Targeted deletion of A(3) adenosine receptors improves tolerance to ischemia-reperfusion injury in mouse myocardium

Adenosine A3 Receptor: From Molecular Signaling to Therapeutic Strategies for Heart Diseases

Cardiovascular diseases (CVDs), particularly heart failure, are major contributors to early mortality globally. Heart failure poses a significant public health problem, with persistently poor long-term outcomes and an overall unsatisfactory prognosis for patients. Conventionally, treatments for heart failure have focused on lowering blood pressure; however, the development of more potent therapies targeting hemodynamic parameters presents challenges, including tolerability and safety risks, which could potentially restrict their clinical effectiveness. Adenosine has emerged as a key mediator in CVDs, acting as a retaliatory metabolite produced during cellular stress via ATP metabolism, and works as a signaling molecule regulating various physiological processes. Adenosine functions by interacting with different adenosine receptor (AR) subtypes expressed in cardiac cells, including A1AR, A2AAR, A2BAR, and A3AR. In addition to A1AR, A3AR has a multifaceted role in the cardiovascular system, since its activation contributes to reducing the damage to the heart in various pathological states, particularly ischemic heart disease, heart failure, and hypertension, although its role is not as well documented compared to other AR subtypes. Research on A3AR signaling has focused on identifying the intricate molecular mechanisms involved in CVDs through various pathways, including Gi or Gq protein-dependent signaling, ATP-sensitive potassium channels, MAPKs, and G protein-independent signaling. Several A3AR-specific agonists, such as piclidenoson and namodenoson, exert cardioprotective impacts during ischemia in the diverse animal models of heart disease. Thus, modulating A3ARs serves as a potential therapeutic approach, fueling considerable interest in developing compounds that target A3ARs as potential treatments for heart diseases.

Purinergic Signaling and its Role in the Stem Cell Differentiation

Purinergic signaling is a mechanism in which extracellular purines and pyrimidines interact with specialized cell surface receptors known as purinergic receptors. These receptors are divided into two families of P1 and P2 receptors, each responding to different nucleosides and nucleotides. P1 receptors are activated by adenosine, while P2 receptors are activated by pyrimidine and purines. P2X receptors are ligand-gated ion channels, including seven subunits (P2X1-7). However, P2Y receptors are the G-protein coupled receptors comprising eight subtypes (P2Y1/2/4/6/11/12/13/14). The disorder in purinergic signaling leads to various health-related issues and diseases. In various aspects, it influences the activity of non-neuronal cells and neurons. The molecular mechanism of purinergic signaling provides insight into treating various human diseases. On the contrary, stem cells have been investigated for therapeutic applications. Purinergic signaling has shown promising effect in stem cell engraftment. The immune system promotes the autocrine and paracrine mechanisms and releases the significant factors essential for successful stem cell therapy. Each subtype of purinergic receptor exerts a beneficial effect on the damaged tissue. The most common effect caused by purinergic signaling is the proliferation and differentiation that treat different health-related conditions.

Purinergic signaling in myocardial ischemia-reperfusion injury

Purines and their derivatives, extensively distributed in the body, act as a class of extracellular signaling molecules via a rich array of receptors, also known as purinoceptors (P1, P2X, and P2Y). They mediate multiple intracellular signal transduction pathways and participate in various physiological and pathological cell behaviors. Since the function in myocardial ischemia-reperfusion injury (MIRI), this review summarized the involvement of purinergic signal transduction in diversified pathological processes, including energy metabolism disorder, oxidative stress injury, calcium overload, inflammatory immune response, platelet aggregation, coronary vascular dysfunction, and cell necrosis and apoptosis. Moreover, increasing evidence suggests that purinergic signaling also mediates the prevention and treatment of MIRI, such as ischemic conditioning, pharmacological intervention, and some other therapies. In conclusion, this review exhibited that purinergic signaling mediates the complex processes of MIRI which shows its promising application and prospecting in the future.

Plasmacytoid Dendritic Cells Mediate Myocardial Ischemia/Reperfusion Injury by Secreting Type I Interferons

Background

We previously demonstrated that ischemically injured cardiomyocytes release cell‐free DNA and HMGB1 (high mobility group box 1 protein) into circulation during reperfusion, activating proinflammatory responses and ultimately exacerbating reperfusion injury. We hypothesize that cell‐free DNA and HMGB1 mediate myocardial ischemia‐reperfusion injury by stimulating plasmacytoid dendritic cells (pDCs) to secrete type I interferon (IFN‐I).

Methods and Results

C57BL/6 and interferon alpha receptor‐1 knockout mice underwent 40 minutes of left coronary artery occlusion followed by 60 minutes of reperfusion (40′/60′ IR) before infarct size was evaluated by 2,3,5‐Triphenyltetrazolium chloride–Blue staining. Cardiac perfusate was acquired in ischemic hearts without reperfusion by antegrade perfusion of the isolated heart. Flow cytometry in pDC‐depleted mice treated with multiple doses of plasmacytoid dendritic cell antigen‐1 antibody via intraperitoneal injection demonstrated plasmacytoid dendritic cell antigen‐1 antibody treatment had no effect on conventional splenic dendritic cells but significantly reduced splenic pDCs by 60%. pDC‐depleted mice had significantly smaller infarct size and decreased plasma interferon‐α and interferon‐β compared with control. Blockade of the type I interferon signaling pathway with cyclic GMP‐AMP synthase inhibitor, stimulator of interferon genes antibody, or interferon regulatory factor 3 antibody upon reperfusion similarly significantly attenuated infarct size by 45%. Plasma levels of interferon‐α and interferon‐β were significantly reduced in cyclic GMP‐AMP synthase inhibitor‐treated mice. Infarct size was significantly reduced by >30% in type I interferon receptor monoclonal antibody–treated mice and interferon alpha receptor‐1 knockout mice. In splenocyte culture, 40′/0′ cardiac perfusate treatment stimulated interferon‐α and interferon‐β production; however, this effect disappeared in the presence of cyclic GMP‐AMP synthase inhibitor.

Conclusions

Type I interferon production is stimulated following myocardial ischemia by cardiogenic cell‐free DNA/HMGB1 in a pDC‐dependent manner, and subsequently activates type I interferon receptors to exacerbate reperfusion injury. These results identify new potential therapeutic targets to attenuate myocardial ischemia‐reperfusion injury.

