Differential effect of a new thyromimetic on triiodothyronine transport into myoblasts and hepatoma and neuroblastoma cells

3,5-Dibromo-3'-pyridazinone-L-thyronine (L-94901), a member of a novel class of thyromimetics, reduces cholesterol plasma levels with little effect on cardiac function in rats. Because receptor binding of L-94901 in isolated heart and liver nuclei is similar but binding in liver nuclei in vivo was 50-fold higher than in cardiac nuclei, we studied its effect on triiodothyronine (T3) transport across the plasma membrane of myoblasts, hepatoma cells and neuroblasts. Previously, we had demonstrated saturable, stereospecific and energy dependent transport of T3 into the three cell lines. After equilibrium of intact cells with hormone, nuclear binding of T3 was decreased by L-94901 in all three cell lines. While whole cell uptake and whole cell binding of T3 was only slightly affected by L-94901, kinetic analysis of the initial rate of uptake showed uncompetitive or noncompetitive inhibition and a differential decrease in Vmax. Furthermore, the Ki for the liver and brain derived cells was 10-fold lower than for the muscle derived cells. This effect on the plasma membrane transport of T3 may explain the differential effect reported in the intact animal.