Metabolism of preconditioned myocardium: effect of loss and reinstatement of cardioprotection

Quantitative Proteomics Analysis Reveals That Cyclooxygenase-2 Modulates Mitochondrial Respiratory Chain Complex IV in Cardiomyocytes

The biochemical mechanisms of cell injury and myocardial cell death after myocardial infarction remain unresolved. Cyclooxygenase 2 (COX-2), a key enzyme in prostanoid synthesis, is expressed in human ischemic myocardium and dilated cardiomyopathy, but it is absent in healthy hearts. To assess the role of COX-2 in cardiovascular physiopathology, we developed transgenic mice that constitutively express functional human COX-2 in cardiomyocytes under the control of the α-myosin heavy chain promoter. These animals had no apparent phenotype but were protected against ischemia-reperfusion injury in isolated hearts, with enhanced functional recovery and diminished cellular necrosis. To further explore the phenotype of this animal model, we carried out a differential proteome analysis of wild-type vs. transgenic cardiomyocytes. The results revealed a tissue-specific proteomic profile dominated by mitochondrial proteins. In particular, an increased expression of respiratory chain complex IV proteins was observed. This correlated with increased catalytic activity, enhanced respiratory capacity, and increased ATP levels in the heart of COX-2 transgenic mice. These data suggest a new link between COX-2 and mitochondria, which might contribute to the protective cardiac effects of COX-2 against ischemia-reperfusion injury.

Pre- and Post-Conditioning of the Heart: An Overview of Cardioprotective Signaling Pathways

Since the discovery of ischemic pre- and post-conditioning, more than 30 years ago, the knowledge about the mechanisms and signaling pathways involved in these processes has significantly increased. In clinical practice, on the other hand, such advancement has yet to be seen. This article provides an overview of ischemic pre-, post-, remote, and pharmacological conditioning related to the heart. In addition, we reviewed the cardioprotective signaling pathways and therapeutic agents involved in the above-mentioned processes, aiming to provide a comprehensive evaluation of the advancements in the field. The advancements made over the last decades cannot be ignored and with the exponential growth in techniques and applications. The future of pre- and post-conditioning is promising.

Review Cerebral Ischemic Tolerance and Preconditioning: Methods, Mechanisms, Clinical Applications, and Challenges

Stroke is one of the leading causes of morbidity and mortality worldwide, and it is increasing in prevalence. The limited therapeutic window and potential severe side effects prevent the widespread clinical application of the venous injection of thrombolytic tissue plasminogen activator and thrombectomy, which are regarded as the only approved treatments for acute ischemic stroke. Triggered by various types of mild stressors or stimuli, ischemic preconditioning (IPreC) induces adaptive endogenous tolerance to ischemia/reperfusion (I/R) injury by activating a multitude cascade of biomolecules, for example, proteins, enzymes, receptors, transcription factors, and others, which eventually lead to transcriptional regulation and epigenetic and genomic reprogramming. During the past 30 years, IPreC has been widely studied to confirm its neuroprotection against subsequent I/R injury, mainly including local ischemic preconditioning (LIPreC), remote ischemic preconditioning (RIPreC), and cross preconditioning. Although LIPreC has a strong neuroprotective effect, the clinical application of IPreC for subsequent cerebral ischemia is difficult. There are two main reasons for the above result: Cerebral ischemia is unpredictable, and LIPreC is also capable of inducing unexpected injury with only minor differences to durations or intensity. RIPreC and pharmacological preconditioning, an easy-to-use and non-invasive therapy, can be performed in a variety of clinical settings and appear to be more suitable for the clinical management of ischemic stroke. Hoping to advance our understanding of IPreC, this review mainly focuses on recent advances in IPreC in stroke management, its challenges, and the potential study directions.

Effects of Running Exercise Combined With Blood Flow Restriction on Strength and Sprint Performance

Chen, YT, Hsieh, YY, Ho, JY, and Lin, JC. Effects of running exercise combined with blood flow restriction on strength and sprint performance. J Strength Cond Res XX(X): 000-000, 2019-We investigated muscle strength and sprint performance after combining running exercise (RE) with blood flow restriction (BFR). Twelve male sprinters received 2 experimental warm-ups: (a) RE (50% heart rate reserve, 2 minutes × 5 sets, 1-minute rest interval) with BFR (occlusion pressure: 1.3 × resting systolic blood pressure) warm-up, namely RE-BFR; and (b) RE without BFR warm-up, namely RE. Isokinetic strength or 60-m sprint performance was assessed after a 5-minute recovery from each experimental warm-up. All subjects completed 4 exercise trials in a counterbalanced order: (a) RE-BFR-strength; (b) RE-strength; (c) RE-BFR-sprint; and (d) RE-sprint. Muscle activation (during RE), blood lactate (BLa) (pre- and post-REs), heart rate (HR), and rating of perceived exertion (RPE) (pre- and post-REs and at a 5-minute recovery) were determined during each experimental warm-up. The isokinetic knee flexor strength and the hamstring-quadriceps (H:Q) ratio observed for the RE-BFR warm-up were significantly higher than those observed for the RE warm-up (p < 0.05). However, no differences (p > 0.05) in the isokinetic knee extensor strength and 60-m sprint performance were observed between the 2 warm-ups. Running exercise-BFR warm-up induced a higher level of vastus lateralis and biceps femoris muscle activation than did RE warm-up (p < 0.05). Furthermore, RE-BFR warm-up induced higher HR, RPE, and BLa values than did RE warm-up after RE and at a 5-minute recovery (p < 0.05). These results suggest that RE-BFR warm-up may augment physiological responses and improve the H:Q ratio and isokinetic knee flexor strength. Thus, RE-BFR warm-up may be considered a practical warm-up strategy for promoting muscle strength and reducing the risk of hamstring injury in male sprinters.

Hexokinase-2 Glycolytic Overload in Diabetes and Ischemia-Reperfusion Injury

Hexokinase-2 (HK2) was recently found to produce increased metabolic flux through glycolysis in hyperglycemia without concurrent transcriptional or other functional regulation. Rather, stabilization to proteolysis by increased glucose substrate binding produced unscheduled increased glucose metabolism in response to high cytosolic glucose concentration. This produces abnormal increases in glycolytic intermediates or glycolytic overload, driving cell dysfunction and vulnerability to the damaging effects of hyperglycemia in diabetes, explaining tissue-specific pathogenesis. Glycolytic overload is also activated in ischemia-reperfusion injury and cell senescence. A further key feature is HK2 displacement from mitochondria by increased glucose-6-phosphate concentration, inducing mitochondrial dysfunction and oxidative stress. This pathogenic mechanism suggested new targets for therapeutics development that gave promising outcomes in initial clinical evaluation.

Ischemic Preconditioning Does Not Alter Performance in Multidirectional High-Intensity Intermittent Exercise

Purpose: Research dealing with ischemic preconditioning (IPC) has primarily focused on variables associated to endurance performance with little research about the acute responses of IPC on repeated multidirectional running sprint performance. Here we aimed to investigate the effects of IPC of the arms and the legs on repeated running sprint performance with changes-of-direction (COD) movements.

Methods: Thirteen moderately-to-well-trained team-sport athletes (7 males; 6 females; age: 24 ± 2 years, size: 175 ± 8 cm, body mass: 67.9 ± 8.1 kg) performed 16 × 30 m all-out sprints (15 s rest) with multidirectional COD movements on a Speedcourt with IPC (3 × 5 min) of the legs (IPC_leg; 240 mm Hg) or of the arms (remote IPC: IPC_remote; 180–190 mm Hg) 45 min before the sprints and a control trial (CON; 20 mm Hg).

