Ischemia/reperfusion-induced death of cardiac myocytes: possible involvement of nitric oxide in the coordination of ATP supply and demand during ischemia

Moderate therapeutic hypothermia induces multimodal protective effects in oxygen-glucose deprivation/reperfusion injured cardiomyocytes

OBJECTIVE

Therapeutic hypothermia has been shown to attenuate myocardial cell death due to ischemia/reperfusion injury. However, cellular mechanisms of cooling remain to be elucidated. Especially during reperfusion, mitochondrial dysfunction contributes to cell death by releasing apoptosis inductors. The aim of the present study was to investigate the effects of moderate therapeutic hypothermia (33.5°C) on mitochondrial mediated apoptosis in ischemia/reperfusion-injured cardiomyocytes.

METHODS

Ischemic injury was simulated by oxygen-glucose deprivation for 6h in glucose/serum-free medium at 0.2% O₂ in mouse atrial HL-1 cardiomyocytes. Simulation of reperfusion was achieved by restoration of nutrients in complete supplemented medium and incubation at 21% O₂. Early application of therapeutic hypothermia, cooling during the oxygen-glucose deprivation phase, was initiated after 3h of oxygen-glucose deprivation and maintained for 24h. Mitochondrial membrane integrity was assessed by cytochrome c and AIF protein releases. Furthermore, mitochondria were stained with MitoTracker Red and intra-cellular cytochrome c localization was visualized by immunofluorescence staining. Moreover, anti-apoptotic Bcl-2 and Hsp70 as well as phagophore promoting LC3-II protein expressions were analyzed by Western-blot analysis.

RESULTS

Therapeutic hypothermia initiated during oxygen-glucose deprivation significantly reduced mitochondrial release of cytochrome c and AIF in cardiomyocytes during reperfusion. Secondly, anti-apoptotic Bcl-2/Bax ratio and Hsp70 protein expressions were significantly upregulated due to hypothermia, indicating an inhibition of both caspase-dependent and -independent apoptosis. Furthermore, cardiomyocytes treated with therapeutic hypothermia showed increased LC3-II protein levels associated with the mitochondria during the first 3h of reperfusion, indicating the initiation of phagophores formation and sequestration of presumably damaged mitochondrion.

CONCLUSION

Early application of therapeutic hypothermia effectively inhibited cardiomyocyte cell death due to oxygen-glucose deprivation/reperfusion-induced injury via multiple pathways. As hypothermia preserved mitochondrial membrane integrity, which resulted in reduced cytochrome c and AIF releases, induction of both caspase-dependent and -independent apoptosis was minimized. Secondly, cooling attenuated intrinsic apoptosis via Hsp70 upregulation and increasing anti-apoptotic Bcl-2/Bax ratio. Moreover, therapeutic hypothermia promoted mitochondrial associated LC3-II during the early phase of reperfusion, possibly leading to the sequestration and degradation of damaged mitochondrion to attenuate the activation of cell death.

The Role of Extracellular Adenosine Triphosphate in Ischemic Organ Injury

OBJECTIVES

Ischemic tissue injury contributes to significant morbidity and mortality and is implicated in a range of pathologic conditions, including but not limited to myocardial infarction, ischemic stroke, and acute kidney injury. The associated reperfusion phase is responsible for the activation of the innate and adaptive immune system, further accentuating inflammation. Adenosine triphosphate molecule has been implicated in various ischemic conditions, including stroke and myocardial infarction.

STUDY SELECTION

Adenosine triphosphate is a well-defined intracellular energy transfer and is commonly referred to as the body's "energy currency." However, Laboratory studies have demonstrated that extracellular adenosine triphosphate has the ability to initiate inflammation and is therefore referred to as a damage-associated molecular pattern. Purinergic receptors-dependent signaling, proinflammatory cytokine release, increased Ca influx into cells, and subsequent apoptosis have been shown to form a common underlying extracellular adenosine triphosphate molecular mechanism in ischemic organ injury.

CONCLUSIONS

In this review, we aim to discuss the molecular mechanisms behind adenosine triphosphate-mediated ischemic tissue injury and evaluate the role of extracellular adenosine triphosphate in ischemic injury in specific organs, in order to provide a greater understanding of the pathophysiology of this complex process. We also appraise potential future therapeutic strategies to limit damage in various organs, including the heart, brain, kidneys, and lungs.

Coronary flow and oxidative stress during local anaphylactic reaction in isolated mice heart: the role of nitric oxide (NO)

The aim of this study was to assess the role of nitric oxide (NO) in cardiac anaphylaxis regarding changes in coronary reactivity and oxidative status of the mice heart. The animals were divided into two groups: experimental group (CBA, iNOS(-/-) mice) and control group: wild-type mice (CBA/H). The hearts of male mice (n = 24; 6-8 weeks old, body mass 20-25 g, 12 in each experimental group) were excised and retrogradely perfused according to the Langendorff technique at a constant perfusion pressure (70 cm H2O). Cardiac anaphylaxis was elicited by injection of solution (1 mg/1 ml) of ovalbumin into the aortic cannula. For the next 10 min, in intervals of 2 min (0-2, 2-4, 4-6, 6-8, 8-10 min) coronary flow (CF) rates were measured and samples of coronary effluent were collected. Markers of oxidative stress including index of lipid peroxidation measured as thiobarbituric acid-reactive substances (TBARS), NO measured in the form of nitrites (NO2(-)), superoxide anion radical (O2(-)), and hydrogen peroxide (H2O2) in the coronary venous effluent were assessed spectrophotometrically. After the ovalbumin challenge, CF was significantly lower in the wild mice group. NO and H2O2 release were significantly higher in iNOS(-/-) mice group. TBARS and O2(-) values did not vary significantly between wild and iNOS(-/-) mice groups. Our results indicate that coronary vasoconstriction during cardiac anaphylaxis does not necessarily depend on inducible nitric oxide synthase (iNOS)/NO activity and that iNOS/NO pathway may not be an only influential mediator of redox changes in this model of cardiac anaphylaxis.

