Involvement of cation transporting systems in myotonic diseases

Excitation-contraction coupling describes the series of events that beginning with propagated action potential on the muscle fiber surface membrane, leads to the twitch contraction of the fiber. Myotonic disorders are inherited diseases whose clinical manifestations include electrophysiological signs such as increased excitability and delayed relaxation of the muscles after voluntary contraction. All these disorders appear to be due to an abnormality of the muscle itself, since they persist after section or blocking of the motor nerve and after curarization. Most experimental and clinical data suggest that human myotonia arises from genetically-induced structural and functional alterations of the muscle cell membrane. The purpose of this review is to summarize the more recent developments in the molecular and pharmacological analysis of cation transporting systems such as ionic channels and (Na+,K+)-ATPase in myotonic disorders.