Signalling pathways and mechanisms of protection in pre- and postconditioning: historical perspective and lessons for the future

Ischaemic pre- and postconditioning are potent cardioprotective interventions that spare ischaemic myocardium and decrease infarct size after periods of myocardial ischaemia/reperfusion. They are dependent on complex signalling pathways involving ligands released from ischaemic myocardium, G-protein-linked receptors, membrane growth factor receptors, phospholipids, signalling kinases, NO, PKC and PKG, mitochondrial ATP-sensitive potassium channels, reactive oxygen species, TNF-α and sphingosine-1-phosphate. The final effector is probably the mitochondrial permeability transition pore and the signalling produces protection by preventing pore formation. Many investigators have worked to produce a roadmap of this signalling with the hope that it would reveal where one could intervene to therapeutically protect patients with acute myocardial infarction whose hearts are being reperfused. However, attempts to date to show efficacy of such an intervention in large clinical trials have been unsuccessful. Reasons for this inability to translate successes in the experimental laboratory to the clinical arena are evaluated in this review. It is suggested that all patients with acute coronary syndromes currently presenting to the hospital and being treated with platelet P2Y12 receptor antagonists, the current standard of care, are indeed already benefiting from protection from the conditioning pathways outlined earlier. If that proves to be the case, then future attempts to further decrease infarction will have to rely on interventions which protect by a different mechanism.

What is Wrong With Cardiac Conditioning? We May be Shooting at Moving Targets

Early recanalization of the occluded culprit coronary artery clearly reduces infarct size in both animal models and patients and improves clinical outcomes. Unfortunately, reperfusion can seldom be accomplished before some myocardium infarcts. As a result there has been an intensive search for interventions that will make the heart resistant to infarction so that reperfusion could salvage more myocardium. A number of interventions have been identified in animal models, foremost being ischemic preconditioning. It protects by activating signaling pathways that prevent lethal permeability transition pores from forming in the heart’s mitochondria at reperfusion. Such conditioning can be accomplished in a clinically relevant manner either by staccato reperfusion (ischemic postconditioning) or by pharmacological activation of the conditioning signaling pathways prior to reperfusion. Unfortunately, clinical trials of ischemic postconditioning and pharmacologic conditioning have been largely disappointing. We suggest that this may be caused by inappropriate use as models intended to mimic the clinical scenario of young healthy animals that receive none of the many drugs currently given to our patients. Patients may be resistant to some forms of conditioning because of comorbidities, for example, diabetes, or they may already be conditioned by adjunct medications, for example, P2Y12 inhibitors or opioids. Incremental technological improvements in patient care may render some approaches to cardioprotection redundant, and thus the clinical target may be continually changing, while our animal models have not kept pace. In remote conditioning, a limb is subjected to ischemia/reperfusion prior to or during coronary reperfusion. Its mechanism is not as well understood as that of ischemic preconditioning, but the results have been very encouraging. In the present article, we will review ischemic, remote, and pharmacologic conditioning and possible confounders that could interfere with their efficacy in clinical trials in 2 settings of myocardial ischemia: (1) primary angioplasty in acute myocardial infarction and (2) elective angioplasty.

Triple therapy greatly increases myocardial salvage during ischemia/reperfusion in the in situ rat heart

BACKGROUND

Cangrelor, a P2Y12 receptor blocker, administered just prior to reperfusion reduced but did not eliminate myocardial infarction in rabbits. Combining cangrelor with ischemic postconditioning offered no additional protection suggesting they protected by a similar mechanism. To determine if cangrelor's protection might be additive to other cardioprotective interventions we tested cangrelor in combination with ischemic preconditioning, cariporide, a sodium-hydrogen exchange blocker, and mild hypothermia.

METHODS

Open-chest rats underwent 30-min coronary occlusion/2-h reperfusion.

