Pressure natriuresis and renal medullary blood flow in dogs

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.

Altered renal medullary blood flow: A key factor or a parallel event in control of sodium excretion and blood pressure?

In the context of the ongoing debate on the mechanism of blood pressure (BP) regulation and pathophysiology of arterial hypertension ("renocentric" vs "neural" concepts), attention is focused on the putative regulatory role of changes in renal medullary blood flow (MBF). Experimental evidence is analysed with regard to the question whether an elevation of BP and renal perfusion pressure (RPP) is likely to increase MBF due to its impaired autoregulation. It is concluded that such increases have been clearly documented only in rats with extracellular fluid volume expansion. A possible translation of this finding to BP regulation in health and hypertension in humans may only be a matter of speculation. Within the "renocentric" theory, the key event leading to restoration of initial BP level is pressure natriuresis. Its relation to elevation of renal interstitial hydrostatic pressure and to the phenomenon of "wash-out" of renal medullary solutes by increasing MBF is discussed. We also assessed the validity of data supporting the putative mechanism of short-term restoration of elevated BP owing to the release of a vasodilator lipid (medullipin) by the medulla. The structure of the proposed medullary lipid is still undefined, and there is no sound evidence on its mediatory role in lowering elevated BP level. In conclusion, MBF change can hardly be regarded as a crucial event in the regulation of BP: it can be involved in the control of sodium excretion and BP only in some circumstances, although its contributory role cannot be excluded.

Evidence against a crucial role of renal medullary perfusion in blood pressure control of hypertensive rats

KEY POINTS

The development of new effective methods of treating arterial hypertension is hindered by uncertainty regarding its causes. According to one widespread concept hypertension is caused by abnormal blood circulation in the kidney, specifically by reduction of blood flow through the kidney medulla; however, this causal relationship has never been rigorously verified. We investigated whether in rats with three different forms of experimental hypertension prolonged selective elevation of renal medullary blood flow using local infusion of the vasodilator bradykinin would lower arterial pressure. We found that increasing medullary blood flow by almost 50% did not result in alleviation of hypertension, which argues against a causal role of such changes in the control of arterial pressure and suggests that attempts at improving renal medullary circulation are not likely to be a promising approach to combating hypertension.

ABSTRACT

The crucial role of renal medullary blood flow (MBF) in the control of arterial pressure (MAP) has been widely accepted but not rigorously verified. We examined the effects of experimental selective MBF elevation on MAP, medullary tissue hypertonicity and renal excretion in hypertensive rats. We used three hypertensive rat models: (1) rats with hypertension induced by chronic angiotensin II infusions (AngII model), (2) rats with hypertension induced by unilateral nephrectomy followed by high salt diet (HS/UNX), and (3) spontaneously hypertensive rats (SHR). In acute experiments, MBF (laser-Doppler measurement) was selectively increased with an intramedullary infusion of bradykinin (Bk) at 0.27 mg h^-1  kg^-1 BW over 4 h. MAP, renal artery blood flow (Transonic probe) and renal excretion parameters were measured simultaneously. In chronic studies with AngII and HS/UNX rats, Bk was infused over 2 weeks and MAP (telemetry probe) and renal excretion were repeatedly determined. In acute studies, with AngII, SHR and HS/UNX groups, Bk infusion caused a 47% increase in MBF (P < 0.01-0.001), whereas solvent infusion was without effect. During the experiments MAP decreased slightly and to the same extent with Bk and solvent infusion. Medullary tissue osmolality and [Na⁺ ] were lower in Bk- than in solvent-infused AngII rats and in SHR. Two weeks of intramedullary Bk infusion tested in AngII and HS/UNX rats did not alter MAP or renal excretion; though in the latter group a significant MBF increase and medullary hypertonicity decrease was observed. Since no decrease in MAP in hypertensive rats was seen with Bk-induced major renal medullary hyperperfusion or with a wash-out of medullary solutes, our data argue against a crucial role of MBF in the pathogenesis of arterial hypertension.

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.

