Role of nitric oxide in mediating renal response to volume expansion

Renal Autocrine and Paracrine Signaling: A Story of Self-protection

Autocrine and paracrine signaling in the kidney adds an extra level of diversity and complexity to renal physiology. The extensive scientific production on the topic precludes easy understanding of the fundamental purpose of the vast number of molecules and systems that influence the renal function. This systematic review provides the broader pen strokes for a collected image of renal paracrine signaling. First, we recapitulate the essence of each paracrine system one by one. Thereafter the single components are merged into an overarching physiological concept. The presented survey shows that despite the diversity in the web of paracrine factors, the collected effect on renal function may not be complicated after all. In essence, paracrine activation provides an intelligent system that perceives minor perturbations and reacts with a coordinated and integrated tissue response that relieves the work load from the renal epithelia and favors diuresis and natriuresis. We suggest that the overall function of paracrine signaling is reno-protection and argue that renal paracrine signaling and self-regulation are two sides of the same coin. Thus local paracrine signaling is an intrinsic function of the kidney, and the overall renal effect of changes in blood pressure, volume load, and systemic hormones will always be tinted by its paracrine status.

Nitric oxide, prostaglandins and angiotensin II in the regulation of renal medullary blood flow during volume expansion

Regulation of medullary blood flow (MBF) is essential in maintaining renal function and blood pressure. However, it is unknown whether outer MBF (OMBF) and papillary blood flow (PBF) are regulated independently when extracellular volume (ECV) is enhanced. The aim of this study was to determine whether OMBF and PBF are differently regulated and whether there is an interaction between nitric oxide (NO), prostaglandins (PGs) and angiotensin II (Ang II) in regulating OMBF and PBF when ECV is enhanced. To achieve these goals, OMBF and PBF were measured by laser-Doppler in volume-expanded rats treated with a cyclooxygenase inhibitor (meclofenamate, 3 mg/kg) and/or a NO synthesis inhibitor (L-nitro-arginine methyl ester (L-NAME), 3 μg/kg/min) and/or Ang II (10 ng/kg/min). OMBF was unchanged by NO or PGs synthesis inhibition but decreased by 36 % (P < 0.05) when L-NAME and meclofenamate were infused simultaneously. PBF was similarly reduced by L-NAME (12 %), meclofenamate (17 %) or L-NAME + meclofenamate (19 %). Ang II did not modify OMBF, but it led to a similar decrease (P < 0.05) in OMBF when it was administered to rats with reduced NO (32 %), PGs (36 %) or NO and PGs (37 %) synthesis. In contrast, the fall in PBF induced by Ang II (12 %) was enhanced (P < 0.05) by the simultaneous PGs (30 %) or PGs and NO (31 %) synthesis inhibition but not in L-NAME-treated rats (20 %). This study presents novel findings suggesting that blood flows to the outer medulla and renal papilla are differently regulated and showing that there is a complex interaction between NO, PGs and Ang II in regulating OMBF and PBF when ECV is enhanced.

Renal Effects of Cyclooxygenase Inhibition When Nitric Oxide Synthesis Is Reduced and Angiotensin II Levels Are Enhanced

The involvement of both cyclooxygenase (COX) isoforms in regulating renal function is well known but their interactions with other regulatory mechanisms, such as angiotensin II (Ang II) and nitric oxide (NO), are not well defined. This study has evaluated the relative contribution of both COX isoforms in regulating renal function when NO synthesis is reduced with and without a simultaneous increment in Ang II levels. The renal responses to a nonselective (meclofenamate) or a selective COX2 (nimesulide) inhibitor were examined in dogs pretreated with L-NAME with or without an intrarenal Ang II infusion. Meclofenamate induced a greater (P < 0.05) renal vasoconstriction than nimesulide in dogs pretreated with L-NAME. This vasoconstriction seems to be Ang II-dependent because it was reduced (P < 0.05) by captopril administration. Meclofenamate also induced a greater (P < 0.05) renal vasoconstriction than that elicited by nimesulide in dogs with reduced NO synthesis and elevated Ang II levels. The renal vasoconstriction induced by nimesulide but not that elicited by meclofenamate in dogs pretreated with L-NAME and Ang II, decreased (P < 0.05) during an extracellular volume expansion. These results demonstrate that the nonselective COX inhibition induces a greater renal vasoconstriction than that elicited by the selective COX2 inhibition when NO synthesis is reduced, and when NO synthesis is reduced and Ang II levels are elevated.

Natriuretic hormones, endogenous ouabain, and related sodium transport inhibitors

The work of deWardener and colleagues stimulated longstanding interest in natriuretic hormones (NHs). In addition to the atrial peptides (APs), the circulation contains unidentified physiologically relevant NHs. One NH is controlled by the central nervous system (CNS) and likely secreted by the pituitary. Its circulating activity is modulated by salt intake and the prevailing sodium concentration of the blood and intracerebroventricular fluid, and contributes to postprandial and dehydration natriuresis. The other NH, mobilized by atrial stretch, promotes natriuresis by increasing the production of intrarenal dopamine and/or nitric oxide (NO). Both NHs have short (<35 min) circulating half lives, depress renotubular sodium transport, and neither requires the renal nerves. The search for NHs led to endogenous cardiotonic steroids (CTS) including ouabain-, digoxin-, and bufadienolide-like materials. These CTS, given acutely in high nanomole to micromole amounts into the general or renal circulations, inhibit sodium pumps and are natriuretic. Among these CTS, only bufalin is cleared sufficiently rapidly to qualify for an NH-like role. Ouabain-like CTS are cleared slowly, and when given chronically in low daily nanomole amounts, promote sodium retention, augment arterial myogenic tone, reduce renal blood flow and glomerular filtration, suppress NO in the renal vasa recta, and increase sympathetic nerve activity and blood pressure. Moreover, lowering total body sodium raises circulating endogenous ouabain. Thus, ouabain-like CTS have physiological actions that, like aldosterone, support renal sodium retention and blood pressure. In conclusion, the mammalian circulation contains two non-AP NHs. Identification of the CNS NH should be a priority.

Altered regulation of renal nitric oxide and atrial natriuretic peptide systems in angiotensin II-induced hypertension

The present study was aimed to determine whether there is an altered role of local nitric oxide (NO) and atrial natriuretic peptide (ANP) systems in the kidney in association with the angiotensin (Ang) II-induced hypertension. Male Sprague-Dawley rats were used. Ang II (100 ng·min⁻¹·kg⁻¹) was infused through entire time course. Thirteenth day after beginning the regimen, kidneys were taken. The protein expression of NO synthase (NOS) and nitrotyrosine was determined by semiquantitative immunoblotting. The mRNA expression of components of ANP system was determined by real-time polymerase chain reaction. The activities of soluble and particulate guanylyl cyclases were determined by the amount of cGMP generated in responses to sodium nitroprusside and ANP, respectively. There developed hypertension and decreased creatinine clearance in the experimental group. The protein expression of eNOS, nNOS and nitrotyrosine was increased in the cortex, while that of iNOS remained unaltered. The urinary excretion of NO increased in Ang II-induced hypertensive rats. The catalytic activity of soluble guanylyl cyclase was blunted in the glomerulus in Ang II-induced hypertensive rats. The mRNA expression of ANP was increased in Ang II-induced hypertensive rats. Neither the expression of NPR-A nor that of NPR-C was changed. The protein expression of neutral endopeptidase was decreased and the activity of particulate guanylyl cyclase was blunted in the glomerulus and papilla in Ang II-induced hypertensive rats. In conclusion, the synthesis of NO and ANP was increased in the kidney of Ang II-induced hypertension, while stimulated cGMP response was blunted. These results suggest desensitization of guanylyl cyclase in the kidney of Ang II-induced hypertensive rats, which may contribute to the associated renal vasoconstriction and hypertension.

Chronic vasodilation produces plasma volume expansion and hemodilution in rats: consequences of decreased effective arterial blood volume

Plasma volume (PV) expansion is required for optimal pregnancy outcomes; however, the mechanisms responsible for sodium and water retention in pregnancy remain undefined. This study was designed to test the "arterial underfill hypothesis" of pregnancy which proposes that an enlarged vascular compartment (due to systemic vasodilation and shunting of blood to the placenta) results in renal sodium and water retention and PV expansion. We produced chronic vasodilation by 14 days administration of nifedipine (NIF; 10 mg·kg(-1)·day(-1)) or sodium nitrite (NaNO2; 70 mg·kg(-1)·day(-1)) to normal, nonpregnant female Sprague-Dawley rats. Mean arterial pressure, monitored by telemetry, was reduced by both NIF and NaNO2 but was unchanged in control rats. At day 14, vasodilator treatment lowered hematocrit and increased PV (determined by Evans blue dye dilution). Plasma osmolarity (Posm), sodium (PNa), and total protein concentrations all fell. These responses resemble the responses to normal pregnancy with hemodilution, marked PV expansion, and decreased Posm and PNa. Our previous work indicates a role of increased inner medullary phosphodiesterase-5 (PDE5) in the sodium retention of pregnancy. Here, we found that inner medullary PDE5A mRNA and protein expression were increased by both NIF and NaNO2 treatment vs. control; however, neither renal cortical nor aortic PDE5 expression was changed by vasodilator treatment. We suggest that a primary, persistent vasodilation drives increased inner medullary PDE5 expression which facilitates continual renal Na retention causing "refilling" of the vasculature and volume expansion.