Glucagon effects on 3H-histamine uptake by the isolated guinea-pig heart during anaphylaxis

We estimated the influence of acute glucagon applications on ³H-histamine uptake by the isolated guinea-pig heart, during a single ³H-histamine passage through the coronary circulation, before and during anaphylaxis, and the influence of glucagon on level of histamine, NO, O₂⁻, and H₂O₂ in the venous effluent during anaphylaxis. Before anaphylaxis, glucagon pretreatment does not change ³H-histamine Umax and the level of endogenous histamine. At the same time, in the presence of glucagon, ³H-histamine Unet is increased and backflux is decreased when compared to the corresponding values in the absence of glucagon. During anaphylaxis, in the presence of glucagon, the values of ³H-histamine Umax and Unet are significantly higher and backflux is significantly lower in the presence of glucagon when compared to the corresponding values in the absence of glucagon. The level of endogenous histamine during anaphylaxis in the presence of glucagon (6.9–7.38 × 10⁻⁸  μM) is significantly lower than the histamine level in the absence of glucagon (10.35–10.45 × 10⁻⁸  μM). Glucagon pretreatment leads to a significant increase in NO release (5.69 nmol/mL) in comparison with the period before glucagon administration (2.49 nmol/mL). Then, in the presence of glucagon, O₂⁻ level fails to increase during anaphylaxis. Also, our results show no significant differences in H₂O₂ levels before, during, and after anaphylaxis in the presence of glucagon, but these values are significantly lower than the corresponding values in the absence of glucagon. In conclusion, our results show that glucagon increases NO release and prevents the increased release of free radicals during anaphylaxis, and decreases histamine level in the venous effluent during cardiac anaphylaxis, which may be a consequence of decreased histamine release and/or intensified histamine capturing by the heart during anaphylaxis.

Nrf2 activation supports cell survival during hypoxia and hypoxia/reoxygenation in cardiomyoblasts; the roles of reactive oxygen and nitrogen species

Adaptive mechanisms involving upregulation of cytoprotective genes under the control of transcription factors such as Nrf2 exist to protect cells from permanent damage and dysfunction under stress conditions. Here we explore of the hypothesis that Nrf2 activation by reactive oxygen and nitrogen species modulates cytotoxicity during hypoxia (H) with and without reoxygenation (H/R) in H9C2 cardiomyoblasts. Using MnTBap as a cell permeable superoxide dismutase (SOD) mimetic and peroxynitrite scavenger and L-NAME as an inhibitor of nitric oxide synthase (NOS), we have shown that MnTBap inhibited the cytotoxic effects of hypoxic stress with and without reoxygenation. However, L-NAME only afforded protection during H. Under reoxygenation, conditions, cytotoxicity was increased by the presence of L-NAME. Nrf2 activation was inhibited independently by MnTBap and L-NAME under H and H/R. The increased cytotoxicity and inhibition of Nrf2 activation by the presence of L-NAME during reoxygenation suggests that NOS activity plays an important role in cell survival at least in part via Nrf2-independent pathways. In contrast, O₂^−• scavenging by MnTBap prevented both toxicity and Nrf2 activation during H and H/R implying that toxicity is largely dependent on O₂^−•.To confirm the importance of Nrf2 for myoblast metabolism, Nrf2 knockdown with siRNA reduced cell survival by 50% during 4 h hypoxia with and without 2 h of reoxygenation and although cellular glutathione (GSH) was depleted during H and H/R, GSH loss was not exacerbated by Nrf2 knockdown. These data support distinctive roles for ROS and RNS during H and H/R for Nrf2 induction which are important for survival independently of GSH salvage.

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Cardiomyoblast toxicity during hypoxia is dependent on O₂^−• and NO^•.

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Nrf2 activation is important for cardiomyoblast survival during hypoxia or hypoxia/reoxygenation, but, restoration of GSH is not required.

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NOS activity is essential for the adaptation of cardiomyoblasts to hypoxia/reoxygenation but survival may be independent of Nrf2.

