Postconditioning in rats and mice

Do We Really Need Aspirin Loading for STEMI?

Aspirin loading (chewable or intravenous) as soon as possible after presentation is a class I recommendation by current ST elevation myocardial infarction (STEMI) guidelines. Earlier achievement of therapeutic antiplatelet effects by aspirin loading has long been considered the standard of care. However, the effects of the loading dose of aspirin (alone or in addition to a chronic maintenance oral dose) have not been studied. A large proportion of myocardial cell death occurs upon and after reperfusion (reperfusion injury). Numerous agents and interventions have been shown to limit infarct size in animal models when administered before or immediately after reperfusion. However, these interventions have predominantly failed to show significant protection in clinical studies. In the current review, we raise the hypothesis that aspirin loading may be the culprit. Data obtained from animal models consistently show that statins, ticagrelor, opiates, and ischemic postconditioning limit myocardial infarct size. In most of these studies, aspirin was not administered. However, when aspirin was administered before reperfusion (as is the case in the majority of studies enrolling STEMI patients), the protective effects of statin, ticagrelor, morphine, and ischemic postconditioning were attenuated, which can be plausibly attributable to aspirin loading. We therefore suggest studying the effects of aspirin loading before reperfusion on the infarct size limiting effects of statins, ticagrelor, morphine, and/ or postconditioning in large animal models using long reperfusion periods (at least 24 h). If indeed aspirin attenuates the protective effects, clinical trials should be conducted comparing aspirin loading to alternative antiplatelet regimens without aspirin loading in patients with STEMI undergoing primary percutaneous coronary intervention.

Effects of ischaemic postconditioning in aortic valve replacement: a multicenter randomized controlled trial

OBJECTIVES

The effect of ischaemic postconditioning (IPost) on postcardioplegic cardiac function is not known. We hypothesized that IPost was cardioprotective in adult patients undergoing elective aortic valve replacement.

METHODS

In a multicentre, prospective, randomized trial, patients (n = 209) were randomized to either a standard operation (controls) or postconditioning. Immediately before the cross-clamp was released, patients in the postconditioning group underwent 3 cycles of flow/non-flow (2 min each) of normothermic blood via the antegrade cardioplegia line. The primary end point was cardiac index. Secondary end points included additional haemodynamic measurements, biomarkers of cardiomyocyte injury, renal function parameters, intra- and postoperative arrhythmias and use of inotropic agents.

RESULTS

There was no significant difference between the groups regarding cardiac index [mean between-group difference, 95% confidence interval (CI), 0.11 (-0.1 to 0.3), P = 0.27]. Postconditioning had no effect on other haemodynamic parametres. There was no between-group difference regarding troponin T or creatine kinase MB. Postconditioning reduced the relative risk for arrhythmias by 45% (P = 0.03) when postoperative atrial fibrillation and intraoperative ventricular fibrillation were combined. There were no differences in patients with/without diabetes, patients above/below 70 years of age or between the centres. However, after postconditioning, the cardiac index [95% CI, 0.46 (0.2-0.7), P = 0.001], cardiac output (P < 0.001), mean arterial pressure (P < 0.001) and left ventricular stroke work index (P < 0.001) were higher in males compared to females.

CONCLUSIONS

IPost had no overall cardioprotective effects in patients undergoing aortic valve replacement but improved postoperative cardiac performance in men compared to women.

Ischaemic post-conditioning in rats: Responder and non-responder differ in transcriptome of mitochondrial proteins