Knockout and Knock-in Mouse Models to Study Purinergic Signaling

Purinergic signaling involves extracellular purines and pyrimidines acting upon specific cell surface purinoceptors classified into the P1, P2X, and P2Y families for nucleosides and nucleotides. This widespread signaling mechanism is active in all major tissues and influences a range of functions in health and disease. Orthologs to all but one of the human purinoceptors have been found in mouse, making this laboratory animal a useful model to study their function. Indeed, analyses of purinoceptors via knock-in or knockout approaches to produce gain or loss of function phenotypes have revealed several important therapeutic targets. None of the homozygous purinoceptor knockouts proved to be developmentally lethal, which suggest that either these receptors are not involved in key developmental processes or that the large number of receptors in each family allowed for functional compensation. Different models for the same purinoceptor often show compatible phenotypes but there have been examples of significant discrepancies. These revealed unexpected differences in the structure of human and mouse genes and emphasized the importance of the genetic background of different mouse strains. In this chapter, we provide an overview of the current knowledge and new trends in the modifications of purinoceptor genes in vivo. We discuss the resulting phenotypes, their applications and relative merits and limitations of mouse models available to study purinoceptor subtypes.

Ability of CP-532,903 to protect mouse hearts from ischemia/reperfusion injury is dependent on expression of A3 adenosine receptors in cardiomyoyctes

A₃ adenosine receptor (A₃AR) agonists are effective at limiting injury caused by ischemia/reperfusion injury of the heart in experimental animal models. However, understanding of their mechanism of action, which is likely multifactorial, remains incomplete. In prior studies, it has been demonstrated that A₃AR-mediated ischemic protection is blocked by glibenclamide and is absent in Kir6.2 gene ablated mice that lack the pore-forming subunit of the ATP-sensitive potassium (K_ATP) channel, suggesting one contributing mechanism may involve accelerated activation of K_ATP channels. However, presence of A₃ARs in the myocardium has yet to be established. Utilizing a whole-cell recording technique, in this study we confirm functional expression of the A₃AR in adult mouse ventricular cardiomyocytes, coupled to activation of ATP-dependent potassium (K_ATP) channels via G_i inhibitory proteins. We further show that ischemic protection provided by the selective A₃AR agonist CP-532,903 in an isolated, buffer-perfused heart model is lost completely in Adora3^{LoxP/LoxP;Myh6-Cre} mice, which is a newly developed model developed and comprehensively described herein whereby the A₃AR gene (Adora3) is deleted exclusively in cardiomyocytes. Our findings, taken together with previously published work, are consistent with the hypothesis that A₃AR agonists provide ischemic tolerance, at least in part, by facilitating opening of myocardial K_ATP channels.

In vivo assessment of coronary flow and cardiac function after bolus adenosine injection in adenosine receptor knockout mice

Bolus injections of adenosine and the A2A adenosine receptor (AR) selective agonist (regadenoson) are used clinically as a substitute for a stress test in people who cannot exercise. Using isolated tissue preparations, our lab has shown that coronary flow and cardiac effects of adenosine are mostly regulated by the AR subtypes A1, A2A, and A2B In this study, we used ultrasound imaging to measure the in vivo effects of adenosine on coronary blood flow (left coronary artery) and cardiac function in anesthetized wild-type, A1 knockout (KO), A2AKO, A2BKO, A3KO, A1, and A3 double KO (A1/3 DKO) and A2A and A2B double KO (A2A/2B DKO) mice in real time. Echocardiographic and Doppler studies were performed using a Visualsonic Vevo 2100 ultrasound system. Coronary blood flow (CBF) baseline data were obtained when animals were anesthetized with 1% isoflourane. Diameter (D) and velocity time integral (VTI) were measured on the left coronary arteries (CBF = ((π/4) × D(2) × VTI × HR)/1000). CBF changes were the highest within 2 min of injection (about 10 mg/kg). Heart rate, cardiac output, and stroke volume were measured by tracing the left ventricle long axis. Our data support a role for the A2 AR in CBF and further support our conclusions of previous studies from isolated tissues. Adenosine-mediated decreases in cardiac output and stroke volume may be A2B and/or A3 AR-mediated; however, the A1 and A2 ARs also play roles in overall cardiac function. These data further provide a powerful translational tool in studying the cardiovascular effects of adenosine in disease states.

Adenosine A3 Receptor: A promising therapeutic target in cardiovascular disease

Cardiovascular complications are one of the major factors for early mortality in the present worldwide scenario and have become a major challenge in both developing and developed nations. It has thus become of immense importance to look for different therapeutic possibilities and treatments for the growing burden of cardiovascular diseases. Recent advancements in research have opened various means for better understanding of the complication and treatment of the disease. Adenosine receptors have become tool of choice in understanding the signaling mechanism which might lead to the cardiovascular complications. Adenosine A3 receptor is one of the important receptor which is extensively studied as a therapeutic target in cardiovascular disorder. Recent studies have shown that A₃AR is involved in the amelioration of cardiovascular complications by altering the expression of A₃AR. This review focuses towards the therapeutic potential of A₃AR involved in cardiovascular disease and it might help in better understanding of mechanism by which this receptor may prove useful in improving the complications arising due to various cardiovascular diseases. Understanding of A₃AR signaling may also help to develop newer agonists and antagonists which might be prove helpful in the treatment of cardiovascular disorder.

