Separation of solubilized A2 adenosine receptors of human platelets from non-receptor [3H]NECA binding sites by gel filtration

8-(3-Isothiocyanatostyryl)caffeine Is a Selective, Irreversible Inhibitor of Striatal A(2)-Adenosine Receptors

8-(3-Isothiocyanatostyryl)caffeine (ISC) was synthesized and shown to inhibit selectively the binding of [(3)H]CGS 21680 (an A(2a)-selective agonist) at adenosine receptors in striatal membranes. The K(i) value at A(2a)-receptors was found to be 110 nM (rat), with selectivity ratios for A(2a) versus A(1)-receptors in rat, guinea pig, bovine, and rabbit striatum of >100-fold. Preincubation of membranes with ISC caused a dose-dependent, irreversible antagonism of the binding of [(3)H]CGS 21680, with an IC(50) value of 3 μM. The irreversibility is likely due to the presence of the chemically reactive isothiocyanate group, since the binding of the corresponding analogue in which the isothiocyanate was replaced with a chloro group was completely reversible. The potency of ISC to irreversibly inhibit the binding of [(3)H]CGS 21680 in several species varied in the order rat ≈ guinea pig > bovine ≈ rabbit. In all four species, binding of the A(1)-selective agonist [(3)H]R-N(6)-phenylisopropyladenosine was not diminished by pre-treatment with 2 μM ISC. The kinetics of irreversible inhibition of rat A(2a)-receptors by 2 μM ISC gave a t(1/2) of approximately 3 min. Following partial inactivation, the remaining rat A(2a)-binding sites retained the same K(d) value as in control membranes for saturation by [(3)H]CGS 21680. Thus, ISC appears to be a selective affinity label for A(2a)- versus A(1)-receptors in the brain.

Study of adenosine A2 receptors in membrane preparations from optic tectum of chicks

Binding properties of the subtypes of adenosine A2 receptors in membrane preparations and the effects of adenosine receptor ligands on cAMP accumulation in slices from the optic tectum of neonatal chicks have been investigated. [3H]2-[4-(2-p-carboxyethyl)phenylamino]-5'-N-ethylcarboxaminoadenosin e (CGS 21680), a selective ligand for adenosine A2a receptors, did not bind to optic tectal membranes, as observed with rat striatal membranes. CGS 21680 also did not induce cyclic AMP accumulation in optic tectum slices. However, 5'-N-ethylcarboxamidoadenosine (NECA), 2-chloro-adenosine or adenosine induced a 2.5- to 3-fold increase on cyclic AMP accumulation in this preparation. [3H]NECA binds to fresh non-washed-membranes obtained from optic tectum of chicks, displaying one population of binding sites, which can be displaced by NECA, 8-phenyltheophylline, 2-chloro-adenosine, but is not affected by CGS 21680. The estimated K(D) value was 400.90 +/- 80.50 nM and the Bmax was estimated to be 2.51 +/- 0.54 pmol/mg protein. Guanine nucleotides, which modulate G-proteins activity intracellularly, are also involved in the inhibition of glutamate responses by acting extracellularly. Moreover, we have previously reported that guanine nucleotides potentiate, while glutamate inhibits, adenosine-induced cyclic AMP accumulation in slices from optic tectum of chicks. However, the guanine nucleotides, GMP or GppNHp and the metabotropic glutamate receptors agonist, 1S,3R-ACPD did not alter the [3H]NECA binding observed in fresh non-washed-membranes. Therefore, the adenosine A2 receptor found in the optic tectum must be the adenosine A2b receptor which is available only in fresh membrane preparations, and its not modulated by guanine nucleotides or glutamate analogs.

[3H]-SCH 58261 labelling of functional A2A adenosine receptors in human neutrophil membranes

1. The present study describes the direct labelling of A2A adenosine receptors in human neutrophil membranes with the potent and selective antagonist radioligand, [3H]-5-amino-7-(2-phenylethyl)-2-(2-furyl)-pyrazolo[4,3-e]-1,2,4 triazolo[l,5-c]pyrimidine, ([3H]-SCH 58261). In addition, both receptor affinity and potency of a number of adenosine receptor agonists and antagonists were determined in binding, adenylyl cyclase and superoxide anion production assays. 2. Saturation experiments revealed a single class of binding sites with Kd and Bmax values of 1.34 nM and 75 fmol mg(-1) protein, respectively. Adenosine receptor ligands competed for the binding of 1 nM [3H]-SCH 58261 to human neutrophil membranes, with a rank order of potency consistent with that typically found for interactions with the A2A adenosine receptors. In the adenylyl cyclase and in the superoxide anion production assays the same compounds exhibited a rank order of potency identical to that observed in binding experiments. 3. Thermodynamic data indicated that [3H]-SCH 58261 binding to human neutrophils is entropy and enthalpy-driven. This finding is in agreement with the thermodynamic behaviour of antagonists binding to rat striatal A2A adenosine receptors. 4. It was concluded that in human neutrophil membranes, [3H]-SCH 58261 directly labels binding sites with pharmacological properties similar to those of A2A adenosine receptors of other tissues. The receptors labelled by [3H]-SCH 58261 mediated the effects of adenosine and adenosine receptor agonists to stimulate cyclic AMP accumulation and inhibition of superoxide anion production in human neutrophils.

