Saturable, stereospecific transport of 3,5,3'-triiodo-L-thyronine and L-thyroxine into GH4C1 pituitary cells.
The mechanism of uptake of the thyroid hormones, 3,5,3'-triiodo-L-thyronine (L-T3) and L-thyroxine (L-T4), was studied in rat pituitary GH4C1 cells. The major portion (approximately 65%) of L-T3 transport was stereospecific and saturable. Transport of L-T3 was 8-10 times more rapid than transport of D-T3. [125I]L-T3 transport was saturable at microM concentrations; a Lineweaver-Burk plot was linear with Km = 0.4 microM and Vmax = 4 pmol/min/10(6) cells. Unlabeled analogs competed with [125I]L-T3 uptake in the order L-T3 > or = L-T4 >> 3,3',5'-triiodo-L-thyronine (reverse-T3), D-T3, D-T4, and L-thyronine. L-T3 and L-T4 also both effectively inhibited [125I]L-T4 transport. Uptake of [125I]L-T3 was inhibited 40-55% by large neutral amino acids and 77% by 80 microM beta-2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid, an inhibitor selective for the L system of amino acid uptake. Conversely, L-T3 inhibited the transport of [3H]leucine by pituitary cells (IC50 = 2 microM), but D-T3 and 3,5,3'-triiodothyroacetic acid (Triac) did not. L-Leucine was transported much more efficiently (Vmax = 0.65 mumol/min/10(6) cells) than L-T3 by GH4C1 cells. The results show that L-T3 and L-T4 share the same stereospecific transport pathway in pituitary cells, that the transport mechanism is saturable at supraphysiological thyroid hormone concentrations, and that the L system is partially responsible for L-T3 transport.
Department of Pharmacology, University of Rochester School of Medicine and Dentistry, New York 14642.
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D000818:Animals; D001667:Binding, Competitive; D001692:Biological Transport; D002460:Cell Line; D002467:Cell Nucleus / Q000378:metabolism; D007457:Iodine Radioisotopes; D007700:Kinetics; D007930:Leucine / Q000378:metabolism*; D010902:Pituitary Gland; D051381:Rats; D011988:Receptors, Thyroid Hormone / Q000378:metabolism; D013237:Stereoisomerism; D013696:Temperature; D013974:Thyroxine / Q000378:metabolism*; D014284:Triiodothyronine / Q000378:metabolism*
D007457:Iodine Radioisotopes; D011988:Receptors, Thyroid Hormone; D014284:Triiodothyronine; D007930:Leucine; D013974:Thyroxine