ENDOGENOUS OPIOIDS MAY MODULATE CATECHOLAMINE SECRETION DURING HIGH-INTENSITY EXERCISE

To determine the effect of endogenous opioids on catecholamine respons e during intense exercise [80% maximal oxygen uptake (VO2max)], nine f it men [mean (SE) VO2max, 63.9 (1.7) ml . kg-1 . min-1; age 27.6 (1.6) years] were studied during two treadmill exercise trials. A double-bl ind experimental design was used with subjects undertaking the two exe rcise trials in counterbalanced order. Exercise trials were 20 min in duration and were conducted 7 days apart. One exercise trial was under taken following administration of naloxone (N; 1.2 mmol . l-1; 3 ml) a nd the other after receiving a placebo (P; 0.9% saline; 3 ml). Prior t o each experimental trial a flexible catheter was placed into an antec ubital vein and baseline blood samples were collected. Immediately aft erwards, each subject received bolus injection of either N or P. Blood samples were also collected after 20 min of continuous exercise while running. Epinephrine and norepinephrine were higher (P < 0.05) in the N than P exercise trial with mean (SE) values of 1679 (196) versus 11 96 (155) pmol . l-1 and 24 (2.2) versus 20 (1.7) nmol . l-1, respectiv ely. Glucose and lactate were higher (P < 0.05) in the N than P exerci se trial with values of 7 (0.37) versus 5.9 (0.31) mmol . l-1 and 6.9 (1.1) versus 5.3 (0.9) mmol . l-1 respectively. These data suggest an opioid inhibition in the release of catecholamines during intense exer cise.