Heterogeneous forms of adenotin-1 of different subcellular localization

The localization of the low-affinity adenosine binding protein adenotin-1 with respect to distribution in rat organs and subcellular compartments was investigated. Adenotin-1 was characterized by 5'-N-ethylcarboxamido[2,8-3H]adenosine ([3H]NECA) binding and Western blotting. Cytosolic as well as membrane fractions of all tissues contained adenotin-1. Highest levels of membrane-bound adenotin-1 were found in the liver (liver > kidney approximately spleen approximately lung > forebrain approximately cerebellum > fat heart - striated muscle), whereas highest levels of cytosolic adenotin-1 were detected in spleen, liver, lung and fat. Subcellular fractions from rat liver were prepared by differential and density gradient centrifugation. Like the homologous proteins endoplasmin or gp96, adenotin-1 is enriched in the endoplasmic reticulum. Cytosolic and membrane-bound adenotin-1 species are pharmacologically distinct, because in the liver particulate fraction adenotin-1 showed a more rapid binding kinetics, a twofold lower affinity for [3H]NECA (KD 227 nM vs. 105 nM) and a sevenfold higher affinity for 2-chloroadenosine than the cytosolic protein (Ki 1.48 microM vs. 9.25 microM). In rat liver cytosol, two different binding sites were found, which differed in [3H]NECA binding kinetics and displayed a hundredfold difference in their affinity for 2-chloro-5'-N-methylcarboxamidoadenosine (Ki 45.8 nM vs. 4.76 microM). The presence of adenotin-1 in subcellular fractions, as determined by radioligand binding, was confirmed by Western blotting. Adenotin-1 was detected as a 98-kDa band in all rat liver subcellular fractions, which agrees with the molecular mass determined for the purified protein. In the cytosol, a 65-kDa hand was labeled more intensely than the 98-kDa band. This additional band probably represents the pharmacologically distinct species of adenotin-1 found in the cytosol.

Characterization of a novel adenosine binding protein sensitive to cyclic AMP in rat brain cytosolic and particulate fractions

A novel binding site for the adenosine receptor agonist 5'-N-ethylcarboxamidoadenosine (NECA), which was enriched in rat forebrain, was characterized in cytosolic and particulate preparations. The site showed a pharmacological profile different from other [3H]NECA binding proteins and was named adenotin 2. [3H]NECA was bound in the presence of 100 microM 2-chloroadenosine with a Kd of 45.4 nM and a Bmax of 4711 fmol/mg in the cytosol and a Kd of 72.4 nM and a Bmax of 4844 fmol/mg in the crude membrane fraction. The presence of two different binding sites on adenotin 2 for [3H]NECA was shown in kinetic experiments. This protein showed identical pharmacological profiles in both subcellular preparations. [3H]NECA was displaced by purine analogues with a rank order of potency of NECA > 3'5' cyclic AMP (cAMP) > 5'-deoxy-5'-chloroadenosine > S-adenosylhomocysteine approximately 5'-deoxy-5'-methylthioadenosine (MeSA) > adenosine approximately adenine. cAMP inhibited [3H]NECA binding allosterically, whereas adenine and MeSA acted competitively. Inhibitors and activators of protein kinases such as N-(2-aminoethyl)-5-isoquinolinesulfonamide, Sp-adenosine cyclic monophophothioate and (8R*, 9S*, 11S*)-(-)-9-hydroxy-9-methoxy -carbonyl-8-methyl-2,3,9, 10-tetrahydro-8,11-epoxy-1H, 8H, 11H-2, 7b, 11a-triazadibenzo-(a,g)cycloocta(cde)-trinden-1-one (K 252a) interacted with [3H]NECA binding to adenotin 2 in nanomolar concentrations. Adenosine-5'-O-(3-thiotriphosphate) (100 microM) increased the affinity of [3H]NECA to a Kd of 9 nM and diminished the affinity of cAMP. The pharmacological characteristics of this novel binding site for [3H]NECA resemble those of the inhibition of phosphorylation processes by adenosine and its derivatives in heart and smooth muscle but are distinct from known adenosine receptors, adenosine binding proteins and protein kinases.

A2 adenosine receptors: their presence and neuromodulatory role in the central nervous system

1. Adenosine is an endogenous neuromodulator that exerts its depressant effect on neurons by acting on the A1 adenosine receptor subtype. Excitatory actions of adenosine, mediated by the activation of the A2 adenosine receptor subtype, have also been shown in the central nervous system. 2. Adenosine A2a receptors are highly localized in the striatum, as demonstrated by the binding assay of the A2a selective agonist, CGS2680, and by analysis of the A2 receptor mRNA localization with in situ hybridization histochemistry. However, adenosine A2a, receptors, albeit at lower levels, are also localized in other brain regions, such as the cortex and the hippocampus. 3. In the striatum, adenosine A2a, receptors are implicated in the control of motor activity. Evidences exists of an antagonistic interaction between adenosine A2a and dopamine D2 receptors. 4. Utilizing selective agonists and antagonists for adenosine A2a receptors, their role in the modulation of the release of several neurotransmitters (acetylcholine, dopamine, glutamate, GABA) has been extensively studied in the brain (striatum, cortex, hippocampus). Controversial results have been obtained and, because the overall effect of endogenous adenosine in the brain is that of an inhibitory tonus, the physiological meaning of the excitatory A2 receptor remains to be clarified.

