Uptake of triiodothyronine into cultured human muscle cells

The uptake of T3 was measured in cultured human muscle cells at 37 degrees C and pH 7.4 in a medium containing albumin and glucose. The initial up]take increased linearly when the total T3 concentration was varied from 10(-9) to 10(-4) M. At prolonged incubation time the uptake decreased to virtually zero in about 30 min. These data indicate a rapid passive transport mechanism of T3 and a fast equilibration of the cellular T3 with the surrounding medium. In agreement with these conclusions the efflux of T3 was rapid and the initial uptake was not altered by pre-incubation in a T3-containing medium.