Effects of NG-nitro-L-arginine on pressure natriuresis in anaesthetized rabbits

Renal sympathetic activity: A key modulator of pressure natriuresis in hypertension

Hypertension is a complex disorder ensuing necessarily from alterations in the pressure-natriuresis relationship, the main determinant of long-term control of blood pressure. This mechanism sets natriuresis to the level of blood pressure, so that increasing pressure translates into higher osmotically driven diuresis to reduce volemia and control blood pressure. External factors affecting the renal handling of sodium regulate the pressure-natriuresis relationship so that more or less natriuresis is attained for each level of blood pressure. Hypertension can thus only develop following primary alterations in the pressure to natriuresis balance, or by abnormal activity of the regulation network. On the other hand, increased sympathetic tone is a very frequent finding in most forms of hypertension, long regarded as a key element in the pathophysiological scenario. In this article, we critically analyze the interplay of the renal component of the sympathetic nervous system and the pressure-natriuresis mechanism in the development of hypertension. A special focus is placed on discussing recent findings supporting a role of baroreceptors as a component, along with the afference of reno-renal reflex, of the input to the nucleus tractus solitarius, the central structure governing the long-term regulation of renal sympathetic efferent tone.

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.

Hormonal regulation of salt and water excretion: a mathematical model of whole kidney function and pressure natriuresis

We present a lumped-nephron model that explicitly represents the main features of the underlying physiology, incorporating the major hormonal regulatory effects on both tubular and vascular function, and that accurately simulates hormonal regulation of renal salt and water excretion. This is the first model to explicitly couple glomerulovascular and medullary dynamics, and it is much more detailed in structure than existing whole organ models and renal portions of multiorgan models. In contrast to previous medullary models, which have only considered the antidiuretic state, our model is able to regulate water and sodium excretion over a variety of experimental conditions in good agreement with data from experimental studies of the rat. Since the properties of the vasculature and epithelia are explicitly represented, they can be altered to simulate pathophysiological conditions and pharmacological interventions. The model serves as an appropriate starting point for simulations of physiological, pathophysiological, and pharmacological renal conditions and for exploring the relationship between the extrarenal environment and renal excretory function in physiological and pathophysiological contexts.

Endogenous angiotensin II modulates nNOS expression in renovascular hypertension

Nitric oxide (NO) influences renal blood flow mainly as a result of neuronal nitric oxide synthase (nNOS). Nevertheless, it is unclear how nNOS expression is modulated by endogenous angiotensin II, an inhibitor of NO function. We tested the hypothesis that the angiotensin II AT1 receptor and oxidative stress mediated by NADPH oxidase contribute to the modulation of renal nNOS expression in two-kidney, one-clip (2K1C) hypertensive rats. Experiments were performed on male Wistar rats (150 to 170 g body weight) divided into 2K1C (N = 19) and sham-operated (N = 19) groups. nNOS expression in kidneys of 2K1C hypertensive rats (N = 9) was compared by Western blotting to that of 2K1C rats treated with low doses of the AT1 antagonist losartan [10 mg x kg(-1) x day(-1); N = 5] or the superoxide scavenger tempol [0.2 mmol x kg(-1) x day(-1); N = 5], which still remain hypertensive. After 28 days, nNOS expression was significantly increased by 1.7-fold in the clipped kidneys of 2K1C rats and by 3-fold in the non-clipped kidneys of 2K1C rats compared with sham rats, but was normalized by losartan. With tempol treatment, nNOS expression increased 2-fold in the clipped kidneys and 1.4-fold in the non-clipped kidneys compared with sham rats. The changes in nNOS expression were not followed by changes in the enzyme activity, as measured indirectly by the cGMP method. In conclusion, AT1 receptors and oxidative stress seem to be primary stimuli for increased nNOS expression, but this up-regulation does not result in higher enzyme activity.

