Adenosine A(2A) and A(2B) receptors in cultured human and porcine coronary artery endothelial cells

CD39 in the development and progression of pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a devastating progressive disease characterised by pulmonary arterial vasoconstriction and vascular remodelling. Endothelial dysfunction has emerged as a contributing factor in the development of PAH. However, despite progress in the understanding of the pathophysiology of this disease, current therapies fail to impact upon long-term outcomes which remain poor in most patients. Recent observations have suggested the disturbances in the balance between ATP and adenosine may be integral to the vascular remodelling seen in PAH. CD39 is an enzyme important in regulating these nucleos(t)ides which may also provide a novel pathway to target for future therapies. This review summarises the role of adenosine signalling in the development and progression of PAH and highlights the therapeutic potential of CD39 for treatment of PAH.

Crosstalk between adenosine receptors and CYP450-derived oxylipins in the modulation of cardiovascular, including coronary reactive hyperemic response

Adenosine is a ubiquitous endogenous nucleoside or autacoid that affects the cardiovascular system through the activation of four G-protein coupled receptors: adenosine A₁ receptor (A₁AR), adenosine A_2A receptor (A_2AAR), adenosine A_2B receptor (A_2BAR), and adenosine A₃ receptor (A₃AR). With the rapid generation of this nucleoside from cellular metabolism and the widespread distribution of its four G-protein coupled receptors in almost all organs and tissues of the body, this autacoid induces multiple physiological as well as pathological effects, not only regulating the cardiovascular system but also the central nervous system, peripheral vascular system, and immune system. Mounting evidence shows the role of CYP450-enzymes in cardiovascular physiology and pathology, and the genetic polymorphisms in CYP450s can increase susceptibility to cardiovascular diseases (CVDs). One of the most important physiological roles of CYP450-epoxygenases (CYP450-2C & CYP2J2) is the metabolism of arachidonic acid (AA) and linoleic acid (LA) into epoxyeicosatrienoic acids (EETs) and epoxyoctadecaenoic acid (EpOMEs) which generally involve in vasodilation. Like an increase in coronary reactive hyperemia (CRH), an increase in anti-inflammation, and cardioprotective effects. Moreover, the genetic polymorphisms in CYP450-epoxygenases will change the beneficial cardiovascular effects of metabolites or oxylipins into detrimental effects. The soluble epoxide hydrolase (sEH) is another crucial enzyme ubiquitously expressed in all living organisms and almost all organs and tissues. However, in contrast to CYP450-epoxygenases, sEH converts EETs into dihydroxyeicosatrienoic acid (DHETs), EpOMEs into dihydroxyoctadecaenoic acid (DiHOMEs), and others and reverses the beneficial effects of epoxy-fatty acids leading to vasoconstriction, reducing CRH, increase in pro-inflammation, increase in pro-thrombotic and become less cardioprotective. Therefore, polymorphisms in the sEH gene (Ephx2) cause the enzyme to become overactive, making it more vulnerable to CVDs, including hypertension. Besides the sEH, ω-hydroxylases (CYP450-4A11 & CYP450-4F2) derived metabolites from AA, ω terminal-hydroxyeicosatetraenoic acids (19-, 20-HETE), lipoxygenase-derived mid-chain hydroxyeicosatetraenoic acids (5-, 11-, 12-, 15-HETEs), and the cyclooxygenase-derived prostanoids (prostaglandins: PGD₂, PGF_2α; thromboxane: Txs, oxylipins) are involved in vasoconstriction, hypertension, reduction in CRH, pro-inflammation and cardiac toxicity. Interestingly, the interactions of adenosine receptors (A_2AAR, A₁AR) with CYP450-epoxygenases, ω-hydroxylases, sEH, and their derived metabolites or oxygenated polyunsaturated fatty acids (PUFAs or oxylipins) is shown in the regulation of the cardiovascular functions. In addition, much evidence demonstrates polymorphisms in CYP450-epoxygenases, ω-hydroxylases, and sEH genes (Ephx2) and adenosine receptor genes (ADORA1 & ADORA2) in the human population with the susceptibility to CVDs, including hypertension. CVDs are the number one cause of death globally, coronary artery disease (CAD) was the leading cause of death in the US in 2019, and hypertension is one of the most potent causes of CVDs. This review summarizes the articles related to the crosstalk between adenosine receptors and CYP450-derived oxylipins in vascular, including the CRH response in regular salt-diet fed and high salt-diet fed mice with the correlation of heart perfusate/plasma oxylipins. By using A_2AAR^-/-, A₁AR^-/-, eNOS^-/-, sEH^-/- or Ephx2^-/-, vascular sEH-overexpressed (Tie2-sEH Tr), vascular CYP2J2-overexpressed (Tie2-CYP2J2 Tr), and wild-type (WT) mice. This review article also summarizes the role of pro-and anti-inflammatory oxylipins in cardiovascular function/dysfunction in mice and humans. Therefore, more studies are needed better to understand the crosstalk between the adenosine receptors and eicosanoids to develop diagnostic and therapeutic tools by using plasma oxylipins profiles in CVDs, including hypertensive cases in the future.

Polydeoxyribonucleotide: A Promising Biological Platform to Accelerate Impaired Skin Wound Healing

The normal wound healing process is characterized by a complex, highly integrated cascade of events, requiring the interactions of many cell types, including inflammatory cells, fibroblasts, keratinocytes and endothelial cells, as well as the involvement of growth factors and enzymes. However, several diseases such as diabetes, thermal injury and ischemia could lead to an impaired wound healing process characterized by wound hypoxia, high levels of oxygen radicals, reduced angiogenesis, decreased collagen synthesis and organization. Polydeoxyribonucleotide (PDRN) has been used to improve wound healing through local and systemic administration thanks to its ability to promote cell migration and growth, angiogenesis, and to reduce inflammation on impaired wound healing models in vitro, in vivo and clinical studies. In light of all these observations, the aim of this review is to provide a full overview of PDRN applications on skin regeneration. We reviewed papers published in the last 25 years on PubMed, inserting “polydeoxyribonucleotide and wound healing” as the main search term. All data obtained proved the ability of PDRN in promoting physiological tissue repair through adenosine A_2A receptor activation and salvage pathway suggesting that PDRN has proven encouraging results in terms of healing time, wound regeneration and absence of side effects.

Adenosine and adenosine receptor-mediated action in coronary microcirculation

Adenosine is an ubiquitous extracellular signaling molecule and plays a fundamental role in the regulation of coronary microcirculation through activation of adenosine receptors (ARs). Adenosine is regulated by various enzymes and nucleoside transporters for its balance between intra- and extracellular compartments. Adenosine-mediated coronary microvascular tone and reactive hyperemia are through receptors mainly involving A_2AR activation on both endothelial and smooth muscle cells, but also involving interaction among other ARs. Activation of ARs further stimulates downstream targets of H₂O₂, K_ATP, K_V and K_Ca2+ channels leading to coronary vasodilation. An altered adenosine-ARs signaling in coronary microcirculation has been observed in several cardiovascular diseases including hypertension, diabetes, atherosclerosis and ischemic heart disease. Adenosine as a metabolite and its receptors have been studied for its both therapeutic and diagnostic abilities. The present review summarizes important aspects of adenosine metabolism and AR-mediated actions in the coronary microcirculation.

Advantages in Wound Healing Process in Female Mice Require Upregulation A2A-Mediated Angiogenesis under the Stimulation of 17β-Estradiol

Estrogenic steroids and adenosine A_2A receptors promote the wound healing and angiogenesis processes. However, so far, it is unclear whether estrogen may regulate the expression and pro-angiogenic activity of A_2A receptors. Using in vivo analyses, we showed that female wild type (WT) mice have a more rapid wound healing process than female or male A_2A-deficient mice (A_2AKO) mice. We also found that pulmonary endothelial cells (mPEC) isolated from female WT mice showed higher expression of A_2A receptor than mPEC from male WT mice. mPEC from female WT mice were more sensitive to A_2A-mediated pro-angiogenic response, suggesting an ER and A_2A crosstalk, which was confirmed using cells isolated from A_2AKO. In those female cells, 17β-estradiol potentiated A_2A-mediated cell proliferation, an effect that was inhibited by selective antagonists of estrogen receptors (ER), ERα, and ERβ. Therefore, estrogen regulates the expression and/or pro-angiogenic activity of A_2A adenosine receptors, likely involving activation of ERα and ERβ receptors. Sexual dimorphism in wound healing observed in the A_2AKO mice process reinforces the functional crosstalk between ER and A_2A receptors.