Results: The mean (±SD) time for the 16 × 30 m multidirectional COD sprints was similar between IPC_leg (Mean t: 16.0 ± 1.8 s), IPC_remote (16.2 ± 1.7 s), and CON (16.0 ± 1.6 s; p = 0.50). No statistical differences in oxygen uptake (mean difference: 0%), heart rate (1.1%) nor muscle oxygen saturation of the vastus lateralis (4.7%) and biceps brachii (7.8%) between the three conditions were evident (all p > 0.05).

Conclusions: IPC (3 × 5 min) of the legs (220 mm Hg) or arms (180–190 mm Hg; remote IPC) applied 45 min before 16 × 30 m repeated multidirectional running sprint exercise does not improve sprint performance, oxygen uptake, heart rate nor muscle oxygen saturation of the vastus lateralis muscle when compared to a control trial.

Ischemic preconditioning vs adenosine vs prostaglandin E1 for protection against liver ischemia/reperfusion injury

Ischemia/reperfusion injury is still a major cause of morbidity and mortality during liver surgery and transplantation. A variety of surgical and pharmacological therapeutic strategies have been investigated to minimize the effects of ischemia/reperfusion. The aim of our study was to analyze and compare preventive influences of ischemic preconditioning, adenosine and prostaglandin E1 in the experimental model of hepatic ischemia/reperfusion injury. Adult chinchilla rabbits were divided into four groups: 10 rabbits subjected to liver ischemic preconditioning (3-min period of inflow occlusion followed by a 5-min period of reperfusion) followed by 45 min of Pringle maneuver; 10 rabbits subjected to pre-treatment with intraportal injection of adenosine followed by 45 min of Pringle maneuver; 10 rabbits subjected to pre-treatment with intraportal injection of prostaglandin E1 followed by 45 min of Pringle maneuver; and control group of 10 rabbits subjected to 45 min of inflow liver ischemia without any preconditioning. On the second postoperative day, blood samples were obtained and biochemical parameters of liver function were measured and compared. Liver tissue samples were also obtained and histopathological changes were compared. Based on biochemical and histopathological parameters, it was demonstrated that ischemic preconditioning provided the best protection against hepatic ischemia/reperfusion injury. This was probably due to a wider range of mechanisms of action of this method oriented to reduce oxidative stress and inflammation, and restore liver microcirculation and hepatocyte energy compared to the examined pharmacological strategies.

Reperfusion Syndrome: Cellular Mechanisms of Microvascular Dysfunction and Potential Therapeutic Strategies

Reperfusion injury is the paradoxical and complex phenomenon of exacerbation of cellular dysfunction and increase in cell death after the restoration of blood flow to previously ischemic tissues. It involves biochemical and cellular changes causing oxidant production and complement activation, which culminates in an inflammatory response, mediated by neutrophil and platelet cell interactions with the endothelium and among the cells themselves. The mounted inflammatory response has both local and systemic manifestations. Despite improvements in imaging, interventional techniques, and pharmacological agents, morbidity from reperfusion remains high. Extensive research has furthered the understanding of the various pathophysiological mechanisms involved and the development of potential therapeutic strategies. Preconditioning has emerged as a powerful method of ameliorating ischemia reperfusion injury to the myocardium and in transplant surgery. More recently, postconditioning has been shown to provide a therapeutic counter to vasoocclusive emergencies. More research and well-designed trials are needed to bridge the gap between experimental evidence and clinical implementation.

Effect of ischemic preconditioning on repeated sprint ability in team sport athletes

This study investigated whether ischemic preconditioning (IPC) in a trained population affected repeated sprint performance. A secondary aim was to assess responses according to gender. Sixteen (nine females and seven males) well trained team sport athletes took part in a randomised crossover study design. Participants underwent an IPC and placebo treatment involving three periods of 5 min occlusion applied unilaterally (3 × 5 min occlusion to each leg) at either 220 mmHg or 50 mmHg. Each period of occlusion was followed by 5 min reperfusion. Following treatment 5 × 6 s maximal effort sprints were undertaken on a cycle ergometer against 7.5% body mass, each interspersed by 24 s recovery. Measured parameters included peak power, total power, percentage decrement, post-exercise blood lactate and ratings of perceived exertion. Nor within subject main effect for IPC was observed, neither was there an interaction effect with gender. Effect sizes were trivial (ES < 0.2) with the exception of a moderate (ES < 1.2) change in post-exercise blood lactate in the female cohort (1.6 ± 0.4 mmol⁻¹ lower following IPC). Results suggest no benefit to team sport players in utilising IPC as a means of enhancing repeated sprint performance. A lower blood lactate response in female participants following IPC may suggest improved blood flow through vasodilation.

The role of hexokinase in cardioprotection - mechanism and potential for translation

Mitochondrial permeability transition pore (mPTP) opening plays a critical role in cardiac reperfusion injury and its prevention is cardioprotective. Tumour cell mitochondria usually have high levels of hexokinase isoform 2 (HK2) bound to their outer mitochondrial membranes (OMM) and HK2 binding to heart mitochondria has also been implicated in resistance to reperfusion injury. HK2 dissociates from heart mitochondria during ischaemia, and the extent of this correlates with the infarct size on reperfusion. Here we review the mechanisms and regulations of HK2 binding to mitochondria and how this inhibits mPTP opening and consequent reperfusion injury. Major determinants of HK2 dissociation are the elevated glucose‐6‐phosphate concentrations and decreased pH in ischaemia. These are modulated by the myriad of signalling pathways implicated in preconditioning protocols as a result of a decrease in pre‐ischaemic glycogen content. Loss of mitochondrial HK2 during ischaemia is associated with permeabilization of the OMM to cytochrome c, which leads to greater reactive oxygen species production and mPTP opening during reperfusion. Potential interactions between HK2 and OMM proteins associated with mitochondrial fission (e.g. Drp1) and apoptosis (B‐cell lymphoma 2 family members) in these processes are examined. Also considered is the role of HK2 binding in stabilizing contact sites between the OMM and the inner membrane. Breakage of these during ischaemia is proposed to facilitate cytochrome c loss during ischaemia while increasing mPTP opening and compromising cellular bioenergetics during reperfusion. We end by highlighting the many unanswered questions and discussing the potential of modulating mitochondrial HK2 binding as a pharmacological target.

Linked Articles

This article is part of a themed section on Conditioning the Heart – Pathways to Translation. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue‐8

Conditional knockout of myocyte focal adhesion kinase abrogates ischemic preconditioning in adult murine hearts

Background

Our laboratory has previously demonstrated the importance of a cytoskeletal‐based survival signaling pathway using in vitro models of ischemia/reperfusion (IR). However, the importance of this pathway in mediating stress‐elicited survival signaling in vivo is unknown.

Methods and Results

The essential cytoskeletal signaling pathway member focal adhesion kinase (FAK) was selectively deleted in adult cardiac myocytes using a tamoxifen‐inducible Cre‐Lox system (α‐MHC‐MerCreMer). Polymerase chain reaction (PCR) and Western blot were performed to confirm FAK knockout (KO). All mice were subjected to a 40‐minute coronary occlusion followed by 24 hours of reperfusion. Ischemic preconditioning (IP) was performed using a standard protocol. Control groups included wild‐type (WT) and tamoxifen‐treated α‐MHC‐MerCreMer^+/−/FAK^WT/WT (experimental control) mice. Infarct size was expressed as a percentage of the risk region. In WT mice IP significantly enhanced the expression of activated/phosphorylated FAK by 36.3% compared to WT mice subjected to a sham experimental protocol (P≤0.05; n=6 hearts [sham], n=4 hearts [IP]). IP significantly reduced infarct size in both WT and experimental control mice (43.7% versus 19.8%; P≤0.001; 44.7% versus 17.5%; P≤0.001, respectively). No difference in infarct size was observed between preconditioned FAK KO and nonpreconditioned controls (37.1% versus 43.7% versus 44.7%; FAK KO versus WT versus experimental control; P=NS). IP elicited a 67.2%/88.8% increase in activated phosphatidylinositol‐3‐kinase (PI3K) p85/activated Akt expression in WT mice, but failed to enhance the expression of either in preconditioned FAK KO mice.

