Abstract 178: Blocking the Acetylcholine Activated Inward Rectifier Potassium Current but not the Rapid Delayed Rectifier Potassium Current Restores Sinus Rhythm in the Chronically Fibrillating Sheep Heart

Abstract

Background: The acetylcholine activated inward rectifier potassium current (IKACh) was shown to be constitutively active in chronic atrial fibrillation. Its blockade has been proposed to be a possible antifibrillatory pharmacotherapy. We therefore tested the hypothesis that blocking IKACh with the bee venom peptide tertiapin, or the small molecule chloroquine, terminates chronic AF in the sheep heart. Methods and Results: We tested our hypothesis using biochemistry, electrophysiology and molecular modeling. In patch clamp, the IC50 of IKACh block by tertiapin, and chloroquine was 60 nM, and 710 nM respectively, while dofetilde, a currently used class III antiarrhythmic, did not block the current. On the other hand, dofetilide and chloroquine blocked the rapid delayed rectifier potassium current (IKr) with an IC50 of 50nM, and 2.3 microM respectively, while tertiapine had no effects. Furthermore, molecular modeling indicated that 1 chloroquine molecule blocks the intracellular ion permeation vestibule of the tetrameric Kir3.1, a molecular correlate of IKACh, by interacting with amino acids important for the channel’s rectification. Dofetilide did not interact with Kir3.1. In vitro fluorescence measurements of chloroquine titration into the purified Kir3.1 intracellular domain protein confirmed the computational results and indicated that chloroquine directly binds Kir3.1 with a stoichiometry of 1 chloroquine molecule per Kir3.1 tetramer. Finally, optical mapping of the chronically fibrillating sheep atria showed that while tertiapine, chloroquine, and dofetilide slowed down chronic AF and prolonged the action potential, only tertiapine and chloroquine (the IKACh blockers) restored normal sinus rhythm. Conclusions: IKACh could be a more powerful anti atrial fibrillation target compared to IKr. Pharmacological blockade of IKACh with a peptide, or a small molecule similar to chloroquine could terminate chronic AF and restore normal sinus rhythm.