Direct evidence of fiber type-dependent GLUT-4 expression in human skeletal muscle

GLUT-4 expression in individual fibers of human skeletal muscles in younger and older adults was studied. Furthermore, the dependency of insulin-stimulated glucose uptake on fiber type distribution was investigated. Fiber type distribution was determined in cryosections of muscle biopsies from 8 younger (29 yr) and 8 older (64 yr) healthy subjects, and estimates of GLUT-4 expression in individual fibers were obtained by combining immunohistochemistry and stereology. GLUT-4 was more abundantly expressed in slow compared with fast muscle fibers in both younger (P < 0.007) and older (P < 0.001) subjects. A 25% reduction of GLUT-4 density in fast fibers (P < 0.001) and an unchanged GLUT-4 density in slow fibers were demonstrated in older compared with younger subjects. Insulin-stimulated glucose uptake rates measured by hyperinsulinemic, euglycemic clamp were not correlated with the fraction of slow fibers in the young (r = -0.45, P > 0.25) or in the elderly(r = 0.11, P > 0.75) subjects. In conclusion, in human skeletal muscle, GLUT-I expression is fiber type dependent and decreases with age, particularly in fast muscle fibers.