Protective effects of low and high doses of cyclosporin A against reoxygenation injury in isolated rat cardiomyocytes are associated with differential effects on mitochondrial calcium levels

Inhibition of the mPTP and Lipid Peroxidation Is Additively Protective Against I/R Injury

BACKGROUND

During myocardial ischemia/reperfusion (I/R) injury, high levels of matrix Ca2+ and reactive oxygen species (ROS) induce the opening of the mitochondrial permeability transition pore (mPTP), which causes mitochondrial dysfunction and ultimately necrotic death. However, the mechanisms of how these triggers individually or cooperatively open the pore have yet to be determined.

METHODS

Here, we use a combination of isolated mitochondrial assays and in vivo I/R surgery in mice. We challenged isolated liver and heart mitochondria with Ca2+, ROS, and Fe2+ to induce mitochondrial swelling. Using inhibitors of the mPTP (cyclosporine A or ADP) lipid peroxidation (ferrostatin-1, MitoQ), we determined how the triggers elicit mitochondrial damage. Additionally, we used the combination of inhibitors during I/R injury in mice to determine if dual inhibition of these pathways is additivity protective.

RESULTS

In the absence of Ca2+, we determined that ROS fails to trigger mPTP opening. Instead, high levels of ROS induce mitochondrial dysfunction and rupture independently of the mPTP through lipid peroxidation. As expected, Ca2+ in the absence of ROS induces mPTP-dependent mitochondrial swelling. Subtoxic levels of ROS and Ca2+ synergize to induce mPTP opening. Furthermore, this synergistic form of Ca2+- and ROS-induced mPTP opening persists in the absence of CypD (cyclophilin D), suggesting the existence of a CypD-independent mechanism for ROS sensitization of the mPTP. These ex vivo findings suggest that mitochondrial dysfunction may be achieved by multiple means during I/R injury. We determined that dual inhibition of the mPTP and lipid peroxidation is significantly more protective against I/R injury than individually targeting either pathway alone.

CONCLUSIONS

In the present study, we have investigated the relationship between Ca2+ and ROS, and how they individually or synergistically induce mitochondrial swelling. Our findings suggest that Ca2+ mediates mitochondrial damage through the opening of the mPTP, although ROS mediates its damaging effects through lipid peroxidation. However, subtoxic levels both Ca2+ and ROS can induce mPTP-mediated mitochondrial damage. Targeting both of these triggers to preserve mitochondria viability unveils a highly effective therapeutic approach for mitigating I/R injury.

Mitochondrial permeability transition pore-dependent necrosis

Mitochondrial permeability transition pore (mPTP)-dependent cell death is a form of necrotic cell death that is driven by mitochondrial dysfunction by the opening of the mPTP and is triggered by increases in matrix levels of Ca2+ and reactive oxygen species. This form of cell death has been implicated in ischemic injuries of the heart and brain as well as numerous degenerative diseases in the brain and skeletal muscle. This review focuses on the molecular triggers and regulators of mPTP-dependent necrosis in the context of myocardial ischemia reperfusion injury. Research over the past 50 years has led to the identity of regulators and putative pore-forming components of the mPTP. Finally, downstream consequences of activation of the mPTP as well as ongoing questions and areas of research are discussed. These questions pose a particular interest as targeting the mPTP could potentially represent an efficacious therapeutic strategy to reduce infarct size following an ischemic event.

Effect of Hexadecyl Azelaoyl Phosphatidylcholine on Cardiomyocyte Apoptosis in Myocardial Ischemia-Reperfusion Injury: A Hypothesis

Reperfusion after myocardial ischemia can induce cardiomyocyte death, known as myocardial reperfusion injury. The pathophysiology of the process of reperfusion suggests the confluence multiple pathways. Recent studies have focused on the inflammatory response, which is considered to be the main mechanism during the process of myocardial ischemia-reperfusion injury and can cause cardiomyocyte apoptosis. Peroxisome proliferator-activated receptors gamma activated by endogenous ligands and exogenous ligand can decrease the inflammatory response in cardiomyocytes. Thiazolidinediones are synthetic, high-affinity, selective ligands for peroxisome proliferator-activated receptors gamma, and can inhibit the inflammatory response, decrease myocardial infarct size, and protect cardiac function. However, thiazolidinediones, including rosiglitazone and pioglitazone, can also contribute to adverse cardiovascular events such as congestive heart failure. Therefore, there are some limitations to the use of thiazolidinediones. Most endogenous ligands were of low affinity until hexadecyl azelaoyl phosphatidylcholine was identified as a high-affinity ligand and agonist for peroxisome proliferator-activated receptors gamma. Hexadecyl azelaoyl phosphatidylcholine binds recombinant peroxisome proliferator-activated receptors with an affinity (Kd(app) ≈40 nM) which is equivalent to rosiglitazone. Therefore, hexadecyl azelaoyl phosphatidylcholine is a specific peroxisome proliferator-activated receptors gamma agonist. Given these findings, we hypothesized that the use of hexadecyl azelaoyl phosphatidylcholine can activate the peroxisome proliferator-activated receptors gamma signal pathways and prevent the inflammatory response process of myocardial ischemia-reperfusion injury, with reduced cardiomyocyte apoptosis and death.

