Hydrogen peroxide decreases effective refractory period in the isolated heart

NADPH Oxidases and Oxidative Stress in the Pathogenesis of Atrial Fibrillation

Atrial fibrillation (AF) is the most common type of cardiac arrhythmia and its prevalence increases with age. The irregular and rapid contraction of the atria can lead to ineffective blood pumping, local blood stasis, blood clots, ischemic stroke, and heart failure. NADPH oxidases (NOX) and mitochondria are the main sources of reactive oxygen species in the heart, and dysregulated activation of NOX and mitochondrial dysfunction are associated with AF pathogenesis. NOX- and mitochondria-derived oxidative stress contribute to the onset of paroxysmal AF by inducing electrophysiological changes in atrial myocytes and structural remodeling in the atria. Because high atrial activity causes cardiac myocytes to expend extremely high energy to maintain excitation-contraction coupling during persistent AF, mitochondria, the primary energy source, undergo metabolic stress, affecting their morphology, Ca2+ handling, and ATP generation. In this review, we discuss the role of oxidative stress in activating AF-triggered activities, regulating intracellular Ca2+ handling, and functional and anatomical reentry mechanisms, all of which are associated with AF initiation, perpetuation, and progression. Changes in the extracellular matrix, inflammation, ion channel expression and function, myofibril structure, and mitochondrial function occur during the early transitional stages of AF, opening a window of opportunity to target NOX and mitochondria-derived oxidative stress using isoform-specific NOX inhibitors and mitochondrial ROS scavengers, as well as drugs that improve mitochondrial dynamics and metabolism to treat persistent AF and its transition to permanent AF.

Age-dependent atrial arrhythmic phenotype secondary to mitochondrial dysfunction in Pgc-1β deficient murine hearts

INTRODUCTION

Ageing and several age-related chronic conditions including obesity, insulin resistance and hypertension are associated with mitochondrial dysfunction and represent independent risk factors for atrial fibrillation (AF).

MATERIALS AND METHODS

Atrial arrhythmogenesis was investigated in Langendorff-perfused young (3-4 month) and aged (>12 month), wild type (WT) and peroxisome proliferator activated receptor-γ coactivator-1β deficient (Pgc-1β-/-) murine hearts modeling age-dependent chronic mitochondrial dysfunction during regular pacing and programmed electrical stimulation (PES).

RESULTS AND DISCUSSION

The Pgc-1β-/- genotype was associated with a pro-arrhythmic phenotype progressing with age. Young and aged Pgc-1β-/- hearts showed compromised maximum action potential (AP) depolarization rates, (dV/dt)max, prolonged AP latencies reflecting slowed action potential (AP) conduction, similar effective refractory periods and baseline action potential durations (APD90) but shortened APD90 in APs in response to extrasystolic stimuli at short stimulation intervals. Electrical properties of APs triggering arrhythmia were similar in WT and Pgc-1β-/- hearts. Pgc-1β-/- hearts showed accelerated age-dependent fibrotic change relative to WT, with young Pgc-1β-/- hearts displaying similar fibrotic change as aged WT, and aged Pgc-1β-/- hearts the greatest fibrotic change. Mitochondrial deficits thus result in an arrhythmic substrate, through slowed AP conduction and altered repolarisation characteristics, arising from alterations in electrophysiological properties and accelerated structural change.

Progression of ventricular remodeling and arrhythmia in the primary hyperoxidative state of glutathione-depleted rats

BACKGROUND

Although oxidative stress is considered to promote arrhythmogenic substrates in diseased model animals, it is difficult to evaluate its primary role. In this study, we evaluated the promotion of arrhythmogenic substrates in the primary hyperoxidative state.

METHODS AND RESULTS

Sprague-Dawley rats were treated with L-buthionine-sulfoximine (BSO, 30 mmol · L(-1) · day(-1)) for 14 days. On day 7 or 14, the serum levels of derivatives of reactive oxygen metabolites (d-ROM) were measured, and immune staining of 8-hydroxy-2'-deoxyguanosine (8O HdG) was performed to assess oxidative stress. The ventricular effective refractory period (ERP), monophasic action potential duration (MAPD), and the inducibility of ventricular arrhythmia were also evaluated. BSO rats exhibited higher serum d-ROM and clearer 8OHdG staining than the controls. The inducibility of ventricular arrhythmia was higher in the BSO rats than in the controls. The ERP was shorter in the BSO rats than the control (day 14, 32 ± 1 vs. 36 ± 1 ms, P<0.05), whereas the MAPD(90) was longer in the BSO rats (day 14, 76 ± 5 vs. 55 ± 4 ms, P<0.05). The mRNA levels of Kv4.2, erg, and SERCA2a were downregulated in the BSO rats (P < 0.05), and Western blot analysis exhibited the downregulation of erg and SERCA2 expression in the BSO rats (P < 0.05).

CONCLUSIONS

Systemic oxidative stress might be one of the primary factors promoting cardiac electrophysiological remodeling and increasing the inducibility of arrhythmia independently of major organ disorders.

