Differential effects of postconditioning on myocardial stunning and infarction: a study in conscious dogs and anesthetized rabbits

Adverse Effects on β-Adrenergic Receptor Coupling: Ischemic Postconditioning Failed to Preserve Long-Term Cardiac Function

BACKGROUND

Ischemic preconditioning (IPC) and ischemic postconditioning (IPoC) are currently among the most efficient strategies protecting the heart against ischemia/reperfusion injury. However, the effect of IPC and IPoC on functional recovery following ischemia/reperfusion is less clear, particularly with regard to the specific receptor-mediated signaling of the postischemic heart. The current article examines the effect of IPC or IPoC on the regulation and coupling of β-adrenergic receptors and their effects on postischemic left ventricular function.

METHODS AND RESULTS

The β-adrenergic signal transduction was analyzed in 3-month-old Wistar rats for each of the intervention strategies (Sham, ischemia/reperfusion, IPC, IPoC) immediately and 7 days after myocardial infarction. Directly after the infarction a cardioprotective potential was demonstrated for both IPC and IPoC: the infarct size was reduced, apoptosis and production of reactive oxygen species were lowered, and the myocardial tissue was preserved. Seven days after myocardial ischemia, only IPC hearts showed significant functional improvement. Along with a deterioration in fractional shortening, IPoC hearts no longer responded adequately to β-adrenergic stimulation. The stabilization of β-adrenergic receptor kinase-2 via increased phosphorylation of Mdm2 (an E3-ubiquitin ligase) was responsible for desensitization of β-adrenergic receptors and identified as a characteristic specific to IPoC hearts.

CONCLUSIONS

Immediately after myocardial infarction, rapid and transient activation of β-adrenergic receptor kinase-2 may be an appropriate means to protect the injured heart from excessive stress. In the long term, however, induction and stabilization of β-adrenergic receptor kinase-2, with the resultant loss of positive inotropic function, leads to the functional picture of heart failure.

Combined morphine and limb remote ischemic perconditioning provides an enhanced protection against myocardial ischemia/reperfusion injury by antiapoptosis

BACKGROUND

Both morphine and limb remote ischemic perconditioning (RIPer) can protect against myocardial ischemia/reperfusion injury (IRI). This experiment was designed to assess whether combined morphine and limb RIPer could provide and enhanced protection against myocardial IRI in an in vivo rat model.

METHODS

One hundred male Sprague-Dawley rats were randomly allocated to six groups: sham, ischemia/reperfusion (IR), ischemic preconditioning, RIPer, morphine (M), and combined morphine and remote ischemic perconditioning (M + RIPer). Ventricular arrhythmias that occurred during ischemia and early reperfusion were scored, and serum creatine kinase isoenzyme and cardiac troponin I levels were assayed. The infarct size was determined by Evans blue and triphenyl tetrazolium chloride staining. The apoptosis in the myocardial ischemic core, ischemic border, and nonischemic areas was assessed through real-time polymerase chain reaction for Bax and Bcl-2 and with the transferase-mediated deoxyuridine triphosphate-biotin nick end labeling assay.

RESULTS

The infarct size, serum cardiac troponin I level, incidence, and score of the arrhythmias during the initial reperfusion were significantly reduced in the M + RIPer group compared with the IR group but did not differ significantly between the ischemic preconditioning and M + RIPer groups. Transferase-mediated deoxyuridine triphosphate-biotin nick end labeling-positive cells were significantly decreased, and the Bcl-2/Bax ratio was significantly increased in the M + RIPer group compared with the IR group.

CONCLUSIONS

This experiment demonstrates that combined morphine and limb RIPer provides an enhanced protection against myocardial IRI by the Bcl-2-linked apoptotic signaling pathway.

Mechanical post-conditioning in STEMI patients undergoing primary percutaneous coronary intervention

Although early myocardial reperfusion via primary percutaneous coronary intervention (PCI) allows the preservation of left ventricular function and improves outcome, the acute restoration of blood flow may contribute to the pathophysiology of infarction, a complex phenomenon called reperfusion injury. First described in animal models of coronary obstruction, mechanical post-conditioning, a sequence of repetitive interruption of coronary blood flow applied immediately after reopening of the occluded vessel, was able to reduce the infarct size. However, evidence of its real benefit remains controversial. This review describes the mechanisms of post-conditioning action and the different protocols employed focusing on its impact on primary PCI outcome.

Myocardial conditioning: opportunities for clinical translation

Myocardial conditioning is an endogenous cardioprotective phenomenon that profoundly limits infarct size in experimental models. The current challenge is to translate this paradigm from the laboratory to the clinic. Accordingly, our goal in this review is to provide a critical summary of the progress toward, opportunities for, and caveats to, the successful clinical translation of postconditioning and remote conditioning, the 2 conditioning strategies considered to have the broadest applicability for real-world patient care. In the majority of phase II studies published to date, postconditioning evoked a ≈35% reduction of infarct size in ST-segment-elevation myocardial infarction patients. Essential criteria for the successful implementation of postconditioning include the appropriate choice of patients (ie, those with large risk regions and negligible collateral flow), timely application of the postconditioning stimulus (immediately on reperfusion), together with proper choice of end points (infarct size, with concomitant assessment of risk region). Remote conditioning has been applied in planned ischemic events (including cardiac surgery and elective percutaneous coronary intervention) and in ST-segment-elevation myocardial infarction patients during hospital transport. Controversies with regard to efficacy have emerged, particularly among surgical trials. These disparate outcomes in all likelihood reflect the remarkable heterogeneity within and among studies, together with a deficit in our understanding of the impact of these variations on the infarct-sparing effect of remote conditioning. Ongoing phase III trials will provide critical insight into the future role of postconditioning and remote conditioning as clinically relevant cardioprotective strategies.