The infarct-sparing effect of IB-MECA against myocardial ischemia/reperfusion injury in mice is mediated by sequential activation of adenosine A3 and A 2A receptors

Conflicting results exist regarding the role of A3 adenosine receptors (A3ARs) in mediating cardioprotection during reperfusion following myocardial infarction. We hypothesized that the effects of the A3AR agonist IB-MECA to produce cardioprotection might involve activation of other adenosine receptor subtypes. C57Bl/6 (B6), A3AR KO, A2AAR KO, and A2AAR KO/WT bone marrow chimeric mice were assigned to 12 groups undergoing either hemodynamic studies or 45 min of LAD occlusion and 60 min of reperfusion. IB-MECA (100 μg/kg) or vehicle was administered by iv bolus 5 min before reperfusion. Radioligand binding assays showed that IB-MECA has high affinity for the mouse A3AR (K i = 0.17 ± 0.05 nM), but also can bind with lower affinity to the A1AR (9.0 ± 2.4 nM) or the A2AAR (56.5 ± 10.2 nM). IB-MECA caused bi-phasic hemodynamic changes, which were completely absent in A3AR KO mice and were modified by A2AAR blockade or deletion. IB-MECA stimulated histamine release, increased heart rate, and significantly reduced IF size in B6 mice from 61.5 ± 1.4 to 48.6 ± 2.4% of risk region (RR; 21% reduction, p < 0.05) but not in A3AR KO mice. Compared to B6, A3AR KO mice had significantly reduced IF size (p < 0.05). In B6/B6 bone marrow chimeras, IB-MECA caused a 47% reduction of IF size (from 47.3 ± 3.9 to 24.7 ± 4.5, p < 0.05). However, no significant cardioprotective effect of IB-MECA was observed in A2AARKO/B6 mice, which lacked A2AARs only on their bone marrow-derived cells. Activation of A3ARs induces a bi-phasic hemodynamic response, which is partially mediated by activation of A2AARs. The cardioprotective effect of IB-MECA is due to the initial activation of A3AR followed by activation of A2AARs in bone marrow-derived cells.

Pharmacological postconditioning of the rabbit heart with non-selective, A1, A2A and A3 adenosine receptor agonists

Objectives

We investigated the effects of novel selective and non-selective adenosine receptor agonists (ARs) on cardioprotection.

Methods

Male rabbits divided into six groups were subjected to 30-min heart ischaemia and 3-h reperfusion: (1) control group, (2) postconditioning (PostC) group, (3) group A: treated with the non-selective agonist (S)-PHPNECA, (4) group B: treated with the A1 agonist CCPA, (5) group C: treated with the A2A agonist VT 7 and (6) group D: treated with the A3 agonist AR 170. The infarcted (I) and the areas at risk (R) were estimated as %I/R. In additional rabbits of all groups, heart samples were taken for determination of Akt, eNOS and STAT 3 at the 10th reperfusion minute.

Key findings

(S)-PHPNECA and CCPA reduced the infarct size (17.2 ± 2.9% and 17.9 ± 2.0% vs 46.8 ± 1.9% in control, P &lt; 0.05), conferring a benefit similar to PostC (26.4 ± 0.3%). Selective A2A and A3 receptor agonists did not reduce the infarct size (39.5 ± 0.8% and 38.7 ± 3.5%, P = NS vs control). Akt, eNOS and STAT 3 were significantly activated after non-selective A1 ARs and PostC.

Conclusions

Non-selective and A1 but not A2A and A3 ARs agonists are essential for triggering cardioprotection. The molecular mechanism involves both RISK and the JAK/STAT pathways.

Adora2b signaling on bone marrow derived cells dampens myocardial ischemia-reperfusion injury

BACKGROUND

Cardiac ischemia-reperfusion (I-R) injury represents a major cause of cardiac tissue injury. Adenosine signaling dampens inflammation during cardiac I-R. The authors investigated the role of the adenosine A2b-receptor (Adora2b) on inflammatory cells during cardiac I-R.

METHODS

To study Adora2b signaling on inflammatory cells, the authors transplanted wild-type (WT) bone marrow (BM) into Adora2b(-/-) mice or Adora2b(-/-) BM into WT mice. To study the role of polymorphonuclear leukocytes (PMNs), neutrophil-depleted WT mice were treated with an Adora2b agonist. After treatments, mice were exposed to 60 min of myocardial ischemia and 120 min of reperfusion. Infarct sizes and troponin I concentrations were determined by triphenyltetrazolium chloride staining and enzyme-linked immunosorbent assay, respectively.

RESULTS

Transplantation of WT BM into Adora2b(-/-) mice decreased infarct sizes by 19 ± 4% and troponin I by 87.5 ± 25.3 ng/ml (mean ± SD, n = 6). Transplantation of Adora2b(-/-) BM into WT mice increased infarct sizes by 20 ± 3% and troponin I concentrations by 69.7 ± 17.9 ng/ml (mean ± SD, n = 6). Studies on the reperfused myocardium revealed PMNs as the dominant cell type. PMN depletion or Adora2b agonist treatment reduced infarct sizes by 30 ± 11% or 26 ± 13% (mean ± SD, n = 4); however, the combination of both did not produce additional cardioprotection. Cytokine profiling showed significantly higher cardiac tumor necrosis factor α concentrations in Adora2b(-/-) compared with WT mice (39.3 ± 5.3 vs. 7.5 ± 1.0 pg/mg protein, mean ± SD, n = 4). Pharmacologic studies on human-activated PMNs revealed an Adora2b-dependent tumor necrosis factor α release.

CONCLUSION

Adora2b signaling on BM-derived cells such as PMNs represents an endogenous cardioprotective mechanism during cardiac I-R. The authors' findings suggest that Adora2b agonist treatment during cardiac I-R reduces tumor necrosis factor α release of PMNs, thereby dampening tissue injury.

Targeting adenosine receptors in the development of cardiovascular therapeutics

Adenosine receptor stimulation has negative inotropic and dromotropic actions, reduces cardiac ischemia-reperfusion injury and remodeling, and prevents cardiac arrhythmias. In the vasculature, adenosine modulates vascular tone, reduces infiltration of inflammatory cells and generation of foam cells, and may prevent the development of atherosclerosis as a result. Modulation of insulin sensitivity may further add to the anti-atherosclerotic properties of adenosine signaling. In the kidney, adenosine plays an important role in tubuloglomerular feedback and modulates tubular sodium reabsorption. The challenge is to take advantage of the beneficial actions of adenosine signaling while preventing its potential adverse effects, such as salt retention and sympathoexcitation. Drugs that interfere with adenosine formation and elimination or drugs that allosterically enhance specific adenosine receptors seem to be most promising to meet this challenge.

