BACKGROUND:
Alterations in the buffering of intracellular Ca
2+
, 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
2+
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
2+
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
2+
buffering was decreased in atrial myocytes of patients with persAF because of a depleted amount of Ca
2+
buffers. In agreement, protein levels of selected Ca
2+
binding myofilament proteins, including cTnC (cardiac troponin C), a major cytosolic Ca
2+
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
2+
buffering observed in persAF. Si-cTnC induced pluripotent stem cell–derived cardiac myocytes exhibited a higher predisposition to spontaneous Ca
2+
release events and developed action potential alternans at low stimulation frequencies. Last, indirect reduction of cytosolic Ca
2+
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
2+
buffering and atrial arrhythmogenesis.
CONCLUSIONS:
Our findings suggest that loss of myofilament proteins, particularly reduced cTnC protein levels, causes diminished cytosolic Ca
2+
buffering in persAF, thereby potentiating the occurrence of spontaneous Ca
2+
release events and AF susceptibility. Strategies targeting intracellular buffering may represent a promising therapeutic lead in AF management.