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

Nitric oxide, endothelium-derived hyperpolarizing factor, and smooth muscle-dependent mechanisms contribute to magnesium-dependent vascular relaxation in mouse arteries

AIM

Magnesium (Mg2+ ) is a vasorelaxant. The underlying physiological mechanisms driving this vasorelaxation remain unclear. Studies were designed to test the hypothesis that multiple signaling pathways including nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF) in endothelial cells as well as Ca2+ antagonization and TRPM7 channels in vascular smooth muscle cells mediate Mg2+ -dependent vessel relaxation.

METHODS

To uncover these mechanisms, force development was measured ex vivo in aorta rings from mice using isometric wire myography. Concentration responses to Mg2+ were studied in intact and endothelium-denuded aortas. Key findings were confirmed in second-order mesenteric resistance arteries perfused ex vivo using pressure myography. Effects of Mg2+ on NO formation were measured in Chinese Hamster Ovary (CHO) cells, isolated mesenteric vessels, and mouse urine.

RESULTS

Mg2+ caused a significant concentration-dependent relaxation of aorta rings. This relaxation was attenuated significantly in endothelium-denuded aortas. The endothelium-dependent portion was inhibited by NO and cGMP blockade but not by cyclooxygenase inhibition. Mg2+ stimulated local NO formation in CHO cells and isolated mesenteric vessels without changing urinary NOx levels. High extracellular Mg2+ augmented acetylcholine-induced relaxation. SKCa and IKCa channel blockers apamin and TRAM34 inhibited Mg2+ -dependent relaxation. The endothelium-independent relaxation in aorta rings was inhibited by high extracellular Ca2+ . Combined blockade of NO, SKCa , and IKCa channels significantly reduced Mg2+ -dependent dilatation in mesenteric resistance vessels.

CONCLUSIONS

In mouse conductance and resistance arteries Mg2+ -induced relaxation is contributed by endothelial NO formation, EDHF pathways, antagonism of Ca2+ in smooth muscle cells, and additional unidentified mechanisms.

Tolerability and efficacy of a reduced dose adenosine stress cardiac magnetic resonance protocol under general anesthesia in infants and children

BACKGROUND

Intravenous adenosine induces pharmacological stress by causing vasodilatation and thus carries the risk of severe hypotension when combined with vasodilatory effects of anesthetic agents.

OBJECTIVE

This study describes our experience with a reduced dose adenosine cardiac magnetic resonance imaging (MRI) protocol in young children under general anesthesia (GA).

MATERIALS AND METHODS

This is a retrospective report of all patients from birth to 18 years who underwent adenosine stress cardiac MRI under GA between August 2018 and November 2022. Based on our anecdotal experience of severe adverse effects in patients receiving adenosine infusion under GA and in discussion with the pediatric anesthesia team, we developed a modified protocol starting at a dose of 110 mcg/kg/min with incremental escalation to a full dose of 140 mcg/kg/min to achieve desired hemodynamic effect.

RESULTS

Twenty-two children (mean age 6.5 years, mean weight 28 kg) satisfied the inclusion criteria. The diagnoses included Kawasaki disease (7), anomalous aortic origin of left coronary artery (3), anomalous aortic origin of right coronary artery (2), coronary fistula (3), repaired d-transposition of great arteries (2), repaired anomalous left coronary artery from pulmonary artery (2), repaired truncus arteriosus with left coronary artery occlusion (1), extracardiac-Fontan with left coronary artery myocardial bridge (1), and post heart transplantation (1). Nine patients needed dose escalation beyond 110 mcg/kg/min. Two patients had transient hypotension during testing (systemic blood pressure drop > 25 mmHg). No patient developed significant heart block or bronchospasm. Six patients (repeat study in one) demonstrated inducible perfusion defects (27%) on stress perfusion sequences-5 of whom had confirmed significant coronary abnormalities on angiography or direct surgical inspection.

CONCLUSION

A reduced/incremental dose adenosine stress cardiac MRI protocol under GA in children is safe and feasible. This avoids severe hypotension which is both unsafe and may result in inaccurate data.

Adenosine signaling as target in cardiovascular pharmacology

Increasing evidence demonstrated the relevance of adenosine system in the onset and development of cardiovascular diseases, such as hypertension, myocardial infarct, ischemia, hypertension, heart failure, and atherosclerosis. In this regard, intense research efforts are being focused on the characterization of the pathophysiological significance of adenosine, acting at its membrane receptors named A1, A2A, A2B, and A3 receptors, in cardiovascular diseases. The present review article provides an integrated and comprehensive overview about current clinical and pre-clinical evidence about the role of adenosine in the pathophysiology of cardiovascular diseases. Particular attention has been focused on current scientific evidence about the pharmacological ligands acting on adenosine pathway as useful tools to manage cardiovascular diseases.

Methotrexate and cardiovascular prevention: an appraisal of the current evidence

New evidence continues to accumulate regarding a significant association between excessive inflammation and dysregulated immunity (local and systemic) and the risk of cardiovascular events in different patient cohorts. Whilst research has sought to identify novel atheroprotective therapies targeting inflammation and immunity, several marketed drugs for rheumatological conditions may serve a similar purpose. One such drug, methotrexate, has been used since 1948 for treating cancer and, more recently, for a wide range of dysimmune conditions. Over the last 30 years, epidemiological and experimental studies have shown that methotrexate is independently associated with a reduced risk of cardiovascular disease, particularly in rheumatological patients, and exerts several beneficial effects on vascular homeostasis and blood pressure control. This review article discusses the current challenges with managing cardiovascular risk and the new frontiers offered by drug discovery and drug repurposing targeting inflammation and immunity with a focus on methotrexate. Specifically, the article critically appraises the results of observational, cross-sectional and intervention studies investigating the effects of methotrexate on overall cardiovascular risk and individual risk factors. It also discusses the putative molecular mechanisms underpinning the atheroprotective effects of methotrexate and the practical advantages of using methotrexate in cardiovascular prevention, and highlights future research directions in this area.

The Importance of Integrated Regulation Mechanism of Coronary Microvascular Function for Maintaining the Stability of Coronary Microcirculation: An Easily Overlooked Perspective

Coronary microvascular dysfunction (CMD) refers to a group of disorders affecting the structure and function of coronary microcirculation and is associated with an increased risk of major adverse cardiovascular events. At present, great progress has been made in the diagnosis of CMD, but there is no specific treatment for it because of the complexity of CMD pathogenesis. Vascular dysfunction is one of the important causes of CMD, but previous reviews mostly considered microvascular dysfunction as a whole abnormality so the obtained conclusions are skewed. The coronary microvascular function is co-regulated by multiple mechanisms, and the mechanisms by which microvessels of different luminal diameters are regulated vary. The main purpose of this review is to revisit the mechanisms by which coronary microvessels at different diameters regulate coronary microcirculation through integrated sequential activation and briefly discuss the pathogenesis, diagnosis, and treatment progress of CMD from this perspective.

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.

Diet, herbs and erectile function: A good friendship!

Plants and plant materials have been used for thousands of years to treat and control erectile dysfunction in men. This practice has spanned many cultures and traditions around the world, with the therapeutic effects of many plants attributed to their phytochemical constituents. This review explains how polyphenols (including phenolic acids, flavonoids, terpenoids, carotenoids, alkaloids and polyunsaturated fatty acids) in plants and plant food products interact with key enzymes (phosphodiesterase-5 [PDE-5], angiotensin-converting enzyme [ACE], acetylcholinesterase [AChE], adenosine deaminase [ADA] and arginase) associated with erectile dysfunction. By modulating or altering the activity of these physiologically important enzymes, various bioactive compounds from plants or plant products can synergistically or additively provide tremendous protection against male erectile problems.

