Oxygen and substrate deprivation on isolated rat cardiac myocytes: temporal relationship between electromechanical and biochemical consequences

Development of an In Vitro Cardiac Ischemic Model Using Primary Human Cardiomyocytes

Developing experimental models to study ischemic heart disease is necessary for understanding of biological mechanisms to improve the therapeutic approaches for restoring cardiomyocytes function following injury. The aim of this study was to develop an in vitro hypoxic/re-oxygenation model of ischemia using primary human cardiomyocytes (HCM) and define subsequent cytotoxic effects. HCM were cultured in serum and glucose free medium in hypoxic condition with 1% O₂ ranging from 30 min to 12 h. The optimal hypoxic exposure time was determined using Hypoxia Inducible Factor 1α (HIF-1α) as the hypoxic marker. Subsequently, the cells were moved to normoxic condition for 3, 6 and 9 h to replicate the re-oxygenation phase. Optimal period of hypoxic/re-oxygenation was determined based on 50% mitochondrial injury via 3-(4,5-dimethylthiazol-2-Yl)-2,5-diphenyltetrazolium bromide assay and cytotoxicity via lactate dehydrogenase (LDH) assay. It was found that the number of cells expressing HIF-1α increased with hypoxic time and 3 h was sufficient to stimulate the expression of this marker in all the cells. Upon re-oxygenation, mitochondrial activity reduced significantly whereas the cytotoxicity increased significantly with time. Six hours of re-oxygenation was optimal to induce reversible cell injury. The injury became irreversible after 9 h as indicated by > 60% LDH leakage compared to the control group cultured in normal condition. Under optimized hypoxic reoxygenation experimental conditions, mesenchymal stem cells formed nanotube with ischemic HCM and facilitated transfer of mitochondria suggesting the feasibility of using this as a model system to study molecular mechanisms of myocardial injury and rescue.

Involvement of microtubules in the tolerance of cardiomyocytes to cold ischemia-reperfusion

Before transplantation, the heart graft is preserved by the use of cold storage in order to limit ischemia-reperfusion stress. However, sustained exposure to low temperature may induce myocardial ultrastructural damage, particularly microtubules (MT) disruption. Previous data suggested that tubulin-binding agents are able to attenuate cold-induced cytoskeleton alterations. Thus, the aim of the present work was to study the influence of docetaxel (DX, a tubulin-binding taxane) on the effects of deep hypothermia (4 degrees C) and of simulated cold ischemia-reperfusion on the MT network and oxidative stress of cardiomyocyte (CM) in monolayer cultures prepared from newborn rat ventricles. The MT network was explored by immunocytochemistry and Western-blotting, the cell stress by tetrazolium dye assay (MTT) and lactate dehydrogenase (LDH) release, and the superoxide production by the dihydroethidium probe (DHE). The MT assembly remained stable after 4 and 8 h of hypothermia. Tubulin acetylation was promoted in CM subjected to 4-h hypothermia. Low temperature reduced the mitochondrial function and increased the basal LDH release. The cold ischemia during 4 and 8 h preserved MT network. Docetaxel promoted MT polymerization and tubulin acetylation in basal and in cold conditions. This drug decreased the release of LDH induced by cold ischemia. Moreover, hypothermia (4 h) significantly raised the anion superoxide production. Docetaxel decreased this oxidative stress in the control CM and in CM submitted to 4 h of hypothermia. These data demonstrated that stabilizing MT with DX exerted a protective effect on CM subjected to hypothermia and to cold ischemia-reperfusion. Tubulin-ligands should be thus considered to improve the tolerance of the heart graft toward stressing conservative conditions.

Tubulin ligands suggest a microtubule–NADPH oxidase relationship in postischemic cardiomyocytes