Small Interfering RNA Targeting Mitochondrial Calcium Uniporter Improves Cardiomyocyte Cell Viability in Hypoxia/Reoxygenation Injury by Reducing Calcium Overload

Intracellular Ca²⁺ mishandling is an underlying mechanism in hypoxia/reoxygenation (H/R) injury that results in mitochondrial dysfunction and cardiomyocytes death. These events are mediated by mitochondrial Ca²⁺ (mCa²⁺) overload that is facilitated by the mitochondrial calcium uniporter (MCU) channel. Along this line, we evaluated the effect of siRNA-targeting MCU in cardiomyocytes subjected to H/R injury. First, cardiomyocytes treated with siRNA demonstrated a reduction of MCU expression by 67%, which resulted in significant decrease in mitochondrial Ca²⁺ transport. siRNA treated cardiomyocytes showed decreased mitochondrial permeability pore opening and oxidative stress trigger by Ca²⁺ overload. Furthermore, after H/R injury MCU silencing decreased necrosis and apoptosis levels by 30% and 50%, respectively, and resulted in reduction in caspases 3/7, 9, and 8 activity. Our findings are consistent with previous conclusions that demonstrate that MCU activity is partly responsible for cellular injury induced by H/R and support the concept of utilizing siRNA-targeting MCU as a potential therapeutic strategy.

Protective effects of fluvoxamine against ischemia/reperfusion injury in isolated, perfused guinea-pig hearts

Serotonin (5-hydroxytryptamine; 5-HT) is known to be activated during ischemia-reperfusion and triggers contractile dysfunction and pathological apoptosis. Here, the beneficial effects of the selective serotonin reuptake inhibitor (SSRI) fluvoxamine was demonstrated on ischemia-reperfusion injury in guinea-pig hearts perfused using the Langendorff technique. The recovery (%) of left ventricular developed pressure (LVDP) by fluvoxamine (5×10(-8) M) was 95.4% (control: 32%), which was consistent with the inhibition of mitochondrial Ca(2+)([Ca(2+)]m) uptake induced by changes in the Ca(2+) content and acidification of the perfusate, and similar to reperfusion following global ischemia in Langendorff-perfused hearts. Fluvoxamine inhibited the increase in [Ca(2+)]m induced by changes in the Ca(2+) content of the perfusate in perfused preparations of mitochondria, which was similar to the results obtained with the mitochondrial permeability transition pore (MPTP) opener atractyroside. The terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labeling (TUNEL)-positive cells were significantly less in fluvoxamine-treated hearts than in control hearts, with decreases in caspase-3 activity. These results suggest that SSRI inhibits opening of the MPTP by preventing [Ca(2+)]m overload-induced apoptosis related to the endogenous accumulation of 5-HT in ischemia-reperfusion hearts.

Citicoline (CDP-choline) protects myocardium from ischemia/reperfusion injury via inhibiting mitochondrial permeability transition

AIMS

Oxidative stress emerges after reperfusion of an organ following an ischemic period and results in tissue damage. In the heart, an amplified generation of reactive oxygen species and a significant Ca(2+) accumulation cause ventricular arrhythmias and mitochondrial dysfunction. This occurs in consequence of increased non-specific permeability. A number of works have shown that permeability transition is a common substrate that underlies the reperfusion-induced heart injury. The aim of this work was to explore the possibility that CDP-choline may circumvent heart damage and mitochondrial permeability transition.

MAIN METHODS

Rats were injected i.p. with CDP-choline at 20 mg/kg body weight. Heart electric behavior was followed during a closure/opening cycle of the left coronary descendent artery. Heart mitochondria were isolated from rats treated with CDP-choline, and their function was evaluated by analyzing Ca(2+) movements, achievement of a high level of the transmembrane potential, and respiratory control. Oxidative stress was estimated following the activity of the enzymes cis-aconitase and superoxide dismutase, as well as the disruption of mitochondrial DNA.

KEY FINDINGS

This study shows that CDP-choline avoided ventricular arrhythmias and drop of blood pressure. Results also show that mitochondria, isolated from CDP-choline-treated rats, maintained selective permeability, retained accumulated Ca(2+), an elevated value of transmembrane potential, and a high ratio of respiratory control. Furthermore, activity of cis-aconitase enzyme and mDNA structure were preserved.

SIGNIFICANCE

This work introduces CDP-choline as a useful tool to preserve heart function from reperfusion damage by inhibiting mitochondrial permeability transition.

ADP protects cardiac mitochondria under severe oxidative stress

ADP is not only a key substrate for ATP generation, but also a potent inhibitor of mitochondrial permeability transition pore (mPTP). In this study, we assessed how oxidative stress affects the potency of ADP as an mPTP inhibitor and whether its reduction of reactive oxygen species (ROS) production might be involved. We determined quantitatively the effects of ADP on mitochondrial Ca(2+) retention capacity (CRC) until the induction of mPTP in normal and stressed isolated cardiac mitochondria. We used two models of chronic oxidative stress (old and diabetic mice) and two models of acute oxidative stress (ischemia reperfusion (IR) and tert-butyl hydroperoxide (t-BH)). In control mitochondria, the CRC was 344 ± 32 nmol/mg protein. 500 μmol/L ADP increased CRC to 774 ± 65 nmol/mg protein. This effect of ADP seemed to relate to its concentration as 50 μmol/L had a significantly smaller effect. Also, oligomycin, which inhibits the conversion of ADP to ATP by F0F1ATPase, significantly increased the effect of 50 μmol/L ADP. Chronic oxidative stress did not affect CRC or the effect of 500 μmol/L ADP. After IR or t-BH exposure, CRC was drastically reduced to 1 ± 0.2 and 32 ± 4 nmol/mg protein, respectively. Surprisingly, ADP increased the CRC to 447 ± 105 and 514 ± 103 nmol/mg protein in IR and t-BH, respectively. Thus, it increased CRC by the same amount as in control. In control mitochondria, ADP decreased both substrate and Ca(2+)-induced increase of ROS. However, in t-BH mitochondria the effect of ADP on ROS was relatively small. We conclude that ADP potently restores CRC capacity in severely stressed mitochondria. This effect is most likely not related to a reduction in ROS production. As the effect of ADP relates to its concentration, increased ADP as occurs in the pathophysiological situation may protect mitochondrial integrity and function.

