Effects of naloxone on hemodynamics and sympathetic activity after exercise.

The effects of highdose naloxone (0.4 mg/kg iv) on hemodynamics and muscle sympathetic nerve activity (MSNA) after exercise were studied in nine normotensive young men randomly allocated the opioid antagonist or vehicle 30 min before treadmill exercise at 70% of resting heart rate reserve. Mean arterial pressure (MAP) was lower after exercise; cardiac output was increased. Mean values for MSNA and plasma norepinephrine were similar before and after exercise, but in individual subjects changes in resting MAP 60 min after exercise were inversely related to changes in sympathetic activity, suggesting that arterial baroreflex regulation of MSNA had been shifted to a lower set point. Naloxone did not prevent postexercise hypotension but transformed these inverse correlations into positive relationships. Naloxone attenuated both calf and systemic vasodilation without altering mean values for MSNA, indicating a peripheral effect of opioid antagonism. In normotensive subjects, naloxone alters the regulation of sympathetic outflow and vascular resistance during recovery from exercise but does not prevent the fall in MAP.