Pharmacological and biochemical characterization of purified A2a adenosine receptors in human platelet membranes by [3H]-CGS 21680 binding

1. The binding properties of human platelet A2a adenosine receptors, assayed with the A2a-selective agonist, [3H]-2-[p-(2-carboxyethyl)-phenethylamino]-5'-N-ethylcarboxamidoad enosine ([3H]-CGS 21680), are masked by a non-receptorial component, the adenotin site. In order to separate A2a receptors from adenotin sites, human platelet membranes were solubilized with 1% 3-[(3-cholamidopropyl)dimethyl-ammonio]-1-propanesulphonate (CHAPS). The soluble platelet extract was precipitated with polyethylene glycol (PEG) and the fraction enriched in adenosine receptors was isolated from the precipitate by differential centrifugation. 2. The present paper describes the binding characteristics of the selective A2a agonist, [3H]-CGS 21680, to this purified platelet membrane preparation. In addition, receptor affinity and potency of several adenosine agonists and antagonists were determined in binding and adenylyl cyclase studies. 3. Saturation experiments revealed a single class of binding site with Kd and Bmax values of 285 nM and 2.07 pmol mg-1 of protein respectively. Adenosine receptor ligands competed for the binding of 50 nM [3H]-CGS 21680 to purified protein, showing a rank order of potency consistent with that typically found for interactions with the A2a adenosine receptors. In the adenylyl cyclase assay the compounds examined exhibited a rank order of potency very close to that observed in binding experiments. 4. Thermodynamic data indicated that [3H]-CGS 21680 binding to the purified receptor is totally entropy-driven in agreement with results obtained in rat striatal A2a adenosine receptors. 5. It is concluded that in the purified platelet membranes there is a CGS 21680 binding site showing the characteristic properties of the A2a receptor. This makes it possible to use this compound for reliable radioligand binding studies on the A2a adenosine receptor of human platelets.

Regulation of the stress-like protein adenotin in PC 12 cells by ethanol exposure

The effects of chronic ethanol exposure on the stress-like protein adenotin were investigated using the radioligand [3H]-5'-N-ethylcarboxamidoadenosine ([3H]NECA). A 4-day exposure to 150 mM ethanol increased both the KD and the density of [3H]NECA binding sites. These changes were not due to residual ethanol as the acute addition of ethanol did not alter [3H]NECA binding. Chronic ethanol exposure of A126-1B2-1 cells, which are a mutant PC 12 cell line deficient in protein kinase A (PKA), increased the cellular density of adenotin, but did not affect the KD for the radioligand. Conversely, when PC 12 cells were exposed to 10 microM forskolin for either 2 or 4 days, the cellular density of adenotin was not altered, but the affinity of adenotin for [3H]NECA was reduced significantly. An increase in KD was not observed after a 1-hr exposure of PC 12 cells to forskolin, indicating that the reduction in affinity for the radioligand was not due simply to a PKA-mediated phosphorylation of adenotin. The present study demonstrated that chronic ethanol regulates adenotin through two different mechanisms. The ethanol-induced increase in the density of adenotin does not involve PKA, while the reduction in affinity ap pears to involve a cAMP-dependent mechanism.

Adenosine physiology and pharmacology: how about A2 receptors?

Adenosine participates in the physiology of central and peripheral tissues through several subtypes of G-protein-coupled receptors. Positively linked to adenylate cyclase, A2 receptors have been subdivided into A2a and A2b sites on the basis of their molecular, biochemical and pharmacological properties. They exhibit selective distribution, and are implicated in the modulation of psychomotor activity, circulation, respiration, and metabolism. Recent data support the evidence that adenosine A2 receptor properties may prove useful in future drug development, and selective manipulation of receptor-associated biologic effects might be relevant in the treatment of various disorders, including psychiatric diseases, hypoxia/ischemia, inflammation or erythrocytosis.

Binding characteristics of the adenosine A2 receptor ligand [3H]CGS 21680 to human platelet membranes

The binding characteristics of the selective adenosine A2 agonist [3H]CGS 21680 ([3H]2-[p-(2-carboxyethyl)-phenethyl-amino]-5'-N- ethylcarboxamidoadenosine) were determined in human platelet membranes. Specific binding was saturable, reversible and dependent upon protein concentration. Saturation experiments revealed a single class of binding sites with Kd and Bmax values of 1.4 microM and 5.9 pmol/mg of protein, respectively. Adenosine receptor agonists and antagonists competed for the binding of [3H]CGS 21680 (50 or 200 nM) to human platelet membranes showing a rank order of potency consistent with that typically found for interactions at the adenosine A2 receptor. Adenylate cyclase stimulation and platelet aggregation inhibition induced by adenosine agonists exhibited a rank order of potency close to that observed in binding experiments. However, the adenosine A1 receptor agonists, R- and S-N6-(2-phenylisopropyl)adenosine, (R-PIA) and (S-PIA), N6-cyclohexyladenosine (CHA) and 2-chloro-N6-cyclopentyladenosine (CCPA), which stimulate adenylate cyclase and inhibit platelet aggregation in the low microM range, displaced [3H]CGS 21680 only in the high microM range. In conclusion, we have found that [3H]CGS 21680, which is widely used as a specific A2 agonist in binding studies on brain tissues, is not appropriate for the characterization of the human platelet adenosine A2 receptor.