The physiological role of mitochondrial calcium revealed by mice lacking the mitochondrial calcium uniporter. Nat Cell Biol. 2013;15:1464-1472. [PMID: 24212091 PMCID: PMC3852190 DOI: 10.1038/ncb2868]

Mitochondrial calcium has been postulated to regulate a wide range of processes from bioenergetics to cell death. Here, we characterize a mouse model that lacks expression of the recently discovered mitochondrial calcium uniporter (MCU). Mitochondria derived from MCU^-/- mice have no apparent capacity to rapidly uptake calcium. While basal metabolism appears unaffected, the skeletal muscle of MCU^-/- mice exhibited alterations in the phosphorylation and activity of pyruvate dehydrogenase. In addition, MCU^-/- mice exhibited marked impairment in their ability to perform strenuous work. We further show that mitochondria from MCU^-/- mice lacked evidence for calcium-induced permeability transition pore (PTP) opening. The lack of PTP opening does not appear to protect MCU^-/- cells and tissues from cell death, although MCU^-/- hearts fail to respond to the PTP inhibitor cyclosporin A (CsA). Taken together, these results clarify how acute alterations in mitochondrial matrix calcium can regulate mammalian physiology.

Dynasore protects mitochondria and improves cardiac lusitropy in Langendorff perfused mouse heart

Background

Heart failure due to diastolic dysfunction exacts a major economic, morbidity and mortality burden in the United States. Therapeutic agents to improve diastolic dysfunction are limited. It was recently found that Dynamin related protein 1 (Drp1) mediates mitochondrial fission during ischemia/reperfusion (I/R) injury, whereas inhibition of Drp1 decreases myocardial infarct size. We hypothesized that Dynasore, a small noncompetitive dynamin GTPase inhibitor, could have beneficial effects on cardiac physiology during I/R injury.

Methods and Results

In Langendorff perfused mouse hearts subjected to I/R (30 minutes of global ischemia followed by 1 hour of reperfusion), pretreatment with 1 µM Dynasore prevented I/R induced elevation of left ventricular end diastolic pressure (LVEDP), indicating a significant and specific lusitropic effect. Dynasore also decreased cardiac troponin I efflux during reperfusion and reduced infarct size. In cultured adult mouse cardiomyocytes subjected to oxidative stress, Dynasore increased cardiomyocyte survival and viability identified by trypan blue exclusion assay and reduced cellular Adenosine triphosphate(ATP) depletion. Moreover, in cultured cells, Dynasore pretreatment protected mitochondrial fragmentation induced by oxidative stress.

Conclusion

Dynasore protects cardiac lusitropy and limits cell damage through a mechanism that maintains mitochondrial morphology and intracellular ATP in stressed cells. Mitochondrial protection through an agent such as Dynasore can have clinical benefit by positively influencing the energetics of diastolic dysfunction.

Mitochondria as a drug target in ischemic heart disease and cardiomyopathy

Ischemic heart disease is a significant cause of morbidity and mortality in Western society. Although interventions, such as thrombolysis and percutaneous coronary intervention, have proven efficacious in ischemia and reperfusion injury, the underlying pathological process of ischemic heart disease, laboratory studies suggest further protection is possible, and an expansive research effort is aimed at bringing new therapeutic options to the clinic. Mitochondrial dysfunction plays a key role in the pathogenesis of ischemia and reperfusion injury and cardiomyopathy. However, despite promising mitochondria-targeted drugs emerging from the laboratory, very few have successfully completed clinical trials. As such, the mitochondrion is a potential untapped target for new ischemic heart disease and cardiomyopathy therapies. Notably, there are a number of overlapping therapies for both these diseases, and as such novel therapeutic options for one condition may find use in the other. This review summarizes efforts to date in targeting mitochondria for ischemic heart disease and cardiomyopathy therapy and outlines emerging drug targets in this field.

The mitochondrial permeability transition pore and its role in anaesthesia-triggered cellular protection during ischaemia-reperfusion injury

This review summarises the most recent data in support of the role of the mitochondrial permeability transition pore (mPTP) in ischaemia-reperfusion injury, how anaesthetic agents interact with this molecular channel, and the relevance this holds for current anaesthetic practice. Ischaemia results in damage to the electron transport chain of enzymes and sets into play the assembly of a non-specific mega-channel (the mPTP) that transgresses the inner mitochondrial membrane. During reperfusion, uncontrolled opening of the mPTP causes widespread depolarisation of the inner mitochondrial membrane, hydrolysis of ATP, mitochondrial rupture and eventual necrotic cell death. Similarly, transient opening of the mPTP during less substantial ischaemia leads to differential swelling of the intermembrane space compared to the mitochondrial matrix, rupture of the outer mitochondrial membrane and release of pro-apoptotic factors into the cytosol. Recent data suggests that cellular protection from volatile anaesthetic agents follows specific downstream interactions with this molecular channel that are initiated early during anaesthesia. Intravenous anaesthetic agents also prevent the opening of the mPTP during reperfusion. Although by dissimilar mechanisms, both volatiles and propofol promote cell survival by preventing uncontrolled opening of the mPTP after ischaemia. It is now considered that anaesthetic-induced closure of the mPTP is the underlying effector mechanism that is responsible for the cytoprotection previously demonstrated in clinical studies investigating anaesthetic-mediated cardiac and neuroprotection. Manipulation of mPTP function offers a novel means of preventing ischaemic cell injury. Anaesthetic agents occupy a unique niche in the pharmacological armamentarium available for use in preventing cell death following ischaemia-reperfusion injury.

