candesartan cilexetil, or triple therapy, significantly improved autoregulatory behavior, as indicated by pressure-mediated vasoconstrictor responses of 30 and 40%; respectively, despite continued angiotensin II infusion.

CONCLUSIONS

These data suggest that chronic elevation of arterial blood pressure, rather than chronic AT1 receptor stimulation, is sufficient to induce hypertensive impairment of renal autoregulatory capability.

The step response: a method to characterize mechanisms of renal blood flow autoregulation

Response of renal vasculature to changes in renal perfusion pressure (RPP) involves mechanisms with different frequency characteristics. Autoregulation of renal blood flow (RBF) is mediated by the rapid myogenic response, by the slower tubuloglomerular feedback (TGF) mechanism, and, possibly, by an even slower third mechanism. To evaluate the individual contribution of these mechanisms to RBF autoregulation, we analyzed the response of RBF to a step increase in RPP. In anesthetized rats, the suprarenal aorta was occluded for 30 s, and then the occlusion was released to induce a step increase in RPP. Three dampened oscillations were observed; their oscillation periods ranged from 9.5 to 13 s, from 34.2 to 38.6 s, and from 100.5 to 132.2 s, respectively. The two faster oscillations correspond with previously reported data on the myogenic mechanism and the TGF. In accordance, after furosemide, the amplitude of the intermediate oscillation was significantly reduced. Inhibition of nitric oxide synthesis by Nomega-nitro-l-arginine methyl ester significantly increased the amplitude of the 10-s oscillation. It is concluded that the parameters of the dampened oscillations induced by the step increase in RPP reflect properties of autoregulatory mechanisms. The oscillation period characterizes the individual mechanism, the dampening is a measure for the stability of the regulation, and the square of the amplitudes characterizes the power of the respective mechanism. In addition to the myogenic response and the TGF, a third rather slow mechanism of RBF autoregulation exists.

Facilitation of renal autoregulation by angiotensin II is mediated through modulation of nitric oxide

AIMS

This study was designed to investigate the influence of angiotensin II (Ang II) and nitric oxide (NO) on autoregulation of renal perfusion.

METHODS

Autoregulation was investigated in isolated perfused kidneys (IPRK) from Sprague-Dawley rats during stepped increases in perfusion pressure.

RESULTS

Ang II (75-200 pM) produced dose-dependent enhancement of autoregulation whereas phenylephrine produced no enhancement and impaired autoregulation of GFR. Enhancement by Ang II was inhibited by the AT1 antagonist, Losartan, and the superoxide scavenger, Tempol. Under control conditions nitric oxide synthase (NOS) inhibition by 10 microm N-omega-nitro-L-arginine methyl ester (L-NAME) facilitated autoregulation in the presence of non-specific cyclooxygenase (COX) inhibition by 10 microm indomethacin. Both COX and combined NOS/COX inhibition reduced the autoregulatory threshold concentration of Ang II. Facilitation by 100 pm Ang II was inhibited by 100 microm frusemide. Methacholine (50 nm) antagonised Ang II-facilitated autoregulation in the presence and absence of NOS/COX inhibition. Infusion of the NO donor, 1 microm sodium nitroprusside, inhibited L-NAME enhancement of autoregulation under control conditions and during Ang II infusion.

CONCLUSIONS

The results suggest than an excess of NO impairs autoregulation under control conditions in the IPRK and that endogenous and exogenous NO, vasodilatory prostaglandins and endothelium-derived hyperpolarizing factor (EDHF) activity antagonise Ang II-facilitated autoregulation. Ang II also produced a counterregulatory vasodilatory response that included prostaglandin and NO release. We suggest that Ang II facilitates autoregulation by a tubuloglomerular feedback-dependent mechanism through AT1 receptor-mediated depletion of nitric oxide, probably by stimulating generation of superoxide.

