Coordinated Variations in Myosin Light Chain and Troponin T Isoforms and in Maximal Shortening Velocity in Human Slow Skeletal Muscle Fibers

Abstract

Recent measurements revealed a 1.7-fold range in maximal shortening velocity (Vmax) among a set of slow fibers from adult human skeletal muscle. The fibers were examined on multiple gel formats to determine whether there were differences in myofibrillar protein isoform composition that might explain the variation in Vmax. All of the fibers in this set were determined to express exclusively slow-type myosin heavy chain (MHC). The fibers were then examined on a gel format to reveal potential differences in myosin light chain (MLC) isoform composition. The faster fibers in this set expressed substantial amounts of fast-type MLC1 (MLC1F), along with slow type MLC1 (MLC1b) and slow type MLC2, and their mean Vmax was significantly higher (24%) than the slower fibers in this set, which had only trace or no detectable amounts of MLC1F. The slower fibers had greater amounts of a protein that is tentatively identified as MLC1Sa, which others have reported to be expressed in human skeletal muscle. The same gel format also indicated differences in troponin T (TnT) isoform composition among the same fibers. Regression analysis revealed that there are strong associations between the expression of specific TnT isoforms and the expression of specific MLC 1 isoforms among human slow skeletal muscle fibers. Conclusion: coordinated variations in MLC isoform and TnT isoform appear to provide a mechanism to expand the range of contractile properties of human slow muscle fibers that express exclusively slow-type MHC.