1. Opioid involvement in the physiological and hormonal responses to acute exercise was investigated in six normal male subjects. Each was exercised to 40% (mild exercise) and 80% (severe exercise) of his previously determined maximal oxygen consumption on two occasions, with and without an infusion of high-dose naloxone. The exercise task was a bicycle ergometer; mild and severe exercise were performed for 20 min each, followed by a recovery period.

2. Exercise produced the expected increases in heart rate, blood pressure, ventilation, tidal volume, respiratory rate, oxygen consumption and carbon dioxide production. After severe exercise, naloxone infusion increased ventilation from 94.8 ± 4.9 litres/min to 105.7 ± 5.0 litres/min (P<0.05), but had no effect on any of the other physiological variables.

3. Exercise-induced changes in several hormones and metabolites were noted, including elevations in circulating lactate, growth hormone (GH), prolactin, cortisol, luteinizing hormone (LH), follicle stimulating hormone (FSH), adrenaline, noradrenaline, plasma renin activity (PRA) and aldosterone. There was no change in plasma met-enkephalin. Naloxone infusion produced the expected increases in LH and cortisol, but also significantly enhanced the elevations in prolactin, adrenaline, noradrenaline, plasma renin activity and aldosterone (P<0.05).

4. Psychological questionnaires revealed minor mood changes after exercise, but no evidence was found for the suggested ‘high’ or euphoria of exercise. Effort was perceived as greater during the naloxone infusion than the saline infusion in every subject.

5. We conclude that endogenous opioids may be important in the control of ventilation and the perception of effort at high levels of power output, and may modulate the responses of circulating catecholamines and the renin-aldosterone system to acute physical stress.

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