GLUCOSE-TRANSPORT AND GLUT4 PROTEIN DISTRIBUTION IN SKELETAL-MUSCLE OF GLUT4 TRANSGENIC MICE

The aim of the present investigation was to determine whether the subc ellular distribution and insulin-stimulated translocation of the GLUT4 isoform of the glucose transporter are affected when GLUT4 is overexp ressed in mouse skeletal muscle, and if the overexpression of GLUT4 al ters maximal. insulin-stimulated glucose transport and metabolism. Rat es of glucose transport and metabolism were assessed by hind-limb perf usion in GLUT4 transgenic (TG) mice and non-transgenic (NTG) controls. Glucose-transport activity was determined under basal (no insulin), s ubmaximal (0.2 m-unit/ml) and maximal (10 m-units/ml) insulin conditio ns using a perfusate containing 8 mM 3-O-methyl-D-glucose. Glucose met abolism was quantified by perfusing the hind limbs for 25 min with a p erfusate containing 8 mM glucose and 10 m-units/ml insulin. Under basa l conditions, there was no difference in muscle glucose transport betw een TG (1.10+/-0.10 mu mol/h per g; mean+/-S.E.M.) and NTG (0.93+/-0.1 6 mu mol/h per g) mice. However, TG mice displayed significantly great er glucose-transport activity during submaximal (4.42+/-0.49 compared with 2.69+/-0.33 mu mol/h per g) and maximal (11.68+/-1.13 compared wi th 7.53+/-0.80 mu mol/h per g) insulin stimulation, Nevertheless, over expression of the GLUT4 protein did not alter maximal rates of glucose metabolism. Membrane purification revealed that, under basal conditio ns, plasma-membrane (similar to 12-fold) and intracellular-membrane (s imilar to 4-fold) GLUT4 protein concentrations were greater in TG than NTG mice. Submaximal insulin stimulation did not increase plasma-memb rane GLUT4 protein concentration whereas maximal insulin stimulation i ncreased this protein in both NTG (4.1-fold) and TG (2.6-fold) mice. T hese results suggest that the increase in insulin-stimulated glucose t ransport following overexpression of the GLUT4 protein is limited by f actors other than the plasma-membrane GLUT4 protein concentration. Fur thermore, GLUT4 overexpression is not coupled to glucose-metabolic cap acity.