Effects of active oxygen generated by DTT/Fe2+ on cardiac Na+/Ca2+ exchange and membrane permeability to Ca2+

Effect of Oxygen Free Radicals on Cardiac Contractile Activity and Sarcolemmal Na+–Ca2+Exchange

Background

Although oxygen free radicals have been shown to induce myocardial cell damage and cardiac dysfunction, the exact mechanism by which these radicals affect the heart function is not clear. Since the occurrence of intracellular Ca2+overload is critical in the genesis of cellular damage and cardiac dysfunction, and since the sarcolemmal Na+–Ca2+exchange is intimately involved in Ca2+movements in myocardium, this study was undertaken to examine the effects of oxygen free radicals on the relationship between changes in cardiac contractile force development and sarcolemmal Na+–Ca2+exchange activity.

Methods and Results

Isolated rat hearts were perfused with a medium containing xanthine plus xanthine oxidase for different times, and changes in contractile force as well as sarcolemmal Na+–Ca2+exchange activity were monitored. Perfusion of the heart with xanthine plus xanthine oxidase resulted in a transient increase followed by a marked decrease in contractile activity; the resting tension was markedly increased. The xanthine plus xanthine oxidase-induced depression in developed tension, rate of contraction, and rate of relaxation, except the transient increase in contractile activity, was prevented by the addition of catalase, but not by superoxide dismutase, in the perfusion medium. A time-dependent depression in sarcolemmal Na+–Ca2+was also evident upon perfusing the heart with xanthine plus xanthine oxidase. This depression in Na+-dependent Ca2+uptake was associated with a decrease in the maximal velocity of reaction without any changes in the affinity of Na+–Ca2+exchanger for Ca2+. The presence of catalase, unlike superoxide dismutase, prevented the decrease in sarcolemmal Na+–Ca2+exchange activity in hearts perfused with xanthine plus xanthine oxidase.

Conclusion

The results support the view that a depression in the sarcolemmal Na+–Ca2+exchange activity may contribute to the occurrence of intracellular Ca2+overload and subsequent decrease in contractile activity. Furthermore, these actions of xanthine plus xanthine oxidase in the whole heart appear to be a consequence of H2O2production rather than the ‘ generation of superoxide radicals.

In dialyzed squid axons oxidative stress inhibits the Na+/Ca2+ exchanger by impairing the Cai2+-regulatory site

The Na(+)/Ca(2+) exchanger, a major mechanism by which cells extrude calcium, is involved in several physiological and physiopathological interactions. In this work we have used the dialyzed squid giant axon to study the effects of two oxidants, SIN-1-buffered peroxynitrite and hydrogen peroxide (H(2)O(2)), on the Na(+)/Ca(2+) exchanger in the absence and presence of MgATP upregulation. The results show that oxidative stress induced by peroxynitrite and hydrogen peroxide inhibits the Na(+)/Ca(2+) exchanger by impairing the intracellular Ca(2+) (Ca(i)(2+))-regulatory sites, leaving unharmed the intracellular Na(+)- and Ca(2+)-transporting sites. This effect is efficiently counteracted by the presence of MgATP and by intracellular alkalinization, conditions that also protect H(i)(+) and (H(i)(+) + Na(i)(+)) inhibition of Ca(i)(2+)-regulatory sites. In addition, 1 mM intracellular EGTA reduces oxidant inhibition. However, once the effects of oxidants are installed they cannot be reversed by either MgATP or EGTA. These results have significant implications regarding the role of the Na(+)/Ca(2+) exchanger in response to pathological conditions leading to tissue ischemia-reperfusion and anoxia/reoxygenation; they concur with a marked reduction in ATP concentration, an increase in oxidant production, and a rise in intracellular Ca(2+) concentration that seems to be the main factor responsible for cell damage.

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.

