Influence of progesterone on the pituitary and plasma levels of LH and FSH in the female rat.

Abstract

In order to study the effects of progesterone on pituitary gonadotrophin levels, the steroid was administered (4 mg/day, subcutaneously) for a period corresponding to the duration of pseudopregnancy (12–14 days) to intact, unilaterally and bilaterally spayed rats (4–15 rats/group). In the bilaterally spayed rats the progesterone treatment was started on the day of ovariectomy, 43 days after ovariectomy or 43 days after ovariectomy + estrogen priming. In all experiments, pituitary levels and in some experiments involving bilaterally spayed rats, blood levels of FSH and LH were determined by HGG augmentation and OAAD methods, respectively. The administration of progesterone to intact and unilaterally spayed rats resulted in increased pituitary LH stores which appeared comparable to those which have been reported to normally occur at proestrus during the estrous cycle. In contrast, pituitary FSH stores in these experiments were increased by more than three-fold (p < .05). Progesterone treatment from the day of bilateral spaying failed to alter ovariectomy-induced increases in pituitary LH and FSH stores but was effective in decreasing postovariectomy rises in the plasma LH and FSH. In the long-term spayed rats, whether primed with estrogen or not, progesterone failed to alter pituitary stores of LH and FSH. It appears that progesterone, at least under the experimental conditions employed, in addition to decreasing the release of LH, also decreases its synthesis in the pituitary. To some extent, it is postulated, the same may be true with regard to FSH.