Spatio-temporal spread of neuronal death after focal photolysis of caged glutamate in neuron/astrocyte co-cultures

Glutamate-mediated excitotoxicity is now accepted as a major mechanism of ischemic neuronal damage. In the infarct core region, massive neuronal death is observed, but neurons in the surroundings of the core (ischemic penumbra) seem viable at the time of stroke. Several hours or days after a stroke, however, many neurons in the penumbra will undergo delayed neuronal death (DND). The mechanisms responsible for such DND are not fully understood. In this study, we investigated whether and how glutamate-mediated localized excitotoxic neuronal death affects surrounding neurons and astrocytes. To induce spatially-restricted excitotoxic neuronal death, a caged glutamate was focally photolyzed by a UV flash in neuron/astrocyte co-cultures. Uncaging of the glutamate resulted in acute neuronal death in the flashed area. After that, DND was observed in the surroundings of the flashed area late after the uncaging. In contrast, DND was not observed in neuron-enriched cultures, suggesting that functional changes in astrocytes, not neurons, after focal acute neuronal death were involved in the induction of DND. The present in vitro study showed that the spatially-restricted excitotoxic neuronal death resulted in DND in the surroundings of the flashed area, and suggested that the nitric oxide (NO)-induced reduction in the expression of astrocytic GLT-1 was responsible for the occurrence of the DND.

Real time measurement of myocardial oxygen dynamics during cardiac ischemia-reperfusion of rats

Because oxygen plays a critical role in the pathophysiology of myocardial injury during subsequent reperfusion, as well as ischemia, the accurate measurement of myocardial oxygen tension is crucial for the assessment of myocardial viability by ischemia-reperfusion (IR) injury. Therefore, we utilized a sol-gel derived electrochemical oxygen microsensor to monitor changes in oxygen tension during myocardial ischemia-reperfusion. We also analyzed differences in oxygen tension recovery in post-ischemic myocardium depending on ischemic time to investigate the correlation between recovery parameters for oxygen tension and the severity of IR injury. An oxygen sensor was built using a xerogel-modified platinum microsensor and a coiled Ag/AgCl reference electrode. Rat hearts were randomly divided into 5 groups: control (0 min ischemia), I-10 (10 min ischemia), I-20 (20 min ischemia), I-30 (30 min ischemia), and I-40 (40 min ischemia) groups (n = 3 per group, respectively). After the induction of ischemia, reperfusion was performed for 60 min. As soon as the ischemia was initiated, oxygen tension rapidly declined to near zero levels. When reperfusion was initiated, the changes in oxygen tension depended on ischemic time. The normalized peak level of oxygen tension during the reperfusion episode was 188 ± 27 in group I-10, 120 ± 24 in group I-20, 12.5 ± 10.6 in group I-30, and 1.24 ± 1.09 in group I-40 (p < 0.001, n = 3, respectively). After 60 min of reperfusion, the normalized restoration level was 129 ± 30 in group I-10, 88 ± 4 in group I-20, 3.40 ± 4.82 in group I-30, and 0.99 ± 0.94 in group I-40 (p < 0.001, n = 3, respectively). The maximum and restoration values of oxygen tension in groups I-30 and I-40 after reperfusion were lower than pre-ischemic values. In particular, oxygen tension in the I-40 group was not recovered at all. These results were also demonstrated by TTC staining. We suggest that these recovery parameters could be utilized as an index of tissue injury and severity of ischemia. Therefore, quantitative measurements of oxygen tension dynamics in the myocardium would be helpful for evaluation of the cardioprotective effects of therapeutic treatments such as drug administration.

Is preconditioning by oxytocin administration mediated by iNOS and/or mitochondrial K(ATP) channel activation in the in vivo anesthetized rabbit heart?

AIMS

Oxytocin (OXT) pretreatment protects the heart during ischemia-reperfusion injury by activating ATP-dependent potassium (K(ATP)) channels. The aim of the current study was to elucidate the roles of nitric oxide synthaseNOS and myocardial biochemistry in the cardioprotective effects of OXT and ischemic preconditioning (IPC).

MAIN METHODS

Male New Zealand White anesthetized rabbits (13 groups) were subjected to 30 min of occlusion of the left coronary artery and 120 min of reperfusion with or without IPC.

KEY FINDINGS

IPC (1 cycle), OXT (0.03 μg/kg, i.p.) or IPC + OXT yield significant infarct size reductions (21.8±1.5%, 20.5±1.2% and 19.4±1.4%, respectively, versus 38.9±3.5% in the S-CONT group; P<0.01) and antiarrhythmic effects, including VF (0%, 0% and 0%, versus 50% in S-CONT group; P<0.05) sustained VT (13%, 13% and 13%, versus 100% in S-CONT group; P<0.005) and other arrhythmias (25%, 13% and 25%, versus 100% in S-CONT group; P<0.005, P<0.01 and P<0.005, respectively). Atosiban (ATO, a selective OXT receptor antagonist), 5-HD and L-NAME (a nonspecific NOS inhibitor) abolished the beneficial effects of IPC and OXT, suggesting that the benefits are achieved via selective activation of OXT receptors, mitochondrial K(ATP) channels and NO. An iNOS inhibitor (1400 W) blocked the beneficial effects of IPC but not OXT. The IPC, OXT, IPC + OXT and 1400 W + OXT interventions significantly preserved ATP levels in the heart.

SIGNIFICANCE

This study demonstrates similarities between acute OXT pretreatment and IPC in terms of infarct size reduction, antiarrhythmic activity, and metabolic status.

