Ageing Increases Cardiac Electrical Remodelling in Rats and Mice via NOX4/ROS/CaMKII-Mediated Calcium Signalling

Objective: Ageing is one of the risk factors associated with cardiovascular diseases including cardiac arrhythmias and heart failure. Ageing-related cardiac dysfunction involves a complicated pathophysiological progress. Abnormal membrane voltage and Ca²⁺ dynamics in aged cardiomyocytes contribute to ageing-related arrhythmias. However, its underlying mechanisms have not been well clarified.

Methods: Young and old rats or mice were included in this study. Cardiac electrophysiological properties and functions were assessed by ECG, echocardiography, and ex vivo heart voltage and Ca²⁺ optical mapping. Proteomics, phosphor-proteomics, Western blotting, Masson staining, and ROS measurement were used to investigate the underlying mechanisms.

Results: Ageing increased the incidence of cardiac hypertrophy and fibrosis in rats. Moreover, ageing increased the occurrence of ventricular tachycardia or ventricular fibrillation induced by rapid pacing and during isoprenaline (ISO) (1 mg/kg i.p.) challenge in mice in vivo. Optical mapping with dual dyes (membrane voltage (V _m ) dye and intracellular Ca²⁺ dye) simultaneously recording revealed that ageing increased the action potential duration (APD) and Ca²⁺ transient duration (CaTD) and slowed the ventricular conduction with the Langendorff-perfused mouse heart. More importantly, ageing increased the ISO-induced (1 μM) changes of APD (ΔAPD80) and CaTD (ΔCaTD50). Ageing also delayed the decay of Ca²⁺ transient by extending the decay time constant from 30% to 90% (τ _30-90). In addition, ageing decreased the V _m /Ca ²⁺ latency which represented the coupling of V _m /Ca ²⁺ including between the midpoint of AP depolarization and Ca²⁺ upstroke, peak transmembrane voltage and peak cytosolic calcium, and time to 50% voltage repolarization and extrusion of cytosolic calcium. Optical mapping also revealed that ageing increased the ISO-induced arrhythmia incidence and occurrence of the excitation rotor. Proteomics and phosphor-proteomics assays from rat hearts demonstrated ageing-induced protein and phosphor-protein changes, suggesting that CaMKII was involved in ageing-induced change. Ageing increased the level of ROS and the expression of NOX4, oxidative CaMKII (ox-CaMKII), phosphorated CaMKII (p-CaMKII), and periostin.

Conclusion: Ageing accelerates cardiac remodelling and increases the susceptibility to ventricular arrhythmias through NOX4/ROS/CaMKII pathway-mediated abnormal membrane voltage and intracellular Ca²⁺ handling and V _m /Ca ²⁺ coupling.