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Effects of Estradiol on Bovine Thecal Cell Function In Vitro: Dependence on Insulin and Gonadotropins^*

Department of Animal Science, Oklahoma State University, Stillwater 74078

E-mail: igf1Leo{at}okstate.edu.

The objective of this study was to evaluate the influence of estradiol (E2) on proliferation and steroid production by thecal cells obtained from large (8 mm) follicles of cattle. Five experiments evaluated the effect of various doses of E2 during a 2-d exposure in serum-free medium on hormone-induced steroidogenesis and cell proliferation. In LH-treated thecal cells of experiment 1, 300 ng/mL of E2 decreased progesterone production by 30% and increased androstenedione production to 5.8-fold of controls. In the absence of LH, both 3 and 300 ng/mL of E2 increased progesterone production. In experiment 2, in the presence of insulin and LH, 3, 30, and 300 ng/mL of E2 decreased progesterone production (by 17 to 36%), whereas 3 ng/mL of E2 decreased and 300 ng/mL of E2 increased androstenedione production. Doses of LH (3 to 30 ng/mL) tested in experiment 3 increased (to as much as 3.7-fold) progesterone production by thecal cells and E2 attenuated this stimulatory effect by 40%. In contrast, E2 amplified the stimulatory effect of LH on androstenedione production in experiment 3. In experiment 4, E2 (300 ng/mL) decreased IGF-I- and insulin-induced thecal cell progesterone production by 70 to 77%, whereas E2 increased basal, IGF-I, and insulin-induced androstenedione production. In experiment 5, in the presence of insulin, 10 to 1000 ng/mL of E2 had no effect on [¹²⁵I]-IGF-I binding to thecal cells, whereas 10 and 100 ng/mL of E2 increased and 1000 ng/mL of E2 decreased progesterone production by thecal cells. Estradiol had no consistent effect on thecal cell numbers among the 5 experiments. These results support the hypothesis that E2 may act as a paracrine factor to directly regulate hormone-induced steroid production by thecal cells without affecting cell numbers or numbers of insulin-like growth factor type I receptors.

Key Words: estradiol • thecal cell • insulin-like growth factor • steroidogenesis

Abbreviation key: E2 = estradiol, ER = estrogen receptor, FCS = fetal calf serum, hCG = human chorionic gonadotropin.