Mononucleated and binucleated cardiomyocytes in left atrium and pulmonary vein have different electrical activity and calcium dynamics

Cardiomyocytes consist of single- and bi-nucleus myocytes. However, the electrophysiological characteristics of mononucleated and binucleated myocytes have never been elucidated. Left atrium (LA) and pulmonary veins (PVs) are important substrate and initiators of atrial fibrillation. The purposes of this study were to evaluate the electrical properties and calcium homeostasis in mononucleated and binucleated cardiomyocytes in the LA and PVs. A whole-cell clamp, fluo-4 fluorescence, and immunocytostaining were used to investigated mononucleated and binucleated cardiomyocytes in the LA and PVs. Both mononucleated PV and LA cardiomyocytes had more positive resting membrane potential than respective PV and LA binucleated cardiomyocytes. Additionally, mononucleated PV cardiomyocytes (n = 36) had faster beating rates (2.1 ± 0.2 Hz versus 1.0 ± 0.2 Hz, P < 0.05) than binucleated (n = 10) PV cardiomyocytes. The PV (n = 18) and LA (n = 15) mononucleated cardiomyocytes had larger [Ca2+]i transients (F/F0 1.64 ± 0.09 versus 1.20 ± 0.03 IU, P < 0.05; 1.52 ± 0.06 versus 1.19 ± 0.05 IU, P < 0.05) than the binucleated PV (n = 10) and LA (n = 10) cardiomyocytes. The immunostaining showed that mononucleated cardiomyocytes had lower Kir 2.3 and higher ryanodine receptor densities than did binucleated cardiomyocytes both in the PV and LA. In conclusions, mononucleated PV and LA cardiomyocytes contain distinctive electrophysiological characteristics with a higher arrhythmogenic activity, which indicates that cell nucleus number may potentially determine the electrical activity and calcium handling in cardiomyocytes.