Background: Previous studies have shown that alterations in the buffering of cytoplasmic Ca²⁺ can promote cardiac arrhythmias. However, the role of cytosolic Ca²⁺ buffering in atrial fibrillation is still poorly understood. In this study, we examine the effect of reduced cytosolic calcium buffering on atrial arrhythmia inducibility and stability in an ex-vivo mouse model. 

Methods: Mouse hearts were harvested and perfused according to the Langendorff-technique. To measure whole heart and atrial electrophysiology, two electrodes were places on the respective atria and one electrode on the heart apex. The hearts were perfused with different solutions of decreasing potassium concentrations (5.4, 3.7 and 2.0 mM) to reduce the threshold for atrial arrhythmia induction. As a final step Diazoxide (300 µM) was added to the perfusing solution. At each potassium concentration the absolute refractory period was measured. In addition, step burst pacing (400-4500bpm) and high frequency burst pacing (6000bpm) was applied to determine the inducibility of arrhythmias (Figure A). 

Results: The myosin ATPase inhibitor blebbistatin is a known desensitizer of troponin C to the binding of Ca²⁺. Application of Blebbisatin (5 µM) significantly increased the incidence of atrial arrhythmias after burst pacing. (Figure B) Furthermore, in a 2 mM potassium and diazoxide solution, the average duration of induced arrhythmias was significantly increased upon perfusion with blebbistatin (Figure C). Blebbistatin-perfused hearts tended to develop atrial arrhythmias at an earlier stage of the escalating protocol than control hearts (Figure D). 

Conclusions: The reduction of cytosolic Ca²⁺ buffering with blebbistatin led to a higher inducibility and longer episode length of atrial arrhythmias. Strategies targeting intracellular Ca²⁺ buffering may represent a promising therapeutic lead in atrial fibrillation management.