Characteristics of 3,5,3′-triiodothyronine (T3)-uptake system of tadpole red blood cells:: effect of endocrine-disrupting chemicals on cellular T3 response

We characterized the 3,5,3'-L-triiodothyronine (T-3)-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 [I-125]T-3 uptake was inhibited by phloretin, monodansylcadaverine, bromosulfoplathalein, 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 10(6) cells at 23 degreesC. These results suggested that a large proportion of T-3 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 [I-125]T-3 uptake, RBCs were incubated with [I-125]T-3 it 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 [I-125]T-3 uptake, with an IC50 of 2.2 muM, which was one order of magnitude greater than that for T-3 (IC50,(,) 0.14 muM), and diethylstilbestrol and ethinylestradiol were modest inhibitors. Tributyltin accelerated saturable initial [I-125]T-3 uptake by 2-fold at 3.2 muM. When RBCs were cultured with 10 nM T-3 at 25 degreesC 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 alpha gene by T-3. However, not all chemicals competed with T-3 binding to the receptor at the same concentrations. Our results raise the possibility that the T-3-uptake system on the plasma membrane of the tadpole RBCs could be a candidate target site for some EDCs and can modulate cellular T-3 response.