Characterising Intracellular Calcium Releases Underlying the Increased Susceptibility to Atrial Fibrillation in Metabolic Syndrome

Abstract

Atrial fibrillation (AF), the most common cardiac arrhythmia, emerges as one of the world’s major health issues associated with significant morbidity and mortality. Metabolic syndrome (MetS), a cluster of cardiovascular and metabolic alterations, is one of the well-established risk factors for AF. The exact mechanism(s) underlying AF in MetS is not well understood. This likely results in a poor prognosis for patients with MetS and/or AF. One of the most critical pathophysiological aspects of AF is defective calcium (Ca2+) homeostasis. Local spontaneous Ca2+ releases through the stochastic opening of the ryanodine receptors (RyRs) from the sarcoplasmic reticulum (SR) give rise to Ca2+ sparks. The adequate release and summation of Ca2+ sparks enable the formation and propagation of global spontaneous SR Ca2+ releases, observed as Ca2+ waves and/or whole-cell Ca2+ transients. Altered Ca2+ -handling proteins can contribute to abnormal Ca2+ spark releases, irregular and asynchronous propagation of global Ca2+ events, and triggered activities, leading to AF. This thesis investigated the potential Ca2+ remodellings involved in the arrhythmogenesis of MetS. Firstly, we systematically reviewed the current literature to understand better the mechanistic Ca2+ -handling in AF and its risk factors. Our review identified that the L-type Ca2+ current was downregulated in both AF and its risk factor groups. In addition, both RyR phosphorylation at Serine 2808 and sodium- Ca2+ exchanger (NCX) expressions were enhanced, as well as frequencies and amplitudes of Ca2+ spark and transient. The remodelling in RyR and NCX potentially led to enhanced Ca2+ functional activities. The main difference between the AF risk factor group and AF was that SR Ca2+ATPase pump (SERCA) expression was elevated in the AF risk factor group and substantially reduced in paroxysmal AF. To further elucidate the Ca2+ pathophysiology, a diet-induced rabbit model of MetS (two control and five MetS) at The University of Auckland was used for characterising Ca2+ releases iii in this study. We developed a robust cell isolation technique to isolate viable and Ca2+ tolerant rabbit atrial myocytes from both the control and MetS animals. The chunk technique was originally based on the protocol developed for human atrial cells. Yet, we were the first to adopt this technique to small animals by adjusting enzyme concentrations accordingly based on animal species and the healthy state of the heart. Thirdly, we recorded spontaneous Ca2+ spark activity using confocal microscopy and compared their spatial-temporal dynamics from the central and subsarcolemmal (SS) cell regions in both control and MetS cells. Spontaneous Ca2+ sparks in MetS displayed higher activities with greater heterogeneities. Ca2+ spark frequencies and amplitudes were significantly enhanced in MetS cells, with quicker Ca2+ spark rise and decay times. Furthermore, a more pronounced Ca2+ spark frequency was revealed in the SS region (4.24  0.32 sparks/s/100 m for control versus 4.76  0.38 sparks/s/100 m for MetS) than their central regions (1.78  0.17 sparks/s/100 m for control versus 3.60  0.26 sparks/s/100 m for MetS). Finally, we characterised global Ca2+ releases. Global Ca2+ events in MetS were significantly elevated. A three-fold increase in Ca2+ event frequency was observed in MetS cells. Likewise, Ca2+ event amplitude was notably larger with quicker rise and decay rates. More than 50% of Ca2+ events in MetS cells consist of Ca2+ waves. In addition, measuring SR Ca2+ with 10 mM caffeine revealed a higher SR Ca2+ content in MetS cells. To our knowledge, this study is the first to shed new light on the key Ca2+ remodellings in MetS, i.e., the upregulated RyR, SR Ca2+ load and SERCA reuptake, which may explain the increased AF risk. The Ca2+ remodelling leads to Ca2+ events with greater magnitude, increased frequency, and quicker rise and decay rates. A larger-scale study is required to validate the altered Ca2+ handling proteins and other key cardiac channels involved in the enhanced spontaneous Ca2+ releases, given the small animal number and heterogeneity of our results.