Functional characterization of three adenosine receptor types

Selective activation of Gαob by an adenosine A1 receptor agonist elicits analgesia without cardiorespiratory depression

The development of therapeutic agonists for G protein-coupled receptors (GPCRs) is hampered by the propensity of GPCRs to couple to multiple intracellular signalling pathways. This promiscuous coupling leads to numerous downstream cellular effects, some of which are therapeutically undesirable. This is especially the case for adenosine A1 receptors (A1Rs) whose clinical potential is undermined by the sedation and cardiorespiratory depression caused by conventional agonists. We have discovered that the A1R-selective agonist, benzyloxy-cyclopentyladenosine (BnOCPA), is a potent and powerful analgesic but does not cause sedation, bradycardia, hypotension or respiratory depression. This unprecedented discrimination between native A1Rs arises from BnOCPA's unique and exquisitely selective activation of Gob among the six Gαi/o subtypes, and in the absence of β-arrestin recruitment. BnOCPA thus demonstrates a highly-specific Gα-selective activation of the native A1R, sheds new light on GPCR signalling, and reveals new possibilities for the development of novel therapeutics based on the far-reaching concept of selective Gα agonism.

Targeting Purinergic Signaling and Cell Therapy in Cardiovascular and Neurodegenerative Diseases

Extracellular purines exert several functions in physiological and pathophysiological mechanisms. ATP acts through P2 receptors as a neurotransmitter and neuromodulator and modulates heart contractility, while adenosine participates in neurotransmission, blood pressure, and many other mechanisms. Because of their capability to differentiate into mature cell types, they provide a unique therapeutic strategy for regenerating damaged tissue, such as in cardiovascular and neurodegenerative diseases. Purinergic signaling is pivotal for controlling stem cell differentiation and phenotype determination. Proliferation, differentiation, and apoptosis of stem cells of various origins are regulated by purinergic receptors. In this chapter, we selected neurodegenerative and cardiovascular diseases with clinical trials using cell therapy and purinergic receptor targeting. We discuss these approaches as therapeutic alternatives to neurodegenerative and cardiovascular diseases. For instance, promising results were demonstrated in the utilization of mesenchymal stem cells and bone marrow mononuclear cells in vascular regeneration. Regarding neurodegenerative diseases, in general, P2X7 and A_2A receptors mostly worsen the degenerative state. Stem cell-based therapy, mainly through mesenchymal and hematopoietic stem cells, showed promising results in improving symptoms caused by neurodegeneration. We propose that purinergic receptor activity regulation combined with stem cells could enhance proliferative and differentiation rates as well as cell engraftment.

Blood Platelet Adenosine Receptors as Potential Targets for Anti-Platelet Therapy

Adenosine receptors are a subfamily of highly-conserved G-protein coupled receptors. They are found in the membranes of various human cells and play many physiological functions. Blood platelets express two (A_2A and A_2B) of the four known adenosine receptor subtypes (A₁, A_2A, A_2B, and A₃). Agonization of these receptors results in an enhanced intracellular cAMP and the inhibition of platelet activation and aggregation. Therefore, adenosine receptors A_2A and A_2B could be targets for anti-platelet therapy, especially under circumstances when classic therapy based on antagonizing the purinergic receptor P2Y₁₂ is insufficient or problematic. Apart from adenosine, there is a group of synthetic, selective, longer-lasting agonists of A_2A and A_2B receptors reported in the literature. This group includes agonists with good selectivity for A_2A or A_2B receptors, as well as non-selective compounds that activate more than one type of adenosine receptor. Chemically, most A_2A and A_2B adenosine receptor agonists are adenosine analogues, with either adenine or ribose substituted by single or multiple foreign substituents. However, a group of non-adenosine derivative agonists has also been described. This review aims to systematically describe known agonists of A_2A and A_2B receptors and review the available literature data on their effects on platelet function.

Design and synthesis of novel xanthine derivatives as potent and selective A2B adenosine receptor antagonists for the treatment of chronic inflammatory airway diseases

Adenosine induces bronchial hyperresponsiveness and inflammation in asthmatics through activation of A_2B adenosine receptor (A_2BAdoR). Selective antagonists have been shown to attenuate airway reactivity and improve inflammatory conditions in pre-clinical studies. Hence, the identification of novel, potent and selective A_2BAdoR antagonist may be beneficial for the potential treatment of asthma and Chronic Obstructive Pulmonary Disease (COPD). Towards this effort, we explored several prop-2-ynylated C8-aryl or heteroaryl substitutions on xanthine chemotype and found that 1-prop-2-ynyl-1H-pyrazol-4-yl moiety was better tolerated at the C8 position. Compound 59, exhibited binding affinity (K_i) of 62 nM but was non-selective for A_2BAdoR over other AdoRs. Incorporation of substituted phenyl on the terminal acetylene increased the binding affinity (K_i) significantly to <10 nM. Various substitutions on terminal phenyl group and different alkyl substitutions on N-1 and N-3 were explored to improve the potency, selectivity for A_2BAdoR and the solubility. In general, compounds with meta-substituted phenyl provided better selectivity for A_2BAdoR compared to that of para-substituted analogs. Substitutions such as basic amines like pyrrolidine, piperidine, piperazine or cycloalkyls with polar group were tried on terminal acetylene, keeping in mind the poor solubility of xanthine analogs in general. However, these substitutions led to a decrease in affinity compared to compound 59. Subsequent SAR optimization resulted in identification of compound 46 with high human A_2BAdoR affinity (K_i = 13 nM), selectivity against other AdoR subtypes and with good pharmacokinetic properties. It was found to be a potent functional A_2BAdoR antagonist with a K_i of 8 nM in cAMP assay in hA_2B-HEK293 cells and an IC₅₀ of 107 nM in IL6 assay in NIH-3T3 cells. Docking study was performed to rationalize the observed affinity data. Structure-activity relationship (SAR) studies also led to identification of compound 36 as a potent A_2BAdoR antagonist with K_i of 1.8 nM in cAMP assay and good aqueous solubility of 529 μM at neutral pH. Compound 46 was further tested for in vivo efficacy and found to be efficacious in ovalbumin-induced allergic asthma model in mice.

Adenosine and adenosine receptors in the pathogenesis and treatment of rheumatic diseases

Adenosine, a nucleoside derived primarily from the extracellular hydrolysis of adenine nucleotides, is a potent regulator of inflammation. Adenosine mediates its effects on inflammatory cells by engaging one or more cell-surface receptors. The expression and function of adenosine receptors on different cell types change during the course of rheumatic diseases, such as rheumatoid arthritis (RA). Targeting adenosine receptors directly for the treatment of rheumatic diseases is currently under study; however, indirect targeting of adenosine receptors by enhancing adenosine levels at inflamed sites accounts for most of the anti-inflammatory effects of methotrexate, the anchor drug for the treatment of RA. In this Review, we discuss the regulation of extracellular adenosine levels and the role of adenosine in regulating the inflammatory and immune responses in rheumatic diseases such as RA, psoriasis and other types of inflammatory arthritis. In addition, adenosine and its receptors are involved in promoting fibrous matrix production in the skin and other organs, and the role of adenosine in fibrosis and fibrosing diseases is also discussed.

