CHOLINERGIC AND OPIOID INVOLVEMENT IN RELEASE OF GROWTH-HORMONE DURING EXERCISE AND RECOVERY

Cholinergic and opioid pathways have been implicated as mediators of t he increased growth hormone (GH) release observed during exercise. Thi s study compared the GH responses induced by a moderate-intensity exer cise bout during treatment with placebo (Plac), the opioid receptor an tagonist naltrexone (Nalt), the indirect cholinergic agonist pyridosti gmine (PD), or a combination of the two drugs (P + N). Ten active male s served as subjects (age, 25.1 +/- 0.6 yr, wt, 79.7 +/- 2.5 kg; %body fat, 14.9 +/- 1.4; peak oxygen consumption, 46.2 +/- 2.7 ml . kg-1 . min-1). Blood samples were drawn at 5-min intervals during the 4.5-h t esting period to determine the GH concentration. The testing period wa s divided as follows: 0600-0700 h = baseline, 0700-0800 h = preexercis e, 0800-0830 h = exercise, and 0830-1030 h = recovery. Drugs were admi nistered 1 h before exercise (at 0700 h). Exercise consisted of 30 min of cycling at an individualized work load previously found to elicit a blood lactate concentration of 2.5 mM. Heart rate, oxygen consumptio n, blood lactate, and blood glucose were measured throughout the exerc ise period. Results indicated that neither the resting GH concentratio n nor the metabolic parameters during exercise were altered by the tre atments. Peak serum GH concentration was not significantly altered by the treatments (range 7.3 +/- 2.0 to 12.6 +/- 4.4 mug/l). However, the integrated GH concentration during the PD (721 +/- 192 mug . l-1 . mi n) and P + N (859 +/- 248 mug . l-1 . min) trials was more than twice that seen in the Plac (362 +/- 123 mug . l-1 . min) and Nalt (297 +/- 87 mug . l-1 . min) trials (P = 0.07). Analysis of the delta scores re vealed that both mean and integrated GH concentrations were significan tly greater with PD and P + N than with Plac or Nalt (P < 0.05). We co nclude that cholinergic pathways can stimulate GH release during and a fter moderate-intensity exercise in young men.