Adenosine A2-receptor activation at reperfusion reduces infarct size and improves myocardial wall function in dog heart

Staphylococcus pseudintermedius 5'-nucleotidase suppresses canine phagocytic activity

Staphylococcus pseudintermedius is a major opportunistic bacterial pathogen and the leading cause of pyoderma in dogs. In canines it is also often associated with infections of the urinary system and wounds and occasionally infects people. Widespread antimicrobial resistance has made the development of alternative treatments a high priority. The development of a staphylococcal vaccine, however, has proven challenging. Identification of virulence factors that inhibit phagocytosis and avoid innate immunity may play a significant role in preventing or treating infection with S. pseudintermedius. In this study, we identified a putative 5'-nucleotidase provisionally named SpAdsA, a S. pseudintermedius cell- wall protein encoded by SpAdsA. SpAdsA shares approximately 52% identity with the orthologous protein of Staphylococcus aureus and 14.8% identity with that of Streptococcus suis type2. It catalyzes the dephosphorylation of adenosine triphosphate and attenuation of this enzyme with critical amino acid substitutions nearly eliminated its hydrolytic activity. Exogenous adenosine inhibited phagocytosis of S. pseudintermedius by canine neutrophils and monocytes. Conversely, the addition of SpAdsA inhibitor or A_2A adenosine receptor antagonist impaired the capacity of S. pseudintermedius to escape from killing by phagocytic cells. The neutralizing ability of canine antibody produced against SpAdsA-M was determined. Taken together, these results suggest that SpAdsA likely plays an important role in S. pseudintermedius virulence and that attenuated SpAdsA may be a good candidate for inclusion in a vaccine against S. pseudintermedius.

Cardiac purinergic signalling in health and disease

This review is a historical account about purinergic signalling in the heart, for readers to see how ideas and understanding have changed as new experimental results were published. Initially, the focus is on the nervous control of the heart by ATP as a cotransmitter in sympathetic, parasympathetic, and sensory nerves, as well as in intracardiac neurons. Control of the heart by centers in the brain and vagal cardiovascular reflexes involving purines are also discussed. The actions of adenine nucleotides and nucleosides on cardiomyocytes, atrioventricular and sinoatrial nodes, cardiac fibroblasts, and coronary blood vessels are described. Cardiac release and degradation of ATP are also described. Finally, the involvement of purinergic signalling and its therapeutic potential in cardiac pathophysiology is reviewed, including acute and chronic heart failure, ischemia, infarction, arrhythmias, cardiomyopathy, syncope, hypertrophy, coronary artery disease, angina, diabetic cardiomyopathy, as well as heart transplantation and coronary bypass grafts.

The association of ADORA2A and ADORA2B polymorphisms with the risk and severity of chronic heart failure: a case-control study of a northern Chinese population

The causes of chronic heart failure (CHF) and its progression are likely to be due to complex genetic factors. Adenosine receptors A2A and A2B (ADORA2A and ADORA2B, respectively) play an important role in cardio-protection. Therefore, polymorphisms in the genes encoding those receptors may affect the risk and severity of CHF. This study was a case-control comparative investigation of 300 northern Chinese Han CHF patients and 400 ethnicity-matched healthy controls. Four common single-nucleotide polymorphisms (SNPs) of ADORA2A (rs2236625, rs2236624, rs4822489, and rs5751876) and one SNP of ADORA2B (rs7208480) were genotyped and an association between SNPs and clinical outcomes was evaluated. Odds ratios (ORs) with 95% confidence intervals (CIs) were used to assess the association. The rs4822489 was significantly associated with the severity of CHF after adjustment for traditional cardiovascular risk factors (p = 0.040, OR = 1.912, 95% CI = 1.029–3.550). However, the five SNPs as well as the haplotypes were not found to be associated with CHF susceptibility. The findings of this study suggest that rs4822489 may contribute to the severity of CHF in the northern Chinese. However, further studies performed in larger populations and aimed at better defining the role of this gene are required.

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 < 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.

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

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.

Targeting of adenosine receptors in ischemia-reperfusion injury

IMPORTANCE OF THE FIELD

Ischemia-reperfusion (IR) injury is a common problem after transplantation as well as myocardial infarction and stroke. IR initiates an inflammatory response leading to rapid tissue damage. Adenosine, produced in response to IR, is generally considered a protective signaling molecule and elicits its physiological responses through four distinct adenosine receptors. The short half-life, lack of specificity and rapid metabolism limits the use of adenosine as a therapeutic agent. Thus, intense research efforts have focused on the synthesis and implementation of specific adenosine receptor agonists and antagonists as potential therapeutic agents for a variety of inflammatory conditions including IR injury.

AREAS COVERED IN THIS REVIEW

Current knowledge on IR injury with a focus on lung, heart and kidney and studies that have advanced our understanding of the role of adenosine receptors and the therapeutic potential of adenosine receptor agonists and antagonists for the prevention of IR injury.

WHAT THE READER WILL GAIN

Insight into the role of adenosine receptor signaling in IR injury.

TAKE HOME MESSAGE

No therapies are currently available that specifically target IR injury; however, targeting of specific adenosine receptors may offer therapeutic strategies in this regard.