Cyperus esculentus L. and Tetracarpidium conophorum Müll. Arg. Supplemented Diet Improved Testosterone Levels, Modulated Ectonucleotidases and Adenosine Deaminase Activities in Platelets from L-NAME-Stressed Rats

In hypertensive individuals, platelet morphology and function have been discovered to be altered, and this has been linked to the development of vascular disease, including erectile dysfunction (ED). The impact of nutritional supplementation with Cyperus esculentus (tiger nut, TN) and Tetracarpidium conophorum (walnut, WN) on androgen levels, ectonucleotidases, and adenosine deaminase (ADA) activities in platelets from L-NAME (Nω-nitro-L-arginine methyl ester hydrochloride) challenged rats were investigated. We hypothesized that these nuts may show a protective effect on platelets aggregation and possibly enhance the sex hormones, thereby reverting vasoconstriction. Wistar rats (male; 250-300 g; n = 10) were grouped into seven groups as follows: basal diet control group (I); basal diet/L-NAME/Viagra (5 mg/kg/day) as positive control group (II); ED-induced group (basal diet/L-NAME) (III); diet supplemented processed TN (20%)/L-NAME (IV); diet supplemented raw TN (20%)/L-NAME (V); diet supplemented processed WN (20%)/L-NAME (VI); and diet supplemented raw WN (20%)/L-NAME (VII). The rats were given their regular diet for 2 weeks prior to actually receiving L-NAME (40 mg/kg/day) for ten days to induce hypertension. Platelet androgen levels, ectonucleotidases, and ADA were all measured. L-NAME considerably lowers testosterone levels (54.5 ± 2.2; p < 0.05). Supplementing the TN and WN diets revealed improved testosterone levels as compared to the control (306.7 ± 5.7), but luteinizing hormone levels remained unchanged. Compared to control groups, the L-NAME-treated group showed a rise in ATP (127.5%) hydrolysis and ADA (116.7%) activity, and also a decrease in ADP (76%) and AMP (45%) hydrolysis. Both TN and WN supplemented diets resulted in substantial (p < 0.05) reversal effects. Enhanced testosterone levels and modulation of the purinergic system in platelets by TN and WN could be one of the mechanisms by which they aid in vasoconstriction control.

Does ischemic preconditioning increase flap survival by ADORA2B receptor activation?

BACKGROUND

Ischemic preconditioning (IPC) is defined as raising tolerance to subsequent ischemic stress by exposing tissues to sub-lethal ischemia. Although many candidates have been suggested, recent studies have clearly demonstrated that adenosine-mediated ADORA2B receptor (ADORA2BR) activation is the main mechanism involved in IPC. While the tissue-protective role of this mechanism has been demonstrated in different ischemia/reperfusion (I/R) models, its role in flap surgery-derived I/R damage has not to date been investigated.

OBJECTIVE

To investigate the role of adenosine and ADORA2BR activation in IPC-mediated tissue protection in an epigastric flap model.

METHODS

Animals were divided into five main groups, all of which were then divided into two subgroups depending on whether or not they were exposed to IPC before the I/R procedure, which consisted of 6 hours of ischemia and 6 days of reperfusion. No drugs were administered in Group 1 (the control group). Animals in Group 2 were pretreated with CD73-inhibitor before IPC application or the ischemic period. Animals in Group 3 were pretreated with adenosine. Animals in Group 4 were pretreated with an ADORA2BR antagonist, and those in Group 5 with an ADORA2BR agonist. After 6 days of reperfusion, tissue survival was evaluated via histological and macroscopic analysis.

RESULTS

IPC application significantly enhanced CD73 expressions and adenosine concentrations (p < 0.01). Flap survivals were increased by IPC in Group 1 (p < 0.05). However, CD73 inhibition blocked this increase (Group 2). In Group 3, adenosine improved flap survival even in the absence of IPC (p < 0.01). While an ADORA2BR antagonist attenuated the tissue-protective effect of IPC (p < 0.01), the ADORA2BR agonist improved flap survival by mimicking IPC in groups 4 and 5.

CONCLUSION

These results provide pharmacological evidence for a contribution of CD73 enzyme-dependent adenosine generation and signaling through ADORA2BR to IPC-mediated tissue protection. They also suggest for the first time that ADORA2BR agonists may be used as a potential preventive therapy against I/R injury in flap surgeries.

Adenosine and the Cardiovascular System: The Good and the Bad

Adenosine is a nucleoside that impacts the cardiovascular system via the activation of its membrane receptors, named A₁R, A_2AR, A_2BR and A₃R. Adenosine is released during hypoxia, ischemia, beta-adrenergic stimulation or inflammation and impacts heart rhythm and produces strong vasodilation in the systemic, coronary or pulmonary vascular system. This review summarizes the main role of adenosine on the cardiovascular system in several diseases and conditions. Adenosine release participates directly in the pathophysiology of atrial fibrillation and neurohumoral syncope. Adenosine has a key role in the adaptive response in pulmonary hypertension and heart failure, with the most relevant effects being slowing of heart rhythm, coronary vasodilation and decreasing blood pressure. In other conditions, such as altitude or apnea-induced hypoxia, obstructive sleep apnea, or systemic hypertension, the adenosinergic system activation appears in a context of an adaptive response. Due to its short half-life, adenosine allows very rapid adaptation of the cardiovascular system. Finally, the effects of adenosine on the cardiovascular system are sometimes beneficial and other times harmful. Future research should aim to develop modulating agents of adenosine receptors to slow down or conversely amplify the adenosinergic response according to the occurrence of different pathologic conditions.

Pharmacological profile of adenosine A2A receptors in patients with lower extremity peripheral artery disease and associated coronary artery disease: A pilot study

BACKGROUND

Altered blood flow occurs in patients with low extremity peripheral artery disease (LE-PAD). LE-PAD is mostly associated with coronary artery disease (CAD). Adenosine is an endogenous nucleoside that affects both coronary and limb artery blood flow, mostly via the adenosine A_2A receptor (A_2AR). We evaluated A_2AR expression and function in peripheral blood mononuclear cells (PBMCs) and the femoral artery tissues of patients with LE-PAD.

METHODS

Artery tissues and PBMCs were sampled in 24 patients with intermittent claudication, and compared with PBMCs in 24 healthy subjects. Expression and function of A_2AR was studied, using a A_2AR monoclonal antibody with agonist properties, allowing determination of A_2AR affinity (K_D) and cAMP production (ie.EC₅₀).

RESULTS

A_2AR expression on PBMCs was lower in patients than controls (median1.3 [range 0.6-1.8] vs 1.75 [1.45-2.1] arbitrary units; P < 0.01), and correlated with A_2AR expression in artery tissues (Pearson's r = 0.71; P < 0.01). Basal and maximally stimulated cAMP production of PBMCs was lower in patients vs controls: 172 [90-310] vs 244 [110-380] pg/10⁶ cells (P < 0.05) and 375 [160-659] vs 670 [410-980] pg/10⁶ cells (P < 0.05), respectively. A high K_D/EC₅₀ ratio, characteristic of spare receptors, was observed in CAD with inducible-myocardial-ischemia.

CONCLUSION

A_2AR expression in the arteries of patients, correlated with their expression in PBMCs. A_2AR expression was lower in patients than in controls. A single blood sample (for measurement of A_2AR expression on PBMCs) may help to screen patients with LE-PAD, whereas the presence of spare receptors may help with risk stratification before vascular surgery in CAD patients with high risk of myocardial ischemia.

Cytoglobin regulates blood pressure and vascular tone through nitric oxide metabolism in the vascular wall

The identity of the specific nitric oxide dioxygenase (NOD) that serves as the main in vivo regulator of O₂-dependent NO degradation in smooth muscle remains elusive. Cytoglobin (Cygb) is a recently discovered globin expressed in fibroblasts and smooth muscle cells with unknown function. Cygb, coupled with a cellular reducing system, efficiently regulates the rate of NO consumption by metabolizing NO in an O₂-dependent manner with decreased NO consumption in physiological hypoxia. Here we show that Cygb is a major regulator of NO degradation and cardiovascular tone. Knockout of Cygb greatly prolongs NO decay, increases vascular relaxation, and lowers blood pressure and systemic vascular resistance. We further demonstrate that downregulation of Cygb prevents angiotensin-mediated hypertension. Thus, Cygb has a critical role in the regulation of vascular tone and disease. We suggest that modulation of the expression and NOD activity of Cygb represents a strategy for the treatment of cardiovascular disease.

The gaseous signalling molecule nitric oxide regulates vascular tone. Here, the authors show that nitric oxide is degraded by the enzyme cytoglobin in the vascular wall, and that mice lacking cytoglobin have reduced blood pressure and are less sensitive to angiotensin-mediated hypertension.

Augmentation of Muscle Blood Flow by Ultrasound Cavitation Is Mediated by ATP and Purinergic Signaling

BACKGROUND

Augmentation of tissue blood flow by therapeutic ultrasound is thought to rely on convective shear. Microbubble contrast agents that undergo ultrasound-mediated cavitation markedly amplify these effects. We hypothesized that purinergic signaling is responsible for shear-dependent increases in muscle perfusion during therapeutic cavitation.

