CONTRACTION STIMULATES TRANSLOCATION OF GLUCOSE-TRANSPORTER GLUT4 IN SKELETAL-MUSCLE THROUGH A MECHANISM DISTINCT FROM THAT OF INSULIN

The acute effects of contraction and insulin on the glucose transport and GLUT4 glucose transporter translocation were investigated in rat soleus muscles by using a 3-O-methylglucose transport assay and the sensitive exofacial labeling technique with the impermeant photoaffinity reagent 2-N-4-(1-azi-2,2,2-trifluoroethyl)benzoyl-1,3-bis (D-mannose-4-yloxy)-2-propylamine (ATB-BMPA), respectively. Addition of wortmannin, which inhibits phosphatidylinositol 3-kinase, reduced insulin-stimulated glucose transport (8.8 +/- 0.5 mu mol per mi per h vs, 1.4 +/- 0.1 mu mol per mi per h) and GLUT4 translocation [2.79 +/- 0.20 pmol/g (wet muscle weight) vs. 0.49 +/- 0.05 pmol/g (wet muscle weight)]. In contrast, even at a high concentration (1 mu M), wortmannin had no effect on contraction-mediated glucose uptake (4.4 +/- 0.1 mu mol per ml per h vs, 4.1 +/- 0.2 mu mol per ml per h) and GLUT4 cell surface content [1.75 +/- 0.16 pmol/g (wet muscle weight) vs, 1.52 +/- 0.16 pmol/g (wet muscle weight)]. Contraction-mediated translocation of the GLUT4 transporters to the cell surface was closely correlated with the glucose transport activity and could account fully for the increment in glucose uptake after contraction, The combined effects of contraction and maximal insulin stimulation were greater than either stimulation alone on glucose transport activity (11.5 +/- 0.4 mu mol per ml per h vs, 5.6 +/- 0.2 mu mol per mi per h and 9.0 +/- 0.2 mu mol per ml per h) and on GLUT4 translocation [4.10 +/- 0.20 pmol/g (wet muscle weight) vs. 1.75 +/- 0.25 pmol/g (wet muscle weight) and 3.15 +/- 0.18 pmol/g (wet muscle weight)]. The results provide evidence that contraction stimulates translocation of GLUT4 in skeletal muscle through a mechanism distinct from that of insulin.