Optimal metabolic regulation of the mammalian heart Na(+)/Ca(2+) exchanger requires a spacial arrangements with a PtdIns(4)-5kinase

In inside-out bovine heart sarcolemmal vesicles, p-chloromercuribenzenesulfonate (PCMBS) and n-ethylmaleimide (NEM) fully inhibited MgATP up-regulation of the Na(+)/Ca(2+) exchanger (NCX1) and abolished the MgATP-dependent PtdIns-4,5P2 increase in the NCX1-PtdIns-4,5P2 complex; in addition, these compounds markedly reduced the activity of the PtdIns(4)-5kinase. After PCMBS or NEM treatment, addition of dithiothreitol (DTT) restored a large fraction of the MgATP stimulation of the exchange fluxes and almost fully restored PtdIns(4)-5kinase activity; however, in contrast to PCMBS, the effects of NEM did not seem related to the alkylation of protein SH groups. By itself DTT had no effect on the synthesis of PtdIns-4,5P2 but affected MgATP stimulation of NCX1: moderate inhibition at 1mM MgATP and 1μM Ca(2+) and full inhibition at 0.25mM MgATP and 0.2μM Ca(2+). In addition, DDT prevented coimmunoprecipitation of NCX1 and PtdIns(4)-5kinase. These results indicate that, for a proper MgATP up-regulation of NCX1, the enzyme responsible for PtdIns-4,5P2 synthesis must be (i) functionally competent and (ii) set in the NCX1 microenvironment closely associated to the exchanger. This kind of supramolecular structure is needed to optimize binding of the newly synthesized PtdIns-4,5P2 to its target region in the exchanger protein.

Guattegaumerine protects primary cultured cortical neurons against oxidative stress injury induced by hydrogen peroxide concomitant with serum deprivation

Guattegaumerine is a natural product with characteristics of being lipophilic and reaching high concentration in the brain, but its function in the central nervous system has not yet been observed. This study was designed to evaluate the neuroprotective effects of guattegaumerine on rat primary cultured cortical neurons. Following a 24-h exposure of the cells to combined serum-starvation and hydrogen peroxide, a significant augment in neuron damage as determined by 3-(4,5-dimethylthiazol-2-yl)-2, 5-diphenyl-tetrazolium bromide (MTT) assay and lactate dehydrogenase (LDH) release were observed. Preincubation of guattegaumerine dramatically improved the cell viability and inhibited LDH release. Preincubation of guattegaumerine also dramatically inhibited malondialhehyde (MDA) production and elevated the decreased total antioxidative capacity in cells caused by the combined injury. Results of flow cytometry and immunohistochemistry showed that pre-addition of guattegaumerine interrupted the apoptosis of the neurons, reversed the up regulation of the pro-apoptotic gene (Bax) and the down regulation of the anti-apoptotic gene (Bcl-2). Furthermore, guattegaumerine suppressed the increase of intracellular calcium ([Ca(2+)](i)) stimulated by either H(2)O(2) or KCl in Ca(2+)-containing extracellular solutions, and high concentration of 2.5 microM guattegaumerine also suppressed the increase of [Ca(2+)](i) induced by H(2)O(2) in Ca(2+)-free solution. These observations suggested that guattegaumerine may possess potential protection against oxidative stress injury, which might be beneficial for neurons.

DDPH ameliorated oxygen and glucose deprivation-induced injury in rat hippocampal neurons via interrupting Ca2+ overload and glutamate release