Adenosine A2A and A2B receptors work in concert to induce a strong protection against reperfusion injury in rat hearts

We aimed to test if stimulation of both adenosine A2A and A2B receptors is required to produce an effective cardioprotection against reperfusion injury. Isolated rat hearts were subjected to 30-min regional ischemia followed by 2 h of reperfusion. The adenosine A1/A2 receptor agonist 5'-(N-ethylcarboxamido) adenosine (NECA) given at reperfusion reduced infarct size, an effect that was reversed by both the adenosine A2A antagonist SCH58261 and the A2B antagonist MRS1706. The A2B agonist BAY 60-6583 but not the selective A2A agonist CGS21680 reduced infarct size. Interestingly, a combination of BAY 60-6583 and CGS21680 further reduced infarct size. These results suggest that both A2A and A2B receptors are involved in NECA's anti-infarct effect at reperfusion. NECA attenuated mitochondrial swelling upon reperfusion and this was blocked by both SCH58261 and MRS1706, indicating that activation of A2 receptors with NECA can modulate reperfusion-induced mitochondrial permeability transition pore (mPTP) opening. In support, NECA also prevented oxidant-induced loss of mitochondrial membrane potential (DeltaPsi(m)) and matrix Ca2+ overload in cardiomyocytes via both the A2 receptors. In addition, NECA increased mitochondrial glycogen synthase kinase-3beta (GSK-3beta) phosphorylation upon reperfusion and this was again blocked by SCH58261 and MRS1706. In conclusion, A2A and A2B receptors work in concert to prevent reperfusion injury in rat hearts treated with NECA. NECA may protect the heart by modulating the mPTP opening through inactivating mitochondrial GSK-3beta. A simultaneous stimulation of A2A and A2B receptors at reperfusion is required to produce a strong cardioprotection against reperfusion injury.

Reduction in myocardial infarct size at 48 hours after brief intravenous infusion of ATL-146e, a highly selective adenosine A2A receptor agonist

This study was undertaken to determine whether the myocardial infarct-sparing effect of ATL-146e, a selective adenosine A(2A) receptor agonist, persists without a rebound effect for at least 48 h and to determine the optimal duration of ATL-146e treatment in anesthetized dogs. Reperfusion injury after myocardial infarction (MI) is associated with inflammation lasting 24-48 h that contributes to ongoing myocyte injury. We previously showed that an ATL-146e infusion, starting just before reperfusion, decreased inflammation and infarct size in dogs examined 2 h after MI without increasing coronary blood flow. In the present study, adult dogs underwent 90 min of left anterior descending coronary artery occlusion. Thirty minutes before reperfusion, ATL-146e (0.01 microg x kg(-1) x min(-1); n = 21) or vehicle (n = 12) was intravenously infused and continued for 2.5 h (protocol 1) or 24 h (protocol 2). At 48 h after reperfusion hearts were excised and assessed for histological risk area and infarct size. Infarct size based on triphenyltetrazolium chloride (TTC) staining as a percentage of risk area was significantly smaller in ATL-146e-treated vs. control dogs (16.7 +/- 3.7% vs. 33.3 +/- 6.2%, P < 0.05; protocol 1). ATL-146e reduced neutrophil accumulation into infarcted myocardium of ATL-146e-treated vs. control dogs (30 +/- 7 vs. 88 +/- 16 cells/high-power field, P < 0.002). ATL-146e infusion for 24 h (protocol 2) conferred no significant additional infarct size reduction compared with 2.5 h of infusion. A 2.5-h ATL-146e infusion initiated 30 min before reperfusion results in marked, persistent (48 h) reduction in infarct size as a percentage of risk area in dogs with a reduction in infarct zone neutrophil infiltration. No significant further benefit was seen with a 24-h infusion.

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.

Protein kinase C protects preconditioned rabbit hearts by increasing sensitivity of adenosine A2b-dependent signaling during early reperfusion

Although protein kinase C (PKC) plays a key role in ischemic preconditioning (IPC), the actual mechanism of that protection is unknown. We recently found that protection from IPC requires activation of adenosine receptors during early reperfusion. We, therefore, hypothesized that PKC might act to increase the heart's sensitivity to adenosine. IPC limited infarct size in isolated rabbit hearts subjected to 30-min regional ischemia/2-h reperfusion and IPC's protection was blocked by the PKC inhibitor chelerythrine given during early reperfusion revealing involvement of PKC at reperfusion. Similarly chelerythrine infused in the early reperfusion period blocked the increased phosphorylation of the protective kinases Akt and ERK1/2 observed after IPC. Infusing phorbol 12-myristate 13-acetate (PMA), a PKC activator, during early reperfusion mimicked IPC's protection. As expected, the protection triggered by PMA at reperfusion was blocked by chelerythrine, but surprisingly it was also blocked by MRS1754, an adenosine A(2b) receptor-selective antagonist, suggesting that PKC was somehow facilitating signaling from the A(2b) receptors. NECA [5'-(N-ethylcarboxamido) adenosine], a potent but not selective A(2b) receptor agonist, increased phosphorylation of Akt and ERK1/2 in a dose-dependent manner. Pretreating hearts with PMA or brief preconditioning ischemia had no effect on phosphorylation of Akt or ERK1/2 per se but markedly lowered the threshold for NECA to induce their phosphorylation. BAY 60-6583, a highly selective A(2b) agonist, also caused phosphorylation of ERK1/2 and Akt. MRS1754 prevented phosphorylation induced by BAY 60-6583. BAY 60-6583 limited infarct size when given to ischemic hearts at reperfusion. These results suggest that activation of cardiac A(2b) receptors at reperfusion is protective, but because of the very low affinity of the receptors endogenous cardiac adenosine is unable to trigger their signaling. We propose that the key protective event in IPC occurs when PKC increases the heart's sensitivity to adenosine so that endogenous adenosine can activate A(2b)-dependent signaling.

