Entry of thyroid hormones into tilapia oocytes

The patterns of entry of thyroid hormones into live tilapia oocytes were examined by incubating ovarian follicles in L-15 medium containing 125I-labeled thyroxine (T4) or 3,5,3'-triiodothyronine (T3). As judged from HPLC profiles, radioactivity in extracts of follicles immersed in T3 was identified to reside in T3, while most of the radioactivity in the extract of T4 immersed follicle was not associated with T4. Radioactivity of T3 immersed follicles reached a constant level after 18 h of incubation. Entry of T3 into the oocytes was non-saturable within the range of 0.5-5000 ng/ml of T3 in the incubation medium, suggesting the absence of specific mechanisms for T3 entry into the oocyte. Presence of female plasma at a level of 20% of incubation medium inhibited the T3 entry into the oocytes by approximately 80%. When follicles were back-transferred to medium without T3, only 15% of T3 in the oocyte disappeared within the following 24 h. From our results, we conclude that free T3 seems to enter oocytes freely across the membranes by diffusion, and that T3 in the oocytes may bind to some molecules in the oocyte. However, during egg formation in vivo, contribution of free T3 entry into the oocytes did not seem to be significant when considering the free T3 ratio in female plasma.