METHODS

Unilateral exposure of the proximal hindlimb of mice (with or without ischemia produced by iliac ligation) to therapeutic ultrasound (1.3 MHz, mechanical index 1.3) was performed for 10 minutes after intravenous injection of 2×10⁸ lipid microbubbles. Microvascular perfusion was evaluated by low-power contrast ultrasound perfusion imaging. In vivo muscle ATP release and in vitro ATP release from endothelial cells or erythrocytes were assessed by a luciferin-luciferase assay. Purinergic signaling pathways were assessed by studying interventions that (1) accelerated ATP degradation; (2) inhibited P2Y receptors, adenosine receptors, or K_ATP channels; or (3) inhibited downstream signaling pathways involving endothelial nitric oxide synthase or prostanoid production (indomethacin). Augmentation in muscle perfusion by ultrasound cavitation was assessed in a proof-of-concept clinical trial in 12 subjects with stable sickle cell disease.

RESULTS

Therapeutic ultrasound cavitation increased muscle perfusion by 7-fold in normal mice, reversed tissue ischemia for up to 24 hours in the murine model of peripheral artery disease, and doubled muscle perfusion in patients with sickle cell disease. Augmentation in flow extended well beyond the region of ultrasound exposure. Ultrasound cavitation produced an ≈40-fold focal and sustained increase in ATP, the source of which included both endothelial cells and erythrocytes. Inhibitory studies indicated that ATP was a critical mediator of flow augmentation that acts primarily through either P2Y receptors or adenosine produced by ectonucleotidase activity. Combined indomethacin and inhibition of endothelial nitric oxide synthase abolished the effects of therapeutic ultrasound, indicating downstream signaling through both nitric oxide and prostaglandins.

CONCLUSIONS

Therapeutic ultrasound using microbubble cavitation to increase muscle perfusion relies on shear-dependent increases in ATP, which can act through a diverse portfolio of purinergic signaling pathways. These events can reverse hindlimb ischemia in mice for >24 hours and increase muscle blood flow in patients with sickle cell disease.

CLINICAL TRIAL REGISTRATION

URL: http://clinicaltrials.gov. Unique identifier: NCT01566890.

A Comparative Study of Vasorelaxant Effects of ATP, ADP, and Adenosine on the Superior Mesenteric Artery of SHR

We investigated superior mesenteric arteries from spontaneously hypertensive rats (SHR) to determine the relaxation responses induced by ATP, ADP, and adenosine and the relationship between the relaxant effects of these compounds and nitric oxide (NO) or cyclooxygenase (COX)-derived prostanoids. In rat superior mesenteric artery, relaxation induced by ATP and ADP but not by adenosine was completely eliminated by endothelial denudation. In the superior mesenteric arteries isolated from SHR [vs. age-matched control Wistar Kyoto rats (WKY)], a) ATP- and ADP-induced relaxations were weaker, whereas adenosine-induced relaxation was similar in both groups, b) ATP- and ADP-induced relaxations were substantially and partly reduced by N(G)-nitro-L-arginine [a NO synthase (NOS) inhibitor], respectively, c) indomethacin, an inhibitor of COX, increased ATP- and ADP-induced relaxations, d) ADP-induced relaxation was weaker under combined inhibition by NOS and COX, and e) adenosine-induced relaxation was not altered by treatment with these inhibitors. These data indicate that levels of responsiveness to these nucleotides/adenosine vary in the superior mesenteric arteries from SHR and WKY and are differentially modulated by NO and COX-derived prostanoids.

Adenosine production: a common path for mesenchymal stem-cell and regulatory T-cell-mediated immunosuppression

Adenosine is an important molecule that exerts control on the immune system, by signaling through receptors lying on the surface of immune cells. This nucleotide is produced, in part, by the action of the ectoenzymes CD39 and CD73. Interestingly, these proteins are expressed on the cell surface of regulatory T-cells (Tregs) and mesenchymal stromal cells (MSCs)-two cell populations that have emerged as potential therapeutic tools in the field of cell therapy. In fact, the production of adenosine constitutes a mechanism used by both cell types to control the immune response. Recently, great scientific progress was obtained regarding the role of adenosine in the inflammatory environment. In this context, the present review focuses on the advances related to the impact of adenosine production over the immune modulatory activity of Tregs and MSCs, and how this nucleotide controls the biological functions of these cells. Finally, we mention the main challenges and hurdles to bring such molecule to clinical settings.

Inhibition of As(III) and Hg(II) caused aortic hypercontraction by eugenol, linalool and carvone

Acute and chronic exposure to arsenic and mercury is known to produce vasoconstriction. There is, however, no clarity concerning the pathways leading to this increased contraction. In this study we elicit and compare maximum contractility of rat aortas under resting conditions in the presence of arsenic and mercury, and delineate pathways mediating this effect. Phenylephrine (PE) induced hypercontraction of 37% and 32% were obtained when isolated aortic segments were exposed to 25 µM As(III) and 6 nM Hg(II), respectively. Isometric contraction measurements in presence of apocynin, verapamil and sodium nitroprusside indicates that the major causes of increased contraction are reactive oxygen species (ROS) and depletion of nitric oxide (NO). Calcium influx plays a minor role in arsenic and mercury caused hypercontraction. In unexposed aorta, eugenol causes relaxation by inhibiting ROS and elevating NO, linalool by blocking voltage dependent calcium channel (VDCC) and elevating NO, and carvone by blocking calcium influx through VDDC. Since the arsenic and mercury hypercontraction is mediated by increased ROS and depleted NO, we hypothesize that molecules which neutralize ROS or elevate NO will be better ameliorators. In line with this argument, we found eugenol to be the best ameliorator of arsenic and mercury hypercontraction followed by linalool and carvone.

The effects of A2B receptor modulators on vascular endothelial growth factor and nitric oxide axis in chronic cyclosporine nephropathy

Introduction:

To investigate the actions of adenosine A_2B receptor modulators on VEGF and NO levels in CsA nephropathy.

Materials and Methods:

Nephropathy was induced by administrating 25 mg/kg (s.c) of CsA for 5 weeks. The VEGF and NO levels were measured in kidney tissue. Serum creatinine, creatinine clearance, urinary albumin excretion, blood urea nitrogen, kidney pathology score were measured to assess renal function. The analysis of mRNA expression of A_2B receptor and VEGF was performed.

Results:

Administration of CsA for 5 weeks induced adverse renal function. The mRNA expression of VEGF was reduced in renal tissue after 5 weeks of CsA treatment. The renal VEGF and NO levels were also reduced in these animals. In vivo administration of A_2B adenosine receptor agonist increased renal VEGF which was inhibited by a selective A_2B AR antagonist (MRS1754) in CsA-treated animals. The increase in VEGF was associated with reversal of adverse renal functions. The effects of A_2B AR modulators were prominent in CsA-treated animals compared with control animals suggesting CsA treatment may upregulate A_2B ARs. The mRNA expression of A_2B AR was increased after 5 weeks of CsA.

Conclusions:

A_2B AR modulators may provide new therapeutic options to retard CsA nephropathy by mediating renal VEGF and NO.

In adenosine A2B knockouts acute treatment with inorganic nitrate improves glucose disposal, oxidative stress, and AMPK signaling in the liver

Rationale: Accumulating studies suggest that nitric oxide (NO) deficiency and oxidative stress are central pathological mechanisms in type 2 diabetes (T2D). Recent findings demonstrate therapeutic effects by boosting the nitrate-nitrite-NO pathway, which is an alternative pathway for NO formation. This study aimed at investigating the acute effects of inorganic nitrate on glucose and insulin signaling in adenosine A_2B receptor knockout mice (A^−/−_2B), a genetic mouse model of impaired metabolic regulation.

Methods: Acute effects of nitrate treatment were investigated in aged wild-type (WT) and A^−/−_2B mice. One hour after injection with nitrate (0.1 mmol/kg, i.p.) or placebo, metabolic regulation was evaluated by intraperitoneal glucose and insulin tolerance tests. NADPH oxidase-mediated superoxide production and AMPK phosphorylation were measured in livers obtained from non-treated or glucose-treated mice, with or without prior nitrate injection. Plasma was used to determine insulin resistance (HOMA-IR) and NO signaling.

