The Effects of 17β-Estradiol on Progesterone Secretion by Bovine Theca and Granulosa Cells*

Abstract

The effects of 17β-estradiol on progesterone secretion by bovine theca interna and granulosa cells were assessed. Granulosa cells and theca interna preparations were isolated from preovulatory follicles obtained from Holstein heifers before the endogenous LH surge. Theca and granulosa cells were cultured separately for 3 days with or without highly purified LH (10 ng/ml) in the presence of one of the following concentrations of estradiol: 0, 0.001, 0.01, 0.1, 1, or 10 μg/ml. During the first day of culture, small amounts of progesterone accumulated in the medium of granulosa and theca cultures; secretion was much greater during the second and third days. All concentrations of estradiol inhibited progesterone secretion by granulosa cells in a dose-dependent fashion. The lowest dose of estradiol (0.001 μg/ml) significantly increased progesterone secretion by theca interna, while higher concentrations (0.1, 1, and 10 μg/ml) significantly inhibited progesterone accumulation. When LH was included in the culture media, the effects of estradiol on progesterone secretion by the theca cultures were unchanged, but in the presence of LH, the lower doses of estradiol (0.001, 0.01, and 0.1 μg/ml) were not inhibitory to progesterone secretion by granulosa cells. The effects of estradioi on progesterone accumulation were reversible and both cell types appeared healthy at the end of the culture period. Therefore, the inhibitory effects of estradiol do not appear to be due to toxic effects on the cells. Estradiol concentration in the follicular fluid from 13 bovine preovulatory follicles (including those used in this study) averaged 1.5 μg/ml ± 0.1 (SEM). It is concluded that both bovine theca interna and granulosa cells are capable of progesterone secretion, that high intrafollicular concentrations of estradiol may inhibit follicular progesterone production in vivo before the LH surge, and that estradiol may be an important modulating factor in the control of follicular progesterone secretion