Pharmacological Activation of IKr Impairs Conduction in Guinea Pig Hearts

Activation of I_Kr Impairs Conduction. Introduction: The hERG (Kv11.1) potassium channel underlies cardiac I_Kr and is important for cardiac repolarization. Recently, hERG agonists have emerged as potential antiarrhythmic drugs. As modulation of outward potassium currents has been suggested to modulate cardiac conduction, we tested the hypothesis that pharmacological activation of I_Kr results in impaired cardiac conduction. Methods and Results: Cardiac conduction was assessed in Langendorff‐perfused guinea pig hearts. Application of the hERG agonist NS3623 (10 μM) prolonged the QRS rate dependently. A significant prolongation (16 ± 6%) was observed at short basic cycle length (BCL 90 ms) but not at longer cycle lengths (BCL 250 ms). The effect could be reversed by the I_Kr blocker E4031 (1 μM). While partial I_Na inhibition with flecainide (1 μM) alone prolonged the QRS (34 ± 3%, BCL 250 ms), the QRS was further prolonged by 19 ± 2% when NS3623 was added in the presence of flecainide. These data suggest that the effect of NS3623 was dependent on sodium channel availability. Surprisingly, in the presence of the voltage sensitive dye di‐4‐ANEPPS a similar potentiation of the effect of NS3623 was observed. With di‐4‐ANEPPS, NS3623 prolonged the QRS significantly (26 ± 4%, BCL 250 ms) compared to control with a corresponding decrease in conduction velocity. Conclusion: Pharmacological activation of I_Kr by the hERG agonist NS3623 impairs cardiac conduction. The effect is dependent on sodium channel availability. These findings suggest a role for I_Kr in modulating cardiac conduction and may have implications for the use of hERG agonists as antiarrhythmic drugs. (J Cardiovasc Electrophysiol, Vol. 21, pp. 923‐929, August 2010)