Cell fractionation studies indicate that dystrophin is a protein of surface membranes of skeletal muscle.

We studied the subcellular localization of dystrophin in rabbit skeletal muscle. In Western-blot analysis of membrane preparations, dystrophin was associated with the sarcolemmal fraction, as indicated by cholesterol content and co-purification with ouabain-binding activity and beta-adrenergic receptor. Dystrophin was also found with junctional T-tubules, but not with 'free' T-tubules, longitudinal portions or terminal cisternae of the sarcoplasmic reticulum. Dystrophin was not solubilized by high salt solutions, but it was solubilized by low concentrations of detergents (Triton X-100 and deoxycholate), suggesting that it is a peripheral membrane protein.

Synthesis of fast myosin induced by fast ectopic innervation of rat soleus muscle is restricted to the ectopic endplate region.

Skeletal muscle fibres, long multinucleated cells, arise by fusion of mononucleated myoblasts to form a myotube that matures into the adult fibre. The two major types of mature fibre, fast and slow fibres, differ physiologically in their rate of isotonic shortening. At the molecular level these type-specific physiological properties are ascribed to different isoforms of myosin, a major protein involved in shortening. Differentiation of fast and slow fibres seems to be under the control of motoneurones, and mature fibres are innervated by only one motoneurone. When rat soleus muscle (SOL, a slow muscle) is dually innervated with a fast nerve, it acquires some properties of a fast muscle, that is, low sensitivity to caffeine and high glycogen content. We report here that in dually innervated soleus muscle the foreign fast nerve induces synthesis of fast isoforms of myosin, but only in the segment of the muscle fibre that is close to the foreign endplate. The localized influence of the nerve endplates suggest that factors controlling the phenotypic expression of the muscle fibre have a short range of activity.

Myosin light chains and muscle pathology.

We evaluated the isoform composition of heavy and light chains of myosin in single muscle fibers from patients with Duchenne dystrophy, myotonic dystrophy, or polymyositis. In all myopathic muscles, there was an increase in the proportion of intermediate fibers which, by analysis of myosin isoforms, fell into two subpopulations, one that contained both fast and slow myosin and another that contained myosin molecular hybrids. The increased proportion of intermediate (or transitional) fibers suggests changes in the equilibrium between fast and slow motor units. These changes could result from regeneration and subsequent maturation of fibers or from direct transformation of mature fibers of one type into the opposite.

Fast to slow transition induced by experimental myotonia in rat EDL muscle.

Experimental myotonia was induced by feeding rats with 20,25-diazacholesterol for up to 8 months. Histochemical analysis of myotonic extensor digitorum longus (EDL) muscle showed a progressive decrease of type IIB fibres and a concomitant increase of type IIA and type I fibres. A transient hypertrophy of type IIA fibres was observed 6 months after beginning the treatment. Analysis of the pattern of myosin light chains of single fibres from EDL showed that myotonia caused a progressive decrease of fibres showing a pure fast myosin light chain pattern and an increase of fibres showing coexistence of fast and slow myosin light chains (intermediate fibres). Only a small percentage of intermediate fibres showed coexistence of fast and slow myosin heavy chains. Myotonic fibres presented an increased sensitivity to caffeine which approached that of normal soleus fibres. Furthermore, sarcoplasmic reticulum (SR) vesicles isolated from hind limb fast muscles of myotonic rats demonstrated a decrease of Ca2+-dependent ATPase and Ca2+-transport activities as well as a decrease of immunoreactivity with anti-rabbit SR fast Ca2+-ATPase antibody. These results suggest that the increased electrical activity brought about by 20,25-diazacholesterol-induced myotonia, caused a fast to slow transition in the phenotypic expression of myosin and sarcoplasmic reticulum proteins.