Mechanism of Na+/Ca2+ exchanger activation by hydrogen peroxide in guinea-pig ventricular myocytes

Implications for neutrophils in cardiac arrhythmias

Cardiac arrhythmias commonly occur as a result of aberrant electrical impulse formation or conduction in the myocardium. Frequently discussed triggers include underlying heart diseases such as myocardial ischemia, electrolyte imbalances, or genetic anomalies of ion channels involved in the tightly regulated cardiac action potential. Recently, the role of innate immune cells in the onset of arrhythmic events has been highlighted in numerous studies, correlating leukocyte expansion in the myocardium to increased arrhythmic burden. Here, we aim to call attention to the role of neutrophils in the pathogenesis of cardiac arrhythmias and their expansion during myocardial ischemia and infectious disease manifestation. In addition, we will elucidate molecular mechanisms associated with neutrophil activation and discuss their involvement as direct mediators of arrhythmogenicity.

Mitochondrial Dysfunction in Cardiac Arrhythmias

Electrophysiological and structural disruptions in cardiac arrhythmias are closely related to mitochondrial dysfunction. Mitochondria are an organelle generating ATP, thereby satisfying the energy demand of the incessant electrical activity in the heart. In arrhythmias, the homeostatic supply-demand relationship is impaired, which is often accompanied by progressive mitochondrial dysfunction leading to reduced ATP production and elevated reactive oxidative species generation. Furthermore, ion homeostasis, membrane excitability, and cardiac structure can be disrupted through pathological changes in gap junctions and inflammatory signaling, which results in impaired cardiac electrical homeostasis. Herein, we review the electrical and molecular mechanisms of cardiac arrhythmias, with a particular focus on mitochondrial dysfunction in ionic regulation and gap junction action. We provide an update on inherited and acquired mitochondrial dysfunction to explore the pathophysiology of different types of arrhythmias. In addition, we highlight the role of mitochondria in bradyarrhythmia, including sinus node dysfunction and atrioventricular node dysfunction. Finally, we discuss how confounding factors, such as aging, gut microbiome, cardiac reperfusion injury, and electrical stimulation, modulate mitochondrial function and cause tachyarrhythmia.

The regulatory role of endoplasmic reticulum chaperone proteins in neurodevelopment

The endoplasmic reticulum (ER) is the largest tubular reticular organelle spanning the cell. As the main site of protein synthesis, Ca2+ homeostasis maintenance and lipid metabolism, the ER plays a variety of essential roles in eukaryotic cells, with ER molecular chaperones participate in all these processes. In recent years, it has been reported that the abnormal expression of ER chaperones often leads to a variety of neurodevelopmental disorders (NDDs), including abnormal neuronal migration, neuronal morphogenesis, and synaptic function. Neuronal development is a complex and precisely regulated process. Currently, the mechanism by which neural development is regulated at the ER level remains under investigation. Therefore, in this work, we reviewed the recent advances in the roles of ER chaperones in neural development and developmental disorders caused by the deficiency of these molecular chaperones.

Role of Oxidative Stress in the Genesis of Ventricular Arrhythmias

Ventricular arrhythmias, mainly lethal arrhythmias, such as ventricular tachycardia and fibrillation, may lead to sudden cardiac death. These are triggered as a result of cardiac injury due to chronic ischemia, acute myocardial infarction and various stressful conditions associated with increased levels of circulating catecholamines and angiotensin II. Several mechanisms have been proposed to underlie electrical instability of the heart promoting ventricular arrhythmias; however, oxidative stress which adversely affects ion homeostasis due to changes in the ion channel structure and function, seems to play a critical role in eliciting different types of ventricular arrhythmias. Prevention or mitigation of the severity of ventricular arrhythmias due to antioxidants has been indicated as the fundamental contribution in the field of preventive cardiology; however, novel interventions have to be developed for greater effectiveness and specificity in attenuating the adverse effects of oxidative stress. In this review, we have attempted to discuss proarrhythmic effects of oxidative stress differing in time and concentration dependence and highlight a molecular and cellular concept how it alters cardiac cell automaticity and conduction velocity sensitizing the probability of ventricular arrhythmias with resultant sudden cardiac death due to ischemic heart disease and other stressful situations. It is concluded that pharmacological approaches targeting multiple mechanisms besides oxidative stress might be more effective in the treatment of ventricular arrhythmias than current antiarrhythmic therapy.

