Beta-endorphin time course response to intensity of exercise: effect of training status.

The concentration of beta-endorphin (B-EP) was measured in 6 trained and 6 untrained cyclists during three intensities of exercise to determine the time course changes of B-EP. B-EP was determined by radioimmunoassay with less than 5% cross reactivity with beta-lipotrophin. A counter-balanced design was used to avoid an order effect from exercise intensity. Resting B-EP values were similar across visits. There were no differences in resting B-EP values comparing the trained (4.61 +/- 0.25 pmol.l-1) to the untrained (4.03 +/- 0.23 pmol.l-1) group. Cycling at 60% VO2max did not increase B-EP in either group at any time measured. Cycling at 70% VO2max increased B-EP by 10 min in both groups p less than 0.05. The rate and magnitude of increase of B-EP were similar for both groups. B-EP changes at 80% VO2max were significantly greater that at 70% VO2max and were identical for the two groups. Both groups demonstrated increases by 5 min and further increases at 30 min of exercise p less than 0.01. These changes occurred despite the fact that lactate levels were lower in the trained group at both 70 and 80% VO2max intensities. It is concluded that the time course change for B-EP is similar for trained and untrained individuals working at the same relative intensity of exercise and does not seem to be related to plasma lactate concentrations.