Pregnant Mare’s Serum Gonadotropin Stimulates Inhibin Subunit Gene Expression in the Immature Rat Ovary: Dose Response Characteristics and Relationships to Serum Gonadotropins, Inhibin, and Ovarian Steroid Content*

We have investigated the temporal changes and dose responses of ovarian inhibin gene expression, circulating inhibin, and ovarian estradiol content in PMSG-primed immature female rats. The relative levels of ovarian inhibin alpha- and beta A-subunit messenger RNA (mRNA), serum inhibin, and ovarian steroid content were measured after stimulation of immature female rats with 20 IU PMSG. Each of these variables rose in parallel and peaked at 48 h (relative ovarian inhibin alpha- and beta A-subunit mRNA increased 3.8-fold and 2.8-fold, respectively, serum inhibin 269 U/ml, vs. control 31, P less than 0.01, ovarian estradiol content 114 pmol/pair of ovaries vs. controls 4.7 P less than 0.001), corresponding to the time of maximal follicular development in this animal model. Subsequently each fell to reach a nadir at 96 h. The data are consistent with the hypothesis that PMSG stimulation of inhibin secretion occurs by a mechanism involving inhibin gene transcription. The increase in ovarian inhibin and estradiol synthesis during folliculogenesis and their simultaneous decline before ovulation suggest that 1) the circulating levels of inhibin and estradiol reflect follicular maturation and 2) these two hormones are regulated via a common mechanism during follicular development in the rat.