Decreased renin release and constant kallikrein secretion after injection of L-NAME in isolated perfused rat kidney

Effect of nitric oxide inhibition on blood pressure and renal sodium handling: a dose-response study in healthy man

Nitric oxide (NO) is a ubiquitous vasodilator and an important regulator of renal sodium excretion. To further investigate the role of NO in renal sodium handling, we studied the effects of the NO synthase inhibitor, NG-monomethyl-L-arginine (L-NMMA), in a crossover dose-response study. During NO inhibition mean arterial pressure increased dose-dependently and reached a plateau after 20 minutes of infusion. On the contrary, the fractional excretion of sodium was reduced equally in all three L-NMMA doses. This indicates that sodium excretion is highly sensitive to even small changes in renal NO bioavailability in healthy human.

Roles of nitric oxide and angiotensin II in the impaired baroreflex gain of pregnancy

The present study tested the hypothesis that nitric oxide (NO) contributes to impaired baroreflex gain of pregnancy and that this action is enhanced by angiotensin II. To test these hypotheses, we quantified baroreflex control of heart rate in nonpregnant and pregnant conscious rabbits before and after: 1) blockade of NO synthase (NOS) with Nomega-nitro-L-arginine (20 mg/kg iv); 2) blockade of the angiotensin II AT1 receptor with L-158,809 (5 microg x kg(-1) x min(-1) iv); 3) infusion of angiotensin II (1 ng x kg(-1) x min(-1) nonpregnant, 1.6-4 ng x kg(-1) x min(-1) pregnant iv); 4) combined blockade of angiotensin II AT(1) receptors and NOS; and 5) combined infusion of angiotensin II and blockade of NOS. To determine the potential role of brain neuronal NOS (nNOS), mRNA and protein levels were measured in the paraventricular nucleus, nucleus of the solitary tract, caudal ventrolateral medulla, and rostral ventrolateral medulla in pregnant and nonpregnant rabbits. The decrease in baroreflex gain observed in pregnant rabbits (from 23.3 +/- 3.6 to 7.1 +/- 0.9 beats x min(-1) x mmHg(-1), P < 0.05) was not reversed by NOS blockade (to 8.3 +/- 2.5 beats x min(-1) x mmHg(-1)), angiotensin II blockade (to 5.0 +/- 1.1 beats x min(-1) x mmHg(-1)), or combined blockade (to 12.3 +/- 4.8 beats x min(-1) x mmHg(-1)). Angiotensin II infusion with (to 5.7 +/- 1.0 beats x min(-1) x mmHg(-1)) or without (to 8.4 +/- 2.4 beats x min(-1) x mmHg(-1)) NOS blockade also failed to improve baroreflex gain in pregnant or nonpregnant rabbits. In addition, nNOS mRNA and protein levels in cardiovascular brain regions were not different between nonpregnant and pregnant rabbits. Therefore, we conclude that NO, either alone or via an interaction with angiotensin II, is not responsible for decrease in baroreflex gain during pregnancy.

Interaction of Endothelial Nitric Oxide and Angiotensin in the Circulation

Discovery of the unexpected intercellular messenger and transmitter nitric oxide (NO) was the highlight of highly competitive investigations to identify the nature of endothelium-derived relaxing factor. This labile, gaseous molecule plays obligatory roles as one of the most promising physiological regulators in cardiovascular function. Its biological effects include vasodilatation, increased regional blood perfusion, lowering of systemic blood pressure, and antithrombosis and anti-atherosclerosis effects, which counteract the vascular actions of endogenous angiotensin (ANG) II. Interactions of these vasodilator and vasoconstrictor substances in the circulation have been a topic that has drawn the special interest of both cardiovascular researchers and clinicians. Therapeutic agents that inhibit the synthesis and action of ANG II are widely accepted to be essential in treating circulatory and metabolic dysfunctions, including hypertension and diabetes mellitus, and increased availability of NO is one of the most important pharmacological mechanisms underlying their beneficial actions. ANG II provokes vascular actions through various receptor subtypes (AT1, AT2, and AT4), which are differently involved in NO synthesis and actions. ANG II and its derivatives, ANG III, ANG IV, and ANG-(1-7), alter vascular contractility with different mechanisms of action in relation to NO. This review article summarizes information concerning advances in research on interactions between NO and ANG in reference to ANG receptor subtypes, radical oxygen species, particularly superoxide anions, ANG-converting enzyme inhibitors, and ANG receptor blockers in patients with cardiovascular disease, healthy individuals, and experimental animals. Interactions of ANG and endothelium-derived relaxing factor other than NO, such as prostaglandin I2 and endothelium-derived hyperpolarizing factor, are also described.

Renal perfusion and the renal hemodynamic response to blocking the renin system in diabetes: are the forces leading to vasodilation and vasoconstriction linked?

In three groups of subjects, those with type 2 diabetes and nephropathy, those with type 1 diabetes without nephropathy, and healthy volunteers subjected to short-term hyperglycemia, we observed a counterintuitive relationship. In all three groups, baseline renal plasma flow (RPF) was positively correlated with the RPF response to blocking the renin-angiotensin system (RAS). This seems paradoxical in that an opposite result would have been expected if angiotensin-dependent renal vasoconstriction was responsible for the renal vasodilator response to RAS blockade. This suggests a link between the renal vasodilator response, mediated by nitric oxide (NO), and the activation of the intrarenal RAS. The complex interrelationships between hyperglycemia, insulin, NO, and the RAS may result in phenotypes that indicate varying risk of diabetic nephropathy and underlying genetic polymorphisms.

Nitric oxide regulates renal cortical cyclooxygenase-2 expression

We have previously shown that cyclooxygenase-2 (COX-2) is localized to the cortical thick ascending limb of the loop of Henle (cTALH)/macula densa of the rat kidney, and expression increases in response to low-salt diet and/or angiotensin-converting enzyme (ACE) inhibition. Because of the localization of neuronal nitric oxide synthase (nNOS) to macula densa and surrounding cTALH, the present study investigated the role of nitric oxide (NO) in the regulation of COX-2 expression. For in vivo studies, rats were fed a normal diet, low-salt diet or low-salt diet combined with the ACE inhibitor captopril. In each group, one-half of them were treated with the nNOS inhibitors 7-nitroinidazole (7-NI) or S-methyl-thiocitrulline. Both of these NOS inhibitors inhibited increases in COX-2 mRNA and immunoreactive protein in response to low salt and low salt+captopril. For in vitro studies, COX-2 expression was studied in primary cultures of rabbit cTALH cells immunodisssected with Tamm-Horsfall antibody. Basal COX-2 immunoreactivity expression was stimulated by S-nitroso-N-acetyl-penicillamine (SNAP), an NO donor, and intracellular cGMP concentration. The cultured cells expressed immunoreactive nNOS, and 7-NI inhibited basal COX-2 immunoreactivity expression, which could be partially overcome by cGMP. In summary, these studies indicate that NO is a mediator of increased renal cortical COX-2 expression seen in volume depletion and suggest important interactions between the NO and COX-2 systems in the regulation of arteriolar tone and the renin-angiotensin system by the macula densa.

