Membrane transport of thyroid hormone in the human choriocarcinoma cell line, JAR.

We studied uptake of L-triiodothyronine (T3) by the human choriocarcinoma cell line, JAR. Uptake was time dependent with a half-time of 56.2 +/- 7.2 min (mean +/- SEM, n = 4). A non-saturable component accounted for about 24% of total uptake. We found a single saturable uptake mechanism with a calculated Michaelis constant (Km) of 586 +/- 206 nM (n = 9) and a corresponding maximum velocity of 17.0 +/- 5.7 pmol/min per mg protein (n = 9), values similar to those we have described recently in cultured normal human trophoblast cells. Uptake was dependent on temperature and intracellular energy, being reduced at lower temperatures and in the presence of potassium cyanide. It was independent of the Na+ gradient across the cell membrane and the presence of Na+ in the external medium, but was affected by the cell membrane potential.