Extracellular renal guanosine cyclic 3'5'-monophosphate modulates nitric oxide and pressure-induced natriuresis

This study addresses the hypothesis that NO- and pressure-induced natriuresis are inhibited when guanosine cyclic 3',5'-monophosphate (cGMP) is prevented from being transported outside its renal synthesizing cells in vivo. Rats received a renal interstitial (RI) infusion of NO donor S-nitroso-N-acetylpenicillamine (SNAP) or SNAP+organic anion transporter inhibitor probenecid (PB) or SNAP+PB+cGMP. SNAP alone increased U(Na)V (P<0.05 at 1 hour and P<0.005 at 2 hours). In contrast, SNAP failed to increase U(Na)V when coinfused with PB, but cGMP coinfused with SNAP+probenecid restored the natriuretic response. SNAP alone increased RI cGMP (P<0.05) during the second experimental period. PB abolished the increase in RI cGMP in response to SNAP (P<0.01), but cGMP levels were restored by coinfusion with cGMP. PB also abolished SNAP-induced increases in fractional excretion of Na(+) (FE(Na)) and lithium (FE(Li)) (both P<0.01). PB also abolished the rise in RI cGMP and natriuresis induced by raising renal perfusion pressure (RPP) from 100 to 160 mm Hg in rats subjected to a standard pressure-natriuresis protocol and the natriuretic response was rescued by coinfusion with cGMP. RI administration of phosphodiesterase type V (PDE V) reduced both RIcGMP and U(Na)V in parallel (both P<0.01) without altering RIcAMP. The data demonstrate that export of cGMP from its renal synthesizing cells into the extracellular RI compartment is critical for the natriuretic action of NO donor SNAP or increased RPP and that RI cGMP controls basal Na(+) excretion. Extracellular cGMP modulates NO- and pressure-induced natriuresis.

Changes in renal hemodynamics and excretory function induced by a reduction of ANG II effects during renal development

The aim was to evaluate whether blockade of ANG II effects during renal development modifies the renal response to an increment of plasma amino acid concentration. It was also examined in anesthetized rats whether the reduction of the renal ability to eliminate an acute volume expansion (VE), elicited by blockade of ANG II during renal development, is sex and/or age dependent. Newborn Sprague-Dawley rats were treated with vehicle or an AT(1)-receptor antagonist (ARA) during postnatal nephrogenesis. Amino acid infusion induced increments (P < 0.05) of glomerular filtration rate (31 +/- 6%) and renal plasma flow (26 +/- 5%) in male but not in female vehicle-treated rats. Natriuretic and diuretic responses to amino acid infusion were similar in male and female vehicle-treated rats. These renal hemodynamics and excretory responses to amino acid infusion were abolished in ARA-treated rats. Renal responses to VE were evaluated at 3-4 and 9-10 mo of age in vehicle and ARA-treated rats. VE-induced natriuresis and diuresis were reduced by more than 38% (P < 0.05) in 3- to 4-mo-old male and female ARA-treated rats. An age-dependent reduction (P < 0.05) in the renal ability to eliminate VE was found in male but not in female rats treated with ARA. Our results demonstrate that the renal effects induced by an increment in amino acids are abolished when ANG II effects have been reduced during nephrogenesis. In addition, this reduction of ANG II effects elicits an impairment of the renal ability to eliminate an acute VE in males and females, which is aggravated by age only in male rats.

Gentamicin decreases guanylyl cyclase activity in rat glomerulus

BACKGROUND

Effects of gentamicin (GM) on the local natriuretic peptide (NP) and nitric oxide (NO) systems in the kidney were investigated.

METHODS

Male Sprague-Dawley rats (180-200 g) were intramuscularly injected with GM (100 mg/kg/day) for 5 days. The expression of NO synthase (NOS) isoforms was determined by Western blot analysis, and that of NPs by real-time polymerase chain reaction. The activity of guanylyl cyclase was also determined by the amount of guanosine 3',5'-cyclic monophosphate (cGMP) generated in responses to atrial natriuretic peptide (ANP) or sodium nitroprusside (SNP).

RESULTS

GM treatment resulted in renal failure in association with increases in urinary flow and the fractional excretion of sodium. Accordingly, the expression of inducible NOS was increased in the cortex, while that of endothelial NOS remained unchanged. The urinary excretion of NO metabolites was increased. The expression of ANP, brain natriuretic peptide and C-type natriuretic peptide mRNA was increased in the kidney. The cGMP production provoked by either ANP or SNP was decreased in the glomerulus, but not in the papilla.

CONCLUSION

GM-induced nephropathy may be causally related with decreased guanylyl cyclase activities in the glomerulus.

Vascular and renal effects of dopamine during extracellular volume expansion: Role of nitric oxide pathway

OBJECTIVE

The aim of the study was to determine the possible role of NO-system activation in vascular and renal effects of the dopaminergic system and the probable interaction between both systems during acute volume expansion in rats.

DESIGN AND METHODS

Expanded (10% bw) and non-expanded anaesthetized male Wistar rats were treated with haloperidol, a DA receptor antagonist (3 mg/kg bw, ip). Mean arterial pressure, diuresis, natriuresis, renal plasma flow, glomerular filtration rate, nitrites and nitrates excretion (NOx) were determined. NADPH diaphorase activity was measured using a histochemistry technique in kidney, aorta and renal arteries. NOS activity in kidney and aorta from expanded and non-expanded animals was determined with L-[U14C]-arginine substrate, in basal conditions and after DA (1 microM) administration.

RESULTS

The hypotensive effect of L-arg and hypertension induced by L-NAME were not modified by haloperidol. This blocker reverted the increase in diuresis, natriuresis and RPF induced by L-arg in both groups. Dopaminergic blockade induced a decrease in NOx excretion and in NADPH-diaphorase activity in glomeruli, proximal tubule and medullar collecting duct and in endothelium and vascular smooth muscle of renal arteries. DA induced an increase in NOS activity in renal medulla and cortex in both groups, but no changes in the aorta were observed.

CONCLUSIONS

Our results suggest that renal DA would be associated with the renal response induced by NO during extracellular volume expansion. NO-system activation would be one of the mechanisms involved in renal DA activity during saline load, but NO appears not to be involved in DA vascular effects.

Effect of acute hyperglycaemia on sodium handling and excretion of nitric oxide metabolites, bradykinin, and cGMP in Type 1 diabetes mellitus

AIMS

The aim of this study was to evaluate the effect of acutely induced hyperglycaemia on renal sodium handling and to explore the role of the bradykinin-nitric oxide-cGMP signalling pathway.

PATIENTS AND METHODS

We compared 20 Type 1 diabetic (DM1) patients without microalbuminuria with 15 weight-, age-, and sex-matched healthy controls (C). Clearances of para-aminohippuric acid (CPAH), inulin (Cin), lithium, sodium, and urinary nitrite/nitrate (NOx), cGMP and bradykinin excretion rates were measured in two 90-min periods: a glycaemic clamp-induced euglycaemia (5 mmol/l-period I) and hyperglycaemia (12 mmol/l-period II) (Study 1) and during time-controlled euglycaemia (5 mmol/l-period I and 5 mmol/l-period II) to avoid the effects of time and volume load (Study 2).

RESULTS

Cin and CPAH were not significantly different during euglycaemia (period I of Study 1) in DM1 and controls, whereas fractional excretion of sodium was decreased in DM1 (1.84 +/- 0.75 vs. 2.36 +/- 0.67%; P < 0.05) due to an increase in fractional distal tubular reabsorption of sodium (94.01 +/- 1.94 vs. 92.24 +/- 2.47%; P < 0.05). A comparison of changes during Study 1 and Study 2 revealed acute hyperglycaemia did not change renal haemodynamics significantly, while fractional distal tubular reabsorption of sodium increased (DM1: P < 0.05; C: P < 0.01) and fractional excretion of sodium decreased (P < 0.01) in both groups. The urinary excretion rates of NOx were comparable during euglycaemia in DM1 and C. While in C, they significantly increased during Study 1 (period I: 382 +/- 217 vs. period II: 515 +/- 254 nmol/min; P < 0.01) and Study 2 (period I: 202.9 +/- 176.8 vs. period II: 297.2 +/- 267.5 nmol/min; P < 0.05) as a consequence of the water load, no changes were found in DM1. The urinary excretion of bradykinin was lower in DM1 compared with C (0.84 +/- 0.68 vs. 1.20 +/- 0.85 micro g/min; P < 0.01) during euglycaemia; it was not affected by hyperglycaemia. There were no significant differences between DM1 and C and in cGMP urinary excretion rates following hyperglycaemia.

CONCLUSION

This study demonstrates that DM1 without renal haemodynamic alterations is associated with impaired renal sodium handling. Moreover, we did not find a relationship between the renal excretion rates of vasoactive mediators and sodium handling due to hyperglycaemia.

