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

BACKGROUND: Alterations in the buffering of intracellular Ca2+, 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 Ca2+ buffering profile in persAF remains obscure. Therefore, we aimed to investigate the intracellular buffering of Ca2+ and its potential arrhythmogenic role in persAF.

METHODS: Transmembrane Ca2+ fluxes (patch-clamp) and intracellular Ca2+ signaling (fluo-3-acetoxymethyl ester) were recorded simultaneously in myocytes from right atrial biopsies of sinus rhythm (Ctrl) 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 were measured on a Langendorff system.

RESULTS: Cytosolic Ca2+ buffering was decreased in atrial myocytes of patients with persAF because of a depleted amount of Ca2+ buffers. In agreement, protein levels of selected Ca2+ binding myofilament proteins, including cTnC (cardiac troponin C), a major cytosolic Ca2+ buffer, were significantly lower in patients with persAF. Small interfering RNA (siRNA)-mediated knockdown of cTnC (si-cTNC) in atrial iPSC-CM phenocopied the reduced cytosolic Ca2+ buffering observed in persAF. Si-cTnC treated atrial iPSC-CM exhibited a higher predisposition to spontaneous Ca2+ release events and developed action potential alternans at low stimulation frequencies. Last, indirect reduction of cytosolic Ca2+ 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 Ca2+ buffering and atrial arrhythmogenesis.

CONCLUSIONS: Our findings suggest that loss of myofilament proteins, particularly reduced cTnC protein levels, causes diminished cytosolic Ca2+ buffering in persAF, thereby potentiating the occurrence of spontaneous Ca2+ release events and atrial fibrillation susceptibility. Strategies targeting intracellular buffering may represent a promising therapeutic lead in persAF management.