NO mediates downregulation of RBF after a prolonged reduction of renal perfusion pressure in SHR

The aim of the study was to investigate mechanisms underlying the downregulation of renal blood flow (RBF) after a prolonged reduction in renal perfusion pressure (RPP) in adult spontaneously hypertensive rats (SHR). We tested the effect on the RBF response of clamping plasma ANG II in sevoflurane-anesthetized SHR. We also tested the effect of general cyclooxygenase (COX) inhibition and inhibition of the inducible COX-2. Furthermore, we assessed the effect of clamping the nitric oxide (NO) system. A prolonged period (15 min) of reduced RPP induced a downregulation of RBF. This was unchanged after clamping of plasma ANG II concentrations, general COX inhibition, and specific inhibition of COX-2. In contrast, clamping the NO system diminished the ability of SHR to downregulate RBF to a lower level. The downregulation of RBF was not associated with a resetting of the lower limit of autoregulation in the control group, in the ANG II-clamped group, or the NO clamped group. However, general COX inhibition and specific COX-2 inhibition enabled downward resetting of the lower limit of autoregulation. In conclusion, in SHR the renin-angiotensin system does not appear to play a major role in the downregulation of RBF after prolonged reduction of RPP. This response appears to be mediated partly by the NO system. We hypothesize that, in SHR, lack of downward resetting of the lower limit of autoregulation in response to a prolonged lowering of RPP could be the result of increased COX-2-mediated production of vasoconstrictory prostaglandins.

Oral administration of L-arginine decreases blood pressure and increases renal excretion of sodium and water in renovascular hypertensive rats

The two-kidney, one-clip renovascular (2K1C) hypertension model is characterized by a reduction in renal flow on the clipped artery that activates the renin-angiotensin system. Endothelium dysfunction, including diminished nitric oxide production, is also believed to play a role in the pathophysiology of this model. Some studies have shown an effect of L-arginine (L-Arg, a nitric oxide precursor) on hypertension. In the present study we determined the ability of L-Arg (7 days of treatment) to reduce blood pressure and alter renal excretions of water, Na+ and K+ in a model of 2K1C-induced hypertension. Under ether anesthesia, male Wistar rats (150-170 g) had a silver clip (0.20 mm) placed around the left renal artery to produce the 2K1C renovascular hypertension model. In the experimental group, the drinking water was replaced with an L-Arg solution (10 mg/ml; average intake of 300 mg/day) from the 7th to the 14th day after surgery. Sham-operated rats were used as controls. At the end of the treatment period, mean blood pressure was measured in conscious animals. The animals were then killed and the kidneys were removed and weighed. There was a significant reduction of mean blood pressure in the L-Arg-treated group when compared to control (129 7 vs 168 6 mmHg, N = 8-10 per group; P<0.05). Concomitantly, a significant enhancement of water and Na+ excretion was observed in the 2K1C L-Arg-treated group when compared to control (water: 13.0 0.7 vs 9.2 0.5 ml/day, P<0.01; Na+: 1.1 0.05 vs 0.8 0.05 mEq/day, respectively, P<0.01). These results show that orally administered L-Arg acts on the kidney, possibly inducing changes in renal hemodynamics or tubular transport due to an increase in nitric oxide formation.

NO dependency of RBF and autoregulation in the spontaneously hypertensive rat

In the spontaneously hypertensive rat (SHR), renal blood flow (RBF) has been reported to be very dependent on nitric oxide (NO); however, autoregulation is normal, albeit shifted to higher perfusion pressures. To test the hypothesis that in the SHR NO dependency of RBF autoregulation is diminished, we investigated RBF autoregulation in anesthetized young male SHR and normotensive Wistar-Kyoto (WKY) rats before and during acute intravenous NO synthase (NOS) inhibition with N(omega)-nitro-L-arginine (L-NNA) and urinary excretion of nitrate plus nitrite (U(NOx)V) at different renal perfusion pressures (RPP). Under baseline conditions, SHR had higher mean arterial pressure (147 +/- 4 mmHg) and renal vascular resistance (16 +/- 1 U) than WKY (105 +/- 4 mmHg and 10 +/- 0.5 U, respectively, P < 0.05). RBF was similar (9.4 +/- 0.5 vs. 10.3 +/- 0.1 ml x min(-1)x g kidney wt(-1)). Acute NOS blockade increased mean arterial pressure similarly, but there was significantly more reduction in RBF and hence an enhanced increase in renal vascular resistance in SHR (to 36 +/- 3 vs. 17 +/- 1 U in WKY, P < 0.001). The renal vasculature of SHR is thus strongly dependent on NO in maintaining basal RBF. The lower limit of autoregulation was higher in SHR than WKY in the baseline situation (85 +/- 3 vs. 71 +/- 2 mmHg, P < 0.05). Acute L-NNA administration did not decrease the lower limit in the SHR (to 81 +/- 3 mmHg, not significant) and decreased the lower limit to 63 +/- 2 mmHg (P < 0.05) in the WKY. The degree of compensation as a measure of autoregulatory efficiency attained at spontaneous perfusion pressures was comparable in SHR vs. WKY but with a shift of the curve toward higher perfusion pressures in SHR. Acute NOS blockade only increased the degree of compensation in WKY. Remarkably, U(NOx)V was significantly lower at spontaneous RPP in SHR. After reduction of RPP, the observed decrease in U(NOx)V was significantly more pronounced in WKY than in SHR. In conclusion, the renal circulation in SHR is dependent on high levels of NO; however, the capacity to modulate NO in response to RPP-induced changes in shear stress seems to be limited.

