3,5,3′-Tri-iodothyronine enhances sugar transport in rat thymocytes by increasing the intrinsic activity of the plasma membrane sugar transporter

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Abstract
We have shown that 3,5,3′-tri-iodothyronine (T3) produces a prompt increase in sugar transport in rat thymocytes by increasing the maximal velocity without changing the Michaelis–Menten constant of the plasma membrane sugar transport system. To elucidate further the mechanism of this effect, we have now assessed the influence of T3 on the number and affinity of sugar transporters in thymocytes, measured as the sugar (2-deoxyglucose; dGlc)-displaceable binding of cytochalasin B. Cytochalasin B inhibited in a dose-related manner the uptake of dGlc by rat thymocytes with inhibition constant values of 0·19 and 0·22 μmol/l in the presence and absence of T3 respectively. Binding of cytochalasin B by the sugar-displaceable sites was rapid and saturable, demonstrating a single class of sites having an apparent dissociation constant of 0·33 ± 0·02 (s.d.) μmol/l and maximal binding capacity of 3·73±0·48 pmol/20 × 106 cells (11·2±1·4 × 104 sites/thymocyte). In the rat thymocyte, sugar transporters were found to be located in two major subcellular pools, the plasma membrane and microsomes, the latter being about twice the size of the former. In these subcellular compartments, as well as in the intact cell, binding of [3H]cytochalasin B by the sugar-displaceable sites constituted about 40% of total cytochalasin B binding. 3,5,3′-Tri-iodothyronine in concentrations that stimulated uptake of dGlc by thymocytes had no effect on [3H]cytochalasin B binding (total and sugar-displaceable) in the intact cell and in the plasma membrane and microsomal compartments, nor did it influence the affinity and number of sugar transporters. From these observations we conclude that, in the rat thymocyte, T3 acts to increase sugar uptake by increasing the intrinsic activity, i.e. the efficiency, rather than the number of sugar transporters. Journal of Endocrinology (1990) 124, 133–140

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