Chronic blood pressure control

Chronic blood pressure is maintained within very narrow limits around an average value. However, the multitude of physiologic processes that participate in blood pressure control present a bewildering array of possibilities to explain how such tight control of arterial pressure is achieved. Guyton and Coleman and colleagues addressed this challenge by creating a mathematical model that integrated the short- and long-term control systems for overall regulation of the circulation. The hub is the renal-body fluid feedback control system, which links cardiac function and vascular resistance and capacitance with fluid volume homeostasis as the foundation for chronic blood pressure control. The cornerstone of that system is renal sodium excretory capability, which is defined by the direct effect of blood pressure on urinary sodium excretion, that is, "pressure natriuresis." Steady-state blood pressure is the pressure at which pressure natriuresis balances sodium intake over time; therefore, renal sodium excretory capability is the set point for chronic blood pressure. However, this often is misinterpreted as dismissing, or minimizing, the importance of nonrenal mechanisms in chronic blood pressure control. This article explains the renal basis for the blood pressure set point by focusing on the absolute dependence of our survival on the maintenance of sodium balance. Two principal threats to sodium balance are discussed: (1) a change in sodium intake or renal excretory capability and (2) a change in blood pressure. In both instances, circulatory homeostasis is maintained because the sodium balance blood pressure set point is reached.

Renal pericytes: regulators of medullary blood flow

Regulation of medullary blood flow (MBF) is essential in maintaining normal kidney function. Blood flow to the medulla is supplied by the descending vasa recta (DVR), which arise from the efferent arterioles of juxtamedullary glomeruli. DVR are composed of a continuous endothelium, intercalated with smooth muscle-like cells called pericytes. Pericytes have been shown to alter the diameter of isolated and in situ DVR in response to vasoactive stimuli that are transmitted via a network of autocrine and paracrine signalling pathways. Vasoactive stimuli can be released by neighbouring tubular epithelial, endothelial, red blood cells and neuronal cells in response to changes in NaCl transport and oxygen tension. The experimentally described sensitivity of pericytes to these stimuli strongly suggests their leading role in the phenomenon of MBF autoregulation. Because the debate on autoregulation of MBF fervently continues, we discuss the evidence favouring a physiological role for pericytes in the regulation of MBF and describe their potential role in tubulo-vascular cross-talk in this region of the kidney. Our review also considers current methods used to explore pericyte activity and function in the renal medulla.

Experimental selective elevation of renal medullary blood flow in hypertensive rats: evidence against short-term hypotensive effect

AIM

Renal medullary blood flow (MBF) can be selectively increased by intrarenal or systemic infusion of bradykinin (Bk) in anaesthetized normotensive rats. We reproduced this effect in a number of rat models of arterial hypertension and examined whether increased perfusion of the renal medulla can cause a short-term decrease in blood pressure (BP) that is not mediated by increased renal excretion and depletion of body fluids.

METHODS

In uninephrectomized Sprague-Dawley rats, BP was elevated to approx. 145 mmHg by acute i.v. infusion of noradrenaline (NA) or angiotensin II (Ang II) (groups 1, 2), 2-week exposure to high-salt diet (3), high-salt diet + chronic low-dose infusion of Ang II using osmotic minipumps (4) or chronic high-dose Ang II infusion on normal diet (5). Uninephrectomized spontaneous hypertensive rats (SHR) were also examined (6,7). To selectively increase medullary perfusion, in anaesthetized rats, bradykinin was infused during 30-75 min into the renal medullary interstitium or intravenously.

RESULTS AND CONCLUSION

Bradykinin increased outer- and inner-medullary blood flow (laser-Doppler fluxes) by 10-20% in groups (1, 2), by 30-50% in groups (3, 4, 5) and approx. 20% in SHR (6, 7). The concurrent increase in total renal blood flow (Transonic probe) was < 3%. A minor (<3%) decrease in BP was seen only in rats acutely rendered hypertensive by NA or Ang II infusions; however, the decreases in BP and increases in medullary perfusion were not correlated. Thus, there was no evidence that in hypertensive rats, substantial selective increases in medullary perfusion can cause a short-term decrease in BP.

Interdependency of cystathione γ-lyase and cystathione β-synthase in hydrogen sulfide-induced blood pressure regulation in rats

Background

Hydrogen sulfide (H₂S), an endogenous vasoactive agent, is produced by cystathionine γ-lyase (CGL) and cystathionine β-synthase (CBS) enzymes. This study was conducted to evaluate the relative contribution of these enzymes in regulating systemic arterial pressure.

Methods

Sprague–Dawley rats were chronically treated with CGL inhibitor, -propargylglycine (PAG, 37.5 mg/kg/day; intraperitoneally, i.p.) or CBS inhibitor, aminooxyacetic acid (AOA, 8.75 mg/kg/day; i.p.) or in combination for 4 weeks and the effects on arterial pressure (tail-cuff plethysmography) and renal excretory function (24 h urine collections using metabolic cages) were assessed once in a week. Changes in renal blood flow (RBF; Ultrasonic flowmetry) and glomerular filtration rate (GFR; Inulin clearance) were assessed in acute experiments in anesthetized rats at the end of treatment period.