Ranolazine may exert its beneficial effects by increasing myocardial adenosine levels

We hypothesized that ranolazine-induced adenosine release is responsible for its beneficial effects in ischemic heart disease. Sixteen open-chest anesthetized dogs with noncritical coronary stenosis were studied at rest, during dobutamine stress, and during dobutamine stress with ranolazine. Six additional dogs without stenosis were studied only at rest. Regional myocardial function and perfusion were assessed. Coronary venous blood was drawn. Murine endothelial cells and cardiomyocytes were incubated with ranolazine and adenosine metabolic enzyme inhibitors, and adenosine levels were measured. Cardiomyocytes were also exposed to dobutamine and dobutamine with ranolazine. Modeling was employed to determine whether ranolazine can bind to an enzyme that alters adenosine stores. Ranolazine was associated with increased adenosine levels in the absence (21.7 ± 3.0 vs. 9.4 ± 2.1 ng/mL, P < 0.05) and presence of ischemia (43.1 ± 13.2 vs. 23.4 ± 5.3 ng/mL, P < 0.05). Left ventricular end-systolic wall stress decreased (49.85 ± 4.68 vs. 57.42 ± 3.73 dyn/cm², P < 0.05) and endocardial-to-epicardial myocardial blood flow ratio tended to normalize (0.89 ± 0.08 vs. 0.76 ± 0.10, P = nonsignificant). Adenosine levels increased in cardiac endothelial cells and cardiomyocytes when incubated with ranolazine that was reversed when cytosolic-5'-nucleotidase (cN-II) was inhibited. Point mutation of cN-II aborted an increase in its specific activity by ranolazine. Similarly, adenosine levels did not increase when cardiomyocytes were incubated with dobutamine. Modeling demonstrated plausible binding of ranolazine to cN-II with a docking energy of -11.7 kcal/mol. We conclude that the anti-adrenergic and cardioprotective effects of ranolazine-induced increase in tissue adenosine levels, likely mediated by increasing cN-II activity, may contribute to its beneficial effects in ischemic heart disease.NEW & NOTEWORTHY Ranolazine is a drug used for treatment of angina pectoris in patients with ischemic heart disease. We discovered a novel mechanism by which this drug may exhibit its beneficial effects. It increases coronary venous levels of adenosine both at rest and during dobutamine-induced myocardial ischemia. Ranolazine also increases adenosine levels in endothelial cells and cardiomyocytes in vitro, by principally increasing activity of the enzyme cytosolic-5'-nucleotidase. Adenosine has well-known myocardial protective and anti-adrenergic properties that may explain, in part, ranolazine's beneficial effect in ischemic heart disease.

Ethanol enhances endothelial ionic currents and nitric oxide release via intermediate-conductance calcium-activated potassium channel

AIMS

Ethanol is known to induce NO release and coronary vasorelaxation. Evidence suggests that K⁺ channels, especially a Ca²⁺-activated K⁺ channel (K_Ca), may regulate endothelial NO production. We aimed to investigate the ethanol effect on K⁺ currents in human coronary artery endothelial cells (HCAECs), identify the K⁺ channel type/subtype and signaling pathway involved, and demonstrate the relevance to ethanol-induced NO release.

MAIN METHODS

Ionic currents of cultured HCAECs were studied using whole-cell patch clamp technique. NO production were measured using the fluorescent probe, 2,3-diaminonaphthalene.

KEY FINDINGS

We found that ethanol significantly potentiated HCAEC current (maximal increase to 155.68 ± 18.93%, 20 mM ethanol, +80 mV; mean ± SEM, n = 9). Ethanol-induced current was significantly inhibited by blockers of IK_Ca or SK_Ca (intermediate- or small-conductance K_Ca), but not by blocking other K⁺ channels. When other known HCAEC channels were inhibited except IK_Ca, 20 mM ethanol significantly increased IK_Ca current to 198 ± 25.11% (n = 6), but it could not enhance SK_Ca current that was similarly isolated. Moreover, ethanol-induced NO release was prevented by blocking IK_Ca channel, adenosine A_2A receptor (A_2AR), G_s protein, or protein kinase A (PKA).

SIGNIFICANCE

This study was the first to demonstrate that acute ethanol exposure could activate endothelial IK_Ca channel, via A_2AR-G_s-PKA signaling, leading to increased whole-cell current and NO release, which could be an important mechanism underlying ethanol-induced NO release and vasodilation.

Multiple adenosine receptor subtypes stimulate wound healing in human EA.hy926 endothelial cells

Wound healing is an important outcome of tissue damage and can be stimulated by adenosine released from cells during events such as tissue injury, ischaemia or tumour growth. The aim of this research was to determine the potency and efficacy of adenosine A₁, A_2A and A_2B receptor agonists on the rate of wound healing and cell proliferation in human EA.hy926 endothelial cells. Real-time PCR data showed that only adenosine A₁, A_2A and A_2B receptor mRNA were expressed in this cell line. All three adenosine receptor agonists, CPA, CGS21680 and NECA, significantly increased the rate of wound healing in human EAhy926 endothelial cells with the following order of potency CGS21680>CPA>NECA and efficacy CPA>NECA>CGS21680. The selective adenosine A₁, A_2A and A_2B receptor antagonists, DPCPX, ZM241385 and MRS1754 (all at 10 nM), reversed the effects of their respective agonists. EAhy926 endothelial cell proliferation was also significantly increased with the adenosine A₁ and A_2B receptor agonists, CPA and NECA. Western blot analysis demonstrated that adenosine A_2A and A₁ receptor protein levels were highly expressed compared with the adenosine A_2B receptors in the EAhy926 endothelial cell lines. While all three adenosine A₁, A_2A and A_2B receptor subtypes contribute to cell proliferation and wound healing in human EAhy926 endothelial cells, treatments selectively targeting receptor subtypes may further enhance wound healing.

Peripheral Microvascular Function Reflects Coronary Vascular Function

Objectives- Coronary endothelial dysfunction is a precursor of atherosclerosis and adverse outcomes. Whether endothelial dysfunction is a localized or generalized phenomenon in humans remains uncertain. We simultaneously measured femoral and coronary vascular function with the hypothesis that peripheral vascular endothelial function will be reflective of coronary endothelial function. Approach and Results- Eighty-five subjects underwent coronary angiography for evaluation of chest pain or abnormal stress tests. Endothelium-dependent and -independent vascular function were measured using intracoronary and intrafemoral infusions of acetylcholine and sodium nitroprusside, respectively. Coronary flow reserve was assessed using intracoronary adenosine infusion. Flow velocity was measured in each circulation using a Doppler wire (FloWire, EndoSonics). Coronary vascular resistance and femoral vascular resistance were calculated as mean arterial pressure (mm Hg)/coronary blood flow (mL/min) and mean arterial pressure (mm Hg)/femoral average peak velocity (cm/s), respectively. Mean age was 53±11 years, 37% were female, 44% had hypertension, 12% had diabetes mellitus, and 38% had obstructive coronary artery disease. There was a correlation between the change in femoral vascular resistance with acetylcholine and acetylcholine-mediated changes in both the coronary vascular resistance ( r=0.27; P=0.014) and in the epicardial coronary artery diameter ( r=-0.25; P=0.021), indicating that subjects with normal endothelial function in the femoral circulation had normal endothelial function in the coronary epicardial and microcirculation and vice versa. The coronary vasodilator response to adenosine also correlated with the femoral vasodilatation with acetylcholine ( r=0.4; P=0.0002). There was no correlation between the coronary and femoral responses to sodium nitroprusside. Conclusions- Endothelial functional changes in the peripheral and coronary circulations were modestly correlated. Thus, peripheral microvascular endothelial function reflects endothelium-dependent coronary epicardial and microvascular function and the coronary flow reserve. Visual Overview- An online visual overview is available for this article.

Adenosine as a Marker and Mediator of Cardiovascular Homeostasis: A Translational Perspective

Adenosine, a purine nucleoside, is produced broadly and implicated in the homeostasis of many cells and tissues. It signals predominantly via 4 purinergic adenosine receptors (ADORs) - ADORA1, ADORA2A, ADORA2B and ADOosine signaling, both through design as specific ADOR agonists and antagonists and as offtarget effects of existing anti-platelet medications. Despite this, adenosine has yet to be firmly established as either a therapeutic or a prognostic tool in clinical medicine to date. Herein, we provide a bench-to-bedside review of adenosine biology, highlighting the key considerations for further translational development of this proRA3 in addition to non-ADOR mediated effects. Through these signaling mechanisms, adenosine exerts effects on numerous cell types crucial to maintaining vascular homeostasis, especially following vascular injury. Both in vitro and in vivo models have provided considerable insights into adenosine signaling and identified targets for therapeutic intervention. Numerous pharmacologic agents have been developed that modulate adenmising molecule.

