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A dystrophin-related protein (DRP) encoded by a gene on chromosome 6 was studied in 14 normal and 79 pathological human skeletal muscle biopsies, as well as in cultured myotubes by light microscopic immunocytochemistry and quantitative immunoblots. In normal muscle immunoreactive DRP was present at the postjunctional surface membrane, at the surface of satellite cells, in the walls of blood vessels, in Schwann cells and in perineurium of intramuscular nerves. All of this produced a weak signal on immunoblots. In Duchenne/Becker dystrophy (DMD/BMD) and in polymyositis (PM) or dermatomyositis (DM) DRP was present throughout the extrajunctional surface membrane of extra- and intrafusal muscle fibers, particularly regenerating ones. This produced a 15-17-fold increase of DRP over normal in DMD/BMD and 4-10-fold increase over normal in PM and DM on immunoblots. In other pathological muscles, DRP localization pattern and quantity was about the same as in normals. Dystrophin-related protein was present in about the same amounts and distribution in normal and DMD cultured myoblasts and myotubes. The molecular stimulus for the marked upregulation of DRP in DMD/BMD and in the inflammatory myopathies is not known. In DMD/BMD the diffuse sarcolemmal DRP may partially compensate for dystrophin deficiency.

Two thousand rads of gamma irradiation delivered to the lower legs of ten day old normal and x-chromosome linked muscular dystrophy (mdx) mice caused significant inhibition of tibial bone and soleus muscle fiber growth. In the irradiated mdx solei, there was a major loss of muscle fibers, lack of central nucleation, and some endomysial fibrosis. These features were caused by a failure of regeneration of muscle fibers due to impaired proliferative capacity of satellite cells. Gamma irradiation transforms the late pathological phenotype of mdx muscles, so that in one major aspect (muscle fiber loss) they resemble muscles in Duchenne muscular dystrophy. However, extensive endomysial fibrosis which is another characteristic feature of Duchenne muscular dystrophy did not develop. This experimental model could be useful for the functional investigation of possible beneficial effects of therapeutic interventions in mdx dystrophy.

Immunoreactive dystrophin was examined in muscle fibers of quadriceps, extraocular muscles and cardiac ventricular muscles of female heterozygote mdx mice at 10, 35 and 60 days of age, with microscopic immunoperoxidase method and by immunoblots. In quadriceps muscle fibers there was a marked gradual diminution of the dystrophin-negative fiber segments between age 10 and 60 days. We suggest that this was partly due to a spontaneous fusion of dystrophin-competent satellite cells into the dystrophin-negative fiber segments and partly to an expansion of the cytoplasmic domain of dystrophin expression related to the original myonuclei. In cardiac muscle that lacks satellite cells, there was persistence of a large number of dystrophin-negative fiber segments even at age 60 days and probably beyond. The findings of this study have implications for the detection of heterozygote female carriers of Duchenne muscular dystrophy (DMD) and for the possible therapy of DMD muscles by myoblast transfer.

Muscle fibers of rat gastrocnemii were punctured in vivo with a tungsten wire whose tip was 10 microns wide. After two hours, most punctured fibers had clearly necrotic segments. The length of necrotic segments appeared to increase between three and four hours. Demarcation of surviving stumps from necrotic segments by a membrane was complete in most muscles by seven hours. At the boundary between necrotic segments and nondemarcated stumps there was a cap of densely aggregated or hypercontracted myofilaments which stained strongly for precipitated calcium. The membrane which demarcated the stumps was found partially formed in the necrotic pulp close to this cap, attached to the plasma membrane. While forming it had a free edge and probably resulted from spontaneous aggregation of phospholipids in the pulp. Other portions of newly formed membrane were found encircling organelles inside necrotic segments. Prompt formation of a demarcating membrane probably limits the extent of necrosis in injured fibers.

Transsected rat soleus muscles incubated in oxygenated Krebs solution in vitro at 37 degrees C develop prominent vacuolation in the vicinity of the cut ends of muscle fibers which extends further along the fiber with increasing time of incubation. Electron microscopy shows that the vacuoles represent dilated segments of T-tubules. Their formation is prevented if the muscle is not cut, or in the following situations: omission of Na+ from the medium, incubation at 10 degrees C, or in media containing either 2,4-dinitrophenol or ouabain. Vacuole formation is considerably reduced by substituting Cl- with organic anions or by adding 9-anthracene carboxylic acid to the medium. These results suggest that a massive influx of sodium into the muscle fibers through their cut ends results in maximal stimulation of the Na+-K+-ATPase of the T-tubular membrane leading to increasing Na+ concentration in the lumen of T-tubules followed by influx of Cl- to maintain the electrical gradient. The accumulation of these ions leads to the entry of water from either the extracellular space or the sarcoplasm and consequent dilatation of T-tubules. Rapid incorporation of lipids into T-tubular membrane to increase its surface is presumably necessary for this to occur. Similar T-tubular dilatation appears to occur in vivo in surviving portions of muscle fibers adjacent to segmental necrosis.