Alterations of the microtubule network, which is involved in many vital processes, occur in several pathological conditions, such as cardiac ischemia. However, the connection between the microtubule assembly state and the factors affecting myocardial reperfusion injury, especially oxidative stress, is unknown. We aimed thus to study the effects of different tubulin ligands on the changes in the microtubule network and in several markers of cell injury and oxidative activity in cardiac muscle cells submitted to a reversible substrate-free, hypoxia-reoxygenation model of ischemia-reperfusion. The microtubule network was visualized by immunocytochemistry. Cell injury was evaluated via lactate dehydrogenase release and the mitochondrial function by the MTT test. Superoxide production was detected using dihydroethidium. The activity of NADPH oxidase and mRNA subunit expression were investigated. The microtubule disassembly induced by simulated ischemia was reversed by placing cardiomyocytes under normoxic conditions. This post-"ischemic" restoration of microtubule assembly was modulated by microtubule stabilizers (taxol: paclitaxel) and by microtubule disrupting drugs (nocodazole, colchicine). In addition, nocodazole decreased superoxide anion production as well as NADPH oxidase activity and mRNA expression of the NADPH oxidase subunit p22phox. These results demonstrated that the "ischemia"-induced microtubule network alteration is reversible and suggest a possible relationship between "reperfusion"-induced reassembly of microtubules and free radical generation in post-"ischemic" cardiomyocytes.

[Development of cardiac physiopathological models from cultured cardiomyocytes]

The cultures of neonatal rat cardiomyocytes represent a very useful tool for the observation and the understanding of the cellular aspects of the electrophysiological, contractile, morphological, metabolic and molecular properties of the myocardium. This model is characterized by a homogeneous population of cardiac muscular cells and by vast possibilities of control of the chemical and physical environment of the cells, allowing the in vitro mimicry of a wide range of cardiac pathological situations. The cardiomyocyte cultures are thus suited to very varied experimental protocols, allowing multiparametric analysis of the cardiocellular effects of different stress such as hypoxia-reoxygenation, of ischemia-reperfusion, of the free radical attack and of thermal shock. These investigations can be combined with the study of the effects and of the cytotoxicity of pharmacological agents, not limited to the putatively cardioactive drugs. The present review proposes an outline of the procedures for the isolation, the culture and the use of neonatal cardiomyocytes. To illustrate the potentialities of this preparation, we describe more specifically the protocols and the various consequences at the cellular scale of an in vitro model of myocardial ischemia reperfusion.

Beta1-adrenergic receptors maintain fetal heart rate and survival

Beta-adrenergic receptor (betaAR) activation has been shown to maintain heart rate during hypoxia and to rescue the fetus from the fetal lethality that occurs in the absence of norepinephrine. This study examines whether the same subtype of betaAR is responsible for survival and heart rate regulation. It also investigates which betaARs are located on the early fetal heart and whether they can be directly activated during hypoxia. Cultured E12.5 mouse fetuses were treated with subtype-specific betaAR antagonists to pharmacologically block betaARs during a hypoxic insult. Hypoxia alone reduced heart rate by 35-40% compared to prehypoxic levels. During hypoxia, heart rate was further reduced by 31% in the presence of a beta(1)AR antagonist, CGP20712A, at 100 nM, but not with a beta2 (ICI118551)- or a beta3 (SR59230A)-specific antagonist at 100 nM. Survival in utero was also mediated by beta1ARs. A beta1 partial agonist, xamoterol, rescued 74% of catecholamine-deficient (tyrosine-hydroxylase-null) pups to birth, a survival rate equivalent to that with a nonspecific betaAR agonist, isoproterenol (87%). Receptor autoradiography showed that beta1ARs were only found on the mouse heart at E12.5, while beta2ARs were localized to the liver and vasculature. To determine if the response to hypoxia was intrinsic to the heart, isolated fetal hearts were incubated under hypoxic conditions in the presence of a betaAR agonist. Heart rate was reduced to 25-30% by hypoxia alone, but was restored to 63% of prehypoxic levels with 100 nM isoproterenol. Restoration was completely prevented if beta1ARs were blocked with CGP20712A at 300 nM, a concentration that blocks beta1ARs, but not beta2- or beta3ARs. Our results demonstrate that beta1ARs are located on the heart of early fetal mice and that beta1AR stimulation maintains fetal heart rate during hypoxia and mediates survival in vivo.

Substrate dependence of the postischemic cardiomyocyte recovery: Dissociation between functional, metabolic and injury markers