Enhanced A1 adenosine receptor-induced vascular contractions in mesenteric artery and aorta of in L-NAME mouse model of hypertension

L-NAME-induced hypertension is commonly used to study endothelial dysfunction and related vascular effects. It has been reported that genetic deletion of A₁ adenosine receptor (AR) reduces blood pressure (BP) increases in mice and thus, suggesting the involvement of A₁AR. Thus, we sought to determine whether A₁AR-induced vascular responses were altered in this mouse model of hypertension. L-NAME (1 mg/ml) was given in the drinking water for 28 days to mice. The BP was monitored using non-invasive tail-cuff system. Muscle tension studies were performed using DMT for mesenteric arteries (MAs) and organ bath for aorta. Protein expression was analyzed by western blot. Significantly, higher systolic and mean arterial blood pressure was noted in L-NAME mice. In MAs, higher 2-Chloro-N⁶-cyclopentyladenosine (CCPA, selective A₁AR agonist) induced contractions in hypertensive mice were observed. This enhanced contraction was inhibited by HET0016 (Cytochrome 450 4A inhibitor, 10 µM, 15 min). Contrary, 5'-(N-Ethylcarboxamido) adenosine (NECA, non-selective AR agonist) induced vascular responses were comparable in both groups. Pinacidil (K_ATP channel opener) induced relaxation was significantly increased in hypertensive mice. In aorta, CCPA-induced contractions were enhanced and inhibited by HET0016 in hypertensive mice. Notably, NECA-induced contractions in aorta were enhanced in hypertensive mice. Higher expressions of A₁AR and Cyp4A were noted in MAs of hypertensive mice. In addition, in aorta, higher A₁AR and comparable Cyp4A levels were observed in hypertensive mice. A₁AR-induced vascular contractions were enhanced in hypertensive mice aorta and MAs. Cyp4A plays a role in altered vascular responses in MAs.

Long-term consequences of disrupting adenosine signaling during embryonic development

There is growing evidence that disruption in the prenatal environment can have long-lasting effects on an individual's health in adulthood. Research on the fetal programming of adult diseases, including cardiovascular disease, focuses on epi-mutations, which alter the normal pattern of epigenetic factors such as DNA methylation, miRNA expression, or chromatin modification, rather than traditional genetic alteration. Thus, understanding how in utero chemical exposures alter epigenetics and lead to adult disease is of considerable public health concern. Few signaling molecules have the potential to influence the developing mammal as the nucleoside adenosine. Adenosine levels increase rapidly with tissue hypoxia and inflammation. Adenosine antagonists including the methlyxanthines caffeine and theophylline are widely consumed during pregnancy. The receptors that transduce adenosine action are the A1, A2a, A2b, and A3 adenosine receptors (ARs). We examined the long-term effects of in utero disruption of adenosine signaling on cardiac gene expression, morphology, and function in adult offspring. One substance that fetuses are frequently exposed to is caffeine, which is a non-selective adenosine receptor antagonist. Over the past several years, we examined the role of adenosine signaling during embryogenesis and cardiac development. We discovered that in utero alteration in adenosine action leads to adverse effects on embryonic and adult murine hearts. We find that cardiac A1ARs protect the embryo from in utero hypoxic stress, a condition that causes an increase in adenosine levels. After birth in mice, we observed that in utero caffeine exposure leads to abnormal cardiac function and morphology in adults, including an impaired response to β-adrenergic stimulation. Recently, we observed that in utero caffeine exposure induces transgenerational effects on cardiac morphology, function, and gene expression. Our findings indicate that the effects of altered adenosine signaling are dependent on signaling through the A1ARs and timing of disruption. In addition, the long-term effects of altered adenosine signaling appear to be mediated by alterations in DNA methylation, an epigenetic process critical for normal development.

New Molecular Insights of Insulin in Diabetic Cardiomyopathy

Type 2 diabetes mellitus (T2DM) is a highly prevalent disease worldwide. Cardiovascular disorders generated as a consequence of T2DM are a major cause of death related to this disease. Diabetic cardiomyopathy (DCM) is characterized by the morphological, functional and metabolic changes in the heart produced as a complication of T2DM. This cardiac disorder is characterized by constant high blood glucose and lipids levels which eventually generate oxidative stress, defective calcium handling, altered mitochondrial function, inflammation and fibrosis. In this context, insulin is of paramount importance for cardiac contractility, growth and metabolism and therefore, an impaired insulin signaling plays a critical role in the DCM development. However, the exact pathophysiological mechanisms leading to DCM are still a matter of study. Despite the numerous questions raised in the study of DCM, there have also been important findings, such as the role of micro-RNAs (miRNAs), which can not only have the potential of being important biomarkers, but also therapeutic targets. Furthermore, exosomes also arise as an interesting variable to consider, since they represent an important inter-cellular communication mechanism and therefore, they may explain many aspects of the pathophysiology of DCM and their study may lead to the development of therapeutic agents capable of improving insulin signaling. In addition, adenosine and adenosine receptors (ARs) may also play an important role in DCM. Moreover, the possible cross-talk between insulin and ARs may provide new strategies to reverse its defective signaling in the diabetic heart. This review focuses on DCM, the role of insulin in this pathology and the discussion of new molecular insights which may help to understand its underlying mechanisms and generate possible new therapeutic strategies.

Angiotensin II stimulation alters vasomotor response to adenosine in mouse mesenteric artery: role for A1 and A2B adenosine receptors

BACKGROUND AND PURPOSE

Stimulation of the A1 adenosine receptor and angiotensin II receptor type-1 (AT1 receptor) causes vasoconstriction through activation of cytochrome P450 4A (CYP4A) and ERK1/2. Thus, we hypothesized that acute angiotensin II activation alters the vasomotor response induced by the non-selective adenosine receptor agonist, NECA, in mouse mesenteric arteries (MAs).

EXPERIMENTAL APPROACH

We used a Danish Myo Technology wire myograph to measure muscle tension in isolated MAs from wild type (WT), A1 receptor and A2B receptor knockout (KO) mice. Western blots were performed to determine the expression of AT1 receptors and CYP4A.

KEY RESULTS

Acute exposure (15 min) to angiotensin II attenuated the NECA-dependent vasodilatation and enhanced vasoconstriction. This vasoconstrictor effect of angiotensin II in NECA-treated MAs was abolished in A1 receptor KO mice and in WT mice treated with the A1 receptor antagonist DPCPX, CYP4A inhibitor HET0016 and ERK1/2 inhibitor PD98059. In MAs from A2B receptor KO mice, the vasoconstrictor effect of angiotensin II on the NECA-induced response was shown to be dependent on A1 receptors. Furthermore, in A2B receptor KO mice, the expression of AT1 receptors and CYP4A was increased and the angiotensin II-induced vasoconstriction enhanced. In addition, inhibition of KATP channels with glibenclamide significantly reduced NECA-induced vasodilatation in WT mice.

CONCLUSIONS AND IMPLICATIONS

Acute angiotensin II stimulation enhanced A1 receptor-dependent vasoconstriction and inhibited A2B receptor-dependent vasodilatation, leading to a net vasoconstriction and altered vasomotor response to NECA in MAs. This interaction may be important in the regulation of BP.

The A2B adenosine receptor colocalizes with adenosine deaminase in resting parietal cells from gastric mucosa

The A2B adenosine receptor (A2BR) mediates biological responses to extracellular adenosine in a wide variety of cell types. Adenosine deaminase (ADA) can degrade adenosine and bind extracellularly to adenosine receptors. Adenosine modulates chloride secretion in gastric glands and gastric mucosa parietal cells. A close functional link between surface A2BR and ADA has been found on cells of the immune system, but whether this occurs in the gastrointestinal tract is unknown. The goal of this study was to determine whether A2BR and ADA are coexpressed at the plasma membrane of the acid-secreting gastric mucosa parietal cells. We used isolated gastric parietal cells after purification by centrifugal elutriation. The membrane fraction was obtained by sucrose gradient centrifugation. A2BR mRNA expression was analyzed by RT-PCR. The surface expression of A2BR and ADA proteins was evaluated by Western blotting, flow cytometry and confocal microscopy. Our findings demonstrate that A2BR and ADA are expressed in cell membranes isolated from gastric parietal cells. They show a high degree of colocalization that is particularly evident in the surface of contact between parietal cells. The confocal microscopy data together with flow cytometry analysis suggest a tight association between A2BR and ADA that might be specifically linked to glandular secretory function.