Nucleotide homeostasis and purinergic nociceptive signaling in rat meninges in migraine-like conditions

Extracellular ATP is suspected to contribute to migraine pain but regulatory mechanisms controlling pro-nociceptive purinergic mechanisms in the meninges remain unknown. We studied the peculiarities of metabolic and signaling pathways of ATP and its downstream metabolites in rat meninges and in cultured trigeminal cells exposed to the migraine mediator calcitonin gene-related peptide (CGRP). Under resting conditions, meningeal ATP and ADP remained at low nanomolar levels, whereas extracellular AMP and adenosine concentrations were one-two orders higher. CGRP increased ATP and ADP levels in meninges and trigeminal cultures and reduced adenosine concentration in trigeminal cells. Degradation rates for exogenous nucleotides remained similar in control and CGRP-treated meninges, indicating that CGRP triggers nucleotide release without affecting nucleotide-inactivating pathways. Lead nitrate-based enzyme histochemistry of whole mount meninges revealed the presence of high ATPase, ADPase, and AMPase activities, primarily localized in the medial meningeal artery. ATP and ADP induced large intracellular Ca(2+) transients both in neurons and in glial cells whereas AMP and adenosine were ineffective. In trigeminal glia, ATP partially operated via P2X7 receptors. ATP, but not other nucleotides, activated nociceptive spikes in meningeal trigeminal nerve fibers providing a rationale for high degradation rate of pro-nociceptive ATP. Pro-nociceptive effect of ATP in meningeal nerves was reproduced by α,β-meATP operating via P2X3 receptors. Collectively, extracellular ATP, which level is controlled by CGRP, can persistently activate trigeminal nerves in meninges which considered as the origin site of migraine headache. These data are consistent with the purinergic hypothesis of migraine pain and suggest new targets against trigeminal pain.

The role of purinergic signaling in the etiology of migraine and novel antimigraine treatment

Etiopathogenesis of migraine involves different structures of the central nervous system: the trigeminal nerve with nuclei located in the brain stem, vascular system, and the cerebral cortex as well as diverse mechanisms and pathological processes. The multidirectional action of purines in different cell types (blood vessels, neurons, and satellite glial cells) and through different types of purinergic receptors contributes to the etiopathogenesis of migraine pain. Adenosine triphosphate (ATP) and its derivatives are involved in initiation and propagation of migrenogenic signals in several ways: they participate in vasomotor mechanism, cortical spreading depression, and in fast transmission or cross-excitation based on the satellite glial cells in trigeminal ganglion. Contribution of purinergic signaling in the conduction of pain is realized through the activation of P1 and P2 receptors expressed widely in the central nervous system: on the neurons and glial cells as well as on the smooth muscles and endothelium in the vascular system. Therefore, the purinergic receptors can be an excellent target for pharmacologists constructing new antimigraine therapeutics. Moreover, the mechanisms facilitating ATP and adenosine degradation may prevent vasodilatation and thus avoid a secondary central sensitization during a migraine attack. Thus, agonists and antagonists of P receptors as well as ecto-enzymes metabolizing nucleotides/nucleosides could gain the growing attention as therapeutic agents.

Cardiac purinergic signalling in health and disease

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

Reciprocal modulation of anti-IgE induced histamine release from human mast cells by A₁ and A(2B) adenosine receptors

BACKGROUND AND PURPOSE

Adenosine is believed to participate in the pathological development of asthma through a mast cell-dependent mechanism. Our study aimed to pharmacologically characterize the functions of adenosine receptor (AR) subtypes (A₁, A(2A) , A(2B) and A₃) in primary human cultured mast cells (HCMC).

EXPERIMENTAL APPROACH

HCMC were derived from progenitor stem cells in buffy coat and the effects of adenosine receptor ligands on basal and IgE-dependent histamine release were evaluated.

KEY RESULTS

Adenosine and analogues alone did not induce HCMC degranulation. When HCMC were activated by anti-IgE after 10 min pre-incubation with adenosine, a biphasic effect on histamine release was observed with enhancement of HCMC activation at low concentrations of adenosine (10⁻⁹-10⁻⁷ mol·L⁻¹) and inhibition at higher concentrations (10⁻⁶-10⁻⁴ mol·L⁻¹). The potentiating action was mimicked by A₁ AR agonists CCPA and 2'MeCCPA, and inhibited by the A₁ AR antagonist PSB36. In contrast, the inhibitory action of adenosine was mimicked by the non-specific A₂ AR agonist CV1808 and attenuated by A(2B) AR antagonists PSB1115 and MRS1760. The non-selective AR antagonist CGS15943 attenuated both the potentiating and inhibitory actions.

CONCLUSIONS AND IMPLICATIONS

We have defined for the first time the contribution of A₁ and A(2B) ARs, respectively, to the potentiating and inhibitory action of adenosine on human mast cell activation. With reference to the current trend of developing novel anti-asthmatic agents from AR ligands, our results suggest that inhibition of human mast cell activation would be a mechanism for A₁ AR antagonists, but not A(2B) AR antagonists.

Wound healing effects of noni (Morinda citrifolia L.) leaves: a mechanism involving its PDGF/A2A receptor ligand binding and promotion of wound closure

Morinda citrifolia L. (Rubiaceae) commonly known as noni, has been used in Polynesia by traditional healers for the treatment of cuts, bruises and wounds. Our objective was to investigate the wound-healing mechanisms of the noni leaf. The investigations of its wound-healing mechanisms were carried out using fresh noni leaf juice (NLJ), noni leaf ethanol extract (NLEE) and its methanol (MFEE) and hexane (HFEE) fractions on the PDGF and A(2A) receptors in vitro and topically in mice. Fresh noni leaf juice showed significant affinity to PDGF receptors, and displayed 166% binding inhibition of the ligand binding to its receptors, while at the same concentration, it only had 7% inhibition of the ligand binding to the A(2A) receptors. NLEE, HFEE and MFEE showed significant affinity to A(2A) receptors, concentration dependently, with IC(50) values of 34.1, 42.9 and 86.7 μg/mL, respectively. However, MFEE significantly increased wound closure and reduced the half closure time in mice with a CT(50) of 5.4 ± 0.2 days compared with control (p < 0.05). These results suggest that noni leaf significantly accelerated wound healing in mice via its ligand binding to the PDGF and A(2A) receptors as its probable mechanisms of wound-healing and also support its traditional usage for wound-healing in Polynesia.

Toward multivalent signaling across G protein-coupled receptors from poly(amidoamine) dendrimers

Activation of the A2A receptor, a G protein-coupled receptor (GPCR), by extracellular adenosine, is antiaggregatory in platelets and anti-inflammatory. Multiple copies of an A2A agonist, the nucleoside CGS21680, were coupled covalently to PAMAM dendrimers and characterized spectroscopically. A fluorescent PAMAM-CGS21680 conjugate 5 inhibited aggregation of washed human platelets and was internalized. We envision that our multivalent dendrimer conjugates may improve overall pharmacological profiles compared to the monovalent GPCR ligands.

