Effect of naloxone on the actions of captopril.

Captopril inhibits the metabolism of endogenous opioids in vitro and potentiates their effects in vivo. We examined the hypothesis that endogenous opioids contribute to the actions of captopril in man. The acute cardiovascular and autonomic effects of oral captopril, intravenous naloxone, and their combination were examined in eight healthy men with normotension in a double-blind, placebo-controlled study of a Latin squares design. Naloxone altered neither blood pressure nor heart rate. There were significant falls in systolic blood pressure during captopril dosing alone, but there was no fall in blood pressure during combination therapy. Heart rates were higher during the combination than during captopril alone. The combination caused sedation, but neither captopril nor naloxone alone had any behavioral effects. Modification of the acute circulatory effects of captopril by naloxone suggests a role for endogenous opioids in the responses to converting enzyme inhibition. The sedation caused by the combination raises the possibility that captopril may exert central nervous actions in man.