Increases in plasma beta-endorphin/beta-lipotropin immunoreactivity after treadmill running in humans

Six well-trained endurance athletes were studied to determine if submaximal treadmill exercise results in increased plasma levels of beta-endorphin/beta-lipotropin (Bh-Ep/Bh-LPH) immunoreactivity. Bh-Ep/Bh-LPH immunoreactivity was measured by radio immunoassay in plasma from six experienced runners before and after 30-min treadmill runs at a self-selected pace, 60 and 80% VO2 max, and a control experiment of 30 min rest. All tests were randomized and occurred during the same time of day for a given subject (0600--1500 h). Preexercise Bh-Ep/Bh-LPH values averaged between 10 and 20 pg/ml and increased two- to fivefold after each run. The increase was statistically significant (P less than 0.05) only after the 60% run when Bh-Ep/Bh-LPH increased to a mean of 58.3 pg/ml. A large individual variation in the Bh-Ep/Bh-LPH response to running was noted. Mood state and perceptual data were also collected, and no significant relationship with Bh-Ep/Bh-LPH was evident. These data suggest that the stress of treadmill running acts as a stimulus to greater Bh-Ep/Bh-LPH secretion, a reduction in its degradation, or a combination of these, which leads to increased levels of these ligands in venous blood. The physiological significance of these increased plasma levels is not clear.