Results

Compared to vehicle treated control group, only the rats with combination therapy showed a decrease in urinary sulfate excretion rate (248 ± 47 vs. 591 ± 70 nmol/24 h; marker for endogenous H₂S level) which was associated with an increase in mean arterial pressure (MAP; 130 ± 2 vs. 99 ± 2 mm Hg). Urine flow and sodium excretion were also increased in combination group as consequent to the increase in MAP. GFR did not alter due to these treatments but RBF was lowered (4 ± 0.3 vs. 7 ± 0.4 ml/min/g) only in the combination group compared to the control group.

Conclusion

These findings indicate that a deficiency in one enzyme's activity could be compensated by the activity of the other to maintain the endogenous H₂S level, the deficiency of which modulates systemic and renal vascular resistances leading to the development of hypertension.

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.

Saline-induced natriuresis and renal blood flow in conscious dogs: effects of sodium infusion rate and concentration

AIM

This study focused on static and dynamic changes in total renal blood flow (RBF) during volume expansion and tested whether a change in RBF characteristics is a necessary effector mechanism in saline-induced natriuresis.

METHODS

The aortic flow subtraction technique was used to measure RBF continuously. Identical amounts of NaCl (2.4 mmol kg(-1)) were given as slow isotonic (Iso, 120 min), slow hypertonic (Hyper, 120 min), and rapid isotonic loads (IsoRapid, 30 min).

RESULTS

During Iso and IsoRapid, arterial blood pressure increased slightly (6-7 mmHg), and during Hyper it remained unchanged. Iso and Hyper increased sodium excretion (4 +/- 1 to 57 +/- 27 and 10 +/- 4 to 79 +/- 28 micromol min(-1), respectively) and decreased plasma renin activity (by 38% and 29%), angiotensin II (by 56% and 58%) and aldosterone (by 47% and 65%), while RBF remained unchanged. IsoRapid caused a similar increase in sodium excretion (to 72 +/- 19 micromol min(-1)), a similar decrease in renin system activity, but a 15% elevation of RBF (282 +/- 22 to 324 +/- 35 mL min(-1)). Selected frequency domain parameters of RBF autoregulation did not change in response to any load.

CONCLUSIONS

In response to slow saline loading simulating daily sodium intake, the rate of sodium excretion may increase 10-20-fold without any change in mean arterial blood pressure or in RBF. Regulatory responses to changes in total body NaCl levels appears, therefore, to be mediated primarily by neurohumoral mechanisms and may occur independent of changes in arterial pressure or RBF.

Mechanisms underlying the antihypertensive functions of the renal medulla

There is good evidence that the renal medulla plays a pivotal role in long-term regulation of blood pressure. 'Renal medullary' blood pressure regulating systems have been postulated to involve both exocrine (pressure natriuresis/diuresis) and endocrine [renal medullary depressor hormone (RMDH)] functions. However, recent studies indicate that pressure diuresis/natriuresis dominates the antihypertensive renal response to increased renal perfusion pressure, suggesting little physiological role for a putative RMDH in compensatory responses to acutely increased blood pressure. The medullary circulation appears to play a key role in mediating pressure diuresis, although the precise mechanisms involved remain controversial. Counter-regulatory vasodilator mechanisms (e.g. nitric oxide), at least partly mediated through cross-talk between the vasculature and the tubular epithelium, protect the medullary circulation from the vasoconstrictor effects of hormonal factors such as angiotensin II. These mechanisms also appear to contribute to compensatory responses to increased salt intake in salt-resistant individuals. Failure of these mechanisms predisposes the organism towards the development of hypertension, appears to underlie the development of some forms of experimental hypertension, and may even contribute to the pathogenesis of essential hypertension.