Cardiac Na+/Ca2+ exchange stimulators among cardioprotective drugs

We previously reviewed our study of the pharmacological properties of cardiac Na+/Ca2+ exchange (NCX1) inhibitors among cardioprotective drugs, such as amiodarone, bepridil, dronedarone, cibenzoline, azimilide, aprindine, and benzyl-oxyphenyl derivatives (Watanabe et al. in J Pharmacol Sci 102:7-16, 2006). Since then we have continued our studies further and found that some cardioprotective drugs are NCX1 stimulators. Cardiac Na+/Ca2+ exchange current (INCX1) was stimulated by nicorandil (a hybrid ATP-sensitive K+ channel opener), pinacidil (a non-selective ATP-sensitive K+ channel opener), flecainide (an antiarrhythmic drug), and sodium nitroprusside (SNP) (an NO donor). Sildenafil (a phosphodiesterase-5 inhibitor) further increased the pinacidil-induced augmentation of INCX1. In paper, here I review the NCX stimulants that enhance NCX function among the cardioprotective agents we examined such as nicorandil, pinacidil, SNP, sildenafil and flecainide, in addition to atrial natriuretic (ANP) and dofetilide, which were reported by other investigators.

Pinacidil, a KATP channel opener, stimulates cardiac Na+/Ca2+ exchanger function through the NO/cGMP/PKG signaling pathway in guinea pig cardiac ventricular myocytes

Pinacidil, a nonselective ATP-sensitive K⁺ (KATP) channel opener, has cardioprotective effects for hypertension, ischemia/reperfusion injury, and arrhythmia. This agent abolishes early afterdepolarizations, delayed afterdepolarizations (DADs), and abnormal automaticity in canine cardiac ventricular myocytes. DADs are well known to be caused by the Na⁺/Ca²⁺ exchange current (I_NCX). In this study, we used the whole-cell patch-clamp technique and Fura-2/AM (Ca²⁺-indicator) method to investigate the effect of pinacidil on I_NCX in isolated guinea pig cardiac ventricular myocytes. In the patch-clamp study, pinacidil enhanced I_NCX in a concentration-dependent manner. The half-maximal effective concentration values were 23.5 and 23.0 μM for the Ca²⁺ entry (outward) and Ca²⁺ exit (inward) components of I_NCX, respectively. The pinacidil-induced I_NCX increase was blocked by L-NAME, a nitric oxide (NO) synthase inhibitor, by ODQ, a soluble guanylate cyclase inhibitor, and by KT5823, a cyclic guanosine monophosphate (cGMP)-dependent protein kinase (PKG) inhibitor, but not by N-2-mercaptopropyonyl glycine (MPG), a reactive oxygen species (ROS) scavenger. Glibenclamide, a nonselective KATP channel inhibitor, blocked the pinacidil-induced I_NCX increase, while 5-HD, a selective mitochondria KATP channel inhibitor, did not. In the Fura-2/AM study pinacidil also enhanced intracellular Ca²⁺ concentration, which was inhibited by L-NAME, ODQ, KT5823, and glibenclamide, but not by MPG and 5-HD. Sildenafil, a phosphodiesterase 5 inhibitor, increased further the pinacidil-induced I_NCX increase. Sodium nitroprusside, a NO donor, also increased I_NCX. In conclusion, pinacidil may stimulate cardiac Na⁺/Ca²⁺ exchanger (NCX1) by opening plasma membrane KATP channels and activating the NO/cGMP/PKG signaling pathway.

Antioxidant defense and protection against cardiac arrhythmias: lessons from a mammalian hibernator (the woodchuck)

