Critical Link between Calcium Regional Heterogeneity and Atrial Fibrillation Susceptibility in Sheep Left Atria

Background: Atrial fibrillation is the most sustained form of arrhythmia in the human population that leads to important electrophysiological and structural cardiac remodeling as it progresses into a chronic form. Calcium is an established key player of cellular electrophysiology in the heart, yet to date, there is no information that maps calcium signaling across the left atrium. Objective: The aim of this study is to determine whether calcium signaling is homogenous throughout the different regions of the left atrium. This work tests the hypothesis that differences across the healthy left atrium contribute to a unique, region-dependent calcium cycling and participates in the pro-arrhythmic activity during atrial fibrillation. Methods: An animal model relevant to human cardiac function (the sheep) was used to characterize both the electrical activity and the calcium signaling of three distinct left atrium regions (appendage, free wall and pulmonary veins) in control conditions and after acetylcholine perfusion (5 μM) to induce acute atrial fibrillation. High-resolution dual calcium-voltage optical mapping on the left atria of sheep was performed to explore the spatiotemporal dynamics of calcium signaling in relation to electrophysiological properties. Results: Action potential duration (at 80% repolarization) was not significantly different in the three regions of interest for the three pacing sites. In contrast, the time to 50% calcium transient decay was significantly different depending on the region paced and recorded. Acetylcholine perfusion and burst pacing-induced atrial fibrillation when pulmonary veins and appendage regions were paced but not when the free wall region was. Dantrolene (a ryanodine receptor blocker) did not reduce atrial fibrillation susceptibility. Conclusion: These data provide the first evidence of heterogenous calcium signaling across the healthy left atrium. Such basal regional differences may be exacerbated during the progression of atrial fibrillation and thus play a crucial role in focal arrhythmia initiation without ryanodine receptor gating modification

Serial block face scanning electron microscopy reveals region dependent remodelling of transverse tubules post myocardial infarction

The highly organized transverse tubule (t-tubule) network facilitates cardiac excitation–contraction coupling and synchronous cardiac myocyte contraction. In cardiac failure secondary to myocardial infarction (MI), changes in the structure and organization of t-tubules result in impaired cardiac contractility. However, there is still little knowledge on the regional variation of t-tubule remodelling in cardiac failure post-MI. Here, we investigate post-MI t-tubule remodelling in infarct border and remote regions, using serial block face scanning electron microscopy (SBF-SEM) applied to a translationally relevant sheep ischaemia reperfusion MI model and matched controls. We performed minimally invasive coronary angioplasty of the left anterior descending artery, followed by reperfusion after 90 min to establish the MI model. Left ventricular tissues obtained from control and MI hearts eight weeks post-MI were imaged using SBF-SEM. Image analysis generated three-dimensional reconstructions of the t-tubular network in control, MI border and remote regions. Quantitative analysis revealed that the MI border region was characterized by t-tubule depletion and fragmentation, dilation of surviving t-tubules and t-tubule elongation. This study highlights region-dependent remodelling of the tubular network post-MI and may provide novel localized therapeutic targets aimed at preservation or restoration of the t-tubules to manage cardiac contractility post-MI. This article is part of the theme issue ‘The cardiomyocyte: new revelations on the interplay between architecture and function in growth, health, and disease’

Interaction of background Ca2+ influx, sarcoplasmic reticulum threshold and heart failure in determining propensity for Ca2+ waves in sheep heart

ABSTRACT: Ventricular arrhythmias can cause death in heart failure (HF). A trigger is the occurrence of Ca(2+) waves which activate a Na(+)‐Ca(2+) exchange (NCX) current, leading to delayed after‐depolarisations and triggered action potentials. Waves arise when sarcoplasmic reticulum (SR) Ca(2+) content reaches a threshold and are commonly induced experimentally by raising external Ca(2+), although the mechanism by which this causes waves is unclear and was the focus of this study. Intracellular Ca(2+) was measured in voltage‐clamped ventricular myocytes from both control sheep and those subjected to rapid pacing to produce HF. Threshold SR Ca(2+) content was determined by applying caffeine (10  mM) following a wave and integrating wave and caffeine‐induced NCX currents. Raising external Ca(2+) induced waves in a greater proportion of HF cells than control. The associated increase of SR Ca(2+) content was smaller in HF due to a lower threshold. Raising external Ca(2+) had no effect on total influx via the L‐type Ca(2+) current, I (Ca‐L), and increased efflux on NCX. Analysis of sarcolemmal fluxes revealed substantial background Ca(2+) entry which sustains Ca(2+) efflux during waves in the steady state. Wave frequency and background Ca(2+) entry were decreased by Gd(3+) or the TRPC6 inhibitor BI 749327. These agents also blocked Mn(2+) entry. Inhibiting connexin hemi‐channels, TRPC1/4/5, L‐type channels or NCX had no effect on background entry. In conclusion, raising external Ca(2+) induces waves via a background Ca(2+) influx through TRPC6 channels. The greater propensity to waves in HF results from increased background entry and decreased threshold SR content. KEY POINTS: Heart failure is a pro‐arrhythmic state and arrhythmias are a major cause of death. At the cellular level, Ca(2+) waves resulting in delayed after‐depolarisations are a key trigger of arrhythmias. Ca(2+) waves arise when the sarcoplasmic reticulum (SR) becomes overloaded with Ca(2+). We investigate the mechanism by which raising external Ca(2+) causes waves, and how this is modified in heart failure. We demonstrate that a novel sarcolemmal background Ca(2+) influx via the TRPC6 channel is responsible for SR Ca(2+) overload and Ca(2+) waves. The increased propensity for Ca(2+) waves in heart failure results from an increase of background influx, and a lower threshold SR content. The results of the present study highlight a novel mechanism by which Ca(2+) waves may arise in heart failure, providing a basis for future work and novel therapeutic targets

Cardiomyocyte-specific loss of plasma membrane calcium ATPase 1 impacts cardiac rhythm and is associated with ventricular repolarisation dysfunction.

Plasma membrane calcium ATPase 1 (PMCA1, Atp2b1) is emerging as a key contributor to cardiac physiology, involved in calcium handling and myocardial signalling. In addition, genome wide association studies have associated PMCA1 in several areas of cardiovascular disease including hypertension and myocardial infarction. Here, we investigated the role of PMCA1 in basal cardiac function and heart rhythm stability. Cardiac structure, heart rhythm and arrhythmia susceptibility were assessed in a cardiomyocyte-specific PMCA1 deletion (PMCA1CKO) mouse model. PMCA1CKO mice developed abnormal heart rhythms related to ventricular repolarisation dysfunction and displayed an increased susceptibility to ventricular arrhythmias. We further assessed the levels of cardiac ion channels using qPCR and found a downregulation of the voltage-dependent potassium channels, Kv4.2, with a corresponding reduction in the transient outward potassium current which underlies ventricular repolarisation in the murine heart. The changes in heart rhythm were found to occur in the absence of any structural cardiomyopathy. To further assess the molecular changes occurring in PMCA1CKO hearts, we performed proteomic analysis. Functional characterisation of differentially expressed proteins suggested changes in pathways related to metabolism, protein-binding, and pathways associated cardiac function including β-adrenergic signalling. Together, these data suggest an important role for PMCA1 in basal cardiac function in relation to heart rhythm control, with reduced cardiac PMCA1 expression resulting in an increased risk of arrhythmia development