RESULTS

Cangrelor, administered as a bolus (60 μg/kg) 10 min before reperfusion and continued as an infusion (6 μg/kg/min) for the duration of the experiment, decreased infarction from 45.3 % of risk zone in control hearts to 25.0 %. Combining cangrelor and ischemic preconditioning offered no additional protection. Mild hypothermia (32-33 °C) instituted by peritoneal lavage with cold saline just prior to coronary occlusion resulted in 25.2 % infarction, and combining cangrelor and hypothermia nearly halved infarction to 14.1 % of risk zone. Cariporide (0.5 mg/kg) just prior to occlusion resulted in 27.2 % infarction and 15.8 % when combined with cangrelor. Combining cangrelor, hypothermia and cariporide further halved infarction to only 6.3 %. We also tested another P2Y12 inhibitor ticagrelor which is chemically similar to cangrelor. Ticagrelor (20 mg/kg) fed 1 h prior to surgery reduced infarct size by an amount similar to that obtained with cangrelor (25.6 % infarction), and this protective effect was abolished by chelerythrine and wortmannin, thus implicating participation of PKC and PI3-kinase, resp., in signaling.

CONCLUSIONS

Cardioprotection from a P2Y12 receptor antagonist can be combined with at least 2 other strategies to magnify the protection. Combining multiple interventions that use different cardioprotective mechanisms could provide powerful protection against infarction in patients with acute coronary thrombosis.

Combined cardioprotectant and antithrombotic actions of platelet P2Y12 receptor antagonists in acute coronary syndrome: just what the doctor ordered

Since the P2Y12 receptor antagonists were first introduced, they have been extensively tested in patients with acute coronary syndrome and are now standard of care. These antiplatelet drugs are very effective in reducing subsequent cardiovascular events, stent thromboses, and mortality in patients with acute myocardial infarction undergoing reperfusion therapy. Although the prevailing view is that their benefit derives from their antithrombotic properties, other unrelated pleiotropic effects appear to be equally beneficial. Accumulating clinical and animal evidence indicates that, if present at the time of reperfusion, these drugs have a direct anti-infarct effect similar to that of ischemic postconditioning. Four oral antagonists have been developed in rapid succession: ticlopidine, clopidogrel, prasugrel, and ticagrelor. Each agent had a more consistent and rapid onset of action than the previous one, and this has correlated with improved clinical outcomes when given early in treatment. Unfortunately, gut absorption causes an appreciable delay in the onset of effect, especially when morphine is used, and the constant push to minimize the door-to-balloon time has made it difficult to achieve adequate platelet inhibition at the time of percutaneous coronary intervention with an oral agent. An intravenous P2Y12 antagonist such as cangrelor may optimize treatment because it produces nearly maximal inhibition of platelet aggregation within minutes. If antiplatelet agents do protect through postconditioning's mechanism, then they would render any other intervention that protects through that mechanism redundant. Indeed, animals treated with cangrelor cannot be further protected by pre- or postconditioning. However, interventions that use a different mechanism such as mild hypothermia or cariporide, a Na(+)-H(+) exchange blocker, do add to cangrelor's protection. Future research should be directed toward identifying interventions that can augment the protection from antiplatelet therapy and finding a way to optimize P2Y12 inhibition at reperfusion in all patients.

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.

Pegfilgrastim administered in an abbreviated schedule, significantly improved neutrophil recovery after high-dose radiation-induced myelosuppression in rhesus macaques

Conventional daily administration of filgrastim is effective in reducing the duration of severe neutropenia and enhancing survival following lethal radiation, myelosuppressive cytotoxic therapy or myeloablation and stem cell transplantation. A sustained-duration form of filgrastim, pegfilgrastim has significantly simplified scheduling protocols after chemotherapy-induced neutropenia to a single injection while maintaining the therapeutic effectiveness of daily administration of filgrastim. We examined the ability of a single or double (weekly) administration of pegfilgrastim to significantly improve neutrophil recovery in a rhesus macaque model of severe radiation-induced myelosuppression. Animals were exposed to potentially lethal 6 Gy total-body X radiation. After irradiation all animals received supportive care and were administered either pegfilgrastim at 300 μg/kg on day 1 or day 1 and day 7 post exposure, or filgrastim at 10 μg/kg/day initiated on day 1 post exposure and continued daily through neutrophil recovery. Pharmacokinetic parameters and neutrophil-related values for duration of neutropenia, neutrophil nadir, time to recovery to an absolute neutrophil count ≥500/μL or ≥2000/μL, and days of antibiotic support were determined. Effective plasma concentrations of pegfilgrastim were maintained in neutropenic animals until after the onset of hematopoietic recovery, which is consistent with neutrophil-dependent properties of elimination. Administration of pegfilgrastim at day 1 and day 7 was most effective at improving neutrophil recovery compared to daily administration of filgrastim or a single injection of pegfilgrastim on day 1, after severe, radiation-induced myelosuppression in rhesus macaques.

