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

Abstract

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 (Rm) following activity. The decrease ofRm 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,Rm 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 ofRm is the basis of the termination of a myotonic burst and for the warm-up phenomenon.