Emerging drugs for migraine prophylaxis and treatment

Migraine is a chronic neurologic disorder with heterogeneous characteristics resulting in a range of symptom profiles, burden, and disability. Migraine affects nearly 12% of the adult population in occidental countries, imposing considerable economic and social losses. The pharmacologic treatment of migraine includes preventive and acute strategies. A better understanding of the migraine pathophysiology along with the discovery of novel molecular targets has lead to a growing number of upcoming therapeutic proposals. This review focuses on new and emerging agents for the treatment of migraine.

New targets in acute migraine treatment

Migraine is a common and highly disabling neurological problem, whose acute treatment was revolutionized by the triptans, serotonin 5-HT1B/1D receptor agonists. Some patients do not respond to triptans, while others are not suitable for them largely because of contraindications based on vascular disease. The exploration of nonvasoconstrictor treatments for acute migraine offers the prospect of dramatic improvements in patient care, as well as important insights into the mechanisms of migraine. Possibilities for such developments include, calcitonin gene-related peptide receptor antagonists, serotonin 5-HT1F and 5-HT1D receptor agonists, glutamate excitatory amino acid receptor antagonists, nitric oxide synthase inhibitors and adenosine A1 receptor agonists. Taken together, the future for migraine and affected patients is bright and promising.

Lack of effect of the adenosine A1 receptor agonist, GR79236, on capsaicin-induced CGRP release in anaesthetized pigs

Migraine is a common neurological disorder that is associated with an increase in plasma calcitonin gene-related peptide (CGRP) levels. CGRP, a potent vasodilator released from the activated trigeminal sensory nerves, dilates intracranial blood vessels and transmits vascular nociception. Hence, inhibition of trigeminal CGRP release may prevent neurotransmission and, thereby, ameliorate migraine headache. Therefore, the present study in anaesthetized pigs investigates the effects of a selective adenosine A(1) receptor agonist, GR79236 (3, 10 and 30 microg/kg, i.v.) on capsaicin-induced carotid haemodynamic changes and on plasma CGRP release. Intracarotid (i.c.) infusion of capsaicin (10 microg/kg/min, i.c.) increased the total carotid blood flow and conductance as well as carotid pulsations, but decreased the difference between arterial and jugular venous oxygen saturations. These responses to capsaicin were dose-dependently attenuated by GR79236. However, the increases in the plasma CGRP concentrations by capsaicin remained essentially unmodified after GR79236 treatment. The above results suggest that GR79236 may have an antimigraine potential due to its postjunctional effects (carotid vasoconstriction) rather than to prejunctional inhibition of trigeminal CGRP release.

A new orally bioavailable dual adenosine A2B/A3 receptor antagonist with therapeutic potential

The synthesis and SAR of 5-heterocycle-substituted aminothiazole adenosine receptor antagonists is described. Several compounds show high affinity and selectivity for the A2B and A3 receptors. One compound (5f) shows good ADME properties in the rat and as such may be an important new compound in testing the current hypotheses proposing a therapeutic role for a dual A2B/A3 antagonist in allergic diseases.

Can we Develop Neurally Acting Drugs for the Treatment of Migraine?

Serotonin (5-HT)(1B/1D) receptor agonists, which are also known as triptans, represent the most important advance in migraine therapeutics in the four millennia that the condition has been recognized. The vasoconstrictive activity of triptans produced a small clinical penalty in terms of coronary vasoconstriction but also raised an enormous intellectual question: to what extent is migraine a vascular problem? Functional neuroimaging and neurophysiological studies have consistently developed the theme of migraine as a brain disorder and, therefore, demanded that the search for neurally acting antimigraine drugs should be undertaken. The prospect of non-vasoconstrictor acute migraine therapies, potential targets for which are discussed here, offers a real opportunity to patients and provides a therapeutic rationale that places migraine firmly in the brain as a neurological problem, where it undoubtedly belongs.

New targets in the acute treatment of headache

PURPOSE OF REVIEW

The aim of this article is to review recently identified targets for the acute treatment of primary headache disorders.

RECENT FINDINGS

Calcitonin gene-related peptide (CGRP) receptor blockade has been shown to be an effective acute anti-migraine strategy and is a non-vasoconstrictor in terms of the mechanism of action. It is likely that direct blockade of CGRP release by inhibition of trigeminal nerves would be similarly effective in both migraine and cluster headache. Options for acute treatment based on preclinical work and initial clinical studies include: serotonin 5HT1F and 5HT1D receptor agonists, glutamate excitatory amino acid receptor antagonists, nitric oxide synthase inhibitors and adenosine A1 receptor agonists. Proof of principle studies with octreotide, a somatostatin receptor agonist, demonstrated it to be better than placebo in the acute treatment of cluster headache but not in the acute management of migraine.

SUMMARY

The prospect of a non-vasoconstrictor acute migraine therapy offers a real opportunity to patients, and perhaps more importantly, provides a therapeutic rationale to plant migraine and cluster headache firmly in the brain as neurological problems.

Ovariectomy and estradiol replacement therapy alters the adenine nucleotide hydrolysis in rat blood serum

The low prevalence of coronary heart disease in premenopausal women and its increase after menopause are well established. Many studies have suggested that steroid hormones can inhibit platelet aggregation, reducing the cardiovascular risk. In addition, a number of studies have shown an effect of estrogen on vascular function. The process of haemostasis and thrombus formation can be also affected by adenine nucleotides and adenosine. Consequently, the regulation of enzymes that hydrolyze these nucleotides in the bloodstream is essential in the modulation of the processes of platelet aggregation, vasodilatation and coronary flow. Thus, in this study, we examined the effect of ovariectomy (OVX), estradiol replacement therapy and the in vitro administration of 17beta-estradiol, dehydroisoandrosterone 3-sulfate (DHEAS) and pregnenolone (PREG) on the activity of the enzymes that degrade adenosine triphosphate (ATP), adenosine diphosphate (ADP) and adenosine monophosphate (AMP) in the blood serum of female rats. OVX significantly increased the hydrolysis of ATP, ADP and AMP, whilst phosphodiesterase activity was unchanged. Estradiol replacement therapy significantly decreased the hydrolysis of the adenine nucleotides and of the substrate marker of phosphodiesterase. In vitro, the addition of steroid hormones did not have any effect on the nucleotide hydrolysis by rat serum. These results suggest the presence of a strong relation between these enzymes and the hormonal system. In addition, the alterations observed are important, because these enzymes control the nucleotides/nucleosides ratio in the circulation and thus the events related to haemostasis.

