Relative importance of adenosine A1 and A3 receptors in mediating physiological or pharmacological protection from ischemic myocardial injury in the rabbit heart

Targeting reperfusion injury in acute myocardial infarction: a review of reperfusion injury pharmacotherapy

INTRODUCTION

Acute myocardial infarction (AMI) (secondary to lethal ischemia-reperfusion [IR]) contributes to much of the mortality and morbidity from ischemic heart disease. Currently, the treatment for AMI is early reperfusion; however, this itself contributes to the final myocardial infarct size, in the form of what has been termed 'lethal reperfusion injury'. Over the last few decades, the discovery of the phenomena of ischemic preconditioning and postconditioning, as well as remote preconditioning and remote postconditioning, along with significant advances in our understanding of the cardioprotective pathways underlying these phenomena, have provided the possibility of successful mechanical and pharmacological interventions against reperfusion injury.

AREAS COVERED

This review summarizes the evidence from clinical trials evaluating pharmacological agents as adjuncts to standard reperfusion therapy for ST-elevation AMI.

EXPERT OPINION

Reperfusion injury pharmacotherapy has moved from bench to bedside, with clinical evaluation and ongoing clinical trials providing us with valuable insights into the shortcomings of current research in establishing successful treatments for reducing reperfusion injury. There is a need to address some key issues that may be leading to lack of translation of cardioprotection seen in basic models to the clinical setting. These issues are discussed in the Expert opinion section.

Adenosine and its receptors in the heart: regulation, retaliation and adaptation

The purine nucleoside adenosine is an important regulator within the cardiovascular system, and throughout the body. Released in response to perturbations in energy state, among other stimuli, local adenosine interacts with 4 adenosine receptor sub-types on constituent cardiac and vascular cells: A(1), A(2A), A(2B), and A(3)ARs. These G-protein coupled receptors mediate varied responses, from modulation of coronary flow, heart rate and contraction, to cardioprotection, inflammatory regulation, and control of cell growth and tissue remodeling. Research also unveils an increasingly complex interplay between members of the adenosine receptor family, and with other receptor groups. Given generally favorable effects of adenosine receptor activity (e.g. improving the balance between myocardial energy utilization and supply, limiting injury and adverse remodeling, suppressing inflammation), the adenosine receptor system is an attractive target for therapeutic manipulation. Cardiovascular adenosine receptor-based therapies are already in place, and trials of new treatments underway. Although the complex interplay between adenosine receptors and other receptors, and their wide distribution and functions, pose challenges to implementation of site/target specific cardiovascular therapy, the potential of adenosinergic pharmacotherapy can be more fully realized with greater understanding of the roles of adenosine receptors under physiological and pathological conditions. This review addresses some of the major known and proposed actions of adenosine and adenosine receptors in the heart and vessels, focusing on the ability of the adenosine receptor system to regulate cell function, retaliate against injurious stressors, and mediate longer-term adaptive responses.

Adenosine receptors and reperfusion injury of the heart

Adenosine, a catabolite of ATP, exerts numerous effects in the heart, including modulation of the cardiac response to stress, such as that which occurs during myocardial ischemia and reperfusion. Over the past 20 years, substantial evidence has accumulated that adenosine, administered either prior to ischemia or during reperfusion, reduces both reversible and irreversible myocardial injury. The latter effect results in a reduction of both necrosis or myocardial infarction (MI) and apoptosis. These effects appear to be mediated via the activation of one or more G-protein-coupled receptors (GPCRs), referred to as A(1), A(2A), A(2B) and A(3) adenosine receptor (AR) subtypes. Experimental studies in different species and models suggest that activation of the A(1) or A(3)ARs prior to ischemia is cardioprotective. Further experimental studies reveal that the administration of A(2A)AR agonists during reperfusion can also reduce MI, and recent reports suggest that A(2B)ARs may also play an important role in modulating myocardial reperfusion injury. Despite convincing experimental evidence for AR-mediated cardioprotection, there have been only a limited number of clinical trials examining the beneficial effects of adenosine or adenosine-based therapeutics in humans, and the results of these studies have been equivocal. This review summarizes our current knowledge of AR-mediated cardioprotection, and the roles of the four known ARs in experimental models of ischemia-reperfusion. The chapter concludes with an examination of the clinical trials to date assessing the safety and efficacy of adenosine as a cardioprotective agent during coronary thrombolysis in humans.

A3 and P2Y2 receptors control the recruitment of neutrophils to the lungs in a mouse model of sepsis

We have recently shown that A3 adenosine receptors and P2Y2 purinergic receptors play an important role in neutrophil chemotaxis. Chemotaxis of neutrophils to sites of infections is critical for immune defense. However, excessive accumulation of neutrophils in the lungs can cause acute lung tissue damage. Here we assessed the role of A3 and P2Y2 receptors in neutrophil sequestration to the lungs in a mouse model of sepsis. Sepsis was induced by cecal ligation and puncture (CLP) using adult male C57BL/6J mice (wild type [WT]), homozygous A3 receptor knockout (A3KO) mice, and P2Y2 receptor knockout (P2Y2KO) mice. Animals were killed 2, 4, 6, or 8 h after CLP, and peritoneal lavage fluid and blood were collected. Lungs were removed, and neutrophil infiltration was evaluated using elastase as a marker. Leukocyte and bacterial counts in peritoneal lavage fluid and blood samples were determined. Survival after sepsis was determined in a separate group. Leukocyte counts in the peritoneum were lower in A3KO and P2Y2KO mice than in WT mice. Conversely, initial leukocyte counts in the peripheral blood were higher in KO mice than in WT mice. Neutrophil sequestration to the lungs reached a maximum 2 h after CLP and remained significantly higher in WT mice compared with A3KO and P2Y2KO mice (P < 0.001). Survival after 24 h was significantly lower in WT mice (37.5%) than in A3KO or P2Y2KO mice (82.5%; P < 0.05). These data suggest that A3 and P2Y2 receptors are involved in the influx of neutrophils into the lungs after sepsis. Thus, pharmaceutical approaches that target these receptors might be useful to control acute lung tissue injury in sepsis.