Defining the substrate that influences the most favourably the myocardial post-ischemic recovery is subject of debates, due to dissociation between functional and biochemical benefits. Hence, we studied the effects of either glucose or different fatty acids on the functional and metabolic recovery of post-ischemic cardiomyocytes in a substrate-free hypoxia model of simulated ischemia-reperfusion. Rat cardiomyocytes were submitted to a 2.5 h simulated ischemia followed by a 2 h reoxygenation without substrate (control), or with either glucose, octanoic acid, oleic acid, or elaidic acid. During simulated ischemia, electromechanical function gradually disappeared while the cellular viability and mitochondrial function declined. During control simulated reperfusion, cardiomyocytes recovered near normal function but a significant reduction in the action potential amplitude and rate persisted. The addition of glucose or oleic acid during simulated reperfusion promoted a faster, better and sustain functional recovery. Amongst the fatty acids, the functional recovery was slower with elaidic and octanoic acids as compared with oleic acid. The mitochondrial function was better improved during simulated reperfusion with glucose than with the tested fatty acids, among which elaidic acid was the less unfavourable. Paradoxically, the addition of whichever substrate during simulated reperfusion tended to worsen the cellular viability. Thus, cardiomyocytes recovery strongly relies on the characteristics of the substrate supplied at the onset of simulated reperfusion: glucidic or lipidic nature, chain-length, insaturation degree. Moreover, these data suggest that defining the appropriateness of a given substrate for the post-ischemic cardiomyocyte recovery is closely related to the functional and the biological endpoints in consideration.

Direct, pleiotropic protective effect of cyclosporin A against simulated ischemia-induced injury in isolated cardiomyocytes

Cyclosporin A is an immunosuppressor that prolongs graft survival but its use is limited by cardiotoxicity. The effects of cyclosporin A on several functional and biological characteristics were thus evaluated in rat cardiomyocytes in normal conditions and in a substrate-free, hypoxia-reoxygenation model of ischemia-reperfusion. Cyclosporin A (100 and 1000 ng/ml) did not induce cardiocytotoxicity in basal conditions. Simulated ischemia gradually decreased and then blocked the spontaneous electromechanical activity. Cyclosporin A at 100 and 1000 ng/ml permitted the maintenance of electromechanical functions that were abolished in control cells. Cyclosporin A also improved the post-"ischemic" functional recovery. Cyclosporin A reduced the "ischemia"-induced lactate dehydrogenase and troponine I releases and the successive rises in heat shock protein mRNA observed after "ischemia" and reoxygenation. Moreover, cyclosporin A improved the resumption of the mitochondrial function. To conclude, cyclosporin A displayed a direct, pleiotropic protection of isolated cardiomyocytes against physiological, metabolic, structural and stress signaling changes induced by ischemia-reperfusion mimicked in vitro.

Microtubule alteration is an early cellular reaction to the metabolic challenge in ischemic cardiomyocytes

Cytoskeleton damage, particularly microtubule (MT) alterations, may play an important role in the pathogenesis of ischemia-induced myocardial injury. However, this disorganization has been scarcely confirmed in the cellular context. We evaluated MT network disassembly in myoblast cell line H9c2 and in neonatal rat cardiomyocytes in an in vitro substrate-free hypoxia model of simulated ischemia (SI). After different duration of SI from 30 up to 180 min, the cells were fixed and the microtubule network was revealed by immunocytochemistry. The microtubule alterations were quantified using a house-developed image analysis program. Additionally, the tubulin fraction were extracted and quantified by Western blotting. The cell respiration, the release of cellular LDH and the cell viability were evaluated at the same periods. An early MT disassembly was observed after 60 min of SI. The decrease in MT fluorescence intensity at 60 and 90 min was correlated with a microtubule disassembly. Conversely, SI-induced significant LDH release (35%) and decrease in cell viability (34%) occurred after 120 min only. These results suggest that the simulated ischemia-induced changes in MT network should not be considered as an ultrastructural hallmark of the cell injury and could rather be an early ultrastructural correlate of the cellular reaction to the metabolic challenge.

Physiological and metabolic actions of mycophenolate mofetil on cultured newborn rat cardiomyocytes in normoxia and in simulated ischemia

