The Effect of Triiodothyronine on Insulin Binding and Action in Rat Adipocytes

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Abstract
The effects of a short term (2 hour) incubation of 5 μM triiodothyronine (T3) on 125I-insulin binding and insulin stimulated (14C)-2-deoxy-D-glucose uptake in rat adipocytes was investigated. In the presence of 5 μM T3, (14C)-2-deoxy-D-glucose uptake was significantly decreased by 11 to 24% at insulin concentrations of 5 to 1000 μU/ml. The concentration of insulin eliciting a half maximal response for insulin stimulated (14C)-2-deoxy-D-glucose uptake was 11.5 μU/ml in the control, and 14.3 μU/ml in the T3, treated cells (P3 treated adipocytes bound 9 to 22% less 125I-labeled insulin yet the concentration of native insulin necessary to displace 50% of the bound 125I-labeled insulin was the same in the control and T3, treated cells (75 and 70 ng/ml, respectively). These studies indicate that the decreased sensitivity of T3, treated cells to insulin is in accordance with a decreased number of receptors with the same binding characteristics as those of the control cells. The decreased maximal uptake of (14C)-2-deoxy-D-glucose at saturating insulin levels is likely to be independent of receptor number and result from a second, undetermined alteration in the hexose transport system of adipocytes treated with T3.

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