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

Summary

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 (P<0.01)

T₃ 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.

Key-Words:

Triiodothyronine - Insulin - Adipocytes - Insulin Binding - Glucose Uptake