Mycophenolate mofetil (MMF) is a new immunosuppressive drug used to reduce acute rejection after heart transplantation. As with other immunosuppressive drugs, MMF therapy is associated with several adverse effects. However, the direct effects of MMF on myocardial tissue has not been yet evaluated. The aim of the work was thus to evaluate the effects of MMF on isolated cardiomyocytes (CM) in normal conditions and in an in vitro model of simulated ischemia (SI; substrate-free hypoxia) and reperfusion (R; reoxygenation). Myocyte-enriched cultures were prepared from newborn rat heart ventricles. The transmembrane potentials were recorded using conventional microelectrodes and the cell contractions were monitored with a photoelectric device. In basal conditions, MMF (10(-6) and 10(-5) M) exerted no significant effects on the survival and on the electrical and contractile activities of CM in culture, even during long-term exposure (up to 48 h). SI per se led to a gradual decrease and then an abortion of the spontaneous automaticity and electromechanical activity of CM. Pretreating CM with either 10(-6) or 10(-5) M MMF was able to reduce the SI-induced cell dysfunctions. The presence of MMF at these concentrations did not hamper the post-SI functional recovery of CM during reoxygenation. At 10(-5) M, MMF applied during reoxygenation only permitted a better recovery of CM. However, the mitochondrial function after reoxygenation, as assessed by 3-(4,5-dimethylthiazol-2-yl)-2,5 diphenyl-tetrazolium bromide (MTT) test, was not significantly influenced by the addition of MMF before as well as after ischemia. Conversely, MMF was able to reduce in this model the postischemic rise in xanthine and hypoxanthine. These data from CM-enriched model show that MMF: (i) had no cytotoxic effect, (ii) displayed a cytoprotective effect during SI, and (iii) exerted its beneficial effect at least partly through the decrease in the xanthine oxidase-dependent free radical production.

Medium perfusion enables engineering of compact and contractile cardiac tissue

We hypothesized that functional constructs with physiological cell densities can be engineered in vitro by mimicking convective-diffusive oxygen transport normally present in vivo. To test this hypothesis, we designed an in vitro culture system that maintains efficient oxygen supply to the cells at all times during cell seeding and construct cultivation and characterized in detail construct metabolism, structure, and function. Neonatal rat cardiomyocytes suspended in Matrigel were cultured on collagen sponges at a high initial density (1.35 x 10(8) cells/cm(3)) for 7 days with interstitial flow of medium; constructs cultured in orbitally mixed dishes, neonatal rat ventricles, and freshly isolated cardiomyocytes served as controls. Constructs were assessed at timed intervals with respect to cell number, distribution, viability, metabolic activity, cell cycle, presence of contractile proteins (sarcomeric alpha-actin, troponin I, and tropomyosin), and contractile function in response to electrical stimulation [excitation threshold (ET), maximum capture rate (MCR), response to a gap junctional blocker]. Interstitial flow of culture medium through the central 5-mm-diameter x 1.5-mm-thick region resulted in a physiological density of viable and differentiated, aerobically metabolizing cells, whereas dish culture resulted in constructs with only a 100- to 200-microm-thick surface layer containing viable and differentiated but anaerobically metabolizing cells around an acellular interior. Perfusion resulted in significantly higher numbers of live cells, higher cell viability, and significantly more cells in the S phase compared with dish-grown constructs. In response to electrical stimulation, perfused constructs contracted synchronously, had lower ETs, and recovered their baseline function levels of ET and MCR after treatment with a gap junctional blocker; dish-grown constructs exhibited arrhythmic contractile patterns and failed to recover their baseline MCR levels.

Specific electromechanical responses of cardiomyocytes to individual and combined components of ischemia

The main factors of myocardial ischemia are hypoxia, substrate deprivation, acidosis, and high extracellular potassium concentration ([K+]e), but the influence of each of these factors has not yet been evaluated in a cardiomyocyte (CM) culture system. Electromechanical responses to the individual and combined components of ischemia were studied in CM cultured from newborn rat ventricles. Action potentials (APs) were recorded using glass microelectrodes and contractions were monitored photometrically. Glucose-free hypoxia initially reduced AP duration, amplitude, and rate and altered excitation-contraction coupling, but AP upstroke velocity (Vmax) remained unaffected. Early afterdepolarizations appeared, leading to bursts of high-rate triggered impulses before the complete arrest of electromechanical activity after 120 min. Acidosis reduced Vmax whereas AP amplitude and rate were moderately decreased. Combining acidosis and substrate-free hypoxia also decreased Vmax but attenuated the effects of substrate-free hypoxia on APs and delayed the cessation of the electrical activity (180 min). Raising [K+]e reduced the maximal diastolic potential and Vmax. Total ischemia (substrate deletion, hypoxia, acidosis, and high [K+]e) decreased AP amplitude and Vmax without changing AP duration. Moreover, delayed afterdepolarizations appeared, initiating triggered activity. Ultimately, 120 min of total ischemia blocked APs and contractions. To conclude, glucose-free hypoxia caused severe functional defects, acidosis delayed the changes induced by substrate-free hypoxia, and total ischemia induced specific dysfunctions differing from those caused by the former conditions. Heart-cell cultures thus represent a valuable tool to scrutinize the individual and combined components of ischemia on CMs.