Oxidative stress, mitochondrial permeability transition pore opening and cell death during hypoxia-reoxygenation in adult cardiomyocytes

Reactive oxygen species production is necessary to induce cell death following hypoxia/reoxygenation but the effect of reactive oxygen species produced during hypoxia on mitochondrial permeability transition pore (mPTP) opening and cell death is not established. Here we designed a model of hypoxia/reoxygenation in isolated cardiomyocytes measuring simultaneously reactive oxygen species production, mPTP opening and cell death in order (i) to establish a causal relationship between them, and (ii) to investigate the roles of various reactive oxygen species in mPTP opening. The percentage of cardiomyocytes exhibiting mPTP opening during reoxygenation increased with the duration of hypoxia. Antioxidants increased the time to mPTP opening when present during hypoxia but not at reoxygenation. This was associated with a drop in hydroxyl radical and hydrogen peroxide during hypoxia and the first minutes of reoxygenation. The increase in time to mPTP opening was accompanied by an improvement in cell viability reflected by maintenance of superoxide production at reoxygenation. Cyclosporin A delayed both the time to mPTP opening and cell death despite maintenance of reactive oxygen species production during hypoxia. These findings demonstrate that reactive oxygen species production precedes mPTP opening and that reactive oxygen species produced during hypoxia, particularly hydroxyl radicals and hydrogen peroxide, are necessary to induce mPTP opening which depends on hypoxia duration.

Protective effects of cyclo(L-Leu-L-Tyr) against postischemic myocardial dysfunction in guinea-pig hearts

The protective effects of cyclic dipeptides in alcoholic beverages were investigated in the perfused guinea-pig hearts subjected to ischemia and reperfusion. Subsequently, in order to determine the importance of cyclic dipeptide structure, the effects of cyclo(L-Leu-L-Tyr) (cLY) were compared with those of the newly synthesized non-cyclic dipeptides, L-Leu-L-Tyr (LY) and L-Tyr-L-Leu (YL). After reperfusion, pressure recovery (%) in the left ventricle reached a peak of over 90% in the presence of cLY (10(-6) M and 10(-5) M) (control: 22.9%). The recovery by LY and YL was significantly lower than that by cLY, and ATP levels simultaneously monitored using (31)P-NMR were already lower during the ischemic end period than those observed with cLY treatment. In perfused mitochondrial preparations, cLY significantly inhibited mitochondrial Ca(2+) ([Ca(2+)](m)) elevation in a similar way to that of the mitochondrial permeability transition pore (MPTP) inhibitor cyclosporin A. In vitro electron paramagnetic resonance (EPR) revealed that the active oxygen radicals quenching activity of cLY was greater than those of non-cyclic dipeptides. cLY inhibited caspase-3-induced apoptosis. The cyclic dipeptide structure inhibits opening of the MPTP by preventing [Ca(2+)](m) overload-induced apoptosis related to mitochondrial active oxygen radical accumulation in ischemia-reperfusion hearts.

New roles for mitochondria in cell death in the reperfused myocardium

Mitochondria play an important role in regulating the life and death of cells. They provide the cell with energy via oxidative phosphorylation but can quickly turn into death-promoting organelles in response to stress by disrupting adenosine triphosphate synthesis, releasing pro-death proteins, and producing reactive oxygen species. Due to their high-energy requirement, cardiac myocytes are abundant in mitochondria and as a result, particularly vulnerable to mitochondrial defects. Myocardial ischaemia and reperfusion are associated with mitochondrial dysfunction and cell death. Therefore, future therapies will focus on preserving mitochondrial integrity and function in hopes of minimizing the impact of ischaemia/reperfusion (I/R) injury. It is well established that myocardial I/R activates both necrosis and apoptosis, and that blocking either process reduces the levels of injury. However, recent studies have demonstrated that alterations in mitochondrial dynamics or clearance of mitochondria via autophagy also can contribute to cell death in the myocardium. In this review, we will discuss these new developments and their impact on the role of cardiac mitochondria in cell death following reperfusion in the heart.

Mitochondrial calcium homeostasis as potential target for mitochondrial medicine

Mitochondria are crucial in different intracellular pathways of signal transduction. Mitochondria are capable of decoding a variety of extracellular stimuli into markedly different intracellular actions, ranging from energy production to cell death. The fine modulation of mitochondrial calcium (Ca²⁺) homeostasis plays a fundamental role in many of the processes involving this organelle. When mitochondrial Ca²⁺ homeostasis is compromised, different pathological conditions can occur, depending on the cell type involved. Recent data have shed light on the molecular identity of the main proteins involved in the handling of mitochondrial Ca²⁺ traffic, opening fascinating and ambitious new avenues for mitochondria-based pharmacological strategies.

What makes the mitochondria a killer? Can we condition them to be less destructive?

Cardioprotection, such as preconditioning and postconditioning, has been shown to result in a significant reduction in cell death. Many of the signaling pathways activated by cardioprotection have been elucidated, but there is still a lack of understanding of the mechanisms by which these signaling pathways reduce cell death. Mitochondria have been reported to be an important player in many types of apoptotic and necrotic cell death. If mitochondria play an important role in cell death, then it seems reasonable to consider that cardioprotective mechanisms might act, at least in part, by opposing mitochondrial cell death pathways. One of the major mechanisms of cell death in ischemia-reperfusion is suggested to be the opening of a large conductance pore in the inner mitochondrial membrane, known as the mitochondrial permeability transition pore. Inhibition of this mitochondrial pore appears to be one of the major mechanisms by which cardioprotection reduces cell death. Cardioprotection activates a number of signaling pathways that reduce the level of triggers (reactive oxygen species and calcium) or enhances inhibitors of the mitochondrial permeability transition pore at the start of reperfusion. This article is part of a Special Issue entitled: Mitochondria and Cardioprotection.

