A post-tetanic decrease of membrane resistance in mammalian skeletal muscle fibres and its antimyotonic effects

Summary The duration of the after-depolarization following an action potential becomes shorter when rat skeletal muscle is stimulated repetitively. This is particularly conspicuous when a muscle has been made myotonic, e.g. by addition of 1 mM 2,4-D to the bathing solution, because in myotonic muscle the duration of the afterdepolarization is longer than in normal muscle. The accelerated decline of afterdepolarization is associated with a temporary decrease of the specific membrane resistance (R_m) following activity. The decrease ofR_m is more pronounced in myotonic than in nonmyotonic muscle. It becomes greater when activity is increased from 3 to 8 action potentials. In myotonic fibres which on the average have membrane resistances more than twice as large as normal,R_m can thus drop to normal values. The decrease of membrane resistance persists for more than a second after activity. We suggest that this temporary change ofR_m is the basis of the termination of a myotonic burst and for the warm-up phenomenon.This work was supported by the Deutsche Forschungsgemeinschaft.