Assessment of the cytoprotective role of adenosine in an in vitro cellular model of myocardial ischemia

This work aimed to detect functional adenosine receptors in isolated rat cardiomyocytes and to study the influence of stimulation of these receptors in an in vitro model of ischemia. Cultures of cardiomyocytes were prepared from newborn rat ventricles. The contractions were photometrically monitored. In this preparation, adenosine induced a positive chronotropic response. This effect was reproduced by CGS 21680 (2-(4-[2-carboxyethyl]-phen-ethyl-amino) adenosine-5'N-ethylunosamide), a specific adenosine A(2) receptor agonist, and antagonized by DMPX (3,7-dimethyl-1-propargylxanthine), an adenosine A(2) receptor antagonist. However, R-PIA (R-N(6)-(2-phenylisopropyl)-adenosine; a specific adenosine A(1) receptor agonist) induced a negative chronotropic effect that was abolished by its corresponding adenosine A(1) antagonist DPCPX (1,3-dipropyl-8-cyclo-pentyl-adenosine). Substrate-free hypoxia, as simulation of ischemia, induced a progressive decrease and then arrest of spontaneous cell contractions. The spontaneous rhythmic contractile activity was restored during reoxygenation following simulated ischemia. Adenosine A(1) receptor stimulation with R-PIA induced a decrease of hypoxia-induced damage. This effect was antagonized by DPCPX, an adenosine A(1) receptor antagonist. Conversely, the cells treated with CGS 21680 did not display complete recovery after reoxygenation. In addition, this effect was abolished by DMPX, since the cells recovered normal function after reoxygenation. To conclude, it appeared that cardiomyocytes possess both functional adenosine A(1) and A(2) receptors and that only the activation of adenosine A(1) receptor had a cytoprotective effect against simulated ischemia-induced cardiac cell injury.

Influence of deep hypothermia on the tolerance of the isolated cardiomyocyte to ischemia-reperfusion

The influence of deep hypothermia (4 degrees C) during a substrate-free, hypoxia-reoxygenation treatment was investigated on cardiomyocytes (CM) prepared from newborn rat heart in culture in an in vitro, substrate-free model of ischemia-reperfusion. The transmembranous potentials were recorded with standard microelectrodes. The contractions were monitored photometrically. The RNA messenger (mRNA) and protein expression for protein (HSP70) were analysed by RT-PCR (reverse transcriptase-polymerase chain reaction) and Western blotting, respectively. Simulated ischemia (SI) caused a gradual decrease and then a cessation of the spontaneous electromechanical activity. During the reoxygenation, the CM recovered normal function, provided that SI did not exceed 2.5 h. When SI duration was increased up to 4 h, reoxygenation failed to restore the spontaneous electromechanical activity. Conversely, the exposure of the CM to SI together with deep hypothermia decreased the functional alterations observed, and provided a complete electromechanical recovery after 2.5 h as well as after 4 h of SI. Deep hypothermia alone failed to induce HSP70 mRNA and protein production. On the contrary, HSP70 mRNA production increased after 2.5 and 4 h of deep hypothermia followed by 1 h of rewarming, proportionally to the duration of the cooling period. This augmentation in mRNA was associated with a rise in HSP70 protein content. In summary, it appeared that deep hypothermia exerts a strong cytoprotective action during SI only, whereas cooling CM before SI has no beneficial effect on subsequent SI. Moreover, these results suggested the persistence of a signaling system and/or transduction in deeply cooled, functionally depressed cells. Finally, CM in culture appeared to be a model of interest for studying heart graft protection against ischemia-reperfusion and contributed to clarifying the molecular and cellular mechanisms of deep hypothermia on myocardium.

