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Inhibitory effect of cibenzoline on Na+/Ca2+ exchange current in guinea-pig cardiac ventricular myocytes.

The effect of cibenzoline, a class I antiarrhythmic drug, on Na(+)/Ca(2+) exchange current (I(NCX)) was investigated using the patch-clamp method. Cibenzoline inhibited the bi-directional I(NCX) in a concentration-dependent manner. The IC(50) values of cibenzoline for the outward and inward components of I(NCX) were 77 and 84 μM, respectively, with Hill coefficients of 1. Intracellular application of trypsin via the pipette solution did not change the inhibitory effect of cibenzoline. The inhibitory effect of cibenzoline on I(NCX) at pH(o) 6.5 was smaller than those at pH(o) 7.4 and pH(o) 8.2. We conclude that cibenzoline inhibits I(NCX) in supra-therapeutic concentrations.