Light-dark dependence of electrocardiographic changes during asphyxia and reoxygenation in a rat model

The aim of this study was to evaluate the effect of ventilation on electrocardiographic time intervals as a function of the light-dark (LD) cycle in an in vivo rat model. RR, PQ, QT and QTc intervals were measured in female Wistar rats anaesthetized with both ketamine and xylazine (100 mg/15 mg/kg, i.m., open chest experiments) after adaptation to the LD cycle (12:12h) for 4 weeks. Electrocardiograms (ECG) were recorded before surgical interventions; after tracheotomy, and thoracotomy, and 5 minutes of stabilization with artificial ventilation; 30, 60, 90 and 120 seconds after the onset of apnoea; and after 5, 10, 15, and 20 minutes of artificial reoxygenation. Time intervals in intact animals showed significant LD differences, except in the QT interval. The initial significant (p<0,001) LD differences in PQ interval and loss of dependence on LD cycle in the QT interval were preserved during short-term apnoea-induced asphyxia (30–60 sec) In contrast, long-term asphyxia (90–120 sec) eliminated LD dependence in the PQ interval, but significant LD differences were shown in the QT interval. Apnoea completely abolished LD differences in the RR interval. Reoxygenation restored the PQ and QT intervals to the pre-asphyxic LD differences, but with the RR intervals, the LD differences were eliminated. We have concluded that myocardial vulnerability is dependent on the LD cycle and on changes of pulmonary ventilation.

Biochemistry and physiology of mitochondrial ion channels involved in cardioprotection

Over the past decades there has been considerable progress in understanding the multifunctional roles of mitochondrial ion channels in metabolism, energy transduction, ion transport, signaling, and cell death. Recent data have suggested that some of these channels function under physiological condition, and others may be activated in response to pathological insults and play a key role in cytoprotection. This review outlines our current understanding of the molecular identity and pathophysiological roles of the mitochondrial ion channels in the heart with particular emphasis on cardioprotection against ischemia/reperfusion injury, and future research on mitochondrial ion channels.

Mechanisms underlying acute protection from cardiac ischemia-reperfusion injury

Mitochondria play an important role in cell death and cardioprotection. During ischemia, when ATP is progressively depleted, ion pumps cannot function resulting in a rise in calcium (Ca(2+)), which further accelerates ATP depletion. The rise in Ca(2+) during ischemia and reperfusion leads to mitochondrial Ca(2+) accumulation, particularly during reperfusion when oxygen is reintroduced. Reintroduction of oxygen allows generation of ATP; however, damage to the electron transport chain results in increased mitochondrial generation of reactive oxygen species (ROS). Mitochondrial Ca(2+) overload and increased ROS can result in opening of the mitochondrial permeability transition pore, which further compromises cellular energetics. The resultant low ATP and altered ion homeostasis result in rupture of the plasma membrane and cell death. Mitochondria have long been proposed as central players in cell death, since the mitochondria are central to synthesis of both ATP and ROS and since mitochondrial and cytosolic Ca(2+) overload are key components of cell death. Many cardioprotective mechanisms converge on the mitochondria to reduce cell death. Reducing Ca(2+) overload and reducing ROS have both been reported to reduce ischemic injury. Preconditioning activates a number of signaling pathways that reduce Ca(2+) overload and reduce activation of the mitochondrial permeability transition pore. The mitochondrial targets of cardioprotective signals are discussed in detail.

Mitochondrial permeability transition pore opening as an endpoint to initiate cell death and as a putative target for cardioprotection

In recent years, mitochondria have been recognized as regulators of cell death via both apoptosis and necrosis in addition to their essential role for cell survival. Cellular dysfunctions induced by intra- or extracellular insults converge on mitochondria and induce a sudden increase in permeability of the inner mitochondrial membrane, the so-called mitochondrial permeability transition. The mitochondrial permeability transition is caused by the opening of permeability transition pores (PTP) in the inner mitochondrial membrane with subsequent loss of ionic homeostasis, matrix swelling and outer membrane rupture. The detailed molecular mechanisms underlying the PTP-induced cellular dysfunction during cardiac pathology such as ischemia/reperfusion or post-infarction remodeling remain to be elucidated. However, a growing body of evidence supports the concept that pharmacological inhibition of the PTP is an effective and promising strategy for the protection of the heart against ischemia/reperfusion injury and for attenuation of the remodeling process which contributes to heart failure. This review summarizes and discusses current data on i) the structure and function of the PTP, ii) possible mechanisms and consequences of PTP opening and iii) the inhibition of PTP opening as a therapeutic approach for treatment of heart disease.

Protective effects of EGCg or GCg, a green tea catechin epimer, against postischemic myocardial dysfunction in guinea-pig hearts

The protective effects of (-)-epigallocatechin-3-gallate (EGCg) or the C-2 epimer, (-)-gallocatechin-3-gallate (GCg), afforded by their antioxidative activity among green tea catechins were investigated in perfused guinea-pig Langendorff hearts subjected to ischemia and reperfusion. The recovery (%) of the left ventricular developed pressure from ischemia by reperfusion was 34.4% in the control, while in the presence of EGCg (3x10(-5) M) or GCg (3x10(-6) M, a more diluted concentration than that of EGCg), it led to a maximal increase of 78.4% or 76.2%, consistent with a significant preservative effect on the tissue level of ATP at the end of ischemia or reperfusion. In the perfused preparation of mitochondria, EGCg (10(-5) M) inhibited mitochondrial Ca(2+) elevation by changes in the Ca(2+) content or the acidification of perfusate, similarly to findings with cyclosporin A, a well known inhibitor of the mitochondrial permeability transition pore. By in vitro electron paramagnetic resonance (EPR), EGCg or GCg was found to directly quench the activity of active oxygen radicals, with the strongest activity in tea catechins. EGCg or GCg decreased the caspase-3 activity induced apoptosis. Therefore, it is concluded that the beneficial effects of EGCg or GCg play an important role in ischemia-reperfusion hearts in close relation with nitric oxide (NO), active oxygen radicals and biological redox systems in mitochondria.