Negative-feedback regulation of ATP release: ATP release from cardiomyocytes is strictly regulated during ischemia

Extracellular ATP acts as a potent agonist on cardiomyocytes, inducing a broad range of physiological responses via P2 purinoceptors. Its concentration in the interstitial space within the heart is elevated during ischemia or hypoxia due to its release from a number of cell types, including cardiomyocytes. However, the exact mechanism responsible for the release of ATP from cardiomyocytes during ischemia is not known. In this study, we investigated whether and how the release of ATP was strictly regulated during ischemia in cultured neonatal rat cardiomyocytes. Ischemia was mimicked by oxygen-glucose deprivation (OGD). Exposure of cardiomyocytes to OGD resulted in an increase in the concentration of extracellular ATP shortly after the onset of OGD (15 min), and the increase was reversed by treatment with blockers of maxi-anion channels. Unexpectedly, at 1 and 2h after the onset of OGD, the blocking of maxi-anion channels increased the concentration of extracellular ATP, and the increase was significantly suppressed by co-treatment with blockers of hemichannels, suggesting that ATP release via maxi-anion channels was involved in the suppression of ATP release via hemichannels during persistent OGD. Here we show the possibility that the release of ATP from cardiomyocytes was strictly regulated during ischemia by negative-feedback mechanisms; that is, maxi-anion channel-derived ATP-induced suppression of ATP release via hemichannels in cardiomyocytes.

The effect of L-arginine on bladder dysfunction following ovariectomy in a rabbit model

INTRODUCTION AND HYPOTHESIS

The present study was designed to investigate the effect of nitric oxide precursor, L: -arginine, on bladder function following ovariectomy.

METHODS

Twenty-eight New Zealand white female rabbits were separated into seven groups. Groups 1 to 6 underwent ovariectomy surgery. Among them, groups 1 and 2 received ovariectomy without treating with L-arginine. Groups 3, 4, 5, and 6 were given high L-arginine diet and were sacrificed 1, 3, 7, and 14 days after ovariectomy, respectively. Group 7 served as the control group. The effects of L: -arginine on the contractility of bladder tissues were determined in response to various stimulations. In addition, L-arginine effects on the expression of Rho kinase (ROK), protein kinase C potentiated inhibitor (CPI-17), caldesmon (CaD), and calponin (CaP) were studied by immunoblotting.

RESULTS

Ovariectomy significantly decreases contractile response to all forms of stimulation. Feeding rabbits L: -arginine significantly increases contractile response at 1 day following ovariectomy, but the response decreases to the control level by 14 days. Ovariectomy increases the expressions of both isoforms of CaD, CaP, and CPI-17; L-arginine treatment induces ROK underexpression, while CaP is overexpressed in the early few days of ovariectomy but returns to the control level at 2 weeks after ovariectomy.

CONCLUSIONS

Ovariectomy appreciably reduced bladder contractility. Treatment with L-arginine reversed the ovariectomy-induced bladder dysfunction. Decreased bladder contractile response was observed in the early days following ovariectomy.

Protective effects of muscone on ischemia-reperfusion injury in cardiac myocytes

ETHNOPHARMACOLOGICAL RELEVANCE

Musk has been traditionally used in Chinese medicine as the main ingredient of many formulations for the treatment of chest pain and angina pectoris.

AIM OF THE STUDY

To investigate the protective effects of muscone (the active ingredient of musk) on ischemia-reperfusion (I/R) injury induced by hypoxia and low glucose in primary cultured rat cardiac myocytes.

MATERIALS AND METHODS

Primary cultures of neonatal rat cardiac myocytes were subjected to ischemia-reperfusion in media, with or without muscone. Cell viability, release of lactic acid dehydrogenase (LDH), superoxide dismutase (SOD) activity, malondialdehyde (MDA) levels, creatine kinase (CK) and caspase-3 activities, as well as intracellular free Ca(2+) concentrations, were measured. Cellular apoptosis and mitochondrial membrane potential (MMP) were assessed by flow cytometry, and the expression of Bcl-2 and Bax proteins was assessed by Western blotting.

RESULTS

Following the exposure of cardiac myocytes to ischemia-reperfusion, there was a marked decrease in pulsating frequency, cell viability, SOD activity, MMP, and the expression of Bcl-2 protein, accompanied by increased LDH release, MDA production, CK and caspase-3 activities, intracellular free Ca(2+) concentrations, rate of apoptosis, and expression of Bax protein. Pretreatment with muscone (0.215, 0.43, 0.86 μg/mL) prior to I/R injury significantly attenuated the above changes.

CONCLUSION

Muscone has a protective effect against I/R injury in cardiac myocytes, indicating that muscone may potentially provide therapeutic benefit in I/R injury by inhibiting cellular oxidative stress and apoptosis.

Oxidative stress-induced formation of a positive-feedback loop for the sustained activation of p38 MAPK leading to the loss of cell division in cardiomyocytes soon after birth