Down-regulation of soluble guanylyl cyclase in the inner medulla of DOCA-salt hypertensive rats

Our laboratory has recently shown increased renal expression of NO synthase 3 (NOS3) in the deoxycorticosterone acetate (DOCA)-salt rat model of hypertension suggesting an up-regulation of the nitric oxide (NO)-cyclic guanosine-3',5'-monophosphate (cGMP) pathway. The present study was designed to determine changes in renal soluble guanylyl cyclase (sGC) activity and expression in the DOCA-salt hypertensive rat. Rats were uninephrectomized and subcutaneously implanted with either a placebo or DOCA-salt pellet. Placebo-treated animals were given tap water ad libitum, while DOCA-treated animals received 0.9% NaCl solution to drink. Each week, rats were placed in metabolic cages for 24 h collection of urine samples. Urine samples were measured for cGMP concentrations using a scintillation proximity method. After 3 weeks, kidneys were removed and dissected into cortex, outer medulla, and inner medulla. Each region of the kidney was further separated into detergent-soluble and detergent-insoluble fractions. DOCA-treated rats exhibited significant increases in urinary cGMP excretion (27.0+/-1.4 fmol/mg creatinine) after 1 week compared to placebo control animals (8.7+/-0.6 fmol/mg creatinine). This was followed by a significant decrease by the second week of treatment (5.4+/-1.0 and 11.4+/-0.6 fmol/mg creatinine in DOCA-salt and placebo, respectively) and a return to placebo values by the third week of treatment (16.2+/-3.1 and 12.9+/-1.0 fmol/mg creatinine in DOCA-salt and placebo, respectively). Western blot analysis of inner medullary detergent-soluble fraction indicated a decrease in the expression of the beta(1)-subunit of sGC in the third week of DOCA-salt-treated animals as compared to placebo controls (n=5 animals per group) while expression of the alpha(1)-subunit was unchanged. Western blot analysis of cortex and outer medullary preparations comparing placebo controls and DOCA-salt-treated animals revealed no difference in alpha(1)- or beta(1)-sGC protein expression. These data suggest an uncoupling of NOS/NO and sGC/cGMP pathways in the renal inner medulla of the DOCA-salt hypertensive rat.

Production and role of extracellular guanosine cyclic 3', 5' monophosphate in sodium uptake in human proximal tubule cells

The present study was designed to determine the capability of human renal proximal tubule (RPT) to generate and export guanosine cyclic 3', 5' monophosphate (cGMP) in response to direct stimulation of soluble guanylyl cyclase by nitric oxide (NO) donors. In addition, we investigated whether cGMP extrusion from human RPT cells is required for inhibition of cellular sodium uptake. RPT cells were cultured from fresh human kidneys (normotensive subjects, n=4, mean age 65+/-4.7 years, 3 men, 1 woman; hypertensive patients, n=6, mean age 64+/-6.1 years, 4 men, 2 women) after unilateral nephrectomy. The fluorescence dye Sodium Green was employed to determine cytoplasmic Na+ concentration. In the presence of the Na+/K+ ATPase inhibitor ouabain, fluorescence was monitored at the appropriate wavelength (excitation 485 nm, emission 535 nm). Nitric oxide donor, S-nitroso-N-acetylpenicillamine (SNAP, 10(-4) M), increased both intracellular and extracellular cGMP (from 1.26+/-0.21 to 88.7+/-12.6 pmol/mg protein and from 0.58+/-0.10 to 9.24+/-1.9 pmol/mL, respectively, P<0.01) and decreased cellular Na+ uptake by 37.4+/-6.8% (P<0.05) compared with control. The effects of SNAP on cGMP production were similar in normotensive and hypertensive subjects. The increases in intracellular and extracellular cGMP concentration because of SNAP were blocked completely by soluble guanylyl cyclase inhibitor ODQ (1-H-[1,2,4] oxadiazolo [4,2-alpha] quinoxalin-1-one). Probenecid, an organic anion transport inhibitor, augmented the SNAP (10(-6) M)-induced increase in intracellular cGMP accumulation (from 4.9+/-0.9 to 9.8+/-1.5 pmol/mg protein, P<0.05), abrogated the SNAP-induced increase in extracellular cGMP extrusion (from 1.07+/-0.4 to 0.37+/-0.1 pmol/L, P<0.05) and blocked the SNAP-induced reduction in cellular Na+ uptake. Neither intracellular nor extracellular cGMP were influenced by l-arginine, the metabolic precursor of NO, or N(G)-nitro-L-arginine methyl ester, an inhibitor of NO synthase. After exogenous administration of cGMP (10(-5) M) or its membrane-permeable analogue 8-Br-cGMP (10(-5) M), only 8-Br-cGMP crossed the cell membrane to increase intracellular cGMP (from 1.36+/-0.19 to 289.7+/-29.4 pmol/mg protein, P<0.01). However, both cGMP and 8-Br-cGMP were effective in decreasing cellular Na+ uptake. In conclusion, human RPT cells contain soluble guanylyl cyclase and are able to generate and export cGMP in response to NO. Because human RPT cells do not themselves contain constitutive NO synthase, the NO-generating cGMP must be derived from sources outside the human RPT. The cGMP cellular export system is critical in the regulation of RPT cellular Na+ absorption in humans.

Paradoxes of nitric oxide in the diabetic kidney

As an important modulator of renal function and morphology, the nitric oxide (NO) system has been extensively studied in the diabetic kidney. However, a number of studies in different experimental and clinical settings have produced often confusing data and contradictory findings. We have reviewed a wide spectrum of findings and issues that have amassed concerning the pathophysiology of the renal NO system in diabetes, pointed out the controversies, and attempted to find some explanation for these discrepancies. Severe diabetes with profound insulinopenia can be viewed as a state of generalized NO deficiency, including in the kidney. However, we have focused our hypotheses and conclusions on the events occurring during moderate glycemic control with some degree of treatment with exogenous insulin, representing more the clinically applicable state of diabetic nephropathy. Available evidence suggests that diabetes triggers mechanisms that in parallel enhance and suppress NO bioavailability in the kidney. We hypothesize that during the early phases of nephropathy, the balance between these two opposing forces is shifted toward NO. This plays a role in the development of characteristic hemodynamic changes and may contribute to consequent structural alterations in glomeruli. Both endothelial (eNOS) and neuronal NO synthase can contribute to altered NO production. These enzymes, particularly eNOS, can be activated by Ca(2+)-independent and alternative routes of activation that may be elusive in traditional methods of investigation. As the duration of exposure to the diabetic milieu increases, factors that suppress NO bioavailability gradually prevail. Increasing accumulations of advanced glycation end products may be one of the culprits in this process. In addition, this balance is continuously modified by actual metabolic control and the degree of insulinopenia.

Volume expansion during acute angiotensin II receptor (AT(1)) blockade and NOS inhibition in conscious dogs

The responses to AT(1)-receptor blockade (candesartan 1 mg/kg) and to concomitant volume expansion (saline 35 ml/kg for 90 min) with and without nitric oxide synthase (NOS) inhibition (N(G)-nitro-L-arginine methyl ester 30 microg small middle dot kg(-1) small middle dot min(-1)) were investigated in separate experiments in normal dogs. AT(1) blockade decreased arterial pressure (106 +/- 4 to 96 +/- 5 mmHg) and increased glomerular filtration rate (GFR) by 17% and sodium excretion threefold. NOS inhibition increased arterial pressure (103 +/- 3 to 116 +/- 3 mmHg) and decreased GFR by 21% and reduced sodium excretion by some 80%. Volume expansion increased arterial pressure significantly in all series involving this procedure, most pronounced during combined AT(1) blockade and NOS inhibition (21 +/- 4 mmHg). Volume expansion during AT(1) blockade elicited marked natriuresis (26 +/- 11 to 274 +/- 55 micromol/min) that was severely reduced by concomitant NOS inhibition (10 +/- 3 to 45 +/- 11 micromol/min), but still much larger than that seen with volume expansion during NOS inhibition alone (2 +/- 1 to 23 +/- 7 micromol/min). Volume expansion during AT(1) blockade increased GFR (+30%), less so during combined AT(1) blockade and NOS inhibition (+13%), but it did not increase GFR significantly (P = 0.07) during NOS inhibition alone. Plasma ANG II increased greater than sevenfold with AT(1) blockade and doubled with NOS inhibition (paired t-test, P < 0.05), whereas it decreased by 50-80% during volume expansion irrespective of pretreatment, i.e., during NOS inhibition, volume expansion did not generate subnormal plasma ANG II concentrations. In conclusion, 1) acute AT(1) blockade leads to hyperfiltration, natriuresis, and hyperresponsiveness to volume expansion, 2) these responses are >85% inhibitable by unspecific NOS inhibition, and 3) NOS inhibition alone is followed by increases in plasma ANG II, hypofiltration, and severe antinatriuresis that may be counterbalanced but not overwhelmed by volume expansion. Thus NOS inhibition virtually abolishes the volume expansion natriuresis, at least in part, due to the lack of appropriate inhibition of the renin-angiotensin-aldosterone system.

Volume expansion during NOS substrate donation with L-arginine: regulatory offsetting of renal response?

The responses to infusion of nitric oxide synthase substrate (L-arginine 3 mg.kg(-1).min(-1)) and to slow volume expansion (saline 35 ml/kg for 90 min) alone and in combination were investigated in separate experiments. L-Arginine left blood pressure and plasma ANG II unaffected but decreased heart rate (6 +/- 2 beats/min) and urine osmolality, increased glomerular filtration rate (GFR) transiently, and caused sustained increases in sodium excretion (fourfold) and urine flow (0.2 +/- 0.0 to 0.7 +/- 0.1 ml/min). Volume expansion increased arterial blood pressure (102 +/- 3 to 114 +/- 3 mmHg), elevated GFR persistently by 24%, and enhanced sodium excretion to a peak of 251 +/- 31 micromol/min, together with marked increases in urine flow, osmolar and free water clearances, whereas plasma ANG II decreased (8.1 +/- 1.7 to 1.6 +/- 0.3 pg/ml). Combined volume expansion and L-arginine infusion tended to increase arterial blood pressure and increased GFR by 31%, whereas peak sodium excretion was enhanced to 335 +/- 23 micromol/min at plasma ANG II levels of 3.0 +/- 1.1 pg/ml; urine flow and osmolar clearance were increased at constant free water clearance. In conclusion, L-arginine 1) increases sodium excretion, 2) decreases basal urine osmolality, 3) exaggerates the natriuretic response to volume expansion by an average of 50% without persistent changes in GFR, and 4) abolishes the increase in free water clearance normally occurring during volume expansion. Thus L-arginine is a natriuretic substance compatible with a role of nitric oxide in sodium homeostasis, possibly by offsetting/shifting the renal response to sodium excess.

