Uptake of L-triiodothyronine into human cultured lymphocytes

The cellular uptake of [125I]L-triiodothyronine and its analogues was investigated in IM-9 human cultured lymphocytes. Uptake of L-triiodothyronine was one half maximal within 15 min of incubation and maximal within 45 min. The efflux of the hormone followed first order kinetics having a one half time of 15 min. Treating the cells with either the mitochondrial inhibitors antimycin-A and potassium cyanide, or lowering the incubation temperature to 12 degrees C, markedly reduced uptake. The uptake of [125I]L-triiodothyronine was saturable having an affinity constant (Kd) of 110 nM. When thyroid hormone analogues were studied D-triiodothyronine and triiodothyroacetic acid (analogues known to bind avidly to nuclear receptors) competed only weakly with [125I]L-triiodothyronine for uptake. These findings indicated the importance of the intact L-alanine side chain of the thyroid hormone molecule for its uptake into lymphocytes. Other studies, with cultured rat hepatocytes, demonstrated a similar saturable uptake system for [125I]L-triiodothyronine. These studies suggest, therefore, that the uptake of thyroid hormones and their analogues into cells may be an important step in the biological actions of these hormones.