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Entry of thyroid hormones into tilapia oocytes
1Division of Applied Biosciences, Graduate School of Agriculture, Kyoto University, Kitashirakawa, Sakyo-ku, 606-8502, Kyoto, Japan. tagawa@kais.kais.kyoto-u.ac.jp
Summary
Thyroid hormone entry into tilapia oocytes occurs via diffusion for triiodothyronine (T3), not thyroxine (T4). T3 uptake is rapid and largely irreversible, suggesting intracellular binding.
Area of Science:
- Endocrinology
- Reproductive Biology
- Fish Physiology
Background:
- Thyroid hormones are crucial for vertebrate development and reproduction.
- Understanding thyroid hormone transport into oocytes is vital for fish reproductive success.
Purpose of the Study:
- To investigate the mechanisms of thyroid hormone (thyroxine and triiodothyronine) entry into live tilapia oocytes.
- To determine the characteristics of thyroid hormone uptake and retention within tilapia oocytes.
Main Methods:
- Incubation of tilapia ovarian follicles with 125I-labeled thyroxine (T4) and 3,5,3'-triiodothyronine (T3).
- Analysis of radioactivity distribution using High-Performance Liquid Chromatography (HPLC).
- Assessment of T3 uptake kinetics, saturation, and effect of female plasma.
Main Results:
- Radioactivity in T3-treated follicles was primarily T3, while T4-treated follicles showed minimal T4 association.
- T3 uptake reached a plateau after 18 hours and was non-saturable, indicating diffusion-driven entry.
- Female plasma significantly inhibited T3 entry (approx. 80%), and T3 retention was high (only 15% loss in 24h).
Conclusions:
- Free triiodothyronine (T3) likely enters tilapia oocytes via passive diffusion.
- T3 within oocytes may bind to intracellular molecules, explaining its high retention.
- The contribution of free T3 diffusion to oocyte T3 content during egg formation in vivo may be limited.