Renal interstitial cGMP mediates natriuresis by direct tubule mechanism

The objective of this study was to test the hypothesis that renal interstitial (RI) cGMP is natriuretic in vivo. In conscious rats (n=8), urinary sodium excretion (U(Na)V) was significantly greater on days 3 and 4 of RI infusion of cGMP (1.17+/-0.14 and 1.61+/-0.11 mmol/24 h, respectively) than during vehicle infusion (0.56+/-0.15 and 0.70+/-0.17 mmol/24 h, respectively) (P<0.01). Similarly, U(Na)V was greater on days 3 and 4 of RI infusion of 8-bromo-cGMP (2.15+/-0.42 and 2.16+/-0.1 mmol/24 h, respectively). Protein kinase G inhibitor Rp-8-pCPT-cGMPS reduced cGMP-induced and 8-bromo-cGMP-induced U(Na)V to control levels. Acute RI infusion of L-arginine (L-Arg, 40 mg. kg(-1). min(-1)), but not D-arginine, caused an increase in U(Na)V from 1.65+/-0.11 to 4.07+/-0.1 micromol/30 min (P<0.01). This increase was blocked by RI infusion of N(G)-nitro-L-arginine methyl ester (100 ng. kg(-1). min(-1)) by the phosphodiesterase (PDE II) activator 5,6DMcBIMP (0.01 micromol/microL), by PDE II (0.03 U. kg(-1). min(-1)) itself, or by the soluble guanylyl cyclase inhibitor 1-H-[1,2,4]oxadiazolo-[4,2-alpha]quinoxalin-1-one (ODQ, 0.12 mg. kg(-1). min(-1)). The PDE II activator also blocked L-Arg-stimulated cGMP levels. The NO donor S-nitroso-N-acetylpenicillamine (SNAP, 0.12 micromol. L(-1). kg(-1). min(-1)) increased U(Na)V from 1.65+/-0.11 to 2.93+/-0.08 micromol/30 min (P<0.01), and this response was blocked completely by ODQ. Renal arterial but not RI administration of the heat-stable enterotoxin of Escherichia coli induced natriuresis. RA infusion of cGMP (3 microg/min) increased U(Na)V, renal blood flow (RBF), and glomerular filtration rate (GFR). Renal cortical interstitial cGMP infusion increased U(Na)V with no effect on total RBF, renal cortical blood flow, or GFR. Similarly, the natriuretic actions of renal interstitial L-Arg or SNAP were not accompanied by any change in RBF or GFR. Medullary cGMP infusion had no effect on U(Na)V, total RBF, or medullary blood flow. Texas red-labeled cGMP infused via the RI space was distributed exclusively to cortical renal tubular cells. The results demonstrate that RI cGMP inhibits renal tubular sodium absorption via protein kinase G independently of hemodynamic changes. These observations indicate that the cortical interstitial compartment provides a potentially important domain for cell-to-cell signaling within the kidney.

Role of cyclooxygenase-2-derived metabolites and nitric oxide in regulating renal function

The aim of this study was to examine the relative contribution of both cyclooxygenase (COX) isoforms in producing the prostaglandins (PG) involved in the regulation of renal function, when nitric oxide (NO) synthesis is reduced. In anesthetized dogs with reduction of NO synthesis, the renal effects of a nonisozyme-specific COX inhibitor (meclofenamate) were compared with those elicited by a selective COX-2 inhibitor (nimesulide) before and during an extracellular volume expansion (ECVE). Intrarenal N(G)- nitro-L-arginine methyl ester (L-NAME) infusion (1 microg x kg(-1) x min(-1); n = 6) did not elicit renal hemodynamic changes and reduced (P < 0.01) the renal excretory response to ECVE. Intravenous nimesulide (5 microg x kg(-1) x min(-1); n = 6) did not modify renal hemodynamic and reduced (P < 0. 05) sodium excretion before ECVE. Simultaneous L-NAME and nimesulide infusion (n = 7) elicited an increment (37%) in renal vascular resistance (RVR; P < 0.05) before ECVE and no hemodynamic changes during ECVE. The reduced excretory response elicited by L-NAME and nimesulide was similar to that found during L-NAME infusion. Finally, simultaneous L-NAME and meclofenamate infusion (10 microg x kg(-1) x min(-1); n = 7) induced an increase in RVR (91%, P < 0.05), a decrease in glomerular filtration rate (35%, P < 0.05), and a reduction of the renal excretory response to ECVE that was greater (P < 0.05) than that elicited by L-NAME alone. The results obtained support the notion that PG involved in regulating renal hemodynamic and excretory function when NO synthesis is reduced are mainly dependent on COX-1 activity.

Nitric oxide synthesis inhibition does not impair water immersion-induced renal vasodilation in humans

Nitric oxide (NO) is tonically released in the kidney to maintain renal perfusion and adequate sodium and water clearance. Little is known about the role of NO in the renal adaptation to an acute volume challenge. This is important for our understanding of pathophysiologic conditions associated with impaired NO activity. This study examined the effects of NO synthesis inhibition on neurohumoral, renal hemodynamic, and excretory responses to head-out immersion (HOI). Seven healthy men underwent four 7-h clearance studies. One study served as a time control study (placebo infusion), and in one study N(G)-monomethyl-L-arginine (L-NMMA; 3 mg/kg priming dose + 3 mg/kg per h) was infused during hours 2 to 5. In a third and fourth clearance study, HOI was applied from hours 3 to 5, during infusion of either placebo or L-NMMA. To assess the degree of NO synthesis inhibition, the effect of L-NMMA on [(15)N]-arginine-to-[(15)N]-citrulline conversion rate was studied in four others. HOI decreased mean arterial pressure (MAP) from 87 +/- 3 to 76 +/- 2 mmHg and renal vascular resistance (RVR) from 82 +/- 6 to 70 +/- 7 mmHg. min/L, and increased sodium excretion (UNaV) from 110 +/- 27 to 195 +/- 29 micromol/min and flow (UV) from 14.4 +/- 1.4 to 15.8 +/- 1.4 ml/min. L-NMMA caused profound and sustained increases in MAP and RVR and decreases in UNaV and UV. HOI superimposed on L-NMMA infusion decreased the elevated MAP from 93 +/- 4 to 83 +/- 2 mmHg and RVR from 111 +/- 9 to 95 +/- 7 mmHg. min/L, and increased UNaV from 41 +/- 8 to 95 +/- 15 micromol/min and UV from 10.0 +/- 1.1 to 12.7 +/- 1.4 ml/min. The relative changes were not significantly different from the effects of HOI without L-NMMA infusion. HOI decreased plasma renin activity and aldosterone and increased plasma atrial natriuretic peptide and urinary cGMP. L-NMMA decreased urinary cGMP, but did not affect the plasma hormones or the changes induced by HOI. L-NMMA decreased the [(15)N]-arginine-to-[(15)N]-citrulline conversion rate to one-third of baseline. The results indicate that in a state of NO deficiency in humans, the kidney can still respond to an acute volume challenge with vasorelaxation, diuresis, and natriuresis.

Effects of sodium intake on the pressor and renal responses to nitric oxide synthesis inhibition in normotensive individuals with different sodium sensitivity

OBJECTIVE

The present study evaluated the role of nitric oxide (NO) in the systemic vascular and renal adaptation to changes in dietary sodium intake.

DESIGN AND METHODS

Seven healthy normotensive male subjects were randomized to high or low sodium diets in a double blind crossover design (7 days on each diet). The NO synthesis inhibitor, NGmonomethyl-L-arginine (L-NMMA) was infused systemically (1.8 mg/kg over 30 min) at the end of each dietary period and its effects on blood pressure, renal plasma flow, glomerular filtration rate, urinary flow rate and sodium excretion were measured.

RESULTS

Blood pressure increased in response to L-NMMA on a high sodium diet only (area under time curve percentage change in mean blood pressure, low sodium = -94.5 +/- 164.3; high sodium = 391.1 +/- 228.6; P < 0.05 low versus high). The increase in blood pressure was directly and significantly associated with the individual salt sensitivity, defined by the difference in systemic mean blood pressure between high and low sodium diets (r = 0.756; P < 0.05). L-NMMA also reduced renal plasma flow and urinary flow rate in subjects on high sodium diet.

CONCLUSIONS

The data support a significant influence of endogenous NO in the systemic and renal vascular adaptation to a high sodium diet in normotensive men. In addition, the direct association between the individual sodium-sensitivity and the pressor response to L-NMMA suggests that there is increased dependence of vascular tone on NO in normotensive subjects whose blood pressure is more sodium sensitive.