Conclusions

Our results indicate that FAK is an essential mediator of IP‐elicited cardioprotection and provide further support for the hypothesis that cytoskeletal‐based signaling is an important component of stress‐elicited survival signaling.

Akt phosphorylates HK-II at Thr-473 and increases mitochondrial HK-II association to protect cardiomyocytes

Hexokinase II (HK-II) is an enzyme that catalyzes the first step in glycolysis and localizes not only in the cytosol but also at mitochondria. Akt, activated by insulin-like growth factor 1 (IGF-1) treatment in neonatal rat ventricular myocytes, translocates to mitochondria and increases mitochondrial HK-II binding. Expression of an HK-II-dissociating peptide diminished IGF-1-induced increases in mitochondrial HK-II as well as protection against hydrogen peroxide treatment, suggesting an important role of mitochondrial HK-II in IGF-1/Akt-mediated protection. We hypothesized, on the basis of an Akt phosphorylation consensus sequence present in HK-II, that Thr-473 is the target of Akt kinase activity. Indeed, recombinant kinase-active Akt robustly phosphorylates WT HK-II, but not Thr-473 mutants. Phosphomimetic (T473D)HK-II, but not non-phosphorylatable (T473A)HK-II, constitutively increased mitochondrial binding compared with WT HK-II and concomitantly confers greater protection against hydrogen peroxide. Glucose 6-phosphate (G-6P), a product of the catalytic activity of HK-II, is well known to dissociate HK-II from mitochondria. Addition of G-6P to isolated mitochondria dose-dependently dissociates WT HK-II, and this response is inhibited significantly in mitochondria isolated from cardiomyocytes expressing T473D HK-II. Pretreatment with IGF-1 also inhibits G-6P-induced overexpressed or endogenous HK-II dissociation, and this response was blocked by Akt inhibition. These results show that Akt phosphorylation of HK-II at Thr-473 is responsible for the Akt-mediated increase in HK-II binding to mitochondria. This increase is, at least in part, due to the decreased sensitivity to G-6P-induced dissociation. Thus, phosphorylation-mediated regulation of mitochondrial HK-II would be a critical component of the protective effect of Akt.

Extent of mitochondrial hexokinase II dissociation during ischemia correlates with mitochondrial cytochrome c release, reactive oxygen species production, and infarct size on reperfusion

Background

The mechanisms by which ischemic preconditioning (IP) inhibits mitochondrial permeability transition pore opening and, hence, ischemia–reperfusion injury remain unclear. Here we investigate whether and how mitochondria‐bound hexokinase 2 (mtHK2) may exert part of the cardioprotective effects of IP.

Methods and Results

Control and IP Langendorff‐perfused rat hearts were subject to ischemia and reperfusion with measurement of hemodynamic function and infarct size. Outer mitochondrial membrane (OMM) permeabilization after ischemia was determined by measuring rates of respiration and H₂O₂ production in the presence and absence of added cytochrome c in isolated mitochondria and permeabilized fibers. IP prevented OMM permeabilization during ischemia and reduced the loss of mtHK2, but not Bcl‐x_L, observed in control ischemic hearts. By contrast, treatment of permeabilized fibers with glucose‐6‐phosphate at pH 6.3 induced mtHK2 loss without OMM permeabilization. However, metabolic pretreatments of the perfused heart chosen to modulate glucose‐6‐phosphate and intracellular pH_i revealed a strong inverse correlation between end‐ischemic mtHK2 content and infarct size after reperfusion. Loss of mtHK2 was also associated with reduced rates of creatine phosphate generation during the early phase of reperfusion. This could be mimicked in permeabilized fibers after mtHK2 dissociation.

Conclusions

We propose that loss of mtHK2 during ischemia destabilizes mitochondrial contact sites, which, when accompanied by degradation of Bcl‐x_L, induces OMM permeabilization and cytochrome c loss. This stimulates reactive oxygen species production and mitochondrial permeability transition pore opening on reperfusion, leading to infarction. Consequently, inhibition of mtHK2 loss during ischemia could be an important mechanism responsible for the cardioprotection mediated by IP and other pretreatments.

Intermittent lower-limb occlusion enhances recovery after strenuous exercise

Repeated cycles of vascular occlusion followed by reperfusion initiate a protective mechanism that acts to mitigate future cell injury. Such ischemic episodes are known to improve vasodilation, oxygen utilization, muscle function, and have been demonstrated to enhance exercise performance. Thus, the use of occlusion cuffs represents a novel intervention that may improve subsequent exercise performance. Fourteen participants performed an exercise protocol that involved lower-body strength and power tests followed by repeated sprints. Occlusion cuffs were then applied unilaterally (2 × 3-min per leg) with a pressure of either 220 (intervention) or 15 mm Hg (control). Participants immediately repeated the exercise protocol, and then again 24 h later. The intervention elicited delayed beneficial effects (24 h post-intervention) in the countermovement jump test with concentric (effect size (ES) = 0.36) and eccentric (ES = 0.26) velocity recovering more rapidly compared with the control. There were also small beneficial effects on 10- and 40-m sprint times. In the squat jump test there were delayed beneficial effects of occlusion on eccentric power (ES = 1.38), acceleration (ES = 1.24), and an immediate positive effect on jump height (ES = 0.61). Thus, specific beneficial effects on recovery of power production and sprint performance were observed both immediately and 24 h after intermittent unilateral occlusion was applied to each leg.

Cardiovascular effects of melatonin receptor agonists

INTRODUCTION

Melatonin synchronizes circadian rhythms with light/dark period and it was demonstrated to correct chronodisruption. Several melatonin receptor agonists with improved pharmacokinetics or increased receptor affinity are being developed, three of them are already in clinical use. However, the actions of melatonin extend beyond chronobiology to cardiovascular and metabolic systems as well. Given the high prevalence of cardiovascular disease and their common occurrence with chronodisruption, it is of utmost importance to classify the cardiometabolic effects of the newly approved and putative melatoninergic drugs.

AREAS COVERED

In the present review, the available (although very sparse) data on such effects, in particular by the approved (circadin, ramelteon, agomelatine) or clinically advanced (tasimelteon, piromelatine = Neu-P11, TIK-301) compounds are summarized. The authors have searched for an association with blood pressure, vascular reactivity, ischemia, myocardial and vascular remodeling and metabolic syndrome.

EXPERT OPINION

The data suggest that cardiovascular effects of melatonin are at least partly mediated via MT(1)/MT(2) receptors and associated with its chronobiotic action. Therefore, despite the sparse direct evidence, it is believed that these effects will be shared by melatonin analogs as well. With the expected approval of novel melatoninergic compounds, it is suggested that the investigation of their cardiovascular effects should no longer be neglected.