Is it time to translate ischemic preconditioning's mechanism of cardioprotection into clinical practice?

After three decades of intense research on cardioprotection, we still do not have an approved intervention for limiting infarct size in the patient with acute myocardial infarction (AMI) aside from reperfusion therapy. Yet approximately 25% of patients with AMI that are reperfused are still at risk for heart failure because of excessive muscle necrosis. This article will try to make the case that ischemic preconditioning (IPC), still the most potent anti-infarct intervention ever described, is ready for serious clinical testing now. Over the past 25 years, IPC's mechanism has been largely elucidated and targets a reperfusion injury. Ischemic preconditioning was never considered an intervention for AMI because of its need for pretreatment. However, knowledge of IPC's mechanism has revealed a large number of drugs and interventions that will activate IPC's signaling pathway at the time of reperfusion. Several small clinical trials suggest that they can be quite effective, but so far industry seems to have little interest in developing them. So, while basic scientists are in a continuous cycle of discovery and publication for new and novel cardioprotectants, there has been little effort devoted to translating these interventions into clinical practice. We believe that there are suitable IPC-based interventions that are ready for clinical testing today and the time has come for large-scale clinical trials.

Myocardial protection with mild hypothermia

Mild hypothermia, 32-35° C, is very potent at reducing myocardial infarct size in rabbits, dogs, sheep, pigs, and rats. The benefit is directly related to reduction in normothermic ischaemic time, supporting the relevance of early and rapid cooling. The cardioprotective effect of mild hypothermia is not limited to its recognized reduction of infarct size, but also results in conservation of post-ischaemic contractile function, prevention of no-reflow or microvascular obstruction, and ultimately attenuation of left ventricular remodelling. The mechanism of the anti-infarct effect does not appear to be related to diminished energy utilization and metabolic preservation, but rather to survival signalling that involves either the extracellular signal-regulated kinases and/or the Akt/phosphoinositide 3-kinase/mammalian target of rapamycin pathways. Initial clinical trials of hypothermia in patients with ST-segment elevation myocardial infarction were disappointing, probably because cooling was too slow to shorten normothermic ischaemic time appreciably. New approaches to more rapid cooling have recently been described and may soon be available for clinical use. Alternatively, it may be possible to pharmacologically mimic the protection provided by cooling soon after the onset of ischaemia with an activator of mild hypothermia signalling, e.g. extracellular signal-regulated kinase activator, that could be given by emergency medical personnel. Finally, the protection afforded by cooling can be added to that of pre- and post-conditioning because their mechanisms differ. Thus, myocardial salvage might be greatly increased by rapidly cooling patients as soon as possible and then giving a pharmacological post-conditioning agent immediately prior to reperfusion.