Effect of Ischaemic Postconditioning on Recovery of Left Ventricular Contractile Function after Acute Myocardial Infarction

OBJECTIVE: This study investigated whether ischaemic postconditioning can improve recovery of myocardial contractile function in acute myocardial infarction patients 1 week and 6 months after angioplasty. METHODS: A total of 62 patients undergoing direct percutaneous coronary intervention after acute myocardial infarction were randomly assigned to receive four episodes of inflation and deflation of the angioplasty balloon in the early reperfusion period (postconditioned group, n = 32), or no additional intervention (control group, n = 30). Two-dimensional size and left ventricular (LV) global and regional contractile functions were then evaluated by echocardiography at 1 week and 6 months after angioplasty. RESULTS: At 1 week, there were no significant differences in left atrial diameter, LV end-diastolic diameter, LV end-diastolic volume, cardiac output, LV ejection fraction or wall motion score index between the two groups. At 6 months, LV ejection fraction was significantly increased and the wall motion score index significantly reduced in the postconditioned group compared with the control group. CONCLUSION: Ischaemic postconditioning can improve long-term LV contractile function 6 months after reperfusion following acute myocardial infarction.

Protective effect of ischemic postconditioning against ischemia reperfusion-induced myocardium oxidative injury in IR rats

Brief episodes of myocardial ischemia-reperfusion (IR) employed during reperfusion after a prolonged ischemic insult may attenuate the total ischemia-reperfusion injury. This phenomenon has been termed ischemic postconditioning. In the present study, we studied the possible effect of ischemic postconditioning on an ischemic reperfusion (IR)-induced myocardium oxidative injury in rat model. Results showed that ischemic postconditioning could improve arrhythmia cordis, reduce myocardium infarction and serum creatin kinase (CK), lactate dehydrogenase (LDH) and aspartate transaminase (AST) activities in IR rats. In addition, ischemic postconditioning could still decrease myocardium malondialdehyde (MDA) level, and increased myocardium Na+-K+-ATPase, Ca2+-Mg2+-ATPase, superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GSH-Px) and glutathione reductase (GR) activities. It can be concluded that ischemic postconditioning possesses strong protective effects against ischemia reperfusion-induced myocardium oxidative injury in IR rats.

Perconditioning and postconditioning: current knowledge, knowledge gaps, barriers to adoption, and future directions

The broad definition of "conditioning" is the application of a series of alternating intervals of brief ischemia (hypoxia) and reperfusion (reoxygenation) applied in the setting of prolonged ischemia causing myocardial infarction. While the conditioning stimulus is applied before the major (index) ischemic event in ischemic preconditioning, it is applied during the event in perconditioning, and applied after the event (reperfusion) in postconditioning. Studies on perconditioning have only recently demonstrated a reduction in infarct size by remote ischemia applied during transport of heart attack victims to the hospital before percutaneous coronary interventions (PCIs). The "conditioning" paradigm has been extended to include remote perconditioning and remote postconditioning. However, the biology of perconditioning is virtually unknown. Postconditioning has enjoyed enthusiastic attention from scientists that have done much to demonstrate that the model of triggers, mediators, and effectors used in preconditioning may also apply to postconditioning, with the addition and important contribution of physiological mechanisms resulting in cardioprotection, including gradual normalization of tissue pH, reduction in generation of reactive oxygen species, and avoidance of hypercontracture. This same schema has not been confirmed in perconditioning. However, the unknowns in both conditioning paradigms far outweigh the knowns. Why postconditioning does not exert cardioprotection in experimental models of comorbidities and aging, yet reduces postischemic injury and contractile dysfunction in older patients with multiple comorbidities is a conundrum for which no answers are forthcoming. The optimal algorithm is unknown, as is the interrelationship between the many molecular, cellular, and physiological pathways that purportedly "mediate" or "trigger" the conditioning responses. Whether there are common pathways engaged in all 3 forms of conditioning, and what nuances separate one form of conditioning from another are unanswered questions. Yet, the translational potential of per- and postconditioning will drive further experimental work and clinical trials, which will ask unprecedented cooperation and information sharing between basic and clinician scientists, and creative developments from industry.

Morphine and remifentanil-induced cardioprotection: its experimental and clinical outcomes

During the past few decades, a large number of animal studies demonstrated that commonly used opioids could provide cardioprotection against ischemia-reperfusion (I/R) injury. Opioid-induced preconditioning or postconditioning mimics ischemic preconditioning (I-Pre) or ischemic postconditioning (I-Post). Both δ- and κ-opioid receptors (OPRs) play a crucial role in opioid-induced cardioprotection (OIC). Down stream signaling effectors of OIC include ATP-sensitive potassium (K_ATP) channels, protein kinase C (PKC), tyrosine kinase, phosphatidylinositol-3-kinase (PI3-kinase), extracellular signal regulated kinase1/2 (ERK1/2), glycogen synthase kinase-3β (GSK-3β), and mitochondrial permeability transition pore (MPTP), among others. Recently, various reports also suggest that opioids could provide cardioprotection in humans. This review will discuss OIC using mostly morphine and remifentanil which are widely used during cardiac anesthesia in addition to the clinical implications of OIC.

