Alpha 1-Adrenoceptor blockade: dissociation of its effects on renin release and arterial blood pressure in man.

1. The possibility that the juxtaglomerular alpha 1-adrenoceptors mediate an inhibitory action on renin release in man was examined in seven patients with essential hypertension, by measuring (i) the acute effects of prazosin (0.25 mg intravenously), a selective alpha 1-adrenoceptor-blocking agent, on arterial pressure and plasma renin activity, the degree of alpha-blockade induced by the drug being assessed by comparing the pressor response with that to a test dose of phenylephrine before and after prazosin administration, and (ii) the increases in plasma renin activity in response to isoprenaline before and during the prazosin-induced alpha-blockade. 2. Twenty minutes after the infusion of prazosin, when the pressor response to phenylephrine was reduced by 80% with respect to control, (i) mean arterial pressure was practically unchanged, (ii) plasma renin activity was almost doubled and (iii) the increases in plasma renin activity in response to isoprenaline were significantly greater, both in absolute and percentage values, than those observed before prazosin. 3. The increments in baseline plasma renin activity induced by prazosin in the absence of decrease in arterial pressure and the enhancement in renin responsiveness to the beta-adrenoceptor stimulus suggest that, in man, the juxtaglomerular alpha 1-adrenoceptors exert a direct, suppressive action on renin release.