Ischaemic post‐conditioning (IPoC) is a clinical applicable procedure to reduce reperfusion injury. Non‐responsiveness to IPoC possibly caused by co‐morbidities limits its clinical attractiveness. We analysed differences in the expression of mitochondrial proteins between IPoC responder (IPoC‐R) and non‐responder (IPoC‐NR). Eighty rats were randomly grouped to sham, ischaemia/reperfusion (I/R), IPoC or ischaemic pre‐conditioning (IPC, as positive cardioprotective intervention) in vivo. Infarct sizes were quantified by plasma troponin I levels 60 minutes after reperfusion. After 7 days, rats were sacrificed and left ventricular tissue was taken for post hoc analysis. The transcriptome was analysed by qRT‐PCR and small RNA sequencing. Key findings were verified by immunoblots. I/R increased plasma troponin I levels compared to Sham. IPC reduced troponin I compared to I/R, whereas IPoC produced either excellent protection (IPoC‐R) or no protection (IPoC‐NR). Twenty‐one miRs were up‐regulated by I/R and modified by IPoC. qRT‐PCR analysis revealed that IPoC‐R differed from other groups by reduced expression of arginase‐2 and bax, whereas the mitochondrial uncoupling protein (UCP)‐2 was induced in IPC and IPoC‐R. IPoC‐R and IPoC‐NR synergistically increased the expression of non‐mitochondrial proteins like VEGF and SERCA2a independent of the infarct size. Cardiac function was more closely linked to differences in mitochondrial proteins than on regulation of calcium‐handling proteins. In conclusion, healthy rats could not always be protected by IPoC. IPoC‐NR displayed an incomplete responsiveness which is reflected by different changes in the mitochondrial transcriptome compared to IPoC‐R. This study underlines the importance of mitochondrial proteins for successful long‐term outcome.

Postconditioning Does Not Reduce Myocardial Infarct Size in an In Vivo Regional Ischemia Rodent Model

In our laboratory, postconditioning reliably reduces lethal ventricular arrhythmias in an in vivo rat model but its effect on necrosis in our model is unknown. In the present analysis, we tested a variety of postconditioning regimens in anesthetized rats subjected to 45 minutes of coronary occlusion and 120 minutes of reperfusion or 30 minutes of coronary occlusion and 120 minutes of reperfusion. In all studies, area at risk was determined by the blue dye technique and area of necrosis was assessed with triphenyl tetrazolium chloride staining and computerized planimetry of ventricular slices. Postconditioning regimens included 4 cycles of 10 seconds of reperfusion/10 seconds of reocclusion, 4 cycles of 20 seconds of reperfusion/20 seconds of reocclusion, 8 cycles of 30 seconds of reperfusion/30 seconds of reocclusion, and 20 cycles of 10 seconds of reperfusion/10 seconds of reocclusion. Postconditioning did not reduce myocardial infarct size with any of these regimens.

ESC working group cellular biology of the heart: position paper: improving the preclinical assessment of novel cardioprotective therapies

Ischaemic heart disease (IHD) remains the leading cause of death and disability worldwide. As a result, novel therapies are still needed to protect the heart from the detrimental effects of acute ischaemia–reperfusion injury, in order to improve clinical outcomes in IHD patients. In this regard, although a large number of novel cardioprotective therapies discovered in the research laboratory have been investigated in the clinical setting, only a few of these have been demonstrated to improve clinical outcomes. One potential reason for this lack of success may have been the failure to thoroughly assess the cardioprotective efficacy of these novel therapies in suitably designed preclinical experimental animal models. Therefore, the aim of this Position Paper by the European Society of Cardiology Working Group Cellular Biology of the Heart is to provide recommendations for improving the preclinical assessment of novel cardioprotective therapies discovered in the research laboratory, with the aim of increasing the likelihood of success in translating these new treatments into improved clinical outcomes.

High plasma concentrations of asymmetric dimethylarginine inhibit ischemic cardioprotection in hypercholesterolemic rats

A low concentration of nitric oxide associated with a high concentration of asymmetric dimethylarginine (ADMA) can explain the lack of ischemic cardioprotection observed in the presence of hypercholesterolemia. The objective of the present study was to evaluate the effect of hypercholesterolemia on ischemic pre- and postconditioning and its correlation with plasma concentrations of ADMA. Male Wistar rats (6-8 weeks old) fed a 2% cholesterol diet (n = 21) for 8 weeks were compared to controls (n = 25) and were subjected to experimental myocardial infarction and reperfusion, with ischemic pre- and postconditioning. Total cholesterol and ADMA were measured in plasma before the experimental infarct and the infarct area was quantified. Weight, total cholesterol and plasma ADMA (means ± SE; 1.20 ± 0.06, 1.27 ± 0.08 and 1.20 ± 0.08 vs 0.97 ± 0.04, 0.93 ± 0.05 and 0.97 ± 0.04 µM) were higher in animals on the hypercholesterolemic diet than in controls, respectively. Cardioprotection did not reduce infarct size in the hypercholesterolemic animals (pre: 13.55% and post: 8% compared to 7.95% observed in the group subjected only to ischemia and reperfusion), whereas infarct size was reduced in the animals on a normocholesterolemic diet (pre: 8.25% and post: 6.10% compared to 12.31%). Hypercholesterolemia elevated ADMA and eliminated the cardioprotective effects of ischemic pre- and postconditioning in rats.