Myocardial AKT: the omnipresent nexus

One of the greatest examples of integrated signal transduction is revealed by examination of effects mediated by AKT kinase in myocardial biology. Positioned at the intersection of multiple afferent and efferent signals, AKT exemplifies a molecular sensing node that coordinates dynamic responses of the cell in literally every aspect of biological responses. The balanced and nuanced nature of homeostatic signaling is particularly essential within the myocardial context, where regulation of survival, energy production, contractility, and response to pathological stress all flow through the nexus of AKT activation or repression. Equally important, the loss of regulated AKT activity is primarily the cause or consequence of pathological conditions leading to remodeling of the heart and eventual decompensation. This review presents an overview compendium of the complex world of myocardial AKT biology gleaned from more than a decade of research. Summarization of the widespread influence that AKT exerts upon myocardial responses leaves no doubt that the participation of AKT in molecular signaling will need to be reckoned with as a seemingly omnipresent regulator of myocardial molecular biological responses.

Ischemic postconditioning inhibits apoptosis after acute myocardial infarction in pigs

OBJECTIVES

Recent studies have shown that ischemic postconditioning reduces myocardial ischemia-reperfusion (I/R) injury; however, the effects of inhibiting apoptosis on cardioprotection induced by ischemic postconditioning remain to be determined. The objective of this study was to investigate whether ischemic postconditioning attenuates myocardial I/R injury by reduced apoptosis in a closed-chest pig model of acute myocardial infarction.

METHODS

Diannan small-ear pigs were randomly divided into 3 groups (5/group): (1) The sham group underwent a sham operation without ischemia; (2) the I/R group received 60 minutes of ischemia and 72 hours of reperfusion; and (3) the ischemic postconditioning (Postcond) group was treated the same as the I/R group except that the pigs received 8 cycles of 30 seconds of reperfusion and 30 seconds of ischemia at the onset of reperfusion. After 72 hours of reperfusion, infarct size was measured by 2,3,5-triphenyltetrazolium chloride staining. Apoptotic cells in the peri-infarct myocardium were evaluated with the terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) method, and apoptosis-related molecules were studied with western blotting analysis.

RESULTS

After 72 hours of reperfusion, mean (±SEM) infarct size was significantly smaller in the Postcond group than in the I/R group (23.26% ± 3.13% versus 10.89% ± 2.02%, P < .05). Apoptotic myocytes in the peri-infarct region were lower in the Postcond group than in the I/R group (15.31% ± 4.58% versus 33.83% ± 4.44%, P < .05). This decrease in the extent of apoptosis was accompanied by a significant decrease in Bax expression (0.306 ± 0.075 versus 0.433 ± 0.102 for the I/R group; P < .05) and a significant increase in Bcl-2 expression (1.801 ± 0.227 versus 1.267 ± 0.308 for the I/R group; P < .05).

CONCLUSIONS

In a clinically relevant closed-chest pig model of myocardial infarction, these data suggest the following: (1) Ischemic postconditioning reduces infarct size following prolonged reperfusion, and (2) this cardioprotective effect is likely achieved via antiapoptotic mechanisms.

The multidimensional physiological responses to postconditioning

Reperfusion is the definitive treatment to reduce infarct size and other manifestations of postischemic injury. However, reperfusion contributes to postischemic injury, and, therefore, reperfusion therapies do not achieve the optimal salvage of myocardium. Other tissues as well undergo injury after reperfusion, notably, the coronary vascular endothelium. Postconditioning has been shown to have salubrious effects on different tissue types within the heart (cardiomyocytes, endothelium) and to protect against various pathologic processes, including necrosis, apoptosis, contractile dysfunction, arrhythmias, and microvascular injury or "no-reflow." The mechanisms by which postconditioning alters the pathophysiology of reperfusion injury is exceedingly complex and involves physiological mechanisms (e.g., delaying re-alkalinization of tissue pH, triggering release of autacoids, and opening and closing of various channels) and molecular mechanisms (activation of kinases) that affect cellular and subcellular targets or effectors. The physiologic responses to postconditioning are not isolated or mutually exclusive, but are interactive, with one response affecting another in an integrated manner. This integrated response on multiple targets differs from the monotherapy approach by drugs that have failed to reduce reperfusion injury on a consistent basis and may underlie the efficacy of this therapeutic approach across species and in human trials.

MG53 participates in ischaemic postconditioning through the RISK signalling pathway

AIMS

Recent studies show that ischaemic postconditioning (PostC), similar to the well-established ischaemic preconditioning (IPC), confers cardioprotection against ischaemia/reperfusion (IR) injury, and both IPC and PostC can activate the reperfusion injury salvage kinase (RISK) pathway and the survivor activating factor enhancement (SAFE) pathway. PostC is clinically more attractive because of its therapeutic application at the predictable onset of reperfusion. Our previous studies have demonstrated that MG53 is a primary component of the IPC machinery. Here, we investigated the potential role of MG53 in PostC-mediated myocardial protection and explored the underlying mechanism.

