Release of growth hormone (GH) from purified somatotrophs: interaction of GH-releasing factor and somatostatin and role of adenosine 3',5'-monophosphate.

An acutely dispersed and purified preparation of somatotrophs obtained from rat adenohypophyses was used to study the mechanism of action of GH-releasing factor (GRF). Synthetic GRF [human pancreatic, hpGRF-(1-40)-OH] stimulated the immediate (within 4 min) release of GH in a dose-related manner, with a preceding or concurrent increase in cAMP in the somatotrophs. Somatostatin, at concentrations as low as 1.0 ng/ml, completely blocked the GRF-induced increase in GH release, with only a partial reduction in the GRF-induced accumulation of cAMP in the somatotrophs. 3-Isobutyl-1-methylxanthine, a phosphodiesterase inhibitor, potentiated the action of GRF in increasing cAMP in the somatotrophs, with subsequent GH release. These results along with those of previous studies suggest that cAMP is an intracellular mediator in the action of GRF and that somatostatin has a major effect on blocking GH release at a step subsequent to cAMP accumulation.