Reperfusion injury: does it exist?

It is well established that reperfusion of the heart is the optimal method of salvaging previously ischemic myocardium. However, the idea of reperfusion injury, i.e. injury caused by the process of reperfusion per se, still remains a controversial issue. In this review, we present mounting evidence supporting the concept that reperfusion injury exists, based on work conducted with adenosine and opioid receptor ligands, and the discovery of two new concepts regarding reperfusion injury: 'postconditioning' (POC) and the reperfusion injury salvage kinase (RISK) signaling pathway.

Adenosine A2A agonists in development for the treatment of inflammation

Extracellular adenosine binds specifically to a family of four G protein-coupled cell-surface adenosine receptors (ARs). As the activation of the A2AAR modulates the activity of multiple inflammatory cells including neutrophils, macrophages and T lymphocytes, the receptor is considered to be a promising pharmacological target for the treatment of inflammatory disorders. Although adenosine binds nonselectively to all four AR subtypes, A2AAR selective agonists have been developed and shown to inhibit multiple manifestations of inflammatory cell activation including superoxide anion generation, cytokine production and adhesion molecule expression. A2AAR agonists are also vasodilators, but the inhibition of inflammation occurs at low doses that produce few or no cardiovascular side effects. Therefore, the selective activation of the A2AAR by these compounds holds significant potential in the treatment of inflammation.

Amelioration of Ischemia/Reperfusion-Induced Myocardial Infarction by the 2-Alkynyladenosine Derivative 2-Octynyladenosine (YT-146)

The present study was aimed at determining whether the novel adenosine A2-agonist YT-146 may have cardioprotective effects against ischemia-reperfusion injury. Anesthetized open-chest dogs underwent 90-min occlusion of the left anterior descending artery and subsequent 300-min reperfusion. The animals were randomly assigned to receive vehicle, 3, or 10 microg/kg YT-146 or ischemic preconditioning (4 episodes of 5 min occlusion followed by 5 min of reperfusion). Blood pressure, heart rate, and regional myocardial blood flow throughout the experiment were measured, as was the myocardial infarct size after reperfusion. The infarct size of the vehicle-treated dog was 56.2% +/- 2.7% (n = 5), whereas that of 3 or 10 microg/kg YT-146-treated dog was smaller (ie, 29.5% +/- 8.7% or 20.2% +/- 7.0%, respectively; n = 5). The infarct size of the dog treated with 10 microg/kg YT-146 was reduced to a degree similar to that of the ischemic preconditioning (19.2% +/- 6.3%, n = 5). YT-146 at both doses elicited a dose-dependent increase in acute hyperemic coronary flow immediately after reperfusion. The cardioprotective effect may be attributed to the limitation of the infarct size, probably via A2-receptor-mediated coronary artery dilatation during the early period of reperfusion.

Cardioprotection with adenosine A2 receptor activation at reperfusion

Pre-ischemic treatment is seldom possible in the clinical setting of acute myocardial infarction. Thus, to successfully save myocardium from infarction, it is required that protective interventions must be effective when applied after ischemia has begun or at the onset of reperfusion. Unfortunately, in spite of a large body of experimental data showing that various interventions are cardioprotective at reperfusion, no specific therapy has yet been established to be clinically applicable. However, recent data from several laboratories have shown that adenosine and its analogues given at reperfusion can markedly protect the heart from ischemia/reperfusion injury. While the experimental data suggest that factors such as adenosine A2 receptor activation, anti-neutrophil effect, attenuation of free radical generation, increased nitric oxide (NO) availability, activation of the PI3-kinase/Akt pathway and ERK, prevention of mitochondrial damage, and anti-apoptotic effects may be involved in the protective effect of adenosine or its analogues, the exact receptor subtype(s), the detailed signaling mechanisms, and interaction between those individual factors are still unknown. A definite answer to these unsolved problems will offer insights into the mechanisms of cardioprotection at reperfusion, and will be critical for developing a successful therapeutic strategy to salvage ischemic myocardium in patients with acute myocardial infarction.

Phosphatidylinositol 3-kinase and ERK1/2 are not involved in adenosine A1, A2Aor A3receptor-mediated preconditioning in rat ventricle strips

Mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase 1 and 2 (ERK1/2) and phosphatidylinositol 3-kinase (PI3-kinase)/protein kinase B (PKB; also known as Akt) are important antiapoptotic signalling pathways which have recently been implicated in cardioprotection. However, at present the involvement of ERK1/2 and PI3-kinase/PKB in adenosine receptor-mediated cardioprotection is poorly understood. In this study we used isolated rat right ventricular strips, contracted by electrical-field stimulation, in order to investigate the role of ERK1/2 and PI3-kinase/PKB in adenosine receptor-induced cardioprotection. Ventricle strips were pretreated for 2 min with the agonists adenosine (non-selective), CPA (A1 selective), CGS 21680 (A2A selective) and Cl-IB-MECA (A3 selective) before 30 min hypoxia followed by 30 min reoxygenation. Each agonist significantly improved posthypoxic percentage contraction recovery compared to control strips. Similarly hypoxic preconditioning (10 min hypoxia followed by 20 min reoxygenation) significantly improved posthypoxic percentage contraction recovery compared to non-preconditioned strips. The selective adenosine receptor antagonists DPCPX (A1), ZM 241385 (A2A) and MRS 1220 (A3) attenuated cardioprotection induced by CPA, CGS 21680 and Cl-IB-MECA, respectively. Pre-incubation (30 min) of ventricle strips with the MEK1 inhibitor PD 98059 (50 microM) or the PI3-kinase inhibitor wortmannin (100 nM) significantly reduced posthypoxic percentage contraction recovery induced by hypoxic preconditioning. In contrast, PD 98059 and wortmannin had no significant effect on cardioprotection induced by CPA, Cl-IB-MECA or CGS 21680. Overall these data indicate that although selective A1, A2A and A3 adenosine receptor agonists induce preconditioning in rat right ventricular strips the effects are independent of ERK1/2- and PI3-kinase-dependent pathways. In contrast ERK1/2 and PI3-kinase-dependent pathways do appear to be involved in early hypoxic preconditioning.

Sustained cardioprotection afforded by A2A adenosine receptor stimulation after 72 hours of myocardial reperfusion

This study was designed to determine whether cardioprotection afforded by A2A adenosine receptor stimulation can be sustained and to determine the effect of an A2A adenosine receptor agonist on Akt and cAMP response element binding protein (CREB) activation, as well as Hsp27 and Hsp70 protein expression in such events. The left anterior descending coronary artery was occluded for 40 minutes in anesthetized rats followed by 72 hours of reperfusion. A2A agonist (CGS21680 at 0.2 microg/kg/min) was administered for 120 minutes, starting either 5 minutes before (early) or after (late) the beginning of reperfusion. Infarct size was reduced significantly in the early compared with the control group (35.2 +/- 1.9% and 52.5 +/- 3.4%, respectively; P < 0.05), whereas no difference was observed with the late group (44.5 +/- 7.1%). After 72 hours of reperfusion, drug administration was accompanied by Akt activation (early, 121.8 +/- 17.6%; late, 118.1 +/- 16.4%; P < 0.05), as well as elevated Hsp27 expression (early, 197.2 +/- 27.7%; late, 203.8 +/- 36.8%; P < 0.05); CREB activation and Hsp70 expression were not altered. In another set of experiments in which reperfusion was limited to 15 minutes, Akt was activated only in the early group (121.8 +/- 17.6%; P < 0.05). Moreover, CREB was activated in both the early and late groups (98.4 +/- 8.3% and 107.0 +/- 6.5%, respectively; P < 0.05), whereas Hsp27 and Hsp70 expression were not altered. These results demonstrate that A2A adenosine receptor activation induces a sustained cardioprotection only if the therapy is instituted before reperfusion. This myocardial protection is associated by an early prosurvival Akt activation. CREB activation and Hsp27 content do not seem to be associated with cardioprotection because they are enhanced in both treated groups, suggesting indirect A2A agonist and pathology-related effects.

Postischemic Administration of CGX-1051, a Peptide from Cone Snail Venom, Reduces Infarct Size in Both Rat and Dog Models of Myocardial Ischemia and Reperfusion

CGX-1051 is a synthetic version of a peptide originally isolated from the venom of cone snails. In the present studies, we tested the potential cardioprotective effect of CGX-1051 in a rat and dog model of myocardial ischemia/reperfusion. CGX-1051 was administered 5 minutes before reperfusion as intravenous bolus doses of 30, 100, and 300 microg/kg. Infarct size (IS) is reported as IS/area at risk (AAR). In the rat, the vehicle control group had an IS/AAR of 59.8+/-2.1%. Postischemic administration of CGX-1051 at doses of 30, 100, and 300 microg/kg resulted in an IS/AAR of 52.6+/-4.2%, 34.6+/-5.6% (P<0.05), and 40.8+/-5.2% (P<0.05), respectively. In the dog, the vehicle control group had an IS/AAR of 18.8+/-1.7%. Postischemic administration of CGX-1051 at doses of 30, 100, and 300 microg/kg resulted in an IS/AAR of 16.9+/-2.5%, 8.4+/-2.9% (P<0.05) and 9.9+/-2.4% (P<0.05), respectively. These results demonstrate that administration of CGX-1051 at a clinically relevant time point results in a dose-dependent reduction in IS in both rats and dogs.

Reduction of infarct size and postischemic inflammation from ATL-146e, a highly selective adenosine A2A receptor agonist, in reperfused canine myocardium

Adenosine and adenosine A(2A) receptor agonists have been shown to limit myocardial infarct size when given at vasodilatory doses during reperfusion. This beneficial effect is thought to be due, in part, to stimulation of adenosine A(2A) receptors on inflammatory cells. The specific aims of this study were to determine whether the anti-inflammatory and cardioprotective properties of a novel adenosine A(2A) receptor agonist, ATL-146e (ATL), alone or in combination with the phosphodiesterase IV inhibitor rolipram would occur using very low, nonvasodilating doses. In a canine model of reperfused myocardial infarction, low-dose ATL given alone reduced infarct size by 45% (P < 0.05 vs. control). When ATL was combined with a very low dose of rolipram (0.001 microg.kg(-1).min(-1)), a marked reduction in P-selectin expression and neutrophil infiltration (51% lower; P < 0.001 vs. control) was seen and the infarct size reduction (58% lower; P < 0.01 vs. control) was greater than observed with ATL (45% lower; P < 0.05) or rolipram (33% lower; P < 0.05) alone. In conclusion, a low, nonvasodilating dose of ATL, a highly selective adenosine A(2A) receptor agonist, reduced infarct size after reperfusion. Furthermore, combining ATL and the phosphodiesterase IV inhibitor rolipram reduced infarct size even more than either agent alone. Such combination therapy may be beneficial clinically by potentiating cardioprotection after coronary reperfusion at doses far below those producing vasodilatation or side effects.