Interactions of the renin-angiotensin system and neuronal nitric oxide synthase in regulation of cyclooxygenase-2 in the macula densa

Cyclooxygenase-2 (COX-2) expression in rat kidney is localized to the macula densa and the immediately proximal cTALH and increases after salt restriction. Either ACE inhibitors or AT1 receptor blockers increase COX-2 expression in both control and salt-restricted animals, suggesting that the RAS activation feedback inhibits renal cortical COX-2 expression. To determine whether increased COX-2 expression in response to ACE inhibition mediated increases in renin production, rats were treated with Captopril for 1 week with or without the specific COX-2 inhibitor, SC58236. Plasma renin activity increased significantly in the Captopril group. This increase was partially reversed by simultaneous treatment with SC58236. Kidney renin activity also increased in the Captopril group compared with control, which was also significantly inhibited by SC58236 treatment. Because of the localization of bNOS to MD and surrounding cTALH, the current study investigated the role of NO in the regulation of COX-2 expression. Rats were fed a normal diet, low salt diet or low salt diet combined with captopril and half of them were treated with the neuronal NOS inhibitor, 7-NI, and half with vehicle. After 7 days, mRNA was extracted and the microsome proteins purified from renal cortex. COX-2 mRNA expression was measured by Northern-blot and normalized with GAPDH. 7-NI treatment decreased COX-2 mRNA and immunoreactive COX-2 expression in each group. In summary, these studies indicate that COX-2 from macula densa/cTALH is a regulator of renin production and release. Angiotensin II may be a negative regulator of cTALH/macula densa COX-2 expression, and NO may mediate increased renal cortical COX-2 expression seen in volume depletion. These studies suggest important interactions between the NO and COX-2 systems in the regulation of arteriolar tone and the renin-angiotensin system by the macula densa.

Role of nitric oxide in the control of renin secretion

Because of the significant constitutive expression of NO synthases in the juxtaglomerular apparatus, nitric oxide (NO) is considered as a likely modulator of renin secretion. In most instances, NO appears as a tonic enhancer of renin secretion, acting via inhibition of cAMP degradation through the action of cGMP. Depending on as yet unknown factors, the stimulatory effect of NO on renin secretion may also switch to an inhibitory one that is compatible with the inhibition of renin secretion by cGMP-dependent protein kinase activity. Whether NO plays a direct regulatory role or a more permissive role in the control of renin secretion remains to be answered.

Role of cGMP-kinase II in the control of renin secretion and renin expression

To investigate the roles of the cGMP-dependent protein kinases (cGKs) in the control of the renin system, we studied the regulation of renin in cGKI- or cGKII-deficient mice in vivo and in vitro. Renal renin mRNA levels both under stimulatory (low-salt diet plus ramipril) and inhibitory (high-salt diet) conditions were not different between wild-type and cGKI-/- mice, but were significantly elevated in cGKII-/- mice under all experimental conditions. In primary cultures of renal juxtaglomerular cells (JG) established from wild-type, cGKI-/-, and cGKII-/- mice, the adenylate cyclase activator forskolin stimulated renin secretion similarly in all genotypes tested. 8-bromo-cGMP attenuated basal and forskolin-stimulated renin secretion in cultures from wild-type and cGKI-/-, but had no effect in cells isolated from cGKII-/- mice. Activation of cGKs by 8-bromo-cGMP decreased renin secretion from the isolated perfused rat kidney, independent of prestimulation by beta-adrenoreceptor activation, macula densa inhibition, reduced perfusion pressure, or by a nominally calcium-free perfusate. Taken together, these findings suggest that activation of cGKII has a general inhibitory effect on renin secretion from renal JG cells.

Dietary sodium affects systemic and renal hemodynamic response to NO inhibition in healthy humans

Animal studies have indicated that increased nitric oxide (NO) synthesis plays a significant role in the renal adaptation to increased sodium intake. To investigate the role of NO during increased sodium intake in humans, we studied the effect of acute, systemic injection of NG-monomethyl-L-arginine (L-NMMA) on renal hemodynamics [glomerular filtration rate and renal plasma flow (GFR and RPF, respectively)], urinary sodium excretion (FENa), systemic hemodynamics [mean arterial blood pressure and heart rate (MAP and HR)], and plasma levels of several vasoactive hormones in 12 healthy subjects during high (250 mmol/day) and low (77 mmol/day) sodium intake in a crossover design. The sodium diets were administered for 5 days before the L-NMMA treatments, in randomized order, with a washout period of 9 days between each diet and L-NMMA treatment. GFR and RPF were measured using the renal clearance of 51Cr-labeled EDTA and 125I-labeled hippuran by the constant infusion technique in clearance periods of 30-min duration. Two baseline periods were obtained, after which L-NMMA was given (3 mg/kg over 10 min), and the effect of treatment was followed over the next five clearance periods. During high sodium intake, L-NMMA induced a more pronounced relative decrease in RPF (P = 0.0417, ANOVA), a more pronounced relative decrease in FENa (P = 0.0032, ANOVA), and a more pronounced relative increase in MAP (P = 0.0231, ANOVA). During low sodium intake, the effect of L-NMMA on FENa was abolished. During low sodium intake, L-NMMA induced a sustained drop in plasma renin (31 +/- 5 vs. 25 +/- 5 microU/ml, P < 0.001), which was not seen during high sodium intake. The data indicate that increased production of NO is an important part of the adaptation to increased dietary sodium intake in healthy humans, with respect to renal hemodynamics, sodium excretion, and the secretion of renin.

