Renal medullary nitric oxide synthase activity in rats on low and high salt diets

We previously determined by Western blotting that the levels of neural, inducible, and endothelial nitric oxide synthase (NOS) in the renal inner medulla arc elevated in rats placed on a high sodium diet (Hypertension 26:570, 1995). In the present studies we have used a NOS enzyme assay to determine total NOS acti\ ily in the tissue of Sprague Dawley rats maintained on a low (0.4% NaCl) or high (4.0% NaCl) salt diet. Total tissue protein homogenate was incubated with ³H-Arginine and appropriate cofactors, and the arginine and converted citrulline were separated by reverse phase HPLC. Using this assay, NOS activity in rat cerebellar tissue homogenate followed first-order kinetics with a K_m=5.5 μM and a V_max=1.28 pinoles citrullme/min/wg protein. Subsequent studies showed that the total NOS activity in the renal inner medulla, outer medulla, renal cortex, and aorta were 7%, 5%, 0.2% and 0.!5%, respectively, of that found in the cerebellum. In the present experiments total NOS activity was increased by 41% in the renal inner medulla of high salt rats (N=5) when compared to age-matched rats maintained on a low salt diet (N=5). No differences in NOS activity were detected in the renal outer medulla of the low and high salt rats. These data confirm our previous protein blotting data and indicate that the renal medullary NOS system may be important in the chronic adaptation to a high sodium diet, (supported by HL 29587 and AHA 95-GB-761.