Kinetic analysis of 99mTc-sestamibi evaluates the protective effects by ischaemic preconditioning on ischaemic myocardium in an isolated rabbit heart

OBJECTIVE

To analyse the kinetic changes of uptake, washout and retention of Tc-sestamibi in order to evaluate the protective effects and possible mechanism of ischaemic preconditioning and adenosine preconditioning on myocardium injured by ischaemia/reperfusion.

METHODS

Isolated ischaemia/reperfusion rabbit heart models, as established by Langendorff, were used. Eighteen rabbit hearts perfused in Krebs-Henseleit (KH) buffer were randomly assigned to three groups: ischaemia/reperfusion (I/R, n=6), adenosine preconditioning (AD, n=6), and ischaemic preconditioning (IPC, n=6). Tc-sestamibi (55.5 MBq) in KH was perfused for 40 min and washed out for 40 min. The kinetic changes of Tc-sestamibi within myocardial tissue was monitored during the uptake and washout phases. Cardiac haemodynamic parameters, creatine kinase and lactate dehydrogenase leakage in coronary effluent, and myocardial infarct size were measured to assess myocardial injuries in rabbit hearts.

RESULTS

In the early phases of uptake, there were no significantly different uptake rates of Tc-sestamibi between AD (before 20 min), IPC (before 15 min) and I/R myocardium (all P>0.05). Uptake rates of Tc-sestamibi in myocardium of the three groups all tended to increase, with the uptake time increasing. In the late phases of uptake, AD and IPC were significantly higher than I/R (all P<0.05). In the washout phases, the retention fractions of Tc-sestamibi in myocardium of the three groups all showed a descending tendency with washout time increasing. The retention fractions in AD and IPC were all higher than I/R (all P<0.05). There were no statistical differences in uptake rates and retention fractions of Tc-sestamibi between AD and IPC (all P>0.05). Cardiac haemodynamic parameters, creatine kinase and lactate dehydrogenase leakage, and myocardial infarct size demonstrated there is lighter injury in AD and IPC myocardium than in I/R (all P<0.05). The retention of Tc-sestamibi and myocardial infarction weight were significantly negatively correlated (r=-0.8384, P<0.001).

CONCLUSION

Adenosine preconditioning has similar myocardial protective effects on ischaemia/reperfusion myocardium as does ischaemic preconditioning. Tc-sestamibi may be a sensitive and reliable measure for evaluating the importance and mechanism of ischaemic preconditioning and adenosine preconditioning.

A3 adenosine receptor-mediated protection of the ischemic heart

The A3 adenosine receptor (A3AR) is attributed with multiple beneficial actions in ischemic-reperfused myocardium, including modulation of oncotic and apoptotic cell death and enhancement of contractile function. Additionally, the A3AR may attenuate vascular dysfunction and improve long-term outcome from myocardial insult (modulating hypertrophy and angiogenesis). Available evidence indicates that this receptor sub-type is minimally activated by endogenous adenosine during ischemia (A3AR antagonists exerting no effects on ischemic outcome), and is thus amenable to activation with exogenous agonists. Protected phenotypes arise with both pre- and post-ischemic treatment with A3AR agonists, and transient A3AR agonism also triggers early and delayed preconditioned states. The molecular basis for the varied protective actions of the A3AR remains poorly defined, and may well vary between species (e.g. rodent vs. human) and protective responses (e.g. acute vs. delayed protection). Nonetheless, A3ARs may be more promising as therapeutic "anti-ischemic" targets compared with other adenosine receptor subtypes, since A3AR agonists elicit fewer and less significant side-effects. This review addresses current knowledge and controversy regarding the protective actions (and associated signaling) of A3ARs in ischemic-reperfused heart.

Effects of Endogenous Adenosine and Adenosine Receptor Agonists on Hypoxia-Induced Myocardial Stunning in Guinea-Pig Atria and Papillary Muscles

The effects of endogenous adenosine and adenosine receptor agonists were examined on hypoxia-induced myocardial stunning of guinea-pig isolated paced left atria and papillary muscles. Hypoxia (30 minutes) reduced developed tension and increased diastolic tension (contracture) of left atria (41.8 +/- 11.5%) and papillary muscles (17.7 +/- 6.2%). Developed tension recovered to 80.8 +/- 3.15 and 77.2 +/- 5.3% 15 minutes after reoxygenation (stunning). Recovery of left atria was unaffected by adenosine deaminase (1 IU mL) but was depressed in papillary muscles (15 minutes, 48.6 +/- 4.3%) and contracture (46.1 +/- 7.5%) increased. Endogenous adenosine therefore protects from ventricular but not atrial stunning. Adenosine receptor agonists were introduced at 10 minutes into hypoxia. CPA (A1 selective, 3 x 10 M) impaired left atrial recovery (5 minutes, 38.1 +/- 5.0%), through direct negative inotropy, but did not affect papillary muscles. CGS21680 (A2A selective, 3 x 10 M) did not affect recovery. APNEA (A1/A3 receptor agonist, 10 M), increased recovery rate of left atria. Improved rate and extent of recovery of papillary muscles by APNEA (15 minutes, 94.8 +/- 3.1%) was prevented by the A3 receptor antagonist, MRS-1220 (10 M). IB-MECA (A3 selective, 3 x 10 M) increased atrial recovery rate but not the maximum developed tension reached in either tissue. However, when added at reoxygenation, IB-MECA caused complete recovery of both tissues, in the absence or presence of adenosine deaminase. Thus, A3 receptor stimulation reverses myocardial stunning of isolated atria and papillary muscles.

Novel N6-substituted adenosine 5'-N-methyluronamides with high selectivity for human adenosine A3 receptors reduce ischemic myocardial injury

