Atresia of Preovulatory Follicles: Gonadotropin Binding and Steroidogenic Activity

Abstract

Atresia of large preovulatory follicles was studied in rats with 5 day reproductive cycles in which ovulation was blocked by 2 injections of sodium pentobarbital (Nembutal) at proestrus and estrus, respectively. The 1st morphological signs of atresia appeared 3 days after the 1st Nembutal injection. After 4 days the oocytes of all atretic follicles showed resumption of meiosis. Specific binding of h[human]CG [chorionic gonadotropin] to expressed granulosa cells was high at proestrus and the next day (day 1), but was decreased at day 2. Specific binding of h follitropin to granulosa cells decreased gradually after proestrus. Autoradiographic analysis revealed localization of hCG binding to granulosa and thecal cells of preovulatory follicles even in those which showed morphological signs of atresia (day 3). hCG binding to follicles in an advanced stage of atresia (day 4) was considerably less. The steroidogenic activity of follicles undergoing atresia was studied by incubating single follicles and measuring accumulation of steroids in the medium after 4 h. The main steroid produced by proestrous follicles (expressed in ng/4 h per follicle) was estradiol (2.53 .+-. 0.56). Estradiol accumulation in the medium by follicles collected at day 1 was considerably lower (0.48 .+-. 0.24). Accumulation of androgen also decreased from 0.34 .+-. 0.05 ng at proestrus to 0.06 .+-. 0.02 ng at day 1. Progesterone accumulation was not different between proestrous follicles and follicles obtained at day 1 (0.41 .+-. 0.09 ng and 0.50 .+-. 0.13 ng, respectively). The decreased estradiol accumulation by follicles that failed to ovulate was not due to a lack of aromatase activity, as addition of testosterone to the medium (50 ng/ml) increased estradiol accumulation 5-fold. Addition of ovine LH [lutropin] (20 .mu.g/ml) increased estradiol production 10-fold in proestrous follicles and only 4-fold in follicles obtained at day 2. Progesterone production was increased over 100-fold in both types of follicles. Before morphological signs of atresia are apparent, follicular estradiol production is reduced. Since preovulatory follicles undergoing atresia show gonadotropin binding and are responsive to LH with increased progestrone production in vitro, and since aromatase activity is still present, enzymes involved in the conversion of progesterone to androgens apparently are deficient in the early process of atresia of preovulatory follicles.