A, MCU and MCU complex measured in cardiac mitochondria after isoproterenol (+ISO, 10 mg/kg/day) or vehicle (−ISO) administration in mice for 1, 2 (*: P=0.018 or 0.0313 vs. −ISO for MCU or MCU complex, n=4), and 4 weeks (*: P=8.6E-05 or 0.0021 vs. −ISO for MCU or MCU complex, n=4). B-C, Dose- and time-dependent changes in MCU and associated proteins in adult mouse cardiomyocytes after ISO incubation. In B, *: P=0.0294, 0.0352, 0.0296, or 0.0176 vs. −ISO for 0.1, 0.5, 1 or 2 μmol/L ISO groups, respectively. In C, *: P=0.046, 0.0309 or 0.0438 vs −ISO for 12, 24 or 48 hr ISO groups, respectively. n=4. D, Mitochondrial matrix free Ca2+ concentrations ([Ca2+]m) determined by rhod-2 and FCCP-induced mitochondrial Ca2+ release in freshly isolated and permeabilized adult mouse cardiomyocytes after 4-week ISO administration. *: P=0.032 vs. −ISO. n=8 mice. E, Steady state mitochondrial Ca2+ concentrations measured by mt-PeriCam in adult rat cardiomyocytes after ISO (0.1 μmol/L, 24 hr) incubation. *: P=0.036 vs. −ISO. n=9 rats. F, Representative traces and summarized data showing Ca2+ uptake by freshly isolated mitochondria from the hearts of WT mice with or without in vivo ISO stimulation (ISO, 10 mg/kg/day, 4 weeks). Arrows: additions of Ca2+ (25 μmol/L each time). *: P=0.0337 vs. −ISO, n=4 mice.