Adenosine signalling in diabetes mellitus--pathophysiology and therapeutic considerations

Adenosine is a key extracellular signalling molecule that regulates several aspects of tissue function by activating four G-protein-coupled receptors, A1, A2A, A2B and A1 adenosine receptors. Accumulating evidence highlights a critical role for the adenosine system in the regulation of glucose homeostasis and the pathophysiology of type 1 diabetes mellitus (T1DM) and type 2 diabetes mellitus (T2DM). Although adenosine signalling is known to affect insulin secretion, new data indicate that adenosine signalling also contributes to the regulation of β-cell homeostasis and activity by controlling the proliferation and regeneration of these cells as well as the survival of β cells in inflammatory microenvironments. Furthermore, adenosine is emerging as a major regulator of insulin responsiveness by controlling insulin signalling in adipose tissue, muscle and liver; adenosine also indirectly mediates effects on inflammatory and/or immune cells in these tissues. This Review critically discusses the role of the adenosine-adenosine receptor system in regulating both the onset and progression of T1DM and T2DM, and the potential of pharmacological manipulation of the adenosinergic system as an approach to manage T1DM, T2DM and their associated complications.

The role of HIFs in ischemia-reperfusion injury

The reduction or cessation of the blood supply to an organ results in tissue ischemia. Ischemia can cause significant tissue damage, and is observed as a result of a thrombosis, as part of a disease process, and during surgery. However, the restoration of the blood supply often causes more damage to the tissue than the ischemic episode itself. Research is therefore focused on identifying the cellular pathways involved in the protection of organs from the damage incurred by this process of ischemia reperfusion (I/R). The hypoxia-inducible factors (HIFs) are a family of heterodimeric transcription factors that are stabilized during ischemia. The genes that are expressed downstream of HIF activity enhance oxygen-independent ATP generation, cell survival, and angiogenesis, amongst other phenotypes. They are, therefore, important factors in the protection of tissues from I/R injury. Interestingly, a number of the mechanisms already known to induce organ protection against I/R injury, including preconditioning, postconditioning, and activation of signaling pathways such as adenosine receptor signaling, converge on the HIF system. This review describes the evidence for HIFs playing a role in I/R protection mediated by these factors, highlights areas that require further study, and discuss whether HIFs themselves are good therapeutic targets for protecting tissues from I/R injury.

The effect of PDRN, an adenosine receptor A2A agonist, on the healing of chronic diabetic foot ulcers: results of a clinical trial

CONTEXT

Foot ulcer is the principal cause of hospitalization for patients with diabetes. Polydeoxyribonucleotide (PDRN), an adenosine A2A receptor agonist, improves wound healing in diabetic mice.

OBJECTIVE

The aim of this study was to evaluate the effect of PDRN on chronic ulcer healing in patients with diabetes.

DESIGN AND SETTING

This randomized, double-blind, placebo-controlled trial, involved two medical centers in Italy.

INTERVENTION

Patients with diabetes showing hard-to-heal ulcers (Wagner grade 1 or 2) were randomly assigned to receive placebo (n = 106) or PDRN (n = 110). The treatments (PDRN and placebo) were performed for 8 weeks by intramuscular and perilesional route [corrected].

MAIN OUTCOME MEASURES

The primary outcome was complete ulcer healing. Secondary outcomes were the days needed to complete wound closure and the reepithelialization of wound surface (as percentage of the original area).

RESULTS

After 8 weeks, 91 placebo and 101 PDRN subjects completed the study. Complete healing was achieved in 18.9% [95% confidence interval (CI) 11.4-26.3] of placebo and in 37.3% (95% CI 28.2-46.3) of PDRN-treated patients (P = .0027). After 8 weeks, PDRN increased the closure of foot ulcers in diabetic subjects (hazard ratio 2.20; 95% CI 1.29-3.75; P = .004). The median time to complete wound healing was 49 days for placebo (range 28-56 d) and 30 days for PDRN-treated subjects (range 14-56 d; P = .0027). The median epithelialized area of the ulcers (expressed as percentage) was 49.3% in the placebo and 82.2% in the PDRN group (P < .001).

CONCLUSIONS

PDRN facilitates the healing of Wagner 1 or 2 diabetic foot ulcers.

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.

A1 adenosine receptor negatively modulates coronary reactive hyperemia via counteracting A2A-mediated H2O2 production and KATP opening in isolated mouse hearts

We previously demonstrated that A2A, but not A2B, adenosine receptors (ARs) mediate coronary reactive hyperemia (RH), possibly by producing H2O2 and, subsequently, opening ATP-dependent K(+) (KATP) channels in coronary smooth muscle cells. In this study, A1 AR knockout (KO), A3 AR KO, and A1 and A3 AR double-KO (A1/A3 DKO) mice were used to investigate the roles and mechanisms of A1 and A3 ARs in modulation of coronary RH. Coronary flow of isolated hearts was measured using the Langendorff system. A1 KO and A1/A3 DKO, but not A3 KO, mice showed a higher flow debt repayment [~30% more than wild-type (WT) mice, P < 0.05] following a 15-s occlusion. SCH-58261 (a selective A2A AR antagonist, 1 μM) eliminated the augmented RH, suggesting the involvement of enhanced A2A AR-mediated signaling in A1 KO mice. In isolated coronary arteries, immunohistochemistry showed an upregulation of A2A AR (1.6 ± 0.2 times that of WT mice, P < 0.05) and a higher magnitude of adenosine-induced H2O2 production in A1 KO mice (1.8 ± 0.3 times that of WT mice, P < 0.05), which was blocked by SCH-58261. Catalase (2,500 U/ml) and glibenclamide (a KATP channel blocker, 5 μM), but not N(G)-nitro-l-arginine methyl ester, also abolished the enhanced RH in A1 KO mice. Our data suggest that A1, but not A3, AR counteracts the A2A AR-mediated CF increase and that deletion of A1 AR results in upregulation of A2A AR and/or removal of the negative modulatory effect of A1 AR, thus leading to an enhanced A2A AR-mediated H2O2 production, KATP channel opening, and coronary vasodilation during RH. This is the first report implying that A1 AR has a role in coronary RH.

Interactions between A(2A) adenosine receptors, hydrogen peroxide, and KATP channels in coronary reactive hyperemia

Myocardial metabolites such as adenosine mediate reactive hyperemia, in part, by activating ATP-dependent K(+) (K(ATP)) channels in coronary smooth muscle. In this study, we investigated the role of adenosine A(2A) and A(2B) receptors and their signaling mechanisms in reactive hyperemia. We hypothesized that coronary reactive hyperemia involves A(2A) receptors, hydrogen peroxide (H(2)O(2)), and KATP channels. We used A(2A) and A(2B) knockout (KO) and A(2A/2B) double KO (DKO) mouse hearts for Langendorff experiments. Flow debt for a 15-s occlusion was repaid 128 ± 8% in hearts from wild-type (WT) mice; this was reduced in hearts from A(2A) KO and A(2A)/(2B) DKO mice (98 ± 9 and 105 ± 6%; P < 0.05), but not A(2B) KO mice (123 ± 13%). Patch-clamp experiments demonstrated that adenosine activated glibenclamide-sensitive KATP current in smooth muscle cells from WT and A(2B) KO mice (90 ± 23% of WT) but not A(2A) KO or A(2A)/A(2B) DKO mice (30 ± 4 and 35 ± 8% of WT; P < 0.05). Additionally, H(2)O(2) activated KATP current in smooth muscle cells (358 ± 99%; P < 0.05). Catalase, an enzyme that breaks down H(2)O(2), attenuated adenosine-induced coronary vasodilation, reducing the percent increase in flow from 284 ± 53 to 89 ± 13% (P < 0.05). Catalase reduced the repayment of flow debt in hearts from WT mice (84 ± 9%; P < 0.05) but had no effect on the already diminished repayment in hearts from A(2A) KO mice (98 ± 7%). Our findings suggest that adenosine A(2A) receptors are coupled to smooth muscle KATP channels in reactive hyperemia via the production of H(2)O(2) as a signaling intermediate.

Impaired A2A adenosine receptor/nitric oxide/VEGF signaling pathway in fetal endothelium during late- and early-onset preeclampsia

To investigate whether fetal endothelial cell proliferation and migration are modulated by the A2A adenosine receptor (A2AAR), nitric oxide (NO) and the vascular endothelial growth factor (VEGF) signaling pathway, we isolated human umbilical vein endothelial cells from normal pregnancy (n = 23), preterm delivery (n = 4), and late-onset (LOPE, n = 10) and early-onset preeclampsia (EOPE, n = 8). We used the non-selective adenosine receptor agonist (NECA) and the selective agonist (CGS-21680) and/or selective antagonist (ZM-241385) for A2AAR. Also, the nitric oxide synthase (NOS) inhibitor, L-NAME, was used in co-incubation with CGS-21680. Compared to normal pregnancy, EOPE exhibited low cell proliferation and migration associated with reduced expressions of A2AAR and VEGF and NO synthesis (i.e., total and phosphorylated serine(1177) endothelial NOS and nitrite formation). In contrast, LOPE exhibited the opposite behavior in all these markers compared to normal pregnancy or EOPE. Cell proliferation and migration were increased by CGS-21680 (or NECA) in all analyzed groups (EOPE>LOPE>normal pregnancy) compared to their respective basal conditions, an effect that was associated with high NO and VEGF synthesis and blocked by ZM-241385 with significantly different IC50 for each group (EOPE>LOPE>normal pregnancy). The differences seem independent of gestational age. L-NAME blocked the CGS-21680-mediated cell proliferation and migration in normal pregnancy and LOPE (IC50 = 36.2 ± 2.5 and 8.6 ± 2.2 nM, respectively) as well as the VEGF expression in normal pregnancy. Therefore, the A2AAR/NO/VEGF signaling pathway exhibits a pro-angiogenic effect in normal pregnancies and LOPE, whereas impairment in this pathway seems related to the reduced angiogenic capacity of the fetal endothelium in EOPE.