Long-term (trophic) purinergic signalling: purinoceptors control cell proliferation, differentiation and death

The purinergic signalling system, which uses purines and pyrimidines as chemical transmitters, and purinoceptors as effectors, is deeply rooted in evolution and development and is a pivotal factor in cell communication. The ATP and its derivatives function as a 'danger signal' in the most primitive forms of life. Purinoceptors are extraordinarily widely distributed in all cell types and tissues and they are involved in the regulation of an even more extraordinary number of biological processes. In addition to fast purinergic signalling in neurotransmission, neuromodulation and secretion, there is long-term (trophic) purinergic signalling involving cell proliferation, differentiation, motility and death in the development and regeneration of most systems of the body. In this article, we focus on the latter in the immune/defence system, in stratified epithelia in visceral organs and skin, embryological development, bone formation and resorption, as well as in cancer.

Adenosine receptor-mediated cardioprotection: are all 4 subtypes required or redundant?

Adenosine is a purine nucleoside, which is produced primarily through the metabolism of adenosine triphosphate (ATP), therefore its levels increase during stressful situations when ATP utilization increases. Adenosine exerts potent cardioprotective effects on the ischemic/reperfused heart, reducing reversible and irreversible myocardial injury. Adenosine receptors (ARs) are G-protein-coupled receptors, and 4 subtypes exist--A(1), A(2A), A(2B), and A(3), all of which have been shown to be cardioprotective. Adenosine receptors are expressed on multiple cardiac cells, including fibroblasts, endothelial cells, smooth muscle cells, and myocytes. Activation of both A(1) and A(3) receptors prior to ischemia has been shown in multiple experimental models to reduce ischemia/reperfusion-induced cardiac injury. Additionally, activation of the A(2A) receptor at the onset of reperfusion has been shown to reduce injury. Most recently, there is evidence that the A(2B) receptor has cardioprotective effects upon its activation. However, controversy remains regarding the precise timing of activation of these receptors required to induce cardioprotection, as well as their involvement in ischemic preconditioning and postconditioning. Adenosine receptors have been suggested to reduce cell death through actions at the mitochondrial ATP-dependent potassium (K(ATP)) channel, as well as protein kinase C and mitogen-activated protein kinase (MAPK) signaling. Additionally, the ability of ARs to interact has been documented, and several recent reports suggest that these interactions play a role in AR-mediated cardioprotection. This review summarizes the current knowledge of the cardioprotective effects of each AR subtype, as well as the proposed mechanisms of AR cardioprotection. Additionally, the role of AR interactions in cardioprotection is discussed.

Adenosine A(3) receptors regulate heart rate, motor activity and body temperature

AIM

To examine the phenotype of mice that lack the adenosine A(3) receptor (A(3)R).

METHODS

We examined the heart rate, body temperature and locomotion continuously by telemetry over several days. In addition, the effect of the adenosine analogue R-N(6)-phenylisopropyl-adenosine (R-PIA) was examined. We also examined heat production and food intake.

RESULTS

We found that the marked diurnal variation in activity, heart rate and body temperature, with markedly higher values at night than during day time, was reduced in the A(3)R knock-out mice. Surprisingly, the reduction in heart rate, activity and body temperature seen after injection of R-PIA in wild type mice was virtually eliminated in the A(3)R knock-out mice. The marked reduction in activity was associated with a decreased heat production, as expected. However, the A(3)R knock-out mice, surprisingly, had a higher food intake but no difference in body weight compared to wild type mice.

CONCLUSIONS

The mice lacking adenosine A(3) receptors exhibit a surprisingly clear phenotype with changes in diurnal rhythm and temperature regulation. Whether these effects are due to a physiological role of A(3) receptors in these processes or whether they represent a role in development remains to be elucidated.

Activation of adenosine low-affinity A3 receptors inhibits the enteric short interplexus neural circuit triggered by histamine

We tested the novel hypothesis that endogenous adenosine (eADO) activates low-affinity A3 receptors in a model of neurogenic diarrhea in the guinea pig colon. Dimaprit activation of H2 receptors was used to trigger a cyclic coordinated response of contraction and Cl(-) secretion. Contraction-relaxation was monitored by sonomicrometry (via intracrystal distance) simultaneously with short-circuit current (I(sc), Cl(-) secretion). The short interplexus reflex coordinated response was attenuated or abolished by antagonists at H2 (cimetidine), 5-hydroxytryptamine 4 receptor (RS39604), neurokinin-1 receptor (GR82334), or nicotinic (mecamylamine) receptors. The A1 agonist 2-chloro-N(6)-cyclopentyladenosine (CCPA) abolished coordinated responses, and A1 antagonists could restore normal responses. A1-selective antagonists alone [8-cyclopentyltheophylline (CPT), 1,3-dipropyl-8-(2-amino-4-chlorophenyl)xanthine (PACPX), or 8-cyclopentyl-N(3)-[3-(4-(fluorosulfonyl)benzoyloxy)propyl]-xanthine (FSCPX)] caused a concentration-dependent augmentation of crypt cell secretion or contraction and acted at nanomolar concentrations. The A3 agonist N(6)-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (IB-MECA) abolished coordinated responses and the A3 antagonist 3-ethyl-5-benzyl-2-methyl-4-phenylethynyl-6-phenyl-1,4-(+/-)-dihydropyridine-3,5-dicarboxylate (MRS1191) could restore and further augment responses. The IB-MECA effect was resistant to knockdown of adenosine A1 receptor with the irreversible antagonist FSCPX; the IC(50) for IB-MECA was 0.8 microM. MRS1191 alone could augment or unmask coordinated responses to dimaprit, and IB-MECA suppressed them. MRS1191 augmented distension-evoked reflex I(sc) responses. Adenosine deaminase mimicked actions of adenosine receptor antagonists. A3 receptor immunoreactivity was differentially expressed in enteric neurons of different parts of colon. After tetrodotoxin, IB-MECA caused circular muscle relaxation. The data support the novel concept that eADO acts at low-affinity A3 receptors in addition to high-affinity A1 receptors to suppress coordinated responses triggered by immune-histamine H2 receptor activation. The short interplexus circuit activated by histamine involves adenosine, acetylcholine, substance P, and serotonin. We postulate that A3 receptor modulation may occur in gut inflammatory diseases or allergic responses involving mast cell and histamine release.