An improved isolation procedure for adult mouse cardiomyocytes

Isolated adult mouse cardiomyocytes are an important tool in cardiovascular research, but are challenging to prepare. Because the energy supply determines cell function and viability, we compared total creatine ([Cr]) and [ATP] in isolated cardiomyocytes with the intact mouse heart. Isolated myocytes suffered severe losses of Cr (-70%) and ATP (-53%). Myocytes were not able to replete [Cr] during a 5 h incubation period in medium supplemented with 1 mM Cr. In contrast, adding 20 mM Cr to the digestion buffers was sufficient to maintain normal [Cr]. Supplementing buffers with 5 mM of inosine (Ino) and adenosine (Ado) to prevent loss of cellular nucleosides partially protected against loss of ATP. To test whether maintaining [ATP] and [Cr] improves contractile function, myocytes were challenged by varying pacing rate from 0.5 to 10 Hz and by adding isoproterenol (Iso) at 5 and 10 Hz. All groups performed well up to 5 Hz, showing a positive cell shortening-frequency relationship; however, only 16% of myocytes isolated under standard conditions were able to sustain pacing with Iso challenge at 10 Hz. In contrast, 30-50% of the myocytes with normal Cr levels were able to contract and maintain low diastolic [Ca(2+)]. Cell yield also improved in Cr and the Cr/Ino/Ado-treated groups (85-90% vs. 70-75% rod shaped in untreated myocytes). These data suggest that viability and performance of isolated myocytes are improved when they are protected from the severe loss of Cr and ATP during the isolation, making them an even better research tool.

Mechanism of cardioprotection by early ischemic preconditioning

A series of brief ischemia/reperfusion cycles (termed ischemic preconditioning, IPC) limits myocardial injury produced by a subsequent prolonged period of coronary artery occlusion and reperfusion. Over the last 2 decades our understanding of IPC's mechanism has increased exponentially. Hearts exposed to IPC have a better metabolic and ionic status during prolonged ischemia compared to naïve hearts. However, this difference is not thought to be the main mechanism by which IPC protects against infarction. Signaling pathways that are activated by IPC distinguish IPC hearts from naïve hearts. During the trigger phase of IPC, adenosine, bradykinin and opioid receptors are occupied. Although these three receptors trigger signaling through divergent pathways, the signaling converges on protein kinase C. We have proposed that at the end of the index ischemia the activated PKC sensitizes the low-affinity A(2b) adenosine receptor (A(2b)AR) through phosphorylation of either the receptor or its coupling proteins so that A(2b)AR can be activated by endogenous adenosine released by the previously ischemic cardiomyocytes. The sensitized A(2b)AR would then be responsible for activation of the survival kinases including PI3 kinase, Akt and ERK which then act to inhibit lethal mitochondrial permeability transition pore formation which normally uncouples mitochondria and destroys many myocytes in the first minutes of reperfusion. Herein we review the evidence for the above mechanisms and their functional details.

Intracellular glucose and binding of hexokinase and phosphofructokinase to particulate fractions increase under hypoxia in heart of the amazonian armored catfish (Liposarcus pardalis)

Armored catfish (Liposarcus pardalis), indigenous to the Amazon basin, have hearts that are extremely tolerant of oxygen limitation. Here we test the hypothesis that resistance to hypoxia is associated with increases in binding of selected glycolytic enzymes to subcellular fractions. Preparations of isolated ventricular sheets were subjected to 2 h of either oxygenated or hypoxic (via nitrogen gassing) treatment during which time the muscle was stimulated to contract. The bathing medium contained 5 mM glucose and was maintained at 25 degrees C. Initial experiments revealed increases in anaerobic metabolism. There was no measurable decrease in glycogen level; however, hypoxic treatment led to a twofold increase in heart glucose and a 10-fold increase in lactate content. It is suggested that the increase in heart glucose content is a result of an enhanced rate of facilitated glucose transport that exceeds the rate of phosphorylation of glucose. Further experiments assessed activities of metabolic enzymes in crude homogenates and subsequently tracked the degree of enzyme binding associated with subcellular fractions. Total maximal activities of glycolytic enzymes (hexokinase [HK], phosphofructokinase [PFK], aldolase, pyruvate kinase, lactate dehydrogenase), and a mitochondrial marker, citrate synthase, were not altered with the hypoxic treatment. A substantial portion (>/=50%) of HK is permanently bound to mitochondria, and this level increases under hypoxia. The amount of HK that is bound to the mitochondrial fraction is at least fourfold higher in hearts of L. pardalis than in rat hearts. Hypoxia also resulted in increased binding of PFK to a particulate fraction, and the degree of binding is higher in hypoxia-tolerant fish than in hypoxia-sensitive mammalian hearts. Such binding may be associated with increased glycolytic flux rates through modulation of enzyme-specific kinetics. The binding of HK and PFK occurs before any significant decrease in glycogen level.

Application of NMR-based metabonomics in the investigation of myocardial ischemia-reperfusion, ischemic preconditioning and antioxidant intervention in rabbits

NMR based metabonomics was applied in rabbit plasma samples during myocardial ischemia-reperfusion injury, with the following interventions: (1) Control (no intervention); (2) ischemic preconditioning (IpC); (3) administration of melatonin; (4) IpC+administration of melatonin; (5) treatment of the indole derivative C6458. The (1)H NMR signal intensity ratio of lactate/glucose was found to increase in Control samples during reperfusion compared to baseline, while lactate+alanine/acetate was decreased suggesting impairment of aerobic glycolysis and concomitant lipid utilization. In contrast, after IpC or treatment with C6458, the lactate/glucose ratio was similar to baseline in accordance with the previously reported decrease in infarct size. Multivariate statistical methods such as Principal Component Analysis (PCA), and Discriminant Analysis (DA) were used for the discrimination of samples. The use of ANOVA variable preselection prior to PCA was advantageous in producing adequate models. PCA could classify the Control group in three clusters according to the condition of the heart (baseline-ischemia-reperfusion) while in the IpC groups no classification was evident. PCA discrimination upon treatment with melatonin and C6458 provided further evidence of their effect on the metabolic profile. The supervised DA resulted in fine discrimination between the different subgroups. Plasma NMR spectra in combination with pattern recognition techniques proved to be an efficient and simple method to depict the metabolic changes produced upon ischemia-reperfusion of the myocardium.

Proteomic analysis of pharmacological preconditioning: novel protein targets converge to mitochondrial metabolism pathways

Ischemic preconditioning is characterized by resistance to ischemia reperfusion injury in response to previous short ischemic episodes, a protective effect that can be mimicked pharmacologically. The underlying mechanism of protection remains controversial and requires greater understanding before it can be fully exploited therapeutically. To investigate the overall effect of preconditioning on the myocardial proteome, isolated rabbit ventricular myocytes were treated with drugs known to induce preconditioning, adenosine or diazoxide (each at 100 micromol/L for 60 minutes). Their protein profiles were then compared with vehicle-treated controls (n=4 animals per treatment) using a multitiered 2D gel electrophoresis approach. Of 28 significantly altered protein spots, 19 nonredundant proteins were identified (5 spots remained unidentified). The majority of these proteins are involved in mitochondrial energetics, including subunits of tricarboxylic acid cycle enzymes and oxidative phosphorylation complexes. These changes were not indiscriminate, with only a small number of enzymes or complex subunits altered, indicating a very specific and targeted affect of these 2 preconditioning mimetics. Among the changes were shifts in the extent of posttranslational modification of 4 proteins. One of these, the adenosine-induced phosphorylation of the ATP synthase beta subunit, was fully characterized with the identification of 5 novel phosphorylation sites. This proteomics approach provides an overall assessment of the cellular response to pharmacological treatment with adenosine and diazoxide and identifies a distinct subset of enzymes and protein complex subunit that may underlie the preconditioned phenotype.

