Myosin isoforms, muscle fiber types, and transitions

Skeletal muscle is an extremely heterogeneous tissue composed of a variety of fast and slow fiber types and subtypes. Moreover, muscle fibers are vers atile entities capable of adjusting their phenotypic properties in response to altered functional demands. Major differences between muscle fiber type s relate to their myosin complement, i.e., isoforms of myosin Light and hea vy chains. Myosin heavy chain (MHC) isoforms appear to represent the most a ppropriate markers for fiber type delineation. On this basis, pure fiber ty pes are characterized by the expression of a single MHC isoform, whereas hy brid fiber type express two or more MHC isoforms. Hybrid fibers bridge the gap between the pure fiber types. The fiber population of skeletal muscles, thus, encompasses a continuum of pure and hybrid fiber types. Under certai n conditions, changes can be induced in MHC isoform expression heading in t he direction of either fast-to-slow or slow-to-fast. Increased neuromuscula r activity, mechanical loading, and hypothyroidism are conditions that indu ce fast-to-slow transitions, whereas reduced neuromuscular activity, mechan ical unloading, and hyperthyroidism cause transitions in the slow-to-fast d irection. (C) 2000 Wiley-Liss, Inc.