Effect of oral contraceptives on plasma beta-endorphin and corticotropin at rest and during exercise

Concentrations of immunoreactive beta-endorphin and corticotropin in plasma were studied in 27 healthy physically active women at rest and after the exercise test on a treadmill requiring 60% and 90% of the maximal oxygen consumption. Eleven of the subjects were on a combination-type of oral contraceptive pills, and the remaining 16 did not use any pills. Plasma immunoreactive beta-endorphin levels at rest were higher in pill non-users than in pill users. Corticotropin levels at rest did not differ between the pill users and non-users. After the 60% exercise test a slight increase was found in the concentrations of corticotropin and beta-endorphin in the pill non-users but not in the pill users. In the 90% exercise test, plasma beta-endorphin and corticotropin levels increased significantly in both groups. We conclude that the use of oral contraceptives may elevate the threshold of the intensity of exercise required to increase beta-endorphin and corticotropin secretion. Decreased resting concentration of beta-endorphin in pill users can be explained by suppression of normal cyclic ovarian function.