Effects of quinidine,procaine amide,and N-propyl-ajmaline on skeletal muscle

Summary Membrane and action potentials of rat diaphragm fibres were measured in vitro as affected by quinidine, procaine amide, and N-propyl-ajmaline in concentrations of 10^–5 to 10^–4 g/ml.All three drugs had immediate effects on the action potential; the effects progressed the faster, the higher the concentration: the de- and repolarization were slowed, the duration increased, the overshoot decreased. Later (e.g., 1 h after application of 10^–5 g/ml quinidine) the resting potential started to decrease and the fibres became inexcitable.Electrical threshold was increased and the refractory period was prolonged by all three drugs, most effectively by N-propyl-ajmaline, least by procaine amide. All drugs abolished myotonic symptoms produced by reduction of extracellular chloride.Quinidine and N-propyl-ajmaline (10^–5 to 10^–4 g/ml) produced a transitory increase of contraction force, on prolonged drug action the muscle went into contracture. Procaine amide always had a negative inotropic effect. All drugs increased contraction time.This work was supported by the Deutsche Forschungsgemeinschaft.