Preconditioning and adenosine protect human proximal tubule cells in an in vitro model of ischemic injury

Renal ischemic reperfusion injury results in unacceptably high mortality and morbidity during the perioperative period. It has been recently demonstrated that ischemic preconditioning or adenosine receptor modulations attenuate renal ischemic reperfusion injury in vivo. An in vitro model of ischemic renal injury was used in cultured human proximal tubule (HK-2) cells to further elucidate the protective signaling cascades against renal ischemic reperfusion injury. ATP depletion preconditioning (1 h of antimycin A and 2-deoxyglucose treatment followed by 1 h of recovery), adenosine, an A(1) adenosine receptor selective agonist, or an A(2a) adenosine receptor selective agonist significantly attenuated subsequent severe ATP depletion injury of HK-2 cells. In contrast, an adenosine receptor antagonist failed to prevent protection induced by ATP depletion preconditioning. Cytoprotection by ATP depletion preconditioning or A(1) adenosine receptor activation was prevented by inhibitors of extracellular signal-regulated mitogen-activated kinases, protein kinase C, and tyrosine kinases. The A(1) and A(2a) adenosine receptor-mediated cytoprotection were also dependent on G(i/o) proteins and PKA activation, respectively. It is concluded that ATP depletion preconditioning and A(1) and A(2a) adenosine receptor activation protect HK-2 cells against severe ATP depletion injury via distinct signaling pathways.

Systemic adenosine given after ischemia protects renal function via A(2a) adenosine receptor activation

Ischemia and reperfusion during renal transplant and aortic surgery result in renal ischemic-reperfusion injury. Previously, we showed that preischemic adenosine treatment protects renal function via A(1) adenosine receptor (AR) activation. In contrast, in the cardiac and pulmonary systems, postischemic adenosine has potent anti-inflammatory attributes and is protective against reperfusion injury via activation of A(2a) ARs. We questioned whether adenosine given after an ischemic insult protects renal function in rats, and we sought to determine the AR subtype and intracellular second messengers involved. Rats were randomized to a sham operation, 45 minutes of renal ischemia and reperfusion and treatments with systemic adenosine or selective AR agonists and antagonists, or treatments of dibutyryl cyclic adenosine monophosphate (cAMP) after 45 minutes of renal ischemia but before reperfusion. Forty-five minutes of renal ischemia followed by 24 hours of reperfusion led to severe renal dysfunction as indicated by marked rises in creatinine and histologically evident renal tubular damage. Adenosine treatment after ischemia protected renal function and improved tubular histology. This protection was mediated via A(2a) AR activation because the A(2a)-selective AR agonist [4-((N-ethyl-5'-carbamoyadenos-2-yl)-aminoethyl)-phenylpropionic acid (CGS-21680)] mimics adenosine-induced renal protection, and the A(2a)-selective AR antagonist [8-(3-chlorostyryl)caffeine (CSC)] blocks adenosine-induced renal protection. A(1) or A(3) AR agonists and antagonists did not mimic and block adenosine-induced renal protection. The signaling intermediates of A(2a) AR-mediated renal protection appear to include cAMP because dibutyryl cAMP mimicked adenosine and CGS-21680 mediated renal protection. Rat kidneys can be protected against reperfusion injury via postischemic A(2a) AR activation or cAMP. These data suggest that A(2a) adenosine agonists may have clinically beneficial implications when renal ischemia is unavoidable.

Beneficial effects of adenosine A(2a) agonist CGS-21680 in infarcted and stunned porcine myocardium

Although there are conflicting results on whether adenosine infusion during reperfusion alters infarct size, there are several reports that indicate adenosine A(2a) agonists reduce infarct size. There are also reports that the A(2a) agonist CGS-21680 increases cAMP and contractility in ventricular myocytes. The purpose of this study was to determine whether low-dose intracoronary infusions of CGS-21680 during reperfusion exert any beneficial effects in irreversibly and reversibly injured myocardium. Open-chest pigs were submitted to 60 min of coronary artery occlusion and 3 h of reperfusion. Treated pigs were administered intracoronary CGS-21680 (0.2 microg x kg(-1) x min(-1)) for the first 60 min of reperfusion. Pigs submitted to regional stunning (15 min ischemia) were treated with intracoronary CGS-21680 (0.15 microg x kg(-1) x min(-1)) after 2 h of reperfusion. In the infarct protocol, CGS-21680 reduced infarct size from 62 +/- 2% of the region at risk to 36 +/- 2%. In stunned myocardium, CGS increased load-independent regional preload recruitable stroke work and area by > or =70%, but the same infusion in normal myocardium was associated with no inotropic effect. Both beneficial effects were associated with little systemic hemodynamic effects. These findings suggest that reperfusion infusions of low doses of the A(2a) agonist CGS-21680 exert beneficial effects in reversibly and irreversibly injured myocardium.

