Abstract 14141: Role of Pak1 in Sinus Node Function

Abstract

Introduction: Sinus node (SN) dysfunction (SND) and atrial arrhythmia frequently occur simultaneously with a hazard ratio of 4.2 for new onset atrial fibrillation (AF) in SND patients. We demonstrated previously that loss of p21-activated kinase 1 (Pak1), a serine/threonine kinase increases the propensity for AF by increasing cellular production of reactive oxygen species (ROS). Since pacemaker current (HCN4) is negatively regulated by cellular ROS we hypothesized that loss of Pak1 leads to attenuated SN function. Methods: Heart rate regulation was quantified in wild type (WT) and Pak1 deficient mice (Pak1 -/- ) in vivo and the isolated Langendorff perfused heart using ECGs, bipolar and multi-electrode array derived atrial electrograms. Unpaired t-test was used to compare means between groups. Results: In vivo WT and Pak1 -/- animals exhibited no difference in basal heart rate (WT: 450 ± 26 bpm, n=16; Pak1 -/- : 440 ± 24 bpm, n=13), but intrinsic HR after autonomic blockage, was reduced in Pak1 -/- (WT: 399 ± 32 bpm, n=16; Pak1 -/- : 365 ± 34 bpm, n=13, p<0.05). The result was confirmed in the Langendorff configuration (WT: 359 ± 47 bpm, n=12; Pak1 -/- : 293 ± 47 bpm, n=21, p<0.01) suggesting attenuated SN impulse generation. To determine the contribution of the Ca clock, hearts were perfused with cyclopiazonic acid (5 μM) a blocker of the sarcoplasmic reticulum Ca ATPase. CPA attenuated the HR in WT and Pak1 -/- (WT: -19.6 ± 6.6 % n=2 ; Pak1 -/- : -15.8 ± 7.5 %, n=5) but did not eliminate the difference in HR (WT: 320 ± 37 bpm, n=2; Pak1 -/- : 255 ± 26 bpm, n=5, p<0.05). Block of HCN4 by ivabradine (3 μM), reduced the HR in WT and Pak1 -/- (WT: -45.1 ± 6.3 %, n=6; Pak1 -/- : -22.2 ± 8.6 %, n=7, p<0.001) and eliminated differences in spontaneous activity (WT: 186 ± 43 bpm, n=6; Pak1 -/- : 234 ± 41 bpm, n=7). In vivo the change in HR in response to ß-blocker propranolol (WT: -13.5 ± 2.5 %, n=6; Pak1 -/- : -18.4 ± 1.5 %; n=4, p<0.01) and muscarinic receptor blocker atropine (WT: 9.7 ± 5.0%, n=10; Pak1 -/- : 4.6 ± 4.4 %; n=9, p<0.05) revealed that the attenuated SN impulse generation in Pak1 -/- mice was compensated by an increased sympathetic and attenuated parasympathetic tone. Conclusion: Attenuated Pak1 expression results in suppressed SN impulse generation likely by ROS dependent attenuation of HCN4 expression.