Effects of acute ranolazine, chronic amiodarone, and its combination on action potential duration measured at 75 % and 90 % repolarization (APD75 and APD90) and effective refractory period (ERP) in coronary-perfused right atrial (A) and left ventricular wedge (B) preparations. Ranolazine significantly prolongs ERP but not APD in the atrium causing significant post repolarization refractoriness (PRR) in atrial, but not ventricular preparations isolated from chronic amiodarone-treated dogs. ERP-APD = PRR. n = 4–17. * p<0.01 vs respective APD75 controls: Chronic amiodarone + Ranolazine vs. Chronic amiodarone alone, Ranolazine (5 μM) vs. control, Chronic amiodarone vs. control. Atria: control n=17 dogs; ranolazine n=10 dogs; chronic amiodarone n =8 dogs; chronic amiodarone+ranolazine n=4 dogs. Ventricle: control n=5 dogs; ranolazine n=5 dogs; chronic amiodarone n =4 dogs; chronic amiodarone+ranolazine n=4 dogs.