Shortly after birth, mammalian cardiomyocytes irreversibly exit from the cell cycle and become terminally differentiated. The cellular mechanisms responsible for the cessation of cell division and terminal differentiation of cardiomyocytes soon after birth have intrigued developmental biologists as well as cardiovascular physicians, but the genetic cues for the irreversible exit from the cell cycle soon after birth remain largely unknown. We examined whether and if so how oxidative stress to mammalian hearts during fetal-neonatal transition produces changes in the proliferative activity and terminal differentiation of cardiomyocytes. Scavenging of reactive oxygen species (ROS) during fetal-neonatal transition, especially after birth, resulted in an increase in the proliferative activity and a decrease in the ratio of binucleated cardiomyocytes. Exposure to ROS in cultured cardiomyocytes increased the activity of p38 MAPK and the expression of connexin 43 (Cx43). Not only knockdown of Cx43 using siRNA but also the inhibition of p38 MAPK activity resulted in a significant decrease in the production of ROS in cardiomyocytes, suggesting that the signaling pathway ROS-p38 MAPK-Cx43 (especially, Cx43 at mitochondria, mtCx43) constituted a closed regulatory system with positive feedback. In addition, continuous scavenging of ROS or suppression of p38 MAPK activity for 4 days after birth resulted in a significant decrease in the expression of mtCx43 and in the number of binucleated cardiomyocytes. This study demonstrated that the ROS-induced formation of a positive-feedback loop ROS-p38 MAPK-mtCx43 for the sustained activation of p38 MAPK soon after birth possibly contributes to the loss of cell division and binucleation in mammalian cardiomyocytes.

Functional significance of the negative-feedback regulation of ATP release via pannexin-1 hemichannels under ischemic stress in astrocytes

The opening of pannexin-1 (Px1) hemichannels is regulated by the activity of P2X(7) receptors (P2X(7)Rs). At present, however, little is known about how extracellular ATP-sensitive P2X(7)Rs regulates the opening and closure of Px1 hemichannels. Several lines of evidence suggest that P2X(7)Rs are activated under pathological conditions such as ischemia, resulting in the opening of Px1 hemichannels responsible for the massive influx of Ca(2+) from the extracellular space and the release of ATP from the cytoplasm, leading to cell death. Here we show in cultured astrocytes that the suppression of the activity of P2X(7)Rs during simulated ischemia (oxygen/glucose deprivation, OGD) resulted in the opening of Px1 hemichannels, leading to the enhanced release of ATP. In addition, the suppression of the activity of P2X(7)Rs during OGD resulted in a significant increase in astrocytic damage. Both the P2X(7)Rs suppression-induced enhancement of the release of ATP and cell damage were reversed by co-treatment with blockers of Px1 hemichannels, suggesting that suppression of the activity of PX(7)Rs resulted in the opening of Px1 hemichannels. All these findings suggested the existence of a negative-feedback loop regulating the release of ATP via Px1 hemichannels; ATP-induced suppression of ATP release. The present study indicates that ATP, released through Px1 hemichannels, activates P2X(7)Rs, resulting in the closure of Px1 hemichannels during ischemia. This negative-feedback mechanism, suppressing the loss of cellular ATP and Ca(2+) influx, might contribute to the survival of astrocytes under ischemic stress.

The opposite roles of nNOS in cardiac ischemia-reperfusion-induced injury and in ischemia preconditioning-induced cardioprotection in mice

The role of neuronal nitric oxide synthase (nNOS) in cardiac ischemia-reperfusion (IR) and ischemia preconditioning (IP) is still controversial. Here, we focused on the possible roles of nNOS in cardiac IR and IP. Wild type C57BL/6 (WT) mice were subjected to coronary artery occlusion for 30 min followed by 24-h reperfusion (IR). Cardiac injury (infarct size and apoptotic cell number) was increased, associated with elevation of oxidative stress (lipid peroxidation) and nitrative stress (nitrotyrosine formation). A potent nNOS inhibitor, L-VNIO, and a superoxide dismutase mimetic and peroxynitrite scavenger, MnTBAP, significantly reduced IR-induced increases of oxidative/nitrative stress and cardiac injury. IR-induced cardiac injury in nNOS(-/-) (KO) mice was significantly lower than that in WT mice. MnTBAP markedly reduced IR-induced cardiac injury by suppression of oxidative/nitrative stress in KO mice. Cardiac IP was performed by three cycles of 5-min IR before 30-min ischemia followed by 24-h reperfusion. IP attenuated IR-induced cardiac injury in WT mice associated with reductions of oxidative/nitrative stress. IP-induced reduction of cardiac injury and oxidative/nitrative stress were eliminated by pretreatment with L-VNIO. In contrast with WT mice, IP had no protective effects in nNOS KO mice. In conclusion, nNOS played a dual role during cardiac IR and IP; nNOS exacerbated IR-induced injury by increasing oxidative/nitrative stress and contributed to IP-induced protection by inhibition of oxidative/nitrative stress.

