Suppression of Basal and GnRH-Stimulated Gonadotropin Secretion Rate in vitro by GnRH Antagonist: Differential Effects on Metestrous and Proestrous Pituitaries

Abstract

LH and FSH basal secretion rates (BSR) in vitro of pituitary fragments from female rats at metestrus or following ovariectomy parallel changes seen in serum gonadotropins. These findings led us to suspect that in vitro BSR of fragments depended on prior exposure to GnRH or lingering GnRH in the culture. We compared pituitaries removed from rats in two cycle stages, proestrus (PRO) and metestrus (MET). Rats were given a single injection of a potent GnRH antagonist, Antag – WY 45760 -[Ac-β(2)-D-Nal¹, 4-F-D-Phe², D-Trp³, D-Arg⁶]-LHRH, or oil prior to sacrifice at 09.00 h on PRO or MET. Pituitary fragments were perifused for 8 h. To test the effects of Antag in vitro some pituitaries from oil-injected rats were perifused with medium containing 1 µM Antag for the first 4 h. Starting at 5 hall chambers received hourly 10-min pulses of 1 µM GnRH. In PRO rats Antag injection in vivo lowered LH BSR to 50% and FSH to 70% of oil-treated controls. Surprisingly, Antag administered in vitro suppressed LH BSR to 30% and FSH to 55% of controls. In pituitaries from MET rats, BSR of LH and FSH were 20 and 70% of control PRO values and were unaffected by Antag treatment in vivo or in vitro. In PRO pituitaries LH and FSH responses to the first GnRH pulse were blunted by Antag in vivo but responses to subsequent pulses were unaffected; MET responses were not different from controls. In vitro Antag suppressed GnRH-stimulated release to 10–20% of controls at both cycle stages. The lower effectiveness of Antag in suppressing FSH has not been shown previously in vitro, but is consistent with in vivo studies in this laboratory. The relative ineffectiveness of the Antag in lowering BSR in MET compared to PRO pituitary fragments suggests that there is less GnRH present, consonant with data showing lower in vivo GnRH release and pituitary receptor levels in MET rats. Our findings suggest that pituitary fragments retain an appreciable amount of endogenous GnRH that stimulates apparent ‘basal’ gonadotropin secretion and that the concept of basal in vitro secretion rate in fragments needs reexamination.