(A) Representative local extracellular action potential (LEAP) tracings recorded at the multicellular monolayer level under control (top) and doxorubicin treatment (bottom) conditions. (B) Box-and-whisker plots representing beating rate (left) and LEAP potential duration (LPD) at 50% and 90% repolarization (right; analogous to APD) at the multicellular monolayer level. n = 18 per group. (C) Representative fluorescent intensities over transmembrane distance and time and action potential tracings at the single-cell level, as measured by the voltage sensor, ASAP2. (D) Box-and-whisker plots depicting beating rate (left) and APD at 50% and 90% repolarization (right). n = 23 per group. (E) Representative AP tracings recorded from patch clamp of single “ventricular-like” control (top left) and doxorubicin-treated ESC-CMs (bottom left) and “atrial-like” control (top right) and doxorubicin-treated ESC-CMs (bottom right). (F) Scatter plot displaying control and doxorubicin-treated “ventricular-like” (n = 10 for untreated, n = 9 for 0.01 μmol/L doxorubicin) and “atrial-like” ESC-CMs (n = 6 for untreated and 0.01 μmol/L doxorubicin) beating rate (left) and APD (right). Action potential durations at 50% and 90% repolarization (APD50, APD90); and LEAP potential duration at 50% and 90% repolarization (LPD50, LPD90). n = 9 per group. Differences between the untreated group and treatment group were not significant. Statistical analysis was performed with a 2-tailed Student’s t test comparing viability of untreated cells to cells treated with each doxorubicin dose. *P < 0.05, **P < 0.0001. Data represent mean ± SEM.