Our previous work has demonstrated that DDPH (1-(2, 6-dimethylphenoxy)-2-(3, 4-dimethoxyphenylethylamino) propane hydrochloride), a competitive alpha(1)-adrenoceptor antagonist, could improve cognitive deficits, reduce histopathological damage and facilitate synaptic plasticity in vivo possibly via increasing NR2B (NMDA receptor 2B) expression and antioxidation of DDPH itself. The present study further evaluated effects of DDPH on OGD (Oxygen and glucose deprivation)-induced neuronal damage in rat primary hippocampal cells. The addition of DDPH to the cultured cells 12 h before OGD for 4 h significantly reduced neuronal damage as determined by MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay and LDH (lactate dehydrogenase) release experiments. The effects of DDPH on intracellular calcium concentration were explored by Fura-2 based calcium imaging techniques and results showed that DDPH at the dosages of 5 microM and 10 microM suppressed the increase of intracellular calcium ([Ca(2+)](i)) stimulated by 50 mM KCl in Ca(2+)-containing extracellular solutions. However, DDPH couldn't suppress the increase of [Ca(2+)](i) induced by both 50 microM glutamate in Ca(2+)-containing extracellular solutions and 20 microM ATP (Adenosine Triphosphate) in Ca(2+)-free solution. These results indicated that DDPH prevented [Ca(2+)](i) overload in hippocampal neurons by blocking Ca(2+) influx (voltage-dependent calcium channel) but not Ca(2+) mobilization from the intracellular Ca(2+) store in endoplasm reticulum (ER). We also demonstrated that DDPH could decrease glutamate release when hippocampal cells were subjected to OGD. These observations demonstrated that DDPH protected hippocampal neurons against OGD-induced damage by preventing the Ca(2+) influx and decreasing glutamate release.

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.

Calcium signaling phenomena in heart diseases: a perspective

Ca(2+) is a major intracellular messenger and nature has evolved multiple mechanisms to regulate free intracellular (Ca(2+))(i) level in situ. The Ca(2+) signal inducing contraction in cardiac muscle originates from two sources. Ca(2+) enters the cell through voltage dependent Ca(2+) channels. This Ca(2+) binds to and activates Ca(2+) release channels (ryanodine receptors) of the sarcoplasmic reticulum (SR) through a Ca(2+) induced Ca(2+) release (CICR) process. Entry of Ca(2+) with each contraction requires an equal amount of Ca(2+) extrusion within a single heartbeat to maintain Ca(2+) homeostasis and to ensure relaxation. Cardiac Ca(2+) extrusion mechanisms are mainly contributed by Na(+)/Ca(2+) exchanger and ATP dependent Ca(2+) pump (Ca(2+)-ATPase). These transport systems are important determinants of (Ca(2+))(i) level and cardiac contractility. Altered intracellular Ca(2+) handling importantly contributes to impaired contractility in heart failure. Chronic hyperactivity of the beta-adrenergic signaling pathway results in PKA-hyperphosphorylation of the cardiac RyR/intracellular Ca(2+) release channels. Numerous signaling molecules have been implicated in the development of hypertrophy and failure, including the beta-adrenergic receptor, protein kinase C, Gq, and the down stream effectors such as mitogen activated protein kinases pathways, and the Ca(2+) regulated phosphatase calcineurin. A number of signaling pathways have now been identified that may be key regulators of changes in myocardial structure and function in response to mutations in structural components of the cardiomyocytes. Myocardial structure and signal transduction are now merging into a common field of research that will lead to a more complete understanding of the molecular mechanisms that underlie heart diseases. Recent progress in molecular cardiology makes it possible to envision a new therapeutic approach to heart failure (HF), targeting key molecules involved in intracellular Ca(2+) handling such as RyR, SERCA2a, and PLN. Controlling these molecular functions by different agents have been found to be beneficial in some experimental conditions.

Cross-talk between calcium and reactive oxygen species signaling

Calcium [Ca2+] and reactive oxygen species (ROS) constitute the most important intracellular signaling molecules participating in the regulation and integration of diverse cellular functions. Here we briefly review cross-talk between the two prominent signaling systems that finely tune the homeostasis and integrate functionality of Ca2+ and ROS in different types of cells. Ca2+ modulates ROS homeostasis by regulating ROS generation and annihilation mechanisms in both the mitochondria and the cytosol. Reciprocal redox regulation of Ca2+ homeostasis occurs in different physiological and pathological processes, by modulating components of the Ca2+ signaling toolkit and altering characteristics of local and global Ca2+ signals. Functionally, interactions between Ca2+ and ROS signaling systems can be both stimulatory and inhibitory, depending on the type of target proteins, the ROS species, the dose, duration of exposure, and the cell contexts. Such extensive and complex cross-talk might enhance signaling coordination and integration, whereas abnormalities in either system might propagate into the other system and undermine the stability of both systems.

