POLYMORPHISM OF MYOSIN AMONG SKELETAL MUSCLE FIBER TYPES GERALDINE F. GAUTHIER and SUSAN LOWEY

An immunocytochemical approach was used to localize myosin with respect to individual fibers in rat skeletal muscle. Transverse cryostat sections of rat diaphragm, a fast-twitch muscle, were exposed to fluorescein-labeled immunoglobulin against purified chicken pectoralis myosin. Fluorescence microscopy revealed a differential response among fiber types, identified on the basis of mitochondrial content. All white and intermediate fibers but only about half of the red fibers reacted with this antimyosin. In addition, an alkali-stable ATPase had the same pattern of distribution among fibers, which is consistent with the existence of two categories of red fibers. The positive response of certain red fibers indicates either that their myosin has antigenic determinants in common with "white" myosin, or that the immunogen contained a " red" myosin. Myosin, extracted from a small region of the pectoralis which consists entirely of white fibers, was used to prepare an immunoadsorbent column to isolate antibodies specific for white myosin. This purified anti-white myosin reacted with the same fibers of the rat diaphragm that had reacted with antibodies against whole pectoralis myosin. This indicates that the myosins of white, intermediate, and some red fibers are sufficiently homologous to share antigenic determinants. In a slow-twitch muscle, the soleus, only a minority of the fibers reacted with antipectoralis myosin. The majority failed to respond; hence, they are not equivalent to intermediate fibers of the diaphragm~ despite their intermediate mitochondrial content. Immunocytochemical analysis of two different musles of the rat has demonstrated that more than one isoenzyme of myosin can exist in a single muscle, and that individual fiber types can be recognized by immunological differences in their myosins. We conclude that, in the rat diaphragm, there are at least two immunochemically distinct types of myosin and four types of muscle fibers: white, intermediate, and two red. We suggest that these fibers correspond to the four types of motor units described by Burke et al. (Burke, R. E., D. N. Levine, P. Tsairis, and F. E. Zajac, III. 1973. J. Physiol. (Lond.) 234:723-748.) in the cat gastrocnemius. 760 THE JOURNAL OF CELL BIOLOGY " VOLUME 74, 1977 9 pages 760-779 on Jauary 4, 2018 jcb.rress.org D ow nladed fom