Cholinergic Mediation of Growth Hormone Secretion Elicited by Arginine, Clonidine, and Physical Exercise in Man

Abstract

The role of acetylcholine in human GH secretion was studied with atropine, which selectively blocks cholinergic muscarinic receptors and crosses the blood-brain barrier. Paired tests were performed in 22 normal subjects divided into 4 groups. The stimuli employed were arginine (30 g/30 min, iv), clonidine (300 μg, orally), physical exercise for 20 min, and saline. In the second test, atropine (1 mg, im) was administered before GH stimulation. Arginine elicited a GH secretory peak of 16.6 ± 5 ng/ml (mean ± SEM), which was completely blocked when atropine was administered with arginine (0.9 ±0.1 ng/ml). Atropine did not, however, modify the PRL secretory response; peak levels after arginine and atropine plus arginine were 16.3 ±3.1 and 16.8 ± 2.5 ng/ml, respectively. Clonidine elicited a GH secretory peak (11.8 ± 2.7 ng/ml) which also was blocked by pretreatment with atropine (1.2 ± 0.2 ng/ml). Neither clonidine nor clonidine plus atropine altered PRL secretion. GH levels also were sharply increased after physical exercise, with a peak level of 19.4 ± 4.9 ng/ml. Atropine completely blocked exercise-induced GH secretion (2 ± 0.9 ng/ml). Atropine alone did not modify GH or PRL values compared with saline administration. The potency of the atropine-induced suppression of GH secretion by three different stimuli, each with presumably different mechanisms of action, suggests that acetylcholine plays an important role in the regulation of GH secretion.