Role of epoxyeicosatrienoic acids in protecting the myocardium following ischemia/reperfusion injury

Cardiomyocyte injury following ischemia-reperfusion can lead to cell death and result in cardiac dysfunction. A wide range of cardioprotective factors have been studied to date, but only recently has the cardioprotective role of fatty acids, specifically arachidonic acid (AA), been investigated. This fatty acid can be found in the membranes of cells in an inactive state and can be released by phospholipases in response to several stimuli, such as ischemia. The metabolism of AA involves the cycloxygenase (COX) and lipoxygenase (LOX) pathways, as well as the less well characterized cytochrome P450 (CYP) monooxygenase pathway. Current research suggests important differences with respect to the cardiovascular actions of specific CYP mediated arachidonic acid metabolites. For example, CYP mediated hydroxylation of AA produces 20-hydroxyeicosatetraenoic acid (20-HETE) which has detrimental effects in the heart during ischemia, pro-inflammatory effects during reperfusion and potent vasoconstrictor effects in the coronary circulation. Conversely, epoxidation of AA by CYP enzymes generates 5,6-, 8,9-, 11,12- and 14,15-epoxyeicosatrienoic acids (EETs) that have been shown to reduce ischemia-reperfusion injury, have potent anti-inflammatory effects within the vasculature, and are potent vasodilators in the coronary circulation. This review aims to provide an overview of current data on the role of these CYP pathways in the heart with an emphasis on their involvement as mediators of ischemia-reperfusion injury. A better understanding of these relationships will facilitate identification of novel targets for the prevention and/or treatment of ischemic heart disease, a major worldwide public health problem.

Different effects of optical isomers of the 5-HT1A receptor antagonist pyrapyridolol against postischemic guinea-pig myocardial dysfunction and apoptosis through the mitochondrial permeability transition pore

The recovery (%) of the left ventricular developed pressure by (S)-(-)-pyrapyridolol (5 x 10(-8) M) (90.7%), an optical isomer of a new 5-HT1A receptor antagonist, was greater than that by (R)-(+)-pyrapyridolol (66.2%, control: 34.4%) against ischemia-reperfusion injury in perfused Langendorff guinea-pig hearts. In the perfused mitochondrial preparation, (S)-(-)-pyrapyridolol inhibited the mitochondrial Ca2+ (Cam) elevation that was brought about by the change of Ca2+ content or pH of perfusate, similar to findings with cyclosporin A, well known to be an inhibitor of the mitochondrial permeability transition pore (MPTP). The mitochondrial K(ATP) channel opener, diazoxide, also inhibited the Cam elevation, but the mitochondrial K(ATP) channel antagonist, 5-hydroxydecanoic acid, attenuated it. There were significantly fewer numbers of TUNEL-positive cells in these (S)-(-)-pyrapyridolol-treated hearts than the control or (R)-(+)-pyrapyridolol, with decreases of the caspase-3 activity. Therefore, these results suggest that (S)-(-)-pyrapyridolol likely inhibits the opening of the MPTP by preventing the Cam overload induced apoptosis related to endogenous 5-HT accumulation in ischemia-reperfusion hearts.

The mitochondrial origin of postischemic arrhythmias

Recovery of the mitochondrial inner membrane potential (DeltaPsi(m)) is a key determinant of postischemic functional recovery of the heart. Mitochondrial ROS-induced ROS release causes the collapse of DeltaPsi(m) and the destabilization of the action potential (AP) through a mechanism involving a mitochondrial inner membrane anion channel (IMAC) modulated by the mitochondrial benzodiazepine receptor (mBzR). Here, we test the hypothesis that this mechanism contributes to spatiotemporal heterogeneity of DeltaPsi(m) during ischemia-reperfusion (IR), thereby promoting abnormal electrical activation and arrhythmias in the whole heart. High-resolution optical AP mapping was performed in perfused guinea pig hearts subjected to 30 minutes of global ischemia followed by reperfusion. Typical electrophysiological responses, including progressive AP shortening followed by membrane inexcitablity in ischemia and ventricular fibrillation upon reperfusion, were observed in control hearts. These responses were reduced or eliminated by treatment with the mBzR antagonist 4'-chlorodiazepam (4'-Cl-DZP), which blocks depolarization of DeltaPsi(m). When applied throughout the IR protocol, 4'-Cl-DZP blunted AP shortening and prevented reperfusion arrhythmias. Inhibition of ventricular fibrillation was also achieved by bolus infusion of 4'-Cl-DZP just before reperfusion. Conversely, treatment with an agonist of the mBzR that promotes DeltaPsi(m) depolarization exacerbated IR-induced electrophysiological changes and failed to prevent arrhythmias. The effects of these compounds were consistent with their actions on IMAC and DeltaPsi(m). These findings directly link instability of DeltaPsi(m) to the heterogeneous electrophysiological substrate of the postischemic heart and highlight the mitochondrial membrane as a new therapeutic target for arrhythmia prevention in ischemic heart disease.

Mitochondrial ion channels: gatekeepers of life and death

Continuous generation of ATP by mitochondrial oxidative phosphorylation is essential to maintain function in mechanically active cells such as cardiomyocytes. Emerging evidence indicates that mitochondrial ion channels activated by reactive oxygen species can induce a mitochondrial "critical" state, which can scale to cause electrical and contractile dysfunction of the cardiac cell and, ultimately, the whole heart. Here we focus on how mitochondrial ion channels participate in life-and-death decisions of the cell and discuss the challenges ahead for translating recent findings into novel therapeutic applications.

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.

