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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 Ca2+ 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. [ABSTRACT FROM AUTHOR]

The presence of ghrelin and its receptor, growth hormone (GH) secretagogue receptor, in the hypothalamus and pituitary, and its ability to stimulate GH release in vivo and in vitro, strongly support a significant role for this peptide in the control of somatotroph function. We previously demonstrated that ghrelin elicits GH secretion directly in somatotrophs by activating two major signalling cascades, which involve inositol phosphate and cAMP. In as much as nitric oxide (NO) and its mediator cGMP have been recently shown to contribute substantially to the response of somatotrophs to key regulatory hormones, including GH-releasing hormone, somatostatin and leptin, we investigated the possible role of this signalling pathway in ghrelin-induced GH release in vitro. Accordingly, cultures of pituitary cells from prepuberal female pigs were challenged with ghrelin (10−8 m, 30 min) in the absence or presence of activators or blockers of key steps of the NO synthase (NOS)/NO/guanylate cyclase (GC)/cGMP route and GH secretion was measured. Two distinct activators of the NO route, S-nitroso- N-acetylpenicillamine (SNAP) (5 × 10−4 m) andl-arginine methyl ester hydrochloride (l-AME) (10−3 m), comparably stimulated GH secretion when applied alone. The presence ofl-AME enhanced ghrelin-stimulated GH secretion, whereas SNAP did not alter its effect. Conversely, two different NOS/NO pathway inhibitors, Nw-nitro-l-arginine methyl ester hydrochloride (10−5 m) or haemoglobin (20 μg/ml), similarly blocked ghrelin-induced (but not basal) GH release, thus indicating that NO contributes critically to ghrelin action in somatotrophs. Moreover, incubation with a permeable cGMP analogue, 8-Br-cGMP (10−8 m) stimulated GH secretion, but did not modify the stimulatory action of ghrelin, suggesting that cGMP could mediate the action of NO. Indeed, inhibition of GC by 10 μm LY-53,583 did not alter basal GH secretion but abolished the GH-releasing action of ghrelin. Taken together, our results provide novel evidence indicating that ghrelin requires activation of the NOS/NO route, and its subsequent GC/cGMP signal transduction pathway, as necessary steps to induce GH secretion from somatotrophs. [ABSTRACT FROM AUTHOR]

There is increasing evidence that nitric oxide (NO) produced by NO synthase (NOS), and their signalling partners, guanylyl cyclase and cGMP, play a relevant role in growth hormone (GH) secretion from somatotrophs. We previously demonstrated that both GH-releasing hormone (GHRH; 10−8 M) and low concentrations of somatostatin (10−15 M) stimulate pig GH release in vitro, whereas a high somatostatin concentration (10−7 M) inhibits GHRH-induced GH secretion. To ascertain the possible contribution of the NOS-NO and guanylyl cyclase-cGMP routes to these responses, cultures of pituitary cells from prepubertal female pigs were treated (30 min) with GHRH (10−8 M) or somatostatin (10−7 or 10−15 M) in the absence or presence of activators or blockers of key steps of these signalling cascades, and GH release was measured. Two distinct activators of NO route, SNAP (5 × 10−4 M) or L-AME (10−3 M), similarly stimulated GH release when applied alone (with this effect being blocked by 10−7 M somatostatin), but did not alter the stimulatory effect of GHRH or 10−15 M somatostatin. Conversely, two NO pathway inhibitors, NAME (10−5 M) or haemoglobin (20 µg/ml) similarly blocked GHRH- or 10−15 M somatostatin-stimulated GH release. 8-Br-cGMP (10−8 to 10−4 M) strongly stimulated GH release, suggesting that cGMP may function as a subsequent step in the NO pathway in this system. Interestingly, 10−7 M somatostatin did not inhibit the stimulatory effect of 8-Br-cGMP. Moreover, although 8-Br-cGMP did not modify the effect of GHRH, it enhanced GH release stimulated by 10−15 M somatostatin. Accordingly, a specific guanylyl cyclase inhibitor, LY-83, 583 (10−5 M) did not alter 10−15 M somatostatin-induced GH release, whereas it blocked GHRH-induced GH secretion. These results demonstrate for the first time that the NOS/NO signalling pathway contributes critically to the stimulatory effects of both GHRH and low-concentration somatostatin on GH release, and that, conversely, the subsequent guanylyl cyclase/cGMP step only mediates GHRH- and not low-concentration somatostatin-induced GH secretion from somatotrophs. [ABSTRACT FROM AUTHOR]

Ghrelin, a recently discovered 28-aa peptide, stimulates GH release through a mechanism involving PLC- and cAMP-related signaling pathways. Recently, nitric oxide (NO) and its mediator, cGMP, have been shown to be required for the response of somatotropes to various regulators (GHRH, somatostatin, leptin). Here, we explore the possible role of the NO synthase (NOS)/NO/guanylate cyclase (GC)/cGMP signaling pathway in ghrelininduced GH release from cultured pig somatotropes using blockers or activators of this route. [ABSTRACT FROM AUTHOR]