Characterization of triiodothyronine transport and accumulation in rat erythrocytes.
The transport of L-T3 was studied in washed rat erythrocytes. L-T3 uptake was temperature sensitive: the initial velocity of uptake at low substrate concentration was 40 times higher at 37 C than at 0C whereas, at equilibrium, the ratio of cell-associated to extracellular L-T3 was about 7 times lower at 37 C than at 0 C. When [125I]L-T3-loaded erythrocytes were diluted into a serum albumin-containing medium, the efflux of L-T3 proceeded at a rate similar to that of influx. A large excess of unlabeled L-T3 in the medium blocked influx and efflux of labeled L-T3, indicating a saturable carrier-mediated transport process across the plasma membrane. the transport obeyed simple Michaelis-Menten kinetics with an apparent Km of 53 nM and a Vmax of 4.3 pmol/min.10(8) cells at 0 C. The Km increased only slightly with temperature whereas the Vmax was 100 times higher at 37 than at 0 C. The Arrhenius activation energy of uptake was 21 Cal/mol. The nonsaturable adsorption of L-T3 to the cells did not exceed 1% of the equilibrium levels at 0 C and 10% at 37 C. Uptake of L-T3 was very specific: unlabeled L-T4, D-T3, triiodothyroacetic acid, rT3, and DL-thyronine inhibited uptake with inhibition constant (Ki) values which were 35, 60, 65, 110, and 250 times, respectively, greater than the Km of L-T3. [125I]L-T4 uptake was negligible. L-T3 uptake and L-T4 inhibition of L-T3 uptake were pH dependent. It is suggested that only the unionized 4'-OH forms of the hormones were recognized by the transport system. At equilibrium, L-T3 was accumulated within the cell (apparent intracellular concentration approximately 50 times higher than that in the medium at 37 C). However, uptake was not dependent on the transmembrane Na+ gradient, suggesting facilitated rather than active transport. Analysis of L-T3 binding to erythrocyte cytosolic proteins suggested that they were implicated in the intracellular trapping of L-T3. At a concentration of 5 x 10(9) erythrocytes/ml (approximately the blood concentration), the amount of L-T3 accumulated in the cells was 13.5 times higher than the extracellular amount. We conclude that L-T3 is solely transported by a saturable, stereospecific, and Na+-independent carrier system. The intracellular accumulation and the rapid transmembrane movements of L-T3 suggest that erythrocytes might play a role in the interorgan transport of L-T3.
Osty
J
J
Unité de Recherche sur la Glande Thyroide et la Régulation Hormonale (U.96) de I'Institut National de la Santé et de la Recherche Médicale, Le Kremlin-Bicêtre, France.
Jego
L
L
Francon
J
J
Blondeau
J P
JP
D000818:Animals; D001667:Binding, Competitive; D001692:Biological Transport / Q000187:drug effects; D004912:Erythrocytes / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008297:Male; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D012044:Regression Analysis; D012964:Sodium / Q000494:pharmacology; D013696:Temperature; D013816:Thermodynamics; D013974:Thyroxine / Q000097:blood; D014284:Triiodothyronine / Q000031:analogs & derivatives / Q000097:blood*
D014284:Triiodothyronine; D012964:Sodium; D013974:Thyroxine; D011188:Potassium