Evidence for a direct stimulatory effect of prostacyclin on renin release in man

THE OBJECTIVES OF THIS INVESTIGATION WERE: (a) to characterize the time and dose dependence of the effects of prostacyclin (PGI(2)) on renin release in healthy men; (b) to define whether PGI(2)-induced renin release is secondary to hemodynamic changes; (c) to determine the plasma and urine concentrations of 6-keto-PGF(1alpha) (the stable breakdown product of PGI(2)) associated with renin release induced by exogenous or pharmacologically enhanced endogenous PGI(2). Intravenous PGI(2) or 6-keto-PGF(1alpha) infusions at nominal rates of 2.5, 5.0, 10.0, and 20.0 ng/kg per min were performed in each of six normal human subjects; in three of them, PGI(2) infusion was repeated after beta-adrenergic blockade and cyclooxygenase inhibition. PGI(2), but not 6-keto-PGF(1alpha), caused a time- and dose-dependent increase of plasma renin activity, which reached statistical significance at 5.0 ng/kg per min and was still significantly elevated 30 min after discontinuing the infusion. Although combined propranolol and indomethacin treatment significantly enhanced the hypotensive effects of infused PGI(2), it did not modify the dose-related pattern of PGI(2)-induced renin release. Plasma 6-keto-PGF(1alpha) levels rose from undetectable levels (<7.5 pg/ml) in a stepwise fashion during increasingly higher infusion rates of PGI(2) or 6-keto-PGF(1alpha). The threshold concentration of plasma 6-keto-PGF(1alpha) associated with a statistically significant stimulation of renin release was approximately 200 pg/ml. Upon discontinuing PGI(2) or 6-keto-PGF(1alpha) infusion, the disappearance of 6-keto-PGF(1alpha) from blood showed an identical biphasic behavior, the initial phase having an apparent t((1/2)) of 3.2 min. The intravenous infusion of furosemide, which is known to stimulate renin release via a cyclooxygenase-dependent mechanism, caused a three-to fourfold increase of urinary 6-keto-PGF(1alpha) excretion rate, concomitant with the elevation of plasma renin activity levels, in six healthy women. 6-Keto-PGF(1alpha) remained undetectable in peripheral venous plasma throughout the study. WE CONCLUDE THAT IN HUMAN SUBJECTS: (a) PGI(2)-induced renin release occurs with a dose and time dependence similar to its reported platelet effects; (b) PGI(2)-induced renin release is not mediated by adrenergic stimuli or cyclooxygenase-dependent mechanisms secondary to hemodynamic changes; (c) furosemide-induced renin release is associated with increased renal PGI(2) formation; and (d) PGI(2) appears to act as a local modulator rather than a circulating hormone in controlling juxtaglomerular function.