Abstract 20407: Restoration of p21-Activated Kinase (Pak1) Activity Attenuates Arrhythmic Activity in a Dog Model of Atrial Fibrillation

Introduction: The mechanisms underlying the genesis of atrial fibrillation (AF) are not fully understood. Activation of the small GTPase Rac1 through production of reactive oxygen species (ROS) is believed to contribute to the development of an AF substrate. We identified Pak1 as an endogenous negative regulator of Rac1 and hypothesized that stimulation of Pak1 activity attenuates atrial arrhythmia by antagonizing ROS induced changes in Ca handling. Method: Tissue and isolated myocytes (left superior pulmonary vein, LSPV) were obtained from dogs with persistent AF (atrial tachypacing, 600bpm, 3 weeks) or sinus rhythm (SR) and changes in Pak1 expression were quantified by western blotting. Changes in [Ca] i (fluo-4/AM) or ROS (fluorescent29,79-dichlorofluorescein, DCF) were monitored by confocal microscopy in isolated atrial myocytes (AMs). AMs from WT and Pak1 -/- mice were used to determine the mechanism by which a decrease in Pak1 enhances arrhythmic activity. Results: For the first time we demonstrate that Pak1 is down regulated in the atria of the canine AF model (adjusted density: SR: 85.6±7.2 % vs. AF 50±8.1%, n=3; p -/- mice and an increased number of delayed after depolarizations during Ctrl and AngII stimulation in isolated Pak1 -/- AMs. On a cellular level Rac1 stimulation by AngII (1microM) induced exaggerated ROS production in Pak1 -/- AMs (DCF(au): WT AngII : 4572±487, n=20 vs. Pak1 -/- AngII : 11231±838, n=16, p i (F/F 0 : WT AngII : 3.4±0.4 n=6 vs. Pak1 -/- AngII : 4.1±0.4 n=9, p AF s prevented the AngII induced increase in DADs that was based on spontaneous Ca release. Conclusion: In AF ROS production is enhanced by down-regulation of Pak1, an endogenous negative regulator of Rac1. Restoring Pak1 activity could be a therapeutic strategy to attenuate ROS induced arrhythmia and remodeling.