Effects of oxygen free radicals on isolated cardiac myocytes from guinea-pig ventricle: electrophysiological studies

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.

Inhibition of cardiac potassium currents by oxidation-activated protein kinase A contributes to early afterdepolarizations in the heart

Reactive oxygen species (ROS) have been shown to prolong cardiac action potential duration resulting in afterdepolarizations, the cellular basis of triggered arrhythmias. As previously shown, protein kinase A type I (PKA I) is readily activated by oxidation of its regulatory subunits. However, the relevance of this mechanism of activation for cardiac pathophysiology is still elusive. In this study, we investigated the effects of oxidation-activated PKA I on cardiac electrophysiology. Ventricular cardiomyocytes were isolated from redox-dead PKA-RI Cys17Ser knock-in (KI) and wild-type (WT) mice and exposed to H₂O₂ (200 µmol/L) or vehicle (Veh) solution. In WT myocytes, exposure to H₂O₂ significantly increased oxidation of the regulatory subunit I (RI) and thus its dimerization (threefold increase in PKA RI dimer). Whole cell current clamp and voltage clamp were used to measure cardiac action potentials (APs), transient outward potassium current (I_to) and inward rectifying potassium current (I_K1), respectively. In WT myocytes, H₂O₂ exposure significantly prolonged AP duration due to significantly decreased I_to and I_K1 resulting in frequent early afterdepolarizations (EADs). Preincubation with the PKA-specific inhibitor Rp-8-Br-cAMPS (10 µmol/L) completely abolished the H₂O₂-dependent decrease in I_to and I_K1 in WT myocytes. Intriguingly, H₂O₂ exposure did not prolong AP duration, nor did it decrease I_to, and only slightly enhanced EAD frequency in KI myocytes. Treatment of WT and KI cardiomyocytes with the late I_Na inhibitor TTX (1 µmol/L) completely abolished EAD formation. Our results suggest that redox-activated PKA may be important for H₂O₂-dependent arrhythmias and could be important for the development of specific antiarrhythmic drugs.NEW & NOTEWORTHY Oxidation-activated PKA type I inhibits transient outward potassium current (I_to) and inward rectifying potassium current (I_K1) and contributes to ROS-induced APD prolongation as well as generation of early afterdepolarizations in murine ventricular cardiomyocytes.

Magnesium, Oxidative Stress, Inflammation, and Cardiovascular Disease

Hypomagnesemia is commonly observed in heart failure, diabetes mellitus, hypertension, and cardiovascular diseases. Low serum magnesium (Mg) is a predictor for cardiovascular and all-cause mortality and treating Mg deficiency may help prevent cardiovascular disease. In this review, we discuss the possible mechanisms by which Mg deficiency plays detrimental roles in cardiovascular diseases and review the results of clinical trials of Mg supplementation for heart failure, arrhythmias and other cardiovascular diseases.

Mitochondrial Dysfunction-Associated Arrhythmogenic Substrates in Diabetes Mellitus

There is increasing evidence that diabetic cardiomyopathy increases the risk of cardiac arrhythmia and sudden cardiac death. While the detailed mechanisms remain incompletely understood, the loss of mitochondrial function, which is often observed in the heart of patients with diabetes, has emerged as a key contributor to the arrhythmogenic substrates. In this mini review, the pathophysiology of mitochondrial dysfunction in diabetes mellitus is explored in detail, followed by descriptions of several mechanisms potentially linking mitochondria to arrhythmogenesis in the context of diabetic cardiomyopathy.

Mitochondrial-Mediated Oxidative Ca2+/Calmodulin-Dependent Kinase II Activation Induces Early Afterdepolarizations in Guinea Pig Cardiomyocytes: An In Silico Study

Background Oxidative stress-mediated Ca2+/calmodulin-dependent protein kinase II (Ca MKII) phosphorylation of cardiac ion channels has emerged as a critical contributor to arrhythmogenesis in cardiac pathology. However, the link between mitochondrial-derived reactive oxygen species (md ROS ) and increased Ca MKII activity in the context of cardiac arrhythmias has not been fully elucidated and is difficult to establish experimentally. Methods and Results We hypothesize that pathological md ROS can cause erratic action potentials through the oxidation-dependent Ca MKII activation pathway. We further propose that Ca MKII -dependent phosphorylation of sarcolemmal slow Na+ channels alone is sufficient to elicit early afterdepolarizations. To test the hypotheses, we expanded our well-established guinea pig cardiomyocyte excitation- contraction coupling, mitochondrial energetics, and ROS - induced- ROS - release model by incorporating oxidative Ca MKII activation and Ca MKII -dependent Na+ channel phosphorylation in silico. Simulations show that md ROS mediated-Ca MKII activation elicits early afterdepolarizations by augmenting the late Na+ currents, which can be suppressed by blocking L-type Ca2+ channels or Na+/Ca2+ exchangers. Interestingly, we found that oxidative Ca MKII activation-induced early afterdepolarizations are sustained even after md ROS has returned to its physiological levels. Moreover, mitochondrial-targeting antioxidant treatment can suppress the early afterdepolarizations, but only if given in an appropriate time window. Incorporating concurrent md ROS -induced ryanodine receptors activation further exacerbates the proarrhythmogenic effect of oxidative Ca MKII activation. Conclusions We conclude that oxidative Ca MKII activation-dependent Na channel phosphorylation is a critical pathway in mitochondria-mediated cardiac arrhythmogenesis.

Mechanisms of sudden cardiac death: oxidants and metabolism

Ventricular arrhythmia is the leading cause of sudden cardiac death (SCD). Deranged cardiac metabolism and abnormal redox state during cardiac diseases foment arrhythmogenic substrates through direct or indirect modulation of cardiac ion channel/transporter function. This review presents current evidence on the mechanisms linking metabolic derangement and excessive oxidative stress to ion channel/transporter dysfunction that predisposes to ventricular arrhythmias and SCD. Because conventional antiarrhythmic agents aiming at ion channels have proven challenging to use, targeting arrhythmogenic metabolic changes and redox imbalance may provide novel therapeutics to treat or prevent life-threatening arrhythmias and SCD.