Characterization of the adenosine receptor in microvascular coronary endothelial cells

In the present work we studied the effect of adenosine and various adenosine analogues on cAMP level in guinea pig coronary endothelial cells of microvascular origin. The tested adenosine agonist mediate a concentration-dependent increase in cAMP level. The rank order of potency was 5'-N-ethylcarboxamidoadenosine (NECA) > CGS 21680 > N6-phenylisopropyladenosine (R-PIA) > 2-chloro-N6-cyclopentyladenosine (CCPA) which is typical for an adenosine A2 receptor. Their respective concentrations for half maximal stimulation of cAMP formation were 0.36 microM, 0.82 microM, 4.7 microM and 9.8 microM. The tested agonists showed differences in efficacy, NECA being the most efficacious. R-PIA, CCPA and adenosine were less efficacious, suggesting partial agonism. The efficacy of adenosine was unchanged by the addition of the nucleoside transport inhibitor S(4-nitrobenzyl)-6-thioinosine (NBTI, 10 microM) suggesting that inhibition of adenylyl cyclase through P-site activation is not responsible for the observed low efficacy of adenosine. We could demonstrate CGS 21680 activation of adenylyl cyclase in a peripheral receptor. We therefore suggest that the endothelial adenosine receptor resembles the striatal adenosine A2a receptor.

Effects of divalent cations on adenosine agonist binding to A1 receptors and non-A1/non-A2 sites in rat cerebral cortex

In the present study results are reported concerning the effects of several divalent cations on the binding characteristics of [3H]-cyclohexyladenosine on A1 adenosine receptors and of [3H]-N-ethylcarboxamidoadenosine on non-A1/non-A2 sites in membranes from cerebral cortex of the rat. The [3H]-cyclohexyladenosine binding to A1 receptors was dose-dependently increased by Mn2+, Co2+, Ca2+. The binding characteristics of the agonist were differently affected by Ca2+/Mn2+ and Mg2+. Ca2+ and Mn2+ increased the Bmax value without any change in Kd, whereas Mg2+ decreased the Kd value without changing the Bmax. In the presence of Ca2+ and Mg2+ the Kd value was similar to that obtained in the presence of Mg2+, whereas the Bmax value was similar to the apparent number of binding sites calculated in the presence of Ca2+. The cations, Cu2+, Cd2+, Zn2+, decreased the A1 binding with IC50 values of 19.6 microM, 39.2 microM and 103.9 microM, respectively. The binding characteristics of [3H]-N-ethylcarboxamidoadenosine to non-A1/non-A2 sites were affected by Ca2+, Mn2+, Co2+ and Mg2+ in the opposite manner to A1 receptors. They decreased the binding with IC50 values of 20.1 mM, 22.8 mM, 93.0 mM and 18.1 mM, respectively. This occurs through an enhancement in Kd values without changes in the number of binding sites. The findings on A1 receptor and non-A1/non-A2 binding site, taken together, suggest that cations could also exert a modulatory action via specific interactions with divalent cation binding sites on the receptor molecule.

In vivo and ex vivo effects of adenosine A1 and A2 receptor agonists on platelet aggregation in the rabbit

We have investigated both in vivo and ex vivo antiaggregatory activity of three adenosine receptor agonists in the anesthetized rabbit: the non-selective, 5'-N-ethyl-carboxamidoadenosine (NECA), the selective adenosine A1 receptor agonist, 2-chloro-N6-cyclopentyladenosine (CCPA) and the new selective A2 receptor agonist, 2-hexynyl-NECA. The drugs were administered by 30-min intravenous infusion at a dose reducing mean blood pressure by 40-50%. NECA and CCPA also markedly decreased heart rate. In ex vivo experiments, NECA (10 micrograms/kg) and 2-hexynyl-NECA (10 micrograms/kg) maximally inhibited adenosine 5'-diphosphate (ADP)-induced platelet aggregation at the end of drug infusion by 26.7 +/- 2.9% and 25.2 +/- 3.5%, respectively. In in vivo studies, the inhibition of platelet aggregation was evaluated using the technique based on selective accumulation of 111In-labeled platelets in pulmonary microcirculation upon challenge with ADP 100 micrograms/kg. NECA (10 micrograms/kg) and 2-hexynyl-NECA (10 micrograms/kg) decreased peak values for platelet accumulation by 35.3 +/- 6.9% and 52.5 +/- 5.9% and the area under curve values by 37.7 +/- 8.7% and 41.2 +/- 12.0%, respectively. In comparison, CCPA (100 micrograms/kg) did not affect platelet responses to ADP in either of the experimental models. Thus, the present study clearly demonstrates for the first time the in vivo antiplatelet activity of adenosine A2 receptor agonists, whereas the adenosine A1 receptor agonist was inactive, in consonance with the in vitro data.

