Increase in action potential duration and inhibition of the delayed rectifier outward current IK by berberine in cat ventricular myocytes

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Abstract
1 In the present work, the effects of the antiarrhythmic drug, berberine, on action potential and ionic currents of cat ventricular myocytes were studied. 2 Berberine prolonged action potential duration in cat ventricular myocytes without altering other variables of the action potential. 3 The drug at concentrations of 0.3–30 μ blocked only the delayed rectifier (IK) current with an IC50 = 4.1 μ. Berberine produced a tonic block and a phasic block that was increased with the duration of the depolarizing pulse. The blocking effect on IK was use-dependent, but not frequency-dependent. 4 In cardiac preparations two delayed rectifier currents have been found: a rapid (IK) current and a slow (IK) current. In the present work it has been found that berberine at the concentrations used, selectively blocked IKr. 5 At concentrations higher than 10 μ it also decreased the transient outward (Ito1) current. The drug did not have effects on the inward rectifier (IK1) or the high threshold calcium current (ICa-L). 6 These results show that berberine is a specific potassium channel blocker. The increase in action potential duration induced by berberine can be explained mainly by its blocking effects on IK.