Mitochondria-derived ROS bursts disturb Ca²⁺ cycling and induce abnormal automaticity in guinea pig cardiomyocytes: a theoretical study

Mitochondria are in close proximity to the redox-sensitive sarcoplasmic reticulum (SR) Ca(2+) release [ryanodine receptors (RyRs)] and uptake [Ca(2+)-ATPase (SERCA)] channels. Thus mitochondria-derived reactive oxygen species (mdROS) could play a crucial role in modulating Ca(2+) cycling in the cardiomyocytes. However, whether mdROS-mediated Ca(2+) dysregulation translates to abnormal electrical activities under pathological conditions, and if yes what are the underlying ionic mechanisms, have not been fully elucidated. We hypothesize that pathological mdROS induce Ca(2+) elevation by modulating SR Ca(2+) handling, which activates other Ca(2+) channels and further exacerbates Ca(2+) dysregulation, leading to abnormal action potential (AP). We also propose that the morphologies of elicited AP abnormality rely on the time of mdROS induction, interaction between mitochondria and SR, and intensity of mitochondrial oxidative stress. To test the hypotheses, we developed a multiscale guinea pig cardiomyocyte model that incorporates excitation-contraction coupling, local Ca(2+) control, mitochondrial energetics, and ROS-induced ROS release. This model, for the first time, includes mitochondria-SR microdomain and modulations of mdROS on RyR and SERCA activities. Simulations show that mdROS bursts increase cytosolic Ca(2+) by stimulating RyRs and inhibiting SERCA, which activates the Na(+)/Ca(2+) exchanger, Ca(2+)-sensitive nonspecific cationic channels, and Ca(2+)-induced Ca(2+) release, eliciting abnormal AP. The morphologies of AP abnormality are largely influenced by the time interval among mdROS burst induction and AP firing, dosage and diffusion of mdROS, and SR-mitochondria distance. This study defines the role of mdROS in Ca(2+) overload-mediated cardiac arrhythmogenesis and underscores the importance of considering mitochondrial targets in designing new antiarrhythmic therapies.

Mitochondria and arrhythmias

Mitochondria are essential to providing ATP, thereby satisfying the energy demand of the incessant electrical activity and contractile action of cardiac muscle. Emerging evidence indicates that mitochondrial dysfunction can adversely affect cardiac electrical functioning by impairing the intracellular ion homeostasis and membrane excitability through reduced ATP production and excessive reactive oxygen species (ROS) generation, resulting in increased propensity to cardiac arrhythmias. In this review, the molecular mechanisms linking mitochondrial dysfunction to cardiac arrhythmias are discussed with an emphasis on the impact of increased mitochondrial ROS on the cardiac ion channels and transporters that are critical to maintaining normal electromechanical functioning of the cardiomyocytes. The potential of using mitochondria-targeted antioxidants as a novel antiarrhythmia therapy is highlighted.

Vitamin E management of oxidative damage-linked dysfunctions of hyperthyroid tissues

INTRODUCTION

Thyroid hormones affect growth, development, and metabolism of vertebrates, and are considered the major regulators of their homeostasis. On the other hand, elevated circulating levels of thyroid hormones are associated with modifications in the whole organism (weight loss and increased metabolism and temperature) and in several body regions. Indeed, tachycardia, atrial arrhythmias, heart failure, muscle weakness and wasting, bone mass loss, and hepatobiliary complications are commonly found in hyperthyroid animals and humans.

RESULTS

Most thyroid hormone actions result from influences on transcription of T3-responsive genes, which are mediated through nuclear receptors. However, there is significant evidence that tissue oxidative stress underlies some dysfunctions produced by hyperthyroidism.

DISCUSSION

During the last decades, increasing interest has been turned to the use of antioxidants as therapeutic agents in various diseases and pathophysiological disorders believed to be mediated by oxidative stress. In particular, because elevated circulating levels of thyroid hormones are associated with tissue oxidative injury, more attention has been paid to explore the application of antioxidants as therapeutic agents in thyroid related disorders.

CONCLUSIONS

At present, vitamin E is among the most commonly consumed dietary supplements due to the belief that it, as an antioxidant, may attenuate morbidity and mortality. This is due to the results of numerous scientific studies, which demonstrate that vitamin E has a primary function to destroy peroxyl radicals, thus protecting polyunsaturated fatty acids biological membranes from oxidative damage. However, results are also available indicating that protective vitamin E effects against oxidative damage can be obtained even through different mechanisms.

Sevoflurane Protects Ventricular Myocytes against Oxidative Stress-induced Cellular Ca2+ Overload and Hypercontracture

Background:

Oxidative stress is implicated in pathogenesis of cardiac reperfusion injury, characterized by cellular Ca2+ overload and hypercontracture. Volatile anesthetics protect the heart against reperfusion injury primarily by attenuating Ca2+ overload. This study investigated electrophysiological mechanisms underlying cardioprotective effects of sevoflurane against oxidative stress-induced cellular injury.

Methods:

The cytosolic Ca2+ levels and cell morphology were assessed in mouse ventricular myocytes, using confocal fluo-3 fluorescence imaging, whereas membrane potentials and L-type Ca2+ current (ICa,L) were recorded using whole-cell patch-clamp techniques. Phosphorylation of Ca2+/calmodulin-dependent protein kinase II was examined by Western blotting.

Results:

Exposure to H2O2 (100 μm) for 15 min evoked cytosolic Ca2+ elevation and hypercontracture in 56.8% of ventricular myocytes in 11 experiments, which was partly but significantly reduced by nifedipine, tetracaine, or SEA0400. Sevoflurane prevented H2O2-induced cellular Ca2+ overload in a concentration-dependent way (IC50 = 1.35%). Isoflurane (2%) and desflurane (10%) also protected ventricular myocytes by a degree similar to sevoflurane (3%). Sevoflurane suppressed H2O2-induced electrophysiological disturbances, including early afterdepolarizations, voltage fluctuations in resting potential, and abnormal automaticities. H2O2 significantly enhanced ICa,L by activating Ca2+/calmodulin-dependent protein kinase II, and subsequent addition of sevoflurane, isoflurane, or desflurane similarly reduced ICa,L to below baseline levels. Phosphorylated Ca2+/calmodulin-dependent protein kinase II increased after 10-min incubation with H2O2, which was significantly prevented by concomitant administration of sevoflurane.

Conclusions:

Sevoflurane protected ventricular myocytes against H2O2-induced Ca2+ overload and hypercontracture, presumably by affecting multiple Ca2+ transport pathways, including ICa,L, Na+/Ca2+ exchanger and ryanodine receptor. These actions appear to mediate cardioprotection against reperfusion injury associated with oxidative stress.