Purification and characterization of an adenotin-like adenosine binding protein from human platelets

A low-affinity adenosine binding protein (adenotin) was purified from human platelet membranes by a four-step procedure. Purification was achieved after extraction from human platelet membranes with 0.3% 3-[3-(cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS). Further purification included Sepharose CL6B gel filtration, DEAE-Sepharose CL6B, and hydroxylapatite chromatography. The protein was purified 884-fold to homogeneity with a 25% yield of binding activity from the membranes. 5'-[8(n)-3H]-N-ethylcarboxamidoadenosine ([3H]NECA) binds to the purified protein with a KD of 155 (144-167) nmol/l and a Bmax of 1.85 +/- 0.10 nmol/mg of protein. Sodium dodecylsulfate polyacrylamide gel electrophoresis of purified protein revealed a single band at 98 kDa. The 2-chloro-substituted adenosine analogs 2-chloro-5'-N-methylcarboxamidoadenosine (CIMECA) and 2-chloro-5'-N-ethylcarboxamidoadenosine (CINECA) were identified as new high affinity ligands of the purified protein showing Ki values of 18 nmol/l and 28 nmol/l, respectively. The low-affinity adenosine binding protein showed a pharmacological profile as follows: CIMECA > 5'-N-ethylcarboxamidoadenosine (NECA) > 2-chloroadenosine (CIA) > 2-[4-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamidoadenosin e (CGS 21,680) > R-N6-phenylisopropyl-adenosine (R-PIA). Amino-terminal sequence analysis revealed homologies to endoplasmin, glucose regulated protein (GRP94), tumor rejection antigen precursor (GP96), and some stress related proteins.

Solubilized rabbit striatal A2a-adenosine receptors: stability and antagonist binding

The A2a-adenosine binding subunit from rabbit striatal membranes was solubilized using 1% 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate and was characterized using the antagonist radioligand [3H]8-[4-[[[[2-aminoethyl)amino]carbonyl]methyl]oxy] phenyl]-1,3-dipropylxanthine (XAC). The solubilized receptor was very stable, with 55% of the specific [3H]XAC binding remaining after storage for 15 days at 4 degrees C. The dissociation constant (Kd) for binding of [3H]XAC to solubilized A2 receptors was determined in saturation studies to be 4.0 nM, with a Bmax of 600 fmol/mg protein. Xanthine inhibitors displaced the specific binding of the adenosine antagonist [3H]XAC (in the presence of 50 nM 8-cyclopentyl-1,3-dipropylxanthine) at 25 degrees C, with Ki values consonant with the expected affinities at A2a receptors. Binding of [3H]XAC (1 nM) or the adenosine agonist [3H]2-(carboxyethylphenylethylamino)adenosine-5'-N-ethyl carboxamide (5 nM) to A2a receptors was diminished in the presence of 0.1 M Na+ in both membranes and solubilized preparations. Agonist binding was increased (by 280% for membranes and 180% for solubilized receptors), and antagonist binding was decreased in the presence of 10 mM Mg2+. Displacement of [3H]XAC by the agonist (R)-N6-phenylisopropyladenosine was biphasic, corresponding to high (IC50 = 188 nM, RH = 30%) and low (IC50 = 9730 nM, RL = 70%) affinity sites. Preincubation with 100 microM GTP (10 mM Mg2+) converted the high affinity binding to low affinity, suggesting that receptor and G-protein are dissociated by the guanine nucleotide. The solubilized receptor was more easily inactivated by exposure to the reducing agent dithiothreitol (IC50 = 3 mM) than in membranes (IC50 = 220 mM), suggesting increased accessibility of structurally essential disulfide bridges.

Structure-activity relationships of 8-styrylxanthines as A2-selective adenosine antagonists

A series of substituted 8-styryl derivatives of 1,3,7-alkylxanthines was synthesized as potential A2-selective adenosine receptor antagonists, and the potency at rat brain A1- and A2-receptors was studied in radioligand binding experiments. At the xanthine 7-position, only small hydrophobic substituents were tolerated in receptor binding. 7-Methyl analogues were roughly 1 order of magnitude more selective for A2 versus A1 receptors than the corresponding 7-H analogues. 1,3-Dimethylxanthine derivatives tended to be more selective for A2-receptors than the corresponding 1,3-diallyl, diethyl, or dipropyl derivatives. Substitutions of the phenyl ring at the 3-(monosubstituted) and 3,5-(disubstituted) positions were favored. 1,3, 7-Trimethyl-8-(3-chlorostyryl)xanthine was a moderately potent (Ki vs [3H]CGS 21680 was 54 nM) and highly A2-selective (520-fold) adenosine antagonist. 1,3,7-Trimethyl-8-[(3-carboxy-1-oxopropyl)amino] styryl]xanthine was highly A2-selective (250-fold) and of enhanced water solubility (max 19 mM). 1,3-Dipropyl-7-methyl-8-(3,5-dimethoxystyryl) xanthine was a potent (Ki = 24 nM) and very A2-selective (110-fold) adenosine antagonist.

Effects of adenosine derivatives on human and rabbit platelet aggregation. Correlation of adenosine receptor affinities and antiaggregatory activity

The inhibitory effects of several adenosine analogues, including the new A2-selective agonists 2-[p-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamido-adenosi ne (CGS 21680) and 2-hexynyl-5'-N-ethylcarbox-amidoadenosine (2-hexynyl-NECA), were investigated in vitro on human and rabbit platelet aggregation. The compounds examined inhibited ADP-induced platelet aggregation over a wide range of potency. The rank order of activity was similar between the two species thus showing that the rabbit is a useful animal model for studying the effects of adenosine derivatives on platelet aggregation. 2-Hexynyl-NECA was found to be the most potent adenosine compound of those currently available, having IC50 values of 0.10 and 0.07 microM in human and rabbit platelets, respectively. Conversely, the A1 agonists R(-)-N6-(2-phenylisopropyl) adenosine (R-PIA), S(+)-N6-(2-phenylisopropyl) adenosine (S-PIA) and 2-chloro-N6-cyclopentyladenosine (CCPA) were the least potent compounds with IC50 values in the micromolar range. The potency of the compounds in inhibiting platelet aggregation was found to be highly correlated with their affinity for A2 receptors as measured using 3H-CGS 21680 binding in rat brain striatum.

