Influence of dietary sodium intake on renal medullary nitric oxide synthase

We previously reported that chronic systemic treatment of rats with a nitric oxide synthase inhibitor leads to a selective decrease in renal medullary blood flow, retention of sodium, and the development of hypertension. In the present studies, we used protein blotting techniques to determine the whole tissue distribution and relative quantitation of the different nitric oxide synthase isoforms in the renal cortex and medulla of Sprague-Dawley rats maintained on a low (0.4% NaCl) or high (4.0% NaCl) dietary salt intake. Neural, endothelial, and inducible nitric oxide synthase were readily detectable in homogenized renal inner and outer medullas. Only endothelial nitric oxide synthase was detectable in the renal cortex. Densitometric comparison of Western blots from equal amounts of total inner medullary tissue protein indicated that endothelial, inducible, and neural nitric oxide synthase were increased by 145%, 49%, and 119%, respectively, in rats maintained on a high NaCl diet compared with rats on a low NaCl diet. No significant differences in nitric oxide synthase levels were detected in the outer medulla, renal cortex, or aorta of rats maintained on low and high NaCl diets. In separate studies, continuous intravenous infusion of N(G)nitro-L-arginine methyl ester (8.6 mg/kg per day) for 11 days in chronically instrumented rats increased mean arterial pressure 32 ± 3 mm Hg in rats on a high NaCl diet (n = 5) but only increased pressure 17 ± 3 mm Hg in rats on a low NaCl diet (n = 6). These data indicate that increased levels of renal medullary nitric oxide synthase may be important in the chronic adaptation to increased sodium intake.