The Role of Opioid Peptides in the Hormonal Responses to Acute Exercise in Man

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Abstract
Opioid involvement in the physiological and hormonal responses to acute exercise was investigated in 6 normal male subjects. Each exercised to 40% (mild exercise) and 80% (severe exercise) of his previously determined maximal O2 consumption on 2 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. Exercise produced the expected increases in heart rate, blood pressure, ventilation, tidal volume, respiratory rate, O2 consumption and CO2 production. After severe exercise, naloxone infusion increased ventilation from 94.8 .+-. 4.9 l/min to 105.7 .+-. 5.0 l/min (P < 0.05), but had no effect on the other physiological variables. Exercise-induced changes in several hormones and metabolites were noted, including elevations in circulating lactate, growth hormone (GH), prolactin, cortisol, luteinizing hormone (LH), FSH, adrenaline, epinephrine, noradrenaline, norepinephrine, plasma renin activity (PRA) and aldosterone. No change in plasma met-enkephalin occurred. Naloxone infusion produced the expected increases in LH and cortisol, but also significantly enhanced the elevations in prolactin, adrenaline, noradrenaline, PRA and aldosterone (P < 0.05).