Plasma beta-endorphin concentration: response to intensity and duration of exercise.

Twelve college-age men exercised on a bicycle ergometer to VO2max and at 60, 70, and 80% VO2max for 30 min to determine the effects of exercise intensity on plasma beta-endorphin (B-EP). The time course for alterations in B-EP and the relationship to lactate were also examined. Following the VO2max test, the three submaximal intensities were completed on separate days using a counter-balanced design. Blood was sampled from an indwelling venous catheter at rest during exercise and recovery to assess the time course response. B-EP content was determined by radioimmunoassay (Immunonuclear) with less than 5% cross-reactivity to B-LPH. At rest, B-EP content was similar across visits, 4.34 +/- 0.36 pmol.l-1. The 60% intensity did not elevate B-EP at any time measured. B-EP content increased by 15 min at 70% VO2max with a further increase at 30 min. B-EP remained elevated during the 20 min recovery. At 80% VO2max B-EP content increased by 5 min. B-EP continued to increase during the exercise and peaked at 21.91 +/- 2.03 pmol.l-1 5 min into the recovery. Lactate showed a mild correlation with B-EP (r = 0.43) at 80% VO2max. A significant correlation (r = 0.78) between lactate and B-EP did occur with the VO2max test. It is concluded that an exercise intensity of at least 70% VO2max for 15 min is needed to increase plasma B-EP. Furthermore, the higher the exercise intensity the more rapid the onset for increases in plasma B-EP.