THYROID-HORMONE AS A REGULATOR OF BASAL AND HUMAN CHORIONIC GONADOTROPIN-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 (T-3) was investigated in vitro using a culture s ystem of porcine theca cells (Tcs) and granulosa cells (Gcs) in mono- and co-culture (GT), the latter resembling follicles in vivo. The cell s were cultured in the absence or presence of human chorionic gonadotr ophin (hCG) with or without T-3 (10(-7), 10(-9) or 10(-11) M). Follicu lar cells were obtained from follicles of different size (small, mediu m and large), and steroid secretion into the culture medium was detect ed by radioimmunoassay. T-3 alone did not influence steroid secretion by Tcs and Gcs isolated from follicles that were small and medium in s ize. In preovulatory follicles, an increase in basal androgen secretio n and a simultaneous decrease in oestradiol secretion were observed wi th Tcs and Gcs in both mono- and co-culture. T-3 together with hCG dec reased hCG-stimulated androgen secretion in Tcs isolated from medium-s ized follicles and had a simultaneous stimulatory effect on hCG-stimul ated oestradiol secretion by Gcs. In cultures of follicular cells obta ined from large follicles, T-3 decreased hCG-stimulated secretion of b oth androgen and oestrogen by Tcs and simultaneously stimulated oestra diol secretion in GT co-cultures. Thus, the interaction of T-3 with go nadotrophin hormone modulated follicular steroidogenesis, depending on follicle size and cell type used in culture. The observed T-3-induced increase in basal androgen secretion by Tcs could account for the atr esia 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 T-3 prevents excessive androgen production by Tcs, probably i nfluencing aromatization processes in the follicle. An increase in hCG -stimulated oestradiol secretion in GT co-cultures is then observed. F urther investigations are required to clarify whether this is linked w ith an effect on the aromatization processes occurring in the follicle .