Hypoxanthine plus xanthine oxidase causes profound natriuresis without affecting renal blood flow autoregulation

BACKGROUND

Enhanced superoxide (O2-.) production by xanthine oxidase in ischemia/reperfusion has been implicated in structural damage. The reperfusion phase is accompanied by decreased tubular sodium reabsorption, which has been partly attributed to enhanced action of O2-. In the present study we assessed whether intrarenal increases of O2-. accomplished by concomitant intrarenal hypoxanthine and intravenous xanthine oxidase (HX/XO) infusion would decrease or increase sodium excretion, and whether HX/XO infusion could be responsible for the diminished efficacy of renal blood flow (RBF) autoregulation in ischemia/reperfusion.

METHODS

In the first group of Sprague-Dawley rats, renal sodium handling was measured before and during O2-. infusion. In the second group, renal hemodynamics and RBF autoregulation were assessed.

RESULTS

Intrarenal O2-. infusion dramatically increased urine flow from 14.5 +/- 2.0 microL/min to 46.3 +/- 4.4 microL/min, urinary excretion of sodium (UNaV) from 1.7 +/- 0.4 micromol/min to 8.6 +/- 0.9 micromol/min, and fractional excretion of sodium FENa from 1.2 +/- 0.4% to 7.6 +/- 1.2%. Urinary excretion of thiobarbituric acid reactive substances (TBARS), a measure of lipid peroxidation, increased during HX/XO infusion. These changes were completely reversible. Glomerular filtration rate (GFR) decreased from 1.12 +/- 0.08 during baseline to 0.79 +/- 0.06 during HX/XO (P < 0.05) and tended to increase toward baseline during recovery (0.84 +/- 0.06 mL/min/g kidney weight). HX/XO did not significantly affect mean arterial pressure (MAP). HX/XO decreased RBF in the second group from 8.4 +/- 0.6 mL/min/g kidney weight to 7.4 +/- 0.5 mL/min/g kidney weight (P < 0.05) and renal vascular resistance (RVR) slightly increased from 13.8 +/- 0.9 units under baseline conditions to 15.1 +/- 1.1 units during HX/XO infusion (P < 0.05). HX/XO did not significantly affect RBF autoregulation. Proteinuria and glucosuria were absent and light microscopy revealed no renal morphologic changes.

CONCLUSION

Intrarenal O2-. infusion (1) dramatically increased sodium and volume excretion and (2) did not affect autoregulation of RBF. Thus, superoxide can markedly affect glomerulotubular balance by diverging actions on renal hemodynamics and reabsorptive function and could mediate the functional tubular consequences of ischemia/reperfusion.

Chronic administration of aqueous extract of Hibiscus sabdariffa attenuates hypertension and reverses cardiac hypertrophy in 2K-1C hypertensive rats

The effect of aqueous extract of petals of Hibiscus sabdariffa (HS) on the established stages of 2-Kidney, 1-Clip renovascular hypertension was investigated in Sprague-Dawley rats. Renovascular hypertension was induced by subjecting the animals to left renal artery clamping using a 0.2mm silver clip under ether anesthesia. Sham-operated (Sh-Op) rats served as controls. Six weeks after renal artery clamping, one group of hypertensive rats (blood pressure (BP) >140 mmHg) received HS (250 mg/kg/day) in drinking water (2K-1C+HS). The second group (2K-1C) and the sham-operated (Sh-Op) controls, received drinking water. BP was monitored weekly using rat-tail plethysmography. After 8 weeks, 2K-1C+HS had a reduction in systolic BP (139.6+/-1.6 mmHg) compared to 2K-1C (174+/-2.4 mmHg, n=5; P<0.001). No significant difference was found in BP of 2K-1C+HS and Sh-Op (139.6+/-1.6 mmHg versus 132+/-3.4 mmHg). A reduction in heart rate in 2K-1C+HS was observed (388+/-3.7 bpm versus 444+/-6.8 bpm in 2K-1C and 416+/-9.3 in Sh-Op, n=5; P<0.001). The hearts of 2K-1C were heavier than those of 2K-1C+HS (0.74+/-0.03 g versus 0.66+/-0.03 g, n=5; P<0.05). Cardiac weight of 2K-1C+HS was comparable to those of Sh-Op (0.57+/-0.04 g). Serum creatinine and plasma electrolytes were not different from controls. This study suggests that HS exhibits antihypertensive and cardioprotective effects in vivo and supports the public belief that HS may be a useful antihypertensive agent.