We recently reported the identification of a novel human adenosine A3 receptor-selective agonist, (2S,3S,4R,5R)-3-amino-5-[6-[5-chloro-2-(3-methylisoxazol-5-ylmethoxy)benzylamino]purin-9-yl]-4-hydroxytetrahydrofuran-2-carboxylic acid methylamide (CP-608,039), with 1,260-fold selectivity for the human A3 versus human A1 receptor (DeNinno et al., J Med Chem 46: 353-355, 2003). However, because the modest (20-fold) rabbit A3 receptor selectivity of CP-608,039 precludes demonstration of A3-mediated cardioprotection in rabbit models, we identified another member of this class, (2S,3S,4R,5R)-3-amino-5-[6-(2,5-dichlorobenzylamino)purin-9-yl]-4-hydroxytetrahydrofuran-2-carboxylic acid methylamide (CP-532,903), which both retained human A3 receptor selectivity (210-fold; human A3/human A1 Ki: 23/4,800 nM) and had improved rabbit A3 receptor selectivity (90-fold; rabbit A3/rabbit A1 Ki: 23/2,000 nM). Infarct size was measured in Langendorff hearts or in vivo after 30 min of regional ischemia and 120 min of reperfusion. Five-minute perfusion with CP-532,903 before ischemia-reperfusion elicited a concentration-dependent reduction in infarct size in isolated hearts (EC50: 0.97 nM; maximum reduction in infarct size: 77%, P < 0.05 vs. control). Furthermore, administration of CP-532,903 (150 nM) at reperfusion also significantly reduced infarct size by 64% (P < 0.05 vs. control), which was not different (P > or = 0.05) from the cardioprotection provided by the same concentration of drug given before ischemia. The selective rabbit A1 receptor antagonist BWA1433 did not affect CP-532,903-dependent cardioprotection. In vivo, CP-532,903 (1 mg/kg) reduced infarct size by 50% in the absence of significant hemodynamic effects (mean arterial pressure, heart rate, rate-pressure product). CP-532,903 and CP-608,039 represent a novel class of human A3 receptor-selective agonists that may prove suitable for investigation of the clinical cardioprotective efficacy of A3 receptor activation.

Acute adenosinergic cardioprotection in ischemic-reperfused hearts

Cells of the cardiovascular system generate and release purine nucleoside adenosine in increasing quantities when constituent cells are "stressed" or subjected to injurious stimuli. This increased adenosine can interact with surface receptors in myocardial, vascular, fibroblast, and inflammatory cells to modulate cellular function and phenotype. Additionally, adenosine is rapidly reincorporated back into 5'-AMP to maintain the adenine nucleotide pool. Via these receptor-dependent and independent (metabolic) paths, adenosine can substantially modify the acute response to ischemic insult, in addition to generating a more sustained ischemia-tolerant phenotype (preconditioning). However, the molecular basis for acute adenosinergic cardioprotection remains incompletely understood and may well differ from more widely studied preconditioning. Here we review current knowledge and some controversies regarding acute cardioprotection via adenosine and adenosine receptor activation.

Protection from myocardial stunning by ischaemia and hypoxia with the adenosine A3 receptor agonist, IB-MECA

Guinea pig isolated working hearts were exposed to 30-min ischaemia by reducing coronary flow to 10%, followed by reperfusion. Aortic output fell to 4.5+/-4.5% of the pre-ischaemic value at reperfusion, recovering to 48.2+/-14.6% at 20-min post-reperfusion; the index of myocardial stunning. IB-MECA (N(6)-(3-iodobenzyl)adenosine-5'-N-methyluronamide, 3 x 10(-7) M), infused from 10 min into ischaemia, did not affect recovery of aortic output 20 min after reperfusion (41.9+/-1.9%). IB-MECA infused at reperfusion, however, significantly protected against stunning, aortic output recovering to 79.6+/-3.9% at 20-min post-reperfusion. Hypoxic gassing (5% CO(2) in nitrogen, 30 min) of guinea pig isolated paced left atria and papillary muscles reduced the developed tension, recovering to 75% 5 min after re-oxygenation. This myocardial stunning was unaffected by IB-MECA (3 x 10(-7) M) added 10 min into hypoxia. IB-MECA added at reoxygenation significantly improved recovery, which was prevented by the adenosine A(3) receptor antagonist, 1-propyl-3-(3-iodo-4-aminobenzyl)-8-(4-oxyacetate)phenylxanthine (I-ABOPX, 1 x 10(-5) M). Thus, stimulation of adenosine A(3) receptors at reperfusion/reoxygenation in guinea pig cardiac preparations protects against myocardial stunning.

Transport characteristics of HL-1 cells: a new model for the study of adenosine physiology in cardiomyocytes

Adenosine is a physiologically important nucleoside in the cardiovascular system where it can act as a cardioprotectant and modulator of energy usage. Adenosine transporters (ATs) modulate cellular adenosine levels, which, in turn, can affect a number of processes such as receptor activation and glucose uptake, but their role in cardiac physiology is poorly understood. Therefore, we have developed a new cell model by determining various adenosine-related characteristics of HL-1, an immortalized atrial cardiomyocyte murine cell line. Adenosine uptake in HL-1 cells is sodium independent, saturable, and inhibitable by nucleoside transport inhibitors (nitrobenzylthioinosine (NBTI), dipyridamole, dilazep). Reverse transcription--polymerase chain reaction analysis confirmed that HL-1 cells possess mouse equilibrative nucleoside transporters 1 and 2 (mENT1, mENT2) and kinetic analyses indicate moderate-affinity (Km = 51.3 +/- 12.9 microM), NBTI-sensitive adenosine transport. NBTI binds at a high-affinity single site (B(max) = 520 +/- 10 fmol/mg protein, Kd = 0.11 +/- 0.04 nM, 1.6 x 10(5) NBTI-binding sites/cell). HL-1 cells possess adenosine receptor, metabolic enzyme, protein kinase C isoform, and insulin-stimulated glucose transport profiles that match normal mouse heart. Therefore, HL-1 is an excellent model to study ATs within cardiomyocytes and the first model for evaluating in detail the role of the ATs in modulating effects of adenosine.

Therapeutic receptor targets of ischemic preconditioning

This review focuses on target receptors that have been shown to have the potential to mimic the cardioprotective effect of ischemic preconditioning (IPC). There is an abundance of information concerning the intracellular mechanisms and membrane-bound receptors responsible for IPC. Important intracellular mediators of this cardioprotection likely reside in the activation of multiple kinase cascades. The major players in IPC are thought to include protein kinase C, tyrosine kinases, and members of the mitogen-activated protein kinase signaling family and these topics will be covered in more detail in other papers of this focused issue. However, many of these kinase-mediated mechanisms are triggered by the activation of transmembrane spanning receptors, some of which may be manipulated therapeutically to induce cardioprotection in humans with unstable angina or who are at risk for myocardial infarction. In this review, we will discuss the evidence supporting the possibility of manipulating several of these G protein-coupled receptors as potential therapeutic targets. Stimulation of numerous receptors has been targeted as possible triggers for IPC. Some of those that have been identified include A(1) adenosine, alpha(1) adrenergic, M(2) muscarinic, B(2) bradykinin, delta(1) opioid, AT(1) angiotensin, and endothelin-1 receptors. In general, these receptors are thought to couple to inhibitory G proteins. In this review, we will focus on the most likely therapeutic candidates for cardioprotection, namely adenosine, opioid, and bradykinin receptors since selective agonists and antagonists, either alone or in combination, have most often been shown to mimic or block IPC in numerous animal models and man, respectively. This is not meant to completely rule out other receptors since it is clear that IPC is a phenomenon with multiple pathways that appear to be responsible for the cardioprotection observed.

