Expression of adenosine receptor subclasses in malignant and adjacent normal human prostate tissues.

BACKGROUND: Adenosine, a purine nucleoside plays important roles in the pathogenesis of cancer initiation and promotion via interaction with four adenosine receptors. In the present study we examined the differential expression pattern of adenosine receptors in the malignant and adjacent normal human prostate tissues. METHODS: Prostate cancer tissue samples and adjacent normal tissues were obtained from 20 patients undergoing radical prostatectomy and histopathological diagnosis was confirmed for each sample. Total RNA was extracted and reverse transcribed into cDNA and the mRNA expression levels of adenosine receptors were investigated by Taq-man real-time RT-PCR experiment. Quantitative protein analysis was done by Western blotting experiment. Moreover, the mRNA and protein expression levels of adenosine receptors were measured after androgen treatment. RESULT: Taq-man real-time RT-PCR measurements show different expression levels of adenosine receptor transcripts. A2B adenosine receptor was predominantly expressed in tumor tissues (2.4-fold) followed by significantly expression of A3 (1.6-fold) and A2A adenosine receptors (1.5-fold) compared to adjacent normal tissues. The presence of adenosine receptors at protein levels in prostate cancer tissues compared with normal tissues was shown the following rank order: A2B > A3 > A2A > A1 . Androgen receptor regulates adenosine receptors mRNA and protein expression in AR-positive LNCaP cells, which was not seen in AR-negative PC-3 cells. CONCLUSION: These results indicated for the first time, the differential mRNA expression profile and protein levels of adenosine receptors in the human prostate cancer. Interestingly, the A2B adenosine receptor followed by A3 is highly expressed in prostate tumor samples in comparison with the adjacent normal tissues. The findings support the possible key role of A2B adenosine receptor in promoting cancer cell growth and suggest that A2B may be a novel target for prostate cancer treatment.

International Union of Basic and Clinical Pharmacology. LXXXI. Nomenclature and classification of adenosine receptors--an update.

In the 10 years since our previous International Union of Basic and Clinical Pharmacology report on the nomenclature and classification of adenosine receptors, no developments have led to major changes in the recommendations. However, there have been so many other developments that an update is needed. The fact that the structure of one of the adenosine receptors has recently been solved has already led to new ways of in silico screening of ligands. The evidence that adenosine receptors can form homo- and heteromultimers has accumulated, but the functional significance of such complexes remains unclear. The availability of mice with genetic modification of all the adenosine receptors has led to a clarification of the functional roles of adenosine, and to excellent means to study the specificity of drugs. There are also interesting associations between disease and structural variants in one or more of the adenosine receptors. Several new selective agonists and antagonists have become available. They provide improved possibilities for receptor classification. There are also developments hinting at the usefulness of allosteric modulators. Many drugs targeting adenosine receptors are in clinical trials, but the established therapeutic use is still very limited.

[Roles of adenosine receptors in Alzheimer's disease].

As an important neurotransmitter, adenosine displays its functions by acting on the adenosine receptors. Recent studies have shown that the distribution, expression and balance among subtypes of adenosine receptors are closely related with cognitive activities, and changes of adenosine receptors play key roles in neurodegenerative disorders including Alzheimer's disease. It has been pointed out that prolonged activation of adenosine receptors by high level adenosine may lead to the disturbance of balance among adenosine receptor subtypes. This imbalance mainly performed as increased expression of A2a receptor and decreased expression of A1 receptor, and enhancement of the excitatory signals mediated by A2a receptor and weakened inhibitory signals mediated by A1 receptor. Changes of these two subtypes of adenosine receptors may lead to a lot of disorders of neurological activities which developed into dysfunction of cognition to the end. These findings imply that the potential of maintaining the balance among adenosine receptors on the treatment of AD would facilitate both the revealing of the mechanism and the cure of AD.

In silico directed chemical probing of the adenosine receptor family.

One of the grand challenges in chemical biology is identifying a small-molecule modulator for each individual function of all human proteins. Instead of targeting one protein at a time, an efficient approach to address this challenge is to target entire protein families by taking advantage of the relatively high levels of chemical promiscuity observed within certain boundaries of sequence phylogeny. We recently developed a computational approach to identifying the potential protein targets of compounds based on their similarity to known bioactive molecules for almost 700 targets. Here, we describe the direct identification of novel antagonists for all four adenosine receptor subtypes by applying our virtual profiling approach to a unique synthesis-driven chemical collection composed of 482 biologically-orphan molecules. These results illustrate the potential role of in silico target profiling to guide efficiently screening campaigns directed to discover new chemical probes for all members of a protein family.

Mechanisms of induction of adenosine receptor genes and its functional significance.

Adenosine is a metabolite generated and released from cells, particularly under injury or stress. It elicits protective or damaging responses via signaling through the adenosine receptors, including the adenylyl cyclase inhibitory A(1) and A(3), and the adenylyl cyclase stimulatory A(2A) and A(2B). Multiple adenosine receptor types, including stimulatory and inhibitory, can be found in the same cell, suggesting that a careful balance of adenosine receptor expression in a particular cell is necessary for a specific adenosine-induced response. This balance could be controlled by differential expression of the adenosine receptor genes under different stimuli. Here, we have reviewed an array of studies that have characterized basal or induced expression of the adenosine receptors and common as well as distinct mechanisms of effect, in hopes that ongoing studies on this topic will further elucidate detailed mechanisms of adenosine receptor regulation, leading to potential therapeutic applications.

Inhibitory purinergic P2 receptor characterisation in rat distal colon.