Effects of diabetes and hypertension on myocardial Na+-Ca2+ exchange

Abnormalities in cardiac function have been extensively documented in experimental and clinical diabetes. These aberrations are well known to be exaggerated when hypertension and diabetes co-exist. The objective of the present study was to examine whether alterations in the activity of the myocardial Na+-Ca2+ exchanger (NCX) can account for the deleterious effects of diabetes and (or) hypertension on the heart. To this aim, the following experimental groups were studied: (i) control; (ii) diabetic; (iii) hypertensive; and (iv) hypertensive-diabetic. Wistar rats served as the control group (C) while Wistar rats injected with streptozotocin (STZ, 55 mg/kg) served as the diabetic (D) group. Spontaneously hypertensive (SH) rats were used as the hypertensive group (H) while SH rats injected with STZ served as the hypertensive-diabetic (HD) group. Sarcolemma was isolated from the ventricles of the C, D, H, and HD groups and NCX activity was examined using rapid quenching techniques to study initial rates over a [Ca2+]o range of 10-160 µM. The Vmax of NCX was lower in the D group when compared with the C group (D, 2.96 ± 0.26 vs. C, 4.0 ± 0.46 nmol·mgprot-1·s-1, P < 0.05), however combined diabetes and hypertension (HD) did not affect the Vmax of NCX activity (HD, 3.84 ± 0.88 vs. H, 3.59 ± 0.24 nmol·mgprot-1·s-1, P > 0.05). However, analysis of the Km values for Ca2+ indicated that both the D and HD groups exhibited a significantly lower Km when compared with their respective control groups (D, 42 ± 4 vs. C, 56 ± 4 µM,P < 0.05; HD, 33 ± 7 vs. H, 51 ± 8 µM, P < 0.05). Immunoblotting using polyclonal antibodies (against canine cardiac NCX) exhibited the typical banding of 160, 120, and 70 kDa. The 120 kDa band is believed to represent the native exchanger with its post-translational modifications. Examination of the blots revealed a lower intensity of the 120 kDa band in the D group when compared with the C group, however, no significant difference in the HD group was observed. We speculate that the lower Vmax in the D group may be due to a reduced concentration of exchanger protein in the membrane. The absence of this defect in the HD group may be a result of compensatory mechanisms to the overall hemodynamic overload, however, this remains to be determined. The increased affinity for Ca2+ in both the D and HD groups (determined by the lower Km values) is an interesting finding and may be due to changes in sarcolemmal lipid bilayer composition secondary to diabetes-induced hyperlipidemia.Key words: diabetes, hypertension, cardiac, Na+-Ca2+ exchange, contractility.

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.

Lysosomal sulphate transport is dependent upon sulphydryl groups

Using thiol blocking agents, we examined the role of sulphydryl groups for function of the lysosomal sulphate transport system. Monothiol binding reagents, p-hydroxymercuribenzoic acid (p-HMB) and p-chloromercuribenzene sulphonic acid (p-CMBS), dithiol binding reagents such as CuCl2, the alkylating agent, N-ethylmaleimide (NEM), and NADH all inhibited lysosomal sulphate transport. The inhibitory effects of NEM and Cu2+ were not additive, suggesting that they both act upon the same critical sulphydryl group(s). Unlike the case for NEM, the inhibitory effects of Cu2+ were reversed by the reducing agent, dithiothreitol. Exposure to NEM resulted in a seven-fold increase in Km to 867 microM versus a control value of 126 microM and a modest decrease in Vmax to 99 pmolperunit beta-hexosaminidase per 30 s versus a control value of 129 pmolperunit beta-hexosaminidase per 30 s. Similar although somewhat less dramatic results were obtained using Cu2+ with an increase of Km to 448 microM and a Vmax of 77 pmolperunit beta-hexosaminidase per 30 s. The sulphate transport activity of detergent solubilized lysosomal membranes could be bound to a p-chloromercuribenzoic acid (p-CMB)-Sepharose sulphydryl affinity resin and eluted with mercaptoethanol. Sulphydryl groups thus appear to play a role in sulphate transport through effects on substrate affinity. Sulphydryl-binding appears to be a strategy that may be useful for purification of the transporter.

