GLUCOSE-TRANSPORT AND GLUCOSE TRANSPORTERS IN MUSCLE AND THEIR METABOLIC-REGULATION

Skeletal muscle is the primary tissue responsible for insulin-dependent glucose uptake in vivo; therefore, glucose uptake by this tissue plays an important role in determining glycemia. Glucose uptake in muscle occurs by a system of facilitated diffusion involving at least two distinct glucose transporters, GLUT-1 and GLUT-4. Both bind the fungal metabolite and inhibitor of glucose transport cytochalasin B. In human skeletal muscle, both types of transporters are detected immunologically, and corresponding mRNA transcripts of both transporter forms are detected. In human skeletal muscle cells in culture, in which contamination by other tissues is ruled out, a 50,000-M, polypeptide is photolabeled with cytochalasin B. In rat skeletal muscle, acute treatment with insulin in vivo increases glucose-transport activity and the number of specific cytochalasin B-binding sites at the plasma membrane. In mildly diabetic (streptozocin-induced) rats, the number of cytochalasin B-binding sites is decreased in total membranes, and preferentially in the plasma membrane. In response to acute insulin treatment, however, there is still recruitment of glucose transporters to the plasma membrane from an intracellular membrane store. Hence, migration of transporters does occur in this form of diabetes. In L6 muscle cells in culture, acute treatment (1 h) with insulin causes recruitment of glucose transporters to the plasma membrane, and prolonged exposure to insulin or to glucose-deprived medium causes increased expression of GLUT-1 mRNA and GLUT-1 protein. Prolonged exposure (24 h) to high glucose in the medium causes a decrease in the number of glucose transporters in the plasma membrane. Hence, in those cells the expression of the GLUT-1 glucose transporter is modulated by insulin.