Inducing late phase of infarct protection in skeletal muscle by remote preconditioning: efficacy and mechanism

We have previously demonstrated that remote ischemic preconditioning (IPC) by instigation of three cycles of 10-min occlusion/reperfusion in a hindlimb of the pig elicits an early phase of infarct protection in local and distant skeletal muscles subjected to 4 h of ischemia immediately after remote IPC. The aim of this project was to test our hypothesis that hindlimb remote IPC also induces a late phase of infarct protection in skeletal muscle and that KATP channels play a pivotal role in the trigger and mediator mechanisms. We observed that pig bilateral latissimus dorsi (LD) muscle flaps sustained 46 ± 2% infarction when subjected to 4 h of ischemia/48 h of reperfusion. The late phase of infarct protection appeared at 24 h and lasted up to 72 h after hindlimb remote IPC. The LD muscle infarction was reduced to 28 ± 3, 26 ± 1, 23 ± 2, 24 ± 2 and 24 ± 4% at 24, 28, 36, 48 and 72 h after remote IPC, respectively ( P < 0.05; n = 8). In subsequent studies, hindlimb remote IPC or intravenous injection of the sarcolemmal KATP (sKATP) channel opener P-1075 (2 μg/kg) at 24 h before 4 h of sustained ischemia (i.e., late preconditioning) reduced muscle infarction from 43 ± 4% (ischemic control) to 24 ± 2 and 19 ± 3%, respectively ( P < 0.05, n = 8). Intravenous injection of the sKATP channel inhibitor HMR 1098 (6 mg/kg) or the nonspecific KATP channel inhibitor glibenclamide (Glib; 1 mg/kg) at 10 min before remote IPC completely blocked the infarct- protective effect of remote IPC in LD muscle flaps subjected to 4 h of sustained ischemia at 24 h after remote IPC. Intravenous bolus injection of the mitochondrial KATP (mKATP) channel inhibitor 5-hydroxydecanoate (5-HD; 5 mg/kg) immediately before remote IPC and 30-min intravenous infusion of 5-HD (5 mg/kg) during remote IPC did not affect the infarct-protective effect of remote IPC in LD muscle flaps. However, intravenous Glib or 5-HD, but not HMR 1098, given 24 h after remote IPC completely blocked the late infarct-protective effect of remote IPC in LD muscle flaps. None of these drug treatments affected the infarct size of control LD muscle flaps. The late phase of infarct protection was associated with a higher ( P < 0.05) muscle content of ATP at the end of 4 h of ischemia and 1.5 h of reperfusion and a lower ( P < 0.05) neutrophilic activity at the end of 1.5 h of reperfusion compared with the time-matched control. In conclusion, these findings support our hypothesis that hindlimb remote IPC induces an uninterrupted long (48 h) late phase of infarct protection, and sKATP and mKATP channels play a central role in the trigger and mediator mechanism, respectively.

Myocardial protection from ischemic preconditioning is not blocked by sub-chronic inhibition of carnitine palmitoyltransferase I

Ischemic preconditioning (IP) triggers cardioprotection via a signaling pathway that converges on mitochondria. The effects of the inhibition of carnitine palmitoyltransferase I (CPT-I), a key enzyme for transport of long chain fatty acids (LCFA) into the mitochondria, on ischemia/reperfusion (I/R) injury are unknown. Here we investigated, in isolated perfused rat hearts, whether sub-chronic CPT-I inhibition (5 days i.p. injection of 25 mg/kg/day of Etomoxir) affects I/R-induced damages and whether cardioprotection by IP can be induced after this inhibition. Effects of global ischemia (30 min) and reperfusion (120 min) were examined in hearts harvested from Control (untreated), Vehicle- or Etomoxir-treated animals. In subsets of hearts from the three treated groups, IP was induced by three cycles of 3 min ischemia followed by 10 min reperfusion prior to I/R. The extent of I/R injury under each condition was assessed by changes in infarct size as well as in myocardial contractility. Postischemic contractility, as indexed by developed pressure and dP/dt(max), was similarly affected by I/R, and was similarly improved with IP in Control, Vehicle or Etomoxir treated animals. Infarct size was also similar in the three subsets without IP, and was significantly reduced by IP regardless of CPT-I inhibition. We conclude that CPT-I inhibition does not affect I/R damages. Our data also show that IP affords myocardial protection in CPT-I inhibited hearts to a degree similar to untreated animals, suggesting that a long-term treatment with the metabolic anti-ischemic agent Etomoxir does not impede the possibility to afford cardioprotection by ischemic preconditioning.

Ischemic preconditioning, insulin, and morphine all cause hexokinase redistribution

Association of hexokinase (HK) with mitochondria preserves mitochondrial integrity and is an important mechanism by which cancer cells are protected against hypoxic conditions. Maintenance of mitochondrial integrity also figures prominently as a major characteristic of many cardioprotective manipulations. In this study, we provide evidence that cardioprotective interventions may promote HK redistribution from the cytosol to the mitochondria in the heart. Isolated Langendorff-perfused rat hearts (n = 6/group) were subjected to normoxic perfusion (control, Con), three 5-min ischemia-reperfusion periods (ischemic preconditioning, IPC), 1 U/l insulin (Ins), or 1 microM morphine (Mor). Hearts were immediately homogenized and centrifuged to obtain whole cell, cytosolic, and mitochondrial fractions. HK, lactate dehydrogenase (LDH), and citrate synthase (CS) enzyme activities were determined. No change in LDH or CS present in the cytosol fraction relative to whole cell activity was observed with any of the cardioprotective interventions. By contrast, HK present in the cytosol fraction relative to whole cell activity decreased significantly (P < 0.05) with all cardioprotective interventions, from 0.58 +/- 0.03 (Con) to 0.46 +/- 0.04 (IPC), 0.41 +/- 0.01 (Ins), and 0.45 +/- 0.02 (Mor). In addition, HK relative to CS activity in the mitochondrial fraction increased significantly with cardioprotection, from 0.15 +/- 0.001 (Con) to 0.21 +/- 0.002 (IPC), 0.18 +/- 0.003 (Ins), and 0.21 +/- 0.005 (Mor). Our novel data suggest that well-known cardioprotective interventions share a common end-effector mechanism of cytosolic HK translocation. Association of HK with mitochondria may promote inhibition of the mitochondrial permeability transition pore and thereby reduce cell death and apoptosis.

Ischaemic Preconditioning Protects Against Ischaemia/Reperfusion Injury: Emerging Concepts

INTRODUCTION

Ischaemic preconditioning (IP) has emerged as a powerful method of ameliorating ischaemia/reperfusion (I/R) injury to the myocardium. This review investigates whether this phenomenon is universally applicable in modulating I/R injury to other tissues.

METHODS

A Medline search was conducted to identify both animal and human studies that described IP-induced protection from I/R injury in a variety of non-cardiac organ systems. Particular emphasis was placed on elucidation of underlying physiological concepts.

RESULTS AND CONCLUSIONS

IP utilises endogenous mechanisms in skeletal muscle, liver, lung, kidney, intestine and brain in animal models to convey varying degrees of protection from I/R injury. To date there are few human studies, but recent reports suggest that human liver, lung and skeletal muscle acquire similar protection after IP. Specifically, preconditioned tissues exhibit reduced energy requirements, altered energy metabolism, better electrolyte homeostasis and genetic re-organisation, giving rise to the concept of 'ischaemia tolerance'. IP also induces 'reperfusion tolerance' with less reactive oxygen species and activated neutrophils released, reduced apoptosis and better microcirculatory perfusion compared to non-preconditioned tissue. Systemic I/R injury is also diminished by preconditioning. IP is ubiquitous but more research is required to fully translate these findings to the clinical arena.