Response of descending vasa recta to luminal pressure

We tested whether luminal perfusion and pressurization induce an endothelial cytoplasmic Ca2+([Ca2+]CYT) response in descending vasa recta (DVR). DVR isolated from the rat outer medulla were cannulated and subjected to free-flow microperfusion (5 nl/min); the onset of which increased [Ca2+]CYTfrom a baseline of 76 ± 13 to 221 ± 65 nM. A graded increase in luminal pressure from 0 to 45 mmHg in stopped-flow experiments induced a parallel increase in [Ca2+]CYTfrom a baseline of 74 ± 24 to 194 ± 33 nM at 45 mmHg, with a tendency for [Ca2+]CYTto plateau at pressures >25 mmHg. The removal of extracellular Ca2+and blockade by either La3+(10 μM) or SKF-96365 (100 μM) eliminated the response. Luminal pressurization to 25 mmHg increased nitric oxide (NO) generation, a response blocked by NO synthase inhibition or removal of extracellular Ca2+. The NO generation was not affected by the superoxide dismutase mimetic tempol. We conclude that DVR endothelia are mechanosensitive and respond to luminal pressure by elevating [Ca2+]CYTand generating NO. That response might augment medullary perfusion and saliuresis.

Role of oxidative stress in angiotensin-induced hypertension

Infusion of ANG II at a rate not sufficient to evoke an immediate vasoconstrictor response, produces a slow increase in blood pressure. Circulating levels of ANG II may be within ranges found in normotensive individuals, although inappropriately high with respect to sodium intake. When ANG II levels are dissociated from sodium levels, oxidative stress (OXST) occurs, which can increase blood pressure by several mechanisms. These include inadequate production or reduction of bioavailability of nitric oxide, alterations in metabolism of arachidonic acid, resulting in an increase in vasoconstrictors and decrease in vasodilators, and upregulation of endothelin. This cascade of events appears to be linked, because ANG II hypertension can be blocked by inhibition of any factor located distally, blockade of ANG II, OXST, or endothelin. Such characteristics are shared by other models of hypertension, such as essential hypertension, hypertension induced by reduction in renal mass, and renovascular hypertension. Thus these findings are clinically important because they reveal 1) uncoupling between ANG II and sodium, which can trigger pathological conditions; 2) the various OXST mechanisms that may be involved in hypertension; and 3) therapeutic interventions for hypertension developed with the knowledge of the cascade involving OXST.

Autoregulation of renal medullary blood flow in rabbits

We examined the extent of renal medullary blood flow (MBF) autoregulation in pentobarbital-anesthetized rabbits. Two methods for altering renal arterial pressure (RAP) were compared: the conventional method of graded suprarenal aortic occlusion and an extracorporeal circuit that allows RAP to be increased above systemic arterial pressure. Changes in MBF were estimated by laser-Doppler flowmetry, which appears to predominantly reflect erythrocyte velocity, rather than flow, in the kidney. We compared responses using a dual-fiber needle probe held in place by a micromanipulator, with responses from a single-fiber probe anchored to the renal capsule, to test whether RAP-induced changes in kidney volume confound medullary laser-Doppler flux (MLDF) measurements. MLDF responses were similar for both probe types and both methods for altering RAP. MLDF changed little as RAP was altered from 50 to >or=170 mmHg (24 +/- 22% change). Within the same RAP range, RBF increased by 296 +/- 48%. Urine flow and sodium excretion also increased with increasing RAP. Thus pressure diuresis/natriuresis proceeds in the absence of measurable increases in medullary erythrocyte velocity estimated by laser-Doppler flowmetry. These data do not, however, exclude the possibility that MBF is increased with increasing RAP in this model, because vasa recta recruitment may occur.

Importance of the renal medullary circulation in the control of sodium excretion and blood pressure

The control of renal medullary perfusion and the impact of alterations in medullary blood flow on renal function have been topics of research interest for almost four decades. Many studies have examined the vascular architecture of the renal medulla, the factors that regulate renal medullary blood flow, and the influence of medullary perfusion on sodium and water excretion and arterial pressure. Despite these studies, there are still a number of important unanswered questions in regard to the control of medullary perfusion and the influence of medullary blood flow on renal excretory function and blood pressure. This review will first address the vascular architecture of the renal medulla and the potential mechanisms whereby medullary perfusion may be regulated. The known extrarenal and local systems that influence the medullary vasculature will then be summarized. Finally, this review will present an overview of the evidence supporting the concept that selective changes in medullary perfusion can have a potent influence on sodium and water excretion with a long-term influence on arterial blood pressure regulation.