Blunted acetylcholine relaxation and nitric oxide release in arteries from renal hypertensive rats

OBJECTIVE

Investigation of the effect of hypertension on endothelium-dependent relaxation and release of nitric oxide (NO) in normotensive and renal hypertensive rats.

DESIGN AND METHODS

Sprague-Dawley rats were randomly allocated into two groups: uninephrectomized controls and one-kidney one-clip (Goldblatt hypertension) hypertensive rats, a non-renin dependent model of hypertension. After 10 weeks and in the presence of the cyclooxygenase inhibitor indomethacin, simultaneous measurements of the NO concentration, measured with a NO-specific microelectrode and endothelium-dependent relaxation were performed in isolated rat superior mesenteric arteries.

RESULTS

Addition of the NO scavenger, oxyhaemoglobin, showed that basal NO concentration was unaltered in arterial segments from hypertensive rats. In norepinephrine-contracted arteries, acetylcholine increased the NO concentration and caused relaxations, and both parameters were significantly reduced in renal hypertensive arteries. Relaxations induced by the NO donor, S-nitroso-N-acetylpenicillamine were reduced. The superoxide scavenger, superoxide dismutase, and the NO synthase substrate, l-arginine, did not change the increase in NO concentration or acetylcholine relaxation in arteries from normotensive or renal hypertensive animals. In contrast, the NO synthase inhibitor, asymmetric dimethyl l-arginine, reduced the NO concentration and acetylcholine relaxation, while these responses were abolished in the presence of oxyhaemoglobin.

CONCLUSIONS

This study provides direct evidence that reduced endothelium-dependent relaxations in the superior mesenteric artery from renal hypertensive rats is due, at least in part, to diminished NO release. The reduced NO release and relaxation persist in the presence of excess of substrate for NO synthase.

Spontaneous renal blood flow autoregulation curves in conscious sinoaortic baroreceptor-denervated rats

These experiments examined whether the conscious sinoaortic baroreceptor-denervated (SAD) rat, owing to its high spontaneous arterial pressure (AP) variability, might represent a model for renal blood flow (RBF) autoregulation studies. In eight SAD and six baroreceptor-intact rats, AP and RBF were recorded (1-h periods) before and after furosemide (10 mg/kg followed by 10 mg. kg(-1). h(-1) iv) administration. In control conditions, AP variability was markedly enhanced in SAD rats (coefficient of variation: 16.0 +/- 1.2 vs. 5.4 +/- 0.5% in intact rats), whereas RBF variability was only slightly increased (8.7 +/- 0.6 vs. 6.1 +/- 0.5% in intact rats), suggesting buffering by autoregulatory mechanisms. In SAD rats, but not in intact rats, the AP-RBF relationships could be modeled with a four-parameter sigmoid Weibull equation (r(2) = 0.24 +/- 0.07, 3,600 data pairs/rat), allowing for estimation of an autoregulatory plateau (10.1 +/- 0.7 ml/min) and a lower limit of RBF autoregulation (P(LL) = 93 +/- 6 mmHg, defined as AP at RBF 5% below the plateau). After furosemide treatment, autoregulation curves (r(2) = 0.49 +/- 0.07) in SAD rats were shifted downward (plateau = 8.6 +/- 0.8 ml/min) and rightward (P(LL) = 102 +/- 5 mmHg). In five of six intact rats, P(LL) became measurable (104 +/- 1 mmHg), albeit with limited accuracy (r(2) = 0.09 +/- 0.03). In conclusion, the conscious SAD rat offers the possibility of describing RBF autoregulation curves under dynamic, unforced conditions. The tubuloglomerular feedback and myogenic mechanisms cooperate in setting P(LL) and thus in stabilizing RBF during spontaneous depressor episodes.

