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Plasmatic levels of beta-endorphin during maximal graded bicycle stress test were measured by RIA on extracted plasma in 10 well-trained (A group) and in 8 untrained subjects (C group). Blood samples were obtained at rest, at peak work load and at the third, 10th and 90th min of recovery. For every stress test the following were evaluated: exercise time, maximum work load, total work load, maximum double product and mean K (an index of velocity of heart rate recovery during the first three minutes after the exercise). Both groups A and C showed a significant rise in beta-endorphin activity at the third minute of recovery; the increase was significantly greater in trained rather than in sedentary subjects (p less than 0.01). Beta-endorphin release was closely related to mean K; no relationship was found between exercise time, maximum work load, total work load, maximum double product and beta-endorphin rise. Our data shows that a release of beta-endorphin occurs during the initial phase of recovery after a maximal stress test; beta-endorphin rise is greater in trained subjects and correlates with the speed of heart rate recovery, but has no relationship with the duration and the grade of the effort. Whether beta-endorphin increase plays a role in the rapid decrease of adrenergic tone which occurs after exercise or represents a secondary phenomenon remains to be determined.

Some recent studies suggest that changes in the endogenous opioid peptides (POE) secretion during stress may be involved in various hemodynamic, respiratory and hormonal responses to exercise. To evaluate the relationship between fitness and POE release, 10 mixed-type exercise trained athletes (A) and 10 sedentary normal controls (C) were examined by bicycle stress testing; maximum oxygen uptake (VO2max) and peak work load (PWL) were greater in A than C (VO2max = 61.8 +/- 5.2 vs 40.2 +/- 6.1 ml/kg; PWL = 1525 +/- 229 vs 915 +/- 305 kgm.both p less than 0.01). After 24 hours A and C underwent rectangular bicycle stress testing (two 20' steps at 60% and 80% VO2max). Total plasmatic beta-endorphin (BEP) and its precursor beta-lipotropin (BLPH) were dosed by radioimmunoassay at rest, at 60% VO2max, at 80% VO2max and after complete recovery. Physical exercise caused a transient rise of BEP + BLPH plasma levels in both A and C. In A the increase was greater and occurred earlier than in C. The POE release under submaximal exercise showed a close correlation with oxygen uptake and therefore with fitness. This relation appeared in A at both low and high effort levels, whereas in C it was more strict at higher effort level. There results suggest that POE system play an important role in the adaptive mechanisms in sport practice.