Inhibitory effect of progesterone on follicular growth and induced superovulation in the rat.

We investigated the effect of exogenous progesterone on follicular growth and pregnant mares' serum gonadotropin and human chorionic gonadotropin (PMS-HCG) induced superovulation in intact and hypophysectomized immature rats. Although the administration of progesterone during PMS-HCG injections had no effect on superovulation, the administration of progesterone beginning 3 days before PMS injection inhibited superovulation in both intact and hypophysectomized rats. In order to study the mechanism of inhibition of superovulation by pretreatment with progesterone, an autoradiographic study using tritiated thymidine was done in hypophysectomized rats just before PMS injection. The labelling indices of granulosa cells were significantly decreased by the administration of progesterone (p < 0.001). Treatment with progesterone resulted in an increase of the follicles in early stages of atresia with pycnotic nuclei in granulosa cells surrounding apparently normal oocytes. Ovarian histology and serum and ovarian tissue concentrations of estradiol were then examined in hypophysectomized rats 20 h after HCG injection. Large Graafian follicles and corpora lutea were rarely seen in rats previously given progesterone, whereas many large Graafian follicles and corpora lutea were observed in the control rats. The concentrations of estradiol in the progesterone-pretreated rats were significantly reduced in serum (p < 0.02) and slightly reduced in ovarian tissues when compared with controls.