Acetylcholine does not play a major role in mediating the endocrine responses to ghrelin, a natural ligand of the GH secretagogue receptor, in humans

OBJECTIVE: Ghrelin is a 28 amino residue peptide produced predominantly by the stomach with substantially lower amounts deriving from other central and peripheral tissues. Ghrelin is a natural ligand of the GH secretagogue (GHS) receptor (GHS-R) and possesses a potent GH-releasing activity for which the acylation in serine 3 is essential. Ghrelin also possesses other endocrine and non-endocrine activities reflecting central and peripheral GHS-R distribution and stimulates PRL, ACTH and cortisol secretion, has been reported able to induce hyperglycaemia and to decrease insulin levels and has orexigenic activity. Moreover, ghrelin stimulates gastric motility and acid secretion and its action is mediated by acetylcholine which, in turn, is known to play a stimulatory influence on GH, ACTH and insulin secretion. SUBJECTS AND METHODS: In order to clarify the influence, if any, of acetylcholine on the endocrine activities of ghrelin, we studied the effects of cholinergic enhancement by pyridostigmine (PD, 120 mg p.o. at -60 minutes) and blockade by pirenzepine (PIR, 100 mg p.o. at -60 minutes) on GH, PRL, cortisol, insulin and glucose responses to human acylated ghrelin (1.0 μg/kg i.v. at 0 minutes) in seven normal young volunteers [age (mean ± SEM): 28.3 ± 3.1 years; BMI: 21.9 ± 0.9 kg/m²]. In the same subjects, the effects of PD and PIR on the GH response to GHRH (1.0 μg/kg i.v. at 0 minutes) have also been studied. RESULTS: The administration of ghrelin induced a prompt increase in circulating GH levels (hAUC: 5452.4 ± 904.9 μg*min/L) which was markedly higher (P < 0.01) than that elicited by GHRH (966.9 ± 20.50 μg*min/L). Ghrelin also induced a significant increase in PRL (1273.5 ± 199.7 μg*min/L) and cortisol levels (15505.1 ± 796.3 μg*min/L) and a decrease in insulin levels (ΔhAUC: -198.1 ± 39.2 mU*min/L) which was preceded by an increase in plasma glucose levels (8743.8 ± 593.0 mg*min/dL). The GH response to GHRH was markedly potentiated by PD (4363.3 ± 917.3 μg*min/L; P < 0.01 vs. GHRH alone). In turn, PD did not modify either the GH response to ghrelin (6564.2 ± 1753.5 μg*min/L) or its stimulatory effect on PRL and cortisol as well as its effects on insulin and glucose levels. The GH response to GHRH was inhibited by PIR (171.5 ± 34.7 μg*min/L, P < 0.01 vs. GHRH alone) which, in turn, did not significantly modify the GH response to ghrelin (4044.0 ± 948.8 μg*min/L). PIR also did not modify the effects of ghrelin on PRL, cortisol, insulin and glucose levels. CONCLUSIONS: The endocrine activities of ghrelin are not affected significantly by cholinergic enhancement and muscarinic blockade. Thus, acetylcholine does not play a major role in the endocrine actions of ghrelin. Moreover, as the cholinergic system influences GH secretion via modulation of somatostatin release, the present data agree with the assumption that ghrelin is partially refractory to the influence of somatostatin.