Post-triptan era for the treatment of acute migraine

There now is one realized and several attractive targets for the treatment of acute attacks of migraine that will follow and augment the use of serotonin 5-HT1B/1D receptor agonists, the triptans. Calcitonin gene-related peptide (CGRP) receptor blockade recently has been shown to be an effective acute antimigraine strategy; therefore, blockade of CGRP release by inhibition of trigeminal nerves would seem a logical approach. A number of targets are reviewed in this article including serotonin 5-HT1F and 5-HT1D receptors, adenosine A1 receptors, nociceptin, vanilloid TRPV1 receptors, and anandamide CB1 receptors. Development of one or more such compound offers the exciting prospect of new non-vasoconstrictor treatments for migraine and cluster headache.

New potent and selective A1 adenosine receptor agonists

Thiirane analogs of ENAdo have been synthesised and found to be extremely potent and selective A(1) adenosine receptor agonists.

Effect of the adenosine A1 receptor agonist GR79236 on trigeminal nociception with blink reflex recordings in healthy human subjects

GR79236 is a highly potent and selective adenosine A1 receptor agonist that has analgesic and anti-inflammatory actions in humans and animals. In animal models it inhibits trigeminal nerve firing and calcitonin gene-related peptide release which play a pivotal role in migraine pathophysiology. Thus GR79236 may have therapeutic potential in migraine. Although there are no validated human models of migraine, the trigeminal nociceptive pathways may be studied with a novel electrode to elicit nociception-specific blink reflex responses. Twelve healthy female volunteers were randomized in a double-blind, placebo-controlled, cross-over trial to investigate the effect of GR79236 on trigeminal nociceptive pathways, as measured by the blink reflex. A secondary objective was to compare the use of two types of electrode, the standard (SE) and nociception-specific electrodes (NE), to investigate human trigeminal pharmacology. Blink reflexes were elicited with SE and NE before and 30 min after GR79236 (10 microg/kg i.v.) or placebo. The median area under the curve of repeated sweeps of the R2 component of the blink reflex was analysed using analysis of covariance with baseline as covariate. Using NE, GR79236 produced a non-significant reduction of the ipsilateral R2 compared with placebo (P = 0.097) and a significant reduction contralaterally (P = 0.008). No significant changes were observed using SE. There were no significant adverse events. The results suggest that NE is more sensitive than SE to detect pharmacological effects in the trigeminal nociceptive system. Furthermore, the adenosine A1 receptor agonist GR79236 inhibits trigeminal nociception in humans. These results support a possible therapeutic role for GR79236 in primary headache disorders.

Endogenous adenosine increases coronary flow by activation of both A2A and A2B receptors in mice

To clarify which adenosine receptor subtype(s) are responsible for regulation of coronary flow through endogenous adenosine, coronary vascular responses were examined in isolated hearts from wild-type (WT) and A(2A) knockout (A(2A)KO) mice. Adenosine deaminase inhibitor, erythro-9-hydroxy-nonyl-adenine (EHNA), and adenosine kinase inhibitor, iodotubericidine (ITU), were used to examine the effects of endogenous adenosine. Combined infusion of EHNA and ITU in Balb/c hearts produced comparable increases in coronary flow as exerted by exogenous adenosine while they markedly decreased the heart rate, and these effects were reversed by adenosine receptor antagonist, 8-p-sulfophenyl-theophylline (8-SPT). Similarly, EHNA and ITU increased coronary flow in WT hearts to 422% of baseline, whereas this response was reduced to 144% of baseline in A(2A)KO hearts. Heart rate was equally reduced (approximately 50% of baseline) in both groups. Alloxazine (A(2B) receptor antagonist) abolished EHNA- and ITU-induced coronary flow in A(2A)KO hearts without altering the reduced heart rate. Selective A(1) receptor antagonist, 8-cyclopentyl-1-1,3-dipropylxanthine (DPCPX), reversed EHNA- and ITU-induced decreases in heart rate without altering the elevated coronary flow. These findings suggest that coronary vascular responses to endogenous adenosine mimic those produced by exogenous adenosine and are mediated at least by activation of both A(2A) and A(2B) receptors in isolated mouse hearts.

Positive coupling of atypical adenosine A3 receptors on human eosinophils to adenylyl cyclase

Adenosine A(3) receptors are reported to couple negatively to adenylyl cyclase (AC) but their mediation of anti-inflammatory effects in human eosinophils prompted us to investigate their coupling to AC. The A(3)-selective agonists IB-MECA and Cl-IB-MECA evoked a concentration-dependent generation of cAMP (EC(50), 3.2 and 1.8 microM, respectively) and were more potent than the A(2A) agonist CGS 21680 (EC(50)=15.4 microM) and adenosine (EC(50)=19.2 microM). The cAMP response was additive to that produced by forskolin (10 microM). The effect of IB-MECA was insensitive to A(1) and A(2A) receptor antagonists, but was antagonized by the A(3)-selective antagonist MRS 1220 (0.1-2.5 microM) in a competitive manner. The estimated K(B) of 190 nM was, however, atypical. The cyclo-oxygenase inhibitor, indomethacin, had no effect on the cAMP response. A general inverse relationship between cAMP generation and inhibition of degranulation was seen. We conclude that in human eosinophils, an atypical form of A(3) receptors positively coupled to AC may exist. The resulting cAMP generation may underlie the anti-inflammatory actions of A(3) agonists in eosinophils.

Adenosine up-regulates cyclooxygenase-2 in human granulocytes: impact on the balance of eicosanoid generation

Polymorphonuclear neutrophils (granulocytes; PMNs) are often the first blood cells to migrate toward inflammatory lesions to perform host defense functions. PMNs respond to specific stimuli by releasing several factors and generate lipid mediators of inflammation from the 5-lipoxygenase and the inducible cyclooxygenase (COX)-2 pathways. In view of adenosine's anti-inflammatory properties and suppressive impact on the 5-lipoxygenase pathway, we addressed in this study the impact of this autacoid on the COX-2 pathway. We observed that adenosine up-regulates the expression of the COX-2 enzyme and mRNA. Production of PGE(2) in response to exogenous arachidonic acid was also increased by adenosine and correlated with COX-2 protein levels. The potentiating effect of adenosine on COX-2 could be mimicked by pharmacological increases of intracellular cAMP levels, involving the latter as a putative second messenger for the up-regulation of COX-2 by adenosine. Specific COX-2 inhibitors were used to confirm the predominant role of the COX-2 isoform in the formation of prostanoids by stimulated PMNs. Withdrawal of extracellular adenosine strikingly emphasized the inhibitory potential of PGE(2) on leukotriene B(4) formation and involved the EP(2) receptor subtype in this process. Thus, adenosine may promote a self-limiting regulatory process through the increase of PGE(2) generation, which may result in the inhibition of PMN functions. This study identifies a new aspect of the anti-inflammatory properties of adenosine in leukocytes, introducing the concept that this autacoid may exert its immunomodulatory activities in part by modifying the balance of lipid mediators generated by PMNs.

