Effects of thyroid hormones on bovine granulosa and thecal cell function in vitro: Dependence on insulin and gonadotropins

The objective of this study was to evaluate the influence of thyroxine (T-4 ) and triiodothryonine (T-3) on steroid production by bovine granulosa and thecal cells. Granulosa and thecal cells were obtained from small (1 to 5 m m) and large (greater than or equal to 8 mm) follicles of cattle, respectiv ely, and cultured for 4 d. We conducted six experiments to evaluate the eff ect of 2 d of exposure to various doses of T-3 or T-4. In insulin- or insul in plus FSH-treated granulosa cells of experiment 1, 30 and 100 ng/ml of T- 4 had no effect on aromatase activity or progesterone production. In experi ment 2, in the presence of insulin and FSH, 1 and 3 ng/ml of Ts weakly (<1. 4-fold) increased aromatase activity of granulosa cells but had no effect o n progesterone production. Low doses of T-4 (3 to 30 ng/ml) tested in exper iment 3 had no effect on aromatase activity but increased (to as much as 1. 4-fold) progesterone production by granulosa cells. In experiment 4, T-4 (3 0 ng/ml) increased (to 1.2-fold) progesterone production by granulosa cells only in the presence of FSH and had no effect on aromatase activity. In th ecal cells of experiment 5, in the presence of insulin and LH, 30 and 100 m g/ml of T-4 increased androstenedione production to 2.3- and 2.8-fold, resp ectively; only 100 ng/ml of T-4 was effective at stimulating progesterone p roduction by thecal cells. In experiment 6, 1 ng/ml of Ts increased thecal cell androstenedione production to 3.9-fold, whereas 3 ng/ml of Tg was with out effect; progesterone production was not affected by Ts. These results s upport the hypothesis that thyroid hormones may have direct stimulatory eff ects on ovarian function in cattle, acting at the level of granulosa and th ecal cells.