Changes in HSP70 and P53 expression are related to the pattern of electromechanical alterations in rat cardiomyocytes during simulated ischemia

The objective was to relate the response of the HSP70 and P53 genes to the cessation and the recovery of cardiac muscle cell functions when submitted to ischemia-reperfusion. We have measured the electromechanical activity, the released enzymes and HSP70 RNA and protein levels in cultured neonatal rat cardiomyocytes (CM) in a substrate-free, hypoxia-reoxygenation model of ischemia-reperfusion. In parallel the expression of the two genes P53 (the key apoptosis regulator gene) and P21/Waf1 (the P53 target gene) has been evaluated. The functional recovery during post-'ischemic' reoxygenation was associated with an overexpression of HSP70 and P53 lasting until the functional parameters reverted back to the normal, prehypoxic values. In contrast, extending the substrate-free hypoxic treatment worsens the dysfunction of the cardiac muscle cell and, in these conditions, reoxygenation failed to restore cell functions and to activate HSP70. Finally, in the conditions of reversible 'ischemic' cell injury, an early and transitory activation of P53 was associated with the functional recovering process of the CM submitted to simulated ischemia. These observations are suggestive of a contributive role of both HSP70 and P53 to a cytoprotective program activated by reoxygenation in post-'ischemic' CM.

Oxidative injury of isolated cardiomyocytes: dependence on free radical species

The contribution of lipid peroxidation to myocardial injury by free radicals (FR) is still unclear. Consequently, we examined the functional damages inflicted on cultured rat cardiomyocytes (CM) during FR stress provoked by the xanthine/xanthine oxidase system (X/XO) or by a hydroperoxidized fatty acid ((9 Z, 11 E, 13 (S), 15 Z)-13-hydroperoxyocta-decatrienoic acid; 13-HpOTrE), in order to simulate in vitro the initial phase and the propagation phase of the FR attack, respectively. Transmembrane potentials were recorded with glass microelectrodes and contractions were monitored photometrically. The EPR spectroscopy showed that X/XO produced superoxide and hydroxyl radicals during 10 min. The X/XO system altered sharply and irreversibly the spontaneous electrical and mechanical activities of the CM. However, the gas chromatographic analysis showed that these drastic functional damages were associated with comparatively moderate membrane PUFA degradation. Moreover, the EPR analysis did not reveal the production of lipid-derived FR. 13-HpOTrE induced a moderate and reversible decrease in electrical parameters, with no change in CM contractions. These results indicate that the functional consequences of FR attack are dependent on the radical species present and do not support the idea that the membrane lipid breakdown is a major factor of myocardial oxidant dysfunction.

Cardiac tissue engineering: cell seeding, cultivation parameters, and tissue construct characterization

Cardiac tissue engineering has been motivated by the need to create functional tissue equivalents for scientific studies and cardiac tissue repair. We previously demonstrated that contractile cardiac cell-polymer constructs can be cultivated using isolated cells, 3-dimensional scaffolds, and bioreactors. In the present work, we examined the effects of (1) cell source (neonatal rat or embryonic chick), (2) initial cell seeding density, (3) cell seeding vessel, and (4) tissue culture vessel on the structure and composition of engineered cardiac muscle. Constructs seeded under well-mixed conditions with rat heart cells at a high initial density ((6-8) x 10(6) cells/polymer scaffold) maintained structural integrity and contained macroscopic contractile areas (approximately 20 mm(2)). Seeding in rotating vessels (laminar flow) rather than mixed flasks (turbulent flow) resulted in 23% higher seeding efficiency and 20% less cell damage as assessed by medium lactate dehydrogenase levels (p < 0.05). Advantages of culturing constructs under mixed rather than static conditions included the maintenance of metabolic parameters in physiological ranges, 2-4 times higher construct cellularity (p &le 0.0001), more aerobic cell metabolism, and a more physiological, elongated cell shape. Cultivations in rotating bioreactors, in which flow patterns are laminar and dynamic, yielded constructs with a more active, aerobic metabolism as compared to constructs cultured in mixed or static flasks. After 1-2 weeks of cultivation, tissue constructs expressed cardiac specific proteins and ultrastructural features and had approximately 2-6 times lower cellularity (p < 0.05) but similar metabolic activity per unit cell when compared to native cardiac tissue.

Cross-influence of membrane polyunsaturated fatty acids and hypoxia-reoxygenation on alpha- and beta-adrenergic function of rat cardiomyocytes