A3 adenosine receptor: pharmacology and role in disease

The study of the A(3) adenosine receptor (A(3)AR) represents a rapidly growing and intense area of research in the adenosine field. The present chapter will provide an overview of the expression patterns, molecular pharmacology and functional role of this A(3)AR subtype under pathophysiological conditions. Through studies utilizing selective A(3)AR agonists and antagonists, or A(3)AR knockout mice, it is now clear that this receptor plays a critical role in the modulation of ischemic diseases as well as in inflammatory and autoimmune pathologies. Therefore, the potential therapeutic use of agonists and antagonists will also be described. The discussion will principally address the use of such compounds in the treatment of brain and heart ischemia, asthma, sepsis and glaucoma. The final part concentrates on the molecular basis of A(3)ARs in autoimmune diseases such as rheumatoid arthritis, and includes a description of clinical trials with the selective agonist CF101. Based on this chapter, it is evident that continued research to discover agonists and antagonists for the A(3)AR subtype is warranted.

Adenosine receptors and the heart: role in regulation of coronary blood flow and cardiac electrophysiology

Adenosine is an autacoid that plays a critical role in regulating cardiac function, including heart rate, contractility, and coronary flow. In this chapter, current knowledge of the functions and mechanisms of action of coronary flow regulation and electrophysiology will be discussed. Currently, there are four known adenosine receptor (AR) subtypes, namely A(1), A(2A), A(2B), and A(3). All four subtypes are known to regulate coronary flow. In general, A(2A)AR is the predominant receptor subtype responsible for coronary blood flow regulation, which dilates coronary arteries in both an endothelial-dependent and -independent manner. The roles of other ARs and their mechanisms of action will also be discussed. The increasing popularity of gene-modified models with targeted deletion or overexpression of a single AR subtype has helped to elucidate the roles of each receptor subtype. Combining pharmacologic tools with targeted gene deletion of individual AR subtypes has proven invaluable for discriminating the vascular effects unique to the activation of each AR subtype. Adenosine exerts its cardiac electrophysiologic effects mainly through the activation of A(1)AR. This receptor mediates direct as well as indirect effects of adenosine (i.e., anti-beta-adrenergic effects). In supraventricular tissues (atrial myocytes, sinuatrial node and atriovetricular node), adenosine exerts both direct and indirect effects, while it exerts only indirect effects in the ventricle. Adenosine exerts a negative chronotropic effect by suppressing the automaticity of cardiac pacemakers, and a negative dromotropic effect through inhibition of AV-nodal conduction. These effects of adenosine constitute the rationale for its use as a diagnostic and therapeutic agent. In recent years, efforts have been made to develop A(1)R-selective agonists as drug candidates that do not induce vasodilation, which is considered an undesirable effect in the clinical setting.

Short- and long-term A3 adenosine receptor activation inhibits the Na+/H+ exchanger NHE3 activity and expression in opossum kidney cells

The renal function of the A(3) adenosine receptor (A3AR) is poorly characterized. In this study, we report that the A3AR-selective agonist, 1-[2-chloro-6-[[(3-iodophenyl)methyl]amino]-9H-purine-9-yl]-1-deoxy-N-methyl-b-D-ribofuranuronamide (2-Cl-IBMECA) regulates the Na+/H+ exchanger-3 (NHE3) in a dose- and time-dependent fashion. In opossum kidney (OK) cells, 2-Cl-IBMECA at high (10(-6) M) and low (10(-8) M) dose inhibits NHE3 by a multiphasic time course with an acute phase of NHE3 inhibition from 15 min to 1 h, followed by a chronic phase of NHE3 inhibition from 24 to 48 h. Pre-incubation with either the selective A3AR-antagonist MRS1523 (10(-7) M) or the protein kinase C inhibitor, Calphostin C (10(-8) M) completely blocked 10(-6) M 2-Cl-IBMECA-induced acute (15 min) and chronic (24 h) phases of NHE3 inhibition. In contrast, the acute inhibitory phase (15 min) of 10(-8) M 2-Cl-IBMECA was completely prevented only when Calphostin C (10(-8) M) was added in conjunction with the protein kinase A inhibitor, H89 (10(-7) M). Acute (15 or 30 min depending on the A3AR-agonist concentration) A3AR-dependent inhibition of NHE3 activity was accompanied by decrease in cell surface NHE3 protein with no change in total NHE3 antigen. Chronic (24 h) A3AR-mediated down-regulation of NHE3 was associated with reduction of surface NHE3, decreased total NHE3 protein (70%) and a paradoxical rise of NHE3 RNA (40%). In summary, these results indicate that A3AR directly regulates NHE3 at multiple levels in a complex pattern. A3AR-dependent short- and long-term inhibition of NHE3 may be a fundamental mechanism of net sodium and fluid balance.

Protective roles of adenosine A1, A2A, and A3 receptors in skeletal muscle ischemia and reperfusion injury

Although adenosine exerts cardio-and vasculoprotective effects, the roles and signaling mechanisms of different adenosine receptors in mediating skeletal muscle protection are not well understood. We used a mouse hindlimb ischemia-reperfusion model to delineate the function of three adenosine receptor subtypes. Adenosine A(3) receptor-selective agonist 2-chloro-N(6)-(3-iodobenzyl)adenosine-5'-N-methyluronamide (Cl-IBMECA; 0.07 mg/kg ip) reduced skeletal muscle injury with a significant decrease in both Evans blue dye staining (5.4 +/- 2.6%, n = 8 mice vs. vehicle-treated 28 +/- 6%, n = 7 mice, P < 0.05) and serum creatine kinase level (1,840 +/- 910 U/l, n = 13 vs. vehicle-treated 12,600 +/- 3,300 U/l, n = 14, P < 0.05), an effect that was selectively blocked by an A(3) receptor antagonist 3-ethyl-5-benzyl-2-methyl-6-phenyl-4-phenylethynyl-1,4-(+/-)-dihydropyridine-3,5-dicarboxylate (MRS-1191; 0.05 mg/kg). The adenosine A(1) receptor agonist 2-chloro-N(6)-cyclopentyladenosine (CCPA; 0.05 mg/kg) also exerted a cytoprotective effect, which was selectively blocked by the A(1) antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 0.2 mg/kg). The adenosine A(2A) receptor agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS-21680; 0.07 mg/kg)-induced decrease in skeletal muscle injury was selectively blocked by the A(2A) antagonist 2-(2-furanyl)-7-[3-(4-methoxyphenyl)propyl]-7H-pyrazolo[4,3-e] [1,2,4]triazolo[1,5-C]pyrimidin-5-amine (SCH-442416; 0.017 mg/kg). The protection induced by the A(3) receptor was abrogated in phospholipase C-beta2/beta3 null mice, but the protection mediated by the A(1) or A(2A) receptor remained unaffected in these animals. The adenosine A(3) receptor is a novel cytoprotective receptor that signals selectively via phospholipase C-beta and represents a new target for ameliorating skeletal muscle injury.