Transmural distribution of antioxidant defences and lipid peroxidation in the rabbit left ventricular myocardium

The left ventricular subendocardial and subepicardial layers of six perfused rabbit hearts were tested for enzymatic and non-enzymatic antioxidant defences and for lipid peroxidation. The subendocardium showed significantly lower catalase activity and contents of non-protein thiol compounds and vitamin E associated with a higher degree of lipid peroxidation. The activities of Cu,Zn- and Mn-superoxide dismutases, glutathione reductase, gamma-glutamylcysteine synthetase and gamma-glutamyl transpeptidase showed no significant transmural differences, and Se-independent glutathione peroxidase activity was not detectable in either layer. Comparable results were observed in another group of six unperfused rabbit hearts. In five H2O2-perfused rabbit hearts, lipid peroxidation was higher, and myocardial creatine phosphokinase activity lower, in the subendocardium than in the subepicardium. In this group, only the subendocardium had significantly higher lipid peroxidation levels than the control hearts. Thus, a lower antioxidant capacity and a greater oxidative stress are present in the rabbit subendocardium. These findings could provide insight into the problem of subendocardial vulnerability to free radical-mediated processes, such as occurs in ischaemia-reperfusion injury.

Hydroperoxide-induced oxidative stress impairs heart muscle cell carbohydrate metabolism

Hydrogen peroxide (H2O2) may incite cardiac ischemia-reperfusion injury. We evaluate herein the influence of H2O2-induced oxidative stress on heart muscle hexose metabolism in cultured neonatal rat cardiomyocytes, which have a substrate preference for carbohydrate. Cardiomyocyte exposure to 50 microM-1.0 mM bolus H2O2 transiently activated the pentose phosphate cycle and thereafter inhibited cellular glucose oxidation and glycolysis. These metabolic derangements were nonperoxidative in nature (as assessed in alpha-tocopherol-loaded cells) and occurred without acute change in cardiomyocyte hexose transport or glucose/glycogen reserves. Glycolytic inhibition was supported by the rapid, specific inactivation of glyceraldehyde-3-phosphate dehydrogenase (GAPDH). The degree of GAPDH inhibition correlated directly with the magnitude of the oxidative insult and was independent of both metal-catalyzed H2O2 reduction to free radicals and lipid peroxidation. Severe GAPDH inhibition was required for a rate-limiting effect on glycolytic flux. Cardiomyocyte pyruvate dehydrogenase was also inhibited by H2O2 overload, but to a lesser degree than GAPDH such that entry of hexose-derived acetyl units into the tricarboxylic acid cycle was not as restrictive as GAPDH inactivation to glycolytic ATP production. An increase in phosphofructokinase activity accompanied GAPDH inactivation, leading to the production and accumulation of glycolytic sugar phosphates at the expense of ATP equivalents. Cardiomyocyte treatment with iodoacetate or 2-deoxyglucose indicated that GAPDH inactivation/glycolytic blockade could account for approximately 50% of the maximal ATP loss following H2O2 overload. Partial restoration of GAPDH activity after a brief H2O2 "pulse" afforded some ATP recovery. These data establish that specific aspects of heart muscle hexose catabolism are H2O2-sensitive injury targets. The biochemical pathology of H2O2 overload on cardiomyocyte carbohydrate metabolism has implications for post-ischemic cardiac bioenergetics and function.

Hydrogen peroxide-induced oxidative stress to the mammalian heart-muscle cell (cardiomyocyte): nonperoxidative purine and pyrimidine nucleotide depletion

Hydrogen peroxide (H2O2) overload may contribute to cardiac ischemia-reperfusion injury. We report utilization of a previously described cardiomyocyte model (J. Cell. Physiol., 149:347, 1991) to assess the effect of H2O2-induced oxidative stress on heart-muscle purine and pyrimidine nucleotides and high-energy phosphates (ATP, phosphocreatine). Oxidative stress induced by bolus H2O2 elicited the loss of cardiomyocyte purine and pyrimidine nucleotides, leading to eventual de-energization upon total ATP and phosphocreatine depletion. The rate and extent of ATP and phosphocreatine loss were dependent on the degree of oxidative stress within the range of 50 microM to 1.0 mM H2O2. At the highest H2O2 concentration, 5 min was sufficient to elicit appreciable cardiomyocyte high-energy phosphate loss, the extent of which could be limited by prompt elimination of H2O2 from the culture medium. Only H2O2 dismutation completely prevented ATP loss during H2O2-induced oxidative stress, whereas various free-radical scavengers and metal chelators afforded no significant ATP preservation. Exogenously-supplied catabolic substrates and glycolytic or tricarboxylic acid-cycle intermediates did not ameliorate the observed ATP and phosphocreatine depletion, suggesting that cardiomyocyte de-energization during H2O2-induced oxidative stress reflected defects in substrate utilization/energy conservation. Compromise of cardiomyocyte nucleotide and phosphocreatine pools during H2O2-induced oxidative stress was completely dissociated from membrane peroxidative damage and maintenance of cell integrity. Cardiomyocyte de-energization in response to H2O2 overload may constitute a distinct nonperoxidative mode of injury by which cardiomyocyte energy balance could be chronically compromised in the post-ischemic heart.