Beta-endorphin immunoreactivity during high-intensity exercise with and without opiate blockade

Nine highly fit men [mean (SE) maximum oxygen uptake, (V) over dotO(2max): 63.9 (1.7) ml.kg(-1).min(-1); age 27.6 (1.6)years] were studied during two treadmill exercise trials to determine plasma beta -endorphin immunoreactivity during intense exercise (80% (V) over dotO(2max)). A double-blind experimental design was used, and subjects performed the two exercise trials in counterbalanced order. Exercise trials were 30 min in duration and were conducted 7 days apart. One exercise trial was undertaken following administration of naloxone (1.2 mg: 3 cm(3)) and the other after receiving a placebo (0.9% NaCl saline; 3 cm(3)). Prior to each experimental trial. a flexible catheter was placed into an antecubital vein and baseline blood samples were collected. Thereafter, each subject received either a naloxone or placebo bolus injection. Blood samples were also collected after 10, 20 and 30 min of continuous exercise. beta -Endorphin was higher (P < 0.05) during exercise when compared to preexercise in both trials. However. no statistically significant difference was found (P > 0.05) between exercise time points within either experimental trial. beta -endorphin immunoreactivity was greater (P < 0.05) in the naloxone than in the placebo trial during each exercise sampling time point [10 min: 63.7 (3.9) pg.ml(-1) vs 78.7 (3.8) pg.ml(-1), 20 min: 68.7 (4.1) pg.ml(-1) vs 83.8 (4.3) pg.ml(-1); 30 min: 71.0 (4.3) pg.ml(-1) vs 82.5 (3.2) pg.ml(-1)]. These data suggest that intense exercise induces significant increases in beta -endorphin that are maintained over time during steady-rate exercise. Exercise and naloxone had an interactive effect on beta -endorphin release that warrants further investigation.