Atrial Connexin (40 and 43) Remodeling in Atrial Fibrillation

Background: Atrial Fibrillation (AF) is an irregular, rapid, and disorganized electrical and contractile activity of the atria that affects approximately 2.5 million Americans and is the most commonly sustained arrhythmia. AF has been shown to be an independent risk factor for thromboembolism. Myocardial electrical continuity is assured by gap junctions, intercellular connections that provide low resistance pathway via specialized hemi channel subunit proteins called connexins. Connexin40 and connexin43 are the principal atrial gap junctional subunits. Reduction in gap junctional proteins (connexin43 and 40) may play an important role in reducing myocyte coupling resulting in reduction in conduction velocity of propagating action potential and providing substrate for AF. In addition, gap junctional impairment may be a link to the reduced contractility of the left atrium in AF. It is known that the posterior left atrium (PLA) is an important source of AF and that the majority of thromboembolic events originate from left atrial appendage (LAA) as LAA contractility is more compromised than other parts of LA. I sought to determine the changes in the protein levels of Cx43 and Cx40 in the PLA and LAA tissues in a canine congestive heart failure (CHF) model associated with AF. Previous studies have reported that AF is associated with increased levels of Reactive Oxygen Species (ROS) and that mitochondrial oxidative stress may decrease connexin levels via activation of a tyrosine kinase, c-Src. Therefore mitochondrial levels of ROS were also measured the PLA and LAA regions of the dog heart. Methods: Control (n=4) and Tachypacing induced CHF dogs (n=7) with increased propensity to AF were studied. Western blot analysis of Cx43 and Cx40 protein levels at PLA, and LAA tissues of both control and CHF dogs were performed. MitoSOX red was used to stain isolated myocytes for detection of mitochondrial superoxide. Stained isolated myocytes were studied under confocal microscope and analyses were performed using Image J software. A student t-test was performed to record statistically significant values. P<0.05 values were considered to be statistically significant. Results: 43.87% decrease in Cx43 level and a 3.49% decrease in Cx40 level in PLA tissue of CHF dogs compared to control (P=not significant) were observed. A more prominent decrease in Cx43 (88.43%) and Cx40 (53.06 %) were detected in LAA tissue of CHF dogs compared to control (P<0.001 and P<0.01 values respectively). AF was also associated with significant increase in mitochondrial ROS levels in both LAA (3.19±0.816) and PLA (1.86±0.313) of CHF dogs, when compared to their controls (0.675±0.133, LAA and 0.434±0.0381, PLA, P<0.01). ROS levels were higher in CHF LAA than in CHF PLA by 43%. Using Hunter, McNaughton and Noble theory of impulse propagation in a basic fiber, a relationship between conduction velocity of the impulse and number of gap junction channels was established. Conclusion: AF is associated with increase in mitochondrial ROS and decrease in both Cx43 and Cx40. These abnormalities were all more prominent in LAA than in PLA of CHF dogs. Mitochondrial oxidative stress and gap junctional remodeling may be important substrate for AF and the associated abnormal contractility. The regional heterogeneity in those abnormalities may provide further insight into the mechanisms of thromboembolism in AF.