Somatostatin Dramatically Stimulates Growth Hormone Release from Primate Somatotrophs Acting at Low Doses Via Somatostatin Receptor 5 and Cyclic AMP

Abstract

Somatostatin and cortistatin have been shown to act directly on pituitary somatotrophs to inhibit growth hormone (GH) release. However, previous results from nonprimate species indicate that these peptides can also directly stimulate GH secretion, at low concentrations. The relevance of this phenomenon in a nonhuman primate model was investigated in the present study by testing the impact of somatostatin/cortistatin on GH release in primary pituitary cell cultures from baboons. High doses (> 10(-10) m) of somatostatin/cortistatin did not alter basal GH secretion but blocked GH-releasing hormone (GHRH)- and ghrelin-induced GH release. However, at low concentrations (10(-17)-10(-13) m), somatostatin/cortistatin dramatically stimulated GH release to levels comparable to those evoked by GHRH or ghrelin. Use of somatostatin receptor (sst) specific agonists/antagonists, and signal transduction blockers indicated that sst2 and sst1 activation via intact adenylate cylcase and mitogen-activated protein kinase systems mediated the inhibitory actions of high-concentration somatostatin. By contrast, the stimulatory actions of low-dose somatostatin on GH release were mediated by sst5 signalling through adenylate cylcase/cAMP/protein kinase A and intracellular Ca(2+) pathways, and were additive with ghrelin (not GHRH). Notably, low-concentrations of somatostatin, similar to sst5-agonists, inhibited prolactin release. These results clearly demonstrate that the ultimate impact of somatostatin/cortistatin on hormone release is dose-dependent, cell type-selective and receptor-specific, where the stimulatory effects of low-concentration somatostatin/cortistatin on GH release extend to primates, thereby supporting the notion that this action is relevant in regulating GH secretion in humans.