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

The uptake of 3,3′,5‐[3′‐ 125 I]triiodo‐L‐thyronine ([ 125 I]L‐T 3 ) and of L‐[3′,5′‐ 125 I]thyroxine ([ 125 I]L‐T 4 ) by cultured rat glial cells was studied under initial velocity ( V i ) conditions. Uptake of both hormones was carrier mediated and obeyed simple Michaelis‐Menten kinetics. The following respective values of K m (μM) and V max (fmol/min/μg of DNA) were obtained at 25°C: 0.52 ± 0.09 and 727 ± 55 for L‐T 3 and 1.02 ± 0.21 and 690 ± 85 for L‐T 4 . K i values (μM) for the inhibition of [ 125 I]L‐T 3 uptake by unlabeled analogues were as follows: L‐T 4 , 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‐T 3 was a better competitor than unlabeled L‐T 4 for the uptake of [ l25 l]L‐T 4 , an observation suggesting that both hormones were taken up by a common carrier system. L‐T 3 and L‐T 4 uptake was pH dependent, a finding suggesting that the phenolic unionized form of the hormones was preferentially taken up. L‐T 3 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‐T 3 binding to isolated nuclei also inhibited L‐T 3 nuclear binding in intact cells, an observation suggesting that uptake could be rate limiting for the access of L‐T 3 to nuclear receptors when transport is severely inhibited.