Dissociation of the effects of alpha 1-adrenergic blockade on blood pressure and renin release in patients with essential hypertension.

Prazosin, a selective antagonist of postsynaptic alpha-adrenoreceptors, was used to investigate the influence mediated by the juxtaglomerular alpha-adrenoreceptors on renin release in man. We studied, in seven patients with essential hypertension, the acute effects of 0.25 mg prazosin, given intravenously, on blood pressure and plasma renin activity, the degree of alpha-blockade induced by the drug being assessed by comparing the increments in blood pressure following a test dose of phenylephrine before and after prazosin administration. We also measured the increments in plasma renin activity in response to beta-adrenergic stimulus consisting of an isoproterenol challenge, before and during the prazosin induced alpha blockade. Prazosin infusion caused, within 20 min, a marked reduction of the pressor response to phenylephrine, a significant increment in plasma renin activity, and no change in blood pressure. The increments in renin in response to isoproterenol were significantly greater, both in absolute and percent values, after rather than before prazosin. These results indicate that the increase in renin during systemic alpha 1-adrenoreceptor blockade may be independent of the fall in blood pressure and support the view that the juxtaglomerular alpha 1-adrenoreceptors participate in the regulation of renin release with an inhibitory action, which antagonizes the stimulating influence of the beta-adrenoreceptors.