Redox regulation of sodium and calcium handling

SIGNIFICANCE

In heart failure (HF), contractile dysfunction and arrhythmias result from disturbed intracellular Ca handling. Activated stress kinases like cAMP-dependent protein kinase A (PKA), protein kinase C (PKC), and Ca/calmodulin-dependent protein kinase II (CaMKII), which are known to influence many Ca-regulatory proteins, are mechanistically involved.

RECENT ADVANCES

Beside classical activation pathways, it is becoming increasingly evident that reactive oxygen species (ROS) can directly oxidize these kinases, leading to alternative activation. Since HF is associated with increased ROS generation, ROS-activated serine/threonine kinases may play a crucial role in the disturbance of cellular Ca homeostasis. Many of the previously described ROS effects on ion channels and transporters are possibly mediated by these stress kinases. For instance, ROS have been shown to oxidize and activate CaMKII, thereby increasing Na influx through voltage-gated Na channels, which can lead to intracellular Na accumulation and action potential prolongation. Consequently, Ca entry via activated NCX is favored, which together with ROS-induced dysfunction of the sarcoplasmic reticulum can lead to dramatic intracellular Ca accumulation, diminished contractility, and arrhythmias.

CRITICAL ISSUES

While low amounts of ROS may regulate kinase activity, excessive uncontrolled ROS production may lead to direct redox modification of Ca handling proteins. Therefore, depending on the source and amount of ROS generated, ROS could have very different effects on Ca-handling proteins.

FUTURE DIRECTIONS

The discrimination between fine-tuned ROS signaling and unspecific ROS damage may be crucial for the understanding of heart failure development and important for the investigation of targeted treatment strategies.

Redox control of cardiac excitability

Reactive oxygen species (ROS) have been associated with various human diseases, and considerable attention has been paid to investigate their physiological effects. Various ROS are synthesized in the mitochondria and accumulate in the cytoplasm if the cellular antioxidant defense mechanism fails. The critical balance of this ROS synthesis and antioxidant defense systems is termed the redox system of the cell. Various cardiovascular diseases have also been affected by redox to different degrees. ROS have been indicated as both detrimental and protective, via different cellular pathways, for cardiac myocyte functions, electrophysiology, and pharmacology. Mostly, the ROS functions depend on the type and amount of ROS synthesized. While the literature clearly indicates ROS effects on cardiac contractility, their effects on cardiac excitability are relatively under appreciated. Cardiac excitability depends on the functions of various cardiac sarcolemal or mitochondrial ion channels carrying various depolarizing or repolarizing currents that also maintain cellular ionic homeostasis. ROS alter the functions of these ion channels to various degrees to determine excitability by affecting the cellular resting potential and the morphology of the cardiac action potential. Thus, redox balance regulates cardiac excitability, and under pathological regulation, may alter action potential propagation to cause arrhythmia. Understanding how redox affects cellular excitability may lead to potential prophylaxis or treatment for various arrhythmias. This review will focus on the studies of redox and cardiac excitation.

Sevoflurane postconditioning attenuates reperfusion-induced ventricular arrhythmias in isolated rat hearts exposed to ischemia/reperfusion injury

Sevoflurane postconditioning has been proven to protect the hearts against ischemia/reperfusion injury, manifested mainly by improved cardiac function, reduced myocardial specific biomarker release, and decreased infarct size. This study is to observe the effects of sevoflurane postconditioning on reperfusion-induced ventricular arrhythmias and reactive oxygen species generation in Langendorff perfused rat hearts. Compared with the unprotected hearts subjected to 25 min of global ischemia followed by 30 min of reperfusion, exposure of 3% sevoflurane during the first 15 min of reperfusion significantly improved cardiac function, reduced cardiac troponin I release, decreased infarct size and attenuated reperfusion-induced ventricular arrhythmia. Further analysis on arrhythmia during the 30 min of reperfusion showed that, sevoflurane postconditioning decreased both the duration and incidence of ventricular tachycardia and ventricular fibrillation. In the meantime, intracellular malondialdehyde and reactive oxygen species levels were also reduced. These above results demonstrate that sevoflurane postconditioning protects the hearts against ischemia/reperfusion injury and attenuates reperfusion-induced arrhythmia, which may be associated with the regulation of lipid peroxidation and reactive oxygen species generation.

High-precision recording of the action potential in isolated cardiomyocytes using the near-infrared fluorescent dye di-4-ANBDQBS

The use of voltage-sensitive fluorescent dyes (VSD) for noninvasive measurement of the action potential (AP) in isolated cells has been hindered by low-photon yield of the preparation, dye toxicity, and photodynamic damage. Here we used a new red-shifted VSD, di-4-ANBDQBS, and a fast electron-multiplied charge-coupled device camera for optical AP (OAP) recording in guinea pig cardiac myocytes. Loading di-4-ANBDQBS did not alter APs recorded with micropipette. With short laser exposures (just enough to record one OAP every 1-5 min), di-4-ANBDQBS yielded fluorescent signals with very high signal-to-background ratios (change in fluorescence on depolarization/fluorescence at resting potential: 19.2 ± 4.1%) and signal-to-noise ratios (40 ± 13.2). Quantum chemical calculations comparing the ANBDQ chromophore to the conventional ANEP chromophore showed that the higher wavelength and the greater voltage sensitivity of the former have the same electro-optical origin: a longer path for electron redistribution in the excited state. OAP closely tracked simultaneously recorded electrical APs, permitting measurement of AP duration within 1% error. Prolonged laser exposure caused progressive AP duration prolongation and instability. However, these effects were alleviated or abolished by reducing the dye concentration and by perfusion with antioxidants. Thus the presented technique provides a unique opportunity for noninvasive AP recording in single cardiomyocytes.

Exercise-induced protection against myocardial apoptosis and necrosis: MnSOD, calcium-handling proteins, and calpain

Exercise provides protection against myocardial ischemia-reperfusion (IR) injury. Understanding the mechanisms of this protection may lead to new interventions for the prevention and/or treatment of heart disease. Although presently these mechanisms are not well understood, reports suggest that manganese superoxide dismutase (MnSOD) and calpain may be critical mediators of this protection. We hypothesized that an exercise-induced increase in MnSOD would provide cardioprotection by attenuating IR-induced oxidative modification to critical Ca(2+)-handling proteins, thereby decreasing calpain-mediated cleavage of these and other proteins attenuating cardiomyocyte death. After IR, myocardial apoptosis and infarct size were significantly reduced in hearts of exercised animals compared with sedentary controls. In addition, exercise prevented IR-induced calpain activation as well as the oxidative modification and calpain-mediated degradation of myocardial Ca(2+)-handling proteins (L-type Ca(2+) channels, phospholamban, and sarcoplasmic/endoplasmic reticulum calcium ATPase). Further, IR-induced activation of proapoptotic proteins was attenuated in exercised animals. Importantly, prevention of the exercise-induced increase in MnSOD activity via antisense oligonucleotides greatly attenuated the cardioprotection conferred by exercise. These results suggest that MnSOD provides cardioprotection by attenuating IR-induced oxidation and calpain-mediated degradation of myocardial Ca(2+)-handling proteins, thereby preventing myocardial apoptosis and necrosis.