Calcineurin is required for translational control of protein synthesis in rat pancreatic acini

CCK increases the rate of net protein synthesis in rat pancreatic acini by activating initiation and elongation factors required for translation. The immunosuppressant FK506 inhibits the Ca2+-calmodulin-dependent phosphatase calcineurin in pancreatic acinar cells and blocks pancreatic growth induced by chronic CCK treatment. To test a requirement for calcineurin in the activation of the translational machinery stimulated by CCK, we evaluated the effects of FK506 on protein synthesis and on regulatory initiation and elongation factors in rat pancreatic acini in vitro. CCK acutely increased protein synthesis in acini from normal rats with a maximum increase at 100 pM CCK to 170 ± 11% of control. The immunosuppressant FK506 dose-dependently inhibited CCK-stimulated protein synthesis over the same concentration range that blocked calcineurin activity, as assessed by dephosphorylation of the calcineurin substrate calcium-regulated heat-stable protein of 24 kDa. Another immunosuppressant, cyclosporin A, inhibited protein synthesis, but its effects appeared more complex. FK506 also inhibited protein synthesis stimulated by bombesin and carbachol. FK506 did not significantly affect the activity of the initiation factor-2B, or the phosphorylation of the initiation factor-2α, ribosomal protein protein S6, or the mRNA cap binding protein eukaryotic initiation factor (eIF) 4E. Instead, blockade of calcineurin with FK506 reduced the phosphorylation of the eIF4E binding protein, reduced the formation of the eIF4F complex, and increased the phosphorylation of eukaryotic elongation factor 2. From these results, we conclude that calcineurin activity is required for protein synthesis, and this action may be related to an effect on the formation of the mRNA cap binding complex and the elongation processes.

Calcineurin-independent inhibition of mitochondrial Ca2+ uptake by cyclosporin A

1. Cyclosporin A (CsA) is a widely used compound because of its potent immunosupressive properties, derived mainly from the inhibition of calcineurin, and also because of its ability to block the mitochondrial permeability transition pore (PTP). This second effect has been involved in the protection against apoptosis mediated by release of mitochondrial factors. We show here that CsA (1-10 microm) has an additional effect on Ca(2+) homeostasis in mitochondria that cannot be attributed to inhibition of PTP. 2. By measuring specifically mitochondrial [Ca(2+)] with targeted aequorin, we show that CsA inhibited Ca(2+) entry into mitochondria both in intact and in permeabilized cells, and this effect was stronger when Ca(2+) entry was triggered by low cytosolic [Ca(2+)], below 5 microm. 3. Inhibition of mitochondrial Ca(2+) uptake required micromolar concentrations of CsA and was not mimicked by other inhibitors of calcineurin such as FK-506 or cypermethrin, nor by a different inhibitor of the PTP, bongkrekic acid. 4. CsA blocked the increase in mitochondrial Ca(2+) uptake rate induced by the mitochondrial Ca(2+) uniporter activator SB202190. 5. Our results suggest that CsA inhibits Ca(2+) entry through the Ca(2+) uniporter by a mechanism independent of the inhibition of PTP or calcineurin. This effect may contribute to reduce depolarization and Ca(2+) overloading in mitochondria after cell stimulation, and thus cooperate with the direct inhibition of PTP to prevent apoptosis.

Cariporide preserves mitochondrial proton gradient and delays ATP depletion in cardiomyocytes during ischemic conditions

The mechanism by which inhibition of Na+/H+ exchanger (NHE) reduces cell death in ischemic-reperfused myocardium remains controversial. This study investigated whether cariporide could inhibit mitochondrial NHE during ischemia, delaying H+ gradient dissipation and ATP exhaustion. Mouse cardiac myocytes (HL-1) were submitted to 1 h of simulated ischemia (SI) with NaCN/deoxyglucose (pH 6.4), with or without 7 microM cariporide, and mitochondrial concentration of Ca2+ (Rhod-2), 2', 7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF) and the charge difference across the mitochondrial membrane potential (Deltapsim, JC-1) were assessed. ATP content was measured by bioluminescence and mitochondrial swelling by spectrophotometry in isolated mitochondria. Cariporide significantly attenuated the acidification of the mitochondrial matrix induced by SI without modifying Deltapsim decay, and this effect was associated to a delayed ATP exhaustion and increased mitochondrial Ca2+ load. These effects were reproduced in sarcolemma-permeabilized cells exposed to SI. In these cells, cariporide markedly attenuated the fall in mitochondrial pH induced by removal of Na+ from the medium. In isolated mitochondria, cariporide significantly reduced the rate and magnitude of passive matrix swelling induced by Na+ acetate. In isolated rat hearts submitted to 40-min ischemia at different temperatures (35.5 degrees, 37 degrees, or 38.5 degrees C) pretreatment with cariporide limited ATP depletion during the first 10 min of ischemia and cell death (lactate dehydrogenase release) during reperfusion. These effects were mimicked when a similar ATP preservation was achieved by hypothermia and were abolished when the sparing effect of cariporide on ATP was suppressed by hyperthermia. We conclude that cariporide acts at the mitochondrial level, delaying mitochondrial matrix acidification and delaying ATP exhaustion during ischemia. These effects can contribute to reduce cell death secondary to ischemia-reperfusion.

Cyclosporine A protects against apoptosis in ischaemic/reperfused rat kidneys

1. Renal ischaemia followed by reperfusion leads to acute renal failure in both native kidneys and renal allografts. Cyclosporine A (CsA) has been used as an immunosuppressive agent in organ transplantation. In the present study, the effect of CsA on ischaemia/reperfusion-induced apoptosis in the kidney was investigated. 2. Ischaemia/reperfusion injury caused widespread apoptosis primarily in the medulla of the kidney. At 1.5 mg/kg per day, CsA significantly reduced the number of apoptotic cells in rat kidney after ischaemia/reperfusion injury. 3. Low-dose CsA treatment did not affect the levels of creatinine in the serum of rats after ischaemia/reperfusion injury.

