Langendorff-Perfused Mouse Heart Arrhythmia Induction

At 1 and 8 weeks after MI, mouse hearts were isolated and Langendorff-perfused with Tyrode solution (37 °C) containing 1 µM isoproterenol (Sigma, Catalogue No.: I6504), ex vivo ECG were continuously measured by placing recording electrodes around the heart as we previously described^{23 (link),54 (link)} (Fig. 1E). PES was applied using a MyoPacer (IonOptix) via a pair of platinum electrodes placed on the left ventricular apex of the heart. The standard stimulation protocol (Fig. 1E) consisted of 10 stimuli at 100 ms intervals (S1, 5V) followed by one extra stimulus (S2) starting at an interval of 80 ms which was then reduced by 2 ms until the effective refractory period (ERP) was reached. If VT or VF was not induced, a second extra stimulus (S3) was added at 80 ms after S2. The S3 interval was then reduced by 2 ms until the ERP was reached. Finally, a third extra stimulus (S4) was added 80 ms after S3 and was then decreased by 2 ms until the ERP was reached. If a heart failed to develop a VT or VF with 3 extra stimuli, the heart was deemed non-inducible.