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.

H2O2-induced chloride currents are indicative of an endogenous Na(+)-Ca2+ exchange mechanism in Xenopus oocytes

1. Defolliculated Xenopus oocytes were voltage clamped in bathing solutions containing 115 mM KCl and 1.8 mM CaCl2. External application of H2O2 transiently elicited voltage-dependent outward rectifying currents within several seconds. Upon depolarization to +50 mV these currents had an activation time constant of 370 ms and reached amplitudes of up to 70 microA. This current was also observed in oocytes without the vitelline membrane. 2. The current was abolished by 500 microM niflumic acid, by the replacement of external Cl- by methanesulphonate, or when extracellular Ca2+ was removed indicating the involvement of Ca2+-activated Cl- channels, which are very abundant in Xenopus oocytes. 3. While the current could be recorded in bathing solutions containing Li+, K+, Rb+, Cs+ and NH4+, extracellular Na+ abolished the current completely (IC50 = 6 mM Na+). 4. The H2O2-induced Cl- current was half-maximally blocked by approximately 25 microM 2'4'-dichlorobenzamil, 250 microM MgCl2, 100 microM CdCl2 and 100 microM NiCl2. These substances have been shown to block Na+-Ca2+ exchangers in various tissues. 5. The data are consistent with the existence of an endogenous Na+-Ca2+ exchanger in the plasma membrane of Xenopus oocytes, which runs in reverse mode in the absence of high external Na+ and the presence of external Ca2+. This endogenous component has to be considered when Xenopus oocytes are used for heterologous expression studies.

Oxygen free radicals and calcium homeostasis in the heart

Many experiments have been done to clarify the effects of oxygen free radicals on Ca2+ homeostasis in the hearts. A burst of oxygen free radicals occurs immediately after reperfusion, but we have to be reminded that the exact levels of oxygen free radicals in the hearts are yet unknown in both physiological and pathophysiological conditions. Therefore, we should give careful consideration to this point when we perform the experiments and analyze the results. It is, however, evident that Ca2+ overload occurs when the hearts are exposed to an excess amount of oxygen free radicals. Through ATP-independent Ca2+ binding is increased, Ca2+ influx through Ca2+ channel does not increase in the presence of oxygen free radicals. Another possible pathway through which Ca2+ can enter the myocytes is Na(+)-Ca2+ exchanger. Although, the activities of Na(+)-K+ ATPase and Na(+)-H(+) exchange are inhibited by oxygen free radicals, it is not known whether intracellular Na(+) level increases under oxidative stress or not. The question has to be solved for the understanding of the importance of Na(+)-Ca2+ exchange in Ca2+ influx process from extracellular space. Another question is 'which way does Na(+)-Ca2+ exchange work under oxidative stress? Net influx or efflux of Ca2+?' Membrane permeability for Ca2+ may be maintained in a relatively early phase of free radical injury. Since sarcolemmal Ca(2+)-pump ATPase activity is depressed by oxygen free radicals, Ca2+ extrusion from cytosol to extracellular space is considered to be reduced. It has also been shown that oxygen free radicals promote Ca2+ release from sarcoplasmic reticulum and inhibit Ca2+ sequestration to sarcoplasmic reticulum. Thus, these changes in Ca2+ handling systems could cause the Ca2+ overload due to oxygen free radicals.