Gene dosage-dependent effects of cardiac-specific overexpression of the A3 adenosine receptor

We used a genetic approach to determine whether increasing the level of A3 adenosine receptors (A3ARs) expressed in the heart confers protection against ischemia without causing cardiac pathology. We generated mice carrying one (A3tg.1) or six (A3tg.6) copies of a transgene consisting of the cardiomyocyte-specific alpha-myosin heavy chain gene promoter and the A3AR cDNA. A3tg.1 and A3tg.6 mice expressed 12.7+/-3.15 and 66.3+/-9.4 fmol/mg of the high-affinity G protein-coupled form of the A3AR in the myocardium, respectively. Extensive morphological, histological, and functional analyses demonstrated that there were no apparent abnormalities in A3tg.1 transgenic mice compared with nontransgenic mice. In contrast, A3tg.6 mice exhibited dilated hearts, expression of markers of hypertrophy, bradycardia, hypotension, and systolic dysfunction. When A3tg mice were subjected to 30 minutes of coronary occlusion and 24 hours of reperfusion, infarct size was reduced approximately 30% in A3tg.1 mice and approximately 40% in A3tg.6 mice compared with nontransgenic littermates. The reduction in infarct size in the transgenic mice was not related to differences in risk region size, systemic hemodynamics, or body temperature, indicating that the cardioprotection was a result of increased A3AR signaling in the ischemic myocardium. The results demonstrate that low-level expression of A3ARs in the heart provides effective protection against ischemic injury without detectable adverse effects, whereas higher levels of A3AR expression lead to the development of a dilated cardiomyopathy.

Preconditioning of rat hearts by adenosine A1 or A3 receptor activation

Our study in rat hearts examined whether activation of adenosine A(1) or A(3) receptors improved functional recovery and reduced apoptosis resulting from low-flow ischemia. Prior to 30 min low-flow ischemia (0.6 ml/min; 6% of baseline flow), Langendorff rat hearts were preconditioned with two 5-min cycles of (a) ischemia (PC; n=7), (b) infusion of 250 nM adenosine A(1) receptor agonist 2-chloro-N(6)-cyclopentyladenosine (CCPA; n=6), or (c) infusion of 50 nM adenosine A(3) receptor agonist N(6)-(3-iodobenzyl)-adenosine-5'-N-methyl-uronamide (IB-MECA; n=8). Recovery of function was improved in PC (71+/-3%), CCPA (68+/-6%) and IB-MECA (68+/-4%) groups compared to control hearts (46+/-5%; P<0.05). Cumulative release of total purines during ischemia-reperfusion was approx. 50% lower in PC, CCPA and IB-MECA groups compared to controls (P<0.05) and was significantly correlated to the percentage functional recovery (R(2)=0.55; P<0.05). The number of cytosolic histone-associated-DNA fragments, a hallmark of apoptosis and measured by Enzyme Linked ImmunoSorbent Assay (ELISA), was small and not different between groups after 30 min reperfusion. However, CCPA (0.6+/-0.1 absorbance units) and MECA (0.7+/-0.1 units; P<0.05 vs. PC) decreased apoptosis after 150 min reperfusion compared to PC (1.4+/-0.3 units) and control (1.2+/-0.1 units) hearts. This study shows that adenosine triggers protection of function in preconditioned rat hearts via both the adenosine A(1) and A(3) receptor. In clinical practice, pharmacological stimulation of adenosine A(3) receptors may be advantageous over adenosine A(1) receptor activation due to a lack of contractile side-effects. In contrast to ischemic preconditioning, pharmacological stimulation of adenosine A(1) or A(3) receptors reduced apoptosis. Furthermore, total purine release may serve as a marker of the degree of functional protection.

p53-Independent induction of Fas and apoptosis in leukemic cells by an adenosine derivative, Cl-IB-MECA

A(3) adenosine receptor (A(3)AR) agonists have been reported to influence cell death and survival. The effects of an A(3)AR agonist, 1-[2-chloro-6-[[(3-iodophenyl)methyl]amino]-9H-purin-9-yl]-1-deoxy-N-methyl-beta-D-ribofuranonamide (Cl-IB-MECA), on apoptosis in two human leukemia cell lines, HL-60 and MOLT-4, were investigated. Cl-IB-MECA (> or =30 microM) increased the apoptotic fractions, as determined using fluorescence-activated cell sorting (FACS) analysis, and activated caspase 3 and poly-ADP-ribose-polymerase. Known messengers coupled to A(3)AR (phospholipase C and intracellular calcium) did not seem to play a role in the induction of apoptosis. Neither dantrolene nor BAPTA-AM affected the Cl-IB-MECA-induced apoptosis. Cl-IB-MECA failed to activate phospholipase C in HL-60 cells, while UTP activated it through endogenous P2Y(2) receptors. Induction of apoptosis during a 48hr exposure to Cl-IB-MECA was not prevented by the A(3)AR antagonists [5-propyl-2-ethyl-4-propyl-3-(ethylsulfanylcarbonyl)-6-phenylpyridine-5-carboxylate] (MRS 1220) or N-[9-chloro-2-(2-furanyl)[1,2,4]triazolo[1,5-c]quinazolin-5-yl]benzeneacetamide (MRS 1523). Furthermore, higher concentrations of MRS 1220, which would also antagonize A(1) and A(2A) receptors, were ineffective in preventing the apoptosis. Although Cl-IB-MECA has been shown in other systems to cause apoptosis through an A(3)AR-mediated mechanism, in these cells it appeared to be an adenosine receptor-independent effect, which required prolonged incubation. In both HL-60 and MOLT-4 cells, Cl-IB-MECA induced the expression of Fas, a death receptor. This induction of Fas was not dependent upon p53, because p53 is not expressed in an active form in either HL-60 or MOLT-4 cells. Cl-IB-MECA-induced apoptosis in HL-60 cells was augmented by an agonistic Fas antibody, CH-11, and this increase was suppressed by the antagonistic anti-Fas antibody ZB-4. Therefore, Cl-IB-MECA induced apoptosis via a novel, p53-independent up-regulation of Fas.