METHODS AND RESULTS

Using Langendorff perfusion, we investigated IR injury in wild-type (wt) and MG53-deficient (mg53(-/-)) mouse hearts with or without PostC. IR-induced myocardial damage was markedly exacerbated in mg53(-/-) hearts compared with wt controls. PostC protected wt hearts against IR-induced myocardial infarction, myocyte necrosis, and apoptosis, but failed to protect mg53(-/-) hearts. The loss of PostC protection in mg53(-/-) hearts was attributed to selectively impaired PostC-activated RISK signalling. Mechanistically, MG53 is required for the interaction between caveolin 3 (CaV3) and the p85 subunit of phosphoinositide 3-kinase (p85-PI3K) and PostC-mediated activation of the RISK pathway. Importantly, a structure-function study revealed that the MG53 tripartite motif (TRIM) domain (aa1-284) physically interacted with CaV3 but not p85-PI3K, whereas the MG53 SPRY domain (aa285-477) interacted with p85-PI3K but not CaV3, indicating that MG53 binds to CaV3 and p85 at its N- and C-terminus, respectively.

CONCLUSIONS

We conclude that MG53 participates in PostC-mediated cardioprotection largely through tethering CaV3 and PI3K and subsequent activation of the RISK pathway.

Catestatin improves post-ischemic left ventricular function and decreases ischemia/reperfusion injury in heart

The Chromogranin A (CgA)-derived anti-hypertensive peptide catestatin (CST) antagonizes catecholamine secretion, and is a negative myocardial inotrope acting via a nitric oxide-dependent mechanism. It is not known whether CST contributes to ischemia/reperfusion injury or is a component of a cardioprotective response to limit injury. Here, we tested whether CST by virtue of its negative inotropic activity improves post-ischemic cardiac function and cardiomyocyte survival. Three groups of isolated perfused hearts from adult Wistar rats underwent 30-min ischemia and 120-min reperfusion (I/R, Group 1), or were post-conditioned by brief ischemic episodes (PostC, 5-cycles of 10-s I/R at the beginning of 120-min reperfusion, Group 2), or with exogenous CST (75 nM for 20 min, CST-Post, Group-3) at the onset of reperfusion. Perfusion pressure and left ventricular pressure (LVP) were monitored. Infarct size was evaluated with nitroblue-tetrazolium staining. The CST (5 nM) effects were also tested in simulated ischemia/reperfusion experiments on cardiomyocytes isolated from young-adult rats, evaluating cell survival with propidium iodide labeling. Infarct size was 61 ± 6% of risk area in hearts subjected to I/R only. PostC reduced infarct size to 34 ± 5%. Infarct size in CST-Post was 36 ± 3% of risk area (P < 0.05 respect to I/R). CST-Post reduced post-ischemic rise of diastolic LVP, an index of contracture, and significantly improved post-ischemic recovery of developed LVP. In isolated cardiomyocytes, CST increased the cell viability rate by about 65% after simulated ischemia/reperfusion. These results suggest a novel cardioprotective role for CST, which appears mainly due to a direct reduction of post-ischemic myocardial damages and dysfunction, rather than to an involvement of adrenergic terminals and/or endothelium.

Bradykinin and adenosine receptors mediate desflurane induced postconditioning in human myocardium: role of reactive oxygen species

BACKGROUND

Desflurane during early reperfusion has been shown to postcondition human myocardium, in vitro. We investigated the role of adenosine and bradykinin receptors, and generation of radical oxygen species in desflurane-induced postconditioning in human myocardium.

METHODS

We recorded isometric contraction of human right atrial trabeculae hanged in an oxygenated Tyrode's solution (34 degrees Celsius, stimulation frequency 1 Hz). After a 30-min hypoxic period, desflurane 6% was administered during the first 5 min of reoxygenation. Desflurane was administered alone or with pretreatment of N-mercaptopropionylglycine, a reactive oxygen species scavenger, 8-(p-Sulfophenyl)theophylline, an adenosine receptor antagonist, HOE140, a selective B2 bradykinin receptor antagonist. In separate groups, adenosine and bradykinin were administered during the first minutes of reoxygenation alone or in presence of N-mercaptopropionylglycine. The force of contraction of trabeculae was recorded continuously. Developed force at the end of a 60-min reoxygenation period was compared (mean +/- standard deviation) between the groups by a variance analysis and post hoc test.

RESULTS

Desflurane 6% (84 +/- 6% of baseline) enhanced the recovery of force after 60-min of reoxygenation as compared to control group (51 +/- 8% of baseline, P < 0.0001). N-mercaptopropionylglycine (54 +/- 3% of baseline), 8-(p-Sulfophenyl)theophylline (62 +/- 9% of baseline), HOE140 (58 +/- 6% of baseline) abolished desflurane-induced postconditioning. Adenosine (80 +/- 9% of baseline) and bradykinin (83 +/- 4% of baseline) induced postconditioning (P < 0.0001 vs control), N-mercaptopropionylglycine abolished the beneficial effects of adenosine and bradykinin (54 +/- 8 and 58 +/- 5% of baseline, respectively).

CONCLUSIONS

In vitro, desflurane-induced postconditioning depends on reactive oxygen species production, activation of adenosine and bradykinin B2 receptors. And, the cardioprotective effect of adenosine and bradykinin administered at the beginning of reoxygenation, was mediated, at least in part, through ROS production.