All preconditioning-related G protein-coupled receptors can be demonstrated in the rabbit cardiomyocyte

G protein-coupled receptors for adenosine (A(1), A(3), A(2A), and A(2B)), bradykinin (B(1)) and opioids (δ) are all involved in the mechanism of ischemic preconditioning. Although the heart is comprised of many tissue types, it has been assumed that preconditioning's protective signaling occurs in the cardiomyocyte. We critically tested that hypothesis by testing for the presence of each of these receptors in isolated adult rabbit ventricular myocytes that had been transfected with cyclic nucleotide-gated (CNG) ion channels. Because subsarcolemmal cyclic adenosine monophosphate (cAMP) opens the CNG channels, we could monitor cAMP levels within a single cardiomyocyte by measuring channel current with a patch pipette. The presence of a receptor would be confirmed if we could alter cAMP in the cell with a selective agonist to the receptor being studied. Superfusion with the β-adrenergic G(s)-coupled receptor agonist isoproterenol (50 nmol/L) transiently increased cAMP levels and, therefore, channel current. Pretreatment with selective agonists to A(1) or A(3) adenosine receptors (ARs) that are G(i)-coupled markedly attenuated the response to isoproterenol, indicating inhibition of adenylyl cyclase by increased G(i) activity. Agonists to bradykinin or δ-opioid receptors also attenuated isoproterenol's response. A(2A)AR and A(2B)AR are G(s)-coupled. The A(2A)AR-selective agonist CGS21680 increased current through CNG channels but only in the presence of phosphodiesterase (PDE) inhibitors, indicating low surface receptor activity and high intracellular PDE activity. As we previously reported, BAY 60-6583, an A(2B)AR-selective agonist which mimics preconditioning's protection in rabbit heart, neither increased nor decreased membrane current in transfected cardiomyocytes, suggesting the absence or a markedly limited number of A(2B)AR in the sarcolemma. However, reverse transcription polymerase chain reaction (RT-PCR) of purified cardiomyocytes yielded an A(2B)AR band, implying that rabbit cardiomyocytes do indeed express A(2B)AR. These data reveal that all receptors reported to be involved in ischemic preconditioning do exist on or within the cardiomyocyte.

Adenosine and Opioid Receptors Do Not Trigger the Cardioprotective Effect of Mild Hypothermia

Mild hypothermia (32°C-34°C) exerts a potent cardioprotection in animal models of myocardial infarction. Recently, it has been proposed that this beneficial effect is related to survival signaling. We, therefore, hypothesized that the well-known cardioprotective pathways dependent on adenosine and/or opioid receptors could be the trigger of hypothermia-induced salvage. Open-chest rabbits were accordingly exposed to 30 minutes of coronary artery occlusion (CAO) under normothermic (NT) or hypothermic ([HT] 32°C) conditions. In the latter, hypothermia was induced by total liquid ventilation with temperature-controlled perfluorocarbons in order to effect ultrafast cooling and to accurately control cardiac temperature. After 4 hours of reperfusion, infarct and no-reflow zone sizes were assessed and quantified as a percentage of the risk zone. In animals experiencing HT ischemia, the infarct size was dramatically reduced as compared to NT animals (9% ± 3% vs 55% ± 2% of the risk zone, respectively). Importantly, administration of opioid and adenosine receptor antagonists (naloxone [6 mg/kg iv] and 8-( p-sulfophenyl) theophylline [20 mg/kg iv], respectively) did not alter the infarct size or affect the cardioprotective effect of hypothermia. Doses of these 2 antagonists were appropriately chosen since they blunted infarct size reduction induced by selective opioid or adenosine receptor stimulation with morphine (0.3 mg/kg iv) or N6-cyclopentyladenosine ([CPA] 100 μg/kg iv), respectively. Therefore, the cardioprotective effect of mild hypothermia is not triggered by either opioid or adenosine receptor activation, suggesting the involvement of other cardioprotective pathways.

A2B adenosine receptors inhibit superoxide production from mitochondrial complex I in rabbit cardiomyocytes via a mechanism sensitive to Pertussis toxin

BACKGROUND AND PURPOSE

A(2B) adenosine receptors protect against ischaemia/reperfusion injury by activating survival kinases including extracellular signal-regulated kinase (ERK) and phosphatidylinositol 3-kinase (PI3K). However, the underlying mechanism(s) and signalling pathway(s) remain undefined.

EXPERIMENTAL APPROACH

HEK 293 cells stably transfected with human A(2B) adenosine receptors (HEK-A(2B) ) and isolated adult rabbit cardiomyocytes were used to assay phosphorylation of ERK by Western blot and cation flux through cAMP-gated channels by patch clamp methods. Generation of reactive oxygen species (ROS) by mitochondria was measured with a fluorescent dye.