Characteristics and function of cardiac mitochondrial nitric oxide synthase

We used laser scanning confocal microscopy in combination with the nitric oxide (NO)-sensitive fluorescent dye DAF-2 and the reactive oxygen species (ROS)-sensitive dyes CM-H(2)DCF and MitoSOX Red to characterize NO and ROS production by mitochondrial NO synthase (mtNOS) in permeabilized cat ventricular myocytes. Stimulation of mitochondrial Ca(2+) uptake by exposure to different cytoplasmic Ca(2+) concentrations ([Ca(2+)](i) = 1, 2 and 5 microm) resulted in a dose-dependent increase of NO production by mitochondria when L-arginine, a substrate for mtNOS, was present. Collapsing the mitochondrial membrane potential with the protonophore FCCP or blocking the mitochondrial Ca(2+) uniporter with Ru360 as well as blocking the respiratory chain with rotenone or antimycin A in combination with oligomycin inhibited mitochondrial NO production. In the absence of L-arginine, mitochondrial NO production during stimulation of Ca(2+) uptake was significantly decreased, but accompanied by increase in mitochondrial ROS production. Inhibition of mitochondrial arginase to limit L-arginine availability resulted in 50% inhibition of Ca(2+)-induced ROS production. Both mitochondrial NO and ROS production were blocked by the nNOS inhibitor (4S)-N-(4-amino-5[aminoethyl]aminopentyl)-N'-nitroguanidine and the calmodulin antagonist W-7, while the eNOS inhibitor L-N(5)-(1-iminoethyl)ornithine (L-NIO) or iNOS inhibitor N-(3-aminomethyl)benzylacetamidine, 2HCl (1400W) had no effect. The superoxide dismutase mimetic and peroxynitrite scavenger MnTBAP abolished Ca(2+)-induced ROS generation and increased NO production threefold, suggesting that in the absence of MnTBAP either formation of superoxide radicals suppressed NO production or part of the formed NO was transformed quickly to peroxynitrite. In the absence of L-arginine, mitochondrial Ca(2+) uptake induced opening of the mitochondrial permeability transition pore (PTP), which was blocked by the PTP inhibitor cyclosporin A and MnTBAP, and reversed by L-arginine supplementation. In the presence of the mtNOS cofactor (6R)-5,6,7,8,-tetrahydrobiopterin (BH(4); 100 microm) mitochondrial ROS generation and PTP opening decreased while mitochondrial NO generation slightly increased. These data demonstrate that mitochondrial Ca(2+) uptake activates mtNOS and leads to NO-mediated protection against opening of the mitochondrial PTP, provided sufficient availability of l-arginine and BH(4). In conclusion, our data show the importance of L-arginine and BH(4) for cardioprotection via regulation of mitochondrial oxidative stress and modulation of PTP opening by mtNOS.

Near infrared light protects cardiomyocytes from hypoxia and reoxygenation injury by a nitric oxide dependent mechanism

Photobiomodulation with near infrared light (NIR) provides cellular protection in various disease models. Previously, infrared light emitted by a low-energy laser has been shown to significantly improve recovery from ischemic injury of the canine heart. The goal of this investigation was to test the hypothesis that NIR (670 nm) from light emitting diodes produces cellular protection against hypoxia and reoxygenation-induced cardiomyocyte injury. Additionally, nitric oxide (NO) was investigated as a potential cellular mediator of NIR. Our results demonstrate that exposure to NIR at the time of reoxygenation protects neonatal rat cardiomyocytes and HL-1 cells from injury, as assessed by lactate dehydrogenase release and MTT assay. Similarly, indices of apoptosis, including caspase 3 activity, annexin binding and the release of cytochrome c from mitochondria into the cytosol, were decreased after NIR treatment. NIR increased NO in cardiomyocytes, and the protective effect of NIR was completely reversed by the NO scavengers carboxy-PTIO and oxyhemoglobin, but only partially blocked by the NO synthase (NOS) inhibitor L-NMMA. Mitochondrial metabolism, measured by ATP synthase activity, was increased by NIR, and NO-induced inhibition of oxygen consumption with substrates for complex I or complex IV was reversed by exposure to NIR. Taken together these data provide evidence for protection against hypoxia and reoxygenation injury in cardiomyocytes by NIR in a manner that is dependent upon NO derived from NOS and non-NOS sources.

Hypoxia-induced regulation of nitric oxide synthase in cardiac endothelial cells and myocytes and the role of the PI3-K/PKB pathway

The roles of endothelial nitric oxide synthase (eNOS), and its putative association with protein kinase B (PKB), and inducible nitric oxide synthase (iNOS) are not well characterized in hypoxic cardiac cells and there is a lack of studies that measure nitric oxide (NO) directly.

OBJECTIVE

To measure NO production in cardiomyocytes and cardiac microvascular endothelial cells (CMECs) under baseline and hypoxic conditions and to evaluate the expression, regulation and activation of eNOS, iNOS and PKB. The effect of PI3-K/PKB inhibition on NO production and eNOS expression/activation was also investigated.

METHODS

Adult rat cardiomyocytes and rat CMECs were made hypoxic by cell pelleting and low PO(2) incubation. Intracellular NO was measured by FACS analysis of DAF-2/DA fluorescence, and eNOS, iNOS and PKB were evaluated by Western blotting or flow cytometry. Upstream PKB inhibition was achieved with wortmannin.

RESULTS

(1) NO levels increased in both cell types after exposure to hypoxia. (2) In hypoxic CMECs, eNOS was upregulated and activated, no iNOS expression was observed and PKB was activated. (3) In myocytes, hypoxia did not affect eNOS expression, but increased its activation. Activated PKB also increased during hypoxia. FACS analysis showed increased iNOS in hypoxic myocytes. (4) Wortmannin resulted in decreased hypoxia-induced NO production and reduced activated eNOS levels.

CONCLUSIONS

Cardiomyocytes and CMECs show increased NO production during hypoxia. eNOS seems to be the main NOS isoform involved as source of the increased NO generation, although there may be a role for iNOS and other non-eNOS sources of NO in the hypoxic myocytes. Hypoxia-induced PKB and eNOS activation occurred simultaneously in both cell types, and the PI3-K/PKB pathway was associated with hypoxia-induced NO production via eNOS activation.

