Involvement of muscarinic cholinergic and α2-adrenergic mechanisms in growth hormone secretion during exercise in humans

The purpose of this study was to examine the role of muscarinic cholinergic and α2-adrenergic mechanisms in growth hormone (GH) secretion during exercise in humans. The GH responses induced during moderate-intensity exercise (using a cycle ergometer at 60% maximal oxygen uptake, V˙O2max, for 30 min) without treatment (control) and after the administration of a muscarinic cholinergic antagonist (atropine 1 mg) or after an α2-adrenergic antagonist (yohimbine 15 mg) were compared in seven healthy men. Although, serum GH concentration had increased significantly after exercise in the control experiment [mean peak GH concentration 52.64 (SEM 18.60) ng · ml−1], the increase was suppressed by the administration of either atropine [mean peak GH concentration 8.64 (SEM 7.47) ng · ml−1] or yohimbine [mean peak GH concentration 17.50 (SEM 7.89) ng · ml−1]. The area under the curve of serum GH concentration against time was significantly lower in the experiment using these drugs [with atropine, mean area 458 (SEM 409) ng · ml−1 · min], with yohimbine mean area 946 (SEM 435) ng · ml−1 · min] than in the control experiment [mean area 3135 (SEM 1098) ng · ml−1 · min]. These results suggest that muscarinic cholinergic and α2-adrenergic mechanisms are involved in GH secretion during exercise in humans.