Carrier‐Mediated Transport of Thyroid Hormones into Rat Glial Cells in Primary Culture

: The uptake of 3,3′,5‐[3′‐125I]triiodo‐L‐thyronine ([125I]L‐T3) and of L‐[3′,5′‐125I]thyroxine ([125I]L‐T4) by cultured rat glial cells was studied under initial velocity (Vi) conditions. Uptake of both hormones was carrier mediated and obeyed simple Michaelis‐Menten kinetics. The following respective values of Km (μM) and Vmax (fmol/min/μg of DNA) were obtained at 25°C: 0.52 ± 0.09 and 727 ± 55 for L‐T3 and 1.02 ± 0.21 and 690 ± 85 for L‐T4. Ki values (μM) for the inhibition of [125I]L‐T3 uptake by unlabeled analogues were as follows: L‐T4, 0.88; 3,3′,5′‐triiodo‐L‐thyronine, 1.4; 3,3′‐diiodo‐L‐thyronine, 2.9; 3,3′,5‐triiodo‐D‐thyronine, 4.8; and triiodothyroacetic acid, 5.3. These values indicate that the uptake system is stereospecific. Unlabeled L‐T3 was a better competitor than unlabeled L‐T4 for the uptake of [l25l]L‐T4, an observation suggesting that both hormones were taken up by a common carrier system. L‐T3 and L‐T4 uptake was pH dependent, a finding suggesting that the phenolic unionized form of the hormones was preferentially taken up. L‐T3 uptake was studied in the presence of various inhibitors; the results suggest that uptake was independent of the transmembrane Na+ gradient and of the cellular energy. Compounds that inhibited cellular uptake but were without effect on L‐T3 binding to isolated nuclei also inhibited L‐T3 nuclear binding in intact cells, an observation suggesting that uptake could be rate limiting for the access of L‐T3 to nuclear receptors when transport is severely inhibited.