20,25‐diazacholesterol myotonia: An electrophysiological study

The electrophysiological characteristics of the myotonic syndrome produced syndrome produced in mammalian skeletal muscle by administration of 20,25‐diazacholesterol (20,25‐D) were studied in detail. In vivo electromyographic recordings confirmed widespread repetitive electrical activity, but delayed relaxation was evanescent and required isotonic rather than issometric conditions, with long recovery periods between stimuli, for clear demonstration. Subsequent administration of a potent inhibitorof membrane chloride conductance (G_ci) induced profound delays in relaxation different from that after chornic 20,25‐D alone. Intracellular passive cable analysis revealed only a small decrease in membrane G_ci and none in potassium conductance. Potassium current‐voltage relationships did not differ in control and treated animals. Intracellular microelectrode recordings consistently showed multiple driven action potentials during long depolarizations but no spontaneous myotonic discharges after cessation of the stimulus. Variations in temperature, buffer, and external ionic concentrations also failed to produce spontaneous activity. Anode break excitation under mild depolarizing conditions, however, did elicit repetitive membrane electrical activity. The myotonia induced by 20,25‐D is not due to low membrane G_ci. The relationship between delayed mechanical relaxation and membrane repetitive electrical activity remains to be clearly established in this myotonic syndrome.