Renal blood flow autoregulation in blood pressure control

Although the kidney strives to maintain its perfusion within tight boundaries, considerable blood flow fluctuations do occur. The reasons for this are the rather slow acting compensatory mechanisms of renal blood flow autoregulation, the effects of renal nerves, hormonal influences, etc. It seems that variations in renal perfusion can exert a major influence on renal excretory functions, on renin release and on blood pressure. The clinical importance of renal blood flow variability is not fully understood. In many situations, the absence of normal cardiovascular oscillations seems to be a risk factor. Large fluctuations in perfusion pressure to the kidney, however, in the long run, may induce target organ damage.

Endothelin resets renal blood flow autoregulatory efficiency during acute blockade of NO in the rat

Renal blood flow (RBF) autoregulatory efficiency may be enhanced during NO inhibition in the rat, as recently reported. Under these conditions, endothelin (ET) synthesis and release may be increased. Our purpose was therefore to determine the role of ET in RBF autoregulatory changes induced by NO inhibition. To address this point, ET(A/B) receptors were blocked in anesthetized rats with bosentan, or selectively with BQ-610 or BQ-788. NO synthesis was inhibited with N(G)-nitro-L-arginine methyl ester (L-NAME). Mean arterial pressure (MAP) was decreased after bosentan (-10 mmHg; P < 0.01) or increased after L-NAME (25 mmHg; P < 0.001). RBF measured with an electromagnetic flow probe was reduced by L-NAME (-50%) and by BQ-788 (-24%). The pressure limits of the autoregulatory plateau (P(A) approximately 100 mmHg) and of no RBF autoregulation (P(o) approximately 80 mmHg) were significantly lowered by 15 mmHg after L-NAME but were unchanged after bosentan, BQ-610, or BQ-788. During NO inhibition, autoregulatory resetting was completely hindered by bosentan (P(A) approximately 100 mmHg) and by ET(B) receptor blockade with BQ-788 (P(A) approximately 106 mmHg), but not by ET(A) receptor blockade with BQ-610 (P(A) approximately 85 mmHg). These results suggest that the involvement of ET in the RBF autoregulatory resetting occurs during NO inhibition, possibly by preferential activation of the ET(B) receptor. However, the relative contribution of ET receptor subtypes remains to be further specified.

Role of angiotensin II and free radicals in blood pressure regulation in a rat model of renal hypertension

One-kidney, 1-clip rats (1K1C) or uninephrectomized controls were treated with either the superoxide dismutase mimetic tempol (0.5 mmol. kg(-1). d(-1)), angiotension type 1 receptor inhibitor losartan (50 mmol. L(-1). kg(-1). d(-1)), or both (n=6 per group) for 2 weeks. At the end of the study, systolic blood pressure (BP) decreased on average by 21% in tempol-treated and 29% in losartan-treated versus untreated 1K1C (217+/-4.4 mm Hg) and was normalized in the losartan plus tempol group. Mean BP also decreased from 159+/-3.7 mm Hg in 1K1C to 93+/-2.8 mm Hg in the losartan plus tempol group. Also, aortic wall area was reduced by 18% in losartan- or tempol-treated 1K1C and by 30% in losartan plus tempol rats compared with untreated 1K1C. Plasma renin activity was increased from 4.8+/-0.3 in untreated 1K1C to 15.9+/-0.9 ng. mL(-1). h(-1) in losartan-treated but not tempol-treated 1K1C. Superoxide generation by the isolated aortic rings assessed by lucigenin chemiluminescence was significantly decreased (by approximately 40%) in all losartan, tempol, and losartan plus tempol groups compared with untreated 1K1C. Nitrotyrosine ELISA in the kidney displayed a significant reduction, from 59+/-13 ng/mg of protein in 1K1C to 12.5+/-5 ng/mg of protein in the losartan plus tempol 1K1C. Western blotting for nNOS in kidney cortex and medulla showed a protein increase in both fractions of 1K1C versus controls and was normalized by losartan plus tempol treatment. Collectively, data show a synergistic effect of losartan and tempol on BP reduction in 1K1C rats. The mechanism may involve reduced superoxide production and nitrotyrosine formation in kidney and decreased kidney neuronal-type NO synthase expression in treated animals. This status in the oxidative balance seems to affect BP in the renal hypertensive rats.

Diabetes-induced microvascular dysfunction in the hydronephrotic kidney: role of nitric oxide

BACKGROUND

Renal hemodynamics in early diabetes are characterized by preglomerular and postglomerular vasodilation and increased glomerular capillary pressure, leading to hyperfiltration. Despite intensive research, the etiology of the renal vasodilation in diabetes remains a matter of debate. The present study investigated the controversial role of nitric oxide (NO) in the renal vasodilation in streptozotocin-induced diabetic rats.