Potential role and mechanisms of subcellular remodeling in cardiac dysfunction due to ischemic heart disease

Several studies have revealed varying degrees of changes in sarcoplasmic reticular and myofibrillar activities, protein content, gene expression and intracellular Ca-handling during cardiac dysfunction due to ischemia-reperfusion (I/R); however, relatively little is known about the sarcolemmal and mitochondrial alterations, as well as their mechanisms in the I/R hearts. Because I/R is associated with oxidative stress and intracellular Ca-overload, it has been indicated that changes in subcellular activities, protein content and gene expression due to I/R are related to both oxidative stress and Ca-overload. Intracellular Ca-overload appears to induce changes in subcellular activities, protein contents and gene expression (subcellular remodeling) by activation of proteases and phospholipases, as well as by affecting the genetic apparatus, whereas oxidative stress is considered to cause oxidation of functional groups of different subcellular proteins in addition to modifying the genetic machinery. Ischemic preconditioning, which is known to depress the development of both intracellular Ca-overload and oxidative stress due to I/R, was observed to attenuate the I/R-induced subcellular remodeling and improve cardiac performance. It is suggested that a combination therapy with antioxidants and interventions, which reduce the development of intracellular Ca-overload, may improve cardiac function by preventing or attenuating the occurrence of subcellular remodeling due to ischemic heart disease. It is proposed that defects in the activities of subcellular organelles may serve as underlying mechanisms for I/R-induced cardiac dysfunction under acute conditions, whereas subcellular remodeling due to alterations in gene expression may explain the impaired cardiac performance under chronic conditions of I/R.

Magnesium deficiency augments myocardial response to reactive oxygen species

Magnesium (Mg) deficiency and oxidative stress are independently implicated in the etiopathogenesis of various cardiovascular disorders. This study was undertaken to examine the hypothesis that Mg deficiency augments the myocardial response to oxidative stress. Electrically stimulated rat papillary muscle was used for recording the contractile variation. Biochemical variables of energy metabolism (adenosine triphosphate (ATP) and creatine phosphate) and markers of tissue injury (lactate dehydrogenase (LDH) release and lipidperoxidation), which can affect myocardial contractility, were assayed in Langendorff-perfused rat hearts. Hydrogen peroxide (100 micromol/L) was used as the source of reactive oxygen species. The negative inotropic response to H2O2 was significantly higher in Mg deficiency (0.48 mmol Mg/L) than in Mg sufficiency (1.2 mmol Mg/L). Low Mg levels did not affect ATP levels or tissue lipid peroxidation. However, H2O2 induced a decrease in ATP; enhanced lipid peroxidation and the release of LDH were augmented by Mg deficiency. Increased lipid peroxidation associated with a decrease in available energy might be responsible for the augmentation of the negative inotropic response to H2O2 in Mg deficiency. The observations from this study validate the hypothesis that myocardial response to oxidative stress is augmented by Mg deficiency. This observation has significance in ischemia-reperfusion injury, where Mg deficiency can have an additive effect on the debilitating consequences.

Differential changes in phospholipase D and phosphatidate phosphohydrolase activities in ischemia–reperfusion of rat heart

Phospholipase D (PLD2) produces phosphatidic acid (PA), which is converted to 1,2 diacylglycerol (DAG) by phosphatidate phosphohydrolase (PAP2). Since PA and DAG regulate Ca(2+) movements, we examined PLD2 and PAP2 in the sarcolemma (SL) and sarcoplasmic reticular (SR) membranes from hearts subjected to ischemia and reperfusion (I-R). Although SL and SR PLD2 activities were unaltered after 30 min ischemia, 5 min reperfusion resulted in a 36% increase in SL PLD2 activity, whereas 30 min reperfusion resulted in a 30% decrease in SL PLD2 activity, as compared to the control value. SR PLD2 activity was decreased (39%) after 5 min reperfusion, but returned to control levels after 30 min reperfusion. Ischemia for 60 min resulted in depressed SL and SR PLD2 activities, characterized with reduced V(max) and increased K(m) values, which were not reversed during reperfusion. Although the SL PAP2 activity was decreased (31%) during ischemia and at 30 min reperfusion (28%), the SR PAP2 activity was unchanged after 30 min ischemia, but was decreased after 5 min reperfusion (25%) and almost completely recovered after 30 min reperfusion. A 60 min period of ischemia followed by reperfusion caused an irreversible depression of SL and SR PAP2 activities. Our results indicate that I-R induced cardiac dysfunction is associated with subcellular changes in PLD2 and PAP2 activities.

Selenium-induced alterations in ionic currents of rat cardiomyocytes

In the present study, rats were treated with sodium selenite (5 micromol/kg body weight/day, ip) for 4 weeks and the parameters of contractile activity, action potential, L-type Ca2+-current (ICaL), as well as transient outward (Ito), inward rectifier (IK1), and steady state (Iss) K+-currents were investigated. Sodium selenite treatment increased rat blood glucose level and lowered plasma insulin level, significantly. This treatment also caused slightly prolongation in action potential with no significant effects on spontaneous contraction parameters and intracellular Ca2+ transients of the heart preparations. These effects were associated with marked alterations in the kinetics of both ICaL and Ito including a significant slowing in both inactivation time constants of ICaL and a significant shift to negative potential at half-inactivation of these channels without any change in the current density. Also, there was a significantly faster inactivation of Ito and no shift in half-inactivation of this channel without any change in its current density. Consequently, there was a approximately 50% increase in total charges carried by Ca2+ current and approximately 50% decrease in total charges carried by K+ currents of the treated rat cardiomyocytes. Additionally we observed a significant inhibition in IK1 density in treated rat cardiomyocytes. Oxidized glutathione level was significantly increased (70%) while the observed decrease in reduced glutathione was much less. Since a shift in redox state of regulatory proteins is related with cell dysfunction, selenium-induced increase in blood glucose and decrease in plasma insulin may correlate these alterations. These alterations, in the kinetics of the channels and in IK1 density, might lead to proarrhythmic effect of chronic selenium supplementation.