The A3 adenosine receptor: an enigmatic player in cell biology

Adenosine is a primordial signaling molecule present in every cell of the human body that mediates its physiological functions by interacting with 4 subtypes of G-protein-coupled receptors, termed A1, A2A, A2B and A3. The A3 subtype is perhaps the most enigmatic among adenosine receptors since, although several studies have been performed in the years to elucidate its physiological function, it still presents in several cases a double nature in different pathophysiological conditions. The 2 personalities of A3 often come into direct conflict, e.g., in ischemia, inflammation and cancer, rendering this receptor as a single entity behaving in 2 different ways. This review focuses on the most relevant aspects of A3 adenosine subtype activation and summarizes the pharmacological evidence as the basis of the dichotomy of this receptor in different therapeutic fields. Although much is still to be learned about the function of the A3 receptor and in spite of its duality, at the present time it can be speculated that A3 receptor selective ligands might show utility in the treatment of ischemic conditions, glaucoma, asthma, arthritis, cancer and other disorders in which inflammation is a feature. The biggest and most intriguing challenge for the future is therefore to understand whether and where selective A3 agonists or antagonists are the best choice.

Adenosine receptors as therapeutic targets

Adenosine receptors are major targets of caffeine, the most commonly consumed drug in the world. There is growing evidence that they could also be promising therapeutic targets in a wide range of conditions, including cerebral and cardiac ischaemic diseases, sleep disorders, immune and inflammatory disorders and cancer. After more than three decades of medicinal chemistry research, a considerable number of selective agonists and antagonists of adenosine receptors have been discovered, and some have been clinically evaluated, although none has yet received regulatory approval. However, recent advances in the understanding of the roles of the various adenosine receptor subtypes, and in the development of selective and potent ligands, as discussed in this review, have brought the goal of therapeutic application of adenosine receptor modulators considerably closer.

ACTIONS OF ADENOSINE AT ITS RECEPTORS IN THE CNS: Insights from Knockouts and Drugs

Adenosine and its receptors have been the topic of many recent reviews ( 1 – 26 ). These reviews provide a good summary of much of the relevant literature—including the older literature. We have, therefore, chosen to focus the present review on the insights gained from recent studies on genetically modified mice, particularly with respect to the function of adenosine receptors and their potential as therapeutic targets. The information gained from studies of drug effects is discussed in this context, and discrepancies between genetic and pharmacological results are highlighted.

Mast cell protease 5 mediates ischemia-reperfusion injury of mouse skeletal muscle

Ischemia with subsequent reperfusion (IR) injury is a significant clinical problem that occurs after physical and surgical trauma, myocardial infarction, and organ transplantation. IR injury of mouse skeletal muscle depends on the presence of both natural IgM and an intact C pathway. Disruption of the skeletal muscle architecture and permeability also requires mast cell (MC) participation, as revealed by the fact that IR injury is markedly reduced in c-kit defective, MC-deficient mouse strains. In this study, we sought to identify the pathobiologic MC products expressed in IR injury using transgenic mouse strains with normal MC development, except for the lack of a particular MC-derived mediator. Histologic analysis of skeletal muscle from BALB/c and C57BL/6 mice revealed a strong positive correlation (R(2) = 0.85) between the extent of IR injury and the level of MC degranulation. Linkage between C activation and MC degranulation was demonstrated in mice lacking C4, in which only limited MC degranulation and muscle injury were apparent. No reduction in injury was observed in transgenic mice lacking leukotriene C(4) synthase, hemopoietic PGD(2) synthase, N-deacetylase/N-sulfotransferase-2 (enzyme involved in heparin biosynthesis), or mouse MC protease (mMCP) 1. In contrast, muscle injury was significantly attenuated in mMCP-5-null mice. The MCs that reside in skeletal muscle contain abundant amounts of mMCP-5 which is the serine protease that is most similar in sequence to human MC chymase. We now report a cytotoxic activity associated with a MC-specific protease and demonstrate that mMCP-5 is critical for irreversible IR injury of skeletal muscle.

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.

Cardioprotection following adenosine kinase inhibition in rat hearts

Adenosine kinase phosphorylates adenosine to AMP, the primary pathway for adenosine metabolism under basal conditions. Inhibition of adenosine kinase results in a site-specific increase in interstitial adenosine. Using a rat model of myocardial infarction, we examined the protective effects of adenosine kinase inhibition. Male Sprague-Dawley rats underwent 30 min regional occlusion followed by 90 min reperfusion. Infarct size, expressed as a percent of the area-at-risk, IS/AAR(%), was 58.0 +/- 2.1 % in untreated rats. Pretreatment with the adenosine kinase inhibitor, 5-iodotubercidin (1 mg/kg), limited infarct development to 37.5+/-3.7% (P < 0.001). The A(1) adenosine receptor (A(1)AR) antagonist, DPCPX (100 microg/kg), abolished the infarct-sparing effect of 5-iodotubercidin (IS, 62.8 +/- 1.3%). Similarly, the A(3) adenosine receptor (A(3)AR) antagonist, MRS-1523 (2 mg/kg), and the delta-opioid receptor (DOR) antagonist, BNTX, (1 mg/kg) abolished the reduction of IS produced by iodotubercidin. Pretreatment with the ROS scavenger, 2-MPG (20 mg/kg), or the PKC-delta antagonist, rottlerin (0.3 mg/kg) also abolished iodotubercidin-mediated cardioprotection. Furthermore, pretreatment with 5-HD, a mitochondrial K(ATP) (mitoK(ATP)) channel inhibitor, but not the sarcolemmal K(ATP) channel blocker, HMR-1098, abrogated the beneficial effects of adenosine kinase inhibition (IS, 59.5 +/- 3.8%). These data suggest that inhibition of adenosine kinase is effective in reducing infarct development via A(1)AR, A(3)AR and DOR activation. Data also suggest that this protection is mediated via ROS, PKC-delta and mitoK(ATP) channels.

