Myocytes were paced at 1 Hz. Pipette solution consisted of (in mM) 125 KCl, 4 MgCl2, 0.06 CaCl2, 10 HEPES, 5 K+-EGTA, 3 Na2ATP, and 5 Na2-creatine phosphate (pH 7.2). External solution consisted of (in mM) 132 NaCl, 5.4 KCl, 1.8 CaCl2, 1.8 MgCl2, 0.6 NaH2PO4, 7.5 HEPES, 7.5 Na+-HEPES, and 5 glucose, pH 7.4. (A). Representative AP from WT-sham, WT-TAC and NCX1-TAC myocytes recorded using current-clamp configuration at 1.5× threshold stimulus, 4-ms duration and at 30°C (4, 17, 46, 47). (B). Means ± SE of resting membrane potential (Em), action potential amplitude, action potential duration at 50% (APD50) and at 90% repolarization (APD90) from 14 WT-sham (4 mice), 5 WT-TAC (2 mice) and 10 NCX1-TAC (4 mice) myocytes are shown. * p<0.0001, WT-sham vs. WT-TAC or NCX1-TAC; #p<0.01, WT-TAC vs. NCX1-TAC.