Pharmacokinetics and safety of single intravenous infusions of the adenosine agonist, AMP 579, in patients with end-stage renal insufficiency

The pharmacokinetics of an adenosine agonist (AMP 579) were characterized in patients with end-stage renal disease compared to sex- and age-matched healthy volunteers. All study participants were administered single AMP 579 doses of 50 micrograms/kg as a 6-hour, constant-rate intravenous infusion. Serial blood samples were obtained for measurement of plasma AMP 579 concentration, and predose samples were collected for determination of AMP 579 plasma protein binding. The safety of AMP 579 administration in renally impaired patients also was evaluated. AMP 579 was rapidly cleared from the systemic circulation in all subjects as plasma concentrations were below the limit of detection by 2 to 4 hours after terminating the infusion. Noncompartmental analysis yielded mean values for the plasma AMP 579 concentration at the end of the 6-hour infusion (C6 h) of 9.6 and 10.5 ng/mL and for systemic clearances (Cl) of 0.91 and 0.72 L/h/kg in renally impaired patients and healthy volunteers, respectively. Mean volumes of distribution (Vss) in the renally impaired and healthy volunteers were 0.92 and 0.84 L/kg, and terminal elimination half-life values (t1/2) were 1.61 and 1.33 hours, respectively. The extent to which AMP 579 is bound to plasma protein was not altered in renally impaired patients since the free fractions were 4.0% and 3.4% for renally impaired and healthy volunteers, respectively. It was concluded that the pharmacokinetic parameters of AMP 579 were similar in both groups. The 6-hour AMP 579 infusion was generally well tolerated by both renal patients and healthy volunteers. There were no serious adverse events, and there were only two mild adverse events in 1 renally impaired patient judged possibly related to the study drug that quickly resolved. There were no clinically significant changes in laboratory values or clinical evaluations during the study. There was a slight increase in heart rate during the infusion of similar magnitude for both the renal patients and healthy volunteers. These data suggest that AMP 579 may be administered to renally impaired patients with minimal cardiovascular effects and adverse events. These results in end-stage renal patients (worst-case scenario) indicate that dose adjustment in patients with renal insufficiency of any degree is not indicated in future studies of AMP 579.

Mechanism of decreased adenosine protection in reperfusion injury of aging rats

The purpose of this study was to determine whether the protective effects of adenosine on myocardial ischemia-reperfusion injury are altered with age, and if so, to clarify the mechanisms that underlie this change related to nitric oxide (NO) derived from the vascular endothelium. Isolated perfused rat hearts were exposed to 30 min of ischemia and 60 min of reperfusion. In the adult hearts, administration of adenosine (5 micromol/l) stimulated NO release (1. 06 +/- 0.19 nmol. min(-1). g(-1), P < 0.01 vs. vehicle), increased coronary flow, improved cardiac functional recovery (left ventricular developed pressure 79 +/- 3.8 vs. 57 +/- 3.1 mmHg in vehicle, P < 0.001; maximal rate of left ventricular pressure development 2,385 +/- 103 vs. 1,780 +/- 96 in vehicle, P < 0.001), and reduced myocardial creatine kinase loss (95 +/- 3.9 vs. 159 +/- 4.6 U/100 mg protein, P < 0.01). In aged hearts, adenosine-stimulated NO release was markedly reduced (+0.42 +/- 0.12 nmol. min(-1). g(-1) vs. vehicle), and the cardioprotective effects of adenosine were also attenuated. Inhibition of NO production in the adult hearts significantly decreased the cardioprotective effects of adenosine, whereas supplementation of NO in the aged hearts significantly enhanced the cardioprotective effects of adenosine. The results show that the protective effects of adenosine on myocardial ischemia-reperfusion injury are markedly diminished in aged animals, and that the loss in NO release in response to adenosine may be at least partially responsible for this age-related alteration.

Pharmacokinetics, safety, and tolerability of single intravenous infusions of an adenosine agonist, AMP 579, in healthy male volunteers

The pharmacokinetics of an adenosine agonist, AMP 579, following intravenous administration were evaluated. Single AMP 579 doses of 20 to 150 micrograms/kg were infused intravenously over 6 hours using a constant-rate or two-step rate of infusion to healthy male volunteers. Plasma and urine samples were collected for measurement of AMP 579 concentration using a validated HPLC assay. An assessment of safety and tolerability was also performed. Based on a noncompartmental method of analysis, the pharmacokinetics of AMP 579 were characterized by rapid systemic clearance (0.77 to 0.85 L/h/kg), a moderate steady-state volume of distribution (0.80 to 0.94 L/kg), and a short terminal elimination half-life (0.84 to 1.13 h). AMP 579 exhibited dose (infusion rate)-proportional pharmacokinetics over the dose range. In addition, little or no unchanged AMP 579 was found in the urine. The primary cardiovascular pharmacodynamic response that was observed was a dose-related increase in mean ventricular heart rate. Fifteen minutes prior to the end of the infusion, volunteers administered the highest dose (150 micrograms/kg) exhibited a mean (range) 59% (36%-69%) increase in heart rate as compared to a mean (range) 18% (0%-73%) increase for the placebo group. No clinically relevant changes in the systolic or diastolic blood pressure were observed. The information obtained in this study should allow the design of AMP 579 dosage regimens that would maximize plasma AMP 579 concentrations and minimize the incidence of adverse events.