Effect of pravastatin on myocardial protection during coronary angioplasty and the role of adenosine

Pravastatin has been shown, in an experimental model of ischemia reperfusion, to increase adenosine levels, which exert a potent and protective effect on the heart. The purpose of this study was to investigate whether pravastatin can provide cardioprotection by increased production of adenosine in patients undergoing coronary angioplasty, a clinical model of ischemia reperfusion. Thirty-five hyperlipidemic patients who underwent elective angioplasty for a major epicardial coronary artery were randomly allocated to either 3-month pravastatin or placebo before catheterization. In the placebo group, the mean ST-segment shift during the second balloon inflation was similar that observed during the first inflation, whereas in the preconditioned patients, the shift was significantly less, which is consistent with ischemic preconditioning. In the pravastatin-treated patients, the changes of ST-segment shift were similar between the first and second balloon inflations. In contrast, the patients who received aminophylline developed higher ST-segment shifts during the first and second inflations than those in the pravastatin-treated group alone. Measurements of chest pain score and myocardial lactate extraction ratios during inflation mirrored those of the ST-segment shift. The present study demonstrates that administration of pravastatin results in a significant gain in tolerance to ischemia during angioplasty. The effect of pravastatin was abolished by aminophylline, suggesting that the cardioprotective effect of pravastatin may result from activation of adenosine receptors.

Influences of adenosine on the fetus and newborn

Few signaling molecules have the potential to influence the developing mammal as the nucleoside adenosine. In contrast to most neurotransmitters, adenosine is released by all cells and is present in all tissues. The adenosinergic system is therefore not dependent on the presence of mature synaptic structures or an intact autonomic nervous system for its release. However, similar to other signaling molecules, adenosine levels are dynamically regulated and increase with increased tissue activity, hypoxia, or stress. Local adenosine concentrations thus provide a "humoral barometer" of acute changes in cellular physiology. The receptors that transduce adenosine action include A1, A2a, A2b, and A3 adenosine receptors. These receptors differ in their affinities for adenosine and in patterns of tissues expression. During development A1 adenosine receptors (A1ARs) are especially important, and A1ARs are among the earliest receptors expressed in the embryonic brain and heart. In the developing heart, the adenosinergic system is the dominant regulator of fetal cardiac function and A1AR activation inhibits cardiac cell division leading to cardiac hypoplasia. In the forming central nervous system, A1AR activation potently inhibits the development of axons and can lead to leukomalacia. These recent data suggest that adenosine is an important modulator of mammalian development.

Adenosine A3 agonist cardioprotection in isolated rat and rabbit hearts is blocked by the A1 antagonist DPCPX

Adenosine A3 agonists have been shown to protect ischemic rat and rabbit myocardium. However, these agonists have been reported to exert A3 independent effects, and no cardiac A3 receptor has yet been identified. We thus tested whether A3 agonist protection is due to A1 receptor activation. Isolated rat and rabbit hearts were subjected to 25 and 45 min of global ischemia, respectively. Rat hearts pretreated with adenosine (100 microM), the A3 agonist 2-chloro-N6-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (Cl-IB-MECA, 50 nM), and vehicle recovered 73 +/- 2%, 75 +/- 4%, and 46 +/- 4%, respectively, of preischemic left ventricular developed pressure (LVDP) after 30 min of reperfusion. The A1 antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 100 nM) blocked the beneficial effects of Cl-IB-MECA (51 +/- 5%) and adenosine (47 +/- 6%). In rabbit hearts, the beneficial effects of the A3 agonist N6-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (50 nM) and the A1 agonist 2-chloro-N6-cyclopentyladenosine (100 nM) on postischemic LVDP (75 +/- 4 and 74 +/- 5%, respectively) were blocked by DPCPX (34 +/- 4 and 36 +/- 3%, respectively). The reduction in infarct size with both agonists was also completely blocked by DPCPX. These results suggest that these A3 agonists protect ischemic myocardium via A1 receptor activation.

Right thing at a wrong time? Adenosine A3 receptors and cerebroprotection in stroke

The involvement of adenosine A3 receptors in normal and pathologic functions of the brain remains to be defined. Previous studies have shown that chronic preischemic administration of the agonist [N6-(3-iodobenzyl)-5'-N-methylcarboxoamidoadenosine or IB-MECA) results in a significant protection of neurons in selectively vulnerable brain regions and in an equally significant reduction of the subsequent mortality. Acute administration of the drug, on the other hand, resulted in a pronounced worsening of these parameters. We now report that the effect of administration of IB-MECA depends on the timing of treatment with respect to the onset of the focal insult, and provide the first data supporting speculation that treatment with adenosine A3 receptor agonists may decrease the infarct size following focal brain ischemia. Treatment with IB-MECA administered 20 min prior to transient middle cerebral ischemia (MCAOt = 30 min) resulted in a significant increase of the infarct size (p < 0.01), whereas administration 20 min after ischemia resulted in statistically significant decrease of the infarct volume. Postischemic treatment results in improved neuronal preservation, decreased intensity of reactive gliosis, and pronounced reduction of microglial infiltration. The data indicate that the effects of adenosine A3 receptor stimulation depend on the differential impact of these receptors on both neuronal and non-neuronal elements of the cerebral tissue, for example, astrocytes, microglia, and vasculature.

Targeted deletion of the A3 adenosine receptor confers resistance to myocardial ischemic injury and does not prevent early preconditioning

We used mice with genetic disruption of the A3 adenosine receptor (AR) gene (A3AR(-/-)mice) to assess the in vivo role of the A3AR in modulating myocardial ischemia/reperfusion injury and preconditioning (PC). Surprisingly, infarct size induced by 30 min of coronary artery occlusion and 24 h of reperfusion was 35% smaller in A3AR(-/-)compared to wild-type mice (A3AR(+/+)). The reduction in infarct size was not the result of differences in heart rate, body temperature or increased cardiac expression of A1ARs. However, neutrophil infiltration within infarcted regions was less in A3AR(-/-)mice. Furthermore, ischemic PC induced by either a single episode (one 5 min occlusion) or multiple episodes (six 4 min occlusions) of ischemia produced equivalent reductions in infarct size in A3AR(-/-)and A3AR(+/+)mice. These results indicate that, in the mouse, (i) A3ARs play an injurious role during acute myocardial ischemia/reperfusion injury, possibly by exacerbating the inflammatory response, and (ii) A3ARs are not necessary for the development of the early phase of ischemic PC.