KEY RESULTS

In HEK-A(2B) cells, the selective A(2B) receptor agonist Bay 60-6583 (Bay 60) increased ERK phosphorylation and cAMP levels, detected by current through cAMP-gated ion channels. However, increased cAMP or its downstream target protein kinase A was not involved in ERK phosphorylation. Pertussis toxin (PTX) blocked ERK phosphorylation, suggesting receptor coupling to G(i) or G(o) proteins. Phosphorylation was also blocked by inhibition of PI3K (with wortmannin) or of ERK kinase (MEK1/2, with PD 98059) but not by inhibition of NO synthase (NOS). In cardiomyocytes, Bay 60 did not affect cAMP levels but did block the increased superoxide generation induced by rotenone, a mitochondrial complex I inhibitor. This effect of Bay 60 was inhibited by PD 98059, wortmannin or PTX. Inhibition of NOS blocked superoxide production because NOS is downstream of ERK.

CONCLUSION AND IMPLICATIONS

Activation of A(2B) adenosine receptors reduced superoxide generation from mitochondrial complex I through G(i/o) , ERK, PI3K, and NOS, all of which have been implicated in ischaemic preconditioning.

Does mild hypothermia protect against reperfusion injury? The debate continues

Mild hypothermia (32-35°C) salvages ischemic myocardium and reduces infarct size in hearts undergoing ischemia/reperfusion. It is clear that a cardioprotective effect is evident when the heart is cooled during ischemia, and the protection is greater as the duration of normothermic ischemia is increasingly limited. The effect of cooling just before and at reperfusion is more controversial. Multiple experimental studies have revealed no effect of mild hypothermia on myocardial infarction when cooling was initiated in the waning minutes of ischemia. But Götberg et al. have demonstrated a small effect in pigs cooled with cold intravenous saline and a venous thermode, although the effect of cooling during ischemia continued to be more prominent. Clinical studies have been disappointing, and possible explanations are offered. Götberg's new data are encouraging, but it is questioned whether this is the correct time to conduct a new large-scale clinical trial.

Cardioprotection by mild hypothermia during ischemia involves preservation of ERK activity

Cooling the ischemic heart by just a few degrees protects it from infarction without affecting its mechanical function, but the mechanism of this protection is unknown. We investigated whether signal transduction pathways might be involved in the anti-infarct effect of mild hypothermia (35°C). Isolated rabbit hearts underwent 30 min of coronary artery occlusion/2 h of reperfusion. They were either maintained at 38.5°C or cooled to 35°C just before and only during ischemia. Infarct size was measured. The effects of the protein kinase C inhibitor chelerythrine, the nitric oxide synthase inhibitor N (ω)-nitro-L: -arginine methyl ester (L: -NAME), the phosphatidylinositol 3-kinase antagonist wortmannin, or either of the mitogen-activated protein kinase kinase 1/2 (MEK1/2) inhibitors PD98059 or U0126 on cooling's protection were examined. Myocardial ATP assays were performed and the level of phosphorylation of extracellular signal-regulated kinase (ERK) and MEK was examined by western blotting. To investigate an effect of cooling on protein phosphatase (PPase), a PPase inhibitor cantharidin was tested in the infarct model and the effect of mild hypothermia on PP2A activity in vitro was measured. Infarct size was 34.4 ± 2.2% of the ischemic zone in normothermic (38.5°C) hearts, but only 15.6 ± 8.7% in hearts cooled to 35°C during ischemia. Mechanical function was unaffected. Neither chelerythrine, L: -NAME, nor wortmannin had any effect, but both PD98059 and U0126 completely eliminated protection. Ischemia rather than reperfusion was the critical time when ERK had to be active to realize protection. Phosphorylation of ERK and MEK fell during normothermic ischemia, but during hypothermic ischemia phosphorylation of ERK remained high while that of MEK was increased. Cooling only slightly delayed the rate at which ATP fell during ischemia, and ERK inhibition did not affect that attenuation suggesting ATP preservation was unrelated to protection. Cantharidin, like cooling, also protected during ischemia but not at reperfusion, and its protection was dependent on ERK phosphorylation. However, mild hypothermia had a negligible effect on PP2A activity in an in vitro assay. Hence, mild hypothermia preserves ERK and MEK activity during ischemia which somehow protects the heart. While a PPase inhibitor mimicked cooling's protection, a direct effect of cooling on PP2A could not be demonstrated.

