Two growth hormone releasing hormone-like peptides (GHRH-LP-1 and GHRH-LP-2) are expressed in chickens. Although GHRH-LP-1 displaces the binding of labelled human GHRH to chicken pituitary membranes and stimulates the accumulation of cAMP in HEK 293 cells transfected with the chicken (c) GHRH receptor it has <1% of the potency of human GHRH132 and has little, if any, growth hormone (GH) - releasing activity in chickens. GHRH-LP-1 is thus unlikely to be the endogenous ligand for the cGHRH receptor, especially as two GHRH-like peptides also exist in goldfish, in which only one is active at the goldfish GHRH receptor. The possibility that GHRH-LP-2 is a GH-releasing factor in chickens has therefore been investigated.
In vivo, circulating GH concentrations in unanaesthetised 46 week old chicks were significantly increased, in a dose-related way, in response to intravenous injections of chicken GHRH-LP-2, with an EC50 of 9.05 μg/kg. Maximally effective doses of GHRH-LP-2 (10 μg/kg and 30 μg/kg) induced GH responses that were comparable to those induced by maximally effective doses of hGHRH144 and thyrotropin-releasing hormone, used as positive controls. The in vivo GH response to GHRH-LP-2 was likely due to direct action, since GHRH-LP-2 promptly induced the release of GH from perifused chicken pituitary glands in vitro.
These results demonstrate that GHRH-LP-2 stimulates GH release in vivo and in vitro in chickens and it is therefore likely to be the endogenous ligand for the recently discovered chicken GHRH receptor.
Prague, Czech Republic
24 - 28 Apr 2010
European Society of Endocrinology