Corelease of dynorphin-like immunoreactivity, luteinizing hormone, and follicle-stimulating hormone from rat adenohypophysis in vitro

Rat anterior pituitary quarters or acutely dispersed rat anterior pituitary cells were incubated in vitro, and the release of dynorphin A_1-13-like immunoreactivity (Dyn A_1-13-IR) into the incubation medium was studied. Addition of LHRH led to a concentration-dependent enhancement of the release of Dyn A_1-13-IR with a maximum secretory rate which was about 4-fold higher than basal secretion. Dyn A_1-13-IR was released by LHRH concomitantly with LH and FSH, and the concentrationresponse relationships as well as the time course were virtually identical. Gel filtration and HPLC revealed a single peak of Dyn A_1-13-IR, with an apparent mol wt of about 6000. In addition to Dyn A_1-13-IR, α-neo-endorphin-like immunoreactivity was released by LHRH. The LHRH-stimulated release of Dyn A_1-13-IR was mimicked by the LHRH analog D-Ala⁶, des-Gly¹⁰-LHRH ethylamide and blocked in a competitive manner by the LHRH antagonist D-pGlu¹, D-Phe², D-Trp^3,6-LHRH. Addition of TRH (5 μM), rat corticotropin-releasing factor (100 μM), arginine vasopressin (1 μM), or synthetic human pancreatic GH-releasing hormone (10 μM) produced no effect on Dyn A_1-13-IR release. An extract of the rat medial basal hypothalamus stimulated the release of Dyn A_1-13-IR and β-endorphin-like immunoreactivity, and the former, but not the latter, effect was blocked by the LHRH antagonist D-pGlu¹, D-Phe², D-Trp^3,6-LHRH. These results demonstrate that dynorphin-like material and other proenkephalin B-derived peptides are released concomitantly with LH and FSH from rat adenohypophysis in vitro upon activation of LHRH receptors. This may indicate that proenkephalin Bderived peptides coexist with LH and/or FSH in at least some gonadotrophs of the normal rat anterior pituitary gland.