Oral supplementation of L-arginine prevents chronic cyclosporine nephrotoxicity in rats

This study was performed to evaluate the effect of L-arginine (L-Arg) on the prevention of chronic cyclosporine (CsA) nephrotoxicity in rats. Rats pair-fed a low-salt diet (0.05%) were given CsA (15 mg/kg/day s.c.), CsA and L-Arg (L-Arg group, 1.25 g/l water), CsA and N-nitro-L-arginine methyl ester (L-NAME group, 70 mg/l water) or vehicle. After 28 days, the L-Arg group had a higher glomerular filtration rate compared to the CsA (0.42 +/- 0.05 vs. 0.31 +/- 0.06 ml/min/100 g, p < 0.05) and the L-NAME groups (vs. 0.19 +/- 0.04 ml/min/100 g, p < 0.05) and a significantly lower serum creatinine level compared to the CsA (0.70 +/- 0.06 vs. 0.92 +/- 0.12 mg/dl, p < 0.05) and the L-NAME groups (vs. 1.21 +/- 0.17 mg/dl, p < 0.05). The L-Arg group had less fibrosis, tubular injury (TI), and arteriolopathy than the CsA (fibrosis 0.39 +/- 0.14 vs. 0.74 +/- 0.15; TI 1.3 +/- 0.3 vs. 2.0 +/- 0.1; arteriolopathy 33 +/- 7 vs. 48 +/- 17, p < 0.05, respectively) and the L-NAME groups (fibrosis vs. 1.67 +/- 0.32, TI vs. 2.6 +/- 0.3, arteriolopathy vs. 63 +/- 10, p < 0.05, respectively). Plasma renin activity in the L-Arg group was less than in the CsA (18 +/- 2 vs. 23 +/- 3 ng Ang I/ml/h, p < 0.05) and the L-NAME groups (vs. 30 +/- 3 ng Ang I/ml/h, p < 0.05). Nitric oxide production in L-Arg group was higher than in the CsA (24.2 +/- 1.7 vs. 11.1 +/- 1.5 mumol/24 h, p < 0.05) and the L-NAME groups (vs. 8.4 +/- 1.0 mumol/24 h, p < 0.05). In conclusion, the nitric oxide pathway is associated with the pathogenesis of chronic CsA nephrotoxicity, and exogenous L-Arg supplementation is effective in reducing chronic CsA nephrotoxicity in rats.

Cell-specific protein and gene expression in the juxtaglomerular apparatus

1. The juxtaglomerular apparatus (JGA) consists of a tubular component, the macula densa (MD), attached to a vascular component consisting of the afferent and efferent arterioles and the extraglomerular mesangium. The JGA is richly innervated by sympathetic fibres. 2. The MD is morphologically, histochemically and functionally different from the ascending thick portion of the loop of Henle where it is located. 3. The vascular component includes the vascular smooth muscle cells of the arteriole, the renin-producing cells or juxtaglomerular cells, extraglomerular mesangial cells (Goormaghtigh cells) and endothelial cells. They are coupled by gap junctions. 4. Physiological evidence indicates that the composition of tubular fluid at the MD regulates renin secretion and glomerular haemodynamics and that the JGA is important in the maintenance of body salt-water homeostasis. Evidence suggests that the MD exerts its action on the vascular component through a paracrine mechanism.

Activation of renin synthesis is dependent on intact nitric oxide production

The present study investigated whether or not nitric oxide (NO) synthesis mediates mechanisms regulating activation of renin formation. Studies were performed on afferent arterioles freshly isolated from the rat kidney. We have shown previously that this preparation is a useful model to study regulation of renin synthesis and secretion. The expression of renin mRNA was assessed by ribonuclease protection assay, and total renin content and renin secretion by radioimmunoassay. In afferent arterioles isolated from rats treated with the angiotensin-converting enzyme inhibitor ramipril, renin mRNA levels, total renin content and renin secretion were increased threefold compared to untreated controls. Inhibition of NO-synthase by NG-nitro-L-arginine methyl ester (L-NAME) in the ramipril-treated rats, abolished the increase in renin mRNA levels, total renin content and renin secretion. In other animals furosemide, a diuretic acting on macula densa cells, activated renin synthesis to a level similar to that found in the ramipril-treated group. Addition of L-NAME to the furosemide-treated rats suppressed the increases in renin mRNA levels, total renin content and renin secretion, suggesting that NO acts on renin activation by a mechanism independent of angiotensin II. In separate experiments, the inhibitory effect of L-NAME on the activation of renin secretion was abolished when afferent arterioles were treated with nicardipine, an L-type Ca2+ channel blocker, suggesting that the suppression of renin activation during NO inhibition is due to increased Ca2+ entry. Since endothelin is a potent mediator of Ca2+ influx and an inhibitor of renin release, we tested whether or not endothelin could be involved in the inhibitory effect of L-NAME on renin secretion. Application of the endothelin receptor antagonist, bosentan, in vitro mimicked the effect of nicardipine. In addition, bosentan coadministered with L-NAME in vivo blunted the inhibitory effect of L-NAME and restored the increases in renin mRNA level, synthesis and secretion. These data indicate that the physiological mechanism(s) regulating activation of renin synthesis and secretion are impaired during NO inhibition, probably because of increased Ca2+ influx. This increase in calcium flux is mediated at least partially by the action of endothelin.

Elevated blood pressures in mice lacking endothelial nitric oxide synthase

Nitric oxide produced in endothelial cells affects vascular tone. To investigate the role of endothelial nitric oxide synthase (eNOS) in blood pressure regulation, we have generated mice heterozygous (+/-) or homozygous (-/-) for disruption of the eNOS gene. Immunohistochemical staining with anti-eNOS antibodies showed reduced amounts of eNOS protein in +/- mice and absence of eNOS protein in -/- mutant mice. Male or female mice of all three eNOS genotypes were indistinguishable in general appearance and histology, except that -/- mice had lower body weights than +/+ or +/- mice. Blood pressures tended to be increased (by approximately 4 mmHg) in +/- mice compared with +/+, while -/- mice had a significant increase in pressure compared with +/+ mice (approximately 18 mmHg) or +/- mice (approximately 14 mmHg). Plasma renin concentration in the -/- mice was nearly twice that of +/+ mice, although kidney renin mRNA was modestly decreased in the -/- mice. Heart rates in the -/- mice were significantly lower than in +/- or +/+ mice. Appropriate genetic controls show that these phenotypes in F2 mice are due to the eNOS mutation and are not due to sequences that might differ between the two parental strains (129 and C57BL/6J) and are linked either to the eNOS locus or to an unlinked chromosomal region containing the renin locus. Thus eNOS is essential for maintenance of normal blood pressures and heart rates. Comparisons between the current eNOS mutant mice and previously generated inducible nitric oxide synthase mutants showed that homozygous mutants for the latter differ in having unaltered blood pressures and heart rates; both are susceptible to lipopolysaccharide-induced death.

