ALPHA-BLOCKING ACTION OF ANTIHYPERTENSIVE AGENT, PRAZOSIN

Abstract

The vasodilatory and .alpha. adrenergic blocking properties of prazosin were studied in anesthetized rats and compared with those of the direct-acting vasodilator, diazoxide. The hypotensive activity of diazoxide was unimpaired after ganglion blockade with pentolinium or .alpha. adrenoreceptor blockade with phentolamine; diazoxide also significantly attenuated angiotensin II pressor responses. The hypotensive action of prazosin was completely abolished, over a 104-fold dose range, after ganglion or .alpha. adrenoreceptor blockade, and this agent failed, even in maximal hypotensive doses, to attenuate angiotensin II pressor responses. In addition, prazosin was shown to possess potent .alpha. adrenoreceptor blocking properties, significantly attenuating norepinephrine pressor responses and causing reversal of epinephrine pressor responses. The hypotensive action of prazosin appears not to be due to a direct relaxant effect on vascular smooth muscle, but is attributable to .alpha. adrenoreceptor blockade.