In vitro exposure of frog (Rana pipiens) pituitaries to gonadotropin-releasing hormone results in a persistent stimulation of gonadotropin biosynthesis.

Abstract

To further investigate the action of gonadotropin-releasing hormone (GnRH) on gonadotropin (Gth) secretion and synthesis, pituitaries from frogs, Rana pipiens, were incubated in the presence or absence of 100 ng/ml GnRH for 12 h. GnRH was then removed from the medium and glands were labeled with 35S-methionine either 12, 24, or 36 h after the conclusion of GnRH exposure. Immunoassayable LH and FSH (iLH and iFSH; both labeled and nonlabeled hormone) were determined by separate radioimmunoassays. Newly synthesized (radiolabeled) Gth was quantified by immunoprecipitation. When GnRH was present in the medium, it stimulated the secretion of both iLH and iFSH (compared to unexposed glands). Following withdrawal of the secretagogue, iLH and iFSH secretion diminished until they returned to control levels (within ca. 12 h). Previous exposure to GnRH enhanced the biosynthesis of Gth; this elevation was detectable up to 36 h after removal of the secretagogue. The magnitude of the elevation in Gth synthesis did not diminish with time, as secretion had, but remained constant. Gth synthesis was comparable in glands that had been exposed to GnRH 12 h and those exposed to the secretagogue 36 h prior. Thus, the GnRH-induced elevation in Gth secretion and synthesis can be dissociated; the elevation in synthesis is more chronic than the elevation in secretion.