Adenosine A1 receptor agonists inhibit trigeminovascular nociceptive transmission

There is a considerable literature to suggest that adenosine A1 receptor agonists may have anti-nociceptive effects, and we sought to explore the role of adenosine A1 receptors in a model of trigeminovascular nociceptive transmission. Cats were anaesthetized (alpha-chloralose 60 mg/kg, intraperitoneally), and prepared for physiological monitoring. The superior sagittal sinus (SSS) was stimulated electrically, and linked units were recorded in the trigeminocervical complex. Post-stimulus histograms were constructed to analyse the responses and the effect of drug administration. Blood was sampled from the external jugular vein to determine levels of calcitonin gene-related peptide (CGRP) release before and after drug administration. Intravenous administration of the highly selective adenosine A1 receptor agonist, GR79236 (3-100 microg/kg) had a dose-dependent inhibitory effect on SSS-evoked trigeminal activity. The maximal effect (80 +/- 6% reduction in probability of firing) was seen at 100 microg/kg. The neuronal inhibitory effect of GR79236 could be inhibited by the selective adenosine A1 receptor antagonist DPCPX (300 microg/kg; P < 0.05). SSS stimulation increased cranial CGRP levels from 33 +/- 2 pmol/l (n = 6) to 64 +/- 3 pmol/l, an effect substantially reduced by pre-treatment with GR79236 (30 microg/kg; P < 0.01). The selective low efficacy adenosine A1 receptor agonist, GR190178 (30-1000 microg/kg i.v.), also inhibited SSS-evoked neuronal activity in a dose-dependent fashion. In this model of trigeminovascular nociception, adenosine A1 receptor activation leads to neuronal inhibition without concomitant vasoconstriction, suggesting a novel avenue for the treatment of migraine and cluster headache.

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

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

Study of an adenosine A1 receptor agonist on trigeminally evoked dural blood vessel dilation in the anaesthetized rat

The purpose of this study was to use intravital microscopy to determine the effect of a selective adenosine A1 receptor agonist, GR79236 (1, 3 and 10 microg/kg i.v.), on neurogenic dural blood vessel dilation in anaesthetized rats. Vasodilation was evoked either by electrical stimulation of perivascular trigeminal nerves or by intravenous CGRP. GR79236 (1-10 microg/kg i.v.) caused a dose-dependent inhibition of neurogenic vasodilation, but had no significant effect on dural vasodilation caused by CGRP. GR79236 (1-3 microg/kg i.v.) had no effect on basal dural vessel diameter, but caused transient dose-dependant bradycardia and hypotension. Bradycardia was more prolonged following 10 microg/kg i.v. GR79236. Pre-treatment with the adenosine A1 receptor antagonist DPCPX (1 mg/kg i.v.) prevented the inhibitory effect of GR79236 (10 microg/kg i.v.) on neurogenic vasodilation as well as GR79236-induced bradycardia and hypotension. These data suggest that the inhibition of neurogenic vasodilation by GR79236 is mediated via the activation of prejunctional adenosine A1 receptors. Provided the systemic cardiovascular effects could be limited, such a mechanism may offer a novel approach to migraine therapy.

1,8-disubstituted xanthine derivatives: synthesis of potent A2B-selective adenosine receptor antagonists

3-Unsubstituted xanthine derivatives bearing a cyclopentyl or a phenyl residue in the 8-position were synthesized and developed as A2B adenosine receptor antagonists. Compounds bearing polar substituents were prepared to obtain water-soluble derivatives. 1-Alkyl-8-phenylxanthine derivatives were found to exhibit high affinity for A2B adenosine receptors (ARs). 1,8-disubstituted xanthine derivatives were equipotent to or more potent than 1,3,8-trisubstituted xanthines at A2B ARs, but generally less potent at A1 and A2A, and much less potent at A3 ARs. Thus, the new compounds exhibited increased A2B selectivity versus all other AR subtypes. 9-Deazaxanthines (pyrrolo[2,3-d]pyrimidindiones) appeared to be less potent at A2B ARs than the corresponding xanthine derivatives. 1-Propyl-8-p-sulfophenylxanthine (17) was the most selective compound of the present series, exhibiting a K(i) value of 53 nM at human A2B ARs and showing greater than 180-fold selectivity versus human A1 ARs. Compound 17 was also highly selective versus rat A1 ARs (41-fold) and versus the other human AR subtypes (A2A > 400-fold and A3 > 180-fold). The compound is highly water-soluble due to its sulfonate function. 1-Butyl-8-p-carboxyphenylxanthine (10), another polar analogue bearing a carboxylate function, exhibited a K(i) value of 24 nM for A2B ARs, 49-fold selectivity versus human and 20-fold selectivity versus rat A1 ARs, and greater than 150-fold selectivity versus human A2A and A3 ARs. 8-[4-(2-Hydroxyethylamino)-2-oxoethoxy)phenyl]-1-propylxanthine (29) and 1-butyl-8-[4-(4-benzyl)piperazino-2-oxoethoxy)phenyl]xanthine (35) were among the most potent A2B antagonists showing K(i) values at A2B ARs of 1 nM, 57-fold (29) and 94-fold (35) selectivity versus human A1, ca. 30-fold selectivity versus rat A1, and greater than 400-fold selectivity versus human A2A and A3 ARs. The new potent, selective, water-soluble A2B antagonists may be useful research tools for investigating A2B receptor function.

Does a selective adenosine A(1) receptor agonist protect against exercise induced ischaemia in patients with coronary artery disease?

BACKGROUND

The "warm up" effect in angina may represent ischaemic preconditioning, which is mediated by adenosine A(1) receptors in most models.

OBJECTIVE

To investigate the effect of a selective A(1) agonist, GR79236 (GlaxoSmithKline), on exercise induced angina and ischaemic left ventricular dysfunction in patients with coronary artery disease.

DESIGN

A double blind crossover study.

PATIENTS

25 patients with multivessel coronary artery disease.

INTERVENTIONS

On mornings one week apart, patients received intravenous GR79236 10 microgram/kg or placebo, and then carried out two supine bicycle exercise tests separated by 30 minutes. Equilibrium radionuclide angiography was done before and during exercise.

RESULTS

The onset of chest pain or 1 mm ST depression was delayed and occurred at a higher rate-pressure product during the second exercise test following either placebo or GR79236. Compared with placebo, GR79236 did not affect these indices during equivalent tests. GR79236 reduced resting global ejection fraction from (mean (SD)) 63 (7)% to 61 (5)% (p < 0.05) by a selective reduction in the regional ejection fraction of "ischaemic" left ventricular sectors (those where the ejection fraction fell during the first exercise test following placebo). Ischaemic sectors showed increased function during the second test following placebo (72 (21)% v 66 (20)%; p = 0.0001), or during the first test following GR79236 (69 (21)% v 66 (20)%; p = 0.0001). Sequential exercise further increased the function of ischaemic sectors even after drug administration.

CONCLUSIONS

GR79236 failed to mimic the warm up effect, and warm up occurred even in the presence of this agent. This suggests that ischaemic preconditioning is not an important component of this type of protection. The complex actions of the drug on regional left ventricular function at rest and during exercise suggest several competing A(1) mediated actions.

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.