Effects of postconditioning with N,N,N'N'-tetrakis-[2-pyridylmethyl]-ethylenediamine in isolated rat hearts

Background

It was reported that N,N,N'N'-tetrakis-[2-pyridylmethyl]-ethylenediamine (TPEN), a transition metal chelator, confers cardioprotection against myocardial ischemic injury. In this study, we investigated the effect of TPEN targeting reperfusion period in isolated rat hearts.

Methods

Langendorff perfused rat hearts were subjected to 30 min of regional ischemia and 2 h of reperfusion. Hearts were randomly assigned to either control (n = 9) or 10 µM of TPEN (n = 8) groups. TPEN was perfused for a period of 5 min before and 30 min after reperfusion.

Results

The ratio of infarct area/ischemic area (AN/AR) was significantly reduced in TPEN treated hearts (6.9 ± 1.7%, P < 0.001) compared to control hearts (29.5 ± 3.2%). Recovery of left ventricular developed pressure (LVDP), rate-pressure product (RPP), +dP/dt_max, and -dP/dt_min in the control group after reperfusion were 53.8 ± 6.2%, 51.0 ± 6.3%, 51.9 ± 5.7%, and 51.4 ± 5.7%, respectively, of the baseline levels. In the TPEN group, LVDP, RPP, +dP/dt_max, and -dP/dt_min returned to 58.5 ± 4.6%, 54.8 ± 6.4%, 61.7 ± 4.9%, and 53.4 ± 3.9%, respectively, of the baseline levels. There were no significant differences in the cardiodynamic variables between the two groups (P > 0.05).

Conclusions

Pharmacological postconditioning with TPEN reduces myocardial infarction however, TPEN does not modify post-ischemic systolic dysfunction in isolated rat hearts.

A servo-controlled canine model of stable severe ischemic left ventricular failure

Reversible left ventricular failure was produced in conscious dogs by compromise of the coronary circulation. In animals with prior left anterior descending coronary artery occlusion, mean left atrial pressure (LAP) was incorporated into an automatic feedback control system used to inflate a balloon cuff on the circumflex (Cfx) coronary artery. The system could produce stable increases in LAP to 15-20 mm Hg. The dominating system transfer function was the ratio of LAP to balloon volume (BV), which was characterized by a fixed delay (5 s), with LAP/BV = (8e(-jomegatau ))/(0.02 + jomega). The system was stabilized by a phase lead network to reduce oscillations of LAP. A total of seven experiments were conducted in three dogs, and testing of inotropic agents was possible in three experiments under stable conditions with the pump off after an hour or more of operation. Problems encountered were 0.003-0.008 Hz oscillations in LAP in three experiments, which could usually be controlled by reducing the system gain. Late stage ventricular fibrillation occurred in all three animals, but defibrillation was easily accomplished after deflating the Cfx balloon. This system produces reversible left ventricular failure solely due to ischemia, thus closely simulating clinical heart failure due to coronary insufficiency.

Molecular aspects of ischaemic postconditioning

Preconditioning, a well established phenomenon had been used since 1980s to attenuate ischaemia-reperfusion induced injury. However, inability to predict the onset of ischaemia in clinical settings led to the discovery of a new concept of postconditioning (PoCo), in 2000s whereby brief repetitive cycles of ischaemia with intermittent reperfusion followed by prolonged ischaemia-elicited tissue protection. There is an impressive array of molecular mechanisms contributing to PoCo-mediated tissue-protection, which include triggers like adenosine (ADO), opioid, erythropoietin (EPO), endogenous nitric-oxide, reactive oxygen species, acetylcholine, tissue factors, pro-inflammatory cytokines and bradykinin; mediators like reperfusion injury salvage kinase pathways including phosphoinositide-3-kinase, extra-cellular signal regulated kinase(1/2) pathway, protein kinase G and protein kinase C; end-effectors like mitochondrial permeability transition pore and mitochondrial potassium ATP channel. The clinical applicability of PoCo has been extended with the use of PoCo mimetic agents like insulin, glucagon like peptide, EPO, statins and ADO before reperfusion in patients with ischaemia reperfusion injury. Remote PoCo has also emerged as a new concept; however, considerable research is required for understanding its molecular mechanisms. In this review, an exhaustive attempt has been made to unearth some molecular aspects of PoCo.

Ischemic postconditioning: experimental models and protocol algorithms

Ischemic postconditioning, a simple mechanical maneuver at the onset of reperfusion, reduces infarct size after ischemia/reperfusion. After its first description in 2003 by Zhao et al. numerous experimental studies have investigated this protective phenomenon. Whereas the underlying mechanisms and signal transduction are not yet understood in detail, infarct size reduction by ischemic postconditioning was confirmed in all species tested so far, including man. We have now reviewed the literature with focus on experimental models and protocols to better understand the determinants of protection by ischemic postconditioning or lack of it. Only studies with infarct size as unequivocal endpoint were considered. In all species and models, the duration of index ischemia and the protective protocol algorithm impact on the outcome of ischemic postconditioning, and gender, age, and myocardial temperature contribute.

