Uptake of reverse T-3 in the human choriocarcinoma cell line, JAr

The uptake and efflux of reverse triiodothyronine (rT(3)) in JAr cells were investigated. Uptake of I-125-rT(3) was time dependent and reversible with a saturable component of around 70 per cent of total uptake after 30 min o f incubation. Efflux was not saturable. Kinetic analysis of the initial spe cific uptake rates revealed an uptake process with a Michaelis constant of 3.04 +/- 0.53 mu M (mean +/- SEM, n=15) and a corresponding maximum velocit y of 9.65 +/- 2.49 pmol/min/mg protein (n=15). Uptake of rT(3) was stereosp ecific, but not specific for rT(3), as unlabelled L stereoisomers of thyroi d hormone analogues were more effective as inhibitors of I-125-rT(3) uptake than rT(3). Unlabelled T-3 and thyroxine (T-4) (10 mu M) reduced cellular uptake of I-125-rT(3) by around 82 and 74 per cent, respectively. The calcu lated inhibition constants K-i were 1.23 +/- 0.29 mu M (n=4) and 0.66 +/- 0 .19 mu M (n=4) for T-3 and T-4, respectively. Similarly, rT(3) reduced cell ular uptake of I-125-T-3 and I-125-T-4 by 34 and 23 per cent, respectively. The calculated inhibition constants K-i were 1.75 +/- 0.55 mu M (n=8) and 1.08 +/- 0.36 mu M (n=8) for the inhibition of I-125-T-3 and I-125-T-4 upta ke, respectively. Reverse T-3 inhibited efflux of I-125-T-3 from the cells by around 20 per cent, but did not inhibit efflux of I-125-T-4. These resul ts suggest that uptake of rT(3) in JAr cells mag; occur via a single, satur able membrane carrier, which also interacts with T-3 and T-4, while efflux of rT(3) map occur by passive diffusion. (C) 1999 W. B. Saunders Company Lt d.