Are hypercontracted muscle fibers artifacts and do they cause rupture of the plasma membrane?

Muscle fibers with hypercontracted zones (contractures) and plasma membrane defects are relatively frequent in Duchenne dystrophy, but artifacts might account for either. To investigate the problem further, we determined the frequency of hypercontracted muscle fibers in longitudinal, trichrome-stained paraffin sections in 143 muscle biopsy specimens in a wide variety of muscle diseases. The specimens were held at rest length during fixation. The incidence of hypercontracted fibers in Duchenne dystrophy was significantly higher than in any of the other diseases or in normal controls. We also induced contractures in rat muscle by fixation without restraint, electrical stimulation, and cutting the muscle fibers in vivo. Electron-microscopy showed no plasma membrane defects associated with the contractures, except in the cut fibers within 25 µm of their cut ends, which were probably overloaded with calcium. The results indicate that (1) in Duchenne dystrophy the contractures in muscle fibers held at rest length during fixation cannot be accounted for by artifact; (2) contractures occurring in normal muscle during fixation do not in and of themselves cause rupture of the muscle fiber plasma membrane.