Reversible blockade of electron transport with amobarbital at the onset of reperfusion attenuates cardiac injury

Mitochondrial dysfunction contributes to myocardial injury during ischemia and reperfusion. Ischemia damages the mitochondrial electron transport chain. Therapeutic intervention during early reperfusion decreases cardiac injury, which suggests that myocardial injury can be attenuated even though mitochondria were already damaged during the preceding ischemia. Our previous study shows that amobarbital given only before ischemia prevents ischemic damage to the electron transport chain and decreases infarct size measured during reperfusion in Langendorff-perfused Fischer 344 rat hearts. In the current study, amobarbital was given at the onset of reperfusion to test whether the blockade of proximal electron transport only during early reperfusion can decrease myocardial injury. Amobarbital administrated during early reperfusion decreased infarct size compared with untreated hearts, which suggests that the modulation of electron transport during early reperfusion attenuates myocardial injury. The increased generation of reactive oxygen species (ROS) contributes to injury. We tested whether the blockade of proximal electron transport prevents ROS release from the mitochondria that sustained ischemic damage. The blockade of the proximal electron transport chain at complex I attenuates maximal ROS generation from ischemia-damaged mitochondria. Thus, the modulation of oxidative function during reperfusion provides a translationally relevant opportunity to prevent a portion of the mitochondrial-dependent injury. The cardiac protection by amobarbital given during reperfusion may result from decreased ROS generation from the electron transport chain.

Cardioprotection: a radical view Free radicals in pre and postconditioning

A series of brief (a few minutes) ischemia/reperfusion cycles (ischemic preconditioning, IP) limits myocardial injury produced by a subsequent prolonged period of coronary artery occlusion and reperfusion. Postconditioning (PostC), which is a series of brief (a few seconds) reperfusion/ischemia cycles at reperfusion onset, attenuates also ischemia/reperfusion injury. In recent years the main idea has been that reactive oxygen species (ROS) play an essential, though double-edged, role in cardioprotection: they may participate in reperfusion injury or may play a role as signaling elements of protection in the pre-ischemic phase. It has been demonstrated that preconditioning triggering is redox-sensitive, using either ROS scavengers or ROS generators. We have shown that nitroxyl triggers preconditioning via pro-oxidative, and/or nitrosative stress-related mechanism(s). Several metabolites, including acetylcholine, bradykinin, opioids and phenylephrine, trigger preconditioning-like protection via a mitochondrial K(ATP)-ROS-dependent mechanism. Intriguingly, and contradictory to the above mentioned theory of ROS as an obligatory part of reperfusion-induced damage, some studies suggest the possibility that some ROS at low concentrations could protect ischemic hearts against reperfusion injury. Yet, we demonstrated that ischemic PostC is also a cardioprotective phenomenon that requires the intervention of redox signaling to be protective. Emerging evidence suggests that in a preconditioning scenario a redox signal is required during the first few minutes of myocardial reperfusion following the index ischemic period. Intriguingly, the ROS signaling in the early reperfusion appear crucial to both preconditioning- and postconditioning-induced protection. Therefore, our and others' results suggest that the role of ROS in reperfusion may be reconsidered as they are not only deleterious.

Acute effects of "delayed postconditioning" with periodic acceleration after asphyxia induced shock in pigs

Asphyxia cardiac arrest and shock are models for whole body ischemia reperfusion injury. Periodic acceleration (pGz) achieved by moving the body on a platform is a novel method for inducing pulsatile vascular shear stress and endogenous production of endothelial nitric oxide, prostaglandin E2, tissue plasminogen activator, and adrenomedullin. The aforementioned are cardioprotective during and after ischemia reperfusion injury. We investigated whether pGz, applied 15 min after return of spontaneous circulation (ROSC) would serve as an effective "delayed" post conditioning tactic to lessen acute reperfusion injury markers in a pediatric swine model of asphyxia induced shock. Asphyxia shock was induced in 20 swine weight 3.9 +/- 0.6 kg. Fifteen minutes after ROSC, the animals were randomized to receive conventional mechanical ventilation (CMV, [Control]) or CMV with pGz. All animals had ROSC and no significant differences in blood gases or hemodynamics after ROSC. pGz treated had significantly less myocardial dysfunction post resuscitation, (i.e. better % ejection fraction (EF), % fractional shortening (FS), and wall motion score index) and lower biochemical indices of reperfusion injury (lower TNF-alpha, IL-6, and Troponin I, and myeloperoxidase activity). Delayed postconditioning with pGz ameliorates acute post resuscitation reperfusion injury and improves myocardial dysfunction after asphyxia-induced shock.

Loss of myocardial ischemic postconditioning in adenosine A1 and bradykinin B2 receptors gene knockout mice

BACKGROUND

Ischemic postconditioning (PostC) is a recently described cardioprotective modality against reperfusion injury, through series of brief reflow interruptions applied at the very onset of reperfusion. It is proposed that PostC can activate a complex cellular signaling cascade, in which cell membrane receptors could serve as the upstream triggers of PostC. However, the exact subtypes of such receptors remain controversial or uninvestigated. To this context, the purpose of present study was to determine the definitive role of adenosine A(1) and bradykinin B(1) and B(2) receptors in PostC.