Delayed ischemic preconditioning activates nuclear-encoded electron-transfer-chain gene expression in parallel with enhanced postanoxic mitochondrial respiratory recovery

BACKGROUND

Delayed ischemic preconditioning promotes cardioprotection via genomic reprogramming. We hypothesize that molecular regulation of mitochondrial energetics is integral to this cardioprotective program.

METHODS AND RESULTS

Preconditioning was induced by use of 3 episodes of 3-minute coronary artery occlusion separated by 5 minutes of reperfusion. Twenty-four hours later, infarct size was reduced by 58% after preconditioning compared with sham-operated controls (P<0.001). Cardiac mitochondria were isolated from sham and preconditioned rat hearts. Mitochondrial respiration and ATP production were similar between the groups; however, preconditioned mitochondria exhibit modest hyperpolarization of the inner mitochondrial membrane potential (> or =22% versus control, P<0.001). After 35-minute anoxia and reoxygenation, preconditioned mitochondria demonstrated a 191+/-12% improvement in ADP-sensitive respiration (P=0.002) with preservation of electron-transfer-chain (ETC) activity versus controls. This augmented mitochondrial recovery was eradicated when preconditioning was abolished by the antioxidant 2-mercaptopropionyl glycine (2-MPG). These biochemical modulations appear to be regulated at the genomic level in that the expression of genes encoding rate-controlling complexes in the ETC was significantly upregulated in preconditioned myocardium, with a concordant induction of steady-state protein levels of cytochrome oxidase, cytochrome c, and adenine nucleotide translocase-1. 2-MPG abolished preconditioning induction of these transcripts. Moreover, transcripts of nuclear regulatory peptides known to orchestrate mitochondrial biogenesis, nuclear respiratory factor-1 and peroxisome-proliferator-activated receptor gamma coactivator 1alpha, were significantly induced in preconditioned myocardium.

CONCLUSIONS

Delayed preconditioned mitochondria display increased tolerance against anoxia-reoxygenation in association with modifications in mitochondrial bioenergetics, with concordant genomic induction of a mitochondrial energetic gene regulatory program. This program appears to be mediated by reactive oxygen species signaling.

F0F1 ATP synthase activity is differently modulated by coronary reactive hyperemia before and after ischemic preconditioning in the goat

The amplitude of coronary reactive hyperemia (CRH), elicited by 15 s of ischemia, is reduced in hearts subjected to 5 min of ischemic preconditioning (IP). F0F1 ATP synthase activity and ATP concentration are also altered by IP. We hypothesized that F0F1 ATP synthase is differently modulated by the inhibitor protein IF(1) during CRH elicited before (CRHnp) and after (CRHprec) IP. Hemodynamic parameters were recorded in 10 anesthetized goats. Myocardial biopsies were obtained before IP (Cnp), during CRHnp, 4 and 6 min after the onset of CRHnp, after IP (Cprec), during CRHprec, and 4 min after CRHprec. F0F1 ATP synthase activity, ATP concentration, and ATP-to-ADP ratio (ATP/ADP) were determined. Compared with CRHnp, IP blunted CRHprec. F0F1 ATP synthase activity transiently increased during CRHnp, decreased 4 min after CRHnp, and returned to control 2 min later; it was lower after IP (Cprec) and did not change during and after CRHprec. All these changes in activity were modulated by IF1. During CRHnp, ATP concentration and ATP/ADP were reduced compared with Cnp and began to rise 6 min thereafter. During Cprec, both parameters were transiently reduced but increased during and after CRHprec. Hence, during CRHnp, F0F1 ATP synthase activity transiently increases and then decreases significantly. The short-lasting inhibition of the enzyme may explain why a few seconds of occlusion do not induce IP. After IP, F0F1 ATP synthase activity is blunted, and it is not affected by a subsequent 15 s of occlusion, which induces a blunted CRHprec. These results suggest that postischemic long-lasting inhibition of F0F1 ATP synthase activity may be a feature of the preconditioned heart. The increase in ATP concentration after preconditioning is in agreement with previous reports of reduced ATP hydrolysis by cytoplasmic ATPases.

Angina pectoris prior to myocardial infarction protects against subsequent left ventricular remodeling

OBJECTIVES

To investigate the hypothesis that prior angina pectoris confers protection from remodeling occurring after myocardial infarction (MI), we analyzed echocardiograms from the Healing and Early Afterload Reducing Therapy (HEART) trial.

BACKGROUND

Ischemia occurring before MI has been shown to reduce infarct size in experimental models and to improve outcomes in patients. The extent to which ischemia occurring before MI influences subsequent changes in ventricular size and function is unclear.

METHODS

We studied 283 patients enrolled in the HEART trial who had echocardiograms at days 1 and 90 after MI. Left ventricular (LV) dilation from days 1 to 90 was used as a measure of LV remodeling. We explored the relationship between symptomatic angina occurring before infarction and subsequent LV remodeling.

RESULTS

In patients who reported angina (n = 111) during the three months preceding MI, LV volume change was -0.73 +/- 2.6 ml over the 90-day post-MI period, compared with 6.8 +/- 2.6 ml for patients (n = 172) without angina (p = 0.017). In contrast, there were no differences in changes in ejection fraction based on prior angina. Maximal creatine kinase was significantly lower in patients with prior angina (2,119 +/- 1,729 vs. 2,701 +/- 2,088, p = 0.016). In a multivariate model, prior angina remained protective for ventricular remodeling after adjusting for age, gender, baseline ejection fraction, Killip class, baseline end-diastolic volume, and drug treatment group (p = 0.042). However, the protective effect of pre-infarction angina appeared to be attenuated in diabetic patients.

CONCLUSIONS

Ischemic symptoms occurring before MI may protect against LV remodeling. These protective effects may be secondary to recruitment of collaterals or ischemic preconditioning of the myocardium, and they appear to be attenuated in diabetic patients.

Frequency and clinical significance of ischemic preconditioning during percutaneous coronary intervention

OBJECTIVES

We sought to examine the short- and long-term clinical consequences of ischemic preconditioning (IP) during percutaneous coronary intervention (PCI).

BACKGROUND

Ischemic preconditioning has been demonstrated in animal models to significantly diminish the extent of myocardial necrosis consequent to coronary occlusion. Surrogate markers of ischemic injury (ST segment shift, lactate release, creatine kinase release) in humans have been shown to be similarly diminished with IP elicited during PCI. There are no studies of the frequency of inducibility of IP during PCI, nor are there longer-term data on the clinical relevance of IP.

METHODS

A total of 382 patients underwent elective PCI employing a previously validated protocol to elicit IP. Procedural, in-hospital, and one-year outcomes were recorded.

RESULTS

Ischemic preconditioning was elicited in 80% of patients and was associated with a significant reduction in the likelihood of in-hospital adverse cardiac events (IP group, 12.1%; non-IP group, 44.1%; p < 0.0001). Women and diabetic patients were less likely to exhibit IP. By one year, patients failing to manifest IP were at significantly greater risk of post-discharge death or non-fatal myocardial infarction (MI) (non-IP group, 25.9%; IP group, 11.1%; p < 0.002). Failure to manifest IP was significantly and independently associated with an increased risk of death or non-fatal MI by one year.

CONCLUSIONS

Clinically relevant short- and long-term cardioprotection can be found in association with IP during PCI. In-hospital adverse ischemic events are significantly diminished in patients with IP, as are the risks of death or non-fatal MI at one year. Failure to elicit IP during PCI serves as an independent marker of increased risk of future ischemic events.

