Entry of thyroid hormones into tilapia oocytes

The patterns of entry of thyroid hormones into live tilapia oocytes were ex amined by incubating ovarian follicles in L-15 medium containing (125)-labe led thyroxine (T4) or 3,5,3'-triiodothyronine (T3). As judged from HPLC pro files, radioactivity in extracts of follicles immersed in T3 was identified to reside in T3, while most of the radioactivity in the extract of T4 imme rsed follicle was not associated with T4. Radioactivity of T3 immersed foll icles reached a constant level after 18 h of incubation. Entry of T3 into t he oocytes was non-saturable within the range of 0.5-5000 ng/ml of T3 in th e incubation medium, suggesting the absence of specific mechanisms for T3 e ntry into the oocyte. Presence of female plasma at a level of 20% of incuba tion medium inhibited the n entry into the oocytes by approximately 80%. Wh en 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 conc lude that free T3 seems to enter oocytes freely across the membranes by dif fusion, 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 ra tio in female plasma. (C) 2001 Elsevier Science Inc. All rights reserved.