Ischemic postconditioning: from receptor to end-effector

Ischemic preconditioning, a robust cardioprotective intervention, has limited clinical efficacy because it must be initiated before myocardial ischemia. Conversely, ischemic postconditioning, repeated brief reocclusions of a coronary artery after release of prolonged coronary occlusion, provides cardioprotection in clinically feasible settings, that is, coronary angioplasty. Ischemic postconditioning's signaling is being investigated to identify pharmacological triggers that could be used without angioplasty. In initial minutes of reperfusion H(+) washes out of previously ischemic cells. pH rises enabling mitochondrial permeability transition pores (MPTPs) to form leading to cessation of ATP production and cell necrosis. Coronary reocclusions maintain sufficient acidosis to keep MPTP closed while signaling is initiated that can generate endogenous antagonists of MPTP formation even after cellular pH normalizes. Reintroduction of oxygen generates reactive oxygen species that activate protein kinase C to increase sensitivity of adenosine A(2b) receptors allowing adenosine released from ischemic cells to bind leading to activation of phosphatidylinositol 3-kinase and extracellular signal-regulated kinase 1/2. Phosphatidylinositol 3-kinase activation results in phosphorylation of Akt promoting activation of nitric oxide synthase and nitric oxide production, which inhibits glycogen synthase kinase-3β, perhaps the final cytosolic signaling step before inhibition of MPTP formation. Interference with MPTP may be the final step that determines cell salvage.

Evidence for an intracellular localization of the adenosine A2B receptor in rat cardiomyocytes

Protection achieved by ischemic preconditioning is dependent on A(2B) adenosine receptors (A(2B)AR) in rabbit and mouse hearts and, predictably, an A(2B)AR agonist protects them. But it is controversial whether cardiomyocytes themselves actually express A(2B)AR. The present study tested whether A(2B)AR could be demonstrated on rat cardiomyocytes. Isolated rat hearts experienced 30 min of ischemia and 120 min of reperfusion. The highly selective, cell-permeant A(2B)AR agonist BAY60-6583 (500 nM) infused at reperfusion reduced infarct size from 40.4 ± 2.0% of the risk zone in control hearts to 19.9 ± 2.8% indicating that A(2B)AR are protective in rat heart as well. Furthermore, BAY60-6583 reduced calcium-induced mitochondrial permeability transition in isolated rat cardiomyocytes. A(2B)AR protein could be demonstrated in isolated cardiomyocytes by western blotting. In addition, message for A(2B)AR was found in individual cardiomyocytes using quantitative RT-PCR. Surprisingly, immunofluorescence microscopy did not show A(2B)AR on the cardiomyocyte's sarcolemma but rather at intracellular sites. Co-staining with MitoTracker Red in isolated cardiomyocytes revealed A(2B)AR are localized to mitochondria. Western blot analysis of a mitochondrial fraction from either rat heart biopsies or isolated cardiomyocytes revealed a strong A(2B)AR band. Thus, the present study demonstrates that activation of A(2B)AR is strongly cardioprotective in rat heart and suppresses transition pores in isolated cardiomyocytes, and A(2B)AR are expressed in individual cardiomyocytes. However, surprisingly, A(2B)AR are present in or near mitochondria rather than on the sarcolemma as are other adenosine receptors. Because A(2B)AR signaling is thought to result in inhibition of mitochondrial transition pores, this convenient location may be important.

The small chill: mild hypothermia for cardioprotection?

