Lack of a Role of Neuronal Nitric Oxide Synthase in the Regulation of the Renal Function in Rats Fed a Low-Sodium Diet

Abstract

Background/Aims: It has been shown that nitric oxide (NO) generated from neuronal NO synthase (nNOS) counteracts angiotensin II mediated vasoconstriction in the pre- and in the postglomerular microcirculation. Previous studies have demonstrated that the nNOS expression in the macula densa of the renal cortex is enhanced by dietary salt restriction. In view of the well-known fact that dietary salt restriction also leads to an activation of the renin-angiotensin system, the present study was performed to assess the role of nNOS-derived NO in the regulation of the renal function in rats maintained on control (C) and low-salt (LS) diets. Methods: Groups of rats were fed either the C or the LS diet. On day 13 after adaptation to the appropriate diet, renal clearance studies were performed to determine the effects of acute nNOS inhibition either by S-methyl-L-thiocitrulline (L-SMTC) or by N^ω-propyl-L-arginine (L-NPA) on renal hemodynamics and sodium excretory function. In separate groups of rats maintained on either the C or the LS diet, the mRNA levels of nNOS and of renin in the renal cortex were examined using the semiquantitative reverse-transcriptase polymerase chain reaction. Results: Intrarenal infusion of vehicle (0.9% saline; 4 µl/min) did not change glomerular filtration rate (GFR), renal plasma flow (RPF), or sodium excretion in either C diet or LS diet rats. Acute intrarenal infusion of L-SMTC (0.3 mg/h) and L-NPA (0.01 mg/h) decreased GFR (–14 ± 5 vs. –13 ± 3%), RPF (–19 ± 6 vs. –17 ± 5%), and sodium excretion (–17 ± 5 vs. –16 ± 4%) in C diet rats as compared with control values (p < 0.05). In contrast, in LS rats, intrarenal administration of either L-SMTC or L-NPA did not cause significant changes in GFR, RPF, and sodium excretion. Furthermore, the mRNA expression for nNOS in the renal cortex was moderately increased in LS rats as compared with C rats (densitometric ratios of nNOS mRNA/GAPDH mRNA 0.31 ± 0.01 vs. 0.22 ± 0.04, p < 0.05), in parallel with the renin expression (renin mRNA/GAPDH mRNA ratios 1.4 ± 0.2 vs. 1.0 ± 0.1, p < 0.05). Conclusions: These results indicate that in normotensive rats kept on a normal salt intake nNOS-derived NO modulates both afferent and efferent arteriolar tones. In contrast, rats on an LS diet exhibit an impaired renal vascular responsiveness to nNOS-derived NO or an impaired ability to release NO by nNOS despite enhanced expression of nNOS mRNA in the renal cortex. In addition, the lack of effect of acute nNOS inhibition on renal function suggests that NO derived by nNOS does not participate in counteracting the vasoconstrictor influences of elevated circulating and/or intrarenal angiotensin II levels on pre- and postglomerular microcirculation in rats on an LS diet.