Protection of cardiac myocytes via delta(1)-opioid receptors, protein kinase C, and mitochondrial K(ATP) channels

The objective of the present study was to investigate the role of delta(1)-opioid receptors in mediating cardioprotection in isolated chick cardiac myocytes and to investigate whether protein kinase C and mitochondrial ATP-sensitive K(+) (K(ATP)) channels act downstream of the delta(1)-opioid receptor in mediating this beneficial effect. A 5-min preexposure to the selective delta(1)-opioid receptor agonist (-)-TAN-67 (1 microM) resulted in less myocyte injury during the subsequent prolonged ischemia compared with untreated myocytes. 7-Benzylidenenaltrexone, a selective delta(1)-opioid receptor antagonist, completely blocked the cardioprotective effect of (-)-TAN-67. Naltriben methanesulfonate, a selective delta(2)-opioid receptor antagonist, had only a slight inhibitory effect on (-)-TAN-67-mediated cardioprotection. Nor-binaltorphimine dihydrochloride, a kappa-opioid receptor antagonist, did not affect (-)-TAN-67-mediated cardioprotection. The protein kinase C inhibitor chelerythrine and the K(ATP) channel inhibitors glibenclamide, a nonselective K(ATP) antagonist, and 5-hydroxydecanoic acid, a mitochondrial selective K(ATP) antagonist, reversed the cardioprotective effect of (-)-TAN-67. These results suggest that the delta(1)-opioid receptor is present on cardiac myocytes and mediates a potent cardioprotective effect via protein kinase C and the mitochondrial K(ATP) channel.

Pharmacological modulation, preclinical studies, and new clinical features of myocardial ischemic preconditioning

The term "ischemic preconditioning (PC)" was first applied to canine myocardium subjected to brief episodes of ischemia and reperfusion that tolerated a more prolonged episode of ischemia better than myocardium not previously exposed to ischemia. Protective effect of myocardial ischemic PC was demonstrated in several animal species, resulting in the strongest endogenous form of protection against myocardial injury, jeopardized myocardium, infarct size, and arrhythmias other than early reperfusion. New onset angina before acute myocardial infarction, episodes of myocardial ischemia during coronary angioplasty or bypass surgery, and the "warm-up" phenomenon may represent clinical counterparts of the PC phenomenon in humans. Here, we have attempted to summarize pharmacological modulation, preclinical studies, and new clinical features of ischemic PC. To date, the pathophysiological basis of the "chemical PC" is still not well established, and "putting PC in a bottle" for clinical applications still remains a new pharmacological venture.

Aldose reductase inhibition alone or combined with an adenosine A(3) agonist reduces ischemic myocardial injury

This study investigated whether aldose reductase (AR) inhibition with zopolrestat, either alone or in combination with an adenosine A(3)-receptor agonist (CB-MECA), reduced myocardial ischemic injury in rabbit hearts subjected to 30 min of regional ischemia and 120 min of reperfusion. Zopolrestat reduced infarct size by up to 61%, both in vitro (2 nM to 1 microM; EC(50) = 24 nM) and in vivo (50 mg/kg). Zopolrestat reduced myocardial sorbitol concentration (index of AR activity) by >50% (control, 15.0 +/- 2.2 nmol/g; 200 nM zopolrestat, 6.7 +/- 1.3 nmol/g). A modestly cardioprotective concentration of CB-MECA (0.2 nM) allowed a 50-fold reduction in zopolrestat concentration while providing a similar reduction in infarct size (infarct area/area at risk: control, 62 +/- 2%; 1 microM zopolrestat, 24 +/- 5%; 20 nM zopolrestat plus 0.2 nM CB-MECA, 20 +/- 4%). In conclusion, AR inhibition is cardioprotective both in vitro and in vivo. Furthermore, combining zopolrestat with an A(3) agonist allows a reduction in the zopolrestat concentration while maintaining an equivalent degree of cardioprotection.

The role of adenosine in preconditioning

Preconditioning is a powerful form of (myocardial) protection that follows brief sublethal ischemia. G-protein-coupled receptors constitute the trigger for entrance to the preconditioned state. In conjunction with other receptors, various membrane adenosine receptors play an important role in the transduction of extracellular signals, leading to protection by preconditioning, lasting 1-3 hr. Adenosine A(1)- and A(3)-receptors mediate inhibition of adenylate cyclase via a guanine nucleotide binding inhibitory protein (G(i/o)). A(2)-receptors couple to a comparable stimulatory protein (G(s)). Adenosine receptors are especially abundant in the central nervous system; in lesser numbers, they are found in many tissues, including the heart. A(1)-receptors are located on cardiomyocytes and vascular smooth muscle cells, A(2)-receptors on endothelial and vascular smooth muscle cells, and A(3)-receptors on ventricular myocytes. Ischemic preconditioning by endogenous adenosine takes place through A(1)- and A(3)-receptors. A(2A/B)-receptor activation results in vasodilation. The relevance of cellular mediators, such as 5'-nucleotidase, to generate adenosine for preconditioning is controversial. In contrast, the role of protein kinase C (PKC) is clearly established. Signals from different receptors converge at PKC, reaching a threshold activation of the kinase necessary to induce protection. Tyrosine and mitogen-activated protein kinases may play a role in addition to PKC. The exact products downstream responsible for the memory of preconditioning are elusive. A prime candidate for the end-effector of preconditioning is the K(ATP) channel. Preconditioning with adenosine-receptor agonists offers the possibility for treatment of coronary artery disease, but research in this field is still in its infancy.

