Desmin protein content differs between fiber types in human single muscle fibers

Abstract

Muscle fibers are composed of various proteins that generate and transmit force. These proteins are integral in determining the structural and functional properties of muscle fibers. Differences in force production and recruitment patterns between type I and II muscle fibers may be a factor in determining cytoskeletal protein content. Contractile and cytoskeletal protein concentrations have been shown to differ on the basis of fiber type in whole muscle homogenates. The purpose of this study was to compare the content of the intermediate filament protein desmin between type I and type IIa single muscle fibers from a mixed muscle in human subjects. Biopsies were taken from the vastus lateralis of six recreationally active males. Approximately 150 single muscle fibers were dissected from each sample and analyzed using SDS-PAGE to determine myosin heavy chain (MHC) composition. Following identification, muscle fibers were pooled into two groups (MHC I and MHC IIa). Desmin and actin content within the pooled samples was determined via immunoblotting. On average, muscles samples were composed of 51+\−7 % type I, 2+\−1% type I/IIa, 27+\−5% type IIa, 19+\−4% type IIa/IIx and 1+\−1% type IIx MHC single fibers. Type II fibers had a larger cross-sectional area compared to type I (5574+\−169 vs. 3955+\−94 μm2, respectively) (P<0.05). Desmin and actin contents were 40% and 37% higher in type I fibers compared to type IIa fibers, respectively (P<0.05). However the desmin to actin ratio was similar between pooled type I and IIa single muscle fibers within the vastus lateralis. These data suggest that desmin and actin content is a function of muscle fiber type. These differences in cytoskeletal protein content may have implications for differences in contractile function and eccentric damage characteristics between fiber types.