Oxygen free radicals and calcium homeostasis in the heart

Many experiments have been done to clarify the effects of oxygen free radicals on Ca2+ homeostasis in the hearts. A burst of oxygen free radicals occurs immediately after reperfusion, but we have to be reminded that the exact levels of oxygen free radicals in the hearts are yet unknown in both physiological and pathophysiological conditions. Therefore, we should give careful consideration to this point when we perform the experiments and analayze the results. It is, however, evident that Ca2+ overload occurs when the hearts are exposed to an excess amount of oxygen free radicals. Though ATP-independent Ca2+ binding is increased, Ca2+ influx through Ca2+ channel does not increase in the presence of oxygen free radicals. Another possible pathway through which Ca2+ can enter the myocytes is Na(+)-Ca2+ exchanger. Although, the activities of Na(+)-K+ ATPase and Na(+)-Ca2+ exchanger. Although, the activities of Na(+)-H+ exchange are inhibited by oxygen free radicals, it is not known whether intracellular Na+ level increases under oxidative stress or not. The question has to be solved for the understanding of the importance of Na(+)-Ca2+ exchange in Ca2+ influx process from extracellular space. Another question is 'which way does Na(+)-Ca2+ exchange work under oxidative stress? Net influx or efflux of Ca2+?' Membrane permeability for Ca2+ may be maintained in a relatively early phase of free radical injury. Since sarcolemmal Ca(2+)-pump ATPase activity is depressed by oxygen free radicals, Ca2+ extrusion from cytosol to extracellular space is considered to be reduced. It has also been shown that oxygen free radicals promote Ca2+ release from sarcoplasmic reticulum and inhibit Ca2+ sequestration to sarcoplasmic reticulum. Thus, these changes in Ca2+ handling systems could cause the Ca2+ overload due to oxygen free radicals.

Excitation-contraction coupling in single guinea-pig ventricular myocytes exposed to hydrogen peroxide

1. The effects of hydrogen peroxide (H2O2), an in vitro free radical generating system, on excitation-contraction (E-C) coupling were studied in isolated adult guinea-pig ventricular myocytes using Ca(2+)-sensitive dyes and the patch-clamp technique. 2. In paced myocytes loaded with indo-1 AM, 1 mM H2O2 briefly increased, then decreased the amplitude of intracellular Ca2+ ([Ca2+]i) transients and cell contractions. Diastolic [Ca2+]i increased in association with cell shortening. Automaticity also developed, followed shortly by inexcitability. In contrast, paced myocytes exposed to the metabolic inhibitors carbonyl cyanide-p-trifluoromethoxyphenylhydrazone (FCCP) and 2-deoxyglucose (DG), rapidly became inexcitable and exhibited marked diastolic shortening prior to increases in diastolic [Ca2+]i. 3. In patch-clamped myocytes loaded with fura-2, H2O2 reduced the amplitude of the Ca2+ current (ICa), the [Ca2+]i transient, and active cell shortening. H2O2 prolonged the relaxation phase of the [Ca2+]i transient, and activated an outward membrane current consistent with the ATP-sensitive K+ current (IK,ATP), but did not change the voltage dependence of ICa, the peak [Ca2+]i transient or active cell shortening. These responses were qualitatively similar to patch-clamped myocytes exposed to FCCP and DG. 4. Following exposure to H2O2, ICa elicited smaller [Ca2+]i transients than under control conditions. This was consistent with the observation that H2O2 reduced sarcoplasmic reticulum (SR) stores of Ca2+ by 42%, when assessed by observing the [Ca2+]i transients elicited by rapid extracellular application of 5 mM caffeine. In contrast FCCP-DG tended to increase SR Ca2+ stores. 5. Despite the decrease in the caffeine-induced Ca2+i release after H2O2, there was an increase in the Na(+)-Ca2+ exchange current associated with the caffeine-induced [Ca2+]i transient. 6. We conclude, therefore, that as with metabolic inhibitors, H2O2 interferes with E-C coupling in guinea-pig myocytes by impairing ICa and activating IK,ATP. However, unlike metabolic inhibitors, H2O2 stimulates Na(+)-Ca2+ exchange and depletes SR Ca2+ stores. Furthermore, diastolic [Ca2+]i becomes elevated while the myocyte is still excitable. These observations suggest that free radicals have primary effects on cardiac E-C coupling independent of their depressant effects on metabolism.

