[Intracellular recording of myotonia in mdx mouse and the effect of Ca antagonist in myotonia].

Bulfield and others found X-linked muscular dystrophic (mdx) mouse by screening C57 BL/10 mice. The serum CK and PK are high in mdx mice, and they develop muscle degeneration 10-15 days after birth. The regeneration is vigorous in mdx mice and almost all the muscle fibers are replaced by regenerated fibers by 60 days after birth. Although mdx mice have been developed as a model for X-linked muscular dystrophy we have found that myotonic bursts are recorded when a glass microelectrode is inserted into the muscle fibers of hemidiaphragm preparations of mdx mice. Insertion myotonia is ceased by addition of the Na channel blocker tetrotoxin. Myotonia is not reduced, nor ceased by lowering the extracellular Ca to 1/15 of the volume of ordinary Tyrode's solution. Calcium antagonist, nicardipine at the dose of 10(-7), and 10(-6)M/L do not reduce myotonic bursts. Higher dose of nicardipine up to 2 x 10(-5)M/L abolished myotonic bursts. These results indicate that myotonic bursts are related to muscle membrane abnormalities, and each action potential occurs through Na channel, but not through Ca channel Higher dose of calcium antagonist can abolish myotonia by affecting Na channel in addition to their primary effects of Ca channel. The clinical effects of the Ca antagonist for myotonia was reported in one study. Since previous medications for myotonia including quinine HCl, procaine amide, diphenylhydantoin, and carbamazepine have some side effects such as tinnitus, headache, nausea, cardiac blocks, and bone marrow suppression, Ca antagonist may be used as a safe therapeutic drug for myotonia.