Uridine adenosine tetraphosphate is a novel vasodilator in the coronary microcirculation which acts through purinergic P1 but not P2 receptors

Uridine adenosine tetraphosphate (Up4A) has been identified as an endothelium-derived contracting factor, which acts through purinergic P2X and P2Y receptors. Since the coronary vascular actions of Up4A are unknown, we investigated the vasoactive profile of Up4A in coronary microvessels, and studied the involvement of purinergic receptor subtypes. Studies were performed in isolated porcine coronary small arteries (diameter∼250 μm), with and without endothelial denudation, mounted on a Mulvany wire myograph. Purinergic receptor expression was assessed by real-time PCR. Up4A (10(-9)-10(-5) M) failed to induce contraction at basal tone, but produced concentration-dependent vasorelaxation in precontracted microvessels. Up4A was slightly less potent than adenosine, ATP, and ADP in producing vasorelaxation, but significantly more potent than UTP and UDP. mRNA expression of P2X(4), P2Y(1), P2Y(2), P2Y(4), P2Y(6) and A(2A), but not P2X(1), receptors was observed. Up4A-induced vasodilation was unaffected by non-selective P2 receptor antagonist PPADS, P2X(1) antagonist MRS2159, P2Y(1) antagonist MRS2179 and P2Y(6) antagonist MRS2578, but was markedly attenuated by non-selective P1 receptor antagonist 8PT and A(2A) antagonist SCH58261. Up4A-induced vasodilation was not affected by ectonucleotidase inhibitor ARL67156, suggesting that A(2A) stimulation was not the result of Up4A breakdown to adenosine. Up4A-induced vasodilation was blunted in denuded vessels; additional A(2A) receptor blockade further attenuated Up4A-induced vasodilation, suggesting that A(2A) receptor-mediated vasodilation is only partly endothelium-dependent. In conclusion, Up4A exerts a vasodilator rather than a vasoconstrictor influence in coronary microvessels, which is mediated via A(2A) receptors and is partly endothelium-dependent.

Regulation of cardiovascular development by adenosine and adenosine-mediated embryo protection

Few signaling molecules have as much potential to influence the developing mammal as the nucleoside adenosine. Adenosine levels increase rapidly with tissue hypoxia and inflammation. Adenosine antagonists include the methylxanthines caffeine and theophylline. The receptors that transduce adenosine action are the A1, A2a, A2b, and A3 adenosine receptors (A1AR, A2aAR, A2bAR, and A3AR). We examined how adenosine acts via A1ARs to influence embryo development. Transgenic mice were studied along with embryo cultures. Embryos lacking A1ARs were markedly growth retarded following intrauterine hypoxia exposure. Studies of mice selectively lacking A1AR in the heart identify the heart as a key site of adenosine's embryo-protective effects. Studies of isolated embryos showed that adenosine plays a key role in modulating embryo cardiac function, especially in the setting of hypoxia. When pregnant mice were treated during embryogenesis with the adenosine antagonist caffeine, adult mice had abnormal heart function. Adenosine acts via A1ARs to play an essential role in protecting the embryo against intrauterine stress, and adenosine antagonists, including caffeine, may be an unwelcome exposure for the embryo.

The A3 adenosine receptor as multifaceted therapeutic target: pharmacology, medicinal chemistry, and in silico approaches

Adenosine is an ubiquitous local modulator that regulates various physiological and pathological functions by stimulating four membrane receptors, namely A(1), A(2A), A(2B), and A(3). Among these G protein-coupled receptors, the A(3) subtype is found mainly in the lung, liver, heart, eyes, and brain in our body. It has been associated with cerebroprotection and cardioprotection, as well as modulation of cellular growth upon its selective activation. On the other hand, its inhibition by selective antagonists has been reported to be potentially useful in the treatment of pathological conditions including glaucoma, inflammatory diseases, and cancer. In this review, we focused on the pharmacology and the therapeutic implications of the human (h)A(3) adenosine receptor (AR), together with an overview on the progress of hA(3) AR agonists, antagonists, allosteric modulators, and radioligands, as well as on the recent advances pertaining to the computational approaches (e.g., quantitative structure-activity relationships, homology modeling, molecular docking, and molecular dynamics simulations) applied to the modeling of hA(3) AR and drug design. 

Contributions of A2A and A2B adenosine receptors in coronary flow responses in relation to the KATP channel using A2B and A2A/2B double-knockout mice

Adenosine plays a role in physiological and pathological conditions, and A(2) adenosine receptor (AR) expression is modified in many cardiovascular disorders. In this study, we elucidated the role of the A(2B)AR and its relationship to the A(2A)AR in coronary flow (CF) changes using A(2B) single-knockout (KO) and A(2A/2B) double-KO (DKO) mice in a Langendorff setup. We used two approaches: 1) selective and nonselective AR agonists and antagonists and 2) A(2A)KO and A(2B)KO and A(2A/2B)DKO mice. BAY 60-6583 (a selective A(2B) agonist) had no effect on CF in A(2B)KO mice, whereas it significantly increased CF in wild-type (WT) mice (maximum of 23.3 ± 9 ml·min(-1)·g(-1)). 5'-N-ethylcarboxamido adenosine (NECA; a nonselective AR agonist) increased CF in A(2B)KO mice (maximum of 34.6 ± 4.7 ml·min(-1)·g(-1)) to a significantly higher degree compared with WT mice (maximum of 23.1 ± 2.1 ml·min(-1)·g(-1)). Also, CGS-21680 (a selective A(2A) agonist) increased CF in A(2B)KO mice (maximum of 29 ± 1.9 ml·min(-1)·g(-1)) to a significantly higher degree compared with WT mice (maximum of 25.1 ± 2.3 ml·min(-1)·g(-1)). SCH-58261 (an A(2A)-selective antagonist) inhibited the NECA-induced increase in CF to a significantly higher degree in A(2B)KO mice (19.3 ± 1.6 vs. 0.5 ± 0.4 ml·min(-1)·g(-1)) compared with WT mice (19 ± 3.5 vs. 3.6 ± 0.5 ml·min(-1)·g(-1)). NECA did not induce any increase in CF in A(2A/2B)DKO mice, whereas a significant increase was observed in WT mice (maximum of 23.1 ± 2.1 ml·min(-1)·g(-1)). Furthermore, the mitochondrial ATP-sensitive K(+) (K(ATP)) channel blocker 5-hydroxydecanoate had no effect on the NECA-induced increase in CF in WT mice, whereas the NECA-induced increase in CF in WT (17.6 ± 2 ml·min(-1)·g(-1)), A(2A)KO (12.5 ± 2.3 ml·min(-1)·g(-1)), and A(2B)KO (16.2 ± 0.8 ml·min(-1)·g(-1)) mice was significantly blunted by the K(ATP) channel blocker glibenclamide (to 0.7 ± 0.7, 2.3 ± 1.1, and 0.9 ± 0.4 ml·min(-1)·g(-1), respectively). Also, the CGS-21680-induced (22 ± 2.3 ml·min(-1)·g(-1)) and BAY 60-6583-induced (16.4 ± 1.60 ml·min(-1)·g(-1)) increase in CF in WT mice was significantly blunted by glibenclamide (to 1.2 ± 0.4 and 1.8 ± 1.2 ml·min(-1)·g(-1), respectively). In conclusion, this is the first evidence supporting the compensatory upregulation of A(2A)ARs in A(2B)KO mice and demonstrates that both A(2A)ARs and A(2B)ARs induce CF changes through K(ATP) channels. These results identify AR-mediated CF responses that may lead to better therapeutic approaches for the treatment of cardiovascular disorders.

