PGLUTAMYLGLUTAMYLPROLINEAMIDE MODULATION OF GROWTH-HORMONE SECRETION IN DOMESTIC-FOWL - ANTAGONISM OF THYROTROPIN-RELEASING-HORMONE ACTION

Pyroglutamylglutamylprolineamide (pGlu-Glu-ProNH2) is a tripeptide wit h structural and immunological similarities to thyrotrophin-releasing hormone (TRH; pGlu-His-ProNH2). Since TRH stimulates GH secretion in d omestic fowl, the possibility that pGlu-Glu-ProNH2 may also provoke GH release was investigated. Unlike TRH, pGlu-Glu-ProNH2 alone had no ef fect on GH release from incubated chicken pituitary glands and did not down-regulate pituitary TRH receptors. However, pGlu-Glu-ProNH2 suppr essed TRH-induced GH release from pituitary glands incubated in vitro and competitively displaced [H-3]methyl3-histidine2-TRH from pituitary membranes. Systemic injections of pGlu-Glu-ProNH2 had no significant effect on basal GH concentrations in conscious birds, but promptly low ered circulating GH levels in sodium-pentobarbitone anaesthetized fowl . Submaximal GH responses of conscious and anaesthetized birds to syst emic TRH challenge were, however, potentiated by prior or concomitant administration of pGlu-Glu-ProNh2. These results demonstrate, for the first time, that pGlu-Glu-ProNH2 has biological activity, with inhibit ory and stimulatory actions within the avian hypothalamo-pituitary axi s. These results indicate that pGlu-Glu-ProNH2 may act as a TRH recept or antagonist within this axis.