Thyroid hormone as a regulator of basal and human chorionic gonadotrophin-stimulated steroidogenesis by cultured porcine theca and granulosa cells isolated at different stages of the follicular phase.

To characterize thyroid hormone action in the ovary, the direct effect of triiodothyronine (T3) was investigated in vitro using a culture system of porcine theca cells (Tcs) and granulosa cells (Gcs) in mono- and co-culture (GT), the latter resembling follicles in vivo. The cells were cultured in the absence or presence of human chorionic gonadotrophin (hCG) with or without T3 (10(-7), 10(-9) or 10(-11) M). Follicular cells were obtained from follicles of different size (small, medium and large), and steroid secretion into the culture medium was detected by radioimmunoassay. T3 alone did not influence steroid secretion by Tcs and Gcs isolated from follicles that were small and medium in size. In preovulatory follicles, an increase in basal androgen secretion and a simultaneous decrease in oestradiol secretion were observed with Tcs and Gcs in both mono- and co-culture. T3 together with hCG decreased hCG-stimulated androgen secretion in Tcs isolated from medium-sized follicles and had a simultaneous stimulatory effect on hCG-stimulated oestradiol secretion by Gcs. In cultures of follicular cells obtained from large follicles, T3 decreased hCG-stimulated secretion of both androgen and oestrogen by Tcs and simultaneously stimulated oestradiol secretion in GT co-cultures. Thus, the interaction of T3 with gonadotrophin hormone modulated follicular steroidogenesis, depending on follicle size and cell type used in culture. The observed T3-induced increase in basal androgen secretion by Tcs could account for the atresia of follicles, since it is accompanied by a decrease in oestradiol secretion in GT co-culture. In its co-activity with hCG, an adequate level of T3 prevents excessive androgen production by Tcs, probably influencing aromatization processes in the follicle. An increase in hCG-stimulated oestradiol secretion in GT co-cultures is then observed. Further investigations are required to clarify whether this is linked with an effect on the aromatization processes occurring in the follicle.