The Regulation of Ovum Implantation in the Rat by Endogenous and Exogenous FSH and Prolactin: Possible Role of Ovarian Follicles1, 2

Abstract

The role of pituitary gonadotropins in controlling implantation was studied in the rat. Serum LH, FSH, and prolactin were measured during the first 8 days (a) of normal pregnancy and (b) lactating pregnancy with“short”and“long”delays in implantation. Of interest was the considerably lower average FSH secretion (506 ng/ml) and high prolactin (PL) (158 ng/ml) during the 8-day experimental period in animals with delay of implantation when compared to the normal pregnant group (FSH = 876 ng/ml; PL = 32 ng/ml). LH concentrations were low throughout the experimental period and there were no significant differences in LH levels among the groups of animals studied. Utilizing highly sensitive methods for the production and detection of short periods of implantational delay, it was demonstrated that FSH and prolactin levels varied inversely with the delay of implantation. Administration of rat FSH (20 µg NIH-FSH-B-1), rat PL (200 µg NIH-PL-RP-1), or 20 µg FSH + 200 µg PL on Day 3 of pregnancy to rats with both“short”and“long”delays in implantation confirmed the ability of FSH to stimulate implantation and PL to antagonize the FSH effect. Destruction of ovarian follicles by X-irradiation eliminated the stimulatory activity of FSH in overcoming implantational delay. The results suggest that (a) FSH is required for induction of implantation, (b) the effectiveness of endogenous and exogenous FSH is dependent upon low serum PL concentrations, (c) the activity of FSH is mediated by the ovarian follicles, possibly due to enhanced estrogen secretion.