Animal models for the study of adenosine receptor function

Adenosine receptors represent a family of G-protein coupled receptors that are ubiquitously expressed in a wide variety of tissues. This family contains four receptor subtypes: A1 and A3, which mediate inhibition of adenylyl cyclase; and A2a and A2b, which mediate stimulation of this enzyme. Currently, all receptor subtypes have been genetically deleted in mouse models except for the A2b adenosine receptor, and some have been overexpressed in selective tissues of transgenic mice. Studies involving these transgenic mice indicated that receptor levels are rate limiting, as effects were amplified upon increases in receptor level. The knockout models pointed to clusters of activities related to the physiologies of the cardiovascular and the nervous systems, which are either reduced or enhanced upon specific receptor deletion. Interestingly, the trend of effects on these systems is similar in the A1 and A3 adenosine receptor knockout mice and opposite to the effects observed in the A2a adenosine receptor knockout model. This review summarizes in vitro studies on pathways affected by each adenosine receptor, and primarily focuses on the above in vivo models generated to investigate the physiologic role of adenosine receptors. Furthermore, it illustrates the need for multiple adenosine receptor subtype deficiency studies in mice and the deletion of the A2b subtype.

Behavioral characterization of mice lacking the A3 adenosine receptor: sensitivity to hypoxic neurodegeneration

1. The potential neuroprotective actions of the A3 adenosine receptor (A3AR) were investigated using mice with functional deletions of the A3AR (A3AR-/-) in behavioral assessments of analgesia, locomotion, tests predictive of depression and anxiety, and the effects of mild hypoxia on cognition and neuronal survival. 2. Untreated A3AR-/- mice were tested in standard behavioral paradigms, including activity in the open field, performance in the hot-plate, tail-flick, tail-suspension, and swim tests, and in the elevated plus maze. In addition, mice were exposed repeatedly to a hypoxic environment containing carbon monoxide (CO). The cognitive effects of this treatment were assessed using the contextual fear conditioning test. After testing, the density of pyramidal neurons in the CA1, 2, and 3 subfields of the hippocampus was determined using standard histological and morphometric techniques. 3. A3AR-/- mice showed increased locomotion in the open field test, elevated plus maze (number of arm entries) and light/dark box (number of transitions). However, they spent more time immobile in two different tests of antidepressant activity (Swim and tail suspension tests). A3AR-/- mice also showed evidence of decreased nociception in the hotplate, but not tail-flick tests. Further, A3AR-/- mice were more vulnerable to hippocampal pyramidal neuron damage following episodes of carbon monoxide (CO)-induced hypoxia. One week after exposure to CO a moderate loss of pyramidal neurons was observed in all hippocampal subfields of both wild-type (A3AR+/+) and A3AR-/- mice. However, the extent of neuronal death in the CA2-3 subfields was less pronounced in A3AR+/+ than A3AR-/- mice. This neuronal loss was accompanied by a decline in cognitive function as determined using contextual fear conditioning. These histological and cognitive changes were reproduced in wild-type mice by repeatedly administering the A3AR-selective antagonist MRS 1523 (5-propyl-2-ethyl-4-propyl-3-(ethylsulfanylcarbonyl)-6-phenylpyridine-5-carboxylate 1 mg/kg i.p.). 4. These results indicate that pharmacologic or genetic suppression of A3AR function enhances some aspects of motor function and suppresses pain processing at supraspinal levels, while acting as a depressant in tests predictive of antidepressant action. Consistent with previous reports of the neuroprotective actions of A3AR agonists, A3AR-/- mice show an increase in neurodegeneration in response to repeated episodes of hypoxia.

A1 adenosine receptor overexpression attenuates ischemia-reperfusion-induced apoptosis and caspase 3 activity

We tested the hypothesis that myocardial ischemia-reperfusion (I/R)-induced apoptosis is attenuated in transgenic mice overexpressing cardiac A(1) adenosine receptors. Isolated hearts from transgenic (TG, n = 19) and wild-type (WT, n = 22) mice underwent 30 min of ischemia and 2 h of reperfusion, with evaluation of apoptosis, caspase 3 activity, function, and necrosis. I/R-induced apoptosis was attenuated in TG hearts. TG hearts had less I/R-induced apoptotic nuclei (0.88 +/- 0.10% vs. 4.22 +/- 0.24% terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling-positive cells in WT, P < 0.05), less DNA fragmentation (3.30 +/- 0.38-fold vs. 4.90 +/- 0.39-fold over control in WT, P < 0.05), and less I/R-induced caspase 3 activity (145 +/- 25% over nonischemic control vs. 234 +/- 31% in WT, P < 0.05). TG hearts also had improved recovery of function and less necrosis than WT hearts. In TG hearts pretreated with LY-294002 (3 microM) to evaluate the role of phosphosinositol-3-kinase in acute signaling, there was no change in the functional protection or apoptotic response to I/R. These data suggest that cardioprotection with transgenic overexpression of A(1) adenosine receptors involves attenuation of I/R-induced apoptosis that does not involve acute signaling through phosphoinositol-3-kinase.

A3 adenosine receptor knockout mice are protected against ischemia- and myoglobinuria-induced renal failure

A(3) adenosine receptor (AR) activation and inhibition worsen and improve, respectively, renal function after ischemia-reperfusion (I/R) injury in rats. We sought to further characterize the role of A(3) ARs in modulating renal function after either I/R or myoglobinuric renal injury. A(3) knockout mice had significantly lower plasma creatinines compared with C57 controls 24 h after I/R or myoglobinuric renal injury. C57 control mice pretreated with the A(3) AR antagonist [3-ethyl-5-benzyl-2-methyl-4-phenylethynyl-6-phenyl-1,4-(+/-)-dihydropyridine-3,5 dicarboxylate] or agonist [0.125 mg/kg N(6)-(3-iodobenzyl)-N-methyl-5'-carbamoyladenosine (IB-MECA)] demonstrated improved or worsened renal function, respectively, after I/R or myoglobinuric renal injury. Higher doses of IB-MECA were lethal in C57 mice subjected to renal ischemia. H(1) but not H(2) histamine receptor antagonist prevented death in mice pretreated with IB-MECA before renal ischemia. Improvement in renal function was associated with significantly improved renal histology. In conclusion, preischemic A(3) AR activation (0.125 mg/kg IB-MECA) exacerbated renal I/R injury in mice. Mice lacking A(3) ARs or blocking A(3) ARs in wild-type mice resulted in significant renal protection from ischemic or myoglobinuric renal failure.