Protective effect of S12340 on cardiac cells exposed to oxidative stress

Oxidative stress induced by reactive oxygen species is one aspect of the deleterious mechanisms involved in myocardial post-ischemic reperfusion injury. The antioxidant properties of the new molecule S12340 (8-[3-(3,5-diterbutyl-4-hydroxyphenyl-thio)propyl]-1-oxa-2- oxo-3,8-diazaspiro[4.5]decane) were evaluated using three successive in vitro approaches mimicking the cardiac cell damages induced by reactive oxygen species released into the reperfused myocardium. (i) The effects of S12340 on lipid peroxidation were evaluated using an original cell-free model of non-enzymatic peroxidation of 1.32 mM arachidonic acid induced by reactive oxygen species generated photochemically. S12340 (13.2 microM) inhibited by 29% the rate of oxidative fragmentation of monohydroperoxidized arachidonic acid into aldehydic products. (ii) S12340 (10 microM) inhibited by 96% and 58% the oxidative necrosis of cultured rat cardiomyocytes induced by xanthine oxidase (20 mU/ml) and monohydroperoxidized arachidonic acid (30 microM), respectively. (iii) Superfusion of guinea-pig papillary muscle with monohydroperoxidized arachidonic acid (20 microM) resulted in marked alterations of their electrophysiological and mechanical activities. These modifications, maximal 15-17 min after the addition of lipid hydroperoxide, were completely abolished by S12340 (30 microM).

Oxygen free radicals and cardiac reperfusion abnormalities

Oxygen free radicals are highly reactive compounds causing peroxidation of lipids and proteins and are thought to play an important role in the pathogenesis of reperfusion abnormalities including myocardial stunning, irreversible injury, and reperfusion arrhythmias. Free radical accumulation has been measured in ischemic and reperfused myocardium directly using techniques such as electron paramagnetic resonance spectroscopy and tissue chemiluminescence and indirectly using biochemical assays of lipid peroxidation products. Potential sources of free radicals during ischemia and reperfusion have been identified in myocytes, vascular endothelium, and leukocytes. In several different experimental models exogenous free radical-generating systems have been shown to produce alterations in cardiac function that resemble the various reperfusion abnormalities described above. Injury to processes involved in regulation of the intracellular Ca2+ concentration may be a common mechanism underlying both free radical-induced and reperfusion abnormalities. Direct effects of free radicals on each of the known Ca(2+)-regulating mechanisms of the cell as well as the contractile proteins and various ionic membrane currents have been described. Free radicals also inhibit critical enzymes in anaerobic and aerobic metabolic pathways, which may limit the metabolic reserve of reperfused myocardium and contribute to intracellular Ca2+ overload. Inhibiting free radical accumulation during myocardial ischemia/reperfusion with free radical scavengers and inhibitors has been demonstrated to reduce the severity of myocardial stunning, irreversible injury, and reperfusion arrhythmias in many, but not all, studies. This evidence strongly implicates free radical accumulation during myocardial ischemia/reperfusion as an important pathophysiological mechanism of reperfusion abnormalities, although many issues remain unresolved.

