Abstract
The in vitro effect of thyroid hormones on glucose transport in insulin-stimulated muscle cells or adipocytes is still unclear. The objective of the present study was to assess the direct effect of 3,3′,5-triiodothyronine (T3) on glucose transport and on the translocation of insulin-regulatable glucose transporter (GLUT4) in insulinstimulated rat adipocytes. This evaluation was performed using an in vitro assay to avoid the well-known systemic effects of this hormone (e.g.: hyperinsulinemia). Adipocytes were isolated from epididymal adipose tissue of Sprague-Dawley rats. Glucose transport assay and immunoblot analysis of GLUT4 were carried out in insulin-stimulated and unstimulated adipocytes after treating with or without T3. The results were as follows; 1) T3 inhibited the glucose transport in insulin-stimulated and unstimulated adipocytes in a dose-dependent manner. 2) T3 decreased the maximal response level (Vmax) but did not alter the sensitivity (Km) of glucose transport to insulin. 3) T3 did not affect the translocation of GLUT4 from the intracellular pool to the plasma membrane. We concluded that T3 inhibits the glucose transport in insulinstimulated adipocytes in a post-receptor level without affecting the translocation of GLUT4 from the intracellular pool to the plasma membrane. This suggests that T3 acts by decreasing the intrinsic activity or the accessibility of GLUT4 in the plasma membrane.
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