Cardioprotective action of CRF peptide urocortin against simulated ischemia in adult rat cardiomyocytes

The major objective of this study was to determine whether urocortin, a member of the corticotrophin-releasing factor (CRF) family, protects adult rat cardiomyocytes from ischemia that has been simulated by glucose deprivation and acidosis. When it was present during simulated ischemia, urocortin (0.1 microM) markedly attenuated the cellular injury, which was assessed by increases in creatine kinase and lactate dehydrogenase levels. This effect was comparable with that observed with adenosine (10 microM). The cardioprotective effect of urocortin was markedly attenuated by the protein kinase C inhibitor chelerythrine and by 5-hydroxydecanoate, an inhibitor of ATP-sensitive K(+) channels. Cardiomyocytes were also protected from injury by pretreatment with urocortin, either by incubation for 5 min with a subsequent 10-min recovery or incubation for 20 min with a 20-h recovery before simulated ischemia. Similar cardioprotective effects were observed with ischemic preconditioning protocols during both immediate and delayed phases. In conclusion, in adult cardiomyocytes, urocortin has immediate and delayed cardioprotective actions that mimic ischemic preconditioning. These actions are mediated via protein kinase C and ATP-sensitive K(+) channels.

Adenosine receptors in the nervous system: pathophysiological implications

Adenosine is a ubiquitous homeostatic substance released from most cells, including neurones and glia. Once in the extracellular space, adenosine modifies cell functioning by operating G-protein-coupled receptors (GPCR; A(1), A(2A), A(2B), A(3)) that can inhibit (A(1)) or enhance (A(2)) neuronal communication. Interactions between adenosine receptors and other G-protein-coupled receptors, ionotropic receptors and receptors for neurotrophins also occur, and this might contribute to a fine-tuning of neuronal function. Manipulations of adenosine receptors influence sleep and arousal, cognition and memory, neuronal damage and degeneration, as well as neuronal maturation. These actions might have therapeutic implications for neurodegenerative diseases such as Parkinson's disease, Alzheimer's disease, as well as for other neurological situations such as epilepsy, idiopathic pain or even drug addition. Peripheral side effects associated with adenosine receptor agonists limit their usefulness in therapeutics; in contrast, adenosine receptor antagonists appear to have less side effects as it is the case of the well-known non-selective antagonists theophylline (present in tea) or caffeine (abundant in coffee and tea), and their emerging beneficial actions in Parkinson's disease and Alzheimer's disease are encouraging. A(1) receptor antagonism may also be useful to enhance cognition and facilitate arousal, as well as in the periphery when deficits of neurotransmitter release occur (e.g. myasthenic syndromes). Enhancement of extracellular adenosine levels through drugs that influence its metabolism might prove useful approaches in situations such as neuropathic pain, where enhanced activation of inhibitory adenosine A(1) receptors is beneficial. One might then consider adenosine as a fine-tuning modulator of neuronal activity, which via subtle effects causes harmonic actions on neuronal activity. Whenever this homeostasis is disrupted, pathology may be installed and selective receptor antagonism or agonism required.

Overexpression of A(3) adenosine receptors decreases heart rate, preserves energetics, and protects ischemic hearts

To determine whether A(3) adenosine receptor (A(3)AR) signaling modulates myocardial function, energetics, and cardioprotection, hearts from wild-type and A(3)AR-overexpressor mice were subjected to 20-min ischemia and 40-min reperfusion while (31)P NMR spectra were acquired. Basal heart rate and left ventricular developed pressure (LVDP) were lower in A(3)AR-overexpressor hearts than wild-type hearts. Ischemic ATP depletion was delayed and postischemic recoveries of contractile function, ATP, and phosphocreatine were greater in A(3)AR-hearts. To determine the role of depressed heart rate and to confirm A(3)AR-specific signaling, hearts were paced at 480 beats/min with or without 60 nmol/l MRS-1220 (A(3)AR-specific inhibitor) and then subjected to ischemia-reperfusion. LVDP was similar in paced A(3)AR-overexpressor and paced wild-type hearts. Differences in ischemic ATP depletion and postischemic contractile and energetic dysfunction remained in paced A(3)AR-overexpressor hearts versus paced wild-type hearts but were abolished by MRS-1220. In summary, A(3)AR overexpression decreased basal heart rate and contractility, preserved ischemic ATP, and decreased postischemic dysfunction. Pacing abolished the decreased contractility but not the ATP preservation or cardioprotection. Therefore, A(3)AR overexpression results in cardioprotection via a specific A(3)AR effect, possibly involving preservation of ATP during ischemia.

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.

Targeted deletion of adenosine A(3) receptors augments adenosine-induced coronary flow in isolated mouse heart

To determine whether adenosine A(3) receptors participate in adenosine-induced changes in coronary flow, isolated hearts from wild-type (WT) and A(3) receptor knockout (A(3)KO) mice were perfused under constant pressure and effects of nonselective and selective agonists were examined. Adenosine and the selective A(2A) agonist 2-[p-(2-carboxyethyl)]phenylethylamino-5'-N-ethylcarboxamidoadenosine (CGS-21680) produced augmented maximal coronary vasodilation in A(3)KO hearts compared with WT hearts. Selective activation of A(3) receptors with 2-chloro-N(6)-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (Cl-IB-MECA) at nanomolar concentrations did not effect coronary flow, but at higher concentrations it produced coronary vasodilation both in WT and A(3)KO hearts. Cl-IB-MECA-induced increases in coronary flow were susceptible to both pharmacological blockade and genetic deletion of A(2A) receptors. Because deletion or blockade of adenosine A(3) receptors augmented coronary flow induced by nonselective adenosine and the selective A(2A) receptor agonist CGS-21680, we speculate that this is due to removal of an inhibitory influence associated with the A(3) receptor subtype. These data indicate that the presence of adenosine A(3) receptors may either inhibit or negatively modulate coronary flow mediated by other adenosine receptor subtypes.