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To evaluate the contribution of angiotensin-(1-7) [Ang-(1-7)] and prostaglandins to the acute and long-term anti hypertensive actions of captopril in mild-to-moderate essential hypertensive patientsBlood pressure, cardiac rate and the plasma concentrations of angiotensin I (Ang I), angiotensin II (Ang II), Ang-(1-7), prostaglandin E2 and 6-keto prostaglandin F1α (the breakdown product of prostacyclin) were determined in the peripheral venous blood of 24 essential hypertensive subjects before and 3 h after administration of 50 mg captopril. Eleven of 24 patients completed a 6- month treatment period with captopril monotherapy (50 mg twice a day). The hemodynamic and hormonal response produced by a last 50 mg dose of captopril was determined once again in the 11 subjects who maintained blood pressure control with captopril monotherapy for 6 months.The fall in blood pressure produced 3 h after drug intake was comparable for the first and the last 50 mg captopril dose. Although the first response to captopril increased plasma levels of Ang I only, the response to the last dose of the drug (6 months after) caused significantly higher levels of Ang I and Ang-(1-7). Neither acute nor chronic therapy with captopril had a significant effect on plasma concentrations of Ang II. Although plasma levels of prostaglandin E2 and 6-keto prostaglandin F1α were not modified by a first exposure to captopril, the concentrations of 6-keto prostaglandin F1α but not prostaglandin E2 rose significantly in subjects treated with the inhibitor for 6 months. A negative correlation was also demonstrated between diastolic blood pressure and plasma Ang-(1-7) levels in the 11 essential hypertensive subjects in whom blood pressure was controlled with captopril monotherapy.Inhibition of angiotensin converting enzyme with captopril had a significant effect on blood pressure that was not directly accounted for by a suppression of plasma Ang II levels. Continuous therapy with captopril unmasked a contribution of Ang-(1-7) and prostacyclin to the antihypertensive actions of this drug.