Effects of lignocaine and quinidine on the persistent sodium current in rat ventricular myocytes.

1. The effects of the Class 1 antiarrhythmic agents lignocaine and quinidine on action potentials, and on sodium currents and potassium currents activated by depolarization, were examined in rat isolated ventricular myocytes by the whole cell, tight seal recording technique. 2. Tetrodotoxin and lignocaine shortened, whereas quinidine prolonged, the duration of the plateau phase of action potentials. 3. At low concentrations, lignocaine and quinidine blocked a persistent sodium current that was resistant to inactivation but they had only a small effect on the transient sodium current. At higher concentrations, they also blocked the transient sodium current. 4. Quinidine, but not tetrodotoxin or lignocaine, depressed potassium currents activated by depolarization and this could account for the prolongation of the plateau phase caused by quinidine. 5. It is suggested that block of the persistent sodium current may be responsible, at least in part, for the antiarrhythmic action of lignocaine and quinidine.