Pressure-dependent renin release during chronic blockade of nitric oxide synthase

We evaluated pressure-dependent stimulation of renin release in rats with sustained hypertension induced by chronic blockade of nitric oxide synthase with N omega-nitro-L-arginine methyl ester (L-NAME) for 5 to 7 days. Rats were anesthetized and catheters were inserted into the carotid artery and abdominal aorta for measurement of arterial pressures. An adjustable snare was placed around the suprarenal aorta, and this snare was tightened to reduce renal perfusion pressure. Pressure-dependent renin release was evaluated in hypertensive rats by reducing renal perfusion pressure to 125, 85, and 65 mm Hg. Renin release was also evaluated in normotensive control rats at these same pressures. Basal systemic arterial pressures averaged 159 +/- 3 and 124 +/- 4 mm Hg (P < .001), respectively, in the L-NAME-treated (n = 22) and normotensive control (n = 18) rats. Basal plasma renin activity was lower in L-NAME than control rats (5.0 +/- 0.3 versus 9.5 +/- 1.3 U, P < .01), and plasma renin activity was markedly attenuated at all comparable levels of renal perfusion pressure. Maximal plasma renin activity levels were achieved at perfusion pressures reduced to 65 mm Hg, and plasma renin activity averaged 14 +/- 2 and 34 +/- 7 U (P < .01) in L-NAME hypertensive and control rats, respectively. However, infusion of the nitric oxide donor sodium nitroprusside similarly stimulated plasma renin activity levels to 39 +/- 3 and 45 +/- 3 U (P > .05), in the hypertensive and normal control groups, respectively. Overall, these findings are consistent with the hypothesis that prolonged L-NAME administration attenuates pressure-dependent renin release by inhibiting nitric oxide formation, which may function as a paracrine mechanism inversely linking renal perfusion pressure with the stimulation of renin release.

Coordinate changes of renin and brain-type nitric-oxide-synthase (b-NOS) mRNA levels in rat kidneys

In our study we have examined the mRNA levels of nitric-oxide-(NO-)synthases in rat kidneys during states of stimulated and reduced renin gene expression, to find out whether renal mRNA levels of NO-synthases are correlated with the activity of the renin system. Stimulation of the renin system was achieved by unilateral renal artery clipping (2-kidney/1-clip rats), treatment with the angiotensin II (ANG II) antagonist losartan (40 mg/kg), application of furosemide (12 mg x kg-1 x day-1) and a low-sodium diet (0.02% w/w Na+), which increased renin mRNA levels to 464%, 495%, 309% and 219% of those of control animals, respectively. Inhibition of the renin system was achieved in the nonclipped (contralateral) kidneys of 2-kidney/1-clip rats and in the kidneys of rats which were fed a high-sodium diet (4% w/w Na+); in both cases renin mRNA levels decreased to about 50% of the control values. First screening of the gene expression of brain-type NO-synthase (b-NOS), endothelial NOS (e-NOS) and inducible NOS (i-NOS) during all these alterations of the renin system was done using the reverse transcriptase-polymerase chain reaction (RT-PCR) technique. Results from such noncompetitive PCR experiments indicated that only b-NOS mRNA levels change concordantly with the levels of renin. These changes in b-NOS mRNA levels were checked by the more reliable method of RNase protection assay. Results of the RNase protection assay proved that the renal levels of b-NOS mRNA were significantly increased by about 50% after a low-sodium diet and hypoperfusion of the kidney. Given a stimulatory role of endothelium-derived relaxing factor (EDRF)/NO on the renin system our findings may provide the first evidence that increases of renal levels of b-NOS mRNA and, as a consequence, of renal EDRF/NO formation could be important mediators of the well-known effect of salt intake and hypoperfusion on the renin system.

Blockade of nitric oxide formation inhibits the stimulation of the renin system by a low salt intake

This study aimed to investigate the possible involvement of endothelial autacoids such as nitric oxide or prostaglandins in the well-known stimulatory effect of a low salt intake on renin secretion and renin gene expression in the kidney. To this end, plasma renin activity (PRA) and kidney renin mRNA levels were determined in male Sprague-Dawley rats fed either a normal (0.6% w/w) or a low (0.03%) NaCl diet for 10 days. To inhibit nitric oxide formation, the animals received L-nitro-argininemethylester (L-NAME, 40 mg/ kg twice a day), to inhibit prostaglandin formation the animals received meclofenamate (8 mg/kg twice a day) during the last 2 days. In animals fed a normal salt diet, L-NAME decreased PRA from 6.5 to 4.9 ng angiotensin I x h(-1) x ml(-1) and decreased renin mRNA levels by about 15%. Meclofenamate did not change PRA or renin mRNA in animals fed on normal salt diet. In vehicle-treated animals fed a low salt diet, PRA increased from 6.5 to 20.2 ng ANGI x h(-1) x ml(-1) and renin mRNA levels increased by 100%. Meclofenamate treatment did not alter these changes of PRA and renin mRNA during the intake of a low salt diet. In animals treated with L-NAME, PRA increased to only 7.2 ng ANGI x h(-1) x ml(-1) and renin mRNA increased by 20%. These findings indicate that inhibition of nitric oxide formation but not of prostaglandin formation substantially attenuates the stimulatory effect of a low salt intake on the renin system, suggesting that nitric oxide is required for this process.

Regulation of renin release is impaired after nitric oxide inhibition

The aim of the present study was dual: first to establish that a preparation of afferent arterioles freshly isolated from the rat kidney is a suitable model to study renin release and synthesis, and second to investigate the effect(s) of nitric oxide (NO) inhibition on renin release in this model. Purification of renal microvessels was based on iron oxide infusion into the kidneys and separation of the afferent arterioles from glomeruli and connective tissue with a magnet. These microvessels express preprorenin mRNA, contain renin granules and release renin as evidenced by RT-PCR, immunocytochemistry and measurement of renin activity, respectively. Renin secretion was increased in isolated afferent arterioles after in vivo treatment with the diuretic furosemide (+300%) or in vitro treatment with the adenylyl cyclase activator forskolin (+50%), indicating that this vascular preparation responds appropriately to regulators of the renin-angiotensin system. Furthermore, in afferent arterioles isolated from control rats, renin release was positively correlated with total renin content (r = 0.85). In afferent arterioles isolated from rats chronically treated with the NO-synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME), forskolin was ineffective in modifying renin release despite stimulation of cAMP levels. In addition, the correlation between renin release and tissue renin content was disrupted. Similar results were obtained when cortical slices were used instead of afferent arterioles, suggesting that this defect in the regulation of renin release is independent of the presence of macula densa cells. To verify that the lack of regulation of renin release after L-NAME treatment was due to NO inhibition, the NO donor 3-morpholino-syndonimin-hydrochloride (SIN-1) was administered in afferent arterioles or cortical slices from kidneys of L-NAME-treated rats. In both preparations, SIN-1 reversed the L-NAME effect and re-established the responsiveness of renin release to forskolin and the relationship between renin release and renin content. These data indicate that the adenylyl cyclase-mediated mechanism regulating renin release is impaired when NO synthesis is inhibited.