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

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

Mechanisms involved in the adenosine-induced vasorelaxation to the pig prostatic small arteries

Benign prostatic hypertrophy has been related with glandular ischemia processes and adenosine is a potent vasodilator agent. This study investigates the mechanisms underlying the adenosine-induced vasorelaxation in pig prostatic small arteries. Adenosine receptors expression was determined by Western blot and immunohistochemistry, and rings were mounted in myographs for isometric force recording. A(2A) and A(3) receptor expression was observed in the arterial wall and A(2A)-immunoreactivity was identified in the adventitia-media junction and endothelium. A(1) and A(2B) receptor expression was not obtained. On noradrenaline-precontracted rings, P1 receptor agonists produced concentration-dependent relaxations with the following order of potency: 5'-N-ethylcarboxamidoadenosine (NECA) = CGS21680 > 2-Cl-IB-MECA = 2-Cl-cyclopentyladenosine = adenosine. Adenosine reuptake inhibition potentiated both NECA and adenosine relaxations. Endothelium removal and ZM241385, an A(2A) antagonist, reduced NECA relaxations that were not modified by A(1), A(2B), and A(3) receptor antagonists. Neuronal voltage-gated Ca(2+) channels and nitric oxide (NO) synthase blockade, and adenylyl cyclase activation enhanced these responses, which were reduced by protein kinase A inhibition and by blockade of the intermediate (IK(Ca))- and small (SK(Ca))-conductance Ca(2+)-activated K(+) channels. Inhibition of cyclooxygenase (COX), large-conductance Ca(2+)-activated-, ATP-dependent-, and voltage-gated-K(+) channel failed to modify these responses. These results suggest that adenosine induces endothelium-dependent relaxations in the pig prostatic arteries via A(2A) purinoceptors. The adenosine vasorelaxation, which is prejunctionally modulated, is produced via NO- and COX-independent mechanisms that involve activation of IK(Ca) and SK(Ca) channels and stimulation of adenylyl cyclase. Endothelium-derived NO playing a regulatory role under conditions in which EDHF is non-functional is also suggested. Adenosine-induced vasodilatation could be useful to prevent prostatic ischemia.

Adverse and protective influences of adenosine on the newborn and embryo: implications for preterm white matter injury and embryo protection

Few signaling molecules have the potential to influence the developing mammal as the nucleoside adenosine. Adenosine levels increase rapidly with tissue hypoxia and inflammation. Adenosine antagonists include the methylxanthines caffeine and theophylline. The receptors that transduce adenosine action are the A1, A2a, A2b, and A3 adenosine receptors (ARs). In the postnatal period, A1AR activation may contribute to white matter injury in the preterm infant by altering oligodendrocyte (OL) development. In models of perinatal brain injury, caffeine is neuroprotective against periventricular white matter injury (PWMI) and hypoxic-ischemic encephalopathy (HIE). Supporting the notion that blockade of adenosine action is of benefit in the premature infant, caffeine reduces the incidence of bronchopulmonary dysplasia and CP in clinical studies. In comparison with the adverse effects on the postnatal brain, adenosine acts via A1ARs to play an essential role in protecting the embryo from hypoxia. Embryo protective effects are blocked by caffeine, and caffeine intake during early pregnancy increases the risk of miscarriage and fetal growth retardation. Adenosine and adenosine antagonists play important modulatory roles during mammalian development. The protective and deleterious effects of adenosine depend on the time of exposure and target sites of action.

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.

Differential effects of adenosine A2a and A2b receptors on cardiac contractility

The mammalian myocardium expresses four adenosine receptor (AR) subtypes: A(1)AR, A(2a)AR, A(2b)AR, and A(3)AR. The A(1)AR is well known for its profound antiadrenergic effects, but the roles of other AR subtypes in modulating contractility remain inconclusive. Thus, the objective of this study was to determine the direct and indirect effects of A(2a)AR and A(2b)AR on cardiac contractility. Experiments were conducted in paced, constant pressure-perfused isolated hearts from wild-type (WT), A(2a)AR knockout (KO), and A(2b)AR KO mice. The A(2a)AR agonist CGS-21680 did not alter basal contractility or β-adrenergic receptor agonist isoproterenol (Iso)-mediated positive inotropic responses, and Iso-induced effects were unaltered in A(2a)AR KO hearts. However, A(2a)AR gene ablation resulted in a potentiation of the antiadrenergic effects mediated by the A(1)AR agonist 2-chloro-N-cyclopentyladenosine. The nonselective AR agonist 5'-N-ethylcarboxamido adenosine and the selective A(2b)AR agonist BAY 60-6583 induced coronary flow-independent increases in contractility, but BAY 60-6583 did not alter Iso-induced contractile responses. The A(1)AR antiadrenergic effect was not potentiated in A(2b)AR KO hearts. The expression of all four AR subtypes in the heart and ventricular myocytes was confirmed using real-time quantitative PCR. Taken together, these results indicate that A(2a)AR does not increase cardiac contractility directly but indirectly alters contractility by modulating the A(1)AR antiadrenergic effect, whereas A(2b)AR exerts direct contractile effects but does not alter β-adrenergic or A(1)AR antiadrenergic effects. These results indicate that multiple ARs differentially modulate cardiac function.

Adenosine A2A receptor is a unique angiogenic target of HIF-2alpha in pulmonary endothelial cells

Hypoxia, through the hypoxia-inducible transcription factors HIF-1alpha and HIF-2alpha (HIFs), induces angiogenesis by up-regulating a common set of angiogenic cytokines. Unlike HIF-1alpha, which regulates a unique set of genes, most genes regulated by HIF-2alpha overlap with those induced by HIF-1alpha. Thus, the unique contribution of HIF-2alpha remains largely obscure. By using adenoviral mutant HIF-1alpha and adenoviral mutant HIF-2alpha constructs, where the HIFs are transcriptionally active under normoxic conditions, we show that HIF-2alpha but not HIF-1alpha regulates adenosine A(2A) receptor in primary cultures of human lung endothelial cells. Further, siRNA knockdown of HIF-2alpha completely inhibits hypoxic induction of A(2A) receptor. Promoter studies show a 2.5-fold induction of luciferase activity with HIF-2alpha cotransfection. Analysis of the A(2A) receptor gene promoter revealed a hypoxia-responsive element in the region between -704 and -595 upstream of the transcription start site. By using a ChIP assay, we demonstrate that HIF-2alpha binding to this region is specific. In addition, we demonstrate that A(2A) receptor has angiogenic potential, as assessed by increases in cell proliferation, cell migration, and tube formation. Additional data show increased expression of A(2A) receptor in human lung tumor cancer samples relative to adjacent normal lung tissue. These data also demonstrate that A(2A) receptor is regulated by hypoxia and HIF-2alpha in human lung endothelial cells but not in mouse-derived endothelial cells.

Altered mechanism of adenosine-induced coronary arteriolar dilation in early-stage metabolic syndrome

Onset of the combined metabolic syndrome (MetS) is a complex progressive process involving numerous cardiovascular risk factors. Although patients with established MetS exhibit reduced coronary flow reserve and individual components of the MetS reduce microvascular vasodilation, little is known concerning the impact of early-stage MetS on the mechanisms of coronary flow control. Therefore, we tested the hypothesis that coronary arteriolar dilation to adenosine is attenuated in early-stage MetS by reduced A2 receptor function and diminished K+ channel involvement. Pigs were fed control or high-fat/cholesterol diet for 9 weeks to induce early-stage MetS. Coronary atheroma was determined in vivo with intravascular ultrasound. In vivo coronary dilation was determined by intracoronary adenosine infusion. Further, apical coronary arterioles were isolated, cannulated and pressurized to 60 cmH2O for in vitro pharmacologic assessment of adenosine dilation. Coronary atheroma was not different between groups, indicating early-stage MetS. Coronary arteriolar dilation to adenosine (in vivo) and 2-chloroadenosine (2-CAD; in vitro) was similar between groups. In control arterioles, 2-CAD-mediated dilation was reduced only by selective A(2A) receptor inhibition, whereas only dual A(2A/2B) inhibition reduced this response in MetS arterioles. Arteriolar A(2B), but not A(2A), receptor protein expression was reduced by MetS. Blockade of voltage-dependent K+ (K(v)) channels reduced arteriolar sensitivity to 2-CAD in both groups, whereas ATP-sensitive K+ (K(ATP)) channel inhibition reduced sensitivity only in control arterioles. Our data indicate that the mechanisms mediating coronary arteriolar dilation to adenosine are altered in early-stage MetS prior to overt decrements in coronary vasodilator reserve.

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.

Wound healing essentials: let there be oxygen

The state of wound oxygenation is a key determinant of healing outcomes. From a diagnostic standpoint, measurements of wound oxygenation are commonly used to guide treatment planning such as amputation decision. In preventive applications, optimizing wound perfusion and providing supplemental O(2) in the perioperative period reduces the incidence of postoperative infections. Correction of wound pO(2) may, by itself, trigger some healing responses. Importantly, approaches to correct wound pO(2) favorably influence outcomes of other therapies such as responsiveness to growth factors and acceptance of grafts. Chronic ischemic wounds are essentially hypoxic. Primarily based on the tumor literature, hypoxia is generally viewed as being angiogenic. This is true with the condition that hypoxia be acute and mild to modest in magnitude. Extreme near-anoxic hypoxia, as commonly noted in problem wounds, is not compatible with tissue repair. Adequate wound tissue oxygenation is required but may not be sufficient to favorably influence healing outcomes. Success in wound care may be improved by a personalized health care approach. The key lies in our ability to specifically identify the key limitations of a given wound and in developing a multifaceted strategy to specifically address those limitations. In considering approaches to oxygenate the wound tissue it is important to recognize that both too little as well as too much may impede the healing process. Oxygen dosing based on the specific need of a wound therefore seems prudent. Therapeutic approaches targeting the oxygen sensing and redox signaling pathways are promising.