The role of intrarenal nitric oxide in the natriuretic response to dopamine-receptor activation

Dopamine and dopamine-1 receptor agonists produce diuresis and natriuresis by causing changes in renal hemodynamics and by the activation of dopamine-1 receptors located within the various regions of the nephron. Nitric oxide plays an important role in the maintenance of systemic and regional hemodynamics. The present study was undertaken to investigate the effect of locally generated nitric oxide on renal function and its potential influence on the renal responses to dopamine-1 receptor agonists. The intrarenal infusion of a nitric oxide synthase inhibitor, L-NAME, (50 microg/kg min for 90 min) in anesthetized rats produced significant decreases in urine volume, urinary sodium excretion, glomerular filtration rate and fractional sodium excretion. These changes in renal function were associated with a concomitant decrease in urinary nitrate excretion, an indicator of nitric oxide release. However, L-NAME at this dose did not produce any significant changes in mean arterial pressure or heart rate. Intravenous infusion of fenoldopam (1 microg/kg min for 30 min), a selective dopamine-1 receptor agonist, produced diuresis and natriuresis without causing any changes in mean arterial pressure and heart rate. These renal effects of fenoldopam were significantly attenuated in animals that received the simultaneous infusion of L-NAME (intrarenal). Similar results were obtained with dopamine in that the natriuretic and diuretic response to dopamine was also attenuated during simultaneous infusion of dopamine with L-NAME. In addition, the diuresis and natriuresis produced by fenoldopam and dopamine was associated with increases in urinary nitrate excretion. Interestingly, these increases in the nitrate levels seen with fenoldopam and dopamine were also significantly reduced in the presence of L-NAME. These results indicate that intrarenal nitric oxide plays an important role in regulating renal sodium excretion and that an intact renal nitric oxide system is required for the full expression of diuretic and natriuretic response seen during dopamine-1 receptor activation.

Abnormal pressure-natriuresis in hypertension: role of nitric oxide

The kidneys have a critical role in long-term control of arterial pressure by regulating extracellular fluid and plasma volume. According to the renal body fluid feedback mechanism for long-term control, persistent hypertension can only occur as a result of a reduction in renal sodium excretory function or a hypertensive shift in the pressure-natriuresis relationship. Although an abnormal relationship between renal perfusion pressure and renal sodium excretion has been identified in every type of hypertension where it has been sought, factors responsible for this effect are still unclear. Nitric oxide (NO) is produced within the kidney and plays an important role in the control of many intrarenal processes which regulate the renal response to changes in perfusion pressure and thus, help determine plasma volume and blood pressure. Numerous studies have shown that long-term inhibition of NO synthesis results in a chronic rightward shift and marked attenuation in renal pressure-natriuresis. Recent studies have shown that certain animal models of genetic hypertension and forms of human hypertension areas are associated with a decrease in NO synthesis. Reductions in NO synthesis reduces renal sodium excretory function not only through direct actions on the renal vasculature, but through modulation of other vasoconstrictor processes and through direct and indirect alterations in tubular sodium transport. The causes and consequences of the dysregulation of NO in hypertension and other renal disease processes remain an important area of investigation.

EFFECTS OF NITRIC OXIDE SYNTHESIS INHIBITION ON THE Na,K-ATPase ACTIVITY IN THE KIDNEY

The present study was aimed at investigating the role of endogenous nitric oxide (NO) in regulating Na,K-ATPase activity in the kidney. The expression of alpha-1 and beta-1 subunits; and the enzymatic activity of Na,K-ATPase were determined in the kidney of rats treated with an NO synthase inhibitor, N(G)-nitro- l -arginine methyl ester (l -NAME). Following the treatment with l -NAME in the drinking water for 4 weeks, Na,K-ATPase activity was increased while tissue nitrite/nitrate levels were decreased in the kidney. Supplementation with l -arginine prevented the l -NAME-induced changes. The expression of either alpha-1 or beta-1 subunit protein of Na,K-ATPase, assessed by Western blot analysis, was not affected by l -NAME-treatment. An acute in vitro treatment of the kidney with l -NAME also caused an increase of Na,K-ATPase activity; which was again prevented by cotreatment with l -arginine. On the contrary, treatment with sodium nitroprusside significantly decreased Na,K-ATPase activity. These results suggest that the endogenous NO plays a direct inhibitory role on Na,K-ATPase activity in the kidney. 2000 Academic Press@p$hr Copyright 2000 Academic Press.

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.

Volume expansion natriuresis during servo control of systemic blood pressure in conscious dogs

The importance of arterial blood pressure (BP) and ANG II for the renal natriuretic response (NaEx) to volume expansion (3.5% body wt) was investigated during converting enzyme blockade (enalaprilate, 2 mg/kg). In separate experiments, BP was clamped either 30 mm Hg above or a few millimeters mercury below baseline by servo-controlled infusion of ANG II or sodium nitroprusside, respectively, so that volume expansion did not change BP. Enalapril decreased BP by 8 mm Hg. Without clamping, volume expansion returned BP to that of preenalapril control and increased NaEx 10-fold (40+/-10 to 377+/-69 micromol/min). During high pressure clamping (133+/-2 mm Hg), peak NaEx after volume expansion was 6% of control experiments. During low pressure clamping, NaEx was 68% of control experiments (45+/-15 to 256+/-64 micromol/min). The results show that 1) in absence of ANG II, volume expansion elicited pronounced natriuresis without increases in BP beyond baseline, 2) in the presence of hypertensive amounts of ANG II, the volume expansion-induced natriuresis was almost eliminated, and 3) nitroprusside prevented the increase in BP but not sodium excretion during volume expansion. ANG II appears to dominate the control of NaEx; however, when absent, volume expansion may still induce marked natriuresis even at constant BP, possibly via nitric oxide-mediated mechanisms.

Influence of hemodilution on the renal blood flow autoregulation during acute expansion in rats

Autoregulation of renal blood flow (RBF) was studied in rats that underwent equivalent blood volume expansion with saline (Sal; 5% body wt), 7% BSA solution (1.4% body wt), and reconstituted whole blood from donor rats (WBL; 1.4% body wt). Renal perfusion pressure (RPP) and renal neural reflexes were prevented by clamping RPP and sectioning the vagus, baro/chemoreceptor, and renal nerves. Sal and BSA expansion increased RBF by approximately 60%, whereas no effect was observed with WBL. RBF autoregulation was markedly attenuated after expansion with cell-free solutions, but no change occurred in WBL-expanded rats. Correction of the fall in hematocrit in Sal- and BSA-expanded rats restored RBF and its autoregulation to control levels. Expansion with Sal or BSA after inhibition of renal vascular tone with intrarenal infusion of papaverine still increased RBF and further changed the RBF-RPP relationship. These findings suggest that the hemodilution plays a central role in the reduction of renal vascular resistance and in the attenuation of the autoregulatory efficiency of renal circulation that accompany expansion with cell-free solutions.

Renal haemodynamic and excretory responses to bradykinin in anaesthetized dogs

1. Effects of bradykinin (BK) on renal haemodynamics and urine formation were examined in anaesthetized dogs. 2. Renal arterial infusion of BK at doses of 5 or 50 ng/kg per min produced dose-dependent increases in renal blood flow (RBF), without affecting systemic arterial pressure or glomerular filtration rate. There were also significant and dose-dependent increases in urine flow (UF), urinary excretion of sodium (UNaV) and fractional excretion of sodium (FENa) and decreases in urine osmolality during BK infusion. 3. Renal haemodynamic and excretory responses to the BK infusion were completely abolished by the simultaneous administration of Hoe 140 (icatibant, 100 ng/kg per min intrarenally), a selective BK B2-receptor antagonist. 4. In the presence of NG-nitro-L-arginine (NOARG; 40 micrograms/kg per min intrarenally), a nitric oxide (NO) synthase inhibitor, BK-induced renal vasodilative and natriuretic effects were markedly attenuated, although responses of UF and urine osmolality to BK remained unchanged. The water diuretic effect of BK was abolished in dogs given both NOARG and ibuprofen (12.5 mg/kg bolus injection plus 12.5 mg/kg per h of sustained infusion intravenously), a cyclooxygenase inhibitor. 5. These results clearly indicate that renal haemodynamic and excretory responses to BK were mediated exclusively by the B2-receptor. Renal vasodilative and natriuretic responses are mainly linked to NO generation, while both NO and prostaglandin biosynthesis are involved in the BK-induced water diuresis.

Strain differences of hypertension induced by dietary NG-nitro-L-arginine in normotensive rats

When the potent inhibitor of nitric oxide (NO) synthesis NG-nitro-L-arginine (L-NNA) was incorporated into the diet, hypertension was induced and sustained due to the effects of the long-term inhibition of endothelium-dependent relaxing factor (EDRF)/NO. The effects of L-NNA on normotensive rats of four strains (Donryu, Sprague-Dawley (SD), Wistar, and Wistar-Kyoto (WKY)) were compared relative to control rats. L-NNA administration caused a sharp initial increase in systolic blood pressure (SBP) at 2 weeks in all animals, and this was followed by a gradual and steady increase until 4 weeks. At the end of the experiments (5 weeks), the mean SBP of Donryu and SD rats was decreased. The maximum blood pressure of Donryu and Wistar rats during the experiments exceeded 200 mmHg, but that of SD and WKY rats was below 200 mmHg. Body weight loss and death were observed only in L-NNA-fed Donryu rats. Pathological changes in the kidneys and the morbidity rates for the lesions were determined, and indicated that the Donryu L-NNA group was 100% positive. These results suggest that the Donryu strain is more sensitive to L-NNA than the other strains. That dietary L-NNA-induced hypertension in normotensive rats of the four strains provides a new artificially-induced hypertensive model in which vasoconstriction occurs mainly due to EDRF deficiency.

