Captopril and opiate antagonism in essential hypertension.

Six patients maintained on 50-100 mg captopril, for 2-25 months, were administered captopril 50 mg orally, together with either naloxone or 0.9% saline vehicle (placebo) given intravenously, in a double-blind crossover study. Naloxone did not appear to modify the circulatory effects of captopril in these patients, in contrast to earlier findings after acute captopril administration in normotensives. The results do not support an important endogenous opioid role in the chronic antihypertensive effect of captopril, but provide evidence that different mechanisms may contribute to the early short term falls in blood pressure, compared to the later long term effects.