Influence of naloxone on muscle sympathetic nerve activity, systemic and calf haemodynamics and ambulatory blood pressure after exercise in mild essential hypertension

OBJECTIVE: To determine the effects of prior exercise and naloxone on haemodynamics, muscle sympathetic nerve activity, pituitary hormones and ambulatory blood pressure. METHODS: We studied 14 mild hypertensive and 14 normotensive subjects on two days. After baseline measurements, subjects were randomly allocated to vehicle or naloxone (0.4 mg/kg) 30 min before 45 min treadmill exercise. RESULTS: In both groups blood pressure, stroke volume, and calf and total peripheral resistances were lower 1 h after exercise, whereas sympathetic activity was unchanged. In normotensive subjects naloxone abolished this calf vasodilation without altering muscle sympathetic nerve activity, and attenuated these haemodynamic aftereffects of exercise, implying a peripheral opioidergic mechanism. Naloxone had no haemodynamic effect in hypertensive subjects. In normotensives there was an inverse relationship between changes in blood pressure and sympathetic activity after vehicle and exercise. This was transformed by naloxone into a positive relationship (r = 0.69, P < 0.02) similar to that observed in hypertensives after vehicle and exercise. Naloxone did not alter the latter positive relationship. Naloxone altered exercise-induced changes in prolactin and luteinizing hormone, but only in normotensive males. In both groups ambulatory blood pressures and heart rates over 2 h after subjects left the laboratory were higher than the values recorded at baseline or 1 h after exercise, and were unaffected by naloxone. CONCLUSIONS: The depressor effect of exercise is due to peripheral vasodilation, occurs in the absence of sympathetic withdrawal and is short-lived. Endogenous opioids, activated by running, participate in the haemodynamic, sympathoneural and pituitary hormone aftereffects of exercise in normotensive subjects, whereas in hypertensives these aftereffects of exercise are achieved through non-opioidergic mechanisms. These observations are consistent with the concept that activation of endogenous opioid systems by exercise is impaired in mild hypertension.