METHODS AND RESULTS

The hearts isolated from adult male C57BL/6J wild-type mice or the mice lacking adenosine A(1), or bradykinin B(1) or B(2) receptors subjected to zero-flow global ischemia and reperfusion in a Langendorff model. PostC, consisting of 6 cycles of 10 seconds of reperfusion and 10 seconds of ischemia, demonstrated significantly reduced myocardial infarct size (22.8+/-3.1%, mean+/-SEM) as compared with the non-PostC wild-type controls (35.1+/-2.8%, P<0.05). The infarct-limiting protection of PostC was absent in adenosine A(1) receptor knockout mice (34.9+/-2.7%) or bradykinin B(2) receptor knockout mice (33.3+/-1.7%) and was partially attenuated in bradykinin B(1) receptor-deficient mice (25.6+/-2.9%; P>0.05). On the other hand, PostC did not significantly alter postischemic cardiac contractile function and coronary flow.

CONCLUSIONS

With the use of three distinctive strains of gene knockout mice, the current study has provided the first conclusive evidence showing PostC-induced infarct-limiting cardioprotection could be triggered by activation of multiple types of cell membrane receptors, which include adenosine A(1) and bradykinin B(2) receptors.

Myocardial ischemic postconditioning against ischemia-reperfusion is impaired in ob/ob mice

Ischemic postconditioning (IPCD) significantly reduces infarct size in healthy animals and protects the human heart. Because obesity is a major risk factor of cardiovascular diseases, the effects of IPCD were investigated in 8- to 10-wk-old leptin-deficient obese (ob/ob) mice and compared with wild-type C57BL/6J (WT) mice. All animals underwent 30 min of coronary artery occlusion followed by 24 h of reperfusion associated or not with IPCD (6 cycles of 10-s occlusion, 10-s reperfusion). Additional mice were killed at 10 min of reperfusion for Western blotting. IPCD reduced infarct size by 58% in WT mice (33+/-1% vs. 14+/-3% for control and IPCD, respectively, P<0.05) but failed to induce cardioprotection in ob/ob mice (53+/-4% vs. 56+/-5% for control and IPCD, respectively). In WT mice, IPCD significantly increased the phosphorylation of Akt (+77%), ERK1/2 (+41%), and their common target p70S6K1 (+153% at Thr389 and +57% at Thr421/Ser424). In addition, the phosphorylated AMP-activated protein kinase (AMPK)-to-total AMPK ratio was also increased by IPCD in WT mice (+64%, P<0.05). This was accompanied by decreases in phosphatase and tensin homolog deleted on chromosome 10 (PTEN), MAP kinase phosphatase (MKP)-3, and protein phosphatase (PP)2C levels. In contrast, IPCD failed to increase the phosphorylation state of all these kinases in ob/ob mice, and the level of the three phosphatases was significantly increased. Thus, although IPCD reduces myocardial infarct size in healthy animals, its cardioprotective effect vanishes with obesity. The lack of enhanced phosphorylation by IPCD of Akt, ERK1/2, p70S6K1, and AMPK might partly explain the loss of cardioprotection in this experimental model of obese mice.

Long-term benefit of postconditioning

BACKGROUND

We previously demonstrated that ischemic postconditioning decreases creatine kinase release, a surrogate marker for infarct size, in patients with acute myocardial infarction. Our objective was to determine whether ischemic postconditioning could afford (1) a persistent infarct size limitation and (2) an improved recovery of myocardial contractile function several months after infarction.

METHODS AND RESULTS

Patients presenting within 6 hours of the onset of chest pain, with suspicion for a first ST-segment-elevation myocardial infarction, and for whom the clinical decision was made to treat with percutaneous coronary intervention, were eligible for enrollment. After reperfusion by direct stenting, 38 patients were randomly assigned to a control (no intervention; n=21) or postconditioned group (repeated inflation and deflation of the angioplasty balloon; n=17). Infarct size was assessed both by cardiac enzyme release during early reperfusion and by 201thallium single photon emission computed tomography at 6 months after acute myocardial infarction. At 1 year, global and regional contractile function was evaluated by echocardiography. At 6 months after acute myocardial infarction, single photon emission computed tomography rest-redistribution index (a surrogate for infarct size) averaged 11.8+/-10.3% versus 19.5+/-13.3% in the postconditioned versus control group (P=0.04), in agreement with the significant reduction in creatine kinase and troponin I release observed in the postconditioned versus control group (-40% and -47%, respectively). At 1 year, the postconditioned group exhibited a 7% increase in left ventricular ejection fraction compared with control (P=0.04).

CONCLUSIONS

Postconditioning affords persistent infarct size reduction and improves long-term functional recovery in patients with acute myocardial infarction.

Postconditioning fails to improve no reflow or alter infarct size in an open-chest rabbit model of myocardial ischemia-reperfusion