Roles of ANG II and bradykinin in the renal regional blood flow responses to ACE inhibition in sodium-depleted dogs

The relative contributions of ANG II and bradykinin (BK) to the renal regional blood flow responses during angiotensin-converting enzyme (ACE) inhibition remain unclear. This study was performed to evaluate renal cortical (CBF) and medullary blood flow (MBF) responses to intrarterial administration of enalaprilat (33 microg. kg(-1). min (-1)) after blockade of the ANG II AT(1 )receptors with candesartan (100 microg) in 7 dogs fed a low-salt diet (0.01%) for 5 days. Laser-Doppler flowmetry was used to measure relative changes in CBF and MBF. Candesartan alone increased CBF (+20 +/- 2%) and MBF (+22 +/- 7%). Enalaprilat infusion after candesartan administration resulted in further increases in both CBF (+21 +/- 5%) and MBF (+41 +/- 8%). However, the relative changes in MBF were significantly greater (P < 0.01) than those in CBF. Administration of the BK B(2) receptor blocker icatibant (300 microg) after enalaprilat returned CBF and MBF to values seen with candesartan alone. These data support a substantive role for BK potentiation during ACE inhibitor-induced renal vasodilation in dogs maintained on a low-sodium diet, with a relatively greater effect on MBF compared to CBF.

Diversity of responses of renal cortical and medullary blood flow to vasoconstrictors in conscious rabbits

The medullary microcirculation receives only about 10% of total renal blood flow, but plays a critical role in long-term arterial pressure regulation, so we need to better understand its regulation. Although there is evidence that circulating and locally acting hormones can differentially affect cortical and medullary blood flow in anaesthetized animals, there is little information from studies in conscious animals. This study is aimed (i) to develop a method for chronic measurement of cortical and medullary blood flow in conscious rabbits, and (ii) to test whether renal cortical and medullary blood flow can be differentially affected by intravenous (i.v.) infusions of various vasoconstrictor hormones in conscious rabbits. At preliminary operations, rabbits were equipped with single-fibre laser-Doppler flowprobes in the (left) renal cortex and medulla, and Transonic flowprobes for measuring cardiac output and renal blood flow. Intravenous angiotensin II (300 ng kg(-1) min(-1)), [Phe2,Ile3,Orn8]-vasopressin (30 ng kg(-1) min(-1)), noradrenaline (300 ng kg(-1) min(-1)), endothelin-1 (20 ng kg(-1) min(-1)) and N G-nitro-L-arginine (10 mg kg(-1)) increased mean arterial pressure (by 10-45% of baseline) and reduced heart rate (by 16-35%) and cardiac output (by 16-45%). Consistent with previous observations in anaesthetized rabbits, all treatments except [Phe2,Ile3, Orn8]-vasopressin reduced renal blood flow (13-63%) and cortical blood flow (16-47%), but medullary blood flow was significantly reduced only by [Phe2,Ile3,Orn8]-vasopressin (41%) and N G-nitro-L-arginine (42%). The diversity of these responses of cortical and medullary blood flow to i.v. infusions of vasoconstrictors provides further evidence for physiological roles of circulating and local hormones in the differential regulation of regional kidney blood flow.

Renal medullary interstitial infusion of norepinephrine in anesthetized rabbits: methodological considerations

We tested methods for delivery of drugs to the renal medulla of anesthetized rabbits. Outer medullary infusion (OMI) of norepinephrine (300 ng. kg(-1). min(-1)), using acutely or chronically positioned catheters, reduced both cortical (CBF; 15%) and medullary perfusion (MBF; 23-31%). Inner medullary infusion (IMI) did not affect renal hemodynamics, whereas intravenous infusion reduced CBF (15%) without changes in MBF. During OMI of [(3)H]norepinephrine, much of the radiolabel (approximately 40% with chronically positioned catheters) spilled over systemically. Nevertheless, autoradiographic analysis showed the concentration of radiolabel was about fourfold greater in the infused medulla than the cortex. In contrast, during IMI, only approximately 5% of the infused radiolabel spilled over into the systemic circulation and approximately 64% was excreted by the infused kidney. The resultant intrarenal levels of radiolabel were considerably less with IMI compared with OMI. In rabbits, OMI therefore provides a useful method for targeting agents to the renal medulla, but given the considerable systemic spillover with OMI, its utility is probably limited to substances that are rapidly degraded in vivo.

