During recent studies of receptor binding of T3 and T4 in tadpole red blood cells (RBCs), it was found that the fractional uptake of T3 was 3-5 times greater than that of T4. The present studies were performed to determine if this difference was due to facilitated uptake of T3. All studies were performed in cells incubated at 22 C in phosphate-buffered amphibian or mammalian Ringer solution containing 10(-11) M [125I]T3 or [125I]T4, with or without nonradioactive L-T3, D-T3, or L-T4 in concentrations ranging from 10(-10) to 2 X 10(-7) M. In tadpole, frog, rat, and human RBCs and in rat thymus cells, the rate of uptake of [125I]T3 and its appearance in the cytosol (extranuclear) fraction of the cell was greatly retarded in the presence of 10(-7) M T3. This effect was not due to saturation of the T3-binding proteins in cytosol, since the presence of 10(-7) M T3 did not influence the percentage of [125I]T3 in cytosol at equilibrium. These data suggest the presence of a saturable system for the cellular uptake of T3. T4 (10(-7) M) had relatively little effect, and D-T3 (10(-7) M) had no effect on this system. No saturable system for T4 uptake could be demonstrated, and at low concentrations of hormone, T4 uptake was only 10-30% of T3 uptake, although the binding activities of cytosol and cellular uptake via nonsaturable systems were quantitatively similar for both hormones. From kinetic studies of [125I]T3 uptake, it was found that at low concentrations of T3 (10(-11)-10(-9) M), the saturable system accounted for more than 60% of the total uptake of [125I]T3. The apparent Km values of the saturable system in RBCs of tadpole, frog, and rat were 4.5 +/- 0.09 (+/- SE), 5.7 +/- 1.10, and 4.6 +/- 0.73 X 10(-8) M T3, respectively. Corresponding values for maximum velocity were 5.92 +/- 1.02, 2.86 +/- 0.70, and 2.08 +/- 0.34 pmol/min X 10(7) cells. In rat thymus cells, the Km was 16.9 +/- 0.93 X 10(-8) M, and the maximum velocity was 9.10 +/- 0.5 pmol/min X 10(7) cells. The saturable uptake system was not influenced by ouabain, dinitrophenol, sodium fluoride, oligomycin, or sodium cyanide. These findings suggest that the uptake of T3, but not T4, into some tissues is facilitated by a specific carrier-mediated system that is not dependent on metabolic energy.(ABSTRACT TRUNCATED AT 400 WORDS)
Animals Biological Transport