The influence of naloxone on exercise-induced increase in plasma pituitary hormones and the subjectively experienced level of exhaustion in healthy males.

Opioid peptides seem to play a role as modulators of the pituitary function in man. In the present study, the effect of naloxone on exercise-induced pituitary hormone release and the subjectively experienced level of exhaustion were investigated in nine healthy males. A submaximal work test was performed on two occasions using a bicycle ergometer: 10 min on 50% of maximal working capacity (MWC), immediately followed by 10 min on 80% of MWC. Ten min before exercise, each subject received, in a single-blind randomized order, either a bolus dose of naloxone (100 micrograms/kg) followed by a slow infusion of naloxone (50 micrograms X kg-1 X h-1) or as a control a corresponding volume of the preservatives in the naloxone preparation as a bolus dose followed by an infusion of diluted preservatives. In the control studies, exercise induced a significant increase in GH, PRL, TSH and ACTH. The increase in ACTH was enhanced following naloxone. Naloxone was without effect on exercise-induced changes in GH, PRL and TSH. An increased level of exhaustion was experienced on 80% of MWC during naloxone. It is concluded that opioid receptors with a moderate sensitivity to naloxone are involved in the regulation of the ACTH response to exercise and also influence the subjectively experienced level of exhaustion.