Involvement of neurohumoral factors in the pressor mechanism of NG-nitro-L-arginine

The mechanism of the NG-nitro-L-arginine (L-NNA)-induced pressor response was examined in pentobarbital-anesthetized dogs. The pressor effect of L-NNA (50 mg/kg, i.v.) was significantly and equally diminished by pretreatment with either hexamethonium (25 mg/kg, i.v.) or phentolamine (5 mg/kg, i.v.). The intracisternal administration of L-NNA (1 mg/kg), which did not cause changes in cardiovascular parameters when administered systemically, produced a significant pressor response and tachycardia. Furthermore, significant suppression of L-NNA-induced pressor responses was observed after treatment of dogs with captopril (5 mg/kg, i.v.) or a non-peptide angiotensin II receptor antagonist, losartan (10 mg/kg, i.v.), or bilateral occlusion of renal veins. The inhibitory effects of hexamethonium and losartan were additive. These results suggest that, in addition to vasoconstriction due to the inhibition of endothelial nitric oxide production, increased activity of the sympathetic nervous and renin-angiotensin systems contributes significantly to the development of pressor responses produced by the intravenous injection of L-NNA in anesthetized dogs.

Tonic stimulation of renin gene expression by nitric oxide is counteracted by tonic inhibition through angiotensin II

This study was designed to examine the possible involvement of prostaglandins and nitric oxide (NO) in the renin stimulatory effect of angiotensin II (AngII) antagonists. To this end, plasma renin activities (PRAs) and renal renin mRNA levels were assayed in rats that were treated with the Ang-converting enzyme inhibitor ramipril or with the AngII AT1-receptor antagonist losartan. Ramipril and losartan increased PRA values from 7.5 +/- 1.6 to 86 +/- 6 and 78 +/- 22 ng of AngI per h per ml and renin mRNA levels from 112 +/- 9% to 391 +/- 20% and 317 +/- 10%, respectively. Inhibition of prostaglandin formation with indomethacin did not influence basal or ramipril-affected PRA. Basal renin mRNA levels also were unchanged by indomethacin, while increases in renin mRNA levels after ramipril treatment were slightly reduced by indomethacin. Inhibition of NO synthase by nitro-L-arginine methyl ester (L-NAME) reduced PRA values to 3.2 +/- 0.9, 34 +/- 13, and 12.1 +/- 2.7 ng of AngI per h per ml in control, ramipril-treated, and losartan-treated animals, respectively. Renin mRNA levels were reduced to 77 +/- 14% under basal conditions and ramipril- and losartan-induced increases in renin mRNA levels were completely blunted after addition of L-NAME. The AngII antagonists, furthermore, induced an upstream recruitment of renin-expressing cells in the renal afferent arterioles, which was also blunted by L-NAME. These findings suggest that renin mRNA levels are tonically increased by NO and that the action of NO is counteracted by AngII.

Nitric oxide and the control of renin secretion

Research during recent years has established nitric oxide as a unique signaling molecule that plays important roles in the regulation of the cardiovascular, nervous, renal, immune and other systems. Nitric oxide has also been implicated in the control of the secretion of hormones by the pancreas, hypothalamus, pituitary and other endocrine glands, and evidence is accumulating that it contributes to the regulation of the secretion of renin by the kidneys. The enzyme nitric oxide synthetase is present in vascular and tubular elements of the kidney, particularly in cells of the macula densa, a structure that plays an important role in the control of renin secretion. Guanylyl cyclase, a major target for nitric oxide, is also present in the kidney and is responsive to changes in nitric oxide levels. Drugs that inhibit nitric oxide synthesis generally suppress renin release in vivo and in vitro, suggesting a stimulatory role for the L-arginine-nitric oxide pathway in the control of renin secretion. Under some conditions, however, blockade of nitric oxide synthesis increases renin secretion. Recent studies indicate that nitric oxide not only contributes to the regulation of basal renin secretion, but also participates in the renin secretory responses to activation of the renal baroreceptor, macula densa and beta adrenoceptor mechanisms that regulate renin secretion. Future research should clarify the mechanisms by which nitric oxide regulates the secretion of renin and establish the physiological significance of this regulation.

Glomerular actions of nitric oxide

NO, a simple molecule synthesized from L-arginine by NO synthases, has been identified to play an important role in cell communication, cell defense and cell injury. The half life of NO is very short because NO either reacts with superoxide anion (O2-), and/or binds to heme molecules or Fe-S groups present in proteins. The biological effects of NO depend on both the concentration of NO at the site of action as well as upon the specific location where NO is generated. Small quantities of NO are generated by cNOS such as that present in the vascular endothelium, while large quantities of nitric oxide are synthesized by iNOS in response to cytokines or bacterial products. Within the kidney NO generated by endothelial cNOS participates in the regulation of the glomerular microcirculation by modifying the tone of the afferent arteriole and mesangial cells (Fig. 4). In addition, NO generated by macula densa and the afferent arteriole control glomerular hemodynamics via TGF and by modulating renin release. Therefore NO is important in the physiologic regulation of glomerular capillary blood pressure, glomerular plasma flow and the glomerular ultrafiltration coefficient. Through its actions on glomerular pressures and flows, NO may also regulate the macro- and micromolecular traffic through the mesangium. Chronic NO insufficiency causes hypertension and glomerular damage and may be causally involved in the genesis of salt dependent hypertension. Increased NO production may be involved in the early pathogenic hemodynamic changes in diabetes and in the physiologic hemodynamic responses to normal pregnancy. Maintenance of the antithrombogenic properties of the endothelium is another important action of NO which inhibits platelet aggregation and adhesion. Large quantities of NO such as that synthesized by either glomerular cells or macrophages during glomerular inflammation may lead to glomerular injury. A better understanding of the physiology and pathophysiology of NO in the kidney will lead to the development of new therapeutic avenues.

The influence of nitric oxide on tumour vascular tone

Acetylcholine and sodium nitroprusside, which vasodilate via release of NO by endothelium-dependent and endothelium-independent mechanisms respectively, had little effect on tumour vascular resistance when administered to tissue-isolated tumours perfused in their normal state. However, under phenylephrine-induced vasoconstriction, sodium nitroprusside induced vasodilation whilst acetylcholine induced a small vasoconstriction. Phenylephrine itself induced an oscillatory change in tumour perfusion pressure. The nitric oxide synthase (NOS) inhibitor N omega-nitro-L-arginine (L-NNA) caused a dose-dependent increase in vascular resistance in ex vivo perfused tumours which was greater than that in normal perfused hindlimbs. Systemic administration of L-NNA caused a 50% decrease in tumour blood flow which was a larger effect than in any of the normal tissues studied except spleen and skeletal muscle. Modification of NOS activity in tumours is a promising means for selective tumour blood flow modification. Investigation of endothelium-dependent versus endothelium-independent methods for modifying tumour blood flow may provide methods for further selectivity.