Fluctuations in the concentration of extracellular ATP synchronized with intracellular Ca(2+) oscillatory rhythm in cultured cardiac myocytes

Isolated and cultured neonatal cardiac myocytes contract spontaneously and cyclically. The intracellular concentration of free Ca(2+) also changes rhythmically in association with the rhythmic contraction of myocytes (Ca(2+) oscillation). Both the contraction and Ca(2+) oscillatory rhythms are synchronized among myocytes, and intercellular communication via gap junctions has been considered primarily responsible for the synchronization. However, a recent study has demonstrated that intercellular communication via extracellular ATP-purinoceptor signaling is also involved in the intercellular synchronization of intracellular Ca(2+) oscillation. In this study, we aim to elucidate whether the concentration of extracellular ATP changes cyclically and contributes to the intercellular synchronization of Ca(2+) oscillation among myocytes. In almost all the cultured cardiac myocytes at four days in vitro (4 DIV), intracellular Ca(2+) oscillations were synchronized with each other. The simultaneous measurement of the concentration of extracellular ATP and intracellular Ca(2+) revealed the extracellular concentration of ATP actually oscillated concurrently with the intracellular Ca(2+) oscillation. In addition, power spectrum and cross-correlation analyses suggested that the treatment of cultured cardiac myocytes with suramin, a blocker of P2 purinoceptors, resulted in the asynchronization of Ca(2+) oscillatory rhythms among cardiac myocytes. Treatment with suramin also resulted in a significant decrease in the amplitudes of the cyclic changes in both intracellular Ca(2+) and extracellular ATP. Taken together, the present study demonstrated the possibility that the concentration of extracellular ATP changes cyclically in association with intracellular Ca(2+), contributing to the intercellular synchronization of Ca(2+) oscillation among cultured cardiac myocytes.

Pretreatment with sarafotoxin 6c prior to coronary occlusion protects against infarction and arrhythmias via cardiomyocyte mitochondrial K(ATP) channel activation in the intact rabbit heart during ischemia/reperfusion

BACKGROUND

Endothelial ET(B) receptor activation by exogenously administered sarafotoxin 6c(a snake venom peptide with a sequence homology to ET-1 prior to ischemia activates release of nitric oxide(NO) and previous studies have shown that NO facilitates mitochondrial K(ATP) activation in cardiac cells and cardioprotection.

OBJECTIVES AND METHODS

The aim of this investigation was to test whether the administration of sarafotoxin 6c(a selective ET(B) receptor agonist) has cardioprotective and antiarrhythmic effects against ischemia and reperfusion injury in a well-standardized model of reperfusion arrhythmias in anesthetized adult male rabbits (n = 53) subjected to 30 min occlusion of the left coronary artery followed by 120 min of reperfusion.

RESULTS

Pretreatment with sarafotoxin 6c (0.24 nmol/kg, i.v.) prior to the period of coronary occlusion offers significant infarct size reduction (19.1 +/- 2.0% versus 39.7 +/- 3.7% in the saline control group; P < 0.01) and antiarrhythmic effects. Sarafotoxin 6c treatment significantly attenuated the incidence of life-threatening arrhythmias like sustained VT (13 versus 100% in the saline control group; P < 0.005) and other arrhythmias (25 versus 100% in the saline control group; P < 0.005), and increased the number of surviving animals without arrhythmias. Pretreatment with 5-HD but not HMR 1883 abolished the beneficial effects of sarafotoxin 6c on reperfusion induced arrhythmias and cardioprotection suggesting that benefits have been achieved via the selective activation of cardiomyocyte mitochondrial K(ATP) channels. Sarafotoxin 6c evoked NO release and selective activation of mitoK(ATP) channels in cardiomyocytes contributes to cardioprotection and antiarrhythmic activity during ischemia-reperfusion in the anesthetized rabbit.

CONCLUSIONS

We conclude that the selective activation of ET(B) receptors by sarafotoxin 6c prior to coronary occlusion contributes to cardioprotective and antiarrhythmic properties.

Homocysteine affects cardiomyocyte viability: concentration-dependent effects on reversible flip-flop, apoptosis and necrosis

BACKGROUND

Hyperhomocysteinaemia (HHC) is thought to be a risk factor for cardiovascular disease including heart failure. While numerous studies have analyzed the role of homocysteine (Hcy) in the vasculature, only a few studies investigated the role of Hcy in the heart. Therefore we have analyzed the effects of Hcy on isolated cardiomyocytes.

METHODS

H9c2 cells (rat cardiomyoblast cells) and adult rat cardiomyocytes were incubated with Hcy and were analyzed for cell viability. Furthermore, we determined the effects of Hcy on intracellular mediators related to cell viability in cardiomyocytes, namely NOX2, reactive oxygen species (ROS), mitochondrial membrane potential (DeltaPsi (m)) and ATP concentrations.

RESULTS

We found that incubation of H9c2 cells with 0.1 mM D,L-Hcy (= 60 microM L-Hcy) resulted in an increase of DeltaPsi (m) as well as ATP concentrations. 1.1 mM D,L-Hcy (= 460 microM L-Hcy) induced reversible flip-flop of the plasma membrane phospholipids, but not apoptosis. Incubation with 2.73 mM D,L-Hcy (= 1.18 mM L-Hcy) induced apoptosis and necrosis. This loss of cell viability was accompanied by a thread-to-grain transition of the mitochondrial reticulum, ATP depletion and nuclear NOX2 expression coinciding with ROS production as evident from the presence of nitrotyrosin residues. Notably, only at this concentration we found a significant increase in S-adenosylhomocysteine which is considered the primary culprit in HHC.

