Ischaemic preconditioning is not dependent on neutrophils or glycolytic substrate at reperfusion in rabbit heart

Initial Despair and Current Hope of Identifying a Clinically Useful Treatment of Myocardial Reperfusion Injury: Insights Derived from Studies of Platelet P2Y12 Antagonists and Interference with Inflammation and NLRP3 Assembly

Myocardial necrosis following the successful reperfusion of a coronary artery occluded by thrombus in a patient presenting with ST-elevation myocardial infarction (STEMI) continues to be a serious problem, despite the multiple attempts to attenuate the necrosis with agents that have shown promise in pre-clinical investigations. Possible reasons include confounding clinical risk factors, the delayed application of protective agents, poorly designed pre-clinical investigations, the possible effects of routinely administered agents that might unknowingly already have protected the myocardium or that might have blocked protection, and the biological differences of the myocardium in humans and experimental animals. A better understanding of the pathobiology of myocardial infarction is needed to stem this reperfusion injury. P2Y12 receptor antagonists minimize platelet aggregation and are currently part of the standard treatment to prevent thrombus formation and propagation in STEMI protocols. Serendipitously, these P2Y12 antagonists also dramatically attenuate reperfusion injury in experimental animals and are presumed to provide a similar protection in STEMI patients. However, additional protective agents are needed to further diminish reperfusion injury. It is possible to achieve additive protection if the added intervention protects by a mechanism different from that of P2Y12 antagonists. Inflammation is now recognized to be a critical factor in the complex intracellular response to ischemia and reperfusion that leads to tissue necrosis. Interference with cardiomyocyte inflammasome assembly and activation has shown great promise in attenuating reperfusion injury in pre-clinical animal models. And the blockade of the executioner protease caspase-1, indeed, supplements the protection already seen after the administration of P2Y12 antagonists. Importantly, protective interventions must be applied in the first minutes of reperfusion, if protection is to be achieved. The promise of such a combination of protective strategies provides hope that the successful attenuation of reperfusion injury is attainable.

A review of myocardial ischaemia/reperfusion injury: Pathophysiology, experimental models, biomarkers, genetics and pharmacological treatment

Cardiovascular diseases are known to be the most fatal diseases worldwide. Ischaemia/reperfusion (I/R) injury is at the centre of the pathology of the most common cardiovascular diseases. According to the World Health Organization estimates, ischaemic heart disease is the leading global cause of death, causing more than 9 million deaths in 2016. After cardiovascular events, thrombolysis, percutaneous transluminal coronary angioplasty or coronary bypass surgery are applied as treatment. However, after restoring coronary blood flow, myocardial I/R injury may occur. It is known that this damage occurs due to many pathophysiological mechanisms, especially increasing reactive oxygen types. Besides causing cardiomyocyte death through multiple mechanisms, it may be an important reason for affecting other cell types such as platelets, fibroblasts, endothelial and smooth muscle cells and immune cells. Also, polymorphonuclear leukocytes are associated with myocardial I/R damage during reperfusion. This damage may be insufficient in patients with co-morbidity, as it is demonstrated that it can be prevented by various endogenous antioxidant systems. In this context, the resulting data suggest that optimal cardioprotection may require a combination of additional or synergistic multi-target treatments. In this review, we discussed the pathophysiology, experimental models, biomarkers, treatment and its relationship with genetics in myocardial I/R injury. SIGNIFICANCE OF THE STUDY: This review summarized current information on myocardial ischaemia/reperfusion injury (pathophysiology, experimental models, biomarkers, genetics and pharmacological therapy) for researchers and reveals guiding data for researchers, especially in the field of cardiovascular system and pharmacology.

Thinking Outside the Box

A broad definition of preconditioning is "the preparation for a subsequent action." Mounting evidence demonstrates that novel remote preconditioning paradigms, in which protective stimuli experienced locally can capacitate systemic tolerance and enhanced cell viability upon exposure to ensuing cellular insults, have been largely successful in the field of cardiovascular ischemia/reperfusion injury. To ensure successful protective preconditioning, some models (including the uterus) have been demonstrated to activate the unfolded protein response (UPR), which is a cellular stress response controlled at the level of the endoplasmic reticulum. However, in the context of remote preconditioning, activation of these intracellular molecular pathways must result in the extracellular transmission of adaptive signals to remote targets. In our recently published manuscript, we have described the activation of the UPR in the pregnant uterine myocyte to be associated with increased uterine myocyte quiescence and normal gestational length. We hypothesize that ubiquitous uterine gestational stresses experienced in every pregnancy, which have been demonstrated in other systems to activate the UPR, may induce a robust paracrine dissemination of a uterine secretome, for example, glucose-regulated protein 78, with preconditioning-like properties. Furthermore, we speculate that the gestational stress-induced uterine secretome acts to promote both local and systemic tolerance to the ensuing gestational insults, allowing for the maintenance of uterine quiescence. In this context, preterm labor may be the result of a pregnant uterus experiencing a stress it cannot accommodate or when it is unable to host an appropriate UPR resulting in insufficient preconditioning and a diminished local and systemic capacity to tolerate pregnancy-dependent increases in normal gestational stress. This is highly attractive from a clinical viewpoint as we ultimately aim to identify local and systemic adaptations that may serve as preconditioning stimuli for use as a strategy to restore appropriate preconditioning profiles to prolong uterine quiescence in pregnancy.