Magnesium orotate elicits acute cardioprotection at reperfusion in isolated and in vivo rat hearts

Orotic acid and its salts chronically administered have been shown to significantly improve cardiac function in pathological settings associated with ischemia–reperfusion (I/R) injury. The aim of our study was to investigate the effect of magnesium orotate (Mg-Or) administration at the onset of post-ischemic reperfusion on myocardial function and infarct size (IS). Ex-vivo experiments performed on isolated perfused rat hearts were used to compare Mg-Or administration with a control group (buffer treated), ischemic post-conditioning, orotic acid treatment, and MgCl2 treatment. Mg-Or administration was also investigated in an in-vivo model of regional I/R performed in rats undergoing reversible coronary ligation. The effect of Mg-Or on mitochondrial permeability transition pore (mPTP) opening after I/R was investigated in vitro to gain mechanistic insights. Both ex-vivo and in-vivo experiments showed a beneficial effect from Mg-Or administration at the onset of reperfusion on myocardial function and IS. In-vitro assays showed that Mg-Or significantly delayed mPTP opening after I/R. Our data suggest that Mg-Or administered at the very onset of reperfusion may preserve myocardial function and reduce IS. This beneficial effect may be related to a significant reduction of mPTP opening, a usual trigger of cardiac cell death following I/R.

Cardioprotective effect of ischemic postconditioning on the model of isolated heart

Irreversible cardiomyocyte damage was induced by 45-min global ischemia followed by 30-min reperfusion in Langendorff-perfused isolated rat heart. Cell damage was assessed by the release of creatine phosphokinase into the perfusate. The hearts were subjected to the following postconditioning protocols: 1) three cycles of 10-sec reperfusion and 10-sec ischemia, total cycle time 20 sec; 2) six cycles of 10-sec reperfusion and 10-sec ischemia, total cycle time 20 sec; 3) three sessions of 20-sec reperfusion and 20-sec ischemia, total cycle time 40 sec; 4) 6 cycles of 20-sec reperfusion and 20-sec ischemia, total cycle time 40 sec; 5) 3 cycles of 30-sec reperfusion and 30-sec ischemia, total cycle time 60 sec. It was found that only postconditioning with a total cycle time of 40 sec or 60 sec prevents myocardial reperfusion injury.

Aquaporin-1 in cardiac endothelial cells is downregulated in ischemia, hypoxia and cardioplegia

