GH-releasing peptide-6 overcomes refractoriness of somatotropes to GHRH after feeding, C D McMahon, Journal of Endocrinology (2001) 170, 235–241
After a meal, somatotropes are temporarily refractory to growth hormone-releasing hormone (GHRH), the principal hormone that stimulates secretion of growth hormone (GH). Refractoriness is particularly evident when free access to feed is restricted to a 2-h period each day. GH-releasing peptide-6 (GHRP-6), a synthetic peptide, also stimulates secretion of GH from somatotropes. Because GHRH and GHRP-6 act via different receptors, we hypothesized that GHRP-6 would increase GHRH-induced secretion of GH after feeding.
Initially, we determined that intravenous injection of GHRP-6 at 1, 3 and 10 ug/kg body weight (BW) stimulated secretion of GH in a dose-dependent manner. Next, we determined that GHRP-6- and GHRH-induced secretion of GH was lower 1 h after feeding (22.5ng/ml and 20 ng/ml respectively) than 1 h before feeding (53.5ng/ml and 64.5 ng/ml respectively).
However, a combination of GHRP-6 at 3 ug/kg BW and GHRH at .2 ug/kg BW synergistically induced an equal and massive release of GH before and after feeding that was fivefold greater than the GHRH-induced release of GH after feeding. Furthermore, the combination of GHRP-6 and GHRH synergistically increased the release of GH from somatotropes cultured in vitro. However, it was not clear if GHRP-6 acted only on somatotropes or also acted at the hypothalamus. Therefore, we wanted to determine if GHRP-6 stimulated secretion of GHRH or inhibited secretion of somatostatin, or both. GHRP-6 stimulated secretion of GHRH from bovine hypothalamic slices but did not alter secretion of somatostatin. We conclude that GHRP-6 acts at the hypothalamus to stimulate secretion of GHRH, and at somatotropes to restore and enhance the responsiveness of somatotropes to GHRH.
“Reduced secretion of GH from somatotropes after feeding is not limited to that induced by GHRH because a 2-adrenergic-induced secretion of GH is also reduced after feeding (Gaynor et al. 1993). How and why somatotropes become refractory to GHRH after feeding is not known. However, given that the combination of GHRH with GHRP-6 induced a rapid and massive release of GH before and after feeding, it seems likely that releasable pools of GH are not reduced and that receptors to GHRH and GHRP-6 are not down-regulated. Rather, it is likely that there is a change in receptor signalling after feeding that is overcome by stimulating GHRH and GHRP-6 receptors together while remaining refractory to either peptide alone.”
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