Abstract 506: Enhanced Mitochondrial Calcium Uptake During Chronic β-adrenergic Stimulation Causes Maladaptive Remodeling and Impaired Left Ventricular Function Independent of Cyclophilin D-mediated Mitochondrial Permeability Transition

The mitochondrial calcium uniporter (MCU) forms the pore of the mitochondrial calcium uniporter channel (mtCU) and is required for rapid mitochondrial Ca 2+ ( m Ca 2+ ) uptake. MCU is necessary to increase cardiac energetics to fuel an increase in cardiac contractility during acute sympathetic stimulation. However, little is known about how MCU-dependent Ca 2+ flux may contribute to the heart’s adaptations to chronic stress. We therefore compared mice with adult cardiomyocyte (ACM)-specific loss ( Mcu fl/fl ; Mcu-cKO) or gain (CAG-CAT-MCU; MCU-Tg) of MCU function to examine the role of MCU-dependent m Ca 2+ uptake in a model of chronic catecholamine overload. In vitro characterization of ACMs confirmed that MCU overexpression enhanced and Mcu deletion inhibited acute m Ca 2+ uptake. Neither loss nor gain of MCU function altered baseline contractile function in vivo . In αMHC-MCM control mice, fractional shortening was transiently increased after 2 days of isoproterenol infusion. This initial increase in contractility was attenuated in MCU-cKO mice. In contrast, MCU-Tg mice exhibited decreased fractional shortening at 7 and 14 days of isoproterenol infusion. This detrimental effect on contractile function was associated with increased LV dilation, HW/BW ratio, and lung edema. MCU-Tg cardiomyocytes in vitro exhibited increased ROS production and a trend towards increased cell death upon elevation of cytosolic Ca 2+ with ionomycin. These data prompted us to test the hypothesis that isoproterenol-induced contractile dysfunction in MCU-Tg hearts is caused by cardiomyocyte dropout due to m Ca 2+ overload and mitochondrial permeability transition. However, genetic deletion of the mPTP component cyclophilin D did not prevent the decline in contractile function, diminish cardiomyocyte death, or attenuate LV remodeling in MCU-Tg animals during chronic isoproterenol infusion. We conclude that although mtCU-dependent m Ca 2+ uptake is essential for early energetic adaptations to high adrenergic load, under conditions of chronic adrenergic stress it is maladaptive and predisposes to heart failure. Our data suggest that this maladaptive response occurs via mechanisms independent of cyclophilin D-mediated permeability transition.