Abstract
Cibenzoline, a class I‐a antiarrhythmic drug, was investigated for the effect on Na+/Ca2+ exchange current (INCX) using the whole‐cell voltage clamp with isolated cardiac ventricular myocytes of guinea pig. INCX was recorded by ramp pulses from the holding potential of −60 mV with the external solution containing 140 mM Na+ and 1 mM Ca2+, and the pipette solution containing 20 mM Na+, 20 mM BAPTA and 10 mM Ca2+ (182 nM free Ca2+). Cibenzoline caused a concentration‐dependent inhibition of INCX. The IC50 values of cibenzoline for the outward and inward components of INCX were 77 microM and 84 microM, 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 INCX at pH 6.5 was smaller than those at pH 7.4 and pH 8.2. Delayed after‐depolarization (DADs) induced by electrical stimulation with ouabain disappeared in the presence of 100 microM cibenzoline. We conclude that cibenzoline inhibits INCX in a supra‐therapeutic range of concentrations possibly from the external or intra‐membrane site of the exchanger molecule.
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