Pilsicainide, a class Ic agent, is known to be an effective drug particularly for treating atrial tachyarrhythmias. However, its electrophysiological effects on the atrium have not been well studied. To characterize the electrophysiologic effects of pilsicainide on atrial myocytes in class Ic drugs, we examined the effects of this drug on membrane currents in single rabbit atrial myocytes using the tight-seal whole cell voltage-clamp technique. Under the current-clamp condition, pilsicainide did not affect the action potential duration at therapeutic ranges (< or = 3 microM) and slightly shortened it at higher concentrations (> or = 10 microM). These observations were quite different from those with other class Ic agents including flecainide and propafenone which prolong the atrial action potential duration. The drug did not affect the resting membrane potential. Under the voltage-clamp condition, pilsicainide inhibited the transient outward current (Ito) that is more prominent in the atrium than in the ventricle in a concentration-dependent manner. However, in contrast to other class Ic agents, the inhibition to Ito by pilsicainide was observed only at much higher concentrations (IC50-300 microM) and did not affect the inactivation time-course of Ito. Moreover, the drug (10 microM) did not significantly affect the Ca2+, delayed rectifier K+, inward rectifying K+, acetylcholine-induced K+ or ATP-sensitive K+ currents. From these results pilsicainide could be differentiated as a pure Na+ channel blocker from other class Ic agents with diverse effects on membrane currents and should be recognized accordingly in clinical situations.