Hibernating animals show resistance to hypothermia-induced cardiac arrhythmias. However, it is not clear whether and how mammalian hibernators are resistant to ischemia-induced arrhythmias. The goal of this investigation was to determine the susceptibility of woodchucks ( Marmota monax) to arrhythmias and their mechanisms after coronary artery occlusion at the same room temperature in both winter, the time for hibernation, and summer, when they do not hibernate. By monitoring telemetric electrocardiograms, we found significantly higher arrhythmia scores, calculated as the severity of arrhythmias, with incidence of ventricular tachycardia, ventricular fibrillation, and thus sudden cardiac death (SCD) in woodchucks in summer than they had in winter. The level of catalase expression in woodchuck hearts was significantly higher, whereas the level of oxidized Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) was lower in winter than it was in summer. Ventricular myocytes isolated from woodchucks in winter were more resistant to H₂O₂-induced early afterdepolarizations (EADs) compared with myocytes isolated from woodchucks in summer. The EADs were eliminated by inhibiting CaMKII (with KN-93), l-type Ca current (with nifedipine), or late Na⁺ current (with ranolazine). In woodchucks, in the summer, the arrhythmia score was significantly reduced by overexpression of catalase ( via adenoviral vectors) or the inhibition of CaMKII (with KN-93) in the heart. This study suggests that the heart of the mammalian hibernator is more resistant to ischemia-induced arrhythmias and SCD in winter. Increased antioxidative capacity and reduced CaMKII activity may confer resistance in woodchuck hearts against EADs and arrhythmias during winter. The profound protection conferred by catalase overexpression or CaMKII inhibition in this novel natural animal model may provide insights into clinical directions for therapy of arrhythmias.-Zhao, Z., Kudej, R. K., Wen, H., Fefelova, N., Yan, L., Vatner, D. E., Vatner, S. F., Xie, L.-H. Antioxidant defense and protection against cardiac arrhythmias: lessons from a mammalian hibernator (the woodchuck).

Regulation of signal transduction by reactive oxygen species in the cardiovascular system

Oxidative stress has long been implicated in cardiovascular disease, but more recently, the role of reactive oxygen species (ROS) in normal physiological signaling has been elucidated. Signaling pathways modulated by ROS are complex and compartmentalized, and we are only beginning to identify the molecular modifications of specific targets. Here, we review the current literature on ROS signaling in the cardiovascular system, focusing on the role of ROS in normal physiology and how dysregulation of signaling circuits contributes to cardiovascular diseases, including atherosclerosis, ischemia-reperfusion injury, cardiomyopathy, and heart failure. In particular, we consider how ROS modulate signaling pathways related to phenotypic modulation, migration and adhesion, contractility, proliferation and hypertrophy, angiogenesis, endoplasmic reticulum stress, apoptosis, and senescence. Understanding the specific targets of ROS may guide the development of the next generation of ROS-modifying therapies to reduce morbidity and mortality associated with oxidative stress.

A role for the sodium pump in H2O2-induced vasorelaxation in porcine isolated coronary arteries

Hydrogen peroxide (H2O2) has been proposed to act as a factor for endothelium-derived hyperpolarization (EDH) and EDH may act as a 'back up' system to compensate the loss of the NO pathway. Here, the mechanism of action of H2O2 in porcine isolated coronary arteries (PCAs) was investigated. Distal PCAs were mounted in a wire myograph and pre-contracted with U46619 (1nM-50μM), a thromboxane A2-mimetic or KCl (60mM). Concentration-response curves to H2O2(1μM-1mM), bradykinin (0.01nM-1μM), sodium nitroprusside (SNP) (10nM-10μM), verapamil (1nM-10μM), KCl (0-20mM) or Ca(2+)-reintroduction (1μM-10mM) were constructed in the presence of various inhibitors. Activity of the Na(+)/K(+)-pump was measured through rubidium-uptake using atomic absorption spectrophotometry. H2O2 caused concentration-dependent vasorelaxations with a maximum relaxation (Rmax) of 100±16% (mean±SEM), pEC50=4.18±0.20 (n=4) which were significantly inhibited by PEG-catalase at 0.1-1.0mM H2O2 (P<0.05). 10mM TEA significantly inhibited the relaxation up to 100μM H2O2 (P<0.05). 60mM K(+) and 500nM ouabain significantly inhibited H2O2-induced vasorelaxation producing a relaxation of 40.8±8.5% (n=5) and 47.5±8.6% (n=6) respectively at 1mM H2O2 (P<0.0001). H2O2-induced vasorelaxation was unaffected by the removal of endothelium, inhibition of NO, cyclo-oxygenase, gap junctions, SKCa, IKCa, BKCa Kir, KV, KATP or cGMP. 100μM H2O2 had no effects on the KCl-induced vasorelaxation or Ca(2+)-reintroduction contraction. 1mM H2O2 inhibited both KCl-induced vasorelaxation and rubidium-uptake consistent with inhibition of the Na(+)/K(+)-pump activity. We have shown that the vascular actions of H2O2 are sensitive to ouabain and high concentrations of H2O2 are able to modulate the Na(+)/K(+)-pump. This may contribute towards its vascular actions.

