Potentiation by enalaprilat of fenoldopam-evoked natriuresis is due to blockade of intrarenal production of angiotensin-II in rats

Abstract

We have previously shown that the natriuretic response to DA-1 receptor agonist fenoldopam is markedly potentiated by angiotensin converting enzyme inhibitor captopril. Since inhibition of angiotensin converting enzyme can lead to decreased production of angiotensin-II and increased levels of kinins (e.g., bradykinin), it is likely that both of these mechanisms might be involved in this phenomenon. However, it is not known whether and to what degree the accumulation of kinins contributes to the overall potentiation of natriuretic response to fenoldopam seen during angiotensin converting enzyme inhibition. In the present study, we have examined the effect of angiotensin converting enzyme inhibitor enalaprilat and angiotensin-II receptor antagonist losartan as well as bradykinin-2 receptor antagonist HOE 140 on fenoldopam-induced natriuresis. Intravenous infusion of fenoldopam (1 μg/kg/min) for 30 min produced significant increases in urine output and urinary sodium excretion without causing any changes in glomerular filtration rate, renal blood flow and mean arterial blood pressure, a phenomenon suggestive of a direct tubular site of action. In animals treated with either the angiotensin converting enzyme inhibitor enalaprilat or angiotensin-II receptor antagonist losartan, the diuretic and natriuretic effects of fenoldopam were potentiated to a similar degree. Whereas no significant changes in glomerular filtration rate occurred when fenoldopam alone was given to control rats, in animals treated with either enalaprilat or losartan, fenoldopam produced a modest but significant increase in glomerular filtration rate. In a separate group of animals, the effects of bradykinin-2 receptor antagonist HOE 140 on potentiation of fenoldopam-induced natriuresis by enalaprilat was examined. It was found that the magnitude of changes in urine output, sodium excretion and glomerular filtration rate in animals receiving pretreatment with both enalaprilat and HOE 140 were similar to those with enalaprilat pretreatment alone. These findings suggest that the potentiation of natriuretic response to fenoldopam by enalaprilat is solely due to blockade of angiotensin-II production and kinins do not appear to contribute to this phenomenon.