Inhibitory effect of captopril on renal responses to frusemide in sodium-restricted rats

Indomethacin (10 mg kg-1 orally) caused a moderate inhibition of the renal responses to frusemide (30 mg kg-1 orally) only in Na+-deficient rats, suggesting that renal prostaglandins (PG) are necessary for optimal effects of frusemide during Na+ restriction. Captopril (1, 3 and 10 mg kg-1 orally) also inhibited frusemide-induced diuresis and natriuresis in Na+-deficient rats; the large effect of captopril at 10 mg kg-1 was accompanied by arterial hypotension. In normal rats, captopril did not disturb blood pressure or affect the renal effects of frusemide. By comparison, minoxidil (10 and 20 mg kg-1 orally) caused hypotension and reduced the natriuretic effects of frusemide in both normal and low-Na+ states. Since circulating angiotensin II (AII) is a stimulus for PG synthesis during Na+ restriction, it is suggested that captopril may impair the renal responses to frusemide through hormonal and haemodynamic changes resulting from inhibition of A II formation.