Cystic ovarian follicles and thyroid activity in the dairy cow.
OBJECTIVE To examine whether the action of triiodothyronine on aromatase activity in porcine follicular cells is related to the modulation of estradiol receptor. METHODS Medium and large preovulatory follicles were incubated in Parker medium (M199) supplemented with 5 % of calf serum as a control medium or with addition of triiodothyronine (T3; 10 -9 M), tamoxifen (TMX; 0.1 mM ) or T3+TMX. The media were collected after 48 h, and assayed for progesterone (P4) and estradiol (E2) secretion by RIA. RESULTS T3 added to the medium decreased E2 secretion by both medium and large preovulatory follicles (119.7 % and 123.8 %, respectively; P<0.05). In contrast, T3 increased the secretion of P4 by medium (136 %; P<0. 05), while decreased the P4 secretion by large preovulatory follicles (123 %; P<0.05). The effect of TMX added alone was also dependent on follicular development. Estradiol secretion by medium follicles was 2.5 fold higher (p<0.01) than in control and 2.9 fold higher (P<0.01) than in T3 treated cells. In preovulatory follicles basal E2 secretion was not affected by TMX, while 1.2 fold higher (P<0.05) secretion compared to T3 treated cells was noted. On the other hand, TMX suppressed basal P4 secretion in medium and preovulatory follicles 1.5 fold (P<0.01) and 1.3 fold (P<0.05), respectively. The same phenomenon was observed in T3 treated cells. TMX added to the culture media decreased P4 secretion by medium follicles 1.8 fold (P<0.01) and that by preovulatory follicles 1.3 fold (P<0.05). CONCLUSIONS The reversed T3 action on estradiol secretion by both medium (P<0.05) and large preovulatory (P<0.01) follicles in TMX treated follicles suggests the up-regulation of ER by triidothyronine.