The role of the cholinergic pathway in growth hormone feedback.

Abstract

Cholinergic pathways play an important role in the regulation of GH secretion. To assess their participation in GH feedback, we investigated the effect of pyridostigmine (an acetylcholinesterase inhibitor) on plasma GH responses to GH-releasing hormone (GHRH) plus TRH, insulin hypoglycemia, and arginine as well as on the inhibition of these responses by exogenous GH. The GH response to each stimulus was inhibited by an infusion of GH (0.55 micrograms/m2/min), started 4 h earlier. Pyridostigmine (120 mg, orally), administered 30 min before the stimulus, enhanced GH responses to GHRH and insulin during both saline and GH infusions. However, GH responses during combined administration of pyridostigmine and GH were less than those during pyridostigmine alone. GH responses to arginine, in contrast, were not affected by pyridostigmine in either the absence or presence of exogenous GH. TSH responses to TRH were unaltered by either GH or pyridostigmine. Pyridostigmine enhancement of GH responses to a maximally stimulatory dose of GHRH suggests that its effect is exerted by inhibition of somatostatin release. The lack of effect of pyridostigmine on plasma GH responses to arginine suggests that arginine and pyridostigmine increase GH secretion through a common pathway. The enhancement by pyridostigmine of GH responses in both the presence and absence of exogenous GH suggests that exogenous GH and pyridostigmine exert their discordant effects on GH secretion through independent mechanisms.