Renal endothelial and macula densa NOS: integrated response to changes in extracellular fluid volume

If, only 20 years ago, anyone had postulated that the absence of nitric oxide gas (NO) would lead to severe hypertension and destruction of the vascular bed of the kidney within weeks, it is not unlikely that smiles of pity would have appeared on the faces of fellow researchers. By now, this has become common knowledge, and hundreds of reports have appeared on the regulation of vascular and renal function by nitric oxide. The amount of information complicates the design of a concept on how NO participates in control of extracellular fluid volume (ECFV) by the kidney. This review analyzes the function of endothelial and macula densa NO synthase (NOS) in the regulation of renal function. From this analysis, endothelial NOS (eNOS)-derived NO is considered a modulator of vascular responses and of renal autoregulation in particular. Increases in renal perfusion pressure and sodium loading will increase eNOS activity, resulting in vasodilatation and depression of tubuloglomerular feedback system responsiveness. Endothelium-derived NO seems important to buffer minute-to-minute variations in perfusion pressure and rapid changes in ANG II activity. In contrast, macula densa NOS is proposed to drive adaptations to long-term changes in distal delivery and is considered a mediator of renin formation. Increases in perfusion pressure and distal delivery will depress the activity and expression of the enzyme that coincides with, and possibly mediates, diminished renin activity. Together, the opposite responses of eNOS and macula densa NOS-derived NO to changes in ECFV lead to an appropriate response to restore sodium balance. The concept that the two enzymes with different localizations in the kidney and in the cell are producing the same product, displaying contrasting responses to the same stimulus but nevertheless exhibiting an integrated response to perturbation of the most important regulated variable by the kidney, i.e., the ECFV, may be applicable to other tissues.

Endothelial cells regulate proximal tubule epithelial cell sodium transport

BACKGROUND

Proximal tubule epithelial cells are in close contact with the renal microvasculature, but the effect of endothelial cells (ECs) on proximal tubule epithelial cell (PTEC) function is not known.

METHODS

To determine if ECs regulate PTECs, we coincubated ECs with PTECs in a system that permitted cross-talk between the two cell types and the vectorial transport of sodium.

RESULTS

In the presence (but not absence) of ECs, adding bradykinin or acetylcholine increased cGMP and decreased sodium transport, as well as Na,K-ATPase in PTECs. Interleukin (IL)1B preconditioning of ECs also increased cGMP and decreased sodium transport and Na,K-ATPase in PTECs. Bradykinin, acetylcholine, and IL1B EC-dependent effects were reversed with the nitric oxide (NO) synthase inhibitor L-NNA. In the absence of ECs, the addition of NO donors to PTECs increased cGMP and decreased sodium transport and Na,K-ATPase. 8Br-cGMP also decreased PTEC sodium transport and Na,K-ATPase.

CONCLUSION

Endothelial cells regulate PTEC function. This effect is mediated by NO synthase-dependent up-regulation of cGMP in PTECs.

Effect of hyperinsulinaemia on renal function and nitrate/nitrite excretion in healthy subjects

1. In addition to its metabolic actions, insulin acts as a vasodilator in certain vascular beds, such as skeletal muscle. It has been shown that this effect is mediated by endothelium-derived nitric oxide (NO). Unlike in the skeletal muscle, insulin-NO interactions in the kidney, another major site of insulin action, have been less studied. The aim of the present study was to explore the role of NO in renal effects of hyperinsulinaemia in healthy subjects. 2. Changes in renal function and urinary nitrate/nitrite (NO2-/NO3-; Griess method) levels as a marker of renal production of NO were assessed during euglycaemic hyperinsulinaemic clamp and compared with normoinsulinaemic isovolaemic conditions (administration of the same amount of insulin/glucose-free vehicle) in 10 healthy male volunteers. 3. Hyperinsulinaemia was associated with a decrease in renal excretion of stable metabolites of NO (mean (+/- SEM) 0.56 +/- 0.12 vs 0.38 +/- 0.05 mumol/min, respectively; P < 0.05). In contrast, administration of the same volume of insulin-free vehicle resulted in elevation of urinary NO2-/NO3- (P < 0.05). The changes in renal sodium handling followed a similar pattern as changes in the renal excretion of NO2-/NO3- with a significantly different response to hyperinsulinaemia when compared with normoinsulinaemia (F = 12.2; P < 0.001). The mean arterial pressure, blood levels of low-density lipoprotein-cholesterol and free fatty acids, possible factors influencing renal and systemic NO production, remained constant throughout both experiments. 4. These results suggest that hyperinsulinaemia is associated, in healthy males, with a decrease in renal generation of NO. In contrast, mild volume expansion with insulin-free vehicle resulted in increased excretion of NO metabolites. This insulin-induced attenuation of renal NO synthesis may contribute to the anti-natriuretic actions of insulin.

Human renal and systemic hemodynamic, natriuretic, and neurohumoral responses to different doses of L-NAME

Experimental evidence indicates that the renal circulation is more sensitive to the effects of nitric oxide (NO) synthesis inhibition than other vascular beds. To explore whether in men the NO-mediated vasodilator tone is greater in the renal than in the systemic circulation, the effects of three different intravenous infusions of NG-nitro-L-arginine methyl ester (L-NAME; 1, 5, and 25 microg. kg-1. min-1 for 30 min) or placebo on mean arterial pressure (MAP), systemic vascular resistance (SVR), renal blood flow (RBF), renal vascular resistance (RVR), glomerular filtration rate (GFR), and fractional sodium and lithium excretion (FENa and FELi) were studied in 12 healthy subjects, each receiving randomly two of the four treatments on two different occasions. MAP was measured continuously by means of the Finapres device, and stroke volume was calculated by a model flow method. GFR and RBF were estimated from the clearances of radiolabeled thalamate and hippuran. Systemic and renal hemodynamics were followed for 2 h after start of infusions. During placebo, renal and systemic hemodynamics and FENa and FELi remained stable. With the low and intermediate L-NAME doses, maximal increments in SVR and RVR were similar: 20.4 +/- 19.6 and 23.5 +/- 16.0%, respectively, with the low dose and 31.4 +/- 26.7 and 31.2 +/- 14.4%, respectively, with the intermediate dose (means +/- SD). With the high L-NAME dose, the increment in RVR was greater than the increment in SVR. Despite a decrease in RBF, FENa and FELi did not change with the low L-NAME dose, but they decreased by 31.2 +/- 11.0 and 20.2 +/- 6.3%, respectively, with the intermediate dose and by 70.8 +/- 8.1 and 31.5 +/- 15.9% with the high L-NAME dose, respectively. It is concluded that in men the renal circulation is not more sensitive to the effects of NO synthesis inhibition than the systemic circulation and that the threshold for NO synthesis inhibition to produce antinatriuresis is higher than the threshold level to cause renal vasoconstriction.

Isoform-specific regulation of nitric oxide synthase mRNA in the kidney by sodium and blood pressure

BACKGROUND

The roles of nitric oxide synthases (NOS) in kidney function are still controversial, principally due to the lack of isoform-specific inhibitors of NOS.

OBJECTIVE

To investigate the relative roles of each isoform of NOS in regulation of sodium and volume homeostasis.

DESIGN

We studied the effects of long-term modifications of sodium diet and blood pressure on expression of NOS mRNA in the renal cortex, where the three isoforms of NOS are present.

METHODS

We used quantitative reverse-transcription-polymerase chain reaction assays specific to each isoform of NOS to determine amounts of their respective mRNA in control rats, deoxycorticosterone acetate (DOCA)-salt hypertensive rats, rats fed a high-salt diet, and furosemide-treated rats fed a low-sodium diet. Nicotinamide adenine nucleotide phosphate H (NADPH) diaphorase staining was performed on DOCA-salt and control rat kidneys.

RESULTS

Levels of NOS I mRNA in DOCA-salt rats were decreased by treatment, those in low-salt-diet rats remained unaffected and those in high-salt diet rats tended to be intermediate between those of the other rat groups. Expression of NOS III mRNA was not significantly modified by either treatment Levels of NOS II mRNA in DOCA-salt rats were increased, those in high-salt-diet rats remained unaffected, and those in low-salt-diet were decreased by treatment, but these levels are more than 100-fold lower than those observed for the other isoforms of NOS. NADPH diaphorase staining in macula densa of DOCA-salt rats was markedly decreased compared with that in macula densa of control rats but staining in renal inflammatory and fibrous lesions became detectable, and staining in the vessels did not differ from that for control rats.

CONCLUSIONS

Our results show that intake of sodium and extracellular fluid volume regulate levels of mRNA of the three NOS isoforms in the renal cortex differently, suggesting that each of them plays a specific role.

Effects of FR139317 on renal responses to acute nitric oxide blockade in anaesthetized rats

1. Effects of FR139317, an endothelin ETA receptor antagonist, on renal haemodynamic and excretory responses to acute nitric oxide (NO) blockade were examined using anaesthetized rats. 2. Intrarenal arterial infusion of NG-nitro-L-arginine (NOARG), the NO synthase inhibitor, at a rate of 40 micrograms/kg per min, produced a significant decrease in renal blood flow, with no change in systemic blood pressure. There were significant decreases in urine flow and urinary excretion of sodium during infusion of NOARG. In animals pretreated with FR139317, similar renal responses to the NOARG infusion were observed. 3. These results suggest that an action of endothelin-1 via ETA receptors does not greatly contribute to the renal haemodynamic and excretory responses to acute blockade of renal NO production.

