Nitric oxide synthase activity in renal cortex and medulla of normotensive and spontaneously hypertensive rats

The medullary portion of the kidney plays a crucial role in the control of sodium and water excretion and arterial pressure. This control is anomalous in hypertension and may be related to an impaired renal nitric oxide (NO) production. We have measured the activity of NO synthase (NOS) in the renal medulla, renal cortex, heart, and aorta from normotensive Wistar Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). Enzyme activity was determined by measuring the conversion of 4C-C-L-arginine to 4C-C-L-citrulline. Ca+-d-dependent NOS activity was considerably higher in the renal medulla than in the other tissues studied, both in WKY and SHR. The medulla and heart of the SHR displayed a higher Ca+-d-dependent NOS activity compared to that of WKY. No differences were found in the Ca+-i-independent NOS activity, except for the renal cortex of the SHR, which was higher than in the rest of the tissues. These observations indicate that the renal medulla has a higher relative capacity to synthesize NO and suggest that the impaired renal medullary control of arterial pressure of genetic hypertension is not due to a reduced NO production by the kidney.