Role of nitric oxide/cyclic GMP in myocardial adenosine A1 receptor-inotropic response

In this study we have determined the different signalling pathways involved in adenosine A(1)-receptor (A(1)-receptor)-dependent inhibition of contractility in rat isolated atria. N-cyclopentyladenosine (CPA) stimulation of A(1)-receptor exerts: negative inotropic response, inositol phosphates accumulation, stimulation of nitric oxide synthase (NOS), increased production of nitric oxide (NO) and cyclic GMP. Inhibitors of phospholipase C (PLC), protein kinase C (PKC), calcium/calmodulin, NOS and guanylate cyclase shifted the dose-response curve of CPA on contractility to the right. Those inhibitors also attenuated the A(1)-receptor-dependent increase in cyclic GMP and activation of NOS. These results suggest that CPA activation of A(1)-receptors exerts a negative inotropic effect associated with increased production of nitric oxide and cyclic GMP. The mechanism appears to occur secondarily to stimulation of phosphoinositide turnover via PLC activation. This, in turn, triggers cascade reactions involving calcium/calmodulin and PKC, leading to activation of NOS and soluble guanylate cyclase.

Pharmacology of adenosine receptors in the vasculature

Adenosine is widely distributed in mammals. One of the primary roles of adenosine within the cardiovascular system is to directly control the functions of both cardiac and vascular tissues. Recently, there has been considerable interest in the subclassification of adenosine receptors. Characterization of a heterogeneous population of receptors for adenosine could provide an opportunity for the development of novel compounds of therapeutic value. Adenosine is released from cells as a result of metabolism, and its release can be increased dramatically from cells that are metabolically stressed. This implies that adenosine can be released from a variety of cells throughout the body, as a result of increased metabolic rates, in concentrations that can have a profound impact on blood vessel function and, consequently, blood flow. It is recognized that the actions of this nucleoside on the vasculature are most prominent when oxygen demand is high and there is a reduction in oxygen tension at the site in question. Therefore, it is not surprising that adenosine has been shown to be an important regulator of blood vessel tone under hypoxic conditions. Furthermore, the activation of adenosine receptors on blood vessels can result in relaxation and/or contractions. The nature of the response subsequent to the activation of adenosine receptors is primarily dependent on the type of blood vessel involved and basal tone. This review will focus on the characterization of subtypes of adenosine receptors in blood vessels, as well as the effect of the stimulation of adenosine receptors on the peripheral circulation.

Functional characterization of coronary vascular adenosine receptors in the mouse

Coronary responses to adenosine agonists were assessed in perfused mouse and rat hearts. The roles of nitric oxide (NO) and ATP-dependent K(+) channels (K(ATP)) were studied in the mouse. Resting coronary resistance was lower in mouse vs rat, as was minimal resistance (2.2+/-0.1 vs 3.8+/-0.2 mmHg ml(-1) min(-1) g(-1)). Peak hyperaemic flow after 20 - 60 s occlusion was greater in mouse. Adenosine agonists induced coronary dilation in mouse, with pEC(50)s of 9.4+/-0.1 for 2-[p-(2-carboxyethyl)phenethylamino]-5'-N-ethyl carboxamidoadenosine (CGS21680, A(2A)-selective agonist), 9.3+/-0.1 for 5'-N-ethylcarboxamidoadenosine (NECA, A(1)/A(2) agonist), 8.4+/-0.1 for 2-chloroadenosine (A(1)/A(2) agonist), 7.7+/-0.1 for N(6)-(R)-(phenylisopropyl)adenosine (R-PIA, A(1)/A(2B) selective), and 6.8+/-0.2 for adenosine. The potency order (CGS21680=NECA>2-chloroadenosine>R-PIA>adenosine) supports A(2A) adenosine receptor-mediated dilation in mouse coronary vessels. 0.2 - 2 microM of the A(2B)-selective antagonist alloxazine failed to alter CGS21680 or 2-chloroadenosine responses. pEC(50)s in rat were 6.7+/-0.2 for CGS21680, 7.3+/-0.1 for NECA, 7.6+/-0.1 for 2-chloroadenosine, 7.2+/-0.1 for R-PIA, and 6.2+/-0.1 for adenosine (2-chloroadenosine>NECA=R-PIA>CGS21680> adenosine), supporting an A(2B) adenosine receptor response. NO-synthase antagonism with 50 microM N(G)-nitro L-arginine (L-NOARG) increased resistance by approximately 25%, and inhibited responses to CGS21680 (pEC(50)=9.0+/-0.1), 2-chloroadenosine (pEC(50)=7.3+/-0.2) and endothelial-dependent ADP, but not sodium nitroprusside (SNP). K(ATP) channel blockade with 5 microM glibenclamide increased resistance by approximately 80% and inhibited responses to CGS21680 in control (pEC(50)=8.3+/-0.1) and L-NOARG-treated hearts (pEC(50)=7.3+/-0.1), and to 2-chloroadenosine in control (pEC(50)=6.7+/-0.1) and L-NOARG-treated hearts (pEC(50)=5.9+/-0.2). In summary, mouse coronary vessels are more sensitive to adenosine than rat vessels. A(2A) adenosine receptors mediate dilation in mouse coronary vessels vs A(2B) receptors in rat. Responses in the mouse involve a sensitive NO-dependent response and K(ATP)-dependent dilation.

Adenosine A(1) receptor-mediated inhibition of protein kinase A-induced calcitonin gene-related peptide release from rat trigeminal neurons

Calcitonin gene-related peptide (CGRP), a potent vasodilator, has been implicated in the pathogenesis of migraine. Its release from adult rat trigeminal neurons in culture was shown to be markedly increased by the activation of adenylate cyclase with forskolin. Modulation of this secretion was investigated by a number of agents with known inhibitory effects on cAMP generation mediated via receptor coupling to G(i/o) proteins. Significantly, forskolin-stimulated CGRP release could be closely correlated with the phosphorylation of the protein kinase A (PKA) substrate cyclic AMP response element-binding protein (CREB). Forskolin-stimulated CGRP release could be potently and effectively inhibited by the adenosine A(1) receptor-selective agonist GR79236X (pIC(50) = 7.7 +/- 0.1, maximal inhibition 65 +/- 2.5% at 300 nM), whereas the A(2A) (CGS21680) and the A(3) (2-chloro-N(6)-(3-iodobenzyl)-adenosine-5'-N-methyluronamide) receptor-selective agonists were without effect. GR79236X-mediated inhibition was abolished by the A(1) receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine. Immunocytochemical studies and Western analysis revealed the presence of adenosine A(1) receptors on trigeminal neurons. However, despite the additional detection of 5-hydroxytryptamine (5-HT)(1B) receptors on these cells, the clinically effective antimigraine 5-HT(1B/1D) agonist sumatriptan did not inhibit forskolin-stimulated CGRP release nor did it show any effect on the concomitant CREB phosphorylation. In contrast, the mu-opioid agonist fentanyl elicited a 74 +/- 4% reduction in CGRP levels. Forskolin-stimulated CGRP release and CREB phosphorylation could be mimicked by incubation of the cells with chlorophenylthio-cAMP and blocked by pretreatment with the PKA inhibitor myrPKI(14-22). Taken together, the present data confirm the PKA-dependence of forskolin-stimulated CGRP release and suggest that A(1) adenosine agonists may warrant further investigation in models of migraine and neurogenic inflammation.