Mitochondrial potassium transport: the role of the mitochondrial ATP-sensitive K(+) channel in cardiac function and cardioprotection

Coronary artery disease and its sequelae-ischemia, myocardial infarction, and heart failure-are leading causes of morbidity and mortality in man. Considerable effort has been devoted toward improving functional recovery and reducing the extent of infarction after ischemic episodes. As a step in this direction, it was found that the heart was significantly protected against ischemia-reperfusion injury if it was first preconditioned by brief ischemia or by administering a potassium channel opener. Both of these preconditioning strategies were found to require opening of a K(ATP) channel, and in 1997 we showed that this pivotal role was mediated by the mitochondrial ATP-sensitive K(+) channel (mitoK(ATP)). This paper will review the evidence showing that opening mitoK(ATP) is cardioprotective against ischemia-reperfusion injury and, moreover, that mitoK(ATP) plays this role during all three phases of the natural history of ischemia-reperfusion injury preconditioning, ischemia, and reperfusion. We discuss two distinct mechanisms by which mitoK(ATP) opening protects the heart-increased mitochondrial production of reactive oxygen species (ROS) during the preconditioning phase and regulation of intermembrane space (IMS) volume during the ischemic and reperfusion phases. It is likely that cardioprotection by ischemic preconditioning (IPC) and K(ATP) channel openers (KCOs) arises from utilization of normal physiological processes. Accordingly, we summarize the results of new studies that focus on the role of mitoK(ATP) in normal cardiomyocyte physiology. Here, we observe the same two mechanisms at work. In low-energy states, mitoK(ATP) opening triggers increased mitochondrial ROS production, thereby amplifying a cell signaling pathway leading to gene transcription and cell growth. In high-energy states, mitoK(ATP) opening prevents the matrix contraction that would otherwise occur during high rates of electron transport. MitoK(ATP)-mediated volume regulation, in turn, prevents disruption of the structure-function of the IMS and facilitates efficient energy transfers between mitochondria and myofibrillar ATPases.

Nonelectrocardiographic evidence that both ischemic preconditioning and adenosine preconditioning exist in humans

OBJECTIVES

The objective of this study was to use electrocardiogram (ECG)-independent parameters to determine whether preconditioning (PC) exists in humans during percutaneous transluminal coronary angioplasty (PTCA).

BACKGROUND

Several studies suggest that both ischemia and adenosine induce PC in the human heart during PTCA. However, because almost all of these studies relied on ST-segment shifts as indicators of the severity of ischemia, their conclusions continue to be questioned, and the very existence of ischemic or adenosine PC in humans remains controversial.

METHODS

Eighteen patients received either intracoronary adenosine (n = 9) or normal saline (n = 9); 10 min later, they underwent PTCA (three 2-min balloon inflations 5 min apart).

RESULTS

Compared with the first inflation, in untreated patients the second and third inflations were associated with less systolic dysfunction (two-dimensional echocardiography), less diastolic dysfunction (color M-mode echocardiography), less lactate production, and less H(+) release into the great cardiac venous blood. In adenosine-treated patients, the extent of all of these abnormalities during the first inflation was less than in untreated patients and did not change with subsequent inflations.

CONCLUSIONS

Previous exposure to a brief episode of ischemia (first balloon inflation) or to adenosine produces concordant decreases in ECG, subjective, mechanical, and metabolic manifestations of ischemia during PTCA. These data support the concept that both ischemic PC and pharmacologic PC exist in humans and that PTCA is a useful clinical setting in which to discern their mechanism.

Metabolic plasticity and the promotion of cardiac protection in ischemia and ischemic preconditioning

The concept of metabolic protection of the ischemic myocardium is in constant evolution and has recently been supported by clinical studies. Historically, enhanced glucose metabolism and glycolysis were proposed as anti-ischemic cardioprotection. This hypothesis is supported by the sub-cellular linkage between key glycolytic enzymes and the activity of two survival-promoting membrane-bound pumps, namely the sodium-potassium ATPase, and the calcium uptake pump of the sarcoplasmic reticulum. Moreover, improved resistance against ischemia follows the administration of glucose-insulin-potassium in a variety of animal models and in patients following acute myocardial infarction. The metabolic plasticity paradigm has now been expanded to include (1) the benefit of improved coupling of glycolysis to glucose oxidation, which explains the action of anti-ischemic fatty acid inhibitors such as trimetazidine and ranolazine; (2) the role of malonyl CoA in the glucose-fatty acid interaction; and (3) the anti-apoptotic role of insulin. Furthermore, we argue for a protective role of increased glucose uptake in the preconditioning paradigm. Additionally, we postulate an adaptive role of mitochondrial respiration in the promotion of cardioprotection in the context of ischemic preconditioning. The mechanisms driving these mitochondrial perturbations are still unknown, but are hypothesized to involve an initial modest uncoupling of respiration from the production of mitochondrial ATP. These perturbations are in turn thought to prime the mitochondria to augment mitochondrial respiration during a subsequent ischemic insult to the heart. In this review we discuss studies that demonstrate how metabolic plasticity can promote cardioprotection against ischemia and reperfusion injury and highlight areas that require further characterization.

Mechanisms by which opening the mitochondrial ATP- sensitive K(+) channel protects the ischemic heart

Diazoxide opening of the mitochondrial ATP-sensitive K(+) (mitoK(ATP)) channel protects the heart against ischemia-reperfusion injury by unknown mechanisms. We investigated the mechanisms by which mitoK(ATP) channel opening may act as an end effector of cardioprotection in the perfused rat heart model, in permeabilized fibers, and in rat heart mitochondria. We show that diazoxide pretreatment preserves the normal low outer membrane permeability to nucleotides and cytochrome c and that these beneficial effects are abolished by the mitoK(ATP) channel inhibitor 5-hydroxydecanoate. We hypothesize that an open mitoK(ATP) channel during ischemia maintains the tight structure of the intermembrane space that is required to preserve the normal low outer membrane permeability to ADP and ATP. This hypothesis is supported by findings in mitochondria showing that small decreases in intermembrane space volume, induced by either osmotic swelling or diazoxide, increased the half-saturation constant for ADP stimulation of respiration and sharply reduced ATP hydrolysis. These effects are proposed to lead to preservation of adenine nucleotides during ischemia and efficient energy transfer upon reperfusion.

Protein kinase C epsilon signaling complexes include metabolism- and transcription/translation-related proteins: complimentary separation techniques with LC/MS/MS

The serine/threonine kinase protein kinase C epsilon (PKC epsilon) has been shown to be a critical component in the heart's resistance to cell death following ischemic insult. Recent studies have indicated that PKC epsilon forms multi-protein signaling complexes to accomplish signal transduction in cardiac protection. Using two-dimensional electrophoresis (2DE), combined with matrix-assisted laser desorption ionization mass spectrometry (MS), the initial analysis of these complexes identified signaling molecules, structural proteins, and stress-activated proteins. The initial analysis, although fruitful, was limited by the number of proteins revealed on the 2D gels. It was also apparent that many known cardiac protective functions of PKC epsilon could not be fully accounted for by the proteins identified in the initial analysis. Here we reported the identification of an additional 57 proteins in PKC epsilon complexes using complimentary separation techniques, combined with high sensitivity MS. These techniques include 2DE or large format 1D SDS-PAGE followed by LC/MS/MS and solution trypsin digestion followed by LC/MS/MS, all of which yielded novel data regarding PKC epsilon protein complexes. Nanoscale LC/MS/MS for the analysis of gel-isolated proteins was performed with sub-femtomole sensitivity. In contrast to 2DE analyses, the identification of proteins from 1D gels was independent of their visualization via staining and allowed for the identification of proteins with high isoelectric points. We found that PKC epsilon complexes contain numerous structural and signaling molecules that had escaped detection by our previous analyses. Most importantly, we identified two new groups of proteins that were previously unrecognized as components of the PKC epsilon complex: metabolism-related proteins and transcription/translation-related proteins.

