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 {\textgreater} or = L-T4 {\textgreater} 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.
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