Insulin and glucagon immunoreactivity during high-intensity exercise under opiate blockade

Eight fit men [maximum oxygen consumption (VO2max) 64.6 (1.4) ml . kg(-1). min(-1), aged 28.3 (1.7) years (SE in parentheses) were studied during two treadmill exercise trials to determine the effect of endogenous opioids on insulin and glucagon immunoreactivity during intense exercise (80% VO2max). A double-blind experimental design was used with subjects undertaking the two exercise trials in counterbalanced order. Exercise trials were 20 min in duration and were conducted 7 days apart. One exercise trial was undertaken following administration of naloxone (N; 1.2 mg; 3 mi) and the other after receiving a placebo (P; 0.9% NaCl saline; 3 mi). Prior to each experimental trial a flexible catheter was placed into an antecubital vein and baseline blood samples were collected. Immediately after, each subject received either a N or P bolus injection. Blood samples were also collected after 20 min of continuous exercise (running). Glucagon was higher (P < 0.05), while insulin was lower (P < 0.05); during exercise compared with pre-exercise values in both trials. However, glucagon was higher (P < 0.05) in the P than in the N exercise trial [141.4 (8.3) ng . l(-1) vs 127.2 (7.6) ng . l(-1)]. There were no differences in insulin during exercise between the P and N trials [50.2 (4.3) pmol . l(-1) is 43.8 (5) pmol l(-1)]. These data suggest that endogenous opioids may augment the glucagon response during intense exercise.