Role of A2A receptor in the modulation of myocardial reperfusion damage

Adenosine protects myocardium from ischemia and reperfusion damage; however, the mechanism of action is still under discussion. We investigated whether (a) adenosine protects isolated crystalloid-perfused rabbit heart from ischemia/ reperfusion injury; (b) this action is receptor mediated and what receptor subtypes are involved, and (c) this action is dependent on an enhanced nitric oxide production. Our results showed a cardioprotective effect of adenosine (10(-4) M), of nonselective adenosine-receptor agonist 5'-N-ethyl-carboxamidoadenosine (NECA; 5 x 10(-6) M), and of A2A agonists CGS 21680 (10(-8) and 10(-6) M), 2-hexynylNECA (10(-7) M). On the contrary, A1 agonist CCPA (10(-8) and 10(-6) M) does not provide any protection. The effect has been achieved in terms of significant reduction in contracture development during reperfusion [diastolic pressure was 46.8 +/- 7.1 mm Hg (p < 0.01); 46.1 +/- 7.8 mm Hg (p < 0.01); 46.9 +/- 5.5 mm Hg (p < 0.01); and 59.3 +/- 6.7 mm Hg (p < 0.05) with 10(-4) M adenosine, 5 x 10(-6) M NECA, 10(-6) M CGS 21680, and 10(-7) M 2-hexynylNECA, respectively, versus 77.6 +/- 5.0 mm Hg in control]; reduced creatine phosphokinase release (13.5 +/- 1.6, 22.2 +/- 7.9, 14.2 +/- 3.3, and 14.1 +/- 4.5 U/gww in treated hearts vs. 34.6 +/- 7.2 U/gww in controls; p < 0.05); improved energy metabolism [adenosine triphosphate (ATP) content is 9.9 +/- 0.5, 10.4 +/- 0.6, 9.8 +/- 0.5, and 10.5 +/- 0.5 micromol/gdw in treated hearts vs. 7.6 +/- 0.2 micromol/gdw; p < 0.05]. Moreover, our data indirectly show a functional presence of A2A receptors on cardiomyocytes as the protection is A2A mediated and exerted only during reperfusion, although in the absence of blood and coronary flow changes. These activities appear independent of nitric oxide pathways, as adenosine and 2-hexynylNECA effects are not affected by the presence of a nitric oxide-synthase inhibitor (10(-4) M L-NNA).

Cardiovascular pharmacology of the adenosine A1/A2-receptor agonist AMP 579: coronary hemodynamic and cardioprotective effects in the canine myocardium

The hemodynamic and cardioprotective properties of the novel adenosine A1/A2 receptor agonist AMP 579 (IS-[1a,2b,3b,4a(S*)]-4-[7-[[1-[(3-chloro-2-thienyl)methyl]propylamino]- 3H-imidazo[4,5-b]pyridin-3-yl]-N-ethyl-2,3-dihydroxy cyclopentanecarboxamide) were studied in two canine models designed to simulate (a) mild single-vessel coronary artery disease, and (b) myocardial ischemia/reperfusion injury. In the first model, a moderate stenosis was placed on the left circumflex coronary artery (LCCA), and the effects of AMP 579 on regional myocardial blood flow were assessed. AMP 579, 10 micrograms/kg/min, i.v., for 10 min, induced coronary dilation without causing endocardial steal. In the model of ischemia/reperfusion injury (60 min LCCA occlusion/5 h reperfusion), AMP 579, 10 micrograms/kg/min, i.v., administered for 15 min before ischemia significantly decreased myocardial infarct size. Control infarct size to area at risk (IS/AAR) equaled 34 +/- 3% (n = 9); IS/AAR for AMP 579-treated dogs equaled 16 +/- 4% (n = 9). Preconditioning (5 min LCCA occlusion + 10 min reperfusion) immediately before the 60-min LCCA occlusion also resulted in a marked decrease in IS/AAR: 9 +/- 3% (n = 6). The selective A1 agonist CPA reduced infarct size when administered at 3 micrograms/kg/min, i.v., for 15 min before LCCA occlusion: IS/AAR = 11 +/- 3% (n = 5). Pretreatment of animals with the adenosine-receptor antagonist 8-SPT, 10 mg/kg, i.v., attenuated the myocardial protective effects associated with preconditioning, CPA, and AMP 579, resulting in IS/AAR values of 28 +/- 7% (n = 7), 28 +/- 4% (n = 8), and 26 +/- 3% (n = 8), respectively. The ability of 8-SPT to block the cardioprotective effects suggests that these effects were mediated through an interaction with adenosine receptors. These experimental results indicate that AMP 579 is an effective coronary vasodilator, which also can protect the heart from ischemic injury. Thus AMP 579 has the potential to be useful in cardiovascular therapeutics.

