Impaired Intracellular Calcium Buffering Contributes to the Arrhythmogenic Substrate in Atrial Myocytes From Patients With Atrial Fibrillation

BACKGROUND: Alterations in the buffering of intracellular Ca , for which myofilament proteins play a key role, have been shown to promote cardiac arrhythmia. It is interesting that although studies report atrial myofibrillar degradation in patients with persistent atrial fibrillation (persAF), the intracellular Ca buffering profile in persAF remains obscure. Therefore, we aim to investigate the intracellular buffering of calcium and its potential arrhythmogenic role in persAF. METHODS: Simultaneous transmembrane fluxes (patch-clamp) and intracellular Ca signaling (fluo-3-acetoxymethyl ester) were recorded in myocytes from right atrial biopsies of sinus rhythm (control) and patients with persAF, alongside human atrial subtype induced pluripotent stem cell-derived cardiac myocytes (iPSC-CMs). Protein levels were quantified by immunoblotting of human atrial tissue and induced pluripotent stem cell-derived cardiac myocytes. Mouse whole heart and atrial electrophysiology was measured on a Langendorff system. RESULTS: Cytosolic Ca buffering was decreased in atrial myocytes of patients with persAF because of a depleted amount of Ca buffers. In agreement, protein levels of selected Ca binding myofilament proteins, including cTnC (cardiac troponin C), a major cytosolic Ca buffer, were significantly lower in patients with persAF. Small interfering RNA (siRNA)-mediated knockdown of cTnC in induced pluripotent stem cell-derived cardiac myocytes (si-cTnC) phenocopied the reduced cytosolic Ca buffering observed in persAF. Si-cTnC induced pluripotent stem cell-derived cardiac myocytes exhibited a higher predisposition to spontaneous Ca release events and developed action potential alternans at low stimulation frequencies. Last, indirect reduction of cytosolic Ca buffering using blebbistatin in an ex vivo mouse whole heart model increased vulnerability to tachypacing-induced atrial arrhythmia, validating the direct mechanistic link between impaired cytosolic Ca buffering and atrial arrhythmogenesis. CONCLUSIONS: Our findings suggest that loss of myofilament proteins, particularly reduced cTnC protein levels, causes diminished cytosolic Ca buffering in persAF, thereby potentiating the occurrence of spontaneous Ca release events and AF susceptibility. Strategies targeting intracellular buffering may represent a promising therapeutic lead in AF management.