Stimulation by Triiodothyronine of the In Vitro Uptake of Sugars by Rat Thymocytes

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Abstract
Studies were conducted to ascertain the in vitro effect of 3,5,3′-triiodothyronine (T3) on the accumulation of the glucose analogue, 2-deoxyglucose (2-DG), by thymocytes freshly isolated from weanling rats. At a concentration of 1 μM, T3 stimulated the 15-min uptake of 3H-2-DG after cells had been exposed to T3 for only 30 min. Significant stimulation of 2-DG accumulation was produced by 1 nM T3, with increasing stimulation at doses ranging up to 10 μM. T3 did not alter the fraction of accumulated 2-DG that was phosphorylated, and kinetic studies indicated that its effect was associated with a significant increase in the apparent Vmax of 2-DG accumulation, but not the apparent Km. T3 also enhanced the accumulation by thymocytes of the nonmetabolized glucose analogue, 3-O-methylglucose (3-O-MG), an effect that was evidently the result of an increase in 3-O-MG transport into the cell, because it was seen in cells incubated with 3H-3-O-MG for only 30 s. The proportionate increase in 2-DG accumulation produced by T3 was not altered by preincubating cells with concentrations of puromycin or cycloheximide sufficient to reduce [3H]-leucine incorporation by 95%, and T3 over a period of >2 h had no effect on [3H]leucine incorporation itself. These results indicate that T3 stimulates the uptake of sugars in rat thymocytes in vitro by an effect on their inward transport. The promptness of the effect and its failure to be inhibited during profound inhibition of protein synthesis further indicate that this effect of T3 is not mediated through a nuclear-dependent mechanism. Rather, the properties of this response, and of the increases in amino acid and 2-DG accumulation produced by T3 in other tissue preparations, strongly suggest that these effects of T3 are mediated at the level of cell membrane.