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

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Abstract
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.