Decreased in vivo glucose uptake but normal expression of GLUT1 and GLUT4 in skeletal muscle of diabetic rats

Abstract

This study was designed to determine whether altered glucose transporter expression is essential for the in vivo insulin-resistant glucose uptake characteristic of streptozocin-induced diabetes. Immunofluorescence in rat skeletal muscle colocalizes GLUT4 with dystrophin, both intrinsic to muscle fibers. In contrast, GLUT1 is extrinsic to muscle fibers, probably in perineurial sheath. Immunoblotting shows that levels of GLUT1 and GLUT4 protein per DNA in hindlimb muscle are unaltered from control levels at 7 d of diabetes but decrease to approximately 20% of control at 14 d of diabetes. This decrease is prevented by insulin treatment. In adipose cells of 7 d diabetic rats, GLUT4 levels are depressed. Thus, GLUT4 undergoes tissue-specific regulation in response to diabetes. GLUT4 and GLUT1 mRNA levels in muscle are decreased 62-70% at both 7 and 14 d of diabetes and are restored by insulin treatment. At 7 d of diabetes, when GLUT4 protein levels in muscle are unaltered, in vivo insulin-stimulated glucose uptake measured by euglycemic clamp is 54% of control. This reflects impairment in both glycogen synthesis and glycolysis and the substrate common to these two pathways, glucose-6-phosphate, is decreased approximately 30% in muscle of diabetic rats. These findings suggest a defect early in the pathway of glucose utilization, probably at the step of glucose transport. Because GLUT1 and GLUT4 levels are unaltered at 7 d of diabetes, reduced glucose uptake in muscle probably reflects impaired glucose transporter translocation or intrinsic activity. Later, at 14 d of diabetes, GLUT1 and GLUT4 protein levels are reduced, suggesting that sequential defects may contribute to the insulin-resistant glucose transport characteristic of diabetes.