Mechanisms mediating pressure natriuresis: what we know and what we need to find out

1. It is well established that pressure natriuresis plays a key role in long-term blood pressure regulation, but our understanding of the mechanisms underlying this process is incomplete. 2. Pressure natriuresis is chiefly mediated by inhibition of tubular sodium reabsorption, because both total renal blood flow and glomerular filtration rate are efficiently autoregulated. Inhibition of active sodium transport within both the proximal and distal tubules likely makes a contribution. Increased renal interstitial hydrostatic pressure (RIHP) likely inhibits sodium reabsorption by altering passive diffusion through paracellular pathways in 'leaky' tubular elements. 3. Nitric oxide and products of cytochrome P450-dependent arachidonic acid metabolism are key signalling mechanisms in pressure natriuresis, although their precise roles remain to be determined. 4. The key unresolved question is, how is increased renal artery pressure 'sensed' by the kidney? One proposal rests on the notion that blood flow in the renal medulla is poorly autoregulated, so that increased renal artery pressure leads to increased renal medullary blood flow (MBF), which, in turn, leads to increased RIHP. An alternative proposal is that the process of autoregulation of renal blood flow leads to increased shear stress in the preglomerular vasculature and, so, release of nitric oxide and perhaps products of cytochrome P450-dependent arachidonic acid metabolism, which, in turn, drive the cascade of events that inhibit sodium reabsorption. 5. Central to the arguments underlying these opposing hypotheses is the extent to which MBF is autoregulated. This remains highly controversial, largely because of the limitations of presently available methods for measurement of MBF.

Role of angiotensin II in the neural control of renal function

The aim of the present study was to distinguish between the direct effects of the renal nerves on renal function and indirect effects via neurally mediated increased systemic angiotensin II. We applied low-level electrical stimulation (1 Hz) to the left renal nerves in pentobarbitone-anesthetized rabbits for 180 minutes and measured renal blood flow, sodium excretion, and urine flow rate from both the stimulated and the nonstimulated contralateral kidney in the presence and the absence of ACE inhibition (enalaprilat). Stimulation resulted in an angiotensin II-mediated rise in arterial pressure and decreases in renal blood flow, urine flow rate, and sodium excretion on the stimulated side. On the nonstimulated denervated side, we found no change in renal blood flow, but found a decrease in urine flow rate. With ACE inhibition, renal stimulation no longer caused an increase in arterial pressure, the antidiuretic responses of the stimulated kidney were attenuated, and, importantly, the decrease in urine flow rate on the nonstimulated kidney was completely abolished. We therefore propose that although a direct effect of the renal nerves on sodium excretion is clearly present, the antidiuresis and antinatriuresis observed during renal activation is further supported by a neurally mediated increase in systemic angiotensin II.

Renal vascular resistance properties and glomerular protection in early established SHR hypertension

OBJECTIVE

To characterize the in vivo vascular properties of the spontaneously hypertensive rat (SHR) renal vascular bed by examining vascular conductance/resistance responsiveness to vasoactive agents in vivo and determining whether the filtration surface area of glomerular capillaries is reduced.

DESIGN AND METHODS

in vivo renal blood flow responses to intrarenally administered angiotensin II, phenylephrine and acetylcholine were compared in 10-week-old SHR and Wistar-Kyoto (WKY) rats using a wide range of doses from near threshold to near maximal effect. Unbiased stereological techniques and high-resolution light microscopy were used to estimate the surface area and length of glomerular capillaries, and evidence of capillary damage.

RESULTS

The SHR renal bed demonstrated significantly enhanced dose-vascular resistance responses to vasoconstrictors. For vascular conductance and calculated radius of resistance vessels, the SHR curves were significantly lower across the full dilator-constrictor range examined, but the dose-related changes were similar to those of WKY rats. There were only modest enhancements of the renal blood flow responses in the SHR, evident only when renal blood flow was reduced by more than 50% SHR and WKY rats did not differ in mean glomerular capillary surface area (0.13+/-0.02 mm2 and 0.14+/-0.02 mm2, respectively) or length (5.76+/-0.85 mm and 5.48+/-0.90 mm, respectively) nor was there evidence of glomerular capillary damage in either strain.

