Increased Ca²⁺ Sequestration by the Sarco-/Endoplasmic Reticulum in Cardiac Purkinje Cells After Myocardial...

During acute coronary occlusion, ischemia is a major determinant of the cell response to subsequent reperfusion and is the major precursor of the typical "ischemia-reperfusion injury" (IRI). Therefore, elucidating the full IRI process primarily relies on a good understanding of ischemia-induced alterations. Ischemic arrhythmias frequently arise during the acute phase of a myocardial infarction (MI) and originate in the terminal arborisations of the cardiac conduction system. These ventricular arrhythmias are triggered by abnormal Ca²⁺-dependent depolarisations (DADs) of Purkinje cells (Pcells) due to increased spontaneous Ca²⁺ release by the sarcoplasmic reticulum (SR). This early alteration of the conduction tissue is also likely to provide a substrate for IRI-related arrhythmogenicity. Recent evidence associates the ischemic phase of the MI with a significant increase in SERCA2 pump expression in Pcells, suggesting that enhanced SR-Ca²⁺ release results from an augmentation of Ca²⁺ sequestration by the SR in those cells. We examined this hypothesis by assessing the impact of ischemia on the dynamics of SR-Ca²⁺ uptake in live Pcells by high-resolution confocal microscopy in a classical canine model of LAD coronary ligation. Pcells from five normal hearts were compared with cells from five hearts 48 Hrs after coronary occlusion. Purkinje-specific Ca²⁺ events, namely peripheral Ca²⁺ wavelets (Wlets) and central cell-wide waves (CWWs), were analysed to assess the regional SR-Ca²⁺ transport of Pcells. A total of 83 normal and 126 MI Wlets, along with 10 normal and 30 MI CWWs, were analysed to compare the peripheral and central SR-Ca²⁺ transports of Pcells between normal and ischemic hearts. Forty-eight hours following the onset of ischemia, individual SR-Ca²⁺ release sites exhibited a 60% increase in Ca²⁺ spark firing rate. However, the site density remained unchanged, indicating an acceleration of intra-SR-Ca²⁺ cycling rather than direct alteration of the SR-Ca²⁺ release channels. While central CWWs remained unchanged, a 37% acceleration of resting Ca²⁺ restoration was readily visible in peripheral Wlets, consistent with enhanced SR-Ca²⁺ uptake at the cell periphery. Computational modelling reproduced these findings when the Ca²⁺ uptake rate was numerically increased by 35%, confirming that augmented SERCA activity is sufficient to explain the pro-arrhythmic SR-Ca²⁺ release of Pcells after MI. Our findings confirm that the augmentation of Ca²⁺ pump density in the periphery of Pcells is associated with an increase in SR-Ca²⁺ uptake, explaining the arrhythmogenicity of Purkinje fibres in an ischemic heart. This ischemia-mediated pro-arrhythmic remodelling of intracellular Ca²⁺ handling in the conduction system is also likely to contribute to triggered activity during subsequent reperfusion.