Hypothesis for the receptors of human blood platelet aggregation and its inhibition by structure-activity relationship

We tried to clarify the size and the common charge distribution of the inhibition or stimulation of human platelet aggregation by structure-activity relationship. Numerous inhibiting and stimulating agents were able to enter the receptors. Inhibitory receptor had recess of 14 x 12.5 A in diameter. Stimulatory receptor had recess of 11 x 12 A in diameter. In the recess, there were three charges, two negative and one positive in the inhibitory receptor, and one negative and two positive in the stimulatory receptor, respectively. Charge distributions and conformation of inhibiting or stimulating agents were similar for the inhibitory agents, prostaglandin I2 (PGI2), PGD2, PGE1 adenosine and isoproterenol and conformation of the stimulating agents, thromboxane A2 (TXA2), platelet activating factor (PAF), adenosine diphosphate (ADP) and adrenaline. Each molecule had 3-10 inhibiting and stimulating conformations. The ratio of the number of conformations for inhibition and stimulating of platelet aggregation was highest for PGI2 which showed the strongest inhibitory activity. TXA2 was opposite in both respects.

Radioligand binding to adenosine receptors and adenosine uptake sites in different brain regions of normal and narcoleptic dogs

The present study compares the characteristics of radioligand binding to adenosine receptors and adenosine uptake sites in 100- and 50-day-old normal and narcoleptic dogs. Binding to A1 receptors was quantified using a selective A1 agonist ([3H]N6-[(R)-1-methyl-2-phenylethyl] adenosine, [3H]R-PIA) and an antagonist ([3H]dipropyl-8-cyclopentyl-xanthine, [3H]CPX). Differences in the binding of [3H]R-PIA and that of [3H]5'-ethylcarboxamide adenosine ([3H]NECA), which binds to both A1 and A2 receptors with similar affinities, were used to quantify A2 receptors. Nucleoside transport sites were labeled with [3H]nitrobenzylthioinosine ([3H]NBTI), a potent inhibitor of nucleoside transport systems. The present study offered no evidence that either adenosine A1 receptors and adenosine uptake sites in the frontal cortex or adenosine A2 receptors in the putamen were altered in narcoleptic dogs. However, we found that adenosine A1 receptors in the dog exist in different affinity states and that the affinity state in which the receptor is found depends on the brain region examined. A characterization of these low- and high-affinity sites was performed and results indicated that these sites cannot be explained by a single interaction of the A1 receptor with a single G-protein population.

Purinergic modulation of field stimulation responses of rat and human vas deferens smooth muscle

1. Guanethidine at 5 x 10(-6) M strongly inhibited rat prostatic but not epididymal vas deferens, reflecting differences in innervation and the neurogenic field stimulation responses of these tissues. 2. Adenosine and ATP inhibited the field stimulation responses of rat prostatic vas deferens by 56 and 50% respectively. A 10-min pretreatment with 10(-4) M caffeine partly reversed this inhibition, by 55% in the case of adenosine and 60% for ATP. 3. Pretreatment for 10 min with 5 microM quinidine failed to significantly alter the extent of either adenosine or ATP inhibition of the field stimulation responses of rat prostatic vas deferens. 4. 8-Phenyltheophylline, the selective blocker of the A1 subtype of the P1 receptor, partly reversed adenosine-induced inhibition of the vas deferens FS responses. NECA, the selective agonist of the A2 subtype of the P1 receptor, very strongly inhibited vas deferens FS responses. 5. Field stimulation responses of human vas deferens were also inhibited by both adenosine and ATP but to a lesser extent and more variably than in rat tissue. 6. Adenosine and ATP inhibition was reversed by caffeine pretreatment, but far more variably than in rat tissue, and quinidine was without significant effect on inhibition of the responses. 7. It is concluded that in these tissues adenosine and ATP may operate via a P1 type receptor of both A1 and A2 subtypes and that a P2 type receptor may be lacking.

Properties of adenotin reconstituted into phospholipid vesicles

Adenotin is a low affinity adenosine binding protein that has amino terminal homology with mammalian and avian stress proteins. Human placental adenotin was solubilized and reconstituted into phospholipid vesicles with an overall yield of 30%. The properties of adenotin in vesicles were similar to the native membranes as follows: association has a Kobs of 0.61 +/- 0.03 minute-1; equilibrium is reached in approximately 15 minutes; and the first order dissociation constant is 5.0 +/- 0.3 minute-1. Displacement analysis reveals an agonist potency order and Ki values as follows: N-ethylcarboxamidoadenosine, 0.35 microM; 2-chloroadenosine, 1.5 microM; R-phenylisopropyladenosine, greater than 1000 microM. The addition of 100 microM 5'-guanylylimidodiphosphate did not decrease binding of 5'-N-ethylcarboxamidoadenosine (NECA) at 37 degrees C or 4 degrees C but did decrease the IC50 for PC12 and JAR cell membrane agonist binding from 9.9 to 3.3 microM and increase the binding to 150-211% of the control value at 37 degrees C. The latter studies at 37 degrees C showed high variability. Using binding sites reconstituted into vesicles and gel filtration chromatography and agonist related guanine nucleotide release, the authors investigated whether these changes were related to an interaction between adenotin and a guanine nucleotide regulatory protein. No evidence for such an interaction was found. These data suggest that adenotin retains its binding properties when reconstituted into phospholipid vesicles. The function of this low affinity adenosine binding site remains to be discovered. However, the reconstitution of adenotin into phospholipid vesicles provides a method to study its function.

