Exercise-thermoregulatory stress and increased plasma beta-endorphin/beta-lipotropin in humans.

Six adult male volunteers of similar body composition and physical fitness were tested to determine plasma immunoreactive beta-endorphin/beta-lipotropin (beta-EN/beta-LPH) response under three exercise-thermoregulatory stress conditions. The experimental protocol consisted of 120 min of stationary upright cycling at 50% VO2max under neutral (24 degrees C, 50% rh)-euhydration (NE), hot (35 degrees C, 50% RH)-euhydration (HE), and hot-dehydration (HD) environmental conditions. beta-EN/beta-LPH was calculated by radioimmunoassay at -30-min, 0-min, and 15-min intervals thereafter. Change in plasma volume (delta PV) was measured to determine its effect on beta-EN/beta-LPH concentration. Preexercise beta-EN/beta-LPH levels averaged 23.7 +/- 2.6 pg X ml-1 in all conditions. The greatest beta-EN/beta-LPH response occurred at 105 min in HD conditions when levels rose to 43.2 +/- 6.9 pg X ml-1. Exercise in HD and HE conditions resulted in significantly (P less than 0.05) elevated beta-EN/beta-LPH above levels observed in NE. delta PV did not account for more than 10% of beta-EN/beta-LPH changes at any time interval. The beta-EN/beta-LPH response pattern closely paralleled rectal temperature changes in all conditions. These data suggest that conditions of increasing exercise thermoregulatory stress are associated with increasing peripheral beta-endorphin concentration.