Alpha adrenergic blockade by prazosin in therapy of essential hypertension.

To elucidate the mechanism of action of prazosin in lowering the blood pressure, we studied the vascular pressor responsiveness to the infusion of norepinephrine and angiotensin II before and after prazosin in patients with essential hypertension. After prazosin, the pressor response to the alpha agonist norepinephrine was markedly attenuated, but the vascular reactivity to angiotensin II was unchanged. There was a threefold increment in the dose of norepinephrine required to elicit a 20 mm Hg rise in diastolic blood pressure after prazosin therapy. These results confirm that prazosin induces significant alpha adrenergic blockade during the treatment of essential hypertension.