Salt intake determines the renal response to L-arginine infusion in normal human subjects

Studies in experimental animals have shown that nitric oxide (NO) generation in the kidney from L-arginine participates in adapting renal function to changes in salt intake, but similar studies in human subjects are lacking. Therefore, we compared the infusion of 30 g of L-arginine to 30 g of branched chain amino acids (control), in eight normal human subjects after 5 to 7 days of equilibration to a low salt (LS; 20 mumol.24 hr-1) or high salt (HS; 200 mumol.24 hr-1) intake. Lithium clearance was used as a marker of proximal tubular reabsorption. Compared to the control infusion, L-arginine did not significantly alter blood pressure, inulin or paraaminohippurate clearance, but significantly increased (P < 0.05) the excretion of NO2 + NO3 (NOx) (LS, 157 +/- 46 to 210 +/- 48 mumol.min-1; HS, 138 +/- 30 to 182 +/- 70) and cGMP (LS, 253 +/- 63 to 337 +/- 76 pmol.min-1; HS, 311 +/- 68 to 563 +/- 52). Renal sodium excretion was decreased by L-arginine infusion during the low salt intake (45 +/- 5 to 21 +/- 3 mumol.min-1; P < 0.05) but was increased by L-arginine during the high salt intake (298 +/- 56 to 537 +/- 84 mumol.min-1; P < 0.05). The calculated fractional reabsorption of sodium in the proximal and distal nephrons, as assessed from lithium and sodium clearances, was increased by L-arginine during the low salt intake but was decreased by L-arginine during the high salt intake. L-arginine increased plasma insulin concentration significantly (P < 0.05). This effect was independent of salt intake (LS, 67 +/- 7 to 92 +/- 13 ng.ml-1; HS, 66 +/- 7 to 76 +/- 9 ng.ml-1). L-arginine did not significantly after plasma renin activity. In conclusion, L-arginine increases the excretion of NOx and cGMP and increases plasma insulin, but the effect on sodium excretion depends upon salt intake. L-arginine enhances Na reabsorption in the proximal and distal nephrons during the low salt intake, but inhibits it during the high salt intake. Effects of L-arginine on NO and cGMP may contribute to its effects on Na reabsorption.

Inhibition of proximal tubular fluid absorption by nitric oxide and atrial natriuretic peptide in rat kidney

Atrial natriuretic factor (ANF) and nitric oxide (NO) stimulate production of guanosine 3',5'-cyclic monophosphate (cGMP) and are natriuretic. Split-drop micropuncture was performed on anesthetized rats to determine the effects of ANF and the NO donor sodium nitroprusside (SNP) on proximal tubular fluid absorption rate (Jva). Compared with control solutions, SNP (10(-4) M) decreased Jva by 23% when administered luminally and by 35% when added to the peritubular perfusate. Stimulation of fluid uptake by luminal angiotensin II (ANG II; 10(-9) M) was abolished by SNP (10(-4) and 10(-6) M). In proximal tubule suspensions, ANF (10(-6) M) increased cGMP concentration to 143%, whereas SNP (10(-6), 10(-5), 10(-4), 10(-3) M) raised cGMP to 231, 594, 687, and 880%, respectively. S-nitroso-N-acetylpenicillamine (SNAP) also raised cGMP concentrations with similar dose-response relations. These studies demonstrate inhibition by luminal and peritubular NO of basal and ANG II-stimulated proximal fluid absorption in vivo. The ability of SNP to inhibit basal fluid uptake whereas ANF only affected ANG II-stimulated transport may be because of production of higher concentrations of cGMP by SNP.

Renal changes induced by nitric oxide and prostaglandin synthesis reduction: effects of trandolapril and verapamil

The benefits of the simultaneous administration of low doses of a calcium antagonist and a converting enzyme inhibitor in the treatment of hypertension and renal vasoconstriction are well established. The objective of this study was to evaluate whether the administration of low doses of a calcium antagonist and a converting-enzyme inhibitor have beneficial effects in treating the renal alterations induced by the acute administration of a cyclooxygenase inhibitor when nitric oxide synthesis is reduced. These effects were examined in anesthetized dogs before and during an acute sodium load. It was found that the intrarenal infusion of meclofenamate (5 microg x kg[-1] x min[-1]), simultaneously with a low dose of NG-nitro-L-arginine methyl ester (1 microg x kg[-1] x min[-1]), produced a 40% decrease of renal blood flow and glomerular filtration rate and a reduction in the renal excretory response to the sodium load. In a second group of dogs, intrarenal verapamil (0.5 microg x kg[-1] x min[-1]) was effective in blocking the effects of nitric oxide and prostaglandin synthesis inhibition on sodium excretion and glomerular filtration rate but did not modify the effects on renal blood flow. An intrarenal infusion of trandolapril (0.3 microg x kg[-1] x min[-1]) was effective in a third group of dogs in reducing the renal hemodynamic effects but not in preventing the antinatriuretic effect observed in the first group. Finally, in a fourth group, the simultaneous administration of verapamil and trandolapril was effective in treating all the renal changes induced by the cyclooxygenase inhibitor when nitric oxide synthesis was reduced. These results suggest that the combination of low doses of trandolapril and verapamil has additive effects in treating the renal vasoconstriction and antinatriuresis induced by the acute administration of a cyclooxygenase inhibitor, when nitric oxide synthesis is reduced.

Systemic inhibition of nitric oxide and prostaglandins in volume-induced natriuresis and hypertension

Nitric oxide (NO) synthesis inhibition with NG-nitro-L-arginine methyl ester (L-NAME) (10 micrograms.kg-1.min-1 i.v.), cyclooxygenase inhibition with meclofenamate (Meclo; 5 mg/kg i.v. bolus), and combination of drugs (L-NAME + Meclo) were used to investigate the roles of NO and prostaglandins (PG) in the hemodynamic and natriuretic responses to isotonic saline volume expansion (VE; 5% body wt over 60 min) in anesthetized dogs. Before VE, L-NAME (n = 6), Meclo (n = 6), and L-NAME + Meclo (n = 6) produced significant increments in mean arterial pressure (MAP) of 12 +/- 2, 15 +/- 3, and 17 +/- 3 mmHg, respectively. VE did not change MAP in Meclo-treated dogs, but produced a significant elevation in the control dogs (14 +/- 6 mmHg), in L-NAME-treated dogs (17 +/- 6 mmHg), and in dogs pretreated with L-NAME + Meclo (12 +/- 5 mmHg). VE alone induced marked natriuretic responses in the control (38 +/- 9 to 562 +/- 86 mumol/min), L-NAME (31 +/- 9 to 664 +/- 65 mumol/min), and Meclo groups (41 +/- 10 to 699 +/- 51 mumol/min). However, this natriuretic response was attenuated in dogs pretreated with L-NAME + Meclo (12 +/- 4 to 185 +/- 52 mumol/ min). These results indicate that 1) blockade of both NO and PGs has significant diminishing effects on volume-induced natriuresis, 2) NO blockade alone impairs volume-induced natriuresis in a manner that requires further increases in MAP to restore the natriuresis, and 3) PG blockade alone does not curtail volume-induced natriuresis.

Dopamine-2 receptor blockade potentiates the renal effects of nitric oxide inhibition in humans

In eight young healthy subjects on a 240 mM Na diet mean arterial pressure (MAP), renal hemodynamics and renal handling of Na and exogenous Li were measured at baseline and during acute nitric oxide (NO) inhibition with 90-minute infusion of 3.0 microg/kg x min(-1) of N(G)-L-arginine methyl ester (L-NAME). The same experiment was repeated with infusion of 50 microg/kg x min(-1) of DA2 receptor blocker L-Sulpiride (L-SULP) alone and, finally, with simultaneous infusion of both L-NAME and L-SULP. L-SULP alone did not elicit any effect. L-NAME alone produced no changes in MAP from 0 to 45 minutes (P1) and a 6.6% increase at 45 to 90 minutes (P2) of infusion. Effective renal plasma flow (ERPF, PAH clearance) and glomerular filtration rate (GFR, inulin clearance) declined by 10.2% and 7.6%, respectively, in P1 and by 15.3% and 11.5% in P2. Filtration Fraction (FF) rose by 4.2% in P2. Calculated renal vascular resistance (RVR) increased by 13.0% to 25.6%. Fractional excretion of Na (FENa) and Li (FELi) fell by 20.0% and by 16.0%, respectively, in P1 and by 40.0% and 25.1% in P2. All these variations, except for MAP and GFR, were significantly greater during coinfusion of L-NAME and L-SULP. ERPF declined by 17.8% to 33.7%, FENa by 26.7% to 53.3%, FELi by 13.8% to 34.8%, while RVR rose by 22.5% to 59.1% and FF by 10.1% to 29.3%. The present data confirm that NO blockade with low-dose systemic infusion of L-NAME produces renal vasoconstriction, reduced GFR with slight increase in FF, and enhanced tubular Li, and Na reabsorption. Since increase in RVR and FF and decrease in FENa and FELi are markedly potentiated by the simultaneous infusion of DA2 blocker L-SULP, which exerts no effects by itself, we suggest that DA interactions between DA system at the level of DA2 receptors and basal NO production play a physiological role in the regulation of renal function in humans.

Actions of nitric oxide on renal epithelial transport

1. In vivo studies have demonstrated that nitric oxide (NO) induces natriuresis. Nitric oxide-induced natriuresis occurs independently of changes in renal perfusion pressure, indicating that it is the result of a tubular effect of NO. 2. In support of this hypothesis investigators have shown that NO inhibits both Na(+)-H+ exchange and Na+/K(+)-ATPase activity in the proximal tubule. In the collecting duct, NO has been shown to decrease sodium flux with no effect on Na+/K(+)-ATPase activity. 3. Thus, direct examination of the actions of NO have shown that NO can inhibit sodium transport in the nephron, which may account for the natriuresis observed in vivo.

