Dronedarone causes greater effective refractory period (ERP) prolongation in murine left atria (LA) with reduced Pitx2c compared to wild type (WT). A: Monophasic action potentials (MAPs) recorded at 100-ms pacing cycle length from WT and Pitx2c+/– intact LA. Action potential duration at 90% repolarization (APD90; blue line) and ERP (red line) before and after dronedarone (10 μM) are indicated. B–D: Mean effects of dronedarone on LA ERP (B), action potential duration at 70% repolarization (APD70) (C), and activation time (AT) in WT (N = 6 hearts; 6 animals) and Pitx2c+/– (N = 9 hearts; 9 animals), measured at pacing cycle lengths at 120, 100, and 80 ms (D). Data are given as mean ± SEM. ∗P <.05 WT vs Pitx2c+/–, 2-way analysis of variance with Bonferroni post hoc analysis.