"Preconditioning at a distance" in the isolated rabbit heart

OBJECTIVE

Brief myocardial ischemia evokes a cardioprotective response, referred to as "ischemic preconditioning" (IP), that limits injury caused by a subsequent prolonged ischemic insult. The myocardial IP effect can be induced by ischemia of "distant" cardiac and noncardiac tissue, implicating the involvement of an as-yet-unidentified humoral trigger. If a preconditioning hormone exists, the authors hypothesize that the IP effect should be transferable, via administration of coronary effluent, from a preconditioned donor heart to a virgin non-preconditioned acceptor heart.

METHODS

Isolated buffer-perfused rabbit hearts were assigned to one of four treatment groups in a donor/acceptor sequence. Donor hearts underwent either three IP cycles or a matched period of uninterrupted perfusion (control donors). Coronary perfusate collected from IP and control donor hearts was reoxygenated and transfused to virgin acceptor hearts. All hearts then underwent 30 minutes of global ischemia followed by 30 minutes of reperfusion. Left ventricular developed pressure (LVDP) (the authors' index of cardioprotection) was monitored throughout the protocol by a left ventricular (LV) balloon.

RESULTS

In donor controls, LVDP assessed at 30 minutes post-reflow was restored to only 49 +/- 5% of baseline values. Recovery of LV function was significantly enhanced in both IP donor hearts (69 +/- 4%*) and IP acceptor hearts (70 +/- 6%*) vs donor controls (*p < 0.05), while, in acceptor controls, intermediate values of LVDP (62 +/- 7%) were obtained.

CONCLUSION

The IP effect can be transferred between rabbit hearts, suggesting the presence of a humoral trigger signal for distant preconditioning. Isolating this hormone may have therapeutic and diagnostic implications in the management of acute myocardial ischemia.

Stimulation of adenosine A3 receptors in cerebral ischemia. Neuronal death, recovery, or both?

The role of the adenosine A3 receptor continues to baffle, and, despite an increasing number of studies, the currently available data add to, rather than alleviate, the existing confusion. The reported effects of adenosine A3 receptor stimulation appear to depend on the pattern of drug administration (acute vs. chronic), dose, and type of the target tissue. Thus, while acute exposure to A3 receptor agonists protects against myocardial ischemia, it is severely damaging when these agents are given shortly prior to cerebral ischemia. Mast cells degranulate when their A3 receptors are stimulated. Degranulation of neutrophils is, on the other hand, impaired. While reduced production of reactive nitrogen species has been reported following activation of A3 receptors in collagen-induced arthritis, the process appears to be enhanced in cerebral ischemia. Indeed, immunocytochemical studies indicate that both pre- and postischemic treatment with A3 receptor antagonist dramatically reduces nitric oxide synthase in the affected hippocampus. Even more surprisingly, low doses of A3 receptor agonists seem to enhance astrocyte proliferation, while high doses induce their apoptosis. This review concentrates on the studies of cerebral A3 receptors and, based on the available evidence, discusses the possibility of adenosine A3 receptor serving as an integral element of the endogenous cerebral neuroprotective complex consisting of adenosine and its receptors.

Crucial role of endogenous interleukin-10 production in myocardial ischemia/reperfusion injury

BACKGROUND

The anti-inflammatory cytokine interleukin-10 (IL-10) has been detected in the plasma of patients with myocardial ischemia/reperfusion. The aim of our study was to investigate the role of endogenously produced IL-10 in myocardial ischemia/reperfusion.

METHODS AND RESULTS

In the present study, we used wild-type and IL-10-deficient mice subjected to myocardial ischemia/reperfusion. Significant levels of IL-10 were produced in wild-type mice at 2 to 6 hours after myocardial reperfusion. The genetic deletion of IL-10 enhanced neutrophil infiltration into the reperfused tissues at 6 hours after reperfusion and increased infarct size and myocardial necrosis. Furthermore, in the absence of IL-10, an enhancement of the inflammatory response was seen, as demonstrated by increased plasma levels of tumor necrosis factor-alpha, nitrite/nitrate (breakdown products of NO), and increased tissue expression of intercellular adhesion molecule-1. Reperfusion for 24 hours was associated with a 75% mortality rate in IL-10-deficient mice, whereas no deaths occurred in the wild-type animals.

CONCLUSIONS

The present findings provide the first direct evidence that endogenous IL-10 inhibits the production of tumor necrosis factor-alpha and NO and serves to protect the ischemic and reperfused myocardium through the suppression of neutrophil recruitment.

Adenosine receptors as potential therapeutic targets

Recent studies indicate a widening role for adenosine receptors in many therapeutic areas. Adenosine receptors are involved in immunological and inflammatory responses, respiratory regulation, the cardiovascular system, the kidney, various CNS-mediated events including sleep and neuroprotection, as well as central and peripheral pain processes. In this review, the physiological role of adenosine receptors in these key areas is described with reference to the therapeutic potential of adenosine receptor agonists and antagonists.

Rabbit heart can be "preconditioned" via transfer of coronary effluent

Brief myocardial ischemia not only evokes a local cardioprotective or "preconditioning" effect but also can render remote myocardium resistant to sustained ischemia. We propose the following hypotheses: remote protection is initiated by a humoral trigger; brief ischemia-reperfusion will result in release of the humoral trigger (possibly adenosine and/or norepinephrine) into the coronary effluent; and transfer of this effluent to a virgin acceptor heart will elicit cardioprotection. To test these concepts, effluent was collected during normal perfusion from donor-control hearts and during preconditioning ischemia-reperfusion from donor-preconditioned (PC) hearts. After reoxygenation occurred and aliquots for measurement of adenosine and norepinephrine content were harvested, effluent was transfused to acceptor-control and acceptor-PC hearts. All hearts then underwent 40 min of global ischemia and 60 min of reperfusion, and infarct size was delineated by tetrazolium staining. Mean infarct size was smaller in both donor- and acceptor-PC groups (9% of left ventricle) than in donor- and acceptor-control groups (36% and 34%; P < 0.01). Protection in acceptor-PC hearts could not, however, be attributed to adenosine or norepinephrine. Thus preconditioning-induced cardioprotection can be transferred between rabbit hearts by transfusion of coronary effluent. Although adenosine and norepinephrine are apparently not responsible, these results suggest that remote protection is initiated by a humoral mechanism.