Nitric oxide, the kidney and hypertension

1. According to the renal body fluid feedback mechanism for long-term control, persistent hypertension can only occur as a result of a reduction in renal sodium excretory function or a hypertensive shift in the pressure natriuresis relationship. Although an abnormal relationship between renal perfusion pressure and renal sodium excretion has been identified in every type of hypertension where it has been sought, factors responsible for this effect are still unclear. 2. Nitric oxide (NO) is produced within the kidney and plays an important role in the control of many intrarenal processes that regulate the renal response to changes in perfusion pressure and, thus, help determine systemic vascular volume and blood pressure. Numerous studies have shown that long-term inhibition of NO synthesis results in a chronic hypertensive shift in renal pressure natriuresis. 3. Recent studies have shown that certain animal models of genetic hypertension and forms of human hypertension areas are associated with a decrease in NO synthesis. Reductions in NO synthesis reduce renal sodium excretory function, not only through direct action on the renal vasculature, but through modulation of other vasoconstrictor processes and through direct and indirect alterations in tubular sodium transport. 4. The causes and consequences of the disregulation of NO in hypertension and other renal disease processes remain an important area of investigation.

Pressure natriuresis and renal medullary blood flow in dogs

In the present study, we evaluated the effects of changes in arterial pressure on regional renal blood flows and sodium excretion in anesthetized dogs during control conditions and after 5% volume expansion with isotonic saline. Medullary and cortical blood flow responses were determined with laser-Doppler needle flow probes inserted into the midmedullary and midcortical regions, and whole-kidney blood flow was assessed with an electromagnetic flow probe. Volume expansion in six dogs caused marked increases in urine flow (20.2 +/- 5.5 to 82.5 +/- 22.7 microL.min-1.g-1) and sodium excretion (3.2 +/- 0.5 to 11.1 +/- 2.7 mumol.min-1.g-1), with slight increases in glomerular filtration rate (0.92 +/- 0.03 to 1.01 +/- 0.02 mL.min-1.g-1) but no significant changes in total renal blood flow (4.7 +/- 0.3 to 5.2 +/- 0.6 mL.min-1.g-1), medullary blood flow (+6 +/- 9%), or cortical blood flow (+12 +/- 10%). During stepwise reductions in renal arterial pressure (150 to 75 mm Hg) elicited with a renal arterial occluder, both before and after volume expansion, medullary, cortical, and total renal blood flows as well as glomerular filtration rate exhibited efficient autoregulation, with slopes not significantly different from zero over this range of arterial pressure. Ther were marked increases in the slopes of the relationships between arterial pressure and urine flow (0.18 +/- 0.05 to 0.78 +/- 0.27 microL.min-1.g-1.mm Hg-1) as well as sodium excretion (0.03 +/- 0.004 to 0.10 +/- 0.03 mumol.min-1.g-1.mm Hg-1) during volume expansion. These data demonstrate that medullary blood flow is efficiently autoregulated in dogs during control and volume-expanded states and indicate that the mechanism responsible for the arterial pressure-induced changes in sodium excretion does not depend on coincident alterations in medullary blood flow.

Renal hemodynamic and excretory responses in anesthetized rats to FK409, a novel nitric oxide donor

Renal hemodynamic and excretory responses to (+/-)-(E)-4-ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexenamide (FK409), a novel nitric oxide (NO) donor, were examined using anesthetized rats. When FK409 was infused into the renal artery of normal rats at 10 micrograms/kg per min, a moderate renal vasodilating effect was observed with a decrease in mean arterial blood pressure. Urine flow, urinary excretion of sodium and fractional excretion of sodium significantly increased by about 85%, 110% and 75%, respectively, compared with each control value. Simultaneously, urinary excretion of NO metabolites (UNOxV) was markedly increased with the administration of FK409. In hypertensive rats treated with NG-nitro-L-arginine (NOARG), the NO synthase inhibitor, FK409 produced a potent renal vasodilation, although the hypotensive effect of the agent was comparable to that seen in normal rats. In addition, glomerular filtration rate was significantly elevated by the agent. There were marked increases in the excretory responses, i.e., levels of urine flow, urinary excretion of sodium and fractional excretion of sodium were increased to about 3-, 6- and 5-fold of each control value, respectively. The extent of increment of UNOxV was similar to that seen in normal rats. These results clearly indicate that FK409 causes renal vasodilation and diuresis, via NO formation. Renal hemodynamic and excretory responses to the agent are sensitive in NO-depleted conditions. FK409 and related compounds may be useful for the treatment of renal diseases, in cases where the basal NO formation is impaired.

Renal effects of rilmenidine in volume-loaded anaesthetized dogs

1. In anaesthetized, fluid expanded rats rilmenidine has diuretic and natriuretic effects. There is strong evidence that the natriuresis is mediated by putative imidazoline receptors. In contrast, in conscious euvolaemic dogs rilmenidine has a diuretic effect that is entirely attributable to activation of alpha 2-adrenoceptors, but no natriuretic effect. To determine whether the effects of rilmenidine are truly species dependent, or merely dependent upon the influences of anaesthesia and volume status, we tested the effects of rilmenidine in pentobarbitone anaesthetized, volume-loaded dogs. 2. The effects of rilmenidine in anaesthetized, volume-loaded dogs were similar to those found in conscious euvolaemic dogs. Compared with vehicle treatment, levels of glomerular filtration rate, urine flow and haematocrit were increased following rilmenidine treatment. No effect of rilmenidine on sodium excretion was observed. 3. We conclude that the renal responses to rilmenidine in dogs are largely unaffected by anaesthesia and plasma volume status. In particular, the natriuretic effect seen in rats was not observed. We conclude that putative imidazoline receptors do not have a major influence on sodium excretion in dogs.

Angiotensin II-induced renal responses in anesthetized rabbits: effects of N omega-nitro-L-arginine methyl ester and losartan

Intrarenal arterial infusion of angiotensin II (4 ng/kg/min) reduced renal blood flow, glomerular filtration rate and urinary Na+ excretion (UNaV) without affecting fractional Na+ excretion (FENa) in anesthetized rabbits. Losartan (10 micrograms/kg/min) abolished these angiotensin II-induced renal responses. The renal blood flow, glomerular filtration rate and UNaV responses were potentiated during intrarenal arterial infusion of N omega-nitro-L-arginine methyl ester (L-NAME, 10 micrograms/kg/min). A high dose of L-NAME (50 micrograms/kg/min) also potentiated the renal blood flow and UNaV responses but not the glomerular filtration rate response. Angiotensin II reduced FENa during L-NAME infusion at either dose. In L-NAME-pretreated rabbits, losartan abolished the angiotensin II-induced renal blood flow and glomerular filtration rate responses, but the reduction in FENa still remained. The present study suggests that in the rabbit kidney (1) nitric oxide attenuates the angiotensin II-induced (angiotensin AT1 receptor-mediated) vasoconstriction and (2) angiotensin II can evoke losartan-resistant tubular Na+ reabsorption, but the tubular action is concealed by nitric oxide.

Role of nitric oxide in short-term and prolonged effects of angiotensin II on renal hemodynamics

Short-term infusions of angiotensin II (Ang II) increase renal vascular resistance and thereby endothelial shear stress and nitric oxide (NO) release. Prolonged stimulation of Ang II can decrease the expression of NO synthase isoforms in the macula densa, but prolonged increases in shear stress can increase transcription of endothelial NO synthase. Therefore, we designed these studies to test the hypothesis that Ang II exerts time-dependent effects on renal NO generation as assessed from renal excretion of nitrate and nitrite, percent increases in renal vascular resistance during inhibition of NO synthase with intravenous NG -nitro-L-arginine methyl ester (L-NAME), or decreases in renal vascular resistance during stimulation of endothelial NO synthase with intravenous acetylcholine. Rats were tested during graded short-term (30 to 90 minutes intravenous) or prolonged (5 to 6 days subcutaneous) Ang II infusions that led to dose-dependent increases in blood pressure and renal vascular resistance and reductions in renal blood flow. Captopril was administered for 3 to 4 days to suppress Ang II generation. The renal excretion of nitrate and nitrite was increased during short-term Ang II infusions (from 205 +/- 22 to 331 +/- 58 pmol.min-1, P < .05) but was unchanged during prolonged Ang II infusion (control group, 197 +/- 33 versus Ang II, 245 +/- 42 pmol.min-1, P=NS). The percent increase in renal vascular resistance with L-NAME was potentiated dose dependently by short-term but not long-term Ang II infusions. The increase in renal vascular resistance with L-NAME in control rats without Ang II infusions was +150 +/- 13%. At an Ang II infusion of 200 ng.kg-1.min-1, the L-NAME-induced percent increase in renal vascular resistance was significantly (P < .01) increased compared with controls in short-term Ang II-infused rats (+369 +/- 70%) but was not significantly different in prolonged infused rats (+190 +/- 33%). Intravenous acetylcholine caused dose-dependent renal vasodilation that was not significantly changed in rats receiving short-term intravenous Ang II but was significantly (P < .005) potentiated in those receiving prolonged Ang II infusions (change in renal vascular resistance with acetylcholine at 10 micrograms.kg-1.min-1 versus control, -21.5 +/- 5.0%; with short-term Ang II, -24.9 +/- 4.5%; with long-term Ang II, -52.1 +/- 7.2%). In conclusion, short- and long-term Ang II infusions caused equivalent changes in blood pressure and renal blood flow and hence presumably equivalent increases in endothelial shear stress. However, only short-term Ang II infusions increased NO generation and the dependence of the renal circulation on NO, whereas acetylcholine-induced NO release was enhanced selectively during long-term Ang II infusions. This suggests that during long-term Ang II, renal NO release may become uncoupled from shear stress yet remains highly responsive to receptor-mediated stimulation.