NO upregulation of a cyclic nucleotide-gated channel contributes to calcium elevation in endothelial cells

We investigated whether nitric oxide (NO) upregulates a cyclic nucleotide-gated (CNG) channel and whether this contributes to sustained elevation of intracellular calcium levels ([Ca(2+)](i)) in porcine pulmonary artery endothelial cells (PAEC). Exposure of PAEC to an NO donor, NOC-18 (1 mM), for 18 h increased the protein and mRNA levels of CNGA2 40 and 50%, respectively (P < 0.05). [Ca(2+)](i) in NO-treated cells was increased 50%, and this increase was maintained for up to 12 h after removal of NOC-18 from medium. Extracellular calcium is required for the increase in [Ca(2+)](i) in NO-treated cells. Thapsigargin induced a rapid cytosolic calcium rise, whereas both a CNG and a nonselective cation channel blocker caused a faster decline in [Ca(2+)](i), suggesting that capacitive calcium entry contributes to the elevated calcium levels. Antisense inhibition of CNGA2 expression attenuated the NO-induced increases in CNGA2 expression and [Ca(2+)](i) and in capacitive calcium entry. Our results demonstrate that exogenous NO upregulates CNGA2 expression and that this is associated with elevated [Ca(2+)](i) and capacitive calcium entry in porcine PAEC.

Heart failure, oxidative stress, and ion channel modulation

The balance of reactive oxygen species (ROS) and nitric oxide, the cell redox state, appears to be important in the mechanisms of heart failure. This balance has significant impact on calcium-handling proteins, affecting excitation-contraction coupling. Both ROS and nitric oxide appear to be elevated in heart failure and are accompanied by significant impairments in the number and function of calcium-handling proteins. These proteins contain sulfhydryl groups or disulfide linkages involving cysteine residues, making them susceptible to the action of oxidizing-reducing agents and nitrosylation, thereby altering their properties. Initial increases in nitric oxide may be an adaptive response to myocardial dysfunction, elevated cytokines, and increases in ROS, while a further increase in nitric oxide and overwhelming ROS can be damaging. Abundant nitric oxide and ROS can cause formation of peroxynitrite, a strong oxidant, or nitric oxide can activate alternate pathways aiding the ROS, causing impaired calcium handling contributing to contractile dysfunction.

Protective role of nebivolol in hydroxyl radical induced injury

Increased oxidative stress has been postulated as one of the main mechanisms underlying stunned myocardium, and may play an important role in and during development of heart failure. Pharmacological interventions may attenuate or prevent detrimental effects of oxygen free radicals on the myocardium. Nebivolol has been shown to attenuate contractile dysfunction in hydroxyl radical mediated injury, but the mechanism(s) remain unknown. It was investigated whether nebivolol could partly attenuate the contractile dysfunction through a direct effect on the myofilaments. In demembranized muscles from explanted human hearts, nebivolol induced a slight desensitization of the myofilaments to calcium. Therefore, during the calcium overload that occurs during reperfusion after an ischemic event, the contractile dysfunction is less severe in the presence of nebivolol. We conclude that the protection of nebivolol in hydroxyl radical induced contractile dysfunction is mediated in part through a direct effect on the myofilaments, in addition to the previously shown preservation of sarcoplasmic reticulum function.

Protamine induces elevation of cytosolic free Ca2+ in cultured porcine aortic endothelial cells

To test the hypothesis that protamine influences calcium movement in endothelial cells, we measured the concentration of intracellular free calcium ([Ca2+]i) in cultured porcine aortic endothelial (PAE) cells in Krebs solution (2.5mM Ca2+, pH 7.4) at 37 degrees C, by fura-2 fluorimetry. The basal [Ca2+]i of PAE cells was 113+/-18 nM (n=6). Protamine increased [Ca2+]i in a concentration-dependent manner (EC50, the concentration having 50% of the maximum effect, 1.4+/-0.3 microg mL(-1), n=6). The response of PAE cells to 100 microg mL(-1) protamine (330+/-80 nM, n=6) was blocked by a Ca2+ chelator, 5 mM glycoletherdiaminetetraacetic acid (EGTA; 131+/-16 nM, n=6), and by a non-selective Ca2+ channel blocker, 3 mM Co2+ (134+/-14 nM, n=6). These results suggest that Ca2+ influx through cell-membrane Ca2+ channels is mainly responsible for the protamine-induced Ca2+ elevation.

Cytosolic Ca2+ movements of endothelial cells exposed to reactive oxygen intermediates: role of hydroxyl radical-mediated redox alteration of cell-membrane Ca2+ channels

1. The mode of action of reactive oxygen intermediates in cysosolic Ca2+ movements of cultured porcine aortic endothelial cells exposed to xanthine/xanthine oxidase (X/XO) was investigated. 2. Cytosolic Ca2+ movements provoked by X/XO consisted of an initial Ca2+ release from thapsigargin-sensitive intracellular Ca2+ stores and a sustained Ca2+ influx through cell-membrane Ca2+ channels. The Ca2+ movements from both sources were inhibited by catalase, cell-membrane permeable iron chelators (o-phenanthroline and deferoxamine), a *OH scavenger (5,5-dimethyl-1-pyrroline-N-oxide), or an anion channel blocker (disodium 4, 4'-diisothiocyano-2, 2'-stilbenedisulphonic acid), suggesting that *O2- influx through anion channels was responsible for the Ca2+ movements, in which *OH generation catalyzed by intracellular transition metals (i.e., Haber-Weiss cycle) was involved. 3. After an initial Ca2+ elevation provoked by X/XO, cytosolic Ca2+ concentration decreased to a level higher than basal levels. Removal of X/XO slightly enhanced the Ca2+ decrease. Extracellular addition of sulphydryl (SH)-reducing agents, dithiothreitol or glutathione, after the removal of X/XO accelerated the decrement. A Ca2+ channel blocker, Ni2+, abolished the sustained increase in Ca2+, suggesting that Ca2+ influx through cell-membrane Ca2+ channels was extracellularly regulated by the redox state of SH-groups. 4. The X/XO-provoked change in cellular respiration was inhibited by Ni2+ or dithiothreitol as well as inhibitors of Haber-Weiss cycle, suggesting that Ca2+ influx was responsible for *OH-mediated cytotoxicity. We concluded that intracellular *OH generation was involved in the Ca2+ movements in endothelial cells exposed to X/XO. Cytosolic Ca2+ elevation was partly responsible for the oxidants-mediated cytotoxicity.

