[Myotonia and channelopathy]

Myotonia occurs with abnormalities of the muscle membrane, and there have been reports of various channel abnormalities including Cl, Na, and Ca channels. Historically in 1966, Lipicky and Bryant found the abnormality of the muscle Cl channel in Thomsen's disease and hereditary myotonia of goat. Thereafter, Na channel abnormality was found in myotonia associated with familial hyperkalemic periodic paralysis and paramyotonia congenita by using patch clamp technique. Myotonic dystrophy is the most common myotonic disorder and genetic studies disclosed the presence of abnormal triplet repeat in the chromosome 19q13.1 and the gene product is known as myotonin protein kinase. However, pathophysiological mechanism of myotonia in myotonic dystrophy is not yet clear except that there are mild decrease of Cl conductance in 5 out of 8 muscle fibers and late opening of Na channels also occurs. As for the treatment of myotonia, medications which work as Na channel blockers have been used and they are effective to reduce myotonia, but they do not improve muscle weakness. Recently dehydroepiandrosterone sulfate which is a male hormone secreted from the adrenal cortex has been used for myotonic dystrophy and the drug reduces myotonia and improves activities of daily living. The author confirmed that myotonia can be induced in the muscle preparations of mice by either anthracene-9-carboxylic acid (Cl channel blocker), or by anemone toxin (blocker of inactivation of Na channel), and dehydroepiandrosterone sulfate can reduce myotonic bursts in both of these muscle preparations by intracellular recordings.