Mitochondrial reactive oxygen species (ROS) play a central role in most aging-related diseases. ROS are produced at the respiratory chain that demands NADH for electron transport and are eliminated by enzymes that require NADPH. The nicotinamide nucleotide transhydrogenase (Nnt) is considered a key antioxidative enzyme based on its ability to regenerate NADPH from NADH. Here, we show that pathological metabolic demand reverses the direction of the Nnt, consuming NADPH to support NADH and ATP production, but at the cost of NADPH-linked antioxidative capacity. In heart, reverse-mode Nnt is the dominant source for ROS during pressure overload. Due to a mutation of the Nnt gene, the inbred mouse strain C57BL/6J is protected from oxidative stress, heart failure, and death, making its use in cardiovascular research problematic. Targeting Nnt-mediated ROS with the tetrapeptide SS-31 rescued mortality in pressure overload-induced heart failure and could therefore have therapeutic potential in patients with this syndrome.

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...

The subiculum was recently shown to be crucially involved in the generation of interictal activity in human temporal lobe epilepsy. Using the pilocarpine model of epilepsy, this study examines the anatomical substrates for network hyperexcitability recorded in the subiculum. Regular- and burst-spiking subicular pyramidal cells were stained with fluorescence dyes and reconstructed to analyze seizure-induced alterations of the dendritic and axonal system. In control animals burst-spiking cells outnumbered regular-spiking cells by about two to one. Regular- and burst-spiking cells were characterized by extensive axonal branching and autapse-like contacts, suggesting a high intrinsic connectivity. In addition, subicular axons projecting to CA1 indicate a CA1-subiculum-CA1 circuit. In the subiculum of pilocarpine-treated rats we found an enhanced network excitability characterized by spontaneous rhythmic activity, polysynaptic responses, and all-or-none evoked bursts of action potentials. In pilocarpine-treated rats the subiculum showed cell loss of about 30%. The ratio of regular- and burst-spiking cells was practically inverse as compared to control preparations. A reduced arborization and spine density in the proximal part of the apical dendrites suggests a partial deafferentiation from CA1. In pilocarpine-treated rats no increased axonal outgrowth of pyramidal cells was observed. Hence, axonal sprouting of subicular pyramidal cells is not mandatory for the development of the pathological events. We suggest that pilocarpine-induced seizures cause an unmasking or strengthening of synaptic contacts within the recurrent subicular network.