Abnormalities of the fast sodium current in myotonic dystrophy, recessive generalized myotonia, and adynamia episodica

Myoballs, i.e., spherical muscle cell regenerates, were cultured from the biopsied muscles of three patients with myotonic dystrophy, three patients with recessive generalized myotonia, and a patient with adynamia episodica. The membrane of these myoballs was voltage-clamped in the whole-cell mode for the recording of sodium currents (at 11, 24, and 37°C). The voltage dependence of the steady-state activation and inactivation curves showed only minor abnormalities in all cases. The time constants of activation (τm) and inactivation (τh), when studied at the three temperatures, showed a characteristic pattern of abnormalities. In myotonic dystrophy, both τm and τh were larger than control; in recessive generalized myotonia and adynamia episodica both τm and τh were smaller than control. In the latter diseases, these time constants also showed a smaller than normal decrease with membrane depolarization. The changes seen for recessive generalized myotonia and adynamia episodica would favour the occurence of myotonia, the opposite results for myotonic dystrophy would oppose myotonia.