Evidence for the involvement of nitric oxide in A2B receptor-mediated vasorelaxation of mouse aorta

We have investigated the role of adenosine and its analogs on vasorelaxation of mouse aorta in intact endothelium with rank order of potency as follows: 5'-N-ethylcarboxamidoadenosine (NECA) > 2-chloroadenosine > adenosine >> CGS-21680, which is consistent with the profile of A(2B)-adenosine receptor (A(2B)AR). In endothelium-intact tissues, acetylcholine produced relaxation ranging from 65 to 80% in phenylephrine (PE, 10(-7) M)-precontracted mouse aorta, whereas no relaxation was observed in endothelium-denuded tissues. The A(2B)AR antagonist alloxazine (10(-5) M) shifted concentration-response curve for NECA (EC(50) = 0.005 x 10(-5) M) to the right with an EC(50) of 2.8 x 10(-5) M, demonstrating that this relaxation is partially dependent on functional endothelium mediated predominantly via A(2B)AR in this tissue. This conclusion was further supported by the following findings: 1) in the endothelium-intact mouse aorta, the EC(50) values for NECA and adenosine were found to be 0.05 and 1.99 x 10(-4) M, respectively; however, in denuded endothelium, these values were 0.098 and 3.55 x 10(-4) M, respectively; 2) NECA-induced relaxation was significantly blocked by N(G)-nitro-l-arginine methyl ester (l-NAME; 10(-4) M) in endothelium-intact tissues, which was reversed by pretreatment with l-arginine (10(-4) M), whereas no significant inhibition was found in endothelium-denuded tissues; 3) total nitrites and nitrates (NOx) in intact endothelium with l-NAME (10(-4) M) alone and in combination with l-arginine were 59% (P < 0.05) and 96%, respectively, in comparison with control (PE + NECA); and 4) endothelial nitric oxide synthase gene expression was found to be 67% (P < 0.05) less in endothelium-denuded as opposed to endothelium-intact mouse aorta. Thus these data demonstrate that adenosine-mediated vasorelaxation is partially dependent on A(2B)AR in mouse aorta.

Adenosine A2A receptor modulation of juvenile female rat skeletal muscle microvessel permeability

Little is known of the regulation of skeletal muscle microvascular exchange under resting or stimulating conditions. Adenosine (ADO) levels in skeletal muscle increase during physiological (exercise) and pathological (hypoxia, inflammation, and ischemia) conditions. Later stages of these pathologies are characterized by the loss of vascular barrier integrity. This study focused on determining which ADO receptor mediates the robust reduction in microvessel permeability to rat serum albumin (P(s)(RSA)) observed in juvenile female rats. In microvessels isolated from abdominal skeletal muscle, ADO suffusion induced a concentration-dependent reduction in arteriolar [log(IC(50)) = -9.8 +/- 0.2 M] and venular [log(IC(50)) = -8.4 +/- 0.2 M] P(s)(RSA). RT-PCR and immunoblot analysis demonstrated mRNA and protein expression of ADO A(1), A(2A), A(2B), and A(3) receptors in both vessel types, and immunofluorescence assay revealed expression of the four subtype receptors in the microvascular walls (endothelium and smooth muscle). P(s)(RSA) responses of arterioles and venules to ADO were blocked by 8-(p-sulphophenyl)theophylline, a nonselective A(1) and A(2) antagonist. An A(2A) agonist, CGS21680, was more potent than the A(1) agonist, cyclopentyladenosine, or the most-selective A(2B) agonist, 5'-(N-ethylcarboxamido)adenosine. The ability of CGS21680 or ADO to reduce P(s)(RSA) was abolished by the A(2A) antagonist, ZM241385. An adenylyl cyclase inhibitor, SQ22536, blocked the permeability response to ADO. In aggregate, these results demonstrate that, in juvenile females (before the production of the reproductive hormones), ADO enhances skeletal muscle arteriole and venule barrier function predominantly via A(2A) receptors using activation of adenylyl cyclase-signaling mechanisms.

Isolation and characterization of coronary endothelial and smooth muscle cells from A1 adenosine receptor-knockout mice

Mice have been used widely in in vivo and in vitro cardiovascular research. The availability of knockout mice provides further clues to the physiological significance of specific receptor subtypes. Adenosine A(1) receptor (A(1)AR)-knockout (A(1)KO) mice and their wild-type (A(1)WT) controls were employed in this investigation. The heart and aortic arch were carefully removed and retroinfused with enzyme solution (1 mg/ml collagenase type I, 0.5 mg/ml soybean trypsin inhibitor, 3% BSA, and 2% antibiotics) through the aortic arch. The efflux was collected at 30-, 60-, and 90-min intervals. The cells were centrifuged, and the pellets were mixed with medium [medium 199-F-12 medium with 10% FBS and 2% antibiotics (for endothelial cells) and advanced DMEM with 10% FBS, 10% mouse serum, 2% GlutaMax, and 2% antibiotics (for smooth muscle cells)] and plated. Endothelial cells were characterized by a cobblestone appearance and positive staining with acetylated LDL labeled with 1,1'-dioctadecyl-3,3,3',3-tetramethylindocarbocyanine perchlorate. Smooth muscle cells were characterized by positive staining of smooth muscle alpha-actin and smooth muscle myosin heavy chain. Homogeneity of the smooth muscle cells was approximately 91%. Western blot analysis showed expression of smoothelin in the cells from passages 3, 7, and 11 in A(1)WT and A(1)KO mice. Furthermore, the A(1)AR was characterized by Western blot analysis using an A(1)AR-specific antibody. To our knowledge, this is the first isolation and successful characterization of smooth muscle cells from the mouse coronary system.

Differential coronary microvascular exchange responses to adenosine: roles of receptor and microvessel subtypes

OBJECTIVE

To assess the role of adenosine receptors in the regulation of coronary microvascular permeability to porcine serum albumin (P(s)(PSA)).

METHODS

Solute flux was measured in single perfused arterioles and venules isolated from pig hearts using fluorescent dye-labeled probes by microspectro-fluorometry. Messenger RNA, protein, and cellular distribution of adenosine receptors in arterioles and venules were analyzed by RT-PCR, immunoblot, and immunofluorescence.

RESULTS

Control venule P(s)(PSA) (10.7 +/- 4.8 x 10(- 7) cm x s(- 1)) was greater than that of arterioles (6.4+/- 2.8 x 10(-7) cm . s(-1); p < .05). Arteriolar P(s)(PSA) decreased (p < .05) with adenosine suffusion over the range from 10(- 8) to 10(-5) M, while venular P(s)(PSA) did not change. The nonselective A(1) and A(2) receptor antagonist, 8-(p-sulfophenyl) theophylline, blocked the adenosine-induced decrease in arteriolar P(s)(PSA). Messenger RNA for adenosine A(1), A(2A), A(2B), and A(3) receptors was expressed in arterioles and venules. Protein for A(1), A(2A), and A(2B), but not A(3), was detected in both microvessel types and was further demonstrated on vascular endothelial cells.

CONCLUSION

Arteriolar P(s)(PSA) decreases with adenosine suffusion but not venular P(s)(PSA). Adenosine A(1), A(2A), and A(2B) receptors are expressed in both arterioles and venules. Selective receptor-linked cellular signaling mechanisms underlying the regulation of permeability remain to be determined.

Signaling pathways involved in adenosine triphosphate-induced endothelial cell barrier enhancement

Endothelial barrier dysfunction caused by inflammatory agonists is a frequent underlying cause of vascular leak and edema. Novel strategies to preserve barrier integrity could have profound clinical impact. Adenosine triphosphate (ATP) released from endothelial cells by shear stress and injury has been shown to protect the endothelial barrier in some settings. We have demonstrated that ATP and its nonhydrolyzed analogues enhanced barrier properties of cultured endothelial cell monolayers and caused remodeling of cell-cell junctions. Increases in cytosolic Ca2+ and Erk activation caused by ATP were irrelevant to barrier enhancement. Experiments using biochemical inhibitors or siRNA indicated that G proteins (specifically Galphaq and Galphai2), protein kinase A (PKA), and the PKA substrate vasodilator-stimulated phosphoprotein were involved in ATP-induced barrier enhancement. ATP treatment decreased phosphorylation of myosin light chain and specifically activated myosin-associated phosphatase. Depletion of Galphaq with siRNA prevented ATP-induced activation of myosin phosphatase. We conclude that the mechanisms of ATP-induced barrier enhancement are independent of intracellular Ca2+, but involve activation of myosin phosphatase via a novel G-protein-coupled mechanism and PKA.