Transient and sustained impacts of hydroxyl radicals on sarcoplasmic reticulum function: protective effects of nebivolol

The hydroxyl radical (*OH) is a very reactive oxygen-free radical species that has profound effects on myocardial contractility. We investigated the impact of *OH on free radical induced injury in right ventricular rabbit cardiac trabeculae. Additionally, we investigated the protective properties of the beta-adrenoceptor antagonist nebivolol. The contractile response to a brief, 2 min exposure to *OH consisted of a severe but transient rigor-like contracture, followed by a new steady state in which diastolic force (Fdia) remained increased and developed force (Fdev) remained decreased. In the new steady state sarcoplasmic reticulum function only partly recovered, reflected by a > 50% blunted force-frequency relationship. In the presence of nebivolol (10(-6) M), during the early phase the increase in Fdia was significantly smaller, and recovered better while Fdev was higher during peak. Moreover, nebivolol completely abolished blunting of the force-frequency relationship, which was observed in the sustained *OH injury phase. The results indicate that hydroxyl radical injury induces systolic and diastolic dysfunction, and that nebivolol can effectively prevent a large part of this *OH injury.

Antioxidant-sensitive shortening of ventricular action potential in hyperthyroid rats is independent of lipid peroxidation

The effects of substances able to reduce peroxidative processes on thyroid hormone-induced electrophysiological changes in ventricular muscle fibres were examined. For this study, 60 day old euthyroid and hyperthyroid rats were used. One group of hyperthyroid rats was untreated and the others were treated with vitamin E, N-acetylcysteine, and cholesterol, respectively. Hyperthyroidism was elicited by 10 day treatment with daily i.p. injections of triiodothyronine (10 microg/100 g body weight). Vitamin E and N-acetylcysteine were administered for 10 days by daily i.m. injections (20 mg/100 g body weight) and daily i.p. injections (100 mg/100 g body weight), respectively. Cholesterol was administered by cholesterol-supplemented diet (4%) from day 30. Hyperthyroidism induced a decrease in the whole antioxidant capacity and an increase in both lipid peroxidation and susceptibility to oxidative stress. Vitamin E and N-acetylcysteine administration to hyperthyroid rats led to reduction in lipid peroxidation and susceptibility to oxidative stress and to increase in antioxidant level, while the diet addition of cholesterol decreased lipid peroxidation but did not modify the other parameters. The hyperthyroid state was also associated with a decrease in the duration of the ventricular action potential recorded in vitro. The vitamin E and N-acetylcysteine administration attenuated the thyroid hormone-induced changes in action potential duration, which was however, significantly different from that of the euthyroid rats. In contrast, cholesterol supplementation did not modify the electrical activity of hyperthyroid heart. These results demonstrate that the triiodothyronine effects on ventricular electrophysiological properties are mediated, at least in part, through a membrane modification involving a free radical mechanism. Moreover, they indicate that the antioxidant-sensitive shortening of action potential duration induced by thyroid hormone is likely independent of enhanced peroxidative processes in sarcolemmal membrane.

Interaction of reactive oxygen species with ion transport mechanisms

The use of electrophysiological and molecular biology techniques has shed light on reactive oxygen species (ROS)-induced impairment of surface and internal membranes that control cellular signaling. These deleterious effects of ROS are due to their interaction with various ion transport proteins underlying the transmembrane signal transduction, namely, 1) ion channels, such as Ca2+ channels (including voltage-sensitive L-type Ca2+ currents, dihydropyridine receptor voltage sensors, ryanodine receptor Ca2+-release channels, and D-myo-inositol 1,4,5-trisphosphate receptor Ca2+-release channels), K+ channels (such as Ca2+-activated K+ channels, inward and outward K+ currents, and ATP-sensitive K+ channels), Na+ channels, and Cl- channels; 2) ion pumps, such as sarcoplasmic reticulum and sarcolemmal Ca2+ pumps, Na+-K+-ATPase (Na+ pump), and H+-ATPase (H+ pump); 3) ion exchangers such as the Na+/Ca2+ exchanger and Na+/H+ exchanger; and 4) ion cotransporters such as K+-Cl-, Na+-K+-Cl-, and Pi-Na+ cotransporters. The mechanism of ROS-induced modifications in ion transport pathways involves 1) oxidation of sulfhydryl groups located on the ion transport proteins, 2) peroxidation of membrane phospholipids, and 3) inhibition of membrane-bound regulatory enzymes and modification of the oxidative phosphorylation and ATP levels. Alterations in the ion transport mechanisms lead to changes in a second messenger system, primarily Ca2+ homeostasis, which further augment the abnormal electrical activity and distortion of signal transduction, causing cell dysfunction, which underlies pathological conditions.

Selective effects of oxygen free radicals on excitation-contraction coupling in ventricular muscle. Implications for the mechanism of stunned myocardium

BACKGROUND

Oxygen free radicals (OFRs) have been implicated in the pathogenesis of myocardial stunning, but the precise mechanism by which OFRs foster stunning remains unclear. We investigated the pathophysiology of the contractile dysfunction that occurs after direct exposure of OFRs to cardiac muscle and compared the results with the pathophysiology of stunned myocardium.

METHODS AND RESULTS

Trabeculae from the right ventricles of rat hearts were loaded iontophoretically with fura-2 to determine [Ca2+]i. Steady-state force-[Ca2+]i relations were obtained by rapid electrical stimulation in the presence of ryanodine. Two exogenous OFR-generating systems were used: H2O2 + Fe(3+)-nitrilotriacetic acid (H2O2 + Fe3+) to produce hydroxyl radical, and xanthine oxidase+purine (XO + P) to produce superoxide. In muscles exposed to H2O2 + Fe3+ for 10 minutes, both twitch force and Ca2+ transients were decreased (eg, in 1.5 mmol/L external [Ca2+], force decreased from 41 +/- 7 to 23 +/- 4 mN/mm2, P < .05, and Ca2+ transient amplitude from 0.96 +/- 0.09 to 0.70 +/- 0.05 mumol/L, P < .05). Maximal Ca(2+)-activated force (Fmax) decreased slightly, from 103 +/- 5 to 80 +/- 12 mN/mm2 (P = NS). Neither the [Ca2+]i required to achieve 50% of Fmax (Ca50) nor the Hill coefficient was changed. In muscles exposed to XO + P for 20 minutes, twitch force was reduced (in 1.5 mmol/L external [Ca2+]) from 50 +/- 9 to 39 +/- 8 mN/mm2 (P < .05). Ca2+ transients, on the other hand, were not affected. Fmax decreased insignificantly from 100 +/- 16 to 81 +/- 14 mN/mm2. Ca50 increased from 0.71 +/- 0.06 to 1.07 +/- 0.07 mumol/L (P < .05), with no change in the Hill coefficient.

