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 (microM) and Vmax (fmol/min/microgram of DNA) were obtained at 25 degrees C: 0.52 +/- 0.09 and 727 +/- 55 for L-T3 and 1.02 +/- 0.21 and 690 +/- 85 for L-T4. Ki values (microM) 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 [125I]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.
Francon
J
J
Unité de Recherche sur la Glande Thyroïde et la Régulation Hormonale (U. 96) de l'INSERM, Le Kremlin-Bicêtre, France.
Chantoux
F
F
Blondeau
J P
JP
D000818:Animals; D013956:Antithyroid Agents; D001692:Biological Transport; D002352:Carrier Proteins / Q000502:physiology*; D002467:Cell Nucleus / Q000378:metabolism; D002478:Cells, Cultured; D003470:Culture Media; D006863:Hydrogen-Ion Concentration; D009457:Neuroglia / Q000378:metabolism*; D051381:Rats; D012964:Sodium / Q000494:pharmacology; D013963:Thyroid Hormones / Q000378:metabolism* / Q000494:pharmacology; D013997:Time Factors
D013956:Antithyroid Agents; D002352:Carrier Proteins; D003470:Culture Media; D013963:Thyroid Hormones; D012964:Sodium