Multiple isoforms of myosin light chain 1 in pig diaphragm slow fibers: Correlation with maximal shortening velocity and force generation

Pig diaphragm slow fibers exhibit heterogeneity in myosin light chain I (MLCI) isoform expression, with many expressing fast-type MLCI (MLCIF), as well as two isoforms of slow-type MLCI (MLCISa and MLCISb). The goal of this study was to test if there is a relationship between MLCI isoform expression and contractile properties among these fibers. Maximal shortening velocity (V-max) and maximal isometric force generation, normalized with fiber cross-sectional area (P-o/CSA), were measured in single fibers. V-max was inversely related to the relative level of MLCISa. The level of MLCI Sa was reciprocally related to the levels of MLClSb and of MLCIF among individual fibers. Fibers expressing MLClSa and in which MLClSb was not detected generated greater P-o/CSA, compared to fibers expressing MLCISb and not MLClSa. The results indicate a complex pattern of MLCI isoform expression among pig diaphragm slow fibers and suggest that shortening velocity and force generation are modulated.. in these fibers, by the MLC I isoform composition. (c) 2006 Elsevier Inc. All rights reserved.