Pathological cardiomyocyte (CMC) hypertrophy and extensive fibrosis are the main constituents of cardiac remodeling, which is associated with an increased incidence of cardiac arrhythmias. Still, anti-arrhythmic therapies are suboptimal due to the incomplete understanding of the underlying pro-arrhythmic mechanisms. This study investigates which factors can underlie this arrhythmogeneity in the remodeled myocardium independently of fibrosis. Thereby we aim to provide a new rationale for specific interventions in the treatment of arrhythmias caused by cardiac remodeling.Methods
Pathological hypertrophy was induced in neonatal rat CMC cultures by treatment with Phenylephrin (PE), while fibrosis was prevented by antiproliferative treatment of endogenous fibroblasts by mitomycin-c in all cultures. At day 9 of culture, arrhythmogeneity of PE treated and control cultures was studied by optical mapping. Cultures were also studied for intracellular electrophysiological properties and protein expression.Results
PE treatment slowed conduction from 21.0 ± 3.8 to 8.2 ± 3.3 cm/s (p < 0.0001). Of all hypertrophic cultures at day 9 of culture, 61.3% showed spontaneous reentrant tachyarrhythmias compared to only 8.0% in control cultures (n=101). All reentrant tachyarrhythmias in the PE treated group were attributable to unidirectional conduction block caused by early after depolarization (EAD) generation. CMCs treated with PE showed a 2-fold decrease in KCND2 and KCND3 mRNA expression (p < 0.05), action potential prolongation and only L-type Ca2+ channel blockade terminated 100% of such reentrant tachyarrhythmias. In addition a significant decrease in connexin43 expression was found while CACNA1c, SCN5a and KCNJ2 mRNA levels did not change significantly after PE treatment.Conclusion
PE-induced pathological hypertrophy in myocardial cultures causes downregulation of multiple potassium channels, which reduces repolarization reserve, allowing the generation of calcium dependent EADs, ultimately resulting in spontaneous reentrant tachyarrhythmias independently of fibrosis. This model could provide a basis for the design of future, more substrate oriented anti-arrhythmic strategies.