CONCLUSIONS

These results indicate that exposure to the H2O2 + Fe3+ free radical-generating system reduces activator Ca2+ availability, whereas XO + P decreases the Ca2+ sensitivity of the myofilaments. Exogenously generated OFRs, particularly those produced by XO + P, mimic the effects of myocardial stunning on cardiac excitation-contraction coupling.

The effect of oxygen free radicals on calcium current and dihydropyridine binding sites in guinea-pig ventricular myocytes

1. We used electrophysiological and binding techniques to determine the effects of oxygen free radicals (OFRs) generated by dihydroxyfumaric acid (DHF, 5 mM) on calcium current and dihydropyridine binding sites in guinea-pig isolated ventricular myocytes. 2. Binding of [3H]-PN200-110 to isolated ventricular myocytes revealed one population of binding sites with a KD of 0.11 +/- 0.01 nM and Bmax of 139.1 +/- 6.9 fmol mg-1 protein (n = 24). After 15 min of exposure to DHF, the density, but not the affinity of [3H]-PN200-110 binding sites was significantly (P < 0.01) reduced to 35% of the control value (Bmax = 49.4 +/- 3.7 fmol mg-1 protein, KD = 0.11 +/- 0.01 nM, n = 15). In the presence of superoxide dismutase (SOD) and catalase (CAT) the reduction in [3H]-PN200-110 binding sites was almost completely prevented (Bmax = 120.5 +/- 7.4 in control, n = 4 and 98.8 +/- 7.4 fmol mg-1 protein in DHF plus SOD and CAT, n = 4). KD values were not modified (0.08 +/- 0.01 in control and 0.09 +/- 0.01 nM in DHF plus SOD and CAT). 3. The time-course of the reduction of [3H]-PN200-110 binding sites by OFRs was paralleled by the decrease in L-type calcium current (Ica,L) measured in patch-clamped guinea-pig ventricular myocytes either in the absence or in the presence of EGTA in the patch pipette. In the former conditions OFRs induced the appearance of calcium-dependent alterations, i.e. the transient inward current, within 10 min. After 30 min of incubation with DHF, [3H]-PN200-110 binding sites were reduced to 25% of the control value. 4. In myocytes incubated with the antilipoperoxidant agent, butylated hydroxytoluene (BHT, 50 microM), the decrease in [3H]-PN200-110 binding sites caused by DHF was partially prevented (Bmax values after 30 min exposure to DHF were 55.5 +/- 1.9 and 23.7 +/- 5.9 fmol mg-1 protein in the presence and in the absence of BHT respectively, P < 0.05). BHT did not affect the decrease in [3H]-PN200-110 binding sites during the first 15 min of exposure to DHF, but was able to prevent completely the further decrease occurring during the following 15 min of incubation with OFRs. 5. Our results demonstrate that the OFR-induced decrease in calcium current is associated with a reduction in DHP binding sites. The decrease in calcium current and in calcium channels may be implicated in the mechanical dysfunction associated with oxidative stress.

Nuclear magnetic resonance studies of cationic and energetic alterations with oxidant stress in the perfused heart. Modulation with pyruvate and lactate

The postischemic generation of oxygen-derived free radicals may contribute to myocardial reperfusion injury by affecting sarcolemmal ion transport. Recent evidence indicates that exposure to reactive oxygen intermediates induces rapid increases in myocardial cytosolic free Ca2+ (Ca2+i). The mechanism is undetermined but may involve disturbances in Na+ homeostasis. We tested this hypothesis by interleaving 23Na and 31P nuclear magnetic resonance (NMR) measurements of Na+i and high-energy phosphates in glucose-perfused rat hearts exposed to hydroxyl radicals generated from H2O2 and Fe3+. In separate experiments, K+i and Ca2+i were measured with 39K and 19F NMR, respectively. The hearts rapidly exhibited contracture. Threefold Na+i increases and substantial K+i depletion were observed. Glycolytic inhibition was indicated by rapid sugar phosphate accumulation and cellular energy depletion. Notably, however, severe functional and energetic deterioration and substantial elevation of Ca2+i occurred before substantial Na+i accumulation or K+i depletion was observed. Further experiments investigated the ability of pyruvate to scavenge H2O2 and to protect the myocardium from oxidant stress. Pyruvate (1 or 2.5 mmol/L) dramatically attenuated functional and energetic alterations and alterations in Na+i and K+i, whereas acetate (2.5 mmol/L) offered no protection. Unlike pyruvate, lactate (5 mmol/L) has little or no capacity to scavenge H2O2 but has similar protective effects. In conclusion, pyruvate effectively protects against H2O2/Fe3+, largely by direct H2O2 scavenging. Protection with lactate may involve intracellular pyruvate augmentation. Without exogenous pyruvate or lactate, myocardial Na+ homeostasis can be substantially altered by oxidant stress, possibly via cellular energy depletion. Excess Na+i accumulation may, in turn, hasten metabolic and functional deterioration, but a causal link with the initial alterations in function or Ca2+i was not supported.

Oxygen-derived free radical stress activates nonselective cation current in guinea pig ventricular myocytes. Role of sulfhydryl groups