p21-Activated kinase1 (Pak1) is a negative regulator of NADPH-oxidase 2 in ventricular myocytes

Ischemic conditions reduce the activity of the p21-activated kinase (Pak1) resulting in increased arrhythmic activity. Triggered arrhythmic activity during ischemia is based on changes in cellular ionic balance and the cells Ca(2+) handling properties. In the current study we used isolated mouse ventricular myocytes (VMs) deficient for the expression of Pak1 (Pak1(-/-)) to determine the mechanism by which Pak1 influences the generation of arrhythmic activity during simulated ischemia. The Ca(2+) transient amplitude and kinetics did not significantly change in wild type (WT) and Pak1(-/-) VMs during 15 min of simulated ischemia. However, Pak1(-/-) VMs exhibited an exaggerated increase in [Ca(2+)]i, which resulted in spontaneous Ca(2+) release events and waves. The Ca(2+) overload in Pak1(-/-) VMs could be suppressed with a reverse mode blocker (KB-R7943) of the sodium calcium exchanger (NCX), a cytoplasmic scavenger of reactive oxygen species (ROS; TEMPOL) or a RAC1 inhibitor (NSC23766). Measurements of the cytoplasmic ROS levels revealed that decreased Pak1 activity in Pak1(-/-) VMs or VMs treated with the Pak1 inhibitor (IPA3) enhanced cellular ROS production. The Pak1 dependent increase in ROS was attenuated in VMs deficient for NADPH oxidase 2 (NOX2; p47(phox-/-)) or in VMs where NOX2 was inhibited (gp91ds-tat). Voltage clamp recordings showed increased NCX activity in Pak1(-/-) VMs that depended on enhanced NOX2 induced ROS production. The exaggerated Ca(2+) overload in Pak1(-/-) VMs could be mimicked by low concentrations of ouabain. Overall our data show that Pak1 is a critical negative regulator of NOX2 dependent ROS production and that a latent ROS dependent stimulation of NCX activity can predispose VMs to Ca(2+) overload under conditions where no significant changes in excitation-contraction coupling are yet evident.

N-n-butyl haloperidol iodide inhibits H2O2-induced Na+/Ca2+-exchanger activation via the Na+/H+ exchanger in rat ventricular myocytes

N-n-butyl haloperidol iodide (F2), a novel compound, has shown palliative effects in myocardial ischemia/reperfusion (I/R) injury. In this study, we investigated the effects of F2 on the extracellular signal-regulated kinase kinase (MEK)/extracellular signal-regulated kinase (ERK)/Na(+)/H(+) exchanger (NHE)/Na(+)/Ca(2+) exchanger (NCX) signal-transduction pathway involved in H2O2-induced Ca(2+) overload, in order to probe the underlying molecular mechanism by which F2 antagonizes myocardial I/R injury. Acute exposure of rat cardiac myocytes to 100 μM H2O2 increased both NHE and NCX activities, as well as levels of phosphorylated MEK and ERK. The H2O2-induced increase in NCX current (I NCX) was nearly completely inhibited by the MEK inhibitor U0126 (1,4-diamino-2,3-dicyano-1,4-bis[o-aminophenylmercapto] butadiene), but only partly by the NHE inhibitor 5-(N,N-dimethyl)-amiloride (DMA), indicating the I NCX increase was primarily mediated by the MEK/mitogen-activated protein kinase (MAPK) pathway, and partially through activation of NHE. F2 attenuated the H2O2-induced I NCX increase in a concentration-dependent manner. To determine whether pathway inhibition was H2O2-specific, we examined the ability of F2 to inhibit MEK/ERK activation by epidermal growth factor (EGF), and NHE activation by angiotensin II. F2 not only inhibited H2O2-induced and EGF-induced MEK/ERK activation, but also completely blocked both H2O2-induced and angiotensin II-induced increases in NHE activity, suggesting that F2 directly inhibits MEK/ERK and NHE activation. These results show that F2 exerts multiple inhibitions on the signal-transduction pathway involved in H2O2-induced I NCX increase, providing an additional mechanism for F2 alleviating intracellular Ca(2+) overload to protect against myocardial I/R injury.

