Purpose: Atrial fibrillation has been associated with risk variants at chromosome 4q25 that presumably modulate the expression or activity of the transcription factor pitx2. Moreover, pitx2 expression is much higher in left than right atrium, and we here tested the hypothesis that pitx2 modulates cardiomyocyte features associated to atrial fibrillation such altered cell size and spontaneous calcium release differentially in right and left atrial myocytes.
Methods: Perforated patch-clamp technique was applied to left and right atrial myocytes from mice with or without atrial chamber-specific deletion of pitx-2 to measure cell capacitance. Current-clamp was used to record the frequency of spontaneous action potentials at different membrane potentials by clamping the holding current at different values.
Results: Comparison of the cell surface area, estimated from the cell capacitance, revealed that left atrial myocytes had a larger cell surface than right atrial myocytes (62±5 vs. 42±3 pC/pF, p=0.01) in wild type mice. Moreover, myocytes from pitx2+/- mice had a larger cell capacitance than those from wild type mice (80±6 vs. 62±5 pC/pF, p=0.06 for left atria and 69±7 vs. 42±3 pC/pF, p=0.03 for right atria). In wild type mice, spontaneous action potentials were rare in both left and right atrial myocytes and were not observed at a normal resting membrane potentials (-80 mV). In pitx2+/- mice, spontaneous action potentials were rarely observed in left atrial myocytes, even when the resting membrane potential was increased to -72±1 mV (0.05±0.05 events/min) or to -63±1 mV (0.3±0.3 events/min). By contrast, spontaneous action potentials were observed even at -77.8±0.4 mV in right atrial myocytes (0.8±0.6 events/min) and their frequency increased dramatically to 2.5±1.5 events/min at -72±1 mV and 13.8±7.92 events/min at -63±1 mV.
Conclusions: Loss of pitx2 expression results in myocytes with features associated to atrial fibrillation such as increased myocyte surface and spontaneous action potentials. Moreover, the frequency of spontaneous action potentials is selectively increased in right atrial myocytes from pitx2+/- mice, suggesting that they are the key source of arrhytmogenic action potentials.