Evaluation of acute haemodynamic response to high-dose naloxone in young hypertensive and normotensive humans.

Cardiac output is increased in many young subjects with mild essential hypertension. The purpose of these experiments was to determine if activation of endogenous endorphin systems contributes to this increase. We investigated the acute effects of the opioid antagonist, naloxone, on Doppler-derived stroke volume, cardiac output and systemic blood pressure in young hypertensive (n = 9) and normotensive (n = 9) subjects. On two separate sessions, naloxone (0.4 mg/kg) was administered intravenously over 10 min to resting subjects according to a random, double-blind study design. Stroke volume and cardiac output were determined before and 10 min after the injection; heart rate and blood pressure were measured at 1 min intervals before and up to 20 min after the injection. Baseline blood pressure, stroke volume, and cardiac output were higher in hypertensive than in normotensive subjects. Naloxone had no immediate effect on blood pressure, heart rate, stroke volume, or total peripheral resistance in either group. These results indicate that: (1) naloxone has no immediate haemodynamic effect in young hypertensive or normotensive subjects, and (2) the higher stroke volume and cardiac output of young subjects with mild essential hypertension cannot be attributed to activation of endogenous opioid systems that are antagonized by naloxone.