Endocrine Abstracts

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 GHRH_1–32 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 4–6 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 hGHRH_1–44 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.