Adenosine A(1) receptor stimulation inhibits alpha(1)-adrenergic activation of the cardiac sarcolemmal Na(+)/H(+) exchanger

Sarcolemmal Na(+)/H(+) exchanger (NHE) activity is increased by stimulation of G(q) protein-coupled receptors (G(q)PCRs), but the roles of other GPCRs are largely unknown. We determined the effects of N-[(1S,trans)-2-hydroxycyclopentyl]adenosine (GR79236), a selective agonist of the G(i)PCR adenosine A(1) receptor, on sarcolemmal NHE activity in adult rat ventricular myocytes (n=8-10 per group). NHE activity was indexed by the H(+) efflux rate after intracellular acidification, measured by microepifluorescence. GR79236 alone (0.01-10 microM) had no effect on NHE activity. However, co-administration of GR79236 inhibited, in a concentration-dependent manner, the stimulation of NHE activity by the alpha(1)-adrenoceptor agonist phenylephrine (10 microM). The inhibitory effect of GR79236 (10 microM) was abolished by (1) the selective A(1) antagonist 1,3-dipropyl-8-cyclopentylxanthine (0.1 microM), confirming an A(1) receptor-mediated action, and (2) pre-treatment with pertussis toxin (5 microgram ml(-1) for 60 min), indicating a G(i) protein-mediated mechanism. Our data suggest the existence of inhibitory crosstalk between the G(i)PCR adenosine A(1) receptor and the G(q)PCR alpha(1)-adrenoceptor in the regulation of sarcolemmal NHE activity.

The discovery and synthesis of highly potent, A2a receptor agonists

A series of N6,2-disubstituted adenosine analogues have been synthesized and their functional activity measured against A2a and A1 receptors. Examples of compounds with both a lipophilic N6-substituent and amino-functionalized 2-position were highly active at the A2a receptor on the human neutrophil.

Role of adenosine A(2B) receptors in vasodilation of rat pial artery and cerebral blood flow autoregulation

This study was aimed to investigate the underlying mechanism of vasodilation induced by the activation of A(2B) adenosine receptors in relation to cerebral blood flow (CBF) autoregulation. Changes in pial arterial diameters were observed directly through a closed cranial window. N(omega)-nitro-L-arginine methyl ester (L-NAME, nitric oxide synthase inhibitor) significantly suppressed the concentration-dependent vasodilations induced by adenosine and 5'-N-ethylcarboxamido-adenosine (NECA) but not the vasodilation by CGS-21680 (A(2A)-receptor agonist). Moreover, NECA-induced vasodilation was suppressed by alloxazine (1 micromol/l) but not by ZM-241385 (1 micromol/l, A(2A) antagonist), which suggests mediation by A(2B)- receptor activation. Otherwise, the level of nitrite/nitrate was concentration dependently increased in the artificial cerebrospinal fluid (CSF) when adenosine and NECA were suffused over the cortical surface. L-NAME and alloxazine, but not ZM-241385, largely inhibited their releases. The lower limit of CBF autoregulation was little affected following pretreatment with L-NAME or alloxazine. Thus it is suggested that adenosine-induced vasodilation via activation of A(2B)-adenosine receptors of the rat pial artery is coupled to the production of nitric oxide, which contributes little to CBF autoregulation.

Adenosine receptor occupancy suppresses chemoattractant-induced phospholipase D activity by diminishing membrane recruitment of small GTPases

Adenosine (Ado) is an important autocrine modulator of neutrophil functions. In this study, we determined the effects of endogenous Ado on fMet-Leu-Phe (fMLP)–induced phospholipase D (PLD) activity in neutrophils. The removal of extracellular Ado by Ado deaminase (ADA) or the blockade of its action by the A2a receptor antagonists 8-(3-chlorostyryl) caffeine (CSC) or CGS15943 markedly increased fMLP-induced PLD activation. The concentration-dependent stimulatory effects of CSC and CGS15943 were abolished by a pretreatment of neutrophil suspensionswith ADA. In contrast, the selective A2a receptor agonist CGS21680 suppressed fMLP-induced PLD activation. Furthermore, inhibition by CGS21680 of fMLP-induced PLD activity was reversed by CSC or CGS15943. The removal of Ado by ADA or the blockade of its action by CSC or CGS15943, markedly increased the membrane recruitment of cytosolic protein kinase C (PKC), RhoA, and ADP-ribosylation factor (ARF) in response to fMLP. As shown for PLD activity, the stimulatory effect of Ado receptor antagonists on PLD cofactors translocation was abolished by a pretreatment of the cells with ADA. Moreover, the membrane translocation of both PKC, RhoA, and ARF in response to fMLP was attenuated by CGS21680 and this effect of the A2a receptor agonist was antagonized by CSC or CGS15943. These data demonstrate that Ado released by neutrophils in the extracellular milieu inhibits PLD activation by blocking membrane association of ARF, RhoA, and PKC through Ado A2a receptor occupancy.

Adenosine Inhibits Macrophage Colony-Stimulating Factor-Dependent Proliferation of Macrophages Through the Induction of p27kip-1 Expression

Adenosine is produced during inflammation and modulates different functional activities in macrophages. In murine bone marrow-derived macrophages, adenosine inhibits M-CSF-dependent proliferation with an IC50 of 45 μM. Only specific agonists that can activate A2B adenosine receptors such as 5′-N-ethylcarboxamidoadenosine, but not those active on A1 (N6-(R)-phenylisopropyladenosine), A2A ([p-(2-carbonylethyl)phenylethylamino]-5′-N-ethylcarboxamidoadenosine), or A3 (N6-(3-iodobenzyl)adenosine-5′-N-methyluronamide) receptors, induce the generation of cAMP and modulate macrophage proliferation. This suggests that adenosine regulates macrophage proliferation by interacting with the A2B receptor and subsequently inducing the production of cAMP. In fact, both 8-Br-cAMP (IC50 85 μM) and forskolin (IC50 7 μM) inhibit macrophage proliferation. Moreover, the inhibition of adenylyl cyclase and protein kinase A blocks the inhibitory effect of adenosine and its analogues on macrophage proliferation. Adenosine causes an arrest of macrophages at the G1 phase of the cell cycle without altering the activation of the extracellular-regulated protein kinase pathway. The treatment of macrophages with adenosine induces the expression of p27kip-1, a G1 cyclin-dependent kinase inhibitor, in a protein kinase A-dependent way. Moreover, the involvement of p27kip-1 in the adenosine inhibition of macrophage proliferation was confirmed using macrophages from mice with a disrupted p27kip-1 gene. These results demonstrate that adenosine inhibits macrophage proliferation through a mechanism that involves binding to A2B adenosine receptor, the generation of cAMP, and the induction of p27kip-1 expression.