Results: A^−/−_2B displayed increased body weight, reduced glucose clearance, and attenuated overall insulin responses compared with age-matched WT mice. Nitrate treatment increased circulating levels of nitrate, nitrite and cGMP in the A^−/−_2B, and improved glucose clearance. In WT mice, however, nitrate treatment did not influence glucose clearance. HOMA-IR increased following glucose injection in the A^−/−_2B, but remained at basal levels in mice pretreated with nitrate. NADPH oxidase activity in livers from A^−/−_2B, but not WT mice, was reduced by nitrate treatment. Livers from A^−/−_2B displayed reduced AMPK phosphorylation compared with WT mice, and this was increased by nitrate treatment. Finally, injection with the anti-diabetic agent metformin induced similar therapeutic effects in the A^−/−_2B as observed with nitrate.

Conclusion: The A^−/−_2B mouse is a genetic mouse model of metabolic syndrome. Acute treatment with nitrate improved the metabolic profile in it, at least partly via reduction in oxidative stress and improved AMPK signaling in the liver.

Relaxation effect of abacavir on rat basilar arteries

BACKGROUND

The use of abacavir has been linked with increased cardiovascular risk in patients with human immunodeficiency virus infection; however, the mechanism involved remains unclear. We hypothesize that abacavir may impair endothelial function. In addition, based on the structural similarity between abacavir and adenosine, we propose that abacavir may affect vascular contractility through endogenous adenosine release or adenosine receptors in blood vessels.

METHODS

The relaxation effect of abacavir on rat basilar arteries was studied using the myograph technique. Cyclic GMP and AMP levels were measured by immunoassay. The effects of abacavir on nucleoside transporters were studied using radiolabeled nucleoside uptake experiments. Ecto-5' nucleotidase activity was determined by measuring the generation of inorganic phosphate using adenosine monophosphate as the substrate.

RESULTS

Abacavir induced the relaxation of rat basilar arteries in a concentration-dependent manner. This relaxation was abolished when endothelium was removed. In addition, the relaxation was diminished by the nitric oxide synthase inhibitor, L-NAME, the guanylyl cyclase inhibitor, ODQ, and the protein kinase G inhibitor, KT5820. Abacavir also increased the cGMP level in rat basilar arteries. Abacavir-induced relaxation was also abolished by adenosine A2 receptor blockers. However, abacavir had no effect on ecto-5' nucleotidase and nucleoside transporters. Short-term and long-term treatment of abacavir did not affect acetylcholine-induced relaxation in rat basilar arteries.

CONCLUSION

Abacavir induces acute endothelium-dependent relaxation of rat basilar arteries, probably through the activation of adenosine A2 receptors in endothelial cells, which subsequently leads to the release of nitric oxide, resulting in activation of the cyclic guanosine monophosphate/protein kinase G-dependent pathway in vascular smooth muscle cells. It is speculated that abacavir-induced cardiovascular risk may not be related to endothelial dysfunction as abacavir does not impair relaxation of blood vessels. The most likely explanation of increased cardiovascular risk may be increased platelet aggregation as suggested by other studies.

Oxidative airway inflammation leads to systemic and vascular oxidative stress in a murine model of allergic asthma

Oxidant-antioxidant imbalance plays an important role in repeated cycles of airway inflammation observed in asthma. It is when reactive oxygen species (ROS) overwhelm antioxidant defenses that a severe inflammatory state becomes apparent and may impact vasculature. Several studies have shown an association between airway inflammation and cardiovascular complications; however so far none has investigated the link between airway oxidative stress and systemic/vascular oxidative stress in a murine model of asthma. Therefore, this study investigated the contribution of oxidative stress encountered in asthmatic airways in modulation of vascular/systemic oxidant-antioxidant balance. Rats were sensitized intraperitoneally with ovalbumin (OVA) in the presence of aluminum hydroxide followed by several intranasal (i.n.) challenges with OVA. Rats were then assessed for airway and vascular inflammation, oxidative stress (ROS, lipid peroxides) and antioxidants measured as total antioxidant capacity (TAC) and thiol content. Challenge with OVA led to increased airway inflammation and oxidative stress with a concomitant increase in vascular inflammation and oxidative stress. Oxidative stress in the vasculature was significantly inhibited by antioxidant treatment, N-acetyl cysteine; whereas hydrogen peroxide (H2O2) inhalation worsened it. Therefore, our study shows that oxidative airway inflammation is associated with vascular/systemic oxidative stress which might predispose these patients to increased cardiovascular risk.

Modulation of Pb(II) caused aortal constriction by eugenol and carvacrol

Exposure to lead is known to cause vasoconstriction, exact mechanism of which remains to be elucidated. In this study, we investigate contractile responses of rat aortal rings equilibrated with Pb(II) in organ bath system, explore pathways responsible for hypercontraction and examine two ameliorators of lead-induced hypercontraction. At 1 μmol L(-1) Pb(II), aortal rings showed an average increase of 50% in isometric contraction. Incubation of rings, unexposed to Pb(II), with 1 μmol L(-1) sodium nitroprusside (nitric oxide (NO) donor), 100 μmol L(-1) apocynin (reactive oxygen species (ROS) inhibitor), and 100 μmol L(-1) indomethacin (cyclooxygenase inhibitor) lead to decrease in phenylephrine-induced contraction by 31, 27, and 29%, respectively. This decrease of contraction for Pb(II)-exposed rings was 48, 53, and 38%, respectively, indicating that ROS- and NO-dependent components of contractions are significantly elevated in Pb(II)-induced hypercontraction. Cyclooxygenase-dependent contractile component did not show significant elevation. Eugenol and carvacrol are plant-derived phenols known to possess antioxidant activity and hence could act as possible ameliorators of hypercontraction. At saturating concentrations of 100 μmol L(-1), eugenol and carvacrol caused a decrease in contraction by 38 and 42% in unexposed rings and 46 and 50% in Pb(II)-exposed rings. Co-incubation of rings with eugenol/carvacrol and various inhibitors suggests that both these active principles exert their relaxant effect via quenching of ROS and stimulation of NO synthesis. To conclude, Pb(II) is shown to induce hypercontraction of aortal rings through elevation of ROS and depletion of NO. This hypercontraction is effectively mitigated by eugenol and carvacrol.

Inhibition of calpain reduces oxidative stress and attenuates endothelial dysfunction in diabetes

Aims

The present study was to investigate the role of calpain in reactive oxygen species (ROS) production in endothelial cells and endothelium-dependent vascular dysfunction under experimental conditions of diabetes.

Methods and results

Exposure to high glucose activated calpain, induced apoptosis and reduced nitric oxide (NO) production without changing eNOS protein expression, its phosphorylation and dimers formation in primary human umbilical vein endothelial cells (HUVECs). These effects of high glucose correlated with intracellular ROS production and mitochondrial superoxide generation. Selectively scavenging mitochondrial superoxide increased NO production in high glucose-stimulated HUVECs. Inhibition of calpain using over-expression of calpastatin or pharmacological calpain inhibitor prevented high glucose-induced ROS production, mitochondrial superoxide generation and apoptosis, which were concurrent with an elevation of NO production in HUVECs. In mouse models of streptozotocin-induced type-1 diabetes and OVE26 type-1 diabetic mice, calpain activation correlated with an increase in ROS production and peroxynitrite formation in aortas. Transgenic over-expression of calpastatin reduced ROS production and peroxynitrite formation in diabetic mice. In parallel, diabetes-induced reduction of endothelium-dependent relaxation in aortic ring was reversed by over-expression of calpastatin in mouse models of diabetes. However, the protective effect of calpastatin on endothelium-dependent relaxation was abrogated by eNOS deletion in diabetic mice.

Conclusions

This study suggests that calpain may play a role in vascular endothelial cell ROS production and endothelium-dependent dysfunction in diabetes. Thus, calpain may be an important therapeutic target to overcome diabetes-induced vascular dysfunction.

Simulation and comparative analysis of binding modes of nucleoside and non-nucleoside agonists at the A2B adenosine receptor

PURPOSE

A2B receptor agonists are studied as possible therapeutic tools for a variety of pathological conditions. Unfortunately, medicinal chemistry efforts have led to the development of a limited number of potent agonists of this receptor, in most cases with a low or no selectivity versus the other adenosine receptor subtypes. Among the developed molecules, two structural families of compounds have been identified based on nucleoside and non-nucleoside (pyridine) scaffolds. The aim of this work is to analyse the binding mode of these molecules at 3D models of the human A2B receptor to identify possible common interaction features and the key receptor residues involved in ligand interaction.

