Fiber type-specific expression of GLUT4 in human skeletal muscle - Influence of exercise training

The fiber type-specific expression of skeletal muscle GLUT4 and the effect of 2 weeks of low-intensity training were investigated in 8 young untrained male subjects. Single muscle fibers were dissected from a vastus lateralis biopsy sample. Based on myosin heavy chain (MHC) expression, fibers were p ooled into 3 groups (MHC I, MHC IIA, and MHC IIX), and the GLUT4 content of 15-40 pooled fibers was determined using SDS-PAGE and immunological detect ion. The GLUT4 content in pooled muscle fibers expressing MHC I was similar to 20% higher (P < 0.05) than that in muscle fibers expressing MHC IIA or MHC IIX. No difference in GLUT4 could be detected between fibers expressing MHC IIA or MHC IIX. Two weeks of exercise training increased (P < 0.05) th e peak power output of the knee extensors by 13%, the maximal activities of citrate synthase and 3-hydroxyacyl-CoA dehydrogenase by 21 and 18%, respec tively, and the GLUT4 protein content by 26% in a muscle homogenate. Furthe rmore, a 23% increase (P < 0.05) in GLUT4 was seen in fibers expressing the MHC I isoform after exercise training for 2 weeks. No change was seen in f ibers expressing MHC IIA or MHC IIX. In conclusion, our data directly demon strate that GLUT4 is expressed in a fiber type-specific manner in human ske letal muscle, although fiber type differences are relatively small. In addi tion, low-intensity exercise training recruiting primarily fibers expressin g MHC I increased GLUT4 content in these fibers but not in fibers expressin g MHC IIA or MHC IIX, indicating that GLUT4 protein content is related more to activity level of the fiber than to its fiber type, which is defined by expression of contractile protein.