Aquaporin-1 (AQP1) is expressed in human and mouse hearts, but little is known about its cellular and subcellular localization and regulation. The aim of this study was to investigate the localization of AQP1 in the mouse heart and to determine the effects of ischemia and hypoxia on its expression. Mouse myocardial cells were freshly isolated and split into cardiomyocyte and non-cardiomyocyte fractions. Isolated, Langendorff-perfused C57Bl6 mouse hearts (n=46) were harvested with no intervention, subjected to 35min of ischemia or ischemia followed by 60min of reperfusion. Eleven mouse hearts were perfusion-fixed for electron microscopy. Forty C57Bl6 mice were exposed to normobaric hypoxia for one or two weeks (n=12). Needle biopsies of human left ventricular myocardium were sampled (n=30) during coronary artery bypass surgery before cardioplegia and after 30min of reperfusion. Human umbilical vein endothelial cells (HUVECs) were subjected to 4h of hypoxia with reoxygenation for either 4 or 24h. AQP1 expression was studied by electron microscopy with immunogold labeling, Western blot, and qPCR. Expression of miR-214 and miR-320 in HUVECs with hypoxia was studied with qPCR. HUVECs were then transfected with precursors and inhibitors of miR-214. AQP1 expression was confined to cardiac endothelial cells, with no signal in cardiomyocytes or cardiac fibroblasts. Immunogold electron microscopy showed AQP1 expression in endothelial caveolae with equal distribution along the basal and apical membranes. Ischemia and reperfusion tended to decrease AQP1 mRNA expression in mouse hearts by 37±9% (p=0.06), while glycosylated AQP1 protein was reduced by 16±9% (p=0.03). No difference in expression was found between ischemia alone and ischemia-reperfusion. In human left ventricles AQP1 mRNA expression was reduced following cardioplegia and reperfusion (p=0.008). Hypoxia in mice reduced AQP1 mRNA expression by 20±7% (p<0.0001), as well as both glycosylated (-47±10%, p=0.03) and glycan-free protein (-34±16%, p=0.05). Hypoxia and reoxygenation in HUVECs downregulated glycan-free AQP1 protein (-34±24%, p=0.04) and upregulated miR-214 (+287±52%, p<0.05). HUVECs transfected with anti-miR-214 had increased glycosylated (1.5 fold) and glycan-free (2 fold) AQP1. AQP1 in mouse hearts is localized to endothelial cell membranes and caveolae. Cardioplegia, ischemia and hypoxia decrease AQP1 mRNA as well as total protein expression and glycosylation, possibly regulated by miR-214.

Type 2 diabetic obese db/db mice are refractory to myocardial ischaemic post-conditioning in vivo: potential role for Hsp20, F1-ATPase δ and Echs1

Ischaemic post-conditioning (PostC) is a clinically relevant cardioprotective modality that has been confirmed in many species including human. It remains unknown if PostC can still protect heart in Type 2 diabetes, a rapidly growing disease in the world. This study investigated the efficacy of PostC in the leptin receptor-deficient db/db mice, which possess Type 2 diabetic characteristics including obesity, hyperglycaemia and hyperleptinaemia. Adult male C57BL/6J wild-type (WT) and db/db mice were anaesthetized, mechanically ventilated and subjected to left coronary artery occlusion for 30 min. followed by 24 hrs of reperfusion. For the PostC groups, the hearts underwent six cycles of 10 sec. of reperfusion and 10 sec. of re-occlusion at the onset of reperfusion. The mice were sacrificed at the end of 24 hrs reperfusion for infarct size measurement. PostC significantly reduced infarct size in WT mice (n = 6/group; P < 0.05), but not in the db/db mice. To identify alterations in protein expression by PostC, proteomic analyses were performed in the heart samples using two-dimensional differential in-gel electrophoresis with three CyDye labelling, followed by mass spectrometry. The results show that mitochondrial proteins (F(1)-ATPase γ and Echs1) were down-regulated by PostC in WT heart. Such change was absent in the db/db heart. On the other hand, PostC reduced Hsp20 in the diabetic heart. In summary, PostC fails to protect Type 2 diabetic mice against ischaemia-reperfusion injury. The potential protein targets for the loss of PostC may include F(1)-ATPase γ, Echs1 and Hsp20 that could regulate cellular ATP consumption/production and defense response to ischaemic stress.

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.

Age, genetic characteristics and number of cycles are critical factors to consider for successful protection of the murine heart with postconditioning

Postconditioning (PostC) is a recently discovered phenomenon whereby brief repetitive cycles of ischaemia with intermittent reperfusion following prolonged ischaemia elicit cardioprotection. This study investigated whether the age, genetic characteristics or number of repetitive cycles influenced the protective effect of PostC in mice. C57BL/6 floxed or non-floxed STAT-3 mice aged between 14-16 weeks (young) or 18-20 weeks (older) were perfused on a Langendorff apparatus and subjected to 35 min global ischaemia and 45 min reperfusion. PostC was elicited by either 3 (PostC-3) or 6 cycles (PostC-6) of 10 s ischaemia and 10 s reperfusion. PostC-3 and PostC-6 in both young and older non-floxed mice reduced the myocardial infarct size. In contrast, only PostC-3 reduced myocardial infarct size in young floxed mice. Neither PostC-3 nor PostC-6 reduced the infarct in older floxed mice. Our data reveal that genetic characteristics, a minute difference in age or the number of postconditioning cycles are critical factors to be considered for the successful effect of ischaemic postconditioning in a murine model. Moreover, these factors should be taken into consideration for future experimental research or clinical applications of this protective phenomenon.