METHODS

The A2B receptor models are built by using two recently published crystal structures of the human A2A receptor in complex with two different agonists. The developed models are used as targets for molecular docking studies of nucleoside and non-nucleoside agonists. The generated docking conformations are subjected to energy minimization and rescoring by using three different scoring functions. Further analysis of top-score conformations are performed with a tool evaluating the interaction energy between the ligand and the binding site residues.

RESULTS

Results suggest a set of common interaction points between the two structural families of agonists and the receptor binding site, as evidenced by the superimposition of docking conformations and by analysis of interaction energy with the receptor residues.

CONCLUSIONS

The obtained results show that there is a conserved pattern of interaction between the A2B receptor and its agonists. These information and can provide useful data to support the design and the development of A2B receptor agonists belonging to nucleoside or non-nucleoside structural families.

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 A1 receptors link to smooth muscle contraction via CYP4a, protein kinase C-α, and ERK1/2

Adenosine A1 receptor (A1AR) activation contracts smooth muscle, although signaling mechanisms are not thoroughly understood. Activation of A1AR leads to metabolism of arachidonic acid, including the production of 20-hydroxyeicosatetraenoic acid (20-HETE) by cytochrome P4504a (CYP4a). The 20-HETE can activate protein kinase C-α (PKC-α), which crosstalks with extracellular signal-regulated kinase (ERK1/2) pathway. Both these pathways can regulate smooth muscle contraction, we tested the hypothesis that A1AR contracts smooth muscle through a pathway involving CYP4a, PKC-α, and ERK1/2. Experiments included isometric tension recordings of aortic contraction and Western blots of signaling molecules in wild type (WT) and A1AR knockout (A1KO) mice. Contraction to the A1-selective agonist 2-chloro-N cyclopentyladenosine (CCPA) was absent in A1KO mice aortae, indicating the contractile role of A1AR. Inhibition of CYP4a (HET0016) abolished 2-chloro-N cyclopentyladenosine-induced contraction in WT aortae, indicating a critical role for 20-HETE. Both WT and A1KO mice aortae contracted in response to exogenous 20-HETE. Inhibition of PKC-α (Gö6976) or ERK1/2 (PD98059) attenuated 20-HETE-induced contraction equally, suggesting that ERK1/2 is downstream of PKC-α. Contractions to exogenous 20-HETE were significantly less in A1KO mice; reduced protein levels of PKC-α, p-ERK1/2, and total ERK1/2 supported this observation. Our data indicate that A1AR mediates smooth muscle contraction via CYP4a and a PKC-α-ERK1/2 pathway.

Functional and RNA expression profile of adenosine receptor subtypes in mouse mesenteric arteries

Concentration-response curves (CRCs) of adenosine receptor (AR) agonists, NECA (nonspecific), CCPA (A1 specific), CGS-216870 (A2A specific), BAY 60-6583 (A2B specific), and Cl-IB-MECA (A3 specific) for mesenteric arteries (MAs) from 4 AR knockout (KO) mice (A1, A2A, A2B, and A3) and their wild type (WT) were constructed. The messenger RNA expression of MAs from KO mice and WT were also studied. Adenosine (10 to 10 M) and NECA (10 to 10 M) induced relaxation in all mice except A2B KO mice, which only showed constriction by adenosine at 10 to 10 and NECA at 10 to 10 M. The CCPA induced a significant constriction at 10 and 10 M in all mice, except A1KO. BAY 60-6583 induced relaxation (10 to 10 M) in WT and no response in A2BKO except at 10 M. The CRCs for BAY 60-6583 in A1, A2A, and A3 KO mice shifted to the left when compared with WT mice, suggesting an upregulation of A2B AR. No responses were noted to CGS-21680 in all mice. Cl-IB-MECA only induced relaxation at concentration greater than 10 M, and no differences were found between different KO mice. The CRC for Bay 60-6583 was not significantly changed in the presence of 10 M of L-NAME, 10 M of indomethacin, or both. Our data suggest that A2B AR is the predominant AR subtype and the effect may be endothelial independent, whereas A1 AR plays a significant modulatory role in mouse MAs.

Nitric oxide synthase/K+ channel cascade triggers the adenosine A(2B) receptor-sensitive renal vasodilation in female rats

Adenosine A2B-receptors mediate the adenosine-evoked renal vasodilations in male rats. Here, we tested whether this finding could be replicated in female renal vasculature and whether K(+) hyperpolarization induced by nitric oxide synthase (NOS) and/or heme oxygenase (HO) accounts for adenosine A2B receptor-sensitive renal vasodilations. In phenylephrine-preconstricted perfused kidneys, vasodilations caused by the adenosine analog 5'-N-ethylcarboxamidoadenosine (NECA, 1.6-50 nmol) were attenuated after blockade of adenosine A2B (alloxazine) but not A2A [8-(3-Chlorostyryl) caffeine, CSC] or A3 receptors (N-(2-methoxyphenyl)-N'-[2-(3-pyridinyl)-4-quinazolinyl]-urea, VUF 5574), confirming the preferential involvement of A2B receptors in NECA responses. NOS activation mediated the A2B receptor-mediated NECA response because: (i) NOS inhibition (N(ω)-nitro-L-arginine-methyl ester, L-NAME) attenuated NECA vasodilations, (ii) concurrent L-NAME/alloxazine exposure caused more inhibition of NECA responses, and (iii) inhibition of NECA responses by alloxazine disappeared in L-arginine-supplemented preparations. Although HO inhibition (zinc protoporphyrin) failed to modify NECA responses, the attenuation of these responses by alloxazine disappeared in hemin (HO inducer)-treated preparations. NECA vasodilations were also attenuated after exposure to BaCl2, glibenclamide but not tetraethylammonium (blockers of inward rectifier, ATP-sensitive, and Ca(2+)-dependent K(+)-channels, respectively). The combined alloxazine/BaCl2/glibenclamide infusion caused no additional attenuation of NECA vasodilations. Vasodilations caused by minoxidil (K(+)-channel opener) were reduced by L-NAME or BaCl2/glibenclamide, supporting the importance of NOS signaling in K(+) hyperpolarization. NECA or minoxidil vasodilations were attenuated by ouabain, Na(+)/K(+)-ATPase inhibitor, and in KCl-preconstricted preparations. Overall, facilitation of adenosine A2B receptor/NOS/K(+) channel/Na(+)/K(+)-ATPase cascade underlies NECA vasodilations in female rats. Enhancing HO activity, albeit not causally related to NECA vasodilations, improves the pharmacologically compromised (alloxazine) NECA response.

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.

CYP-epoxygenases contribute to A2A receptor-mediated aortic relaxation via sarcolemmal KATP channels

Previously, we have shown that A(2A) adenosine receptor (A(2A)AR) mediates aortic relaxation via cytochrome P-450 (CYP)-epoxygenases. However, the signaling mechanism is not understood properly. We hypothesized that ATP-sensitive K(+) (K(ATP)) channels play an important role in A(2A)AR-mediated relaxation. Organ bath and Western blot experiments were done using isolated aorta from A(2A)KO and corresponding wild-type (WT) mice. Aortic rings from WT and A(2A) knockout (KO) mice were precontracted with submaximal dose of phenylephrine (PE, 10(-6) M), and concentration-response curves for pinacidil, cromakalim (nonselective K(ATP) openers), and diazoxide (mitochondrial K(ATP) opener) were obtained. Diazoxide did not have any relaxation effect on PE-precontracted tissues, whereas relaxation to pinacidil (48.09 ± 5.23% in WT vs. 25.41 ± 2.73% in A(2A)KO; P < 0.05) and cromakalim (51.19 ± 2.05% in WT vs. 38.50 ± 2.26% in A(2A)KO; P < 0.05) was higher in WT than A(2A)KO aorta. This suggested the involvement of sarcolemmal rather than mitochondrial K(ATP) channels. Endothelium removal, treatment with SCH 58651 (A(2A)AR antagonist; 10(-6) M), N(G)-nitro-l-arginine methyl ester (l-NAME, nitric oxide synthase inhibitor) and methylsulfonyl-propargyloxyphenylhexanamide (MS-PPOH, CYP-epoxygenases inhibitor; 10(-5) M) significantly reduced pinacidil-induced relaxation in WT compared with controls, whereas these treatments did not have any effect in A(2A)KO aorta. Glibenclamide (K(ATP) channel inhibitor, 10(-5) M) blocked 2-p-(2-carboxyethyl)phenethylamino-5'N-ethylcarboxamido adenosine hydrochloride (CGS 21680, A(2A)AR agonist)-induced relaxation in WT and changed 5'-N-ethylcarboxamide (NECA) (nonselective adenosine analog)-induced response to higher contraction in WT and A(2A)KO. 5-Hydroxydecanoate (5-HD, mitochondrial K(ATP) channel inhibitor, 10(-4) M) had no effect on CGS 21680-mediated response in WT aorta. Our data suggest that A(2A)AR-mediated vasorelaxation occurs through opening of sarcolemmal K(ATP) channels via CYP-epoxygenases and possibly, nitric oxide, contributing to pinacidil-induced responses.