The purpose of the present investigation was to determine whether the beneficial effects of polyunsaturated fatty acids (PUFA) may influence ischemia-reperfusion-induced alterations of myocardial alpha- and beta-adrenoceptor (alpha-AR, beta-AR) responsiveness. This study was carried out using monolayer cultures of neonatal rat ventricular myocytes in a substrate-free, hypoxia-reoxygenation model of ischemia. The cardiomyocytes (CM) were incubated during 4 days in media enriched either with n-6 PUFA (arachidonic acid, AA) or with n-3 PUFA (eicosapentaenoic acid, EPA, and docosahexaenoic acid, DHA). The n-6/n-3 ratio in n-3 CM was close to 1.2, compared to 20.1 in n-6 CM. The contractile parameters of n-6 CM and n-3 CM were similar in basal conditions as well as during hypoxia and reoxygenation. In basal conditions, the phospholipid (PL) enrichment with long chain n-3 PUFA resulted in an increased chronotropic response to isoproterenol (ISO) and to phenylephrine (PHE). After posthypoxic reoxygenation, the chronotropic response to beta-AR activation in n-6 CM was significantly enhanced as compared with the control response in normoxia. In opposition, the ISO-induced rise in frequency in n-3 CM in control normoxia and after reoxygenation was similar. In these n-3 CM, the changes in contractile parameters, which accompanied the chronotropic response, were also similar in reoxygenation and in normoxic periods, although the rise in shortening velocity was slightly increased after reoxygenation. In response to PHE addition, only the chronotropic effect of n-6 CM appeared significantly enhanced after hypoxic treatment. These results suggested that increasing n-3 PUFA in PL reduced the increase in alpha- and beta-AR functional responses observed after hypoxia-reoxygenation. This effect may partly account for the assumed cardiac protective effect of n-3 PUFA, through the attenuation of the functional response to catecholamines in the ischemic myocardium.

Correlation between direct ESR spectroscopic measurements and electromechanical and biochemical assessments of exogenous free radical injury in isolated rat cardiac myocytes

Reactive free radical species appear to be involved in the ischemic injury of cardiac muscle, although the mechanisms by which oxygen-derived free radicals affect the heart cell function are not known. In the present study, cultured ventricular myocytes were exposed to an exogenous oxygen radical generating system. The myocyte-enriched, primary cultures were prepared from ventricles of new-born rat heart and exposed to a xanthine/xanthine oxidase (X+XO) system. The transmembrane potentials were recorded with glass microelectrodes. Cell contractions were monitored photometrically. The release of lactate dehydrogenase (LDH) in the medium was analysed. Quantitative measurement and the time course of the radical generation were performed by the electron paramagnetic resonance (EPR) spin trapping technique with the spin trap 5,5-dimethyl-1-pyroline-N-oxide (DMPO). We verified that X and XO alone had no significant functional and biochemical effects. The X+XO system produced a rapid decrease in the action potential amplitude. This effect was accompanied by a strong decrease in contractility and spontaneous rate. The time course of these functional defects were correlated with a progressive efflux of LDH from the cardiomyocytes. Prolonging the exposure to the X+XO system provoked the cessation of the spontaneous beatings and the progressive loss of the resting diastolic potential, together with a near total release of the cellular LDH. The LDH release and the functional depression were both efficiently prevented by catalase. On the contrary, superoxide dismutase (SOD) slowed down but did not protect against the functional and biochemical effects of the free radicals. In comparison, the EPR spectra obtained indicated that the X+XO system was associated with an important generation of superoxide anions but also with a small hydroxyl production. SOD scavenged the superoxide but a small .OH production persisted. Catalase (CAT) did not modify the superoxide generation but decreased the hydroxyl adduct formation. These results suggest that, although the generation of superoxide anions by the X+XO system was higher than the hydroxyl production, the functional injury and enzyme leakage seemed mainly mediated through a hydrogen peroxide-hydroxyl radical pathway. Cultured ventricular myocytes can be thus used as a valuable model to investigate the cellular mechanism of oxidant-induced damage in the heart.

