Fiber type-specific differences in glucose uptake by single fibers from skeletal muscles of 9- and 25-month-old rats.

The primary purpose of this study was to assess the feasibility of applying a novel approach to measure myosin heavy chain (MHC) isoform expression, glucose uptake, fiber volume, and protein abundance in single muscle fibers of adult (9 months) and old (25 months) rats. Epitrochlearis muscle fibers were successfully isolated and analyzed for MHC isoform expression, glucose uptake, fiber volume, and protein (COXIV, APPL1, IκB-β) abundance. Insulin-stimulated glucose uptake by single fibers did not differ between age groups, but there was a significant difference between fiber types (IIA > IIX > IIB/X ≈ IIB). There were also significant main effects of fiber type on APPL1 (IIX > IIB) and COXIV (IIA > IIX > IIB/X ≈ IIB) abundance, and IIB fibers were significantly larger than IIA fibers. This study established the feasibility of a new approach for assessing age-related differences in muscle at the single-fiber level and demonstrated the magnitude and rank order for fiber-type differences in insulin-stimulated glucose uptake of 9-month-old and 25-month-old rats.