Targeting adenosine receptors in the development of cardiovascular therapeutics

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

Role of ω-hydroxylase in adenosine-mediated aortic response through MAP kinase using A2A-receptor knockout mice

Previously, we have shown that A(2A) adenosine receptor (A(2A)AR) knockout mice (KO) have increased contraction to adenosine. The signaling mechanism(s) for A(2A)AR is still not fully understood. In this study, we hypothesize that, in the absence of A(2A)AR, ω-hydroxylase (Cyp4a) induces vasoconstriction through mitogen-activated protein kinase (MAPK) via upregulation of adenosine A(1) receptor (A(1)AR) and protein kinase C (PKC). Organ bath and Western blot experiments were done using isolated aorta from A(2A)KO and corresponding wild-type (WT) mice. Isolated aortic rings from WT and A(2A)KO mice were precontracted with submaximal dose of phenylephrine (10(-6) M), and concentration responses for selective A(1)AR, A(2A)AR agonists, angiotensin II and cytochrome P-450-epoxygenase, 20-hydroxyeicosatrienoic acid (20-HETE) PKC, PKC-α, and ERK1/2 inhibitors were obtained. 2-p-(2-Carboxyethyl)-phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride (CGS-21680, A(2A)AR agonist) induced concentration-dependent relaxation in WT, which was blocked by methylsulfonyl-propargyloxyphenylhexanamide (cytochrome P-450-epoxygenase inhibitor; 10(-5) M) and also with removal of endothelium. A(1) agonist, 2-chloro-N(6)-cyclopentyladenosine (CCPA) produced higher contraction in A(2A)KO aorta than WT (49.2 ± 8.5 vs. 27 ± 5.9% at 10(-6) M, P < 0.05). 20-HETE produced higher contraction in A(2A)KO than WT (50.6 ± 8.8 vs. 21.1 ± 3.3% at 10(-7) M, P < 0.05). Contraction to CCPA in WT and A(2A)KO aorta was inhibited by PD-98059 (p42/p44 MAPK inhibitor; 10(-6) M), chelerythrine chloride (nonselective PKC blocker; 10(-6) M), Gö-6976 (selective PKC-α inhibitor; 10(-7) M), and HET0016 (20-HETE inhibitor; 10(-5) M). Also, contraction to 20-HETE in WT and A(2A)KO aorta was inhibited by PD-98059 and Gö-6976. Western blot analysis indicated the upregulation of A(1)AR, Cyp4a, PKC-α, and phosphorylated-ERK1/2 in A(2A)KO compared with WT (P < 0.05), while expression of Cyp2c29 was significantly higher in WT. CCPA (10(-6) M) increased the protein expression of PKC-α and phosphorylated-ERK1/2, while HET0016 significantly reduced the CCPA-induced increase in expression of these proteins. These data suggest that, in the absence of A(2A)AR, Cyp4a induces vasoconstriction through MAPK via upregulation of A(1)AR and PKC-α.

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.

Adenosine elicits an eNOS-independent reduction in arterial blood pressure in conscious mice that involves adenosine A2A receptors

AIMS

Adenosine plays an important role in the regulation of heart rate (HR) and vascular reactivity. However, the mechanisms underlying the acute effect of adenosine on arterial blood pressure in conscious mice are unclear. Therefore, this study investigated the effect of the nucleoside on mean arterial blood pressure (MAP) and HR in conscious mice.

METHODS

Chronic indwelling catheters were placed in C57Bl/6J (WT) and endothelial nitric oxide synthase knockout (eNOS(-/-)) mice for continuous measurements of MAP and HR. Using PCR and myograph analysis, involvement of adenosine receptors was investigated in human and mouse renal blood vessels.

RESULTS

Bolus infusion of 0.5 mg kg(-1) adenosine elicited significant transient decreases in MAP (99.3 ± 2.3 to 70.4 ± 4.5 mmHg) and HR (603.2 ± 18.3 to 364.3 ± 49.2 min(-1)), which were inhibited by the A(2A) receptor antagonist ZM 241385. Activation of adenosine A(2A) receptors with CGS 21680 (0.02 mg kg(-1)) caused a significant reduction in MAP from 99.6 ± 1.2 to 73.1 ± 3.6 mmHg accompanied by tachycardia (610.5 ± 9.3 to 677.5 ± 9.5 min(-1)). The reduction in MAP observed after adenosine or CGS 21680 administrations was not significantly different in WT and eNOS(-/-) mice. In isolated human and mouse intrarenal arteries, adenosine caused a relaxation dependent on A(2A) adenosine receptor activation. A(2A) receptors were present in both human and mouse arteries whereas A(1) and A(2B) receptors were only present in mouse arteries.

CONCLUSION

In conclusion, acute adenosine administration and selective stimulation of adenosine A(2A) receptors results in an immediate, transient eNOS-independent reduction in MAP. A(2A) receptor activation causes relaxation of human and mouse arteries.

NADPH oxidase pathway is involved in aortic contraction induced by A3 adenosine receptor in mice

The NADPH oxidase (Nox) subunits 1, 2 (gp91 phox), and 4 are the major sources for reactive oxygen species (ROS) in vascular tissues. In conditions such as ischemia-reperfusion and hypoxia, both ROS and adenosine are released, suggesting a possible interaction. Our aim in this study was to examine the A(3) adenosine receptor (A(3)AR)-induced vascular effects and its relation to ROS and Nox1, 2, and 4 using aortic tissues from wild-type (WT) and A(3)AR knockout (A(3)KO) mice. The selective A(3)AR agonist 2-chloro-N(6)-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (Cl-IBMECA) (10(-10)-10(-5) M) induced contraction of the aorta from WT but not from A(3)KO mice, and this contraction was inhibited by the Nox inhibitor apocynin (10(-5) M) and the ROS scavengers superoxide dismutase-polyethylene glycol and catalase-polyethylene glycol (100 U/ml each). Cl-IBMECA-induced contraction was not affected by the mast cell degranulator compound 48/80 (100 μg/ml) or the stabilizer cromolyn sodium (10(-4) M). In addition, Cl-IBMECA (10(-7) M) increased intracellular ROS generation by 35 ± 14% in WT but not in A(3)KO aorta, and this increase was inhibited by apocynin (10(-5) M), diphenyleneiodonium chloride (10(-5) M), and the A(3)AR antagonist 3-propyl-6-ethyl-5-[(ethylthio)carbonyl]-2 phenyl-4-propyl-3-pyridine carboxylate (MRS1523) (10(-5) M). Furthermore, Cl-IBMECA selectively increased the protein expression of the Nox2 subunit by 150 ± 15% in WT but not in A(3)KO mice without affecting either Nox1 or 4, and this increase was inhibited by apocynin. The mRNA of Nox2 was unchanged by Cl-IBMECA in either WT or A(3)KO aortas. In conclusion, A(3)AR enhances ROS generation, possibly through activation of Nox2, with subsequent contraction of the mouse aorta.

Role of adenosine signaling in penile erection and erectile disorders

INTRODUCTION

Penile erection is a hemodynamic process, which results from increased flow and retention of blood in the penile organ due to the relaxation of smooth muscle cells. Adenosine, a physiological vasorelaxant, has been shown to be a modulator of penile erection.

AIM

To summarize the research on the role of adenosine signaling in normal penile erection and erectile disorders.

