Steroidogenesis in Isolated Cells of Preovulatory Rat Follicles*

Abstract

Preovulatory follicles were isolated from immature rats 2 days after they were given a single injection of PMS [pregnant mares'' serum] gonadotrophin (PMSG). The follicles were either incubated intact or mechanically separated to obtain theca and granulosa cells. The samples were incubated for 2 h in a modified Krebs bicarbonate buffer containing albumin. The accumulation of progesterone, androstenedione, testosterone, and estradiol in the medium was determined by RIA [radioimmunoassay]. When the cells were isolated from rats killed before the LH [luteinizing hormone] surge, the intact follicles secreted predominantly estradiol in the hormone-free medium. The theca cells produced mainly androstenedione, and the granulosa cells produced mainly progesterone and estradiol. When LH was added to the incubation medium, the intact follicles increased the production of progesterone, androstenedione, and estradiol. In the theca cells, the in vitro addition of LH increased only the production of androstenedione while in the granulosa cells only progesterone was increased during the 2 h incubation. When the cells were obtained from rats killed 4-8 h after the LH surge, the intact follicles, as well as the theca and granulosa cells, produced large amounts of progesterone, whereas the production of androstenedione, testosterone and estradiol was low in all of these preparations. When the cells were obtained both before and after the LH surge, the addition of testosterone in vitro increased the formation of estradiol from intact follicles and theca and granulosa cells. When 17-hydroxyprogesterone was added in vitro to theca cells, an increased formation of androstenedione was found only in cells isolated before the LH surge. LH probably has an acute and prolonged stimulatory effect on some early step(s) of steroidogenesis in both the granulosa and theca cells of the preovulatory rat follicle. After the prolonged influence of LH, a block develops in the formation of androstenedione in the theca cells leading to a decreased formation of both androgens and estrogens by the whole follicle.