Atrial fibrillation in the elderly: the potential contribution of reactive oxygen species

Atrial fibrillation (AF) is the most commonly encountered cardiac arrhythmia, and is a significant source of healthcare expenditures throughout the world. It is an arrhythmia with a very clearly defined predisposition for individuals of advanced age, and this fact has led to intense study of the mechanistic links between aging and AF. By promoting oxidative damage to multiple subcellular and cellular structures, reactive oxygen species (ROS) have been shown to induce the intra- and extra-cellular changes necessary to promote the pathogenesis of AF. In addition, the generation and accumulation of ROS have been intimately linked to the cellular processes which underlie aging. This review begins with an overview of AF pathophysiology, and introduces the critical structures which, when damaged, predispose an otherwise healthy atrium to AF. The available evidence that ROS can lead to damage of these critical structures is then reviewed. Finally, the evidence linking the process of aging to the pathogenesis of AF is discussed.

N-n-Butyl haloperidol iodide inhibits the augmented Na+/Ca2+ exchanger currents and L-type Ca2+ current induced by hypoxia/reoxygenation or H2O2 in cardiomyocytes

N-n-butyl haloperidol iodide (F(2)), a novel quaternary ammonium salt derivative of haloperidol, was reported to antagonize myocardial ischemia/reperfusion injuries. To investigate its mechanisms, we characterized the effects of F(2) on Na(+)/Ca(2+) exchanger currents (I(NCX)) and the L-type Ca(2+) channel current (I(Ca,L)) of cardiomyocytes during either hypoxia/reoxygenation or exposure to H(2)O(2). Using whole-cell patch-clamp techniques, the I(NCX) and I(Ca,L) were recorded from isolated rat ventricular myocytes. Exposure of cardiomyocytes to hypoxia/reoxygenation or H(2)O(2) enhanced the amplitude of the inward and outward of I(NCX) and I(Ca,L). F(2) especially inhibited the outward current of Na(+)/Ca(2+) exchanger, as well as the I(Ca,L), in a concentration-dependent manner. F(2) inhibits cardiomyocyte I(NCX) and I(Ca,L) after exposure to hypoxia/reoxygenation or H(2)O(2) to antagonize myocardial ischemia/reperfusion injury by inhibiting Ca(2+) overload.

Metabolic stress, reactive oxygen species, and arrhythmia

Cardiac arrhythmias can cause sudden cardiac death (SCD) and add to the current heart failure (HF) health crisis. Nevertheless, the pathological processes underlying arrhythmias are unclear. Arrhythmic conditions are associated with systemic and cardiac oxidative stress caused by reactive oxygen species (ROS). In excitable cardiac cells, ROS regulate both cellular metabolism and ion homeostasis. Increasing evidence suggests that elevated cellular ROS can cause alterations of the cardiac sodium channel (Na(v)1.5), abnormal Ca(2+) handling, changes of mitochondrial function, and gap junction remodeling, leading to arrhythmogenesis. This review summarizes our knowledge of the mechanisms by which ROS may cause arrhythmias and discusses potential therapeutic strategies to prevent arrhythmias by targeting ROS and its consequences. This article is part of a Special Issue entitled "Local Signaling in Myocytes".

The effects of hydrogen peroxide on intracellular calcium handling and contractility in the rat ventricular myocyte

Elevations in reactive oxygen species are implicated in many disease states and cause systolic and diastolic myocardial dysfunction. To understand the underlying cellular dysfunction, we characterised the effects of H₂O₂ on [Ca(2+)](i) handling and contractility in the rat ventricular myocyte. This was achieved using patch clamping, [Ca(2+)](i) measurement using Fluo-3, video edge detection and confocal microscopy. All experiments were performed at 37°C. 200 μM H₂O₂ resulted in a 44% decrease in the [Ca(2+)](i) transient amplitude, a 30% increase in diastolic [Ca(2+)](i) and an 18% decrease in the rate of systolic Ca(2+) removal. This was associated with a 61% reduction in systolic shortening, a contracture of 3 μm and a 42% increase in relaxation time respectively. The decrease in the [Ca(2+)](i) transient amplitude could be explained by a 27% decrease in SR Ca(2+) content. This, in turn results from a 22% decrease of SERCA activity. The decreased SR Ca(2+) content also provides a mechanism for a reduction in [Ca(2+)](i) spark frequency with no evidence for a Ca(2+) independent modification of ryanodine receptor open probability. We conclude that decreased SERCA activity is the major factor responsible for the changes of the systolic [Ca(2+)](i) transient.