Reducing the heart's temperature by 2-5°C is a potent cardioprotective treatment in animal models of coronary artery occlusion. The anti-infarct benefit depends upon the target temperature and the time at which cooling is instituted. Protection primarily results from cooling during the ischaemic period, whereas cooling during reperfusion or beyond offers little protection. In animal studies, protection is proportional to both the depth and duration of cooling. An optimal cooling protocol must appreciably shorten the normothermic ischaemic time to effectively salvage myocardium. Patients presenting with acute myocardial infarction could be candidates for mild hypothermia since the current door-to-balloon time is typically 90 min. But they would have to be cooled quickly shortly after their arrival. Several strategies have been proposed for ultra-fast cooling, but most like liquid ventilation and pericardial perfusion are too invasive. More feasible strategies might include cutaneous cooling, peritoneal lavage with cold solutions, and endovascular cooling with intravenous thermodes. This last option has been investigated clinically, but the results have been disappointing possibly because the devices lacked capacity to cool the patient quickly or cooling was not implemented soon enough. The mechanism of hypothermia's protection has been assumed to be energy conservation. However, whereas deep hypothermia clearly preserves ATP, mild hypothermia has only a modest effect on ATP depletion during ischaemia. Some evidence suggests that intracellular signalling pathways might be responsible for the protection. It is unknown how cooling could trigger these pathways, but, if true, then it might be possible to duplicate cooling's protection pharmacologically.

Cardioprotective PKG-independent NO signaling at reperfusion

Cell models of ischemic preconditioning (IPC) indicate nitric oxide (NO) is involved in protection accruing during reoxygenation but disagree whether it acts through PKG. Using a more relevant intact heart model, we studied isolated rabbit hearts subjected to 30-min coronary artery occlusion/120-min reperfusion. We previously found protection from PKG activator 8-(4-chlorophenylthio)-guanosine 3',5'-cyclic monophosphate (CPT-cGMP) at reperfusion was blocked by A(2b) adenosine receptor (A(2b)AR), ERK, or phosphatidylinositol 3-kinase (PI3-kinase) blockers. In this investigation A(2b)AR agonist BAY 60-6583 or CPT-cGMP at reperfusion reduced infarction comparably to IPC. Their protection was abrogated by N(ω)-nitro-l-arginine methyl ester (l-NAME), suggesting a PKG-independent NO synthase in IPC's mediator pathway downstream of PKG and A(2b)AR. NO donor S-nitroso-N-acetyl-d,l-penicillamine (SNAP) at reperfusion also protected. This protection was not blocked by PI3-kinase inhibitor wortmannin or ERK antagonist PD-98059, suggesting NO acted downstream of these kinases. Protection from SNAP was not affected by mitochondrial ATP-sensitive K(+) channel closer 5-hydroxydecanoate, PKC antagonist chelerythrine, reactive oxygen species scavenger N-2-mercaptopropionylglycine, or soluble guanylyl cyclase antagonist 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ). Absence of ODQ effect indicated NO was acting independently of PKG. BAY 58-2667, a soluble guanylyl cyclase activator, was protective, and l-NAME blocked its infarct-sparing effect, indicating a second signaling event dependent on NO generation but independent of PKG. SB216763, a blocker of glycogen synthase kinase-3β (GSK-3β), decreased infarct size, and its infarct-sparing effect was not affected by l-NAME, suggesting GSK-3β acted downstream or independently of NO. Hence, NO signaling occurs in IPC's mediator pathway downstream of Akt and ERK, and its protection is independent of PKG.

