Elevated cytosolic Na⁺ increases mitochondrial formation of reactive oxygen species in failing cardiac myocytes

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⁺ ([Na²⁺]_i) reduces mitochondrial Ca ([Ca]m) by accelerating Ca efflux via the mitochondrial Na/Ca 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]m, we speculated that in failing myocytes, elevated [Na ²⁺]_i promotes oxidative stress. METHODS AND RESULTS-: We used a patch-clamp-based approach to simultaneously monitor cytosolic and mitochondrial Ca and, alternatively, mitochondrial H₂O₂ 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 β-adrenergic stimulation. During this transition, NAD(P)H initially oxidized but recovered when [Ca]m increased. The transient oxidation of NAD(P)H was closely associated with an increase in mitochondrial H₂O ₂ formation. This reactive oxygen species formation was potentiated when mitochondrial Ca uptake was blocked (by Ru360) or Ca efflux was accelerated (by elevation of [Na²⁺]_i). In failing myocytes, H2O2 formation was increased, which was prevented by reducing mitochondrial Ca efflux via the mitochondrial Na/Ca exchanger. CONCLUSIONS-: Besides matching energy supply and demand, mitochondrial Ca uptake critically regulates mitochondrial reactive oxygen species production. In heart failure, elevated [Na ²⁺]_i promotes reactive oxygen species formation by reducing mitochondrial Ca 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.