Na+ -Ca2+ exchanger expression and its modulation

Here we reviewed our recent work on the chronic effects of nicotine on the Na+ -Ca2+ exchanger (NCX) gene and protein expressions in various organs of rats treated with nicotine in the drinking water for 4-12 weeks. Microarray analysis and reverse transcriptase-polymerase chain reaction (RT-PCR) did not detect significant changes in NCX mRNA expression in cerebral cortex, hippocampus, heart and skeletal muscle. However, NCX1 protein was up-regulated by nicotine in cerebral cortex and hippocampus, but was down-regulated in the heart. NCX2 protein was up-regulated by nicotine in hippocampus. We suggest that although mRNA change was insignificant, NCX protein expression was altered by chronic nicotine administration in brain and heart in rats. We also reviewed our work on modulators of NCX gene expression and function in cardiac myocytes.

Apocynin: a potent NADPH oxidase inhibitor for the management of atrial fibrillation

Oxidative stress in atrial tissue may be causally related to atrial fibrillation as suggested by clinical and animal studies. Reactive oxygen species (ROS) are known to play a key role in fibrosis and the induction of after-depolarization and triggered activity. Therefore, suppressing oxidative stress may have a potential beneficial role in the management of atrial fibrillation. Since increased NADPH oxidase activity is shown to play a key role in generation of ROS in atrial tissue and in atrial fibrillation, our proposed strategy to target upstream inhibition of ROS production by inhibition of NADPH oxidase activity may provide a novel approach to prevent atrial fibrillation recurrences. We hypothesize that apocynin could be effective against atrial fibrillation, by virtue of its potent inhibitory effect of a major oxidative system (i.e. NADPH oxidase) combined with its demonstrated anti-inflammatory, antifibrotic and antihypertensive effects which partially are driven from its antioxidant property. Atrial fibrillation is known to be initiated by the interaction of these multiple factors.

Stretch-activated channel activation promotes early afterdepolarizations in rat ventricular myocytes under oxidative stress

Mechanical stretch and oxidative stress have been shown to prolong action potential duration (APD) and produce early afterdepolarizations (EADs). Here, we developed a simulation model to study the role of stretch-activated channel (SAC) currents in triggering EADs in ventricular myocytes under oxidative stress. We adapted our coupling clamp circuit so that a model ionic current representing the actual SAC current was injected into ventricular myocytes and added as a real-time current. This current was calculated as I(SAC) = G(SAC) * (V(m) - E(SAC)), where G(SAC) is the stretch-activated conductance, V(m) is the membrane potential, and E(SAC) is the reversal potential. In rat ventricular myocytes, application of G(SAC) did not produce sustained automaticity or EADs, although turn-on of G(SAC) did produce some transient automaticity at high levels of G(SAC). Exposure of myocytes to 100 microM H(2)O(2) induced significant APD prolongation and increase in intracellular Ca(2+) load and transient, but no EAD or sustained automaticity was generated in the absence of G(SAC). However, the combination of G(SAC) and H(2)O(2) consistently produced EADs at lower levels of G(SAC) (2.6 +/- 0.4 nS, n = 14, P < 0.05). Pacing myocytes at a faster rate further prolonged APD and promoted the development of EADs. SAC activation plays an important role in facilitating the development of EADs in ventricular myocytes under acute oxidative stress. This mechanism may contribute to the increased propensity to lethal ventricular arrhythmias seen in cardiomyopathies, where the myocardium stretch and oxidative stress generally coexist.

