Beta-endorphin and adrenocorticotropin response to supramaximal treadmill exercise in trained and untrained males.

The response of plasma beta-endorphin (beta-EP) and adrenocorticotropin (ACTH) was studied in seven well-trained (T) young endurance athletes and seven untrained (UT) age- and weight-matched males during treadmill exercise. Subjects ran continuously for 7 min at 60% VO2max, 3 min at 100% VO2max and 2 min at 110% VO2max. Arterialized blood was obtained periodically from a cannulated heated (41 degrees C) hand vein. Plasma beta-EP was measured by radio-immunoassay (RIA) which incorporated an antibody that did not cross-react (less than 1.5%) with beta-lipotropin. Plasma beta-EP was similar between groups at rest (T = 4.3 +/- 0.8 fmol ml-1, mean +/- SE, UT = 3.3 +/- 0.6 fmol ml-1) and did not change at the 60% VO2max stage. Beta-endorphin significantly increased at 100% VO2max with both groups responding similarly. A further increase occurred at 110% VO2max (T = 10.8 + 2.0 and UT = 6.6 + 1.0 fmol ml-1, P less than 0.05 for between group differences). This between group difference persisted 1 min after exercise when the highest beta-EP levels were reached (T = 18.7 +/- 4.7 and UT = 12.8 +/- 3.1 fmol ml-1, P less than 0.05). Plasma ACTH responses were similar to beta-EP with the highest values (T = 61.5 +/- 7.2, UT = 45.7 +/- 6.8 fmol ml-1, P less than 0.05 for between group differences) occurring at 1 min post-exercise. A positive correlation, r = 0.85, P less than 0.05, was found between beta-EP and ACTH using the 1 min post-exercise values. The enhanced response of beta-EP and ACTH in T may indicate a training-induced adaptation which increases the response capacity to extreme levels of stress.