Postconditioning (PoC) with brief intermittent ischemia after myocardial reperfusion has been shown to lessen some elements of postischemic injury including arrhythmias and, in some studies, the size of myocardial infarction. We hypothesized that PoC could improve reflow to the risk zone after reperfusion. Anesthetized, open-chest rabbits were subjected to 30 min of coronary artery occlusion followed by 3 h of reperfusion. In protocol 1, rabbits were randomly assigned to the control group ( n = 10, no further intervention after reperfusion) or to the PoC group, which consisted of four cycles of 30-s reocclusions with 30 s of reperfusion in between starting at 30 s after the initial reperfusion (4 × 30/30, n = 10). In protocol 2, rabbits were assigned to the control group ( n = 7) or the PoC group, which received PoC consisting of four cycles of 60-s intervals of ischemia and reperfusion starting at 30 s after the initial reperfusion (4 × 60/60, n = 7). No reflow was determined by injecting thioflavine S (a fluorescent marker of capillary perfusion), risk zone by blue dye, and infarct size by triphenyltetrazolium chloride. In protocol 1, there were no statistical differences in hemodynamics, ischemic risk zone, or infarct size (35 ± 6% of the risk zone in the PoC group vs. 29 ± 4% in the control group, P = 0.38) between the groups. Similarly, in protocol 2, PoC failed to reduce infarct size compared with the control group (45 ± 4% of the risk zone in the PoC group vs. 42 ± 6% in the control group, P = 0.75). There was a strong correlation in both protocols between the size of the necrotic zone and the portion of the necrotic zone that contained an area of no reflow. However, PoC did not affect this relationship. PoC did not reduce infarct size in this model, nor did it reduce the extent of the anatomic zone of no reflow, suggesting that this intervention may not impact postreperfusion microvascular damage due to ischemia.

The paradigm of postconditioning to protect the heart

Ischaemic preconditioning limits the damage induced by subsequent ischaemia/reperfusion (I/R). However, preconditioning is of little practical use as the onset of an infarction is usually unpredictable. Recently, it has been shown that the heart can be protected against the extension of I/R injury if brief (10–30 sec.) coronary occlusions are performed just at the beginning of the reperfusion. This procedure has been called postconditioning (PostC). It can also be elicited at a distant organ, termed remote PostC, by intermittent pacing (dyssynchrony-induced PostC) and by pharmacological interventions, that is pharmacological PostC. In particular, brief applications of intermittent bradykinin or diazoxide at the beginning of reperfusion reproduce PostC protection. PostC reduces the reperfusion-induced injury, blunts oxidant-mediated damages and attenuates the local inflammatory response to reperfusion. PostC induces a reduction of infarct size, apoptosis, endothelial dysfunction and activation, neutrophil adherence and arrhythmias. Whether it reduces stunning is not clear yet. Similar to preconditioning, PostC triggers signalling pathways and activates effectors implicated in other cardioprotective manoeuvres. Adenosine and bradykinin are involved in PostC triggering. PostC triggers survival kinases (RISK), including A t and extracellular signal-regulated kinase (ERK). Nitric oxide, via nitric oxide synthase and non-enzymatic production, cyclic guanosine monophosphate (cGMP) and protein kinases G (PKG) participate in PostC. PostC-induced protection also involves an early redox-sensitive mechanism, and mitochondrial adenosine-5′ -triphosphate (ATP)-sensitive K⁺ and PKC activation. Protective pathways activated by PostC appear to converge on mitochondrial permeability transition pores, which are inhibited by acidosis and glycogen synthase kinase-3β (GSK-3β). In conclusion, the first minutes of reperfusion represent a window of opportunity for triggering the aforementioned mediators which will in concert lead to protection against reperfusion injury. Pharmacological PostC and possibly remote PostC may have a promising future in clinical scenario.

Connexin 43 in ischemic pre- and postconditioning

Connexin 43 (Cx43) is the predominant protein forming gap junctions and non-junctional hemichannels in ventricular myocardium, but Cx43 is also localized at the inner membrane of cardiomyocyte mitochondria. In cardiomyocytes, Cx43 is involved in the formation of reactive oxygen species, which are central to the signal transduction cascade of ischemic preconditioning's protection. Accordingly, genetically-induced or age-related loss of Cx43 abolishes infarct size reduction by ischemic preconditioning. Similarly, mitochondrial import inhibition of Cx43 completely blocks infarct size reduction by pharmacological preconditioning with diazoxide. In contrast to its importance for preconditioning-induced cardioprotection, Cx43 is not important for infarct size reduction by ischemic postconditioning. In summary, Cx43--especially Cx43 localized in mitochondria--appears to be one key element of the signal transduction cascade of the protection by preconditioning.

Preconditioning and postconditioning: innate cardioprotection from ischemia-reperfusion injury

Reperfusion is the definitive treatment to salvage ischemic myocardium from infarction. A primary determinant of infarct size is the duration of ischemia. In myocardium that has not been irreversibly injured by ischemia, reperfusion induces additional injury in the area at risk. The heart has potent innate cardioprotective mechanisms against ischemia-reperfusion that reduce infarct size and other presentations of postischemic injury. Ischemic preconditioning (IPC) applied before the prolonged ischemia exerts the most potent protection observed among known strategies. It has been assumed that IPC exerts protection during ischemia. However, recent data suggest that cardioprotection is also exerted during reperfusion. Postconditioning (PoC), defined as brief intermittent cycles of ischemia alternating with reperfusion applied after the ischemic event, has been shown to reduce infarct size, in some cases equivalent to that observed with IPC. Although there are similarities in mechanisms of cardioprotection by these two interventions, there are key differences that go beyond simply exerting these mechanisms before or after ischemia. A significant limitation of IPC has been the inability to apply this maneuver clinically except in situations where the ischemic event can be predicted. On the other hand, PoC is applied at the point of service in the hospital (cath-lab for percutaneous coronary intervention, coronary artery bypass grafting, and other cardiac surgery) where and when reperfusion is initiated. Initial clinical studies are in agreement with the success and extent to which PoC reduces infarct size and myocardial injury, even in the presence of multiple comorbidities.