Platelet P2Y₁₂ blockers confer direct postconditioning-like protection in reperfused rabbit hearts

BACKGROUND

Blockade of platelet activation during primary percutaneous intervention for acute myocardial infarction is standard care to minimize stent thrombosis. To determine whether antiplatelet agents offer any direct cardioprotective effect, we tested whether they could modify infarction in a rabbit model of ischemia/reperfusion caused by reversible ligation of a coronary artery.

METHODS AND RESULTS

The P2Y₁₂ (adenosine diphosphate) receptor blocker cangrelor administered shortly before reperfusion in rabbits undergoing 30-minute regional ischemia/3-hour reperfusion reduced infarction from 38% of ischemic zone in control hearts to only 19%. Protection was dose dependent and correlated with the degree of inhibition of platelet aggregation. Protection was comparable to that seen with ischemic postconditioning (IPOC). Cangrelor protection, but not its inhibition of platelet aggregation, was abolished by the same signaling inhibitors that block protection from IPOC suggesting protection resulted from protective signaling rather than anticoagulation. As with IPOC, protection was lost when cangrelor administration was delayed until 10 minutes after reperfusion and no added protection was seen when cangrelor and IPOC were combined. These findings suggest both IPOC and cangrelor may protect by the same mechanism. No protection was seen when cangrelor was used in crystalloid-perfused isolated hearts indicating some component in whole blood is required for protection. Clopidogrel had a very slow onset of action requiring 2 days of treatment before platelets were inhibited, and only then the hearts were protected. Signaling inhibitors given just prior to reperfusion blocked clopidogrel's protection. Neither aspirin nor heparin was protective.

CONCLUSIONS

Clopidogrel and cangrelor protected rabbit hearts against infarction. The mechanism appears to involve signal transduction during reperfusion rather than inhibition of intravascular coagulation. We hypothesize that both drugs protect by activating IPOC's protective signaling to prevent reperfusion injury. If true, patients receiving P2Y₁₂ inhibitors before percutaneous intervention may already be postconditioned thus explaining failure of recent clinical trials of postconditioning drugs.

Alpha-melanocyte-stimulating hormone inhibits renal injury in the absence of neutrophils

BACKGROUND

We previously showed that alpha-melanocyte stimulating hormone (alpha-MSH) decreases ischemia/reperfusion injury even when started six hours after ischemia. Alpha-MSH inhibits both neutrophil accumulation and nitric oxide production. To determine the relative importance of alpha-MSH on the neutrophil pathway, we examined the effects of alpha-MSH in injury models where neutrophil effects are minimal or absent.

METHODS

We studied the effects of alpha-MSH in (1) intercellular adhesion molecule-1 (ICAM-1) knock-out and background mice that were subjected to 40 minutes of ischemia and 24 hours reperfusion, and (2) isolated kidneys that were subjected to in vivo ischemia for 20 minutes and then perfused ex vivo for one hour without neutrophils. To begin to search for direct tubule effects of alpha-MSH, we studied the effect of alpha-MSH on nitric oxide (NO) in endotoxin/interferon-gamma-treated mouse cortical tubule cells.

RESULTS

ICAM-1 knock-out mice had 75% less neutrophil infiltration than background mice after ischemia. Despite the relative lack of neutrophils, alpha-MSH inhibited renal injury in ICAM-1 knock-out mice. Alpha-MSH also significantly preserved GFR and tubular sodium reabsorption in the isolated perfused ischemic kidney model. Alpha-MSH and a nitric oxide inhibitor did not exhibit synergy. Finally, alpha-MSH inhibited nitrite production by 20% in the mouse cortical tubule cells (MCT), similar to parallel observations in a cultured mouse macrophage line (RAW cells).

CONCLUSIONS

We conclude that alpha-MSH decreases renal injury when neutrophil effects are minimal or absent, indicating that alpha-MSH inhibits neutrophil-independent pathways of renal injury. The preservation of sodium absorption ex vivo and inhibition of nitrite production in cultured MCT cells suggests that alpha-MSH inhibits tubular injury by direct tubular effects.

Endogenous protective mechanisms in myocardial ischemia: hibernation and ischemic preconditioning

Myocardial ischemia, even if it persists for a prolonged period of time, does not inevitably induce irreversible damage. Recent studies have identified 2 phenomena that are characterized by endogenous cardioprotective features, i.e., myocardial hibernation and ischemic preconditioning. Myocardial hibernation is characterized by chronic contractile dysfunction during persistent ischemia. The myocardium remains viable, and function is restored upon reperfusion. Ischemic preconditioning is characterized by delayed development of infarct size when prolonged and severe myocardial ischemia is preceded > or = 1 short-lasting episodes of ischemia and reperfusion. While ischemic preconditioning involves the activation of the adenosine A1 receptor, the bradykinin receptor, and activation of adenosine triphosphate (ATP)-dependent potassium channels, the mechanisms underlying myocardial hibernation are still unclear.

