Effect of human growth hormone-releasing hormone on the release of dynorphin-like immunoreactivity, luteinizing hormone, and follicle-stimulating hormone from rat adenohypophysis in vitro

The effect of GH-releasing hormone (GHRH) on the release of the endogenous opioid dynorphin from rat adenohypophysis was investigated in vitro. Rat anterior pituitary quarters were incubated in vitro, and hormone release into the incubation medium was measured by RIAs. Human pancreatic GHRH [hpGHRH-(l–44)] as well as human Leu²⁷, Gly⁴⁵-GHRH [GHRH-(l–45)] enhanced the secretion of dynorphin Ai_13-like immunoreactivity (Dyn A1–13-IR) in a concentration-dependent manner. The concentrations of hpGHRH-(l–44) that stimulated the release of Dyn A1–13-IR were about 100-fold higher than those that enhanced GH secretion. GH release induced by hpGHRH-(l–44) was blocked by somatostatin (IC50, -10 nM) without affecting hpGHRH-(l–44)-induced release of Dyn A1-13-IR. GH release was elicited by prostaglandin E2, while Dyn A1–13-IR secretion remained unchanged. At concentrations that enhanced Dyn A1–13-IR release, hpGHRH-(l–44) also elicited LH and FSH secretion. The LHRH antagonist D-pGlu¹, DPhe², D-Trp^3,6-LHRH blocked the secretion of Dyn A1-13-IRf LH, and FSH induced by hpGHRH-(l–44), whereas the LHRH antagonist did not influence the simultaneous GH release elicited by hpGHRH-(l–44). A possible direct effect of GHRH on the LHRH receptor was examined in radioligand binding studies using iodinated D-Ala⁶, des-Gly¹⁰-LHRH ethylamide (LHRHA). The binding of [¹²⁵I]iodo-LHRH-A to rat anterior pituitary membranes was completely displaced by hpGHRH-(l–44) and GHRH-(1–45). The deduced apparent dissociation constants were about 3 orders of magnitude higher than that of LHRH-A, but were close to those concentrations that enhanced Dyn A1–13- IR release. We conclude that GHRH-induced release of Dyn A1-13-IR is unrelated to GH release. High concentrations of GHRH may interact directly with LHRH receptors on gonadotrophs and thereby enhance the release of LH, FSH, and Dyn A1-13-IR.