Consequences of brief ischemia: stunning, preconditioning, and their clinical implications: part 2

In experimental studies in the dog, total proximal coronary artery occlusions of up to 15 minutes result in reversible injury, meaning that the myocytes survive this insult. The 15 minutes of ischemia, however, induce numerous changes in the myocardium, including certain monuments to the brief episode of ischemia that may persist for days. One of these monuments is stunned myocardium, which represents "prolonged postischemic contractile dysfunction of myocardium salvaged by reperfusion." The mechanism of stunning involves generation of oxygen radicals as well as alteration in calcium homeostasis and possibly alteration in contractile protein structure. Stunning has been observed in several clinical scenarios, including after percutaneous transluminal coronary angioplasty, unstable angina, stress-induced ischemia, after thrombolysis, and after cardiopulmonary bypass. Oxygen radical scavengers and calcium channel blockers have been shown to enhance function of stunned myocardium in experimental studies, and in a few clinical studies, calcium channel blockers have been shown to ameliorate stunning. Although brief periods of ischemia can contribute to prolonged left ventricular dysfunction and even heart failure, they paradoxically play a cardioprotective role. Episodes of ischemia as short as 5 minutes, followed by reperfusion, protect the heart from a subsequent longer coronary artery occlusion by markedly reducing the amount of necrosis that results from the test episode of ischemia. This phenomenon, called ischemic preconditioning, has been observed in virtually every species in which it has been studied and is a powerful cardioprotective effect. The mechanism of ischemic preconditioning involves both triggers and mediators and involves complex second messenger pathways that appear to involve such components as adenosine, adenosine receptors, the epsilon isoform of protein kinase C, the ATP-dependent potassium channels, as well as others, including a paradoxical protective role of oxygen radicals. Both an early and a late phase of preconditioning have been described, and the mechanisms underlying their induction are under investigation. That preconditioning may occur in humans is suggested by the observations that repetitive balloon inflations in the coronary artery are associated with progressively less chest pain, ST-segment elevation, lactate production, the protective effects of preinfarction angina, the anginal "warm-up phenomenon," and studies performed on human cardiac biopsies that show metabolic properties suggesting preconditioning. Development of pharmacological agents that stimulate second messenger pathways thought to be involved in preconditioning, but without causing ischemia, could result in novel approaches to treating ischemia. Hence, on one hand, brief episodes of ischemia can have a negative effect on the heart: stunning; and on the other hand, they have a protective effect: preconditioning. The future challenge is how to minimize the stunning phenomenon and maximize the preconditioning phenomenon in clinical practice.

Consequences of brief ischemia: stunning, preconditioning, and their clinical implications: part 1

In experimental studies in the dog, total proximal coronary artery occlusions of up to 15 minutes result in reversible injury, meaning that the myocytes survive this insult. The 15 minutes of ischemia, however, induce numerous changes in the myocardium, including certain monuments to the brief episode of ischemia that may persist for days. One of these monuments is stunned myocardium, which represents "prolonged postischemic contractile dysfunction of myocardium salvaged by reperfusion." The mechanism of stunning involves generation of oxygen radicals as well as alteration in calcium homeostasis and possibly alteration in contractile protein structure. Stunning has been observed in several clinical scenarios, including after percutaneous transluminal coronary angioplasty, unstable angina, stress-induced ischemia, after thrombolysis, and after cardiopulmonary bypass. Oxygen radical scavengers and calcium channel blockers have been shown to enhance function of stunned myocardium in experimental studies, and in a few clinical studies, calcium channel blockers have been shown to ameliorate stunning. Although brief periods of ischemia can contribute to prolonged left ventricular dysfunction and even heart failure, they paradoxically play a cardioprotective role. Episodes of ischemia as short as 5 minutes, followed by reperfusion, protect the heart from a subsequent longer coronary artery occlusion by markedly reducing the amount of necrosis that results from the test episode of ischemia. This phenomenon, called ischemic preconditioning, has been observed in virtually every species in which it has been studied and is a powerful cardioprotective effect. The mechanism of ischemic preconditioning involves both triggers and mediators and involves complex second messenger pathways that appear to involve such components as adenosine, adenosine receptors, the epsilon isoform of protein kinase C, the ATP-dependent potassium channels, as well as others, including a paradoxical protective role of oxygen radicals. Both an early and a late phase of preconditioning have been described, and the mechanisms underlying their induction are under investigation. That preconditioning may occur in humans is suggested by the observations that repetitive balloon inflations in the coronary artery are associated with progressively less chest pain, ST-segment elevation, lactate production, the protective effects of preinfarction angina, the anginal "warm-up phenomenon," and studies performed on human cardiac biopsies that show metabolic properties suggesting preconditioning. Development of pharmacological agents that stimulate second messenger pathways thought to be involved in preconditioning, but without causing ischemia, could result in novel approaches to treating ischemia. Hence, on one hand, brief episodes of ischemia can have a negative effect on the heart: stunning; and on the other hand, they have a protective effect: preconditioning. The future challenge is how to minimize the stunning phenomenon and maximize the preconditioning phenomenon in clinical practice.

Duration and reinstatement of myocardial protection against infarction by ischemic preconditioning in open chest dogs

These studies were undertaken to determine the duration of protection against myocardial infarction provided by ischemic preconditioning in the canine heart, and to learn if cardioprotection can be restored by another preconditioning stimulus when the initial effect is lost. Control and four preconditioning groups of anesthetized, open-chest dogs were compared. All underwent a test 60 min episode of ischemia, induced by occlusion of the anterior descending (LAD) artery, followed by 3 h of reperfusion. Preconditioning was induced by one 10 min LAD occlusion, followed by either 10 min, 2, 3, or 5 h of reperfusion. In order to test whether preconditioning could be reinstated, another group of dogs with preconditioning plus 3 h reperfusion underwent a second 10 min preconditioning stimulus with 10 min reperfusion before the 60 min test-occlusion. Infarct size (as percent of area-at-risk) was analyzed (using analysis of covariance) with respect to coronary collateral blood flow measured with radioactive microspheres. Infarct size was limited markedly by preconditioning (23+/-6 v 6+/-2%, P<0.05) but the protective effect was dissipated partially after 2 h reperfusion and was dissipated completely after 3 h reperfusion (20+/-4%, non-significant v Control and significant P<0.05 v preconditioning). Protection was restored in three of six dogs with preconditioning +5 h reperfusion, suggesting that the second window of protection appears early in some canine hearts. When preconditioning was repeated after 3 h reperfusion, cardioprotection was reinstated fully (7+/-2%, P<0.05 v Control and NS v preconditioning). The results show that maximal preconditioning cardioprotection is present in the dog heart after 10 min of reperfusion and is dissipated totally following 3 h of reperfusion. However, a second preconditioning stimulus of 10 min of ischemia followed by 10 min of reperfusion to the dissipated preconditioned heart reinstates full preconditioning. Thus, this model provides a system to test for theoretical causes of the preconditioned state. Final mediators should be present when preconditioning is present and absent when preconditioning is dissipated. It is noteworthy that a second window of protection appeared in 50% of dogs when the period of reperfusion was extended to 5 h.