Background-Oxidative stress is causally linked to the progression of heart failure, and mitochondria are critical sources of reactive oxygen species in failing myocardium. We previously observed that in heart failure, elevated cytosolic Na ϩ ([Na ϩ ] i ) reduces mitochondrial Ca 2ϩ ([Ca 2ϩ ] m ) by accelerating Ca 2ϩ efflux via the mitochondrial Na ϩ /Ca 2ϩ exchanger. Because the regeneration of antioxidative enzymes requires NADPH, which is indirectly regenerated by the Krebs cycle, and Krebs cycle dehydrogenases are activated by [Ca 2ϩ ] m , we speculated that in failing myocytes, elevated [Na ϩ ] i promotes oxidative stress. Methods and Results-We used a patch-clamp-based approach to simultaneously monitor cytosolic and mitochondrial Ca 2ϩ and, alternatively, mitochondrial H 2 O 2 together with NAD(P)H in guinea pig cardiac myocytes. Cells were depolarized in a voltage-clamp mode (3 Hz), and a transition of workload was induced by -adrenergic stimulation. During this transition, NAD(P)H initially oxidized but recovered when [Ca 2ϩ ] m increased. The transient oxidation of NAD(P)H was closely associated with an increase in mitochondrial H 2 O 2 formation. This reactive oxygen species formation was potentiated when mitochondrial Ca 2ϩ uptake was blocked (by Ru360) or Ca 2ϩ efflux was accelerated (by elevation of [Na ϩ ] i ). In failing myocytes, H 2 O 2 formation was increased, which was prevented by reducing mitochondrial Ca 2ϩ efflux via the mitochondrial Na ϩ /Ca 2ϩ exchanger. Conclusions-Besides matching energy supply and demand, mitochondrial Ca 2ϩ uptake critically regulates mitochondrial reactive oxygen species production. In heart failure, elevated [Na ϩ ] i promotes reactive oxygen species formation by reducing mitochondrial Ca 2ϩ uptake. This novel mechanism, by which defects in ion homeostasis induce oxidative stress, represents a potential drug target to reduce reactive oxygen species production in the failing heart. (Circulation. 2010;121:1606-1613.) Key Words: heart failure Ⅲ sodium Ⅲ calcium Ⅲ free radicals Ⅲ ion channels O xidative stress plays a fundamental role in many cardiovascular diseases and aging. 1,2 In chronic heart failure, oxidative stress is causally linked to the progression of the disease, 1,3,4 and mitochondria were identified as critical sources of reactive oxygen species (ROS) in the heart. 5 ROS impair excitation-contraction (EC) coupling, 6 -8 cause arrhythmias, 9 and contribute to cardiac remodeling by activating signaling pathways that induce hypertrophy, apoptosis, and necrosis. 10 -13 The precise mechanisms that regulate mitochondrial ROS formation, however, are incompletely understood.
Clinical Perspective on p 1613In cardiac myocytes, the processes of EC coupling consume large amounts of ATP, which is replenished by oxidative phosphorylation in mitochondria. Because the heart undergoes frequent changes in workload, precise matching of ATP supply and demand is essential to maintain cardiac function. 14 Two key regulators of oxidative phosphorylation are ADP and...
