Role of Glucose Transporters in Glucocorticoid-Induced Insulin Resistance: GLUT4 Isoform in Rat Skeletal Muscle is Not Decreased by Dexamethasone

Abstract

The diabetogenic effects of glucocorticoid excess are due in part to peripheral resistance to insulin. To test the hypothesis that glucocorticoid-induced peripheral insulin resistance might be attributable to a decreased number of glucose transporters, we examined the effects of dexamethasone treatment on the expression of the GLUT4 (insulin regulatable) glucose transporter in skeletal muscle, the major site of insulin-mediated glucose uptake. Dexamethasone treatment of rats (1 mg/day for 1 wk) induced hyperglycemia and hyperinsulinemia. At dosages of either 0.1 or 1 mg/day, insulin-stimulated 2-deoxyglucose uptake in isolated soleus muscle was reduced by ≥ 50%, demonstrating the presence of insulin resistance in skeletal muscle. Immunoblots of crude membranes from deep quadriceps muscle showed that dexamethasone treatment (1 mg/day) increased the amount of GLUT4 protein by 84%. GLUT4 mRNA abundance was similarly increased when expressed per unit RNA but was unchanged when expressed on a DNA basis because the tissue RNA content was decreased by dexamethasone. In contrast to quadriceps, GLUT4 protein concentration in soleus and extensor digitorum longus extracts was not significantly increased by dexamethasone treatment. Because glucocorticoids cause selective atrophy of type IIb muscle fibers, which express relatively less GLUT4 protein, the apparent increase in GLUT4 content in quadriceps muscle from dexamethasone-treated animals may have resulted from inadvertent increased sampling of GLUT4-enriched type I and IIA fibers, caused by a glucocorticoid-induced decrease in the relative mass of the GLUT4-poor type IIb fibers. We conclude that glucocorticoids do not decrease GLUT4 content in skeletal muscle and that glucocorticoid-induced insulin resistance in this tissue is not due to suppression of glucose transporter gene expression.