DISSOCIATION OF GLUT4 TRANSLOCATION AND INSULIN-STIMULATED GLUCOSE-TRANSPORT IN TRANSGENIC MICE OVEREXPRESSING GLUT1 IN SKELETAL-MUSCLE

Overexpression of the human GLUT1 glucose transporter protein in skele tal muscle of transgenic mice results in large increases in basal gluc ose transport and metabolism, but impaired stimulation of glucose tran sport by insulin, contractions, or hypoxia (Gulve, E, A., Ren, J,-M,, Marshall, B, A., Gao, J,, Hansen, P, A., Holloszy, J. O., and Mueckler , M. (1994) J, Biol, Chem, 269, 18366-18370). This study examined the relationship between glucose transport and cell-surface glucose transp orter content in isolated skeletal muscle from wild-type and GLUT1-ove rexpressing mice using 2-deoxyglucose, 3-O-methylglucose, and the -N-[ 4-(1-azi-2,2,2-trifluoroethyl)benzoyl]-1,3-bis (D-mannos-4-yloxy)-2-pr opylamine exofacial photolabeling technique. Insulin (2 milliunits/ml) stimulated a 3-fold increase in a-deoxyglucose uptake in extensor dig itorum longus muscles of control mice (0.47 +/- 0.07 mu mol/ml/20 min in basal muscle versus 1.44 mu mol/ml/20 min in insulin-stimulated mus cle; mean +/- S.E.), Insulin failed to increase a-deoxyglucose uptake above basal rates in muscles overexpressing GLUT1 (4.00 +/- 0.40 mu mo l/ml/20 min in basal muscle versus 3.96 +/- 0.37 mu mol/ml/20 min in i nsulin-stimulated muscle). A similar lack of insulin stimulation in mu scles overexpressing GLUT1 was observed using 3-O-methylglucose. Howev er, the magnitude of the insulin-stimulated increase in cell-surface G LUT4 photolabeling was nearly identical (similar to 3-fold) in wild-ty pe and GLUT1-overexpressing muscles. This apparently normal insulin-st imulated translocation of GLUT4 in GLUT1-overexpressing muscle was con firmed by immunoelectron microscopy. Our findings suggest that GLUT4 a ctivity at the plasma membrane can be dissociated from the plasma memb rane content of GLUT4 molecules and thus suggest that the intrinsic ac tivity of GLUT4 is subject to regulation.