Occurrence and characteristics of the mitochondrial permeability transition in plants

The behavior of purified potato mitochondria toward the main effectors of the animal mitochondrial permeability transition has been studied by light scattering, fluorescence, SDS-polyacrylamide gel electrophoresis, and immunoblotting techniques. The addition of Ca(2+) induces a phosphate-dependent swelling that is fully inhibited by cyclosporin A if dithioerythritol is present. Mg(2+) cannot be substituted for Ca(2+) but competes with it. Disruption of the outer membrane and release of several proteins, including cytochrome c, occur upon completion of swelling. Ca(2+)-induced swelling is delayed and its rate is decreased when pH is shifted from 7.4 to 6.6. It is accelerated by diamide, phenylarsine oxide, and linolenic acid. In the absence of Ca(2+), however, linolenic acid (< or =20 microm) rapidly dissipates the succinate-driven membrane potential while having no effect on mitochondrial volume. Anoxic conditions favor in vitro swelling and the concomitant release of cytochrome c and of other proteins in a pH-dependent way. These data indicate that the classical mitochondrial permeability transition occurs also in plants. This may have important implications for our understanding of cell stress and death processes.

Role of calcineurin in insulin-like growth factor-1-induced hypertrophy of cultured adult rat ventricular myocytes

The present study examined the role of calcineurin in insulin-like growth factor (IGF)-1-induced hypertrophy in primary cultures of adult rat ventricular myocytes (ARVM), prepared from the ventricles of 14-16-week-old male Sprague-Dawley rats. The effects of several humoral factors, including phenylephrine, angiotensin II, endothelin-1, IGF-1 and interleukin-6, on the morphology of ARVM were studied. Myocyte surface area was significantly increased by IGF-1 (2,268 +/- 571 to 3,018 +/- 836 microm2, p < 0.01), but not by other humoral factors. This hypertrophic effect of IGF-1 was blocked by genistein (tyrosine kinase inhibitor), PD98059 (MEK inhibitor). These findings suggest that IGF-1 produces ARVM hypertrophy by a tyrosine kinase-MEK mediated pathway as has been reported in neonatal cardiomyocytes. IGF-1-mediated ARVM hypertrophy was also attenuated by cyclosporine A (calcineurin inhibitor), and staurosporine and chelerythrine (protein kinase C inhibitors). IGF-1 markedly increased calcineurin activity (8.7 +/- 1.2 to 98.0 +/- 54.3 pmol x h(-1) mg(-1), p < 0.01), and this activation was completely blocked by pre-treatment with cyclosporine A (8.5 +/- 11.4pmol x h(-1) x mg(-1), p < 0.01) and chelerythrine (2.3 +/- 2.7 pmol x h(-1) mg(-1), p < 0.01). It appears that IGF-1 activates calcineurin by a protein kinase C-dependent pathway. Increased mRNA expression of atrial natriuretic factor by IGF-1 was inhibited by cyclosporine A (p < 0.01). The findings indicate that IGF-1 induces ARVM hypertrophy by protein kinase C and calcineurin-related mechanisms. The fact that elevated calcineurin activity and induced atrial natriuretic factor mRNA expression by IGF-1 were blocked by cyclosporine A further supports the hypothesis that calcineurin is critically involved in IGF-1-induced ARVM hypertrophy.

Cold preservation-warm reoxygenation increases hepatocyte steady-state Ca(2+) and response to Ca(2+)-mobilizing agonist

Although the role of Ca(2+) in liver transplantation injury has been the object of several studies, direct evidence for alterations in intracellular Ca(2+) homeostasis after cold preservation-warm reoxygenation (CP/WR) has never been presented. We thus investigated the effects of CP/WR on steady-state Ca(2+) and responses to a Ca(2+)-mobilizing agonist. Isolated rat hepatocytes were suspended in University of Wisconsin solution, stored at 4 degrees C for 0, 24, and 48 h, and reoxygenated at 37 degrees C for 1 h. Cytosolic Ca(2+) was measured in single cells by digitized fluorescence videomicroscopy. CP/WR caused a significant increase in steady-state cytosolic Ca(2+), which was inversely proportional to cell viability. Pretreatment of hepatocytes with an agent that protects mitochondrial function attenuated the increase in steady-state cytosolic Ca(2+) and improved hepatocyte viability. Ca(2+) responses to the purinergic agonist ATP also increased significantly as a function of cold storage time. This increase was related to an increase in the size of inositol 1,4,5-trisphosphate-sensitive Ca(2+) stores and subsequent capacitative Ca(2+) entry. Thus CP/WR significantly perturbs steady-state hepatocellular Ca(2+) and responses to Ca(2+)-mobilizing agonists, which may contribute to hepatocyte metabolic dysfunction observed after CP/WR.

Mitochondrial intermembrane junctional complexes and their role in cell death

A mitochondrial complex comprising the voltage-dependent anion channel (outer membrane), the adenine nucleotide translocase (inner membrane) and cyclophilin-D (matrix) assembles at contact sites between the inner and outer membranes. Under pathological conditions associated with ischaemia and reperfusion the junctional complex 'deforms' into the permeability transition (PT) pore, which can open transiently, allowing free permeation of low Mr solutes across the inner membrane. This may be a critical step in the pathogenesis of lethal cell injury in ischaemia and reperfusion. Moreover, it is argued, the degree of pore opening may be an important determinant of the relative extent of apoptosis and necrosis under these conditions. In addition, mitochondria are the major sites of action of Bax and other apoptotic regulatory proteins of the Bcl-2 family. These proteins control a mitochondrial amplificatory loop in the apoptotic signalling pathway in which cytochrome c and other apoptogenic proteins of the mitochondrial intermembrane space are released into the cytosol. There are indications that the junctional complex, or components of it, may also mediate the action of Bax, but in a way that does not involve PT pore formation.