Introduction

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an inherited arrhythmogenic cardiomyopathy most commonly caused by mutations in the ryanodine receptor (RyR2) Ca²⁺ release channel resulting in a hyperactive or “leaky” phenotype. Patients typically have normal resting ECGs but experience dangerous ventricular tachycardias during exercise- or emotional stress.  Delayed afterdepolarizations (DADs) are believed to be the underlying mechanism of arrhythmia in CPVT and occur as a result of increased spontaneous Ca²⁺ release in cardiac myocytes in the form of Ca²⁺ waves during diastole that produces an inward current via the Na+/Ca²⁺ exchanger. Repolarization abnormalities are critical in other arrhythmogenic diseases, such as long QT syndrome, however this has received less attention in CPVT. This work aimed to investigate whether altered repolarization contributes to arrhythmogenesis in CPVT.

Methods

All experiments were conducted with UK Home Office and local ethical approval. Adult male and female mice with CPVT-causing heterozygous RyR2-R420Q mutation (R420Q; N=9) or wildtype littermate controls (WT; N=10) were killed by stunning and cervical dislocation. Hearts were perfused with physiological Tyrode’s solution +/- isoproterenol (100 nM) using a Langendorff apparatus. Hearts were electrically paced using a pair of platinum electrodes positioned on the right ventricle. Monophasic action potentials (MAP) were recorded from the left ventricle apex using custom-made Ag/AgCl electrodes and signals digitizedand recorded at 5kHz.

Results

Mean action potential duration at 90% repolarization (APD90) during steady state (sinus rhythm) was not different between WT and R420Q mice in normal Tyrode’s solution (P=0.90). Isoproterenol shortened APD90 in WT (P<0.05) but not R420Q hearts. Hearts were subjected to a short burst (1s) of rapid electrical pacing then the APD was measured during subsequent sinus beats. The first APD following cessation of pacing was prolonged in R420Q hearts compared to WT (P<0.05). Hearts were paced to steady state at 10 Hz then an extrastimulus (S2) was delivered during repolarization. S2 elicited runs of premature ventricular complexes in 40-75% of R420Q hearts in normal Tyrode’s and isoproterenol solutions, whereas these did not occur in any WT hearts. Similar electrophysiological abnormalities were observed in whole cell patch clamp recordings from isolated ventricular myocytes from R420Q mice compared to WT. Confocal Ca²⁺ imaging revealed abnormal patterns of spontaneous Ca²⁺ release occurring in R420Q myocytes that could contribute to the altered repolarization at the single cell and whole heart levels.

Conclusion

Our findings suggest that repolarization abnormalities during dynamic changes in heart activity may occur in and contribute to CPVT pathology. The functional consequences on arrhythmogenesis in the heart are currently under investigation.