CONCLUSION

We found concentration-dependent effects of Hcy in cardiomyocytes, varying from induction of reversible flip-flop of the plasma membrane phospholipids, to apoptosis and necrosis.

Changes in the fluctuation of the contraction rhythm of spontaneously beating cardiac myocytes in cultures with and without cardiac fibroblasts

The heart functions as a syncytium of cardiac myocytes and surrounding supportive non-myocytes such as fibroblasts. There is a possibility that a variety of non-myocyte-derived factors affect the maturation of cardiac myocytes in the development of the heart. Cultured neonatal cardiac myocytes contract spontaneously and cyclically. The fluctuation of beating rhythm varies depending on the strength of coupling through gap junctions among cardiac myocytes, indicating that the development of intercellular communication via gap junctions is crucial to the stability of contraction rhythm in cardiac myocytes. In this study, we aimed at elucidating whether and how cardiac fibroblasts affect the development of cardiac myocytes from the point of view of the changes in the fluctuation of the contraction rhythm of cardiac myocytes in cardiac myocyte-fibroblast co-cultures. The present study suggested that cardiac fibroblasts co-cultured with cardiac myocytes enhanced the intercellular communication among myocytes via gap junctions, thereby stabilizing the spontaneous contraction rhythm of cultured cardiac myocytes.

Pharmacological preconditioning by levosimendan is mediated by inducible nitric oxide synthase and mitochondrial KATP channel activation in the in vivo anesthetized rabbit heart model

BACKGROUND

Provocation of fatal cardiac arrhythmias has limited the use of inotropic agents as heart failure therapy. Levosimendan (LEV) is a new inodilator, whose mechanism of action includes calcium sensitization of contractile proteins and the opening of ATP-dependent potassium channels.

OBJECTIVES AND METHODS

The aim of this investigation was to test whether the administration of LEV has cardioprotective and antiarrhythmic effects against ischemia and reperfusion injury in a manner similar to ischemic preconditioning (IPC) in a well-standardized model of reperfusion arrhythmias in anesthetized adult male rabbits (n=122) subjected to 30 min occlusion of the left coronary artery followed by 120 min of reperfusion.

RESULTS

Pretreatment with either 1 cycle of IPC, LEV (0.1 micromol/kg, i.v.), or IPC+LEV prior to the period of coronary occlusion offers significant infarct size reduction (21.6+/-1.6%, 22.1+/-2.2%, and 21.4+/-1.4%, respectively vs 38.7+/-3.6% in saline control group; P<0.01) and antiarrhythmic effects. IPC, LEV and IPC+LEV treatment significantly attenuated the incidence of life-threatening arrhythmias like sustained VT (13%, 13% and 13%, respectively vs 100% in saline control group; P<0.005) and other arrhythmias (25%, 25% and 13%, respectively vs 100% in saline control group; P<0.005), and increased the number of surviving animals without arrhythmias. Pretreatment with 5-HD, N(omega)-nitro-L-arginine methyl ester (L-NAME, a nonspecific NOS inhibitor) and the specific iNOS inhibitor 1400 W [N-(-3-(aminomethyl)benzyl) acetamidine] abolished the beneficial effects of IPC, and LEV on reperfusion induced arrhythmias and cardioprotection suggesting that benefits have been achieved via both the selective activation of cardiomyocyte mitochondrial K(ATP) channels and NO. One cycle of IPC and LEV pretreatment significantly preserved the level of ATP in the 30 min ischemic heart and 120 min reperfused heart.

CONCLUSIONS

The present study demonstrates similarities between acute LEV treatment and IPC of the rabbit myocardium in terms of survival, cardioprotection, antiarrhythmic activity, and metabolic status.

Possible involvement of ATP-purinoceptor signalling in the intercellular synchronization of intracellular Ca2+ oscillation in cultured cardiac myocytes

Isolated and cultured neonatal cardiac myocytes contract spontaneously and cyclically. The contraction rhythms of two isolated cardiac myocytes, each of which beats at different frequencies at first, become synchronized after the establishment of mutual contacts, suggesting that mutual entrainment occurs due to electrical and/or mechanical interactions between two myocytes. The intracellular concentration of free Ca(2+) also changes rhythmically in association with the rhythmic contraction of myocytes (Ca(2+) oscillation), and such a Ca(2+) oscillation was also synchronized among cultured cardiac myocytes. In this study, we investigated whether intercellular communication other than via gap junctions was involved in the intercellular synchronization of intracellular Ca(2+) oscillation in spontaneously beating cultured cardiac myocytes. Treatment with either blockers of gap junction channels or an un-coupler of E-C coupling did not affect the intercellular synchronization of Ca(2+) oscillation. In contrast, treatment with a blocker of P2 purinoceptors resulted in the asynchronization of Ca(2+) oscillatory rhythms among cardiac myocytes. The present study suggested that the extracellular ATP-purinoceptor system was responsible for the intercellular synchronization of Ca(2+) oscillation among cardiac myocytes.