Cardioprotection associated with preconditioning in the anesthetized ferret

The cardioprotective effect of ischemic preconditioning (PC) was investigated in the anesthetized ferret model of myocardial ischemia followed by reperfusion. PC of 2, 5, or 10-min duration, followed by 10-min reflow, was studied in animals subjected to 60-min sustained LAD coronary artery ischemia followed by 5-h reperfusion. Infarct size was determined by tetrazolium staining. Sham PC ferrets had a mean infarct of 72% of risk zone. A 2-min or 5-min cycle of PC significantly reduced tissue damage to 54% (p < 0.05) and 44% (p < 0.01), respectively. Infarct reduction associated with 10-min ischemic PC was not significant (57% of AAR). The cardioprotective effects of 5-min PC were lost when sustained ischemia was prolonged to 75 or 90-min. Myocardial salvage afforded by 5-min PC was also abolished by both a) inhibition of ATP-sensitive potassium channels using either glyburide or 5-HD and b) blockade of adenosine receptors with the A1 selective agent DPCPX. In the absence of PC, activation of ATP-sensitive potassium channels with the cardiac-selective agonist BMS-180448 significantly (p < 0.01) reduced infarct size from 66% to 37% of the risk zone. Cardioprotection, or its loss, was not the result of hemodynamic alterations occurring during PC, drug administration, or the coronary occlusion and reperfusion phases. Based upon its body size and lack of extensive myocardial collateral circulation the ferret offers a usefull alternative small species for study of ischemia and reperfusion salvage. It is concluded in the ferret that: a) the threshold for PC is less than in either the rat, rabbit, or dog; unlike the dog and pig, the beneficial effects of PC are b) reduced when the ischemic PC interval is extended to 10-min or c) lost if sustained coronary occlusion is maintained for a period of 75-min or longer; and last, a role in PC for both d) ATP-sensitive potassium channels and e) adenosine A1 receptors can be demonstrated.

Acute pharmacologic preconditioning as a new concept and alternative approach for prevention of skeletal muscle ischemic necrosis

The phenomenon of ischemic preconditioning for augmentation of ischemic tolerance has been well documented in the myocardium of common laboratory animals and human cardiomyocytes. The cellular mechanism of ischemic preconditioning is unclear, but adenosine is most likely the mediator in the rabbit, dog, pig and human. We have demonstrated recently that the protective effect of ischemic preconditioning and adenosine against ischemic injury can also be induced in pig skeletal muscles [116]. We speculate that adenosine is a potential treatment modality for prevention of skeletal muscle ischemic injury in vascular and musculoskeletal reconstructive surgery and in muscle and limb procurement for transplantation in the future. It is hoped that this review will stimulate workers at other laboratories to join the adventure in exploring the cellular mechanism and clinical application of adenosine for augmentation of skeletal muscle ischemic tolerance.

Effects of Bordetella pertussis toxin pretreatment on the antiarrhythmic action of ischaemic preconditioning in anaesthetized rats

1. Bordetella pertussis toxin, which catalyses the ADP-ribosylation of certain guanine nucleotide binding proteins (G proteins), thus functionally uncoupling them from associated receptors, was examined to determine whether it modified the antiarrhythmic effect of ischaemic preconditioning in anaesthetized rats. 2. Pertussis toxin (25 micrograms kg-1, i.p., 48 h prior to heart isolation) attenuated the negative chronotropic effect of acetylcholine (ACh) in rat isolated Langendorff perfused hearts. ACh (10 microM) reduced heart rate by 4% in hearts taken from pertussis toxin-treated animals, compared to a reduction of 57% in hearts taken from animals treated only with vehicle. 3. In anaesthetized rats, ischaemic preconditioning (a single 3 min occlusion of the left main coronary artery followed by 10 min reperfusion) had a pronounced antiarrhythmic effect during a subsequent 30 min period of regional myocardial ischaemia. Compared to hearts receiving only a 30 min period of left coronary occlusion, there was a reduced mortality (67% and 0% for control and preconditioned groups, respectively; P < 0.01) and decreased incidences of ventricular tachycardia (VT) and ventricular fibrillation (VF). Pretreatment with pertussis toxin (25 micrograms kg-1, i.p., 48 h previously) did not modify the arrhythmias associated with a 30 min period of regional myocardial ischaemia, neither did it modify the reduction in mortality (from 56% to 0%; P < 0.05) associated with preconditioning. Furthermore, the decrease in total ventricular premature beat count induced by preconditioning seen in controls (from 427 +/- 130 to 95 +/- 45) was also seen in pertussis toxin-treated rats (from 252 +/- 190 to 57 +/- 25). 4. These results suggest that receptor coupling to pertussis toxin-sensitive G proteins is not necessary for the antiarrhythmic effect of ischaemic preconditioning in this model.