Both A2a and A2b adenosine receptors at reperfusion are necessary to reduce infarct size in mouse hearts

Pre- and postconditioning depend on the activation of adenosine receptors (ARs) at the end of the index ischemia. The aim of this study was to determine which receptor subtypes must be activated. In situ mouse hearts underwent 30 min of regional ischemia, followed by 2 h of reperfusion. As expected, either ischemic postconditioning (6 cycles of 10 s of reperfusion and 10 s of coronary occlusion) or infusion of the selective A(2b) adenosine receptor (A(2b)AR) agonist BAY60-6583 (BAY60) for 60 min, starting 5 min before reperfusion reduced infarct size in wild-type C57Bl/6N mice. Protection from either was abolished by the selective A(2b)AR antagonist MRS-1754, confirming a role for A(2b)AR. Additionally, the coadministration of ischemic postconditioning and a selective A(2a)AR antagonist led to the loss of protection as well. 5'-Ectonucleotidase (CD73) is thought to be necessary for the production of adenosine during ischemia. As predicted, ischemic postconditioning did not protect CD73 knockout mice. Selective agonists of either A(2b)AR (BAY60) or A(2a)AR (CGS-21680), as well as the coadministration of ischemic postconditioning and BAY60, also failed to protect hearts of the CD73 knockout mice. But the nonselective A(1)/A(2)AR agonist 5'-(N-ethylcarboxamido)adenosine (NECA) was protective, suggesting that the activation of multiple AR subtypes might be required. The coadministration of CGS-21680 and BAY60 also elicited profound protection, indicating that two AR subtypes, A(2a) and A(2b), must be simultaneously activated for protection to occur.

A(2b) adenosine receptors can change their spots

Recently, a central role for the A(2b) adenosine receptor in a variety of cardiovascular functions including inflammation, erectile function, coronary artery dilation, asthma and cardioprotection has been demonstrated. Despite this evidence, the low-affinity A(2b) adenosine receptor is still poorly understood. This receptor appears to be very promiscuous in its coupling. In most tissues, it couples to G(s) much like its cousin, the A(2a) adenosine receptor, but in mast cells and now, most recently, in cardiac fibroblasts, the A(2b) receptor also couples to G(q). Because of its low affinity, this receptor was originally thought unlikely to play any important physiological role. But the sensitivity of A(2b) adenosine receptors can be greatly increased by interaction with protein kinase C (PKC) making this receptor, under various conditions, both an activator and a target of PKC. We have recently documented a third coupling involving G(i). This plasticity and versatility of A(2b) adenosine receptors position them as potential triggers of signalling in multiple signalling cascades in many physiological responses, making this a most interesting receptor indeed.

BAY 58-2667, a nitric oxide-independent guanylyl cyclase activator, pharmacologically post-conditions rabbit and rat hearts

AIMS

BAY 58-2667 (BAY-58) directly activates soluble guanylyl cyclase without tolerance in a nitric oxide (NO)-independent manner, and its haemodynamic effect is similar to that of nitroglycerin. We tested whether BAY-58 could make both rabbit and rat hearts resistant to infarction when given at the end of an ischaemic insult.

METHODS AND RESULTS

All hearts were exposed to 30 min regional ischaemia followed by 120-(isolated hearts) or 180-(in situ hearts) min reperfusion. BAY-58 (1-50 nM) infused for 60 min starting 5 min before reperfusion significantly reduced infarction from 33.0 +/- 3.2% in control isolated rabbit hearts to 9.5-12.7% (P < 0.05). In a more clinically relevant in situ rabbit model, infarct size was similarly reduced with a loading dose of 53.6 microg/kg followed by a 60 min infusion of 1.25 microg/kg/min (41.1 +/- 3.1% infarction in control hearts to 16.0 +/- 4.4% in treated hearts, P < 0.05). BAY-58 similarly decreased infarction in the isolated rat heart, and protection was abolished by co-treatment with a protein kinase G (PKG) antagonist, or a mitochondrial K(ATP) channel antagonist. Conversely, N(omega)-nitro-L-arginine-methyl-ester-hydrochloride, a NO-synthase inhibitor, failed to block BAY-58's ability to decrease infarction, consistent with the latter's putative NO-independent activation of PKG. Finally, BAY-58 increased myocardial cGMP content in reperfused hearts while cAMP was unchanged.

CONCLUSION

When applied at reperfusion, BAY-58 is an effective cardioprotective agent with a mechanism similar to that of ischaemic pre-conditioning and, hence, should be a candidate for treatment of acute myocardial infarction in man.