Evidence for the involvement of cyclic GMP in adenosine-induced, age-dependent vasodilatation

1. Adenosine-induced dilatation of rat aorta was present in aorta taken from 4 week-old rats, attenuated with increase in age of rats to 8 weeks, and was virtually absent in the aorta from 12 week-old rats. 2. Removal of the endothelium by mechanical rubbing attenuated adenosine-induced dilatation. 3. Haemoglobin and methylene blue partly reversed the adenosine-induced endothelium-dependent dilatation. 4. The order of potency of adenosine derivatives was 5'-(N-ethylcarboxamido)adenosine (NECA) greater than 2-phenylaminoadenosine (CV-1808) greater than 2-chloroadenosine greater than N6-([R]-[-]-phenylisopropyl)adenosine (R-PIA) greater than adenosine greater than N6-cyclohexyladenosine (CHA) greater than N6-([S]-[+]-phenylisopropyl)adenosine (S-PIA), indicating that adenosine receptors mediating the dilatation are of the A2 subtype. 5. [3H]-NECA bound to preparations of membranes from rats of 4 weeks old; it was displaced more effectively by NECA and the A2 ligand CV-1808 than by the A1 ligands CHA and S-PIA. ligands CHA and S-PIA. 6. The number but not affinity of specific binding sites for NECA decreased considerably with increase in age of rats to 8 weeks, and binding sites for [3H]-NECA were hardly detected in membrane preparations from rats of 20 weeks old. 7. Adenosine caused a marked increase in cyclic GMP production, but did not induce an increase in the cyclic AMP level. 8. This increase in cyclic GMP production induced by adenosine was abolished by methylene blue or 8-phenyltheophylline, or by removal of the endothelium. 9. The age-associated decrease in adenosine-induced dilatation was found to be associated with a reduction in the formation of cyclic GMP, but not of cyclic AMP. 10. These results suggest that adenosine causes dilatation via A2 receptors by inducing production of an endothelium-derived relaxing factor (EDRF), which in turn stimulates soluble guanylate cyclase, and so increases production of cyclic GMP. It is also suggested that the main reason for the age-associated decrease in adenosine-induced dilatation is a decrease in the number of A2-receptors or the ability of the endothelium to produce EDRF, leading to decreased production of cyclic GMP.

Agonist derived molecular probes for A2 adenosine receptors

The adenosine agonist 2-(4-(2-carboxyethyl)phenylethylamino)-5'-N- ethylcarboxamidoadenos ine (CGS21680) was recently reported to be selective for the A2 adenosine receptor subtype, which mediates its hypotensive action. To investigate structure/activity relationships at a distal site, CGS21680 was derivatized using a functionalized congener approach. The carboxylic group of CGS21680 has been esterified to form a methyl ester, which was then treated with ethylenediamine to produce an amine congener. The amine congener was an intermediate for acylation reactions, in which the reactive acyl species contained a reported group, or the precursor for such. For radioiodination, derivatives of p-hydroxyphenylpropionic, 2-thiophenylacetic, and p-aminophenylacetic acids were prepared. The latter derivative (PAPA-APEC) was iodinated electrophilically using [125I]iodide resulting in a radioligand which was used for studies of competition of binding to striatal A2 adenosine receptors in bovine brain. A biotin conjugate and an aryl sulfonate were at least 350-fold selective for A2 receptors. For spectroscopic detection, a derivative of the stable free radical tetramethyl-1-piperidinyloxy (TEMPO) was prepared. For irreversible inhibition of receptors, meta- and para-phenylenediisothiocyanate groups were incorporated in the analogs. We have demonstrated that binding at A2 receptors is relatively insensitive to distal structural changes at the 2-position, and we report high affinity molecular probes for receptor characterization by radioactive, spectroscopic and affinity labelling methodology.

A selective binding site for 3H-NECA that is not an adenosine A2 receptor

In homogenates of NG108-15 cells, adenosine analogues activate adenylate cyclase with the following order of potency: N-ethylcarboxamidoadenosine (NECA) greater than 2-chloroadenosine greater than N6-(L-phenylisopropyl)-adenosine (PIA) = cyclohexyladenosine = 2-phenylaminoadenosine. Adenosine receptor antagonists inhibit NECA-stimulated adenylate cyclase activity with the order of potency 3-isobutyl-1-methyl-xanthine (IBMX) greater than theophylline greater than caffeine. These data suggest that these ligands act at an adenosine A2 receptor. There is an apparently homogenous population of saturable 3H-NECA binding sites in homogenates of NG108-15 cells. These sites have an affinity for 3H-NECA of approximately 1 microM, and are present at a density of approximately 10 pmol/mg protein. Unlabelled NECA, 2-chloroadenosine, IBMX and theophylline displace 3H-NECA binding, with an order of potency that suggests that the 3H-NECA binding site may represent an adenosine A2 receptor. However, PIA, cyclohexyladenosine and 2-phenylaminoadenosine produce no detectable displacement of 3H-NECA binding at concentrations that produce a maximal stimulation of adenylate cyclase activity. Pretreatment of NG108-15 cells with either NECA or PIA produces a homologous desensitization of subsequent responses to all the adenosine analogues, with no effect on subsequent responses to a prostacyclin receptor agonist or NaF. This suggests that all the adenosine analogues examined activate an adenosine A2 receptor. Therefore, the 3H-NECA site at which PIA is inactive cannot represent this receptor.