CONCLUSIONS

The renal vascular bed of the SHR in vivo exhibits reduced vascular conductance across a wide vasomotor range, compatible with findings in other vascular beds. We have further shown no evidence of reduced glomerular capillary surface area or damage. These findings are compatible with the hypothesis that the reduced conductance of the SHR pre-glomerular vasculature increases the aorta-capillary pressure gradient thus protecting the glomerular capillaries from systemic hypertension at this age.

Responses of regional kidney perfusion to vasoconstrictors in anaesthetized rabbits: dependence on agent and renal artery pressure

1. We tested the effects of intravenous infusions of angiotensin II (AngII; 300 ng/kg per min) and the vasopressin V1 receptor agonist [Phe2,Ile3,Orn8]-vasopressin (30 ng/kg per min) on regional kidney perfusion in an extracorporeal circuit model in anaesthetized rabbits in which renal artery pressure (RAP) can be set independently of systemic mean arterial pressure. To test whether the level of RAP can influence the renal vascular response to [Phe2,Ile3,Orn8]-vasopressin, we compared its effects when RAP was initially set at approximately 65 mmHg with those when RAP was set at approximately 130 mmHg. 2. When RAP was initially set at approximately 65 mmHg, a 20min infusion of AngII increased RAP (13%) and reduced renal blood flow (RBF; 50%) and cortical perfusion (CBF; 43%). Medullary perfusion (MBF) transiently increased during the first 10 min of infusion, but was not significantly different from control levels during the final 5 min of infusion. 3. When RAP was initially set at approximately 65 mmHg, a 20 min infusion of [Phe2,Ile3,Orn8]-vasopressin increased RAP (9%) and reduced RBF (21%); MBF was reduced by 57%, but CBF was reduced by only 15%. In contrast, when RAP was initially set at approximately 130 mmHg, infusion of [Phe2,Ile3,Orn8]-vasopressin reduced RAP (7%) and increased RBF (13%). In these experiments, MBF was reduced by 38%, but CBF increased by 6%. 4. Our experiments show that AngII preferentially reduces CBF, while [Phe2,Ile3,Orn8]-vasopressin preferentially reduces MBF. The renal vascular responses to [Phe2,Ile3,Orn8]-vasopressin appear to be profoundly affected by the level of RAP, because increasing RAP from approximately 65 to approximately 130 mmHg transforms its cortical vasoconstrictor effect into cortical vasodilatation while leaving the response of the medullary microvasculature relatively unchanged. Whether renal vascular responses to other vasoactive agents (e.g. AngII) are similarly affected by the level of RAP remains to be determined.

Sex differences in pressure diuresis/natriuresis in rabbits

We tested for sex-related differences in the pressure diuresis/natriuresis relationships in anaesthetized, renally denervated rabbits, using an extracorporeal circuit to perfuse the left kidney with the rabbit's own blood, through a series of step-wise increases in renal artery pressure (RAP) (from 65 to 130 mmHg). Urine flow, sodium excretion, and the fractional excretions of sodium and urine increased with increasing RAP, and were greater in male than in female rabbits at all levels of RAP-tested. However, these apparent sex-related differences in the acute pressure diuresis/natriuresis relationships were not reflected in alterations in chronic regulation of mean arterial pressure (MAP). Thus, in rabbits on a normal salt diet (0.85 g day(-1)), resting conscious MAP was significantly greater in males (87 +/- 3 mmHg) compared with females (77+/-1 mmHg). Chronically increasing daily salt intake to 4.98 g day(-1) for 28 days had no significant effect on resting conscious MAP in either sex. Thus, although our observations indicate sex differences, at least under the present experimental conditions, in the factors regulating extracellular fluid volume, these do not appear to have a major impact in setting the level of MAP in the long term.