Oxidative-stress-induced afterdepolarizations and calmodulin kinase II signaling

In the heart, oxidative stress caused by exogenous H(2)O(2) has been shown to induce early afterdepolarizations (EADs) and triggered activity by impairing Na current (I(Na)) inactivation. Because H(2)O(2) activates Ca(2+)/calmodulin kinase (CaMK)II, which also impairs I(Na) inactivation and promotes EADs, we hypothesized that CaMKII activation may be an important factor in EADs caused by oxidative stress. Using the patch-clamp and intracellular Ca (Ca(i)) imaging in Fluo-4 AM-loaded rabbit ventricular myocytes, we found that exposure to H(2)O(2) (0.2 to 1 mmol/L) for 5 to 15 minutes consistently induced EADs that were suppressed by the I(Na) blocker tetrodotoxin (10 micromol/L), as well as the I(Ca,L) blocker nifedipine. H(2)O(2) enhanced both peak and late I(Ca,L), consistent with CaMKII-mediated facilitation. By prolonging the action potential plateau and increasing Ca influx via I(Ca,L), H(2)O(2)-induced EADs were also frequently followed by DADs in response to spontaneous (ie, non-I(Ca,L)-gated) sarcoplasmic reticulum Ca release after repolarization. The CaMKII inhibitor KN-93 (1 micromol/L; n=4), but not its inactive analog KN-92 (1 micromol/L, n=5), prevented H(2)O(2)-induced EADs and DADs, and the selective CaMKII peptide inhibitor AIP (autocamtide-2-related inhibitory peptide) (2 micromol/L) significantly delayed their onset. In conclusion, H(2)O(2)-induced afterdepolarizations depend on both impaired I(Na) inactivation to reduce repolarization reserve and enhancement of I(Ca,L) to reverse repolarization, which are both facilitated by CaMKII activation. Our observations support a link between increased oxidative stress, CaMKII activation, and afterdepolarizations as triggers of lethal ventricular arrhythmias in diseased hearts.

H(2)O(2)-induced left ventricular dysfunction in isolated working rat hearts is independent of calcium accumulation

Reactive oxygen species (ROS) and intracellular Ca(2+) overload play key roles in myocardial ischemia-reperfusion (IR) injury but the relationships among ROS, Ca(2+) overload and LV mechanical dysfunction remain unclear. We tested the hypothesis that H(2)O(2) impairs LV function by causing Ca(2+) overload by increasing late sodium current (I(Na)), similar to Sea Anemone Toxin II (ATX-II). Diastolic and systolic Ca(2+) concentrations (d[Ca(2+)](i) and s[Ca(2+)](i)) were measured by indo-1 fluorescence simultaneously with LV work in isolated working rat hearts. H(2)O(2) (100 microM, 30 min) increased d[Ca(2+)](i) and s[Ca(2+)](i). LV work increased transiently then declined to 32% of baseline before recovering to 70%. ATX-II (12 nM, 30 min) caused greater increases in d[Ca(2+)](i) and s[Ca(2+)](i). LV work increased transiently before declining gradually to 17%. Ouabain (80 microM) exerted similar effects to ATX-II. Late I(Na) inhibitors, lidocaine (10 microM) or R56865 (2 microM), reduced effects of ATX-II on [Ca(2+)](i) and LV function, but did not alter effects of H(2)O(2). The antioxidant, N-(2-mercaptopropionyl)glycine (MPG, 1 mM) prevented H(2)O(2)-induced LV dysfunction, but did not alter [Ca(2+)](i). Paradoxically, further increases in [Ca(2+)](i) by ATX-II or ouabain, given 10 min after H(2)O(2), improved function. The failure of late I(Na) inhibitors to prevent H(2)O(2)-induced LV dysfunction, and the ability of MPG to prevent H(2)O(2)-induced LV dysfunction independent of changes in [Ca(2+)](i) indicate that impaired contractility is not due to Ca(2+) overload. The ability of further increases in [Ca(2+)](i) to reverse H(2)O(2)-induced LV dysfunction suggests that Ca(2+) desensitization is the predominant mechanism of ROS-induced contractile dysfunction.

Acidic preconditioning inhibits Na+/H+ and Na+/Ca2+ exchanger interaction via PKCepsilon in guinea-pig ventricular myocytes

An interaction between the Na(+)/Ca(2+) exchanger (NCX) and the Na(+)/H(+) exchanger (NHE) induces reperfusion injury. We investigated the effect of brief repetitive acidosis as acidic preconditioning on NCX and NHE interaction during recovery from acidosis. NCX current with the reversal potential was measured in guinea-pig ventricular myocytes using the whole-cell voltage clamp. The cells were exposed to 5 min of acidosis preceded by two episodes of brief acidosis as acidic preconditioning. Acidosis inhibited NCX current and upon recovery shifted its reversal potential in the negative direction. The shift was prevented by cariporide, but was augmented by a high concentration of phorbol 13-myristate acetate (PMA). Acidic preconditioning prevented the shift, but not in the presence of a selective PKCepsilon inhibitor. A low concentration of PMA, which activates PKCepsilon selectively, prevented the shift, but together with PKCepsilon inhibitor (epsilonV1-2) restored the shift during recovery. 5-Hydroxydecanoate inhibited the effects of acidic preconditioning and those of both low and high concentrations of PMA. The negative shift of NCX reversal potential during recovery from acidosis may be due to [Na(+)](i) accumulation by the NHE. Acidic preconditioning prevented the shift most likely by activating PKCepsilon, which in turn inhibited the NHE. The NHE-NCX interaction may be one of the important end-effectors of preconditioning.

