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Department of Biology, Faculty of Science, Shizuoka University, 836 Ohya, Shizuoka 422-8529, Japan

(Requests for offprints should be addressed to K Yamauchi; Email: sbkyama{at}ipc.shizuoka.ac.jp)

We characterized the 3,5,3'-L-triiodothyronine (T₃)- uptake system on the plasma membrane of Rana catesbeiana tadpole red blood cells (RBCs) in the presence of a variety of inhibitors and potentially competing amino acids. Saturable [¹²⁵I]T₃ uptake was inhibited by phloretin, monodansylcadaverine, bromosulfophthalein, sodium taurocholate and tryptophan. Saturable uptake obeyed simple Michaelis–Menten kinetics with an apparent K_m of 110 nM and a V_max of 2.5 pmol/min per 10⁶ cells at 23 °C. These results suggested that a large proportion of T₃ transported into RBCs was mediated by the aromatic amino acid transporter (System T)-linked transporter. To investigate the effect of endocrine-disrupting chemicals (EDCs) on [¹²⁵I]T₃ uptake, RBCs were incubated with [¹²⁵I]T₃ in the presence of each chemical. Among the test chemicals, di-n-butyl phthalate, n-butylbenzyl phthalate and the miticide, dicofol, were the most powerful inhibitors of [¹²⁵I]T₃ uptake, with an IC₅₀ of 2.2 µM, which was one order of magnitude greater than that for T₃ (IC₅₀, 0.14 µM), and diethylstilbestrol and ethinylestradiol were modest inhibitors. Tributyltin accelerated saturable initial [¹²⁵I]T₃ uptake by 2-fold at 3.2 µM. When RBCs were cultured with 10 nM T₃ at 25 °C for 2 days in the presence of monodansylcadaverine, ethinylestradiol, ioxynil or dicofol at the defined concentrations, these compounds inhibited significantly the induction of the thyroid hormone receptor gene by T₃. However, not all chemicals competed with T₃ binding to the receptor at the same concentrations. Our results raise the possibility that the T₃-uptake system on the plasma membrane of the tadpole RBCs could be a candidate target site for some EDCs and can modulate cellular T₃ response.

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