Study of fluxes at low concentrations of l-tri-iodothyronine with rat liver cells and their plasma-membrane vesicles. Evidence for the accumulation of the hormone against a gradient

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Abstract
Influx and efflux of L-tri-[125I]iodothyronine with isolated rat liver parenchymal cells and their plasma-membrane vesicles were studied by a rapid centrifugation technique. At 23.degree. C and in the concentration range that included the concentration of free L-triiodothyronine in rat plasma, (3-5 pM) influx into cells was saturable; an apparent Kt [half maximal uptake] value of 8.6 .+-. 1.6 pM was obtained. At 5 pM-L-tri-[125I]iodothyronine in the external medium the ratios of the concentration inside to outside in cells and plasma-membrane vesicles were 38:1 and 366:1, respectively, after 7 s of incubation. At equilibrium (60 s at 23.degree. C) uptake of L-tri-[125I]iodothyronine by cells was linear with the hormone concentration, whereas that by plasma-membrane vesicles exhibited an apparent saturation with a Kd value of 6.1 .+-. 1.3 pM. Efflux of L-tri[125I]iodothyronine from cells equilibrated with the hormone (5-123 pM) was constant up to 21 s; the amount that flowed out was 17.7 .+-. 3.8% when cells were equilibrated with 5 pM-hormone. When plasma-membrane vesicles were equilibrated with L-tri-[125I]iodothyronine (556-1226 pM) 66.8 .+-. 5.8% flowed out after 21 s. From a consideration of the data on efflux from cells and binding of L-tri-[125I]iodothyronine to the liver homogenate, as studied by the charcoal-adsorption and equilibrium-dialysis methods, it appears that 18-22% of the hormone exists in the free form in the cell. Vinblastine and colchicine diminished the uptake of L-tri-[125I]iodothyronine by cells but not by plasma-membrane vesicles; binding to the cytosol fraction was not affected. Phenylbutazone, 6-n-propyl-2-thiouracil, methimazole and corticosterone diminished the uptake by cells, plasma-membrane vesicles and binding to the cytosol fraction to different extents. At low concentrations of L-tri-[125I]iodothyronine rat liver cells and their plasma-membrane vesicles accumulated the hormone against an apparent gradient by a membrane-mediated process. Contribution of cytoplasmic proteins to uptake by plasma-membrane vesicles was negligible. The amount of L-tri-[125I]iodothyronine required to achieve half-maximal uptake agrees with that occurring in the free form in the blood conferring physiological importance to the transporting system in the plasma membrane of the liver cell.

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