Uptake of Reverse T3in the Human Choriocarcinoma Cell Line, JAr

The uptake and efflux of reverse triiodothyronine (rT3) in JAr cells were investigated. Uptake of125I-rT3was time dependent and reversible with a saturable component of around 70 per cent of total uptake after 30min of incubation. Efflux was not saturable. Kinetic analysis of the initial specific uptake rates revealed an uptake process with a Michaelis constant of 3.04±0.53μm(mean±sem,n=15) and a corresponding maximum velocity of 9.65±2.49pmol/min/mg protein (n=15). Uptake of rT3was stereospecific, but not specific for rT3, as unlabelledlstereoisomers of thyroid hormone analogues were more effective as inhibitors of125I-rT3uptake than rT3. Unlabelled T3and thyroxine (T4) (10μm) reduced cellular uptake of125I-rT3by around 82 and 74 per cent, respectively. The calculated inhibition constantsKiwere 1.23±0.29μm(n=4) and 0.66±0.19μm(n=4) for T3and T4, respectively. Similarly, rT3reduced cellular uptake of125I-T3and125I-T4by 34 and 23 per cent, respectively. The calculated inhibition constantsKiwere 1.75±0.55μm(n=8) and 1.08±0.36μm(n=8) for the inhibition of125I-T3and125I-T4uptake, respectively. Reverse T3inhibited efflux of125I-T3from the cells by around 20 per cent, but did not inhibit efflux of125I-T4. These results suggest that uptake of rT3in JAr cells may occur via a single, saturable membrane carrier, which also interacts with T3and T4, while efflux of rT3may occur by passive diffusion.