Endothelium-mediated regulation of renin secretion

The aim of the present study was to investigate the endothelial influence on renin secretion of isolated juxtaglomerular cells. Specifically the role of nitric oxide (NO) and of endothelin was studied. Coculture of primary cultures of juxtaglomerular cells with aortic and microvascular endothelial cells decreased renin secretion. Inhibition of NO formation by absence of l-arginine or presence of N omega-nitro-l-arginine caused a marked decrease in cGMP accumulation and a reduction in renin secretion in cocultures. Exogenous NO (NO liberators sodium nitroprusside/SIN 1) stimulated the 20-hour renin secretion from juxtaglomerular cells markedly, too. The effect of NO on renin secretion was biphasic: short-time inhibition and long-time stimulation of renin release. NO's stimulatory effect on renin secretion is dependent on extracellular calcium, but independent on cAMP or cGMP accumulation. Endothelin 1, 2, and 3 did not affect basal renin secretion, but inhibited cAMP stimulated renin release to a similar extent. Endothelin's action is not mediated via the subtype A endothelin receptor, but seems to involve calcium mobilization in juxtaglomerular cells that is dependent on extracellular calcium and associated with prominent calcium activated chloride channels. Taken together, coculture of juxtaglomerular cells with endothelial cells inhibits renin secretion despite the stimulatory effect of native NO released from endothelial cells. cAMP stimulated renin secretion is inhibited by all three endothelin isoforms thus contributing to the inhibition of renin secretion in coculture.

Cyclic GMP-linked pathway for renin secretion

The role of cGMP as a second messenger for renin secretion is contentious. This was investigated using a superfused collagenase-dispersed rat kidney cortex cell preparation devoid of indirect influences on renin secretion. Nitroprusside, atriopeptin II and 8-Br-cGMP all increased renin release but the dose-response relationships were biphasic. At low dose ranges there was a positive correlation between increasing drug concentration and renin secretion, but at high drug concentrations, a negative correlation was apparent. Methylene blue, a guanylate cyclase inhibitor, also suppressed baseline renin release at 10(-5) and 10(-6) M, but stimulated release at 10(-3) M. Using mid-range drug concentrations, the cGMP specific phosphodiesterase inhibitor MB22948 potentiated renin release in response to nitroprusside and 8-Br-cGMP. Inhibition of guanylate cyclase with either methylene blue or LY83583 attenuated renin release in response to nitroprusside, but, as expected, had no effect on 8-Br-cGMP induced release. We conclude that, under physiological conditions, cGMP is a stimulatory second messenger for renin release. This activity is mimicked at low dose ranges by 8-Br-cGMP, nitroprusside and atriopeptin II. In response to high doses of these drugs an unknown inhibitory pathway is activated and this opposes, in a dose-related manner, the stimulatory actions of cGMP for renin release.

Endothelium derived relaxing factor is involved in the pressure control of renin gene expression in the kidney

To study the influence of endothelium derived relaxing factor/nitric oxide (EDNO) on renin gene expression, the effects of a 2-day treatment with the NO-synthase inhibitor nitro-L-arginine-methylester (L-NAME, 40 mg/kg twice a day) on plasma renin activity (PRA) and renal and adrenal renin m-RNA levels were examined in conscious rats with and without unilateral renal clips (0.2 mm). In sham-clipped animals L-NAME led to a decrease of PRA from 7.5 to 2.5 ng angiotensin (ANGI).h-1.ml-1 and to a 35% decrease of renal renin m-RNA levels. Unilateral renal artery clipping increased PRA to 35 and to 13 ng ANGI.h-1.ml-1 in vehicle and in L-NAME-treated rats, respectively. In the clipped kidneys renin m-RNA levels increased to 450% of control values in vehicle-treated animals and to 220% of control values in L-NAME-treated animals. In the contralaterals as opposed to clipped kidneys, renin m-RNA levels decreased to 16% and 50% of the control values in vehicle- and in L-NAME-treated animals, respectively. In the adrenal glands renin m-RNA levels were not significantly changed either by clipping of one renal artery or by treatment of animals with L-NAME. The NO-donor sodium nitroprusside (100 microM) was found to increase renin secretion and renin m-RNA levels in primary cultures of renal juxtaglomerular cells. These findings suggest that EDNO is involved in the control of the renin gene by the renal perfusion pressure.

Role of calcium ions in the pressure control of renin secretion from the kidneys

In this study we examined the role of calcium ions in the control of renin release by the renal artery pressure. For this purpose renin secretion rates (RSR) were measured in isolated rat kidneys perfused at pressures of 140, 100, 80 and 40 mmHg (19, 13, 11, 5 kPa) with media containing either 1.5 mmol/l ("normal") or zero calcium concentrations (calcium-free perfusate with 0.5 mmol/l EGTA). At normal calcium the RSR was inversely related to the renal artery pressure, whereas calcium withdrawal resulted in an almost linear and proportional relationship between RSR and perfusion pressure. As a consequence, RSR at 140 mm Hg (19 kPa) with a calcium-free medium was similar to renin release at 40 mm Hg (5 kPa) with normal calcium. The nitric oxide (NO) donor sodium nitroprusside (1 mumol/l) stimulated RSR in a pressure-dependent fashion at a calcium concentration of 1.5 mmol/l. With a calcium-free perfusate, sodium nitroprusside did not restore the inverse pressure dependence of RSR seen with normal calcium but almost doubled the RSR across the whole pressure range. Whilst RSR was significantly reduced by angiotensin II (1 nmol/l) in the range between 40 mmHg and 140 mmHg (5-19 kPa) with normal calcium, withdrawal of extracellular calcium ions practically abolished the inhibitory action of angiotensin II. Since angiotensin II attenuated RSR especially at low renal perfusion pressure, our results indicate that renin release in this pressure range is still inhibitable by calcium mobilization in renal juxtaglomerular cells. Thus, the enhancement of renin secretion at lower pressures cannot be explained by a decreased sensitivity of renin release towards calcium ions.(ABSTRACT TRUNCATED AT 250 WORDS)

Interrelation between renin mRNA levels, renin secretion, and blood pressure in two-kidney, one clip rats

To examine the interrelation between renin mRNA levels, renin secretion, and blood pressure in rats, we clipped the left renal arteries of rats and measured renin mRNA levels in both kidneys, plasma renin activity, and blood pressure. One and 2 days after clipping, renin mRNA levels increased 3-fold and 4.3-fold in the stenosed kidney and were suppressed to 52% and 26% of controls in the intact kidneys; plasma renin activity increased from 8 to 16.5 and to 30.5 ng angiotensin I.h-1.mL-1 and systolic blood pressure rose from 114 to 123 and to 137 mm Hg. We found a strong correlation (P < .001) between plasma renin activity and renin mRNA levels in the clipped kidneys. We also found significant correlations (P < .05) between mRNA levels in the clipped and intact kidneys and between plasma renin activity and blood pressure for the individual animals. Treatment of normal rats with the converting enzyme inhibitor ramipril (5 mg/kg twice a day) for 2 days increased renin mRNA levels in both kidneys fourfold. In animals with unilateral clips, additional treatment with ramipril increased renin mRNA levels 6.4-fold in the stenosed and 3.3-fold in the intact kidneys. These findings suggest that endogenous angiotensin II exerts an inhibitory effect on renin mRNA expression in normal kidneys, clipped kidneys, and their contralaterals. Suppression of the renin gene in contralateral kidneys seems not to be directly mediated by the rise of plasma renin activity or by the rise of blood pressure in two-kidney, one clip rats.