Effects of renal medullary and intravenous norepinephrine on renal antihypertensive function

Increasing renal arterial pressure activates at least 3 antihypertensive mechanisms: reduced renin release, pressure natriuresis, and release of a putative renal medullary depressor hormone. To examine the role of renal medullary perfusion in these mechanisms, we tested the effects of the infusion of norepinephrine, either infusion into the renal medullary interstitium or intravenous infusion, on responses to increased renal arterial pressure in pentobarbital-anesthetized rabbits. We used an extracorporeal circuit, which allows renal arterial pressure to be set to any level above or below systemic arterial pressure. With renal arterial pressure initially set at 65 mm Hg, intravenous and medullary interstitial norepinephrine (300 ng. kg(-1). min(-1)) similarly increased mean arterial pressure (by 12% to 17% of baseline) and reduced total renal blood flow (by 16% to 17%) and cortical perfusion (by 13% to 19%), but only medullary norepinephrine reduced medullary perfusion (by 28%). When renal arterial pressure was increased to approximately 160 mm Hg, in steps of approximately 65 mm Hg, urine output and sodium excretion increased exponentially, and plasma renin activity and mean arterial pressure fell. Medullary interstitial but not intravenous norepinephrine attenuated the increased diuresis and natriuresis and the depressor response to increased renal arterial pressure. This suggests that norepinephrine can act within the renal medulla to inhibit these renal antihypertensive mechanisms, perhaps by reducing medullary perfusion. These observations support the concept that medullary perfusion plays a critical role in the long-term control of arterial pressure by its influence on pressure diuresis/natriuresis mechanisms and also by affecting the release of the putative renal medullary depressor hormone.

Effects of the vasopressin V1 agonist [Phe2,Ile3,Orn8]] vasopressin on regional kidney perfusion and renal excretory function in anesthetized rabbits

To test whether renal V1-receptors selectively influence blood flow in the renal medulla, we compared the effects of infusion of [Phe2,Ile3,Orn8]vasopressin (3-30 ng/kg/min) by the intravenous, renal arterial, and renal medullary interstitial routes in anesthetized rabbits. Intravenous [Phe2,Ile3,Orn8]vasopressin (30 ng/kg/min) reduced renal medullary perfusion (MBF) by 36 +/- 5% but did not significantly affect cortical perfusion (CBF). MBF was also reduced with the renal arterial (35 +/- 5%) and renal medullary interstitial (40 +/- 7%) routes but, in contrast to the intravenous infusion, CBF was also reduced, by 21 +/- 3% and 15 +/- 3%, respectively. Urine flow and sodium excretion were increased by [Phe2,Ile3,Orn8]vasopressin, and with direct intrarenal administration, this effect was similar for both the infused (left) and noninfused (right) kidneys. After a 20-min renal medullary interstitial infusion of [3H]norepinephrine, radiolabel concentration was approximately fivefold greater in the left medulla than in the left cortex. We conclude that [Phe2,Ile3,Orn8]vasopressin acts on V1-receptors to alter regional kidney blood flow and tubular salt and water handling. The V1-receptors involved are almost certainly within the kidney itself, but given the contrasting effects of the different infusion routes on MBF and CBF, we cannot exclude the possibility that some of the observed effects of [Phe2,Ile3,Orn8]vasopressin are mediated by activation of extra-renal V1-receptors.

Effects of renal medullary infusion of a vasopressin V1 agonist on renal antihypertensive mechanisms in rabbits

The factors responsible for the development of hypertension during chronic activation of intrarenal V1 receptors are unknown. We therefore tested whether medullary interstitial infusion of the selective V1-receptor agonist [Phe2,Ile3,Orn8]vasopressin (V1 agonist) influences renal antihypertensive mechanisms initiated by increased renal perfusion pressure (RPP). In intact anesthetized rabbits, the V1 agonist (10 ng . kg-1 . min-1) reduced medullary perfusion by 36 +/- 7%, whereas cortical perfusion was reduced by only 14 +/- 2%. An extracorporeal circuit was used to increase RPP in a stepwise manner from 65 to 85, 110, 130, and 160 mmHg for consecutive 20-min periods. Increased RPP reduced mean arterial pressure by 35 +/- 8% in vehicle-treated rabbits, but by only 10 +/- 3% in V1 agonist-treated rabbits. Simultaneously, pressure-diuresis-natriuresis was induced; urine flow and sodium excretion increased similarly in the two groups of rabbits, but hematocrit did not change. We suggest that the depressor response to increased RPP is mainly due to release of a putative renal medullary depressor hormone (RMDH). Suppression of the release and/or actions of RMDH may therefore contribute to the hypertensive effect of chronic V1 receptor activation.

