Myosin light chain isoform expression among single mammalian skeletal muscle fibers: species variations

Extensive heterogeneity in myosin heavy chain and light chain (MLC) isoform expression in skeletal muscle has been well documented in several mammalian species. The initial objective of this study was to determine the extent of heterogeneity in myosin isoform expression among single fibers in limb muscles of dogs, a species for which relatively little has been reported. Fibers were isolated from muscles that have different functions with respect to limb extension and limb flexion and were analyzed on SDS gels, with respect to myosin isoform composition. The results of this part of the Study indicate that there are at least four distinct fiber types in dog limb and diaphragm muscles. on the basis of MLC isoform expression: conventional fast (expressing fast-type isoforms of MLCI (MLClF) and MLC2 (MLC2F), plus MLC3), conventional slow (expressing slow-type MLCI (MLClS) and MLC2 (MLC2S)), hybrid (expressing MLClS, MLClF, MLC2S, MLC2F and MLC3) and a second slow fiber type, designated LIS S-IF. S-IF fibers express MLClF, along with MLClS and MLC2S and relatively low levels of MLC3. The fraction of slow fibers that Lire SIF fibers varies among dog limb muscles, being greater in limb extensors than flexors. Furthermore, the mean level of MLClF in SIF fibers is greater in extensors than flexors (mean levels ranee from similar to 3% to 50% of total MLCl). The study was, therefore, extended to Include six additional species, spanning a broad range in adult body size to more thoroughly characterize heterogeneity in MLC isoform expression among mammals. The results indicate that there Lire distinct patterns in MLC isoform expression among fast and slow fibers among different species. Specifically, large-size mammals have two distinct types of slow fibers, based upon MLC isoform composition (conventional and SIF fibers), whereas small mammals exhibit variations ill MLC isoforms between different types of fast fibers, including a fast fiber type that expresses MLClS (designated as F-IS fibers). S-IF fibers were absent in rodent Muscles and F-IS fibers were not found in large mammals. We conclude that extensive variation exists in MLC isoform expression in mammalian skeletal muscle fibers, yet there are distinct patterns among different species and among muscles within all individual species.