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

Abstract

A B S T R A C T 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 ,uM, 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 ,uM. 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, 3-0-methylglucose (3-0-MG), an effect that was evidently the result of an increase in 3-0-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.

In vivo stimulation of sugar uptake in rat thymocytes. An extranuclear action of 3,5,3'-triiodothyronine.

“…Blood was collected, from which serum was obtained, and thymus glands were quickly removed and placed into ice-cold Tris-Krebs-Ringer buffer, pH 7.4. Thymocytes were then isolated by methods previously described (1). The isolated cells were resuspended in ice-cold buffer, and 200-,gl aliquots (-30 X 1O6 cells/ ml) were transferred to microtubes and centrifuged in a Beckman Microfuge (Beckman Instruments, Inc., Palo Alto, CA) for 30 s. The supernatant was aspirated, and the pelleted cells were transferred to vials containing liquid scintillation cocktail, as were aliquots of serum.…”

“…4 In Cell viability. Cell viability was assessed by means ofthe trypan blue technique (1), in which a viable cell is one that excludes the dye.…”

“…We have previously demonstrated that 3,5,3'-triiodothyronine (T3)' increases the uptake ofthe glucose analogue 2-deoxyglucose (2-DG) in isolated rat thymocytes in vitro (1)(2)(3)(4)(5). This effect of T3 is evident within 20 min after the addition of the hormones; it is independent of new protein synthesis, is calcium-dependent, and is mediated by an increase in cellular cyclic AMP (cAMP) concentration secondary to an increase in adenylate cyclase activity.…”

Thyroid hormones and the central nervous system of mammals (Review)

“…By the 1960s and 1970s, the existence of TH-binding sites at the plasma membrane had already been reported (discussed in ref. 46), and some of these sites were proposed to be involved in triggering early hormonal effects, such as increased uptake of amino acids, nucleosides and glucose into target cells (107)(108)(109)(110)(111)(112). However, over the following decades, after the identification and cloning of the genes encoding nTRs, most work focused on the nuclear pathway of TH action.…”

Effect of tryptophan on the early tri-iodothyronine uptake in mouse thymocytes

“…Isolation of thymocytes was performed as previously described (21). Cell viability was tested before and after experiments and, as assessed by trypan blue dye exclusion, was always greater than 95%.…”