Uptake of L-triiodothyronine sulphate by human choriocarcinoma cell line, JAr

This study investigated uptake of triiodothyronine sulphate (T3S) and inter actions between uptake of T3S and triiodothyronine (T-3) using the human ch oriocarcinoma cell line (JAr) as a model of placental transport. Cells were incubated at 37 degrees C with 30 pM I-125-T-3 for 2 min with unlabelled T -3 (0-30 mu M) or T3S (0-1 mM). Addition of an excess unlabelled T-3 (30 mu M) or T3S (1 mM) reduced the initial rate of I-125-T-3 uptake by 69.3 +/- 3.6 per cent (P<0.0001) and 52.9 +/- 7.8 per cent (P<0.0001), respectively. The calculated Michaelis constant (K-m) for T-3 uptake was 0.378 +/- 0.133 mu M (n = 3) with a corresponding maximum velocity (V-max) of 15.4 +/- 6.9 pmol/min/mg protein. Uptake of I-125-T-3 was inhibited in a dose-dependent way by the addition of unlabelled T3S (0-1 mM). The calculated inhibition constant (K-i) for the inhibition of I-125-T-3 uptake by T3S was 121.8 +/- 35.2 mu M (n=6). Saturable uptake of I-125-T3S by JAr cells was negligible. The T3S preparation incubated with the cells contained about 0.1 per cent T-3, sufficient to explain the apparent inhibition of I-125-T-3 uptake by u nlabelled T3S. These results suggest that, in contrast to T-3 uptake in the se cells, JAr cells do not have a saturable uptake mechanism for T3S, and t hat T3S does not interact with the T-3 transporter in these cells. (C) 1999 W. B. Saunders Company Ltd.