Direct autoradiographic localization of adenosine A2 receptors in the rat brain using the A2-selective agonist, [3H]CGS 21680

The regional distribution of adenosine A2 receptors in the rat brain was determined using the A2-selective agonist ligand [3H](2-p-carboxyethyl)phenylamino)-5'-N-carboxamidoadenosine (CGS 21680) by quantitative receptor autoradiography. [3H]CGS 21680 binding was highly localized in the striatal region of the rat brain with the greatest density of binding found in the caudate-putamen, nucleus accumbens and olfactory tubercle. Additionally, lower levels of binding were also found in the globus pallidus. No significant amounts of [3H]CGS 21,680 binding were detected in other brain regions. This localization of brain adenosine A2 receptors was markedly different from the known regional distribution of A1 receptors which are highly concentrated in cerebellum, hippocampus, thalamus and cortex. The present results provide further evidence for a specific contribution of adenosine in the modulation of central neurotransmission.

Influence of the molecular structure of N6-(omega-aminoalkyl)adenosines on adenosine receptor affinity and intrinsic activity

The affinities of a series of N6-(omega-aminoalkyl)adenosines as probes for A1 and A2 adenosine receptors were determined in various radioligand binding assays and the intrinsic activities were measured in adenylate cyclase assays. Clear species differences were noticed for A1 receptor affinity of these adenosine receptor agonists, the compounds being more active in calf than in rat brain tissue. The affinity profile within the series was, however, rather similar in both membrane preparations, with N6-9-aminononyladenosine displaying highest affinity. The A2 receptor affinities were comparable to values measured for the A1 receptor in its low affinity state, as assessed with a radiolabelled antagonist in the presence of 1 mM GTP. Calculation of the intrinsic activities of the adenosine analogues from their modulating action on adenylate cyclase showed almost all the compounds to be equally effective to (-)-N6-(R-phenylisopropyl) adenosine, on either A1 or A2 adenosine receptors. N6-3-Aminopropyl- and N6-12-aminododecyladenosine, however, proved to be partial agonists, the first on A1 and the second on A2 adenosine receptors. The data are used as the basis for a discussion of adenosine receptor subtype selectivity and intrinsic activity in general.

2',3'-Dideoxy-N6-cyclohexyladenosine: an adenosine derivative with antagonist properties at adenosine receptors

The 2',3'-dideoxy analogue of the potent A1 receptor agonist, N6-cyclohexyladenosine (CHA), was synthesized as a potential antagonist for the A1 adenosine receptor. In studies on adenylate cyclase 2',3'-dideoxy-N6-cyclohexyladenosine (ddCHA) did not show agonist properties at A1 or at A2 receptors. However, it antagonized the inhibition by R-PIA of adenylate cyclase activity of fat cell membranes via A1 receptors with a Ki value of 13 microM. ddCHA competed for the binding of the selective A1 receptor antagonist, [3H]8-cyclopentyl-1,3-dipropylxanthine ([3H]DPCPX), to rat brain membranes with a Ki value of 4.8 microM; GTP did not affect the competition curve. In contrast to the marked stereoselectivity of the A1 receptor for the alpha- and the natural beta-anomer of adenosine, the alpha-anomer of ddCHA showed a comparable affinity for the A1 receptor (K1 value 13.9 microM). These data indicate that the 2'- and 3'-hydroxy groups of adenosine and its derivatives are required for agonist activity at and high affinity binding to A1 adenosine receptors and for the distinction between the alpha- and beta-forms.

2-Chloro-N6-cyclopentyladenosine: a highly selective agonist at A1 adenosine receptors

2-Chloro-N6-cyclopentyladenosine (CCPA) was synthesized as a potential high affinity ligand for A1 adenosine receptors. Binding of [3H]PIA to A1 receptors of rat brain membranes was inhibited by CCPA with a Ki-value of 0.4 nM, compared to a Ki-value of 0.8 nM for the parent compound N6-cyclopentyladenosine (CPA). Binding of [3H]NECA to A2 receptors of rat striatal membranes was inhibited with a Ki-value of 3900 nM, demonstrating an almost 10,000-fold A1-selectivity of CCPA. CCPA inhibited the activity of rat fat cell membrane adenylate cyclase, a model for the A1 receptor, with an IC50-value of 33 nM, and it stimulated the adenylate cyclase activity of human platelet membranes with an EC50-value of 3500 nM. The more than 100-fold A1-selectivity compares favourably with a 38-fold selectivity of CPA. Thus, CCPA is an agonist at A1 adenosine receptors with a 4-fold higher selectivity and 2-fold higher affinity than CPA, and a considerably higher selectivity than the standard A1 receptor agonist R-N6-phenylisopropyladenosine (R-PIA). CCPA represents the agonist with the highest selectivity for A1 receptors reported so far.