This study aimed to characterize the cellular pathways along which nitric oxide (NO) stimulates renin secretion from the kidney. Using the isolated perfused rat kidney model we found that renin secretion stimulated 4-to 8-fold by low perfusion pressure (40 mmHg), by macula densa inhibition (100 mol͞liter of bumetanide), and by adenylate cyclase activation (3 nmol͞liter of isoproterenol) was markedly attenuated by the NO synthase inhibitor nitro-L-arginine methyl ester (L-Name) (1 mM) and that the inhibition by L-Name was compensated by the NO-donor sodium nitroprusside (SNP) (10 mol͞liter). Because renal juxtaglomerular (JG) cells as the organism's main site of renin production are surrounded by cells with a high capacity for NO formation such as endothelial cells and macula densa cells (1, 2) the influence of NO on renin expression and renin secretion has been investigated in a series of in vivo and in vitro experiments. With only one exception (3), renin secretion and renin gene expression in vivo have been found to be attenuated by NO synthase inhibitors in conscious (4-7) and anesthetized animals (8-10) and in isolated perfused kidneys (11). Conversely, NO donors were reported to stimulate renin secretion in vivo and in isolated kidneys (11-14), suggesting that on the level of the kidney NO acts as a stimulator of renin secretion. The pathways along which NO could stimulate renin secretion are poorly understood. The best-characterized general intracellular signaling pathway of NO comprises the stimulation of soluble guanylate cyclase, leading to an increase of intracellular cGMP levels and to the induction of cGMP-induced reactions (15). Surprisingly, activators of cGMP-dependent protein kinases inhibit renin secretion not only in isolated JG cells but also in isolated perfused kidneys and even inhibit NO-stimulated renin secretion there (14,16,17), suggesting (i) that G-kinase activation unlikely accounts for the stimulatory effect of NO on renin secretion and (ii) that NO could, in principle, also inhibit renin secretion via G-kinase activation. In fact, for more isolated experimental preparations such as kidney slices or isolated JG cells not only stimulation but also inhibition of renin secretion by NO has been reported (18-23). A possible explanation for such divergent effect of NO in intact kidneys and in isolated JG cells could be a complex signaling mechanism that also depends on factors different from NO itself.In this context we recently found that the stimulatory effect of NO on renin secretion in isolated perfused kidneys was markedly attenuated when cAMP-dependent kinase was inhibited (14). Because cAMP is the best-established second messenger stimulating renin secretion from JG cells, it is conceivable that a linkage of NO with the cAMP pathway could account for the stimulatory effect of NO on renin secretion. In principle, NO could cause a stimulation of A kinase either through a transactivation of the kinase by cGMP (24) or through a cGMP-induced inhibition of cAMP degradation via cAM...

This study sought to examine the involvement of prostaglandins and of nitric oxide (NO) in the macula densa-dependent activation of the renin system in vivo. For this purpose, male Sprague-Dawley rats were chronically infused with furosemide (12 mg/day) for 6 days to inhibit macula densa salt transport. To inhibit the synthesis of prostaglandins and of NO, animals were injected with indomethacin (2 mg/kg twice daily) and with nitro-L-arginine methyl ester (L-NAME; 40 mg/kg twice daily) for the last 2 days of the experiment, respectively. Furosemide infusion increased plasma renin activity (PRA) from 8.8 +/- 1.4 to 41 +/- 5.2 ng angiotensin I (ANG I).h-1.ml-1 and renin mRNA levels from 112 +/- 8 of standard to 249 +/- 18% of standard. After treatment with indomethacin, the furosemide-induced increases in renin mRNA levels was attenuated to 190 +/- 11% of standard. After injections of L-NAME, both the furosemide-induced increases of renin mRNA levels and of PRA were reduced to 126 +/- 14% of standard and 22 +/- 5 ng ANG I.h-1.ml-1, respectively. These findings suggest that activation of renin gene expression by blockade of the macula densa function is dependent on intact NO and prostaglandin formation, whereas for stimulation of renin secretion mainly intact NO formation appears to be necessary.

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.

This study was designed to examine the possible involvement of prostaglandins and nitric oxide (NO) in the renin stimulatory effect ofangiotensin 11 (AngII) antagonists. To this end, plasma renin activities (PRAs) and renal renin mRNA levels were assayed in rats that were treated with the Angconverting enzyme inhibitor ramipril or with the AnglI 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%o and 317 ± 10%o, respectively. Inhibition of prostaglandin formation with indomethacin did not influence basal or ramiprilaffected 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 AngIH antagonists, furthermore, induced an upstream recruitment of reninexpressing 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.One of the most prominent in vivo effects of angiotensin (Ang)-converting enzyme (ACE) inhibitors is a marked stimulation of renin secretion and renin gene expression in the kidneys (1-5). Since ANGII is known as a negative feedback regulator of renin secretion and of renin gene expression (6, 7), it is thought that the renin stimulatory effects of ACE inhibitors are related to the inhibition of AnglI formation and consequently to the lowering of circulating and tissue AnglI levels (8).There is evidence that several in vivo effects of ACE inhibitors are not, as originally thought, due to the inhibition of AnglI formation but to the inhibition of kinin degradation (9-12). Inhibition of kinin degradation results in elevated tissue levels of autacoids such as prostaglandins and nitric oxide (NO), the formation of which is stimulated by kinins (11)(12)(13)(14). Although NO is reported to inhibit the renin system (15-17), contradictory findings suggest that NO stimulates the renin system (18-25) and also that prostaglandins stimulate the renin system (for review, see ref. 26). With such a stimulatory effect of NO and prostaglandins on the renin system, it is possible that the renin stimulatory effect of ACE inhibitors could be related to an enhanced formation of prostaglandins and NO.To investigate the mode of action of ACE inhibitors on the renin system, we studied renin release and renin mRNA levels in the kidneys of conscious rats that were treated with the ACE inhibitor ramipril (27) or with the Angll AT1-rec...

This study aimed to characterize the cellular pathways along which nitric oxide (NO) influences the secretion of renin from the kidney. Using the isolated perfused rat kidney model, we found that the NO donor sodium nitroprusside (SNP) (1–30 μmol/l) induced a prompt, concentration-dependent fourfold increase of basal renin secretion. The membrane-permeable cGMP analogs 8-bromo-cGMP and 8-(4-chlorophenylthio)-cGMP (8-pCPT-cGMP; each 5–50 μmol/l) inhibited basal renin secretion and attenuated the stimulation of renin secretion by SNP. Conversely, the renin stimulatory effect of SNP was enhanced in the presence of the G kinase inhibitor Rp-8-CPT-cGMPS (10 μmol/l). The renin stimulatory effect of SNP was amplified in nominally calcium-free perfusate and was abolished in the presence of angiotensin II (1 nmol/l). Renin secretion stimulated by SNP was clearly attenuated by the A kinase inhibitor Rp-8-CPT-cAMPS (25 μmol/l). These findings indicate that the renin stimulatory effect of NO donors in renal juxtaglomerular cells cannot be explained by activation of G kinase and is also less likely to be causally related to the regulation of renin secretion by calcium. Because A kinase activity is required for the stimulation of renin secretion by SNP, it appears as if the renin stimulatory effect is causally related to the cAMP pathway controlling renin secretion.