Selective downregulation of mitochondrial electron transport chain activity and increased oxidative stress in human atrial fibrillation. Am J Physiol Heart Circ Physiol 311: H54 -H63, 2016. First published May 6, 2016 doi:10.1152/ajpheart.00699.2015Mitochondria are critical for maintaining normal cardiac function, and a deficit in mitochondrial energetics can lead to the development of the substrate that promotes atrial fibrillation (AF) and its progression. However, the link between mitochondrial dysfunction and AF in humans is still not fully defined. The aim of this study was to elucidate differences in the functional activity of mitochondrial oxidative phosphorylation (OXPHOS) complexes and oxidative stress in right atrial tissue from patients without (non-AF) and with AF (AF) who were undergoing open-heart surgery and were not significantly different for age, sex, major comorbidities, and medications. The overall functional activity of the electron transport chain (ETC), NADH:O2 oxidoreductase activity, was reduced by 30% in atrial tissue from AF compared with non-AF patients. This was predominantly due to a selective reduction in complex I (0.06 Ϯ 0.007 vs. 0.09 Ϯ 0.006 nmol·min Ϫ1 ·citrate synthase activity Ϫ1 , P ϭ 0.02) and II (0.11 Ϯ 0.012 vs. 0.16 Ϯ 0.012 nmol·min Ϫ1 ·citrate synthase activity Ϫ1 , P ϭ 0.003) functional activity in AF patients. Conversely, complex V activity was significantly increased in AF patients (0.21 Ϯ 0.027 vs. 0.12 Ϯ 0.01 nmol·min Ϫ1 ·citrate synthase activity Ϫ1 , P ϭ 0.005). In addition, AF patients exhibited a higher oxidative stress with increased production of mitochondrial superoxide (73 Ϯ 17 vs. 11 Ϯ 2 arbitrary units, P ϭ 0.03) and 4-hydroxynonenal level (77.64 Ϯ 30.2 vs. 9.83 Ϯ 2.83 ng·mg Ϫ1 protein, P ϭ 0.048). Our findings suggest that AF is associated with selective downregulation of ETC activity and increased oxidative stress that can contribute to the progression of the substrate for AF. atrial fibrillation; humans; mitochondria; electron transport chain complexes; oxidative phosphorylation; oxidative stress; superoxide; 4-hydroxynonenal protein adducts NEW & NOTEWORTHYThe study provides evidence of a selective downregulation of mitochondrial electron transport chain functional activity predominantly affecting complexes I and II and associated increase ATRIAL FIBRILLATION (AF), a rapid irregular rhythm of the atria, is associated with electrical, functional, and structural changes in the atria that promote the substrate for its recurrence and progression (36, 53, 60). The incidence and prevalence of AF increase with advancing age and aging-associated diseases such as hypertension, ischemic heart disease, and heart failure (2, 40) and contribute to increased morbidity, particularly an increased risk for stroke, heart failure, and death (25,52). Although the pathophysiology of AF has been well characterized, the underlying mechanisms that contribute to the progression of AF in human atria have not been fully defined (33,35,57,58,60). Mitochondria, occupying 30% ...

Background: Type II diabetes mellitus is a major health problem contributing to increased morbidity and mortality with associated cardiovascular diseases. Metformin, an oral antidiabetic agent, has cardioprotective properties independent of their glucose lowering effect; however, mechanisms underlying cardioprotection remain poorly defined. We hypothesized that the cardioprotective effect of metformin appears to be associated with inhibition of mitochondrial permability transition pore (mPTP) that could be beneficial in diabetic hearts with attenuated endogenous cardioprotective responses. Purpose: The aim of the study was to determine the protective effect of metformin on mPTP opening in mitochondria from human myocardium.. Methods: Mitochondria freshly isolated from the left atrial appendage of nondiabetic patients undergoing cardiac surgery were loaded with calcium- (fluo-3) and membrane potential-sensitive (Safranin-O) fluorescent dyes and challenged with sequential pulses of Ca2+ (10 μM every 3 m) in the absence and presence of different concentrations of metformin (2.5, 5, 10 mM). The sensitivity of mitochondria toward mPTP opening was assessed by abrupt release of mitochondrial Ca2+ (fluo-5N fluorescence), with simultaneous dissipation of mitochondrial membrane potential (safranin O fluorescence) and mitochondrial swelling (decrease in light scattering). Results: Metformin caused a dose-dependent inhibition of Ca2+-induced mPTP opening with delayed mitochondrial Ca2+ release, depolarization and swelling. The tolerance of metformin-treated mitochondria to Ca2+-induced mPTP opening when compared to controls was increased from 457±71 to 600±74 nmol/mg protein at 2.5mM, 674±10 nmol/mg protein at 5 mM and 750±77 nmol/mg protein at 10 mM. Cyclosporin A (1 μM), a known mPTP inhibitor, has marginal incremental effect on metformin-induced inhibition of mPTP opening. Conclusion: In human cardiac mitochondria, metformin inhibited calcium-overload-mediated mPTP opening that otherwise leads to mitochondrial energetic failure and cell death. Thus, metformin may help restore attenuated cardioprotection in diabetic hearts by increasing tolerance to stress-induced mPTP opening and, thus, preventing cell death.

Introduction: Dronedarone (DR) is a new antiarrhythmic agent that was design to minimize side effects of Amiodarone. However, in clinical trials was shown to worsen heart failure (HF) and increased mortality. The mechanisms for these adverse effects are not known. Since mitochondria are critical for maintaining cardiac energetics and regulate, we hypothesized that DR increases sensitivity towards opening of mitochondrial permeability transition pore (mPTP) that underlies mitochondrial failure and cell death. Methods: Mitochondria were isolated from left atrial appendage tissue of patients undergoing bypass cardiac surgery (n=3). Patients did not have any history of HF or atrial fibrillation. To induce mPTP, mitochondria were exposed with sequential additions of Ca 2+ (Fluo-5N) in the presence of 5, 10, 20 μM of DR. The opening of mPTP was determined by monitoring abrupt release of Ca 2+ , rapid loss of membrane potential (ΔΨ) and mitochondrial swelling. Results: DR accelerated Ca 2+ -induced mPTP opening starting at 5 μM concentration (Fig). At concentration of 20 μM, it completely depolarized mitochondria abolishing capacity for Ca 2+ handling and responsiveness to cyclosporine A (CsA), an inhibitor of mPTP. Conclusion: In isolated human cardiac mitochondria from patients without history of HF DR at clinically relevant concentration increased sensitivity of mitochondria to Ca 2+ induced mPTP opening. In failing heart this can be compromised and needs to be studied. Dronedarone accelerates Ca 2+ -induced mPTP opening in mitochondria isolated from human left atrial appendage.