Abstract
Ischemic heart disease is the leading cause of death in the world. Importantly, CaMKII activity is upregulated during ischemia/reperfusion (I/R). However, the mechanisms by which CaMKII induces arrhythmias remains incompletely understood. We tested the hypothesis that CaMKII-dependent upregulation of the late Na+ current (INa,L) via direct phosphorylation of Nav1.5 at Ser571 is responsible for increased arrhythmia susceptibility during I/R. The role of CaMKII-dependent regulation of INa,L was explored using mice harboring Nav channel variants Nav1.5-Ser571E (S571E, phosphomimetic) and Nav1.5-Ser571A (S571A, phosphorylation incompetent). Isolated, perfused WT, S571E and S571A hearts were subjected to global ischemia (15 min) followed by reperfusion (15 min). Immunoblots demonstrated a significant increase in phosphorylated Nav1.5 at Ser571 in WT hearts during I/R compared to baseline (2.2±0.3 vs 1.0±0.3, respectively; P<0.05, n=5). Optical mapping using the voltage-sensitive dye di-4-ANEPPs (4 μM) revealed a significant increase in action potential duration in WT and S571E mice compared to S571A during I/R (WT: 55.2±0.6 ms*, S571E: 70.0±2.3 ms*, S571A: 46.0±2.9 ms; *P<0.05 vs. S571A, n=5/genotype). Furthermore, spontaneous arrhythmia incidence was increased in WT and S571E hearts (WT: 0.60±0.06 events/min*, S571E: 0.81±0.14 events/min*, S571A : 0.23±0.03 events/min; *P<0.05 vs. S571A, n=5/genotype). Interestingly, arrhythmia inducibility using an S1-S2 protocol was also increased in WT and S571E hearts (WT: 80%*, S571E: 100%*, S571A: 0%; *P<0.05 vs. S571A, n=5/genotype). The Na+ channel blocker mexiletine (10 μM) normalized APD and decreased arrhythmia inducibility in WT hearts during I/R (0% arrhythmia incidence in WT hearts treated with mexiletine). These data indicate a central role for CaMKII-dependent phosphorylation of Nav1.5 and augmentation of INa,L in the development of arrhythmogenesis during I/R.
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