Cardiovascular, humoral, and renal effects of phenoxybenzamine in hypertension.

We examined the effects of alpha adrenergic receptor blockade with phenoxybenzamine on various parameters related to blood pressure regulation in 10 normotensive and 12 essential hypertensive subjects. The responses were observed before and after phenoxybenzamine treatment during standardized maneuvers of volume expansion with saline infusion and volume contraction with a diuretic. Alpha adrenergic blockade produced a significantly greater (P less than 0.02) reduction in blood pressure in the hypertensive group than in the normotensive subjects. The baroreceptor response, evaluated by comparing the change in plasma norepinephrine concentration in relation to the change in blood pressure, was significantly reduced (P less than 0.05) in hypertensives compared to normal subjects. Plasma and urinary norepinephrine concentration, which were similar in the two groups, increased in both during phenoxybenzamine treatment. There was no significant change in the mean levels of plasma renin activity or plasma aldosterone concentration in either group after phenoxybenzamine treatment. However, the relationship between plasma renin activity and plasma aldosterone levels, as judged by linear regression analysis, was significantly altered (P less than 0.05) by phenoxybenzamine therapy in both groups. The latter may reflect an effect of phenoxybenzamine on the metabolism or secretion of aldosterone. These results, utilizing an alpha adrenergic antagonist, confirm the enhanced vascular reactivity and diminished baroreceptor function in essential hypertension; no evidence for an alpha adrenergic effect on plasma renin activity could be demonstrated in normotensive or hypertensive subjects.