INFLUENCE OF DOPAMINERGIC, ADRENERGIC AND CHOLINERGIC BLOCKADE AND TRH ADMINISTRATION ON GH RESPONSES TO GRF 1–29

Abstract

In order to establish the influence of dopaminergic, á‐adrenergic and cholinergic pathways on GRF‐mediated GH release we have studied the GH responses to GRF 1–29 (100 or 50 μg as i.v. bolus) alone and in combination with metoclopramide (MCP, 10 mg, i.v.), thymoxamine (THYM, 210 μg/min, 150 min infusion), and atropine (1.2 mg, i.v.). We have also investigated any possible interaction between TRH and GRF in view of the reported inhibitory effects of TRH infusion on stimulated GH release. Dopaminergic and á‐adrenergic blockade with MCP and THYM respectively, did not have any effect on the GH responses to GRF. This lack of effect strongly suggests that any action which these neurotransmitters may exert on GH secretion is not at a pituitary level. TRH did not modify the GH response to GRF suggesting that the inhibitory effect on stimulted GH secretion is exerted at a hypothalamic level. In contrast, GH responses to GRF were significantly reduced by prior administration of atropine. These data support the view that cholinergic pathways play an important role in the regulation of GH secretion and such control may be exerted at both hypothalamic and pituitary levels.