Effect of acidotic blood reperfusion on reperfusion injury after coronary artery occlusion in the dog heart

A prolongation of the intracellular acidosis after myocardial ischemia can protect the myocardium against reperfusion injury. In isolated hearts, this was achieved by prolongation of the extracellular acidosis. The aim of this study was to investigate whether regional reperfusion with acidotic blood after coronary artery occlusion can reduce infarct size and improve myocardial function in vivo. Anesthetized open-chest dogs were instrumented for measurement of regional myocardial function, assessed by sonomicrometry as systolic wall thickening (sWT). Infarct size was determined by triphenyltetrazolium staining after 3 h of reperfusion. The left anterior descending coronary artery (LAD) was perfused through a bypass from the left carotid artery. The animals underwent 1 h of LAD occlusion and subsequent bypass-reperfusion with normal blood (control, n = 6) or blood equilibrated to pH = 6.8 by using 0.1 mM HCl during the first 30 min of reperfusion (HCl, n = 5). Regional collateral blood flow (RCBF) at 30-min occlusion was measured by using colored microspheres. There was no difference in recovery of sWT in the LAD-perfused area between the two groups at the end of the experiments [-2.8+/-1.2% (HCl) vs. -4.4+/-2.5% (control); mean +/- SEM; p = NS]. RCBF was comparable in both groups. Infarct size (percentage of area at risk) was reduced in the treatment group (12.8+/-2.8%) compared with the control group (26.2+/-4.8%; p < 0.05). These results indicate that reperfusion injury after coronary artery occlusion can be reduced by a prolonged local extracellular acidosis in vivo.

Adenosine inhibition of neutrophil damage during reperfusion does not involve K(ATP)-channel activation

This study tests the hypothesis that cardioprotection exerted by adenosine A2-receptor activation and neutrophil-related events involves stimulation of ATP-sensitive potassium (K(ATP)) channels on neutrophils during reperfusion. The adenosine A2 agonist CGS-21680 (CGS) inhibited superoxide radical generation from isolated rabbit polymorphonuclear neutrophils (PMNs) in a dose-dependent manner from 17.7 +/- 2.1 to 7.4 +/- 1.3 nmol/5 x 10(6) PMNs (P < 0.05). Pinacidil, a K(ATP)-channel opener, partially inhibited superoxide radical production, which was completely reversed by glibenclamide (Glib). Incremental doses of Glib in combination with CGS (1 microM) did not alter CGS-induced inhibition of superoxide radical generation. CGS significantly reduced PMN adherence to the endothelial surface of aortic segments in a dose-dependent manner from 189 +/- 8 to 50 +/- 6 PMNs/mm2 (P < 0.05), which was also not altered by incremental doses of Glib. Infusion of CGS (0.025 mg/kg) before reperfusion reduced infarct size from 29 +/- 2% in the Vehicle group to 15 +/- 1% in rabbits undergoing 30 min of ischemia and 120 min of reperfusion (P < 0.05). Glib (0.3 mg/kg) did not change the infarct size (28 +/- 2%) vs. the Vehicle group and did not attenuate infarct size reduction by CGS (16 +/- 1%). Glib did not change blood glucose levels. Cardiac myeloperoxidase activity was decreased in the ischemic tissue of the CGS group (0.15 +/- 0.03 U/100 mg tissue) compared with the Vehicle group (0.37 +/- 0.05 U/100 mg tissue; P < 0.05). We conclude that adenosine A2 activation before reperfusion partially reduces infarct size by inhibiting neutrophil activity and that this effect does not involve K(ATP)-channel stimulation.

Intracoronary magnesium is not protective against acute reperfusion injury in the regional ischaemic-reperfused dog heart

Intravenous magnesium lowers mortality in patients with suspected myocardial infarction. We tested the hypothesis that the protective effect may be due to a direct, local influence of magnesium on myocardial reperfusion injury in a dog model of ischaemia/reperfusion. Ten anaesthetized open chest dogs underwent 1 h of left anterior descending artery (LAD) occlusion and 6 h of reperfusion. The animals received intracoronary (i.c.) magnesium aspartate (Mg, n = 5) or vehicle infusion (n = 5) for the first hour of reperfusion. Mg infusion was adapted to actual LAD flow (ultrasonic flow probe) to increase regional plasma concentration by 4 mmol L-1. Regional myocardial function was measured as percent systolic wall thickening (sWTh, sonomicrometry). Intracoronary Mg increased LAD flow during application (at 15 min reperfusion; Mg, 194 +/- 44 (mean +/- SD); control, 116 +/- 41 mL min-1 100 g-1, P < 0.01). sWTh decreased during coronary occlusion from 14.3 +/- 7.1% to -4.7 +/- 2.7% in the control group and from 14.8 +/- 2.5% to -4.1 +/- 3.1% in the Mg group. Throughout the reperfusion period wall function remained depressed in both groups to a similar degree (control, -3.5 +/- 1.8%; Mg, -3.0 +/- 1.9% at 6 h reperfusion). Global haemodynamics were not different. Infarct size after 6 h reperfusion (TTC staining) was similar in both groups (Mg, 20.6 +/- 5.0; control, 24.4 +/- 8.7% of area at risk). Regional magnesium application (i.c.) to post-ischaemic reperfused myocardium had no influence on infarct size or post-ischaemic regional wall function in this model.(ABSTRACT TRUNCATED AT 250 WORDS)

Harnessing nature's own cardiac defense mechanism with acadesine, an adenosine regulating agent: importance of the endothelium

Although the effects of adenosine on the heart, including the clinical suppression of cardiac arrhythmias, have been recognized for more than half a century, it is only in the last decade that the therapeutic potential of adenosine has been recognized. Research related to the clinical application of adenosine has concentrated on two areas. The first came directly from early observations about the use of adenosine in treating cardiac arrhythmias, in particular supraventricular tachycardias. The second relates to the use of adenosine to protect the heart from the deleterious consequences of myocardial ischemia and reperfusion. This review will focus on the latter cardioprotective properties of adenosine, particularly those shown by a novel group of drugs termed adenosine regulating agents, the prototype of which is acadesine (Protara).