Plasma endorphin species during dynamic exercise in humans

Summary. β‐Endorphin is metabolized to γ‐ and α‐endorphin. In order to evaluate endorphin metabolism during exercise, radioimmunoassay blood levels of α, β‐ and γ‐endorphins were recorded during exercise for 2 h on a cycling ergometer in 12 endurance‐trained and 11 untrained male subjects. In untrained subjects, mild exercise (49±4% V O 2max , mean±SD) did not show an increase in plasma β‐endorphin, while the levels of its metabolites rose. No changes were noted in the endurance‐trained subjects. More intensive exercise (66±6% V O 2 max in untrained and 57±7% V O 2max in trained subjects) resulted in an increase in p‐endorphin concentration in association with elevation of the a‐endorphin level. While before and during exercise the β‐endorphin levels did not differ significantly between athletes and untrained subjects, the levels of γ‐ and α‐endorphins, as well as the molar ratios α/β and γ/β, were significantly higher in untrained subjects. In conclusion, blood levels of β‐endorphin metabolites in the resting state and during exercise are dependent on previous training. In untrained subjects, mild exercise may result in accumulation of γ‐ and α‐endorphins in blood without a concomitant change in β‐endorphin level.