N-substituted adenosines as novel neuroprotective A(1) agonists with diminished hypotensive effects

The synthesis and pharmacological profile of a series of neuroprotective adenosine agonists are described. Novel A(1) agonists with potent central nervous system effects and diminished influence on the cardiovascular system are reported and compared to selected reference adenosine agonists. The novel compounds featured are derived structurally from two key lead structures: 2-chloro-N-(1-phenoxy-2-propyl)adenosine (NNC 21-0041, 9) and 2-chloro-N-(1-piperidinyl)adenosine (NNC 90-1515, 4). The agonists are characterized in terms of their in vitro profiles, both binding and functional, and in vivo activity in relevant animal models. Neuroprotective properties assessed after postischemic dosing in a Mongolian gerbil severe temporary forebrain ischemia paradigm, using hippocampal CA1 damage endpoints, and the efficacy of these agonists in an A(1) functional assay show similarities to some reference adenosine agonists. However, the new compounds we describe exhibit diminished cardiovascular effects in both anesthetized and awake rats when compared to reference A(1) agonists such as (R)-phenylisopropyladenosine (R-PIA, 5), N-cyclopentyladenosine (CPA, 2), 4, N-[(1S,trans)-2-hydroxycyclopentyl]adenosine (GR 79236, 26), N-cyclohexyl-2'-O-methyladenosine (SDZ WAG 994, 27), and N-[(2-methylphenyl)methyl]adenosine (Metrifudil, 28). In mouse permanent middle cerebral artery occlusion focal ischemia, 2-chloro-N-[(R)-[(2-benzothiazolyl)thio]-2-propyl]adenosine (NNC 21-0136, 12) exhibited significant neuroprotection at the remarkably low total intraperitoneal dose of 0.1 mg/kg, a dose at which no cardiovascular effects are observed in conscious rats. The novel agonists described inhibit 6, 7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate-induced seizures, and in mouse locomotor activity higher doses are required to reach ED(50) values than for reference A(1) agonists. We conclude that two of the novel adenosine derivatives revealed herein, 12 and 5'-deoxy-5'-chloro-N-[4-(phenylthio)-1-piperidinyl]adenosine (NNC 21-0147, 13), representatives of a new series of P(1) ligands, reinforce the fact that novel selective adenosine A(1) agonists have potential in the treatment of cerebral ischemia in humans.

The effect of body temperature on myocardial protection conferred by ischaemic preconditioning or the selective adenosine A1 receptor agonist GR79236, in an anaesthetized rabbit model of myocardial ischaemia and reperfusion

1 The cardioprotective effect of N-[(1S, trans)-2-hydroxycyclopentyl]adenosine (GR79236), an adenosine A1 receptor agonist, was compared with that produced by ischaemic preconditioning in an anaesthetized rabbit model of myocardial ischaemia and reperfusion. In addition, we examined the effect of different body core temperatures on GR79236- or ischaemic preconditioning-induced cardioprotection when administered prior to ischaemia, and on cardioprotection induced by GR79236 administered 10 min prior to the onset of reperfusion. 2 When rabbits were subjected to 30 min occlusion of the left coronary artery, followed by 2 h reperfusion, GR79236 (3 x 10(-8) mol kg-1 i.v. (10.5 microg kg-1 i.v.)) or ischaemic preconditioning (5 min ischaemia followed by 5 min reperfusion), administered or applied 10 min prior to the occlusion, significantly limited the development of infarction. The cardioprotective effect of ischaemic preconditioning was significantly greater than that seen after administration of GR79236. Pre-treatment with the selective adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 3.3 x 10(-6) mol kg-1 (1 mg kg-1 i.v.)), prevented the cardioprotective effect of GR79236, but not that of ischaemic preconditioning. 3 Maintaining body core temperature at 38.5 degrees C rather than at 37.0 degrees C did not influence infarct size in control groups of rabbits, but reduced the cardioprotective effect of GR79236 when administered 10 min prior to occlusion or 10 min prior to the onset of reperfusion. The cardioprotective effect of ischaemic preconditioning was not temperature-dependent. 4 In conclusion, myocardial protection conferred by GR79236 in anaesthetized rabbits is mediated via adenosine A1 receptors. Myocardial protection can be conferred when GR79236 is administered before the onset of ischaemia or reperfusion, and is reduced when body core temperature is maintained at 38.5 degrees C rather than at 37.0 degrees C. In contrast, myocardial protection conferred by ischaemic preconditioning is not reduced by adenosine A1 receptor blockade, or by maintaining body core temperature at 38.5 degrees C rather than at 37.0 degrees C. These findings point to distinct differences in the mechanisms of induction of myocardial protection by adenosine A1 receptor agonist and ischaemic preconditioning. They also highlight the need for careful control of body core temperature when investigating the phenomenon of cardioprotection.

Examination of adenosine receptor-mediated relaxation of the pig coronary artery

1. The adenosine receptors mediating relaxation of porcine isolated left anterior descending coronary arteries (LAD) and the effects of the level and type of preconstriction on the responses to adenosine analogues were examined in the present study. 2. Relaxation responses to the non-selective adenosine receptor agonist N-ethylcarboxamidoadenosine (NECA) were endothelium independent. N-Ethylcarboxamidoadenosine, GR 79236 (A1 receptor selective) and 8-cyclopentyl-1,3-dipropylxanthine (CGS 21680) (A2A receptor selective) produced full relaxation in LAD precontracted to 50% of the response to potassium depolarization with the thromboxane receptor agonist U46619. The order of potency was CGS 21680 = NECA > GR 79236, consistent with that defining the A2A receptor subtype. 3. 3,7-Dimethyl-1-propargylxanthine (DMPX; A2 receptor selective) competitively antagonized NECA and CGS 21680 with pKB values of 4.95 +/- 0.09 and 5.06 +/- 0.22, respectively. The A1 receptor selective antagonist 1,3-[3H]-dipropyl-8-cyclopentylxanthine (DPCPX) had no effect on NECA relaxation, even in the presence of DMPX. 4. The sensitivity to relaxation by NECA was dependent on the precontracting agent. Arteries precontracted with endothelin (ET)-1 were most sensitive to NECA, U46619-precontracted arteries were intermediate and KCl-precontracted arteries were least sensitive. 5. The potency of NECA was reduced when the preconstriction level was increased from 50 to 90% of maximum in U46619-precontracted arteries (pEC50 7.94 +/- 0.12 and 7.35 +/- 0.04, respectively) and, in KCl-precontracted arteries, both the potency and maximum effect of NECA were reduced when the preconstriction level increased from 50 to 80% of maximum (pEC50 7.52 +/- 0.13 and 6.91 +/- 0.26, respectively; maximum responses 82.5 +/- 10.2 and 23.9 +/- 3.6%, respectively, of the preconstricted tone). Relaxation responses to NECA were independent of the level of precontraction in ET-1-precontracted arteries. 6. In porcine LAD, relaxation responses to adenosine analogues were endothelium independent and were mediated via A2A adenosine receptors. Responses to NECA were dependent on both the level and type of preconstriction.