Neutrophils both reduce and increase permeability in a cell culture model of the blood-brain barrier

This study was carried out to determine the effects that human neutrophils have on permeability across a model of the blood-brain barrier (BBB) formed by primary cultures of bovine brain microvessel endothelial cells (BBMEC). Transendothelial electrical resistance (TEER) was used to measure changes in permeability across BBMEC monolayers in a dual compartment system, during neutrophil interactions. When neutrophils (5 x 10(6)/ml) were applied to monolayers, TEER increased (permeability decreased). Adenosine was implicated, since the TEER increase was blocked by adenosine deaminase (1 U/ml) and the adenosine A2 receptor antagonist ZM 241385 (at 10(-6) M but not 10(-8) M, implicating A2B receptors). Oxygen free radicals were implicated as the TEER increase was blocked by combined catalase (100 U/ml) and superoxide dismutase (60 U/ml). When a gradient of the bacterial chemoattractant peptide formyl methionyl leucine phenylalanine (fMLP, 10(-7) M) was applied to neutrophils, the TEER decreased (permeability increased), concurrent with migration. When fMLP (10(-7) M) was added to the neutrophils, without migration, no change occurred. The TEER decrease was blocked by loading endothelium with the calcium buffer BAPTA (10 microM) and partially blocked by the serine protease inhibitor aprotinin (20 microg/ml). Measures to block the potential extracellular triggers heparin binding protein, glutamate, oxygen free radicals and binding to intercellular cell adhesion molecule-1 (ICAM-1) were ineffective. These data indicate that neutrophils both reduce and increase permeability in a cell culture model of the BBB, correlated to their proximity and migration through the endothelium. They explore the role of neutrophils in BBB breakdown, and the formation or amelioration of vasogenic cerebral edema.

Modulation of A2A adenosine receptors and associated Galphas proteins by ZM 241385 treatment of porcine coronary artery

Functional regulation and expression of the adenosine A2A receptor and associated G-protein were investigated in porcine coronary artery exposed to an A2A receptor antagonist, ZM 241385 (4-(2-[7-amino-2-(2-furyl)[1,2,4]-triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol). The arteries were incubated for 3 days in culture medium in the absence (control) and presence (treated) of 10 microM ZM 241385. Changes in isometric tension by adenosine receptor agonists were evaluated in endothelium-free tissues. ZM 241385-treatment produced a statistically significant rightward displacement of CGS-21680, NECA, and CAD concentration-response curves compared with the respective controls (P < 0.05). The EC50, expressed in nM, values in treated and control tissues were: 617.3 +/- 23 versus 24.9 +/- 1.5 for CGS-21680 (2-(p-(2-carboxyethyl)phenethylamino)-5'N-ethylcarboxamidoadenosine), 27.4 +/- 6.3 versus 3.06 +/- 0.8 for NECA (5'-N-ethylcarboxamidoadenosine), and 5786.2 +/- 160 versus 89.2 +/- 24.1 for CAD (chloroadenosine). However, the relaxing effect of forskolin remained unchanged in treated and control tissues. The concentration-response curves for NECA, CAD, and CGS-21680 were also displaced to the right when cAMP levels were measured in treated and control smooth muscle cells while no differences were observed with forskolin. Quantitative Western blot analysis demonstrated that the density of A2A receptors increased in ZM 241385-treated artery. We also showed a significant decrease in Galphas protein levels after ZM 241385 treatment compared with control. Taken together, these data indicate that prolonged blockade of A2A receptors in the coronary artery leads to desensitization of the functional effects of adenosine agonists by a mechanism that involves decreases in cAMP production. This was associated with an up-regulation of A2A receptors and a decrease in Galphas protein expression.

Differential expression of adenosine receptors in human endothelial cells: role of A2B receptors in angiogenic factor regulation

Adenosine has been reported to stimulate or inhibit the release of angiogenic factors depending on the cell type examined. To test the hypothesis that differential expression of adenosine receptor subtypes contributes to endothelial cell heterogeneity, we studied microvascular (HMEC-1) and umbilical vein (HUVEC) human endothelial cells. Based on mRNA level and stimulation of adenylate cyclase, we found that HUVECs preferentially express A2A adenosine receptors and HMEC-1 preferentially express A2B receptors. Neither cells expressed A1 or A3 receptors. The nonselective adenosine agonist 5'-N-ethylcarboxamidoadenosine (NECA) increased expression of interleukin-8 (IL-8), basic fibroblast growth factor (bFGF), and vascular endothelial growth factor (VEGF) in HMEC-1, but had no effect in HUVECs. In contrast, the selective A2A agonist 2-p-(2-carboxyethyl)phenylethylamino-NECA (CGS 21680) had no effect on expression of these angiogenic factors. Cotransfection of each type of adenosine receptors with a luciferase reporter in HMEC-1 showed that A2B receptors, but not A1, A2A, or A3, activated IL-8 and VEGF promoters. These effects were mimicked by constitutively active alphaG(q), alphaG12, and alphaG13, but not alphaG(s) or alphaG(i1-3). Furthermore, stimulation of phospholipase C indicated coupling of A2B receptors to G(q) proteins in HMEC-1. Thus, differential expression of adenosine receptor subtypes contributes to functional heterogeneity of human endothelial cells. A2B receptors, predominantly expressed in human microvascular cells, modulate expression of angiogenic factors via coupling to G(q), and possibly via G12/13.

Cardiac effects of adenosine in A(2A) receptor knockout hearts: uncovering A(2B) receptors

To clarify the relative roles of A(2) adenosine receptor subtypes in the regulation of coronary flow and myocardial contractility, coronary vascular and functional responses to adenosine and its analogs were examined in isolated wild-type (WT) and A(2A) receptor knockout (A(2A)KO) mouse hearts. Nonselective agonists adenosine and 5'-N-ethyl-carboxamido-adenosine (NECA) increased coronary flow in A(2A)KO hearts, albeit with a rightward shift of concentration-response curves and decreased maximal vasodilation compared with WT hearts. 2-p-(2-Carboxy-ethyl)phenethylamino-5'-N-ethyl-carboxamidoadenosine (CGS-21680, a selective A(2A) receptor agonist) increased coronary flow in WT hearts but did not affect A(2A)KO hearts. Adenosine and NECA each elicited equal maximal increases in developed pressure in WT and A(2A)KO hearts, whereas CGS-21680 did not affect developed pressure in A(2A)KO hearts. Alloxazine, a selective A(2B) receptor antagonist, attenuated NECA-induced coronary vasodilation (from 202 +/- 14% to 128 +/- 9% of baseline, P < 0.05) and NECA-induced increases in developed pressure (from 133 +/- 8% to 112 +/- 7% of baseline, P < 0.05) in A(2A)KO hearts. Together, these findings support the conclusion that A(2B) adenosine receptor activation increases coronary flow and developed pressure in isolated murine hearts.

Comparison of the vascular effects of adenosine in isolated mouse heart and aorta

The present study was designed to characterize and compare the vascular effects of adenosine and its analogs in the murine heart and aorta. Mouse hearts perfused under constant pressure in standard Langendorff fashion demonstrated concentration-dependent increases in coronary flow to adenosine, 2-chloradenosine (CAD), 5'-(N-ethyl-carboxamido)-adenosine (NECA), and 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxam-idoadenosine (CGS-21680). All agonists produced comparable increases in coronary flow with the following order of potency: CGS-21680 = NECA >> CAD > or = adenosine. In l-phenylephrine hydrochloride (phenylephrine) precontracted aortic rings, all nonselective agonists (NECA, CAD, and adenosine) produced marked concentration-dependent relaxation, whereas the adenosine A(2A) selective agonist CGS-21680 did not. Adenosine receptor agonists were >100 times more potent for coronary vasodilation than aortic vasorelaxation. The selective A(2A) receptor antagonist 5-amino-7-(beta-phenylethyl)-2-(8-furyl)pyrazolo-[4,3-e]-1,2,4-triazolo-[1,5-c]pyrimidine (SCH-58261) blocked both CGS-21680- and NECA-induced increases in coronary flow, whereas the A(2B) receptor antagonist benzo[g]pteridine-2,4(1H,3H)-dione (alloxazine) inhibited NECA-induced aortic relaxation. These data indicate a differential response to adenosine agonists in murine coronary vasculature and aorta where coronary vasodilation is mediated predominantly by activation of A(2A) adenosine receptors.