Ischaemic postconditioning revisited: lack of effects on infarct size following primary percutaneous coronary intervention

AIMS

To assess the short- and long-term effects of postconditioning (p-cond) on infarct size, extent of myocardial salvage, and left ventricular ejection fraction (LVEF) in a series of patients presenting with evolving ST-elevation myocardial infarction (STEMI). Previous studies have shown that p-cond during primary percutaneous coronary intervention (PCI) confers protection against ischaemia-reperfusion injury and thus might reduce myocardial infarct size.

METHODS AND RESULTS

Seventy-nine patients undergoing PCI for a first STEMI with TIMI grade flow 0-1 and no collaterals were randomized to p-cond (n= 39) or controls (n= 40). Postconditioning was performed by applying four consecutive cycles of 1 min balloon inflation, each followed by 1 min deflation. Infarct size, myocardial salvage, and LVEF were assessed by cardiac-MRI 1 week and 6 months after MI. Postconditioning was associated with lower myocardial salvage (4.1 ± 7.2 vs. 9.1 ± 5.8% in controls; P= 0.004) and lower myocardial salvage index (18.9 ± 27.4 vs. 30.9 ± 20.5% in controls; P= 0.038). No significant differences in infarct size and LVEF were found between the groups at 1 week and 6 months after MI.

CONCLUSION

This randomized study suggests that p-cond during primary PCI does not reduce infarct size or improve myocardial function recovery at both short- and long-term follow-up and might have a potential harmful effect.

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.

Contribution of delayed intracellular pH recovery to ischemic postconditioning protection

Ischemic postconditioning (PoCo) has been proven to be a feasible approach to attenuate reperfusion injury and enhance myocardial salvage in patients with acute myocardial infarction, but its mechanisms have not been completely elucidated yet. Recent studies demonstrate that PoCo may delay the recovery of intracellular pH during initial reperfusion, and that its ability to limit infarct size critically depends on this effect. Prolongation of postischemic intracellular acidosis inhibits hypercontracture, mitochondrial permeability transition, calpain-mediated proteolysis, and gap junction-mediated spread of injury during the first minutes of reflow. This role of prolonged acidosis does not exclude the participation of other pathways in PoCo-induced cardioprotection. On the contrary, it may allow these pathways to act by preventing immediate reperfusion-induced cell death. Moreover, the existence of interactions between intracellular acidosis and endogenous protection signaling cannot be excluded and needs to be investigated. The role of prolonged acidosis in PoCo cardioprotection has important implications in the design of optimal PoCo protocols and in the translation of cardioprotective strategies to patients with on-going myocardial infarction receiving coronary reperfusion.

Inhibition of contractile activity during postconditioning enhances cardioprotection by restoring sarcolemmal dystrophin through phosphatidylinositol 3-kinase

BACKGROUND

Although ischemic postconditioning (IPost) confers cardioprotection by protecting the mitochondria though the activation of phosphatidylinositol 3-kinase (PI3K), a potential drawback of IPost is impairment of aerobic ATP generation during reperfusion by repeated ischemia. This decrease in ATP might inhibit the restoration of sarcolemmal dystrophin, which is translocated during ischemia, and render cardiomyocytes susceptible to contraction-induced oncosis.

METHODS AND RESULTS

Isolated rat hearts were subjected to 30 min ischemia and 120 min reperfusion. IPost induced by 20 cycles of 10-s reperfusion and 10-s ischemia enhanced the activation of PI3K as evidenced by the increased phosphorylation of Akt, but had no effect on myocardial ATP, restoration of sarcolemmal dystrophin, or cardiomyocyte oncosis during IPost. Administration of the contractile blocker, 2,3-butanedione monoxim (BDM), during IPost increased myocardial ATP and facilitated the redistribution of dystrophin to the sarcolemma. This led to reduced cardiomyocyte oncosis and infarct size, and improved the left ventricular function. The anti-oncotic effect of BDM occurred without changing the anti-apoptotic effect of IPost. The PI3K inhibitor, LY294002, prevented the phosphorylation of Akt, decreased the recovery of ATP and restoration of sarcolemmal dystrophin, and blocked the anti-oncotic and anti-apoptotic effects of IPost.