Effects of intrarenal infusion of 17-octadecynoic acid on renal antihypertensive mechanisms in anesthetized rabbits

To characterize the role of cytochrome P450 metabolism of fatty acids in the renal response to increased renal perfusion pressure, we tested the effects of renal arterial infusion of 17-octadecynoic acid (17-ODYA, 450 nmol/min) on renal and systemic hemodynamic, and renal excretory responses to step-wise increases in renal perfusion pressure (RPP) in anesthetized rabbits, using an extracorporeal circuit for renal autoperfusion. Inhibition of cytochrome P450-dependent fatty acid metabolism was estimated by comparing the metabolism of arachidonic acid in microsomes prepared from the kidneys of control and 17-ODYA-treated animals. Step-wise increases in RPP decreased mean arterial pressure, which previous studies have indicated is attributable to the release of a depressor hormone from the renal medulla. Elevations in RPP also increased renal blood flow and glomerular filtration rate, and the absolute and fractional excretions of urine and sodium. Intrarenal infusion of 17-ODYA reduced the metabolism of arachidonic acid to 20-hydroxyeicosatetraenoic acid by 41%, but it did not significantly influence the responses to increased renal perfusion pressure. We conclude that either the responses elicited by increased renal perfusion pressure in anesthetized rabbits do not depend on cytochrome P450-dependent fatty acid metabolism, or that cytochrome P450 activity must be inhibited by more than was achieved in the present study (41%), before functional effects on the response to increased renal perfusion pressure are observed.

The kidney in blood pressure regulation and development of hypertension

Systemic arterial pressure is a dynamic and responsive physiologic parameter that can be influenced by many different factors. In particular, short-term changes in arterial pressure are caused by a myriad of mechanisms that affect cardiac output, total peripheral resistance, and cardiovascular capacitance. In the long run, however, most of these actions can be buffered or compensated by appropriate renal adjustments of sodium balance, ECFV, and blood volume. As long as the mechanisms regulating sodium excretion can maintain sodium balance by appropriately modulating the sensitivity of the pressure-natriuresis relationship, normal arterial pressure can be sustained. Derangements that compromise the ability of the kidneys to maintain sodium balance, however, can result in the kidney's need for an elevated arterial pressure to reestablish net salt and water balance.

Renal responses to increases in renal sympathetic nerve activity induced by brainstem stimulation in rabbits

We have stimulated the rostral ventrolateral medulla of the central nervous system to increase renal sympathetic nerve activity (RSNA), and measured the effect on renal blood flow, glomerular filtration rate, and urinary excretion. Increases in RSNA were produced by infusion of 0.02 M glutamate at a rate of 30-50 nl/min into the subretrofacial nucleus for 40 min, in 10 urethane anaesthetized rabbits. Changes in RSNA were quantified as the mean nerve activity per 1 s period and as the frequency and amplitude of individual discharges (reflecting the number of activated nerve fibres). Glutamate infusion increased RSNA 59 +/- 11% over control levels. This was predominantly due to a 65 +/- 15% increase in the frequency of discharges (3.0 +/- 0.35 to 4.6 +/- 0.4 Hz), rather than the amplitude of the discharges (+9 +/- 3% over control). The effects of these changes on the kidney were made against data collected in the last 20 min of the infusion and the 40 min pre-and post-stimulation periods, when arterial pressure and heart rate were unchanged from control levels. Renal blood flow fell significantly from 31.3 +/- 4.5 to 17.7 +/- 5.1 ml/min (47% decrease) and filtration fraction significantly increased from 12.7 +/- 1.1 to 15.7 +/- 2.1% (24% increase) during glutamate infusion. Each of these variables returned to their pre-stimulus levels after ceasing the central stimulation. Fluid, sodium and potassium excretion were not changed by this stimulus. In conclusion, the results in this study suggest that a selective increase in sympathetic nerve activity to the kidney without change in renal perfusion pressure can cause constriction of the renal vasculature without alteration in sodium and water excretion.