Nitric oxide in the kidney: synthesis, localization, and function

The characterization and cloning of constitutive and inducible nitric oxide (NO)-synthesizing enzymes and the development of specific inhibitors of the L-arginine NO pathway have provided powerful tools to define the role of NO in renal physiology and pathophysiology. There is increasing evidence that endothelium-derived NO is tonically synthesized within the kidney and that NO plays a crucial role in the regulation of renal hemodynamics and excretory function. Bradykinin and acetylcholine induce renal vasodilation by increasing NO synthesis, which in turn leads to enhancement of diuresis and natriuresis. The blockade of basal NO synthesis has been shown to result in decreases of renal blood flow and sodium excretion. These effects are partly mediated by an interaction between NO and the renin angiotensin system. Intrarenal inhibition of NO synthesis leads to reduction of sodium excretory responses to changes in renal arterial pressure without an effect on renal autoregulation, suggesting that NO exerts a permissive or a mediatory role in pressure natriuresis. Nitric oxide released from the macula densa may modulate tubuloglomerular feedback response by affecting afferent arteriolar constriction. Nitric oxide produced in the proximal tubule possibly mediates the effects of angiotensin on tubular reabsorption. In the collecting duct, an NO-dependent inhibition of solute transport is suggested. The L-arginine NO pathway is also active in the glomerulus. Under pathologic conditions such as glomerulonephritis, NO generation is markedly enhanced due to the induction of NO synthase, which is mainly derived from infiltrating macrophages. An implication of NO in the mechanism of proteinuria, thrombosis mesangial proliferation, and leukocyte infiltration is considered. In summary, the data presented on NO and renal function have an obvious clinical implication. A role for NO in glomerular pathology has been established. Nitric oxide is the only vasodilator that closely corresponds to the characteristics of essential hypertension. Using chronic NO blockade, models of systemic hypertension will provide new insights into mechanisms of the development of high blood pressure.

Effect of endothelium-derived relaxing factor on renin secretion from isolated mouse renal juxtaglomerular cells

This study aimed to examine the direct influence of native endothelium derived relaxing factor (EDRF) on renin secretion. To this end isolated mouse renal juxtaglomerular cells were cocultured with bovine aortic endothelial cells which produced and released significant amounts of EDRF as assayed by guanylate cyclase activities which were measured separately in endothelial and juxtaglomerular cells as well as in the cocultures of juxtaglomerular with endothelial cells. EDRF production was blunted in the absence of extracellular L-arginine and in the presence of N omega-nitro-L-arginine (L-NAG; 200 microM). Inhibition of endothelial EDRF production by removal of arginine or addition of L-NAG was associated with a significant decrease of renin secretion from the cocultures while the same regimen had no effect on renin secretion from JG cells alone. Exogenous generation of nitric oxide by the addition of sodium nitroprusside (100 microM) stimulated renin secretion in the cocultures both at normal and inhibited EDRF formation as well as from juxtaglomerular cells alone. These findings suggest that native EDRF released from vascular endothelial cells is a stimulatory signal for renin secretion from renal juxtaglomerular cells.

Eicosanoid-dependence of responses of pre- but not postglomerular vessels to noradrenaline in rat isolated kidneys

1. We investigated the role of nitric oxide (NO) and of vasoactive eicosanoids in the control of renal vascular resistance (RVR) and glomerular filtration rate (GFR) and of their responses to noradrenaline (NA). This study was conducted in single-pass perfused, isolated kidney preparations of the rat. 2. NA (63, 110 and 160 nM) dose-dependently increased RVR and to a lesser degree GFR. 3. In baseline conditions, N omega-nitro-L-arginine methylester (L-NAME, 100 microM) increased GFR more than RVR, thus demonstrating a basal release of NO which predominates in postglomerular vessels. 4. In kidneys stimulated with NA, L-NAME potentiated the increases in RVR but not in GFR. Indomethacin (1.5, 150 nM and 15 microM) did not alter GFR but markedly and dose-dependently reduced the NA-induced increase in RVR. Similar results were obtained with GR 32191B (10 and 100 microM), a prostaglandin H2/thromboxane A2 (PGH2/TxA2) receptor antagonist. 5. Indomethacin (15 microM) suppressed the enhancing effects of L-NAME on RVR responses to NA but did not affect those on GFR. 6. It is concluded that the mechanisms of the response to NA differ among pre- and postglomerular vessels. In preglomerular vessels the vasoconstrictor action and the NO release depend upon the activation of PGH2/TxA2 receptors, while both are eicosanoid-independent in the postglomerular vessels.

Cardiac weight in hypertension induced by nitric oxide synthase blockade

Wistar rats given a nitric oxide synthase inhibitor, NG-nitro-L-arginine-methyl ester (L-NAME), for 4 weeks develop time- and dose-dependent hypertension without cardiac hypertrophy. This initial study of the relation between left ventricular weight and L-NAME-induced hypertension has now been extended by giving 50 mg/kg per day L-NAME to Wistar rats (n = 30) for 8 weeks and comparing results with those from control rats (n = 10) and two-kidney, one clip rats (n = 14). Although L-NAME rats and two-kidney, one clip rats had increased systolic blood pressures during the last 3 weeks of the experiment (202 +/- 24 and 224 +/- 16 mm Hg, respectively), the ratio of left ventricular weight to body weight of L-NAME rats (2.12 +/- 0.32 mg/g) was not statistically different from that of control rats (1.93 +/- 0.13 mg/g), whereas that of two-kidney, one clip rats was increased (2.85 +/- 0.20 mg/g). The plasma renin activity of L-NAME rats was not significantly different from that of control rats. Two L-NAME rat subgroups were defined according to the presence of left ventricular hypertrophy (ratio of left ventricular weight to body weight > 2.19 mg/g, control mean +2 SD) (6 of 25) or its absence (19 of 25). Systolic blood pressure, plasma renin activity, and cardiac angiotensin converting enzyme activity of L-NAME rats with left ventricular hypertrophy were significantly higher than those of the subgroup without.(ABSTRACT TRUNCATED AT 250 WORDS)

Temporal evolution of endothelial dysfunction in a rat model of chronic heart failure

OBJECTIVES

The goal of this study was to investigate the evolution of endothelial dysfunction and plasma renin activity in a rat model of heart failure.

