Cholinergic and opioid involvement in release of growth hormone during exercise and recovery

Cholinergic and opioid pathways have been implicated as mediators of the increased growth hormone (GH) release observed during exercise. This study compared the GH responses induced by a moderate-intensity exercise bout during treatment with placebo (Plac), the opioid receptor antagonist naltrexone (Nalt), the indirect cholinergic agonist pyridostigmine (PD), or a combination of the two drugs (P + N). Ten active males 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 testing period to determine the GH concentration. The testing period was divided as follows: 0600-0700 h = baseline, 0700-0800 h = preexercise, 0800-0830 h = exercise, and 0830-1030 h = recovery. Drugs were administered 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 consumption, blood lactate, and blood glucose were measured throughout the exercise period. Results indicated that neither the resting GH concentration nor the metabolic parameters during exercise were altered by the treatments. Peak serum GH concentration was not significantly altered by the treatments (range 7.3 ± 2.0 to 12.6 ± 4.4 μg/l). However, the integrated GH- concentration during the PD (721 ± 192 μg · l^-1 · min) and P + N (859 ± 248 μg · l^-1 · min) trials was more than twice that seen in the Plac (362 ± 123 μg · l^-1 · min) and Nalt (297 ± 87 μg · l^-1 · min) trials (P = 0.07). Analysis of the delta scores revealed that both mean and integrated GH concentrations were significantly greater with PD and P + N than with Plac or Nalt (P < 0.05). We conclude that cholinergic pathways can stimulate GH release during and after moderate-intensity exercise in young men.