METHODS

In the renal microcirculation, basal tone and response to NO synthase blockade were studied using the in vivo hydronephrotic kidney technique. L-arginine analog N-nitro-L-arginine methyl ester (L-NAME) was administered locally to avoid confounding by systemic blood pressure effects. The expression of endothelial NO synthase (eNOS) was investigated in total kidney by immunocytochemistry and in isolated renal vascular trees by Western blotting. Urinary excretion of nitrites/nitrates was measured.

RESULTS

Diabetic rats demonstrated a significant basal vasodilation of all preglomerular and postglomerular vessels versus control rats. Vasoconstriction to L-NAME was significantly increased in diabetic vessels. After high-dose L-NAME, there was no difference in diameter between diabetic and control vessels, suggesting that the basal vasodilation is mediated by NO. Immunocytochemically, the expression of eNOS was mainly localized in the endothelium of preglomerular and postglomerular vessels and glomerular capillaries, and was increased in the diabetic kidneys. Immunoblots on isolated renal vascular trees revealed an up-regulation of eNOS protein expression in diabetic animals. The urinary excretion of nitrites/nitrates was elevated in diabetic rats.

CONCLUSION

The present study suggests that an up-regulation of eNOS in the renal microvasculature, resulting in an increased basal generation of NO, is responsible for the intrarenal vasodilation characteristic of early diabetes.

Interaction between nitric oxide and renal myogenic autoregulation in normotensive and hypertensive rats

Blood pressure fluctuates continuously throughout life and autoregulation is the primary mechanism that isolates the kidney from this fluctuation. Compared with Wistar rats, Brown Norway (B-N) rats display impaired renal myogenic autoregulation when blood pressure fluctuation is increased. They also are very susceptible to hypertension-induced renal injury. Because blockade of nitric oxide augments myogenic autoregulation in Wistar rats, we compared the response of the myogenic system in B-N rats to nitric oxide blockade with that of other strains [Wistar, Sprague-Dawley, Long-Evans, spontaneously hypertensive (SHR)]. Renal blood flow dynamics were assessed in isoflurane anesthetized rats before and after inhibition of nitric oxide synthase by Lω-nitro-arginine methyl-ester (L-NAME, 10 mg/kg, iv). Under control conditions, myogenic autoregulation in the B-N rats was weaker than in the other strains. Myogenic autoregulation was not augmented after L-NAME administration in the SHR, but was augmented in all the normotensive rats. The enhancement was significantly greater in B-N rats so that after L-NAME the efficiency of autoregulation did not differ among the strains. The data suggest that nitric oxide is involved in the impaired myogenic autoregulation seen in B-N rats. Furthermore, the similarity of response in Wistar, Long-Evans, and Sprague-Dawley rats suggests that modulation by nitric oxide is a fundamental property of renal myogenic autoregulation.Key words: renal blood flow, transfer function, dynamics, SHR, Wistar, Long-Evans, Sprague-Dawley, Brown-Norway, L-NAME.

Concerted actions of renal endothelial and macula densa NO systems in the maintenance of extracellular fluid volume

It is now clear that nitric oxide (NO) exerts a substantial influence on renal function and that the kidney has a high capacity to produce NO. However, there are at least two different NO systems in the kidney. The interplay between NO generated by the endothelium and by the macula densa is considered in this review. It seems that endothelial NO increases in response to an increase in perfusion pressure and an increase in distal delivery, whereas macula densa NO decreases upon a sustained increase in distal delivery. Furthermore, evidence is accumulating that macula densa NO may well mediate renin release. Though seemingly in contrast, both the response of the endothelial NO and of the macula densa NO system seem appropriate to restore a perturbation of fluid balance. The function of the tubuloglomerular feedback (TGF) mechanism is likely to be influenced by both sources of NO, because of the close proximity of these NO producing cells to the vascular smooth muscle cells of the afferent arteriole. The endothelial NO system seems to be responsible for short-term, dampening actions to increased afferent arteriolar tone elicited by activation of the TGF system. The macula densa NO system, on the other hand, is probably adapting TGF responses to sustained increases in distal delivery. The analysis presented in this paper is an attempt to integrate the function of the two NO systems into physiological regulation. The exact role of the medullary NOS enzymes remains to be further elucidated.