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 aimed to investigate the intracellular buffering of Ca and its potential arrhythmogenic role in persAF.

Methods: Transmembrane Ca fluxes (patch-clamp) and intracellular Ca 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 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 (si-cTNC) in atrial iPSC-CM phenocopied the reduced cytosolic Ca buffering observed in persAF. Si-cTnC treated atrial iPSC-CM 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 atrial fibrillation susceptibility. Strategies targeting intracellular buffering may represent a promising therapeutic lead in persAF management.