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We have studied the uptake of ¹²⁵I-thyroxine (¹²⁵I-T₄) in the human choriocarcinoma cell line JAR. Uptake of ¹²⁵I-T₄ was time-dependent, stereospecific and reversible, with a saturable component of 33% after 120 min of incubation. Kinetic analysis of the initial specific uptake rates indicated the presence of a single uptake process with a Michaelis constant of 59·4 ± 13·9 nm (n=12) and maximum velocity of 0·29 ± 0·06 pmol/min per mg protein. Uptake was dependent on intracellular energy as, in the presence of 2 mm potassium cyanide, saturable uptake was reduced to 60·6 ± 8·5% (n=4) of control uptake. Uptake was also temperature-dependent. Saturable ¹²⁵I-T₄ uptake after 60 min of incubation was 26·1 ± 3·0% at 25 °C (n=6) and 27·3 ± 5·7% at 4 °C of control uptake at 37 °C. Ouabain did not inhibit ¹²⁵I-T₄ uptake indicating that the uptake was independent of the Na gradient across the cell membrane. Although T₄ uptake was stereospecific, as D-T₄ failed to inhibit ¹²⁵I-L-T₄ uptake, it was not specific for T₄, as tri-iodothyronine (T₃) and reverse T₃ also inhibited ¹²⁵I-T₄ uptake. We conclude that JAR cells have a saturable, stereospecific and reversible membrane transport mechanism for T₄ which is dependent on intracellular energy, but independent of the Na⁺ gradient across the cell membrane.

Journal of Endocrinology (1995) 146, 233–238

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