Endocrine Abstracts

It is known that the synthesis and release of pituitary GH is controlled by complex neuroendocrine mechanisms that involve several neuropeptides, such as GHRH, SST, PACAP, TRH, GnRH, Ghrelin, among other regulators. Previous reports indicate that, during vertebrate evolution, the potency and efficacy of these secretagogues may vary and play differential effects upon GH regulation. In this work we aimed to study, in vitro, the capacity of these peptides to control the expression and secretion of pituitary GH in three vertebrate models: rat (mammals), chicken (birds) and iguana (reptiles), employing pituitary cultures at different incubation periods (0–6 h) and two doses of the secretagogues (1 and 10 nM). Results showed that GHRH significantly stimulated GH mRNA expression as well as GH secretion in the three species within the first hour of incubation, in comparison to the controls. However, its effect upon GH mRNA was 60 times greater in iguana than in the other species. TRH had no effect on GH secretion in any incubation period, but it stimulated GH mRNA expression in all species and, in the case of iguana, its effect was 150 times higher than in the others. PACAP stimulated GH mRNA expression at 4 h in chicken pituitary cultures, whereas no significant differences were observed in rats and iguanas. Ghrelin increased GH secretion in chickens, but had no effect in its mRNA synthesis, contrary to what was found in iguana cultures where GH mRNA significantly diminished. GnRH stimulated both GH mRNA expression and GH release in chicken pituitary cultures, while in iguana only GH secretion was significantly increased. On the other hand, SST strongly inhibited GH mRNA expression and GH release in the iguana, while no significant effect was directly observed in rats and chickens, at the doses and time-frame conditions employed. Results indicate that there is a differential effect of these secretagogues upon GH synthesis and secretion during vertebrate evolution, and further studies are needed to understand how these mechanisms have evolved.