Effects of halothane on renal hemodynamics and interstitial nitric oxide in rabbits

The effects of halothane on renal hemodynamics and the nitric oxide (NO)-guanylate cyclase signaling pathway were examined in anesthetized rabbits using a renal microdialysis method. Halothane (0.5 and 2 vol%) caused dose-dependent decreases in blood pressure, renal blood flow and the renal interstitial concentrations of guanosine 3',5'-cyclic monophosphate (cGMP) or nitrate (NO2)/nitrite (NO3). Sodium nitroprusside (20 microg kg(-1) min(-1), i.v.) under the inhalation of halothane (2 vol%) increased the renal interstitial concentration of cGMP. L-Arginine (priming dose, 300 mg kg(-1) 10 min(-1); sustaining dose, 50 mg kg(-1) min(-1), i.v.) did not reverse halothane-induced reductions of cGMP and NO2/NO3. These findings demonstrate that halothane caused a renal vasoconstriction and inhibited the NO-guanylate cyclase signaling pathway in the kidney. Moreover, it is possible that the renal hemodynamic responses to halothane might have been induced, in part, through this inhibition. Finally, it can be assumed that halothane did not interfere with the activation process of guanylate cyclase by NO.

Kinin influences on renal regional blood flow responses to angiotensin-converting enzyme inhibition in dogs

The relative roles of ANG II and bradykinin (BK) in the regulation of renal medullary circulation have remained unclear. We compared the contributions of ANG II and BK to the renal medullary blood flow (MBF) responses to angiotensin-converting enzyme (ACE) inhibition (enalaprilat, 33 micrograms . kg-1. min-1) in dogs maintained on a normal-salt diet (0.63%, 3 days, n = 14; group 1) with those fed a low-salt diet (0.01%, 5 days, n = 14; group 2), which upregulates both the kallikrein-kinin and the renin-angiotensin systems. MBF responses to ACE inhibition were evaluated either before (n = 7) or after (n = 7) treatment with the BK B2 receptor blocker icatibant (100-300 micergrams) in both groups. Laser-Doppler needle flow probes were used to determine relative changes in MBF and cortical blood flow (CBF). ACE inhibition increased MBF (group 1, 33 +/- 9%, P </= 0.01; group 2, 24 +/- 9%, P </= 0.005) as well as CBF (group 1, 23 +/- 2%, P </= 0.006; group 2, 28 +/- 10%, P </= 0.05). These responses were prevented by prior blockade of B2 receptors in group 2, but not in group 1. These data indicate that under normal sodium intake, increases in MBF and CBF caused by ACE inhibition are primarily due to reduced intrarenal ANG II levels. In contrast, the renal vasodilatory responses to ACE inhibition in dogs on low salt intake were markedly dependent on the activation of BK B2 receptors.

Nitric oxide synthase activity and renal injury in genetic hypertension

Nitric oxide (NO) is an endogenous vasodilator synthesized in the endothelium by constitutive NO synthase (cNOS). We have shown that upregulation of cNOS activity in hypertension may contribute to forestalling left ventricular and aortic hypertrophy (Hypertension. 29: 235, 1997). NO has been shown to inhibit growth-related responses affecting vascular smooth muscle, and mesangial cells, as well as reduce production of extracellular matrix in response to injury. Here, we investigated the relationship between renal cNOS activity (conversion of [14C] L-arginine to [14C] L-citrulline) and glomerular (GIS) and tubulointerstitial (TIS) injury scores and urinary protein excretion, indices of renal injury, in age and blood pressure matched spontaneously hypertensive rats (SHR, SBP 220+/-9 mm Hg) fed 0.5% NaCl diet and Dahl salt-sensitive (DS) rats fed 4% NaCl diet (DS-4%, SBP 228+/-8 mm Hg) as well as their normotensive counterparts Wistar Kyoto rats fed 0.5% NaCl diet (WKY, 137+/-3 mm Hg) and DS rats fed 0.5% NaCl diet (DS-0.5%, SBP 135+/-4 mm Hg). In SHR, renal medullary cNOS activity was 89% higher than in WKY (8.91+/-0.98 vs 4.71+/-0.37 nmol/min/g protein, P<0.05) whereas, in hypertensive DS-4% rats cNOS activity was 43% lower than in DS-0.5% rats (1.98+/-0.16 vs 3.48+/-0.29 nmol/min/g protein, P<0.05). Renal cortical cNOS was lower than in medulla but similar in all groups; inducible NOS activity was not detected. Despite hypertension of similar severity and duration, hypertensive DS-4% developed 9 fold more GIS (190+/-42 vs 21+/-11), 20 fold more TIS (4.0+/-0.7 vs 0.2+/-0.3), and 5 fold more proteinuria (54+/-11 vs 8.5+/-3.0 mg/day), all P<0.05. The current studies, in conjunction with our recent studies in heart and aorta, strongly suggest that in hypertension, increased cNOS activity may provide a protective homeostatic role in all the end-organs that are targets of hypertensive injury.