Protective effects of trimetazidine on hypoxic cardiac myocytes from the rat

The electrophysiological effects of the antianginal drug trimetazidine (TMZ) were investigated in cultured rat ventricular myocytes using a substrate-free hypoxia model of ischemia. The transmembrane potentials were recorded with glass microelectrodes and the contractions were simultaneously monitored with a video motion detector. The cardiomyocytes were treated with TMZ (1-5.10(-4) M final concentration) in the bath. The untreated and the drug-treated cells were submitted either to 150 min normoxia or to 150 min hypoxia followed by 90 min reoxygenation in the absence of oxidizable substrate. In normoxic conditions, TMZ did not affect the maximal diastolic potential (MDP) but significantly lowered the plateau potential level (OS) and decreased the upstroke velocity (Vmax) and the spontaneous action potential rate (APR). Conversely, TMZ significantly increased action potential duration at 80% repolarization (APD80). Under substrate-free hypoxia, the untreated cells displayed a progressive contractile failure and an important decrease in OS and APD. In parallel, early postdepolarizations triggering high rate spikes were observed. Prolonging oxygen depletion led to the cessation of the spontaneous electrical activity and thereafter to a gradual decrease in MDP. Near normal rhythmic action potentials and contractions resumed after reoxygenation. Comparatively, the treatment by 5.10(-4) M TMZ almost completely prevented the decrease in plateau amplitude, resting membrane potential, Vmax, APD80, and rate caused by substrate-free hypoxia. Moreover, the hypoxia-induced arrhythmias and the cessation of spontaneous electromechanical activities did not occur in the presence of TMZ (5.10(-4) M). After reoxygenation, the TMZ-treated cells exhibited a higher action potential amplitude than that of the untreated cells, although the TMZ-induced depressive effects on the spontaneous frequency and the Vmax persisted. In conclusion, this study shows that TMZ (5.10(-4) M) is efficient in protecting the isolated cardiac myocytes against the functional alterations induced by substrate-free hypoxia and led thus to a better recovery upon reoxygenation. The cytoprotective action may be linked, at least in part, to apparent ion channel blocking effects of the drug, which appeared in basal conditions at concentrations used in this study.

Influence of phospholipid long chain polyunsaturated fatty acid composition on neonatal rat cardiomyocyte function in physiological conditions and during glucose-free hypoxia-reoxygenation

There is evidence that dietary polyunsaturated fatty acids (PUFA) may protect against cardiovascular diseases, but the involvement of the cardiac muscle cell in this beneficial action remain largely unknown. The present study compared the respective influence of n-3 and n-6 PUFA on the function of cultured neonatal rat cardiomyocytes (CM). Cells were grown for 4 days in media enriched either n-3 (eicosapentaenoic acid, EPA and docosahexaenoic acid, DHA) or n-6 (arachidonic acid, AA) PUFA. The PUFA n-6/n-3 ratio in the phospholipids was close to 1 and 20 in the n-3 and n-6 cells, respectively. The transmembrane potentials were recorded using microelectrodes and the contractions were monitored with a photoelectric device. In physiological conditions, the increase of n-6 PUFA level in the phospholipids resulted in a significant decrease in the maximal rate of initial depolarization (-16%). In opposition, the action potential amplitude and duration were not altered, and the cell contraction outline was not affected. Ischemia was simulated in vitro using a substrate-free, hypoxia-reoxygenation procedure in a specially designed gas-flow chamber. The progressive loss of electrical activity induced by the substrate-free, hypoxic treatment was affected by the n-6/n-3 ratio, since the n-6 rich CM displayed a slower depression of the AP amplitude and duration parameters. Conversely, the recovery of the resting potential (MDP) during reoxygenation was faster in n-3 CM, whereas the recovery of the contraction parameters was unaffected by the fatty acid composition of the cells. These results suggested that, in physiological conditions, the modification of long chain PUFA balance in the phospholipids of cardiac muscle cells may modulate the initial AP upstroke, which is governed by sodium channels. Moreover, the presence of n-3 PUFA appeared to accelerate the electrical depression during substrate-free hypoxia but in turn to allow a faster recovery upon reoxygenation.

Differential regulation of cardiac heme oxygenase-1 and vascular endothelial growth factor mRNA expressions by hemin, heavy metals, heat shock and anoxia

Increasing attention has been paid to the effects of hypoxia, heavy metals and heat shocks on gene expression and to the similarities in their actions. This paper compares mRNA levels of two putative hypoxia, heavy metal and heat shock sensitive genes: heme oxygenase-1 (HO-1) and vascular endothelial growth factor (VEGF) in myocyte-enriched cultures of neonatal rat heart cells. HO-1 mRNA expression is stimulated by hemin, Cd2+, Co2+ and heat shocks but not by Ni2+ or Mn2+. It is stimulated by long (13h) but not by short (4h) periods of anoxia. Conversely, VEGF mRNA expression is stimulated by short as well as long periods of anoxia, by Cd2+, Co2+, Ni2+ and Mn2+ but not by hemin or heat shocks. The results suggest that heavy metals, anoxia and heat shocks exert their effects on VEGF and HO-1 mRNA expression through separate though potentially overlapping mechanisms. Increased expressions of HO-1 and VEGF may be both cardioprotective under hypoxic/ischemic conditions.