MAIN OUTCOME MEASURES

Evidence in the literature on the association between adenosine signaling and normal and abnormal penile erection, i.e., erectile dysfunction (ED) and priapism.

METHODS

The article reviews the literature on the role of endogenous and exogenous adenosine in normal penile erection, as well as in erectile disorders namely, ED and priapism.

RESULTS

Adenosine has been shown to relax corpus cavernosum from various species including human in both in vivo and in vitro studies. Neuromodulatory role of adenosine in corpus cavernosum has also been demonstrated. Impaired adenosine signaling through A(2B) receptor causes partial resistance of corpus cavernosum, from men with organic ED, to adenosine-mediated relaxation. Increased level of adenosine has been shown to be a causative factor for priapism.

CONCLUSION

Overall, the research reviewed here suggests a general role of exogenous and endogenous adenosine signaling in normal penile erection. From this perspective, it is not surprising that impaired adenosine signaling is associated with ED, and excessive adenosine signaling is associated with priapism. Adenosine signaling represents a potentially important diagnostic and therapeutic target for the treatment of ED and priapism.

Inhibition of vascular ectonucleotidase activities by the pro-drugs ticlopidine and clopidogrel favours platelet aggregation

BACKGROUND AND PURPOSE

After conversion to their active forms by the liver, ticlopidine and clopidogrel exert antiplatelet effects through irreversible inhibition of the P2Y₁₂ receptor. Concentrations of nucleotides such as ADP, the physiological agonist at platelet P2Y₁ and P2Y₁₂ receptors, are regulated by vascular ectonucleotidases, mainly nucleoside triphosphate diphosphohydrolase (NTPDase)1 and ecto-5'-nucleotidase. Here we evaluate the effect of these pro-drugs on vascular ectonucleotidase activity and on the natural function of these enzymes in regulating platelet aggregation.

EXPERIMENTAL APPROACH

Nucleotidase assays were performed by HPLC and by P(i) determination, using human umbilical vein endothelial cells (HUVEC) and protein extracts from transfected COS-7 cells as sources of enzymes. Platelet aggregation was assayed using human platelet-rich plasma.

KEY RESULTS

Each pro-drug inhibited endothelial ectonucleotidase activities and decreased their ability to block platelet aggregation in vitro. At their therapeutic concentrations, ticlopidine (60 µM) and clopidogrel (20 µM) inhibited ADP hydrolysis by HUVEC by about 80%, and AMP hydrolysis by one-third. Accordingly, these compounds showed a mixed-type inhibition of recombinant human NTPDase1 with an apparent K(i) (K(i,app) ) of 10 µM (clopidogrel) and 14 µM (ticlopidine). Recombinant rat ecto-5'-nucleotidase, but not its human orthologue, was inhibited by ticlopidine with a K(i,app) of 4.5 mM.

CONCLUSIONS AND IMPLICATIONS

These pro-drugs facilitated platelet aggregation via the inhibition of vascular NTPDase1 in vitro. Further studies should be performed to assess whether this effect also occurs in vivo, especially at the beginning of treatment, before sufficient levels of active metabolites are produced by the liver.

Adenosine receptors and vascular inflammation

Epidemiological studies have shown a positive correlation between poor lung function and respiratory disorders like asthma and the development of adverse cardiovascular events. Increased adenosine (AD) levels are associated with lung inflammation which could lead to altered vascular responses and systemic inflammation. There is relatively little known about the cardiovascular effects of adenosine in a model of allergy. We have shown that A(1) adenosine receptors (AR) are involved in altered vascular responses and vascular inflammation in allergic mice. Allergic A(1)wild-type mice showed altered vascular reactivity, increased airway responsiveness and systemic inflammation. Our data suggests that A(1) AR is pro-inflammatory systemically in this model of asthma. There are also reports of the A(2B) receptor having anti-inflammatory effects in vascular stress; however its role in allergy with respect to vascular effects has not been fully explored. In this review, we have focused on the role of adenosine receptors in allergic asthma and the cardiovascular system and possible mechanism(s) of action.

Involvement of A1 adenosine receptors in altered vascular responses and inflammation in an allergic mouse model of asthma

Poor lung function and respiratory disorders like asthma have a positive correlation with the development of adverse cardiovascular events. Increased adenosine levels are associated with lung inflammation that could lead to altered vascular responses and systemic inflammation. We hypothesized that asthmatic lung inflammation has systemic effects through A(1) adenosine receptors (A(1)AR) and investigated the effects of aerosolized adenosine on vascular reactivity and inflammation, using A(1)AR knockout (A(1)KO) and corresponding wild-type (A(1)WT) mice that were divided into three experimental groups each: control (CON), allergen sensitized and challenged (SEN), and SEN + aerosolized adenosine (SEN + AD). Animals were sensitized with ragweed (200 microg ip; days 1 and 6), followed by 1% ragweed aerosol challenges (days 11 to 13). On day 14, the SEN + AD groups received one adenosine aerosol challenge (6 mg/ml) for 2 min, and aortae were collected on day 15. 5'-N-ethylcarboxamidoadenosine (NECA; nonselective adenosine analog) induced concentration-dependent aortic relaxation in the A(1)WT CON group, which was impaired in the A(1)WT SEN and SEN + AD groups. All groups of A(1)KO mice showed similar (no significant difference) concentration-dependent relaxation to NECA. The A(1)WT SEN and SEN + AD groups had a significantly higher contraction to selective A(1) agonist 2-chloro-N(6)-cyclopentyladenosine (CCPA) compared with the CON group. Western blot data showed that aortic A(1)AR expression was significantly increased in WT SEN and SEN + AD mice compared with CON mice. Gene expression of ICAM-1 and IL-5 was significantly increased in allergic A(1)WT aorta and were undetected in the A(1)KO groups. A(1)WT allergic mice had significantly higher airway hyperresponsiveness (enhanced pause) to NECA, with adenosine aerosol further enhancing it. In conclusion, allergic A(1)WT mice showed altered vascular reactivity, increased airway hyperresponsiveness, and systemic inflammation. These data suggest that A(1)AR is proinflammatory systemically in this model of allergic asthma.

Absence of adenosine-mediated aortic relaxation in A(2A) adenosine receptor knockout mice

Adenosine mediates vascular responses through four receptor subtypes: A(1), A(2A), A(2B), and A(3). The role of A(2A) receptors in aortic vascular tone was investigated using A(2A) adenosine receptor (AR) knockout (A(2A)KO) and corresponding wild-type (A(2A)WT) mice. Isolated aortic rings from A(2A)WT and A(2A)KO mice were precontracted with phenylephrine (10(-7) M), and concentration responses for adenosine analogs and selective agonists/antagonists were obtained. Nonselective adenosine analog (NECA; EC(50) = 6.78 microM) and CGS-21680 (A(2A)AR selective agonist; EC(50) = 0.013 microM) produced concentration-dependent relaxation (maximum of 25% and 28% relaxation at 10(-5) M NECA and CGS-21680, respectively) in A(2A)WT aorta. In A(2A)KO aorta, NECA (EC(50) = 0.075 microM) induced concentration-dependent contraction (maximum contraction of 47% at 10(-6) M; P < 0.05 compared with A(2A)WT), whereas CGS-21680 produced no response. SCH-58261 (10(-6) M; A(2A)AR selective antagonist) abolished both NECA- and CGS-21680-mediated vasorelaxation in A(2A)WT (P < 0.05), whereas no change was observed in A(2A)KO. When DPCPX (10(-5) M; A(1) selective antagonist) was used in NECA concentration response, greater vasorelaxation was observed in A(2A)WT (50% vs. 25% in controls at 10(-5) M; P < 0.05), whereas lower contraction was seen in A(2A)KO tissues (5% vs. 47% in controls at 10(-6) M; P < 0.05). Aortic endothelial function, determined by response to acetylcholine, was significantly higher in WT compared with KO (66% vs. 51%; P < 0.05). BAY 60-6583 (A(2B) selective agonist) produced similar relaxation in both KO and WT tissues. In conclusion, A(2A)AR KO mice had significantly lower aortic relaxation and endothelial function, suggesting that the A(2A)AR plays an important role in vasorelaxation, probably through an endothelium-dependent mechanism.