BACKGROUND

Endothelial dysfunction has been demonstrated in heart failure and may play a significant role in this pathophysiologic process. Studies have also suggested a physiologic interaction between the renin-angiotensin system and endothelium-derived relaxing factor. However, the evolution of endothelial dysfunction and plasma renin activity in heart failure has not been studied to date.

METHODS

Endothelium-dependent and -independent relaxations were studied at 1, 4 and 16 weeks after coronary artery ligation in a rat model of heart failure. Thoracic aortic rings were placed in isolated organ baths and acetylcholine and sodium nitroprusside concentration response curves were generated. Plasma renin activity was assessed at each time point.

RESULTS

Aortic rings from rats with heart failure demonstrated no evidence of endothelial dysfunction at week 1, although progressive rightward shifts in the acetylcholine curves and decreasing maximal relaxation over time compared with findings in sham-operated control rats were evident at weeks 4 and 16. The sodium nitroprusside curves were not different between rats with heart failure and sham-operated rats. Plasma renin activity was elevated at week 1 and progressively increased through week 16, even though it correlated poorly with endothelial dysfunction.

CONCLUSIONS

This study suggests that endothelial dysfunction in heart failure is a progressive time-dependent process that probably plays a minor role early in heart failure. Although plasma renin activity increased significantly in rats with heart failure, it was poorly predictive of endothelial dysfunction.

Effect on renin release of inhibiting renal nitric oxide synthesis in anaesthetized dogs

Nitric oxide plays an important role in the regulation of basal renal blood flow. This study was performed to examine whether selective inhibition of renal nitric oxide synthesis affects renin release in vivo. Accordingly, in six barbiturate-anaesthetized dogs renin release was examined before and after intrarenal infusion of the selective inhibitor of nitric oxide synthesis, NG-nitro-L-arginine (NOARG). NOARG was infused into the renal artery to yield a renal arterial blood concentration of 0.4 mumol ml-1. NOARG did not change systemic arterial blood pressure and glomerular filtration rate, but reduced basal renal blood flow by 26 +/- 2%. Urine flow, sodium and potassium excretion were reduced after inhibition of renal nitric oxide synthesis. Basal renin release (3 +/- 2 micrograms AI min-1) was not altered by NOARG infusion (1 +/- 1 micrograms AI min-1). To stimulate renin release the renal artery was constricted to a renal perfusion pressure of 50 mmHg. At this perfusion pressure infusion of NOARG reduced renin release significantly from 48 +/- 11 micrograms AI min-1 to 14 +/- 4 micrograms AI min-1. In conclusion, inhibition of renal nitric oxide synthesis reduces basal renal blood flow and reduces renin release stimulated by renal arterial constriction. These findings indicate that renal nitric oxide modulates both renal blood flow and renin release in vivo.

Involvement of endothelium-derived relaxing factor in the pressure control of renin secretion from isolated perfused kidney

Using isolated rat kidneys perfused at controlled pressure, we examined a potential role of endothelium-derived relaxing factor (EDRF) in the pressure control of renin secretion. We found that stimulation of EDRF release by acetylcholine (1 mumol/liter) increased mean perfusate flow rates from 15.0 +/- 0.5 to 18.0 +/- 0.5 ml/min per g and average renin secretion rates from 3.5 +/- 0.5 to 16.0 +/- 2.0 ng angiotensin I/h per min per g at a perfusion pressure of 100 mmHg (mean +/- SEM, n = 6). Those effects of acetylcholine were significantly reduced during inhibition of EDRF formation with NG-nitro-L-arginine (100 mumol/liter), but they were not affected with the cyclooxygenase inhibitor indomethacin (10 mumol/liter). Lowering of the perfusion pressure from 100 mmHg to 40 mmHg resulted in an increase of average renin secretion rates from 3.5 +/- 0.5 to 79 +/- 12 ng AngI/h per min per g under control conditions (n = 8), and to 171 +/- 20 ng AngI/h per min per g in the presence of 10 mumol/liter acetylcholine (n = 3). The rise of renin secretion in response to a reduction of the renal artery pressure was markedly attenuated with inhibitors of EDRF formation such as NG-nitro-L-arginine (100 mumol/liter) and related compounds. During inhibition of EDRF formation, addition of sodium nitroprusside (10 mumol/liter) increased mean perfusate flow rates from 12.0 +/- 0.5 to 23.0 +/- 2.0 ml/min per g and average renin secretion rates from 2.0 +/- 0.5 to 18.0 +/- 1.5 ng AngI/h per min per g at 100 mmHg (n = 5). Lowering of the perfusion pressure from 100 mmHg to 40 mmHg under those conditions increased average renin secretion rates to 220 +/- 14 ng AngI/h per min per g (n = 5). Taken together, our findings suggest that EDRF and related activators of soluble guanylate cyclase stimulate renin secretion from isolated kidneys, predominantly at lower perfusion pressure. Moreover, pressure control of renin secretion appears to require the tonical stimulation by intrarenal EDRF.

Determinants of aortic cyclic guanosine monophosphate in hypertension induced by chronic inhibition of nitric oxide synthase

Nitric oxide (NO) and atrial natriuretic factor (ANF) cause vascular relaxation by generating cyclic guanosine monophosphate (cGMP) via activation of the soluble and particulate guanylate cyclases, respectively. The chronic effects of NG-nitro-L-arginine methyl ester (L-NAME), an L-arginine antagonist and NO synthase inhibitor, on the blood pressure and plasma and aortic cGMP levels of rats were tested. Wistar rats (n = 10 per group) were given doses of L-NAME (0, 1, 5, 10, 20, 50, and 100 mg/kg.d) by gavage twice a day for 4 wk. Chronic L-NAME induced a time- and dose-dependent increase in blood pressure. The total heart weight/body weight ratio did not change in any group, despite the hypertension. The plasma levels of cGMP did not change significantly in any group, and were correlated with the plasma ANF levels (r = 0.51, P less than 0.0001). Aortic cGMP decreased in negative correlation with increasing L-NAME from 0 to 10 mg/kg.d, culminating in a 10-fold drop arterial wall cGMP. The aortic cGMP content of rats in the four highest dose groups (from 10 to 100 mg/d) tended to increase slightly and was positively correlated with endogenous ANF (r = 0.48, P less than 0.002, n = 40). Intravenous L-arginine decreased arterial blood pressure and reversed the decline in aortic cGMP. Exogenous ANF and sodium nitroprusside both significantly increased aortic cGMP. Neither the arterial wall concentrations of cGMP-dependent kinase nor cAMP was changed by L-NAME. Thus, chronic blockade of NO synthase with L-NAME induces a dose-dependent increase in blood pressure and decrease in aortic cGMP. The in vivo basal aortic cGMP seems to be mainly dependent on NO synthase: soluble guanylate cyclase activity and to a minor extent on particulate guanylate cyclase activity.