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

Free oxygen radicals are formed during early reperfusion and are thought to contribute to some types of reperfusion abnormalities, including arrhythmias and myocardial stunning. The purpose of this study was to investigate electrophysiological effects of oxygen free radicals using voltage clamped single ventricular myocytes from guinea-pig hearts. Oxygen free radicals were produced enzymatically by the direct addition of xanthine oxidase (XOD, 0.04 U/ml) in the experimental chamber to a solution containing hypoxanthine (0.96 mM). The generation of oxygen radicals was confirmed by the formation of adrenochrome from adrenaline. Oxygen radicals caused automaticity of isolated myocytes within 20-30 min, followed by later hypercontracture. The percentage of rod-shaped cells declined sigmoidally as a function of time, with a half maximal value at 40.9 +/- 1.6 min, and a Hill slope of -0.10 +/- 0.01 (n = 26). These effects were prevented by a combination of superoxide dismutase (10(5) U/L) plus catalase (10(6) U/L). The rate at which cells underwent morphological shape changes was unchanged by ryanodine (0.5 microM) which is thought to act on the sarcoplasmic reticulum or by the Ca2+ channel blockers nisoldipine (1 microM) or Cd2+ (30 microM). Cellular automaticity and hypercontracture were delayed by variable degrees, and sometimes completely prevented, by zero (1 mM EGTA) extracellular Ca2+, MnCl2 (2 mM) and LaCl3 (50 microM), and amiloride (1 mM). On the other hand, in the presence of a low extracellular Na+ (30 mM) or caffeine (10 mM), hypercontracture occurred at a faster time scale. Whole cell voltage clamping revealed a decrease of the inward rectifying K+ current (IK1), and a decrease of the peak of the L-type Ca2+ current (ICa,L). The total ICa,L during the clamp step was increased, mainly because of an increased time constant of inactivation (47.6 +/- 4.7 ms to 72.7 +/- 15.5 ms after 30 min, n = 4, P less than 0.05). We conclude that oxygen radicals cause automaticity and hypercontracture of isolated myocytes, that these effects may be due to an increased intracellular Ca2+ concentration ([Ca2+]i), and despite an increased ICa,L, that the enhanced Ca2+ influx may occur predominantly via the Na/Ca exchange.

Stunning: a radical re-view

The recovery from trauma, whether ischemia or some other form of tissue injury, is never instantaneous; time is always required for repair and the return of normal metabolism and function. To what extent the delay in recovery of contractile activity (stunning) after a brief period of ischemia represents convalescence from ischemia-induced injury, as opposed to the expression of reperfusion-induced injury, is perhaps not as clear as the proponents of stunning would hope. Definitive evidence for a distinct reperfusion-induced pathology, which compromises the recovery of contractile function from the depressed state induced by ischemia, is elusive. If reperfusion-induced injury accounts for a significant proportion of stunning, then the molecular mechanisms responsible for initiating the event and those responsible for orchestrating the event at the level of the contractile protein are far from clear. Perturbations of calcium homeostasis are frequently cited as responsible for the depressed contractile state, however, some metabolic derangement must precede any pathologically induced ionic disturbance. In this connection, evidence indicates that free-radical-induced oxidant stress, during the early moments of reperfusion, may modify the activity of a number of thiol-regulated proteins that are directly, or indirectly, responsible for controlling the movement of calcium. Sarcolemmal sodium-calcium exchange and the calcium release channel of the sarcoplasmic reticulum may be activated, whereas the sarcolemmal calcium pump and sodium-potassium ATPase, together with the calcium pump of the sarcoplasmic reticulum, may be inhibited. Under the conditions prevailing during ischemia and reperfusion, this would be expected to promote an early intracellular calcium overload. It is difficult to reconcile such a change with the decreased inotropic state that characterizes stunning; however, it seems likely that the calcium overload is transient and that the stunned myocardium rapidly reestablishes normal levels of intracellular calcium. It is still difficult to explain adequately the reduced inotropic state; clearly, the mechanism of stunning is not quite as simple as its definition.

Effects of free radicals on the electrophysiological function of cardiac membranes

Abnormal electrical activity in heart cells can result in irregular heart rhythms or arrhythmias. Any form of pathological or toxicological damage to the sarcolemmal membrane presents the risk of precipitating arrhythmias and compromise of the heart's function as a pump. An array of cardiovascular conditions from coronary artery disease and myocardial infarction to cardiomyopathies and hypertrophy, can induce arrhythmias. Many of these conditions recently have been linked to increases in free radical production. Early studies suggesting a role for free radicals in the abnormal function of ischemic and reperfused hearts use anti-free radical interventions to reduce arrhythmias. More recent works have taken advantage of different free radical-generating systems to show a reproducible sequence of changes in the cellular action potential; these data suggest changes in the transmembrane movement of ions through membrane channels. Biochemical evidence supports a possible involvement of ion exchange mechanisms in the cardiac sarcolemma. All the evidence indicate that free radical injury may have profound effects on the electrical function of myocardial cells.