Cardiovascular effects of chronic inhibition of nitric oxide synthesis and dietary salt in spontaneously hypertensive rats

Abstract

The cardiovascular effects of chronic inhibition of nitric oxide synthesis and dietary salt were studied in 9-wk-old spontaneously hypertensive rats (SHR). Nω-nitro-L-arginine methyl ester (L-NAME, 0.025% in food, about 20 mg/kg/d) was given to rats receiving diets containing low, moderate, and high salt levels (NaCI 0.2%, 1.1%, and 6.0% of the dry weight of the chow) for 3 wk. L-NAME increased systolic blood pressure by 50 to 60 mmHg in all treated groups, as compared with an average rise of 10 to 20 mmHg in the control SHR. The high-salt diet did not further increase blood pressure. L-NAME also induced cardiac and renal hypertrophy, and these changes were aggravated by the high-salt diet. In addition, 19 of the 30 rats treated with L-NAME suffered strokes and all of them had several myocardial infarctions and renal damage, while the rats not treated with L-NAME had no evidence of stroke or myocardial or renal injury. Responses of mesenteric arterial rings in vitro were studied at the end of the experiment. The vascular contractile responses to noradrenaline were increased, and the relaxation responses to acetylcholine were inhibited in the L-NAME treated groups. In addition, the high-salt diet alone tended to inhibit the response to acetylcholine. Plasma renin activity was markedly increased by L-NAME treatment and decreased by the high-salt diet. The 24-h urine protein excretion was increased both by the L-NAME treatment and by the high-salt diet. The combination of L-NAME and the high-salt diet markedly raised the serum creatinine concentration. Our findings show that the coronary and renal functions are particularly vulnerable in SHR during impaired nitric oxide synthesis, and that the end-organ damage is worsened by an increased intake of dietary salt. We suggest that dysfunction of the endothelium is the primary cause of the effects observed in this study.