The effects of Cl- channel blockers 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) and niflumic acid (NFA) on aconitine-induced arrhythmias were investigated. Left ventricular pressure and electrocardiogram were monitored in Langendorff-perfused rat hearts. Whole-cell patch-clamp and current-clamp techniques were used to measure sodium current (I(Na)) and action potential (AP), respectively, in single rat cardiac ventricular myocytes. Addition of the Na+ channel agonist aconitine (0.1 microM) to the perfusion solution produced polymorphic ventricular arrhythmias with a latent period of 25.5 +/- 6.3 s. NPPB could reverse aconitine-induced arrhythmias. A similar effect was observed by using NFA. NPPB and NFA reversibly depressed the upstroke of the AP in a dose-dependent manner with IC50 values of approximately 12.3 and approximately 73.1 microM, respectively, without significantly affecting the resting potential of rat ventricular myocytes. Both Cl- channel blockers inhibited I(Na) and induced a leftward shift of the steady-state inactivation of I(Na). In conclusion, the results of this study demonstrate that NPPB as well as NFA can suppress aconitine-induced arrhythmias in rat hearts mainly by inhibiting cardiac I(Na).