Adult zebrafish ventricular electrical gradients as tissue mechanisms of ECG patterns under baseline vs. oxidative stress.

AIMS In mammalian ventricles, electrical gradients establish electrical heterogeneities as essential tissue mechanisms to optimize mechanical efficiency and safeguard electrical stability. Electrical gradients shape mammalian electrocardiographic patterns; disturbance of electrical gradients is proarrhythmic. The zebrafish heart is a popular surrogate model for human cardiac electrophysiology thanks to its remarkable recapitulation of human electrocardiogram and ventricular action potential features. Yet, zebrafish ventricular electrical gradients are largely unexplored. The goal of this study is to define the zebrafish ventricular electrical gradients that shape the QRS complex and T wave patterns at baseline and under oxidative stress. METHODS AND RESULTS We performed in vivo electrocardiography and ex vivo voltage-sensitive fluorescent epicardial and transmural optical mapping of adult zebrafish hearts at baseline and during acute H2O2 exposure. At baseline, apicobasal activation and basoapical repolarization gradients accounted for the polarity concordance between the QRS complex and T wave. During H2O2 exposure, differential regional impairment of activation and repolarization at the apex and base disrupted prior baseline electrical gradients, resulting in either reversal or loss of polarity concordance between the QRS complex and T wave. KN-93, a specific calcium/calmodulin-dependent protein kinase II inhibitor (CaMKII), protected zebrafish hearts from H2O2 disruption of electrical gradients. The protection was complete if administered prior to oxidative stress. CONCLUSIONS Despite remarkable apparent similarities, zebrafish and human ventricular electrocardiographic patterns are mirror images supported by opposite electrical gradients. Like mammalian ventricles, zebrafish ventricles are also susceptible to H2O2 proarrhythmic perturbation via CaMKII activation. Our findings suggest that the adult zebrafish heart may constitute a clinically relevant model to investigate ventricular arrhythmias induced by oxidative stress. However, the fundamental ventricular activation and repolarization differences between the two species that we demonstrated in this study highlight the potential limitations when extrapolating results from zebrafish experiments to human cardiac electrophysiology, arrhythmias, and drug toxicities. TRANSLATIONAL PERSPECTIVE Zebrafish electrocardiograms remarkably recapitulate human electrocardiograms in health and disease. Yet, the underlying tissue mechanisms were unknown. This study provides the first demonstration that the baseline electrical gradients that shape zebrafish ventricular electrocardiograms are opposite human electrical gradients. Additionally, like mammalian hearts, zebrafish hearts are susceptible to CaMKII-mediated H2O2 proarrhythmic perturbations. Despite its vital role in zebrafish cardiac regeneration following injury, H2O2 can disrupt zebrafish sinus node activity, cause all three types of atrioventricular blocks, and reverse ventricular electrical gradients. These findings highlight the clinical relevance, utility, and limitations of the adult zebrafish heart as model for mammalian arrhythmogenesis studies.