The positive inotropic effect of endothelin-1 is mediated by mitochondrial reactive oxygen species

We have previously demonstrated the participation of reactive oxygen species (ROS) in the positive inotropic effect of a physiological concentration of Angiotensin II (Ang II, 1 nM). The objective of the present work was to evaluate the role and source of ROS generation in the positive inotropic effect produced by an equipotent concentration of endothelin-1 (ET-1, 0.4 nM). Isolated cat ventricular myocytes were used to measure sarcomere shortening with a video-camera, superoxide anion (()O(2)(-)) with chemiluminescence, and ROS production and intracellular pH (pH(i)) with epifluorescence. The ET-1-induced positive inotropic effect (40.4+/-3.1%, n=10, p<0.05) was associated to an increase in ROS production (105+/-29 fluorescence units above control, n=6, p<0.05). ET-1 also induced an increase in ()O(2)(-) production that was inhibited by the NADPH oxidase blocker, apocynin, and by the blockers of mitochondrial ATP-sensitive K(+) channels (mK(ATP)), glibenclamide and 5 hydroxydecanoic acid. The ET-1-induced positive inotropic effect was inhibited by apocynin (0.3 mM; 6.3+/-6.6%, n=13), glibenclamide (50 microM; 8.8+/-3.5%, n=6), 5 hydroxydecanoic acid (500 microM; 14.1+/-8.1, n=9), and by scavenging ROS with MPG (2 mM; 0.92+/-5.6%, n=8). ET-1 enhanced proton efflux (J(H)) carried by the Na(+)/H(+) exchanger (NHE) after an acid load, effect that was blocked by MPG. Consistently, the ET-induced positive inotropic effect was also inhibited by the NHE selective blocker HOE642 (5 microM; 9.37+/-6.07%, n=7). The data show that the effect of a concentration of ET-1 that induces an increase in contractility of about 40% is totally mediated by an intracellular pathway triggered by mitochondrial ROS formation and stimulation of the NHE.

Characterization of noradrenaline-induced increases in intracellular Ca2+ levels in Chinese hamster ovary cells stably expressing human alpha1A-adrenoceptor

The mechanism for noradrenaline (NA)-induced increases in intracellular Ca(2+) concentration ([Ca(2+)](i)) and physiological significance of Na(+) influx through receptor-operated channels (ROCs) and store-operated channels (SOCs) were studied in Chinese hamster ovary (CHO) cells stably expressing human alpha(1A)-adrenoceptor (alpha(1A)-AR). [Ca(2+)](i) was measured using the Ca(2+) indicator fura-2. NA (1 microM) elicited transient and subsequent sustained [Ca(2+)](i) increases, which were inhibited by YM-254890 (G(alphaq/11) inhibitor), U-73122 (phospholipase C (PLC) inhibitor), and bisindolylmaleimide I (protein kinase C (PKC) inhibitor), suggesting their dependence on G(alphaq/11)/PLC/PKC. Both phases were suppressed by extracellular Ca(2+) removal, SK&F 96365 (inhibitor of SOC and nonselective cation channel type-2 (NSCC-2)), LOE 908 (inhibitor of NSCC-1 and NSCC-2), and La(3+) (inhibitor of transient receptor potential canonical (TRPC) channel). Reduction of extracellular Na(+) and pretreatment with KB-R7943, a Na(+)/Ca(2+) exchanger (NCX) inhibitor, inhibited both phases of [Ca(2+)](i) increases. These results suggest that 1) stimulation of alpha(1A)-AR with NA elicits the transient and sustained increases in [Ca(2+)](i) mediated through NSCC-2 that belongs to a TRPC family; 2) Na(+) influx through these channels drives NCX in the reverse mode, causing Ca(2+) influx in exchange for Na(+) efflux; and 3) the G(alphaq/11)/PLC/PKC-dependent pathway plays an important role in the increases in [Ca(2+)](i).