A(3) adenosine receptor activation attenuates neutrophil function and neutrophil-mediated reperfusion injury

This study tested the hypothesis that A(3) adenosine receptors inhibit neutrophil (PMN) function and PMN-mediated reperfusion injury. 2-Chloro-N(6)-(3-iodobenzyl)adenosine-5'-N-methyluronamide (Cl-IB-MECA), an A(3) agonist, did not attenuate superoxide production or myeloperoxidase release from stimulated PMNs. However, Cl-IB-MECA reduced platelet-activating factor-stimulated PMN adherence to coronary endothelium at low concentrations: 52 +/- 27, 45 +/- 10, and 87 +/- 23 PMNs/mm(2) at 0.1, 1.0, and 10 nM vs. 422 +/- 64 PMNs/mm(2) with platelet-activating factor alone. This inhibition was not blocked by A(1) (5 microM KW-3902) or A(2a) (5 microM KF-21326) antagonists: 44 +/- 3 and 43 +/- 2 PMNs/mm(2), respectively. Endothelial pretreatment with 1 nM Cl-IB-MECA reduced PMN adherence, which was reversed by the A(3) antagonist MRS-1220 (100 nM). PMN-mediated reperfusion injury was initiated in isolated rabbit hearts by infusion of 28 x 10(6) PMNs/min for 10 min early in reperfusion. PMNs caused a significant decrease in recovery of left ventricular developed pressure and positive and negative time derivatives of pressure (23 +/- 3, 25 +/- 3, and 23 +/- 3% of baseline, respectively) vs. buffer-perfused hearts (43 +/- 7, 44 +/- 7, and 45 +/- 6%, respectively). Cl-IB-MECA (10 nM) given at reperfusion attenuated the PMN-mediated loss of contractile recovery (40 +/- 3, 46 +/- 5, and 42 +/- 4% of baseline). Cl-IB-MECA reduced myeloperoxidase release activity (5.3 +/- 0.6 absorbance units/min) and CD18-positive cells (54 +/- 9 cells/slide) compared with the untreated PMN group (17.9 +/- 1.7 absorbance units/min and 183 +/- 68 cells/slide). We conclude that Cl-IB-MECA attenuates reperfusion injury by decreasing PMN-endothelial cell interactions. These results suggest that the A(3) adenosine receptor may be a novel therapeutic target for treatment of myocardial ischemia and reperfusion.

Species-dependent hemodynamic effects of adenosine A3-receptor agonists IB-MECA and Cl-IB-MECA

The purpose of this study was to compare the hemodynamic effects of the adenosine A3-receptor agonists N6-(3-iodobenzyl)-9-[5-(methylcarbamoyl)-beta-D-ribofuranosyl]aden ine (IB-MECA) and 2-chloro-N6-(3-iodobenzyl)-9-[5-(methylcarbamoyl)-beta-D-ribofu ranosy l]adenine (Cl-IB-MECA) in isolated rat and rabbit hearts and in the intact, open-chest pig. Isolated hearts perfused with Krebs-Henseleit buffer at a constant pressure (70 mmHg) were treated with 50 nM of either IB-MECA or Cl-IB-MECA. Neither IB-MECA nor Cl-IB-MECA altered ventricular function or heart rate in the isolated rat and rabbit hearts, and neither agent altered coronary flow in the rabbit. However, 2 min of IB-MECA treatment in the isolated rat heart increased coronary flow by 25%, an effect that did not exhibit tachyphylaxis. The IB-MECA-induced coronary dilation was only partially attenuated by the adenosine A3-receptor antagonist MRS-1191 (50 nM). IB-MECA-induced coronary dilation was completely blocked by the adenosine A2a-receptor antagonist 7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-[4,3-e]-1,2, 4-triazolo[1,5-c]pyrimidine (Sch-58261, 50 nM). Cl-IB-MECA (50 nM) did not increase coronary flow in the rat, but 100 nM did increase flow by 18%. In pentobarbital sodium-anesthetized pigs IB-MECA (5 micrograms/kg iv) decreased systemic blood pressure and increased pulmonary artery pressure, effects that did exhibit tachyphylaxis. These results illustrate that adenosine A3-receptor agonists produce species-dependent effects, which in the rat heart appear to be caused by adenosine A2a-receptor activation.

Adenosine and cerebral ischemia: therapeutic future or death of a brave concept?

Numerous studies have consistently shown that agonist stimulation of adenosine A1 receptors results in a significant reduction of morbidity and mortality associated with global and focal brain ischemia in animals. Based on these observations, several authors have suggested utilization of adenosine A1 receptors as targets for the development of clinically viable drugs against ischemic brain disorders. Recent advent of adenosine A1 receptor agonists characterized by lowered cardiovascular effects added additional strength to this argument. On the other hand, although cardioprotective, adenosine A3 receptor agonists proved severely cerebrodestructive when administered prior to global ischemia in gerbils. Moreover, stimulation of adenosine A3 receptors appears to reduce the efficacy of some of the neuroprotective actions mediated by adenosine A1 receptors. The review discusses the possible role of adenosine receptor subtypes (A1, A2, and A3) in the context of their involvement in the pathology of cerebral ischemia, and analyzes putative strategies for the development of clinically useful strategies based on adenosine and its receptors. It also stresses the need for further experimental studies before definitive conclusions on the usefulness of the adenosine concept in the treatment of brain ischemia can be made.

Adenosine and cerebral ischemia: therapeutic future or death of a brave concept?

Numerous studies have consistently shown that agonist stimulation of adenosine A1 receptors results in a significant reduction of morbidity and mortality associated with global and focal brain ischemia in animals. Based on these observations, several authors have suggested utilization of adenosine A1 receptors as targets for the development of clinically viable drugs against ischemic brain disorders. Recent advent of adenosine A1 receptor agonists characterized by lowered cardiovascular effects added additional strength to this argument. On the other hand, although cardioprotective, adenosine A3 receptor agonists proved severely cerebrodestructive when administered prior to global ischemia in gerbils. Moreover, stimulation of adenosine A3 receptors appears to reduce the efficacy of some of the neuroprotective actions mediated by adenosine A receptors. The review discusses the possible role of adenosine receptor subtypes (A1, A2, and A3) in the context of their involvement in the pathology of cerebral ischemia, and analyzes putative strategies for the development of clinically useful strategies based on adenosine and its receptors. It also stresses the need for further experimental studies before definitive conclusions on the usefulness of the adenosine concept in the treatment of brain ischemia can be made.