Oxygen-derived free radicals (O-Rs) cause alterations in cardiac electrical activity, including sustained depolarization, which may contribute to arrhythmic activity in reperfusion after ischemia. The ionic current(s) and cellular mechanism(s) underlying the sustained depolarization are not well defined. We used the whole-cell variant of the patch-clamp technique to study sustained depolarization in guinea pig ventricular myocytes during the extracellular application of O-Rs (generating system: dihydroxyfumaric acid, 3 to 6 mmol/L; FeCl3/ADP, 0.05:0.5 mmol/L). Myocytes superfused with O-Rs (pipette EGTA, 0.1 mmol/L) showed (1) sustained depolarization to between -40 and -10 mV, (2) oscillations in membrane potential, and (3) triggered activity. The depolarization resulted from an increase in quasi-steady state difference current reversing at approximately -18 mV, and the oscillations were due to transient inward current. The latter were inhibited with ryanodine (10 mumol/L) or high pipette EGTA (5 mmol/L), but the steady state current was unaffected. Nonselective cation current (INSC) (recorded with Cs+, Li+, and Mg2+ replacing K+, Na+, and Ca2+, respectively; 20 mmol/L tetraethylammonium chloride [TEA] and 5 mmol/L BAPTA in the pipette solution and 10 mmol/L TEA, 10 mumol/L tetrodotoxin, and 10 mumol/L nicardipine in the bath solution) was activated by O-Rs; the increase in current was unaffected by preventing changes in [Ca2+]i but was inhibited with dithiothreitol. Oxidizing agents (diamide and thimerosal) or caffeine (pipette EGTA, 0.1 mmol/L) produced a similar increase in membrane conductance. INSC activated with O-Rs, oxidizing agents, or caffeine was sensitive to SK&F 96365. O-R treatment was without effect when INSC was already activated with caffeine. The data suggest that (1) extracellular O-Rs activate a Ca(2+)-sensitive INSC in the absence of changes in [Ca2+]i, (2) oxidative modification of extracellular sulfhydryl groups may be involved, and (3) this mechanism is different from the Ca(2+)-dependent activation of INSC by intracellular O-Rs, indicating that O-Rs may alter ion channel activity by differential mechanisms, depending on the compartment, extracellular or intracellular, in which they are present.

Biochemical mechanism of irreversible cell injury caused by free radical-initiated reactions

Effects of oxidative stress on isolated rat ventricular myocytes were studied. Myocyte viability was determined by the ability of these cells to retain rod-shaped morphology and to exclude trypan blue. The mean life time of myocytes was quantitated using the Weibull distribution function. Superfusion with 200 microM tert-butyl hydroperoxide (t-BHP) led to a time-dependent loss of cell viability, generation of the products of lipid peroxidation, oxidation of protein and non-protein thiols, a decrease in [ATP]i and in the cellular energy charge. Dithiothreitol (DTT, 5 mM) prolonged survival of myocytes exposed to t-BHP, attenuated oxidation of protein and non-protein thiols, and preserved the energy charge. Exposure to DTT did not affect the concentration of t-BHP-generated lipid peroxidation products. Promethazine (1 microM) prevented t-BHP-induced increase in the concentration of lipid peroxidation products, but did not prevent either loss of thiols or loss of cell viability. Superfusion with N-ethylmaleimide (NEM, 5 microM) also led to loss of cell viability, with accompanying decreases in protein and non-protein thiols, ATP and energy charge without the accumulation of the products of lipid peroxidation. Superfusion with FeSO4 (400 microM) and ascorbate (1 mM), (Fe-Asc) did not result in loss of cell viability or a decrease protein thiols or the energy charge. Superfusion with Fe-Asc, did, however, lead to a slight decrease in the concentration of non-protein thiols and ATP and a large increase in the concentration of lipid peroxidation products. Accumulation of lipid peroxidation products induced by Fe-Asc was prevented by promethazine.(ABSTRACT TRUNCATED AT 250 WORDS)

Cerebral vasospasm and free radicals

The literature implicating free radical reactions in the genesis of cerebral vasospasm following aneurysmal subarachnoid hemorrhage is reviewed. While this condition has features of a prototypical free radical-mediated disease and a plausible theory can be outlined, data to support the theory are limited. An association of lipid peroxidation with vasospasm has been observed, but more sophisticated techniques for detection of free radicals and for detection of free radical damage to arterial wall proteins and nucleic acids have not been used. There are conflicting reports about efficacy of various antioxidant treatments for vasospasm. In these studies, concomitant experiments have usually not confirmed that the treatments have decreased free radicals or lipid peroxides in cerebrospinal fluid. Because smooth muscle contraction is involved in vasospasm, it would be interesting to investigate the actions of free radicals on smooth muscle cells using, for example, isometric tension recordings and patch clamp techniques. Studies of cardiac myocytes indicate that free radicals alter conductances through potassium and calcium channels and through the sodium-calcium exchanger and may result in elevations in intracellular calcium. Few studies have been performed on cerebral smooth muscle cells. In one study, exposure of cerebrovascular smooth muscle cells to free radicals resulted in increased outward currents, decreased membrane resistance, cell contraction, appearance of membrane blebs, and cell death. In summary, more investigations using better experimental techniques are required before free radicals and reactions induced by them can be said with certainty to be the primary cause of vasospasm.

Alterations in electrical activity and membrane currents induced by intracellular oxygen-derived free radical stress in guinea pig ventricular myocytes

Oxygen-derived free radicals (O-Rs) are thought to induce alterations in cardiac electrical activity; however, the underlying membrane ionic currents affected by O-Rs and the mechanisms by which O-Rs induce their effects on ion channels in the heart are not well defined. In this study, we investigated the time-dependent changes in resting membrane potential and action potential configuration and changes in steady-state membrane currents in guinea pig ventricular myocytes after intracellular application of an O-R-generating system. O-Rs were generated from the combination of dihydroxyfumaric acid (3 mM) and FeCl3:ADP (0.05:0.5 mM) added to the pipette solution that was used to record membrane potential and currents via the whole-cell variant of the patch-clamp technique. Intracellular exposure of myocytes to the O-R-generating solution induced three stages of changes: 1) an early depolarization (5-10 mV) and an increase in action potential duration accompanied by a decrease in resting inward rectifying K+ current conductance, 2) delayed afterdepolarizations and triggered activity caused by the activation of transient inward current mediated by Na(+)-Ca2+ exchange, with failure to repolarize and sustained depolarization between -35 and -20 mV, reflecting the stimulation of nonselective cation current, and 3) a late stage of marked decline in action potential duration, hyperpolarization, and loss of excitability accompanied by activation of the outward current through ATP-sensitive K+ channels. These alterations in electrical activity and membrane currents could be prevented by pretreatment with N-(2-mercaptopropionyl)glycine (500 microM), a scavenger of hydroxyl free radicals. The alterations associated with stages 1 and 2 but not stage 3 were completely abolished on intracellular Ca2+ chelation (5 mM EGTA in the pipette solution) or disruption of sarcoplasmic reticulum Ca2+ handling with ryanodine (10 microM). This study shows that intracellular O-R stress causes specific alterations in membrane ionic currents, leading to changes in resting membrane potential and action potential configuration. Moreover, the data indicate that an elevation in intracellular Ca2+ due to abnormal Ca2+ handling by the sarcoplasmic reticulum is a cause of some of the alterations in membrane currents during O-R stress.