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 GHRH-induced release of GH after feeding. Furthermore, the combination of GHRP-6 and GHRH synergistically increased 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 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 signaling after feeding that is overcome by stimulating GHRH and GHRP-6 receptors together, while remaining refractory to either peptide alone.”
“The data suggest that exercise-induced GH release can be profoundly modulated by a single meal taken before exercise. In our study, a high-fat drink attenuated postexercise peak GH levels and AUC by about 50%. It is worth noting that a typical fast-food meal would likely contain similar calories as fat as our high-fat protocol [e.g. a Big Mac hamburger and regular french fries contains 446 kcal as fat alone (15)]. A smaller, insignificant decrease in the GH response was seen after the ingestion of a high-glucose meal. The increase in metabolic rate and cellular redox state, as judged by the increases in vo, and the lactate to pyruvate ratio, were virtually the same after placebo, high- glucose, and high-fat meals. Thus, the reduced GH response to exercise must have resulted from a direct effect of the ingested substrate (i.e. fat) on the hypothalamic pituitary axis or by indirect effects mediated by the hormonal response to the meal itself.”