A2A adenosine receptor deficiency leads to impaired tracheal relaxation via NADPH oxidase pathway in allergic mice

A(2A) adenosine receptor (A(2A)AR) has been shown to suppress superoxide generation in leukocytes via the cAMP-protein kinase A (PKA) pathway. However, no study has yet explored the role of A(2A)AR in relation to NADPH oxidase in murine tracheas in vitro, which may lead to altered smooth muscle relaxation in asthma. Therefore, the present study evaluated the effects of A(2A)AR deficiency on the NADPH oxidase pathway in tracheas of A(2A) wild-type (WT) and A(2A) knockout (KO) mice. A(2A)WT mice were sensitized with ovalbumin (30 microg i.p.) on days 1 and 6, followed by 5% ovalbumin aerosol challenge on days 11, 12, and 13. A(2A)AR (gene and protein expression), cAMP, and phosphorylated PKA (p-PKA) levels were decreased in A(2A)WT sensitized mice compared with controls. A(2A)KO mice also showed decreased cAMP and p-PKA levels. A(2A)WT sensitized and A(2A)KO control mice had increased gene and protein expression of NADPH oxidase subunits (p47phox and gp91phox) compared with the controls. Tracheal relaxation to specific A(2A)AR agonist, 4-[2-[[6-amino-9-(N-ethyl-beta-d-ribofuranuronamidosyl)-9H-purin-2-yl]amino]ethyl]benzenepropanoic acid hydrochloride (CGS 21680), decreased in A(2A)WT sensitized mice compared with the controls, although it was absent in A(2A)KO mice. Pretreatment with NADPH oxidase inhibitors apocyanin/diphenyliodonium reversed the attenuated relaxation to CGS 21680 in A(2A)WT sensitized tracheas, whereas specific PKA inhibitor (9S,10S,12R)-2,3,9,10,11,12-hexahydro-10-hydroxy-9-methyl-1-oxo-9,12-epoxy-1H-diindolo[1,2,3-fg:3',2',1'-kl]pyrrolo[3,4-i] [1,6]benzodiazocine-10-carboxylic acid hexyl ester (KT 5720) blocked CGS 21680-induced relaxation. Tracheal reactive oxygen species (ROS) generation was also increased in A(2A)WT sensitized and A(2A)KO control mice compared with the controls. In conclusion, this study shows that A(2A)AR deficiency causes increased NADPH oxidase activation leading to decreased tracheal relaxation via altered cAMP-PKA signaling and ROS generation.

Recent improvements in the development of A(2B) adenosine receptor agonists

Adenosine is known to exert most of its physiological functions by acting as local modulator at four receptor subtypes named A(1), A(2A), A(2B) and A(3) (ARs). Principally as a result of the difficulty in identifying potent and selective agonists, the A(2B) AR is the least extensively characterised of the adenosine receptors family. Despite these limitations, growing understanding of the physiological meaning of this target indicates promising therapeutic perspectives for specific ligands. As A(2B) AR signalling seems to be associated with pre/postconditioning cardioprotective and anti-inflammatory mechanisms, selective agonists may represent a new therapeutic group for patients suffering from coronary artery disease. Herein we present an overview of the recent advancements in identifying potent and selective A(2B) AR agonists reported in scientific and patent literature. These compounds can be classified into adenosine-like and nonadenosine ligands. Nucleoside-based agonists are the result of modifying adenosine by substitution at the N (6)-, C(2)-positions of the purine heterocycle and/or at the 5'-position of the ribose moiety or combinations of these substitutions. Compounds 1-deoxy-1-{6-[N'-(furan-2-carbonyl)-hydrazino]-9H-purin-9-yl}-N-ethyl-beta-D-ribofuranuronamide (19, hA(1) K (i) = 1050 nM, hA(2A) K (i) = 1550 nM, hA(2B) EC(50) = 82 nM, hA(3) K (i) > 5 muM) and its 2-chloro analogue 23 (hA(1) K (i) = 3500 nM, hA(2A) K (i) = 4950 nM, hA(2B) EC(50) = 210 nM, hA(3) K (i) > 5 muM) were confirmed to be potent and selective full agonists in a cyclic adenosine monophosphate (cAMP) functional assay in Chinese hamster ovary (CHO) cells expressing hA(2B) AR. Nonribose ligands are represented by conveniently substituted dicarbonitrilepyridines, among which 2-[6-amino-3,5-dicyano-4-[4-(cyclopropylmethoxy)phenyl]pyridin-2-ylsulfanyl]acetamide (BAY-60-6583, hA(1), hA(2A), hA(3) EC(50) > 10 muM; hA(2B) EC(50) = 3 nM) is currently under preclinical-phase investigation for treating coronary artery disorders and atherosclerosis.

Endogenous methyl palmitate modulates nicotinic receptor-mediated transmission in the superior cervical ganglion

Nitric oxide (NO) is identified as the endothelium-derived relaxing factor and a neurotransmitter with a superfusion bioassay cascade technique. By using a similar technique with rat superior cervical ganglion (SCG) as donor tissue and rabbit endothelium-denuded aortic ring as detector tissue, we report here that a vasodilator, which is more potent than NO, is released in the SCG upon field electrical stimulation (FES) or addition of nicotine. Release of this vasodilator was enhanced by arginine analogs, including N(omega)-nitro-l-arginine (a NO synthase inhibitor), suggesting that it is not NO. Analysis by gas chromatography/mass spectrometry identified 2 saturated fatty acids, palmitic acid methyl ester (PAME) and stearic acid methyl ester (SAME), being released from the SCG upon FES in the presence of arginine analogs. Exogenous PAME but not SAME induced significant aortic dilation (EC(50) = 0.19 nM), indicating that PAME is the potent vasodilator. Release of PAME and SAME was significantly diminished in chronically decentralized SCG but not denervated SCG, suggesting the preganglionic origin. Furthermore, release of both fatty acids was calcium- and myosin light chain kinase-dependent, suggesting that both were released from axoplasmic vesicular stores. Electrophysiological studies further demonstrated that PAME but not SAME inhibited nicotine-induced inward currents in cultured SCG and the alpha7-nicotinic acetylcholine receptor-expressing Xenopus oocytes. Endogenous PAME appears to play a role in modulation of the autonomic ganglionic transmission and to complement the vasodilator effect of NO.

Recent improvements in the development of A(2B) adenosine receptor agonists

Adenosine is known to exert most of its physiological functions by acting as local modulator at four receptor subtypes named A(1), A(2A), A(2B) and A(3) (ARs). Principally as a result of the difficulty in identifying potent and selective agonists, the A(2B) AR is the least extensively characterised of the adenosine receptors family. Despite these limitations, growing understanding of the physiological meaning of this target indicates promising therapeutic perspectives for specific ligands. As A(2B) AR signalling seems to be associated with pre/postconditioning cardioprotective and anti-inflammatory mechanisms, selective agonists may represent a new therapeutic group for patients suffering from coronary artery disease. Herein we present an overview of the recent advancements in identifying potent and selective A(2B) AR agonists reported in scientific and patent literature. These compounds can be classified into adenosine-like and nonadenosine ligands. Nucleoside-based agonists are the result of modifying adenosine by substitution at the N (6)-, C(2)-positions of the purine heterocycle and/or at the 5'-position of the ribose moiety or combinations of these substitutions. Compounds 1-deoxy-1-{6-[N'-(furan-2-carbonyl)-hydrazino]-9H-purin-9-yl}-N-ethyl-beta-D-ribofuranuronamide (19, hA(1) K (i) = 1050 nM, hA(2A) K (i) = 1550 nM, hA(2B) EC(50) = 82 nM, hA(3) K (i) > 5 muM) and its 2-chloro analogue 23 (hA(1) K (i) = 3500 nM, hA(2A) K (i) = 4950 nM, hA(2B) EC(50) = 210 nM, hA(3) K (i) > 5 muM) were confirmed to be potent and selective full agonists in a cyclic adenosine monophosphate (cAMP) functional assay in Chinese hamster ovary (CHO) cells expressing hA(2B) AR. Nonribose ligands are represented by conveniently substituted dicarbonitrilepyridines, among which 2-[6-amino-3,5-dicyano-4-[4-(cyclopropylmethoxy)phenyl]pyridin-2-ylsulfanyl]acetamide (BAY-60-6583, hA(1), hA(2A), hA(3) EC(50) > 10 muM; hA(2B) EC(50) = 3 nM) is currently under preclinical-phase investigation for treating coronary artery disorders and atherosclerosis.