CONCLUSIONS

These results suggest that the inhibition of contractile activity during IPost prevents cardiomyocyte oncosis and enhances cardioprotection through PI3K-dependent restoration of sarcolemmal dystrophin.

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.

Regulating RISK: a role for JAK-STAT signaling in postconditioning?

Postconditioning (POC), a novel strategy of cardioprotection against ischemia-reperfusion injury, is clinically attractive because of its therapeutic application at the predictable onset of reperfusion. POC activates several intracellular kinase signaling pathways, including phosphatidylinositol 3-kinase (PI3K)-Akt (RISK). The regulation of POC-induced survival kinase signaling, however, has not been fully characterized. JAK-STAT activation is integral to cardiac ischemic tolerance and may provide upstream regulation of RISK. We hypothesized that POC requires the activation of both JAK-STAT and RISK signaling. Langendorff-perfused mouse hearts were subjected to 30 min of global ischemia and 40 min of reperfusion, with or without POC immediately after ischemia. A separate group of POC hearts was treated with AG 490, a JAK2 inhibitor, Stattic, a specific STAT3 inhibitor, or LY-294002, a PI3K inhibitor, at the onset of reperfusion. Cardiomyocyte-specific STAT3 knockout (KO) hearts were also subjected to non-POC or POC protocols. Myocardial performance (+dP/dt(max), mmHg/s) was assessed throughout each perfusion protocol. Phosphorylated (p-) STAT3 and Akt expression was analyzed by Western immunoblotting. POC enhanced myocardial functional recovery and increased expression of p-STAT3 and p-Akt. JAK-STAT inhibition abrogated POC-induced functional protection. STAT3 inhibition decreased expression of both p-STAT3 and p-Akt. PI3K inhibition also attenuated POC-induced cardioprotection and reduced p-Akt expression but had no effect on STAT3 phosphorylation. Interestingly, STAT3 KO hearts undergoing POC exhibited improved ischemic tolerance compared with KO non-POC hearts. POC induces myocardial functional protection by activating the RISK pathway. JAK-STAT signaling, however, is insufficient for effective POC without PI3K-Akt activation.

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.

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.

Targeted deletion of A2A adenosine receptors attenuates the protective effects of myocardial postconditioning

Endogenous adenosine is an important ligand trigger for the cardioprotective effects of postconditioning (POC), yet it is unclear which adenosine receptor subtype is primarily responsible. To evaluate the role of A2A adenosine receptors in POC-induced protection, global ischemia-reperfusion was performed with and without POC in isolated wild-type (WT) and A2A adenosine receptor knockout (A2AKO) mouse hearts. Injury was measured in terms of postischemic functional recovery and release of cardiac troponin I (cTnI). Activation of protective signaling with POC was assessed by Akt and extracellular signal-regulated kinase (ERK) 1/2 phosphorylation. In WT hearts, POC improved recovery of postischemic developed pressure in early (81.6 ± 6.4% of preischemic baseline vs. 37.5 ± 5.6% for non-POC WT at 1 min) and late (62.2 ± 4.2% of baseline vs. 45.5 ± 5.3% for non-POC WT at 30 min) reperfusion, reduced cTnI release by 37%, and doubled the phosphorylation of both Akt and ERK1/2. These beneficial effects of POC were blocked by treatment with the selective A2A adenosine receptor antagonist ZM-241385 during reperfusion. Postischemic functional recovery, cTnI release, and phosphorylation of Akt and ERK1/2 were not different between non-POC WT and A2AKO hearts. In A2AKO hearts, POC did not improve functional recovery, reduce cTnI release, nor increase phosphorylation of Akt or ERK1/2. Thus the protective effects of POC are attenuated by both selective A2A receptor antagonism and targeted deletion of the gene encoding A2A adenosine receptors. These observations support the conclusion that endogenous activation of A2A adenosine receptors is an essential trigger leading to the protective effects of POC in isolated murine hearts.