Mechanisms of spontaneous Ca release-mediated arrhythmia in a novel 3D human atrial myocyte model: I. Transverse-axial tubule variation.

Intracellular calcium (Ca ) cycling is tightly regulated in the healthy heart ensuring effective contraction. This is achieved by transverse (t)-tubule membrane invaginations that facilitate close coupling of key Ca -handling proteins such as the L-type Ca channel and Na -Ca exchanger (NCX) on the cell surface with ryanodine receptors (RyRs) on the intracellular Ca store. Although less abundant and regular than in the ventricle, t-tubules also exist in atrial myocytes as a network of transverse invaginations with axial extensions known as the transverse-axial tubule system (TATS). In heart failure and atrial fibrillation, there is TATS remodelling that is associated with aberrant Ca -handling and Ca -induced arrhythmic activity; however, the mechanism underlying this is not fully understood. To address this, we developed a novel 3D human atrial myocyte model that couples electrophysiology and Ca -handling with variable TATS organization and density. We extensively parameterized and validated our model against experimental data to build a robust tool examining TATS regulation of subcellular Ca release. We found that varying TATS density and thus the localization of key Ca -handling proteins has profound effects on Ca handling. Following TATS loss, there is reduced NCX that results in increased cleft Ca concentration through decreased Ca extrusion. This elevated Ca increases RyR open probability causing spontaneous Ca releases and the promotion of arrhythmogenic waves (especially in the cell interior) leading to voltage instabilities through delayed afterdepolarizations. In summary, the present study demonstrates a mechanistic link between TATS remodelling and Ca -driven proarrhythmic behaviour that probably reflects the arrhythmogenic state observed in disease. KEY POINTS: Transverse-axial tubule systems (TATS) modulate Ca handling and excitation-contraction coupling in atrial myocytes, with TATS remodelling in heart failure and atrial fibrillation being associated with altered Ca cycling and subsequent arrhythmogenesis. To investigate the poorly understood mechanisms linking TATS variation and spontaneous Ca release, we built, parameterized and validated a 3D human atrial myocyte model coupling electrophysiology and spatially-detailed subcellular Ca handling governed by the TATS. Simulated TATS loss causes diastolic Ca and voltage instabilities through reduced Na -Ca exchanger-mediated Ca removal, cleft Ca accumulation and increased ryanodine receptor open probability, resulting in spontaneous Ca release and promotion of arrhythmogenic waves and delayed afterdepolarizations. At fast electrical rates typical of atrial tachycardia/fibrillation, spontaneous Ca releases are larger and more frequent in the cell interior than at the periphery. Our work provides mechanistic insight into how atrial TATS remodelling can lead to Ca -driven instabilities that may ultimately contribute to the arrhythmogenic state in disease.