Electrical myotonia and cataract in X-linked muscular dystrophy (mdx) mouse.

An X chromosome-linked mouse mutant (mdx) has been investigated as an animal model of Duchenne's muscular dystrophy, and has been found to have the same defect of dystrophin in the muscle surface membrane. Intracellular recordings from the mdx mouse hemidiaphragm preparations revealed low resting membrane potentials and electrical myotonia which occurred at the time of microelectrode insertion and withdrawal. Electrical myotonia of the mdx mouse was observed in 30-50% of the impaled muscle fibers at low temperature, which decreased to only 7.8% at 37 degrees C. Electrical myotonia of mdx mice was not abolished by (+)-tubocurarine. Though there was no behavioral myotonia in mdx mice, repetitive bursts of action potentials in mdx mice were based on the abnormalities of the muscle membrane since neuromuscular blockade did not abolish the repetitive bursts. Also close observation of the lenses of mdx mice revealed cataracts from the newborn stage to the adult age. Slit lamp examination of the lenses of the mdx mice revealed nuclear cataracts followed by anterior subcapsular cataract as they grew. The cataract of mdx mice is different from that of myotonic dystrophy which is usually posterior subcapsular.