The aim of this study was to characterise the P2 receptors involved in purinergic relaxant responses in rat distal colon circular muscle. Concentration-response curves for purinergic agonists were constructed on methacholine-precontracted circular muscle strips of rat distal colon in the absence and presence of the nerve blocker TTX and the ecto-nucleotidase inhibitor ARL67156. The effects of the P2 receptor antagonists RB2, PPADS, suramin, MRS2179 and NF279, the NO-synthase inhibitor L-NAME and the small conductance K(+) channel blocker apamin were investigated. The localisation of the different P2 receptors was examined immunocytochemically. Immunocytochemistry demonstrated the expression of P2Y(1), P2Y(6) and P2X(1) receptors on smooth muscle cells and P2Y(2), P2Y(12), P2X(2) and P2X(3) receptors in the myenteric plexus; almost a quarter of the P2Y(2)-immunopositive neurons co-expressed nNOS. The P2X-selective agonist alphabetameATP and the P2Y-selective agonist ADPbetaS were the most potent relaxants; their effects were abolished by apamin. The effect of ADPbetaS was antagonised by the P2Y(1)-selective antagonist MRS2179 pointing to interaction with the muscular P2Y(1)-receptors. The relaxant effect of alphabetameATP was partially reduced by TTX and concentration-dependently antagonised by PPADS, suramin, RB2 and the P2X(1)-selective antagonist NF279; this correlates with an interaction with neuronal P2X(3) and muscular P2X(1) receptors. UTP was the least potent agonist; its effect was markedly increased by ARL67156, nearly abolished by TTX and reduced by L-NAME. This points to interaction with the neuronal P2Y(2)-receptors inducing relaxation, at least partially, by NO release.

Effects of adenosine on adhesion molecule expression and cytokine production in human PBMC depend on the receptor subtype activated.

BACKGROUND AND PURPOSE: Adenosine suppresses immune responses through adenosine(2A) (A(2A)) receptors, by raising intracellular cAMP. Interleukin (IL)-18 up-regulates the expression of intercellular adhesion molecule (ICAM)-1 on monocytes, leading to production of pro-inflammatory cytokines such as IL-12, interferon (IFN)-gamma and tumor necrosis factor (TNF)-alpha by human peripheral blood mononuclear cells (PBMC). We have previously demonstrated that elevation of cAMP inhibits this IL-18-induced expression of adhesion molecules. In the present study, we examined the effect of adenosine on the IL-18-induced up-regulation of ICAM-1 on human monocytes and production of IL-12, IFN-gamma and TNF-alpha by PBMC. EXPERIMENTAL APPROACH: The expression of ICAM-1 was examined by flow cytometry. IL-12, IFN-gamma and TNF-alpha were determined by ELISA assay. KEY RESULTS: Adenosine inhibited the IL-18-induced up-regulation of ICAM-1 on human monocytes and it abolished the IL-18-enhanced production of IL-12, IFN-gamma and TNF-alpha. While an A(2A) receptor antagonist reversed the action of adenosine, an A(1) or A(3) receptor antagonist enhanced them. An A(2A) receptor agonist, CGS21680, mimicked the effects of adenosine and its effects were abolished not only by the A(2A) receptor antagonist but also by A(1) or A(3) receptor agonists. Activation via A(2A) receptors resulted in elevation of cAMP in monocytes, whereas the stimulation of A(1) or A(3) receptors inhibited it, suggesting that intracellular signal transduction following ligation of A(2A) receptors might be blocked by activation of A(1) or A(3) receptors. CONCLUSIONS AND IMPLICATIONS: Adenosine differentially regulates IL-18-induced adhesion molecule expression and cytokine production through several subtypes of its receptors.

Computer aided comparative analysis of the binding modes of the adenosine receptor agonists for all known subtypes of adenosine receptors.

Molecular models of all known subtypes (A1, A2A, A2B, and A3) of the human adenosine receptors were built in homology with bovine rhodopsin. These models include the transmembrane domain as well as all extracellular and intracellular hydrophilic loops and terminal domains. The molecular docking of adenosine and 46 selected derivatives was performed for each receptor subtype. A binding mode common for all studied agonists was proposed, and possible explanations for differences in the ligand activities were suggested.

A Novel Method for Screening Adenosine Receptor Specific Agonists for Use in Adenosine Drug Development.

Agonists that target the A1, A2A, A2B and A3 adenosine receptors have potential to be potent treatment options for a number of diseases, including autoimmune diseases, cardiovascular disease and cancer. Because each of these adenosine receptors plays a distinct role throughout the body, obtaining highly specific receptor agonists is essential. Of these receptors, the adenosine A2AR and A2BR share many sequence and structural similarities but highly differ in their responses to inflammatory stimuli. Our laboratory, using a combination of specially developed cell lines and calcium release analysis hardware, has created a new and faster method for determining specificity of synthetic adenosine agonist compounds for the A2A and A2B receptors in human cells. A2A receptor expression was effectively removed from K562 cells, resulting in the development of a distinct null line. Using HIV-lentivector and plasmid DNA transfection, we also developed A2A and A2B receptor over-expressing lines. As adenosine is known to cause changes in intracellular calcium levels upon addition to cell culture, calcium release can be determined in these cell lines upon compound addition, providing a functional readout of receptor activation and allowing us to isolate the most specific adenosine agonist compounds.

Potent adenosine A1 and A2A receptors antagonists: recent developments.

This review summarizes the current tendencies observed in the past 5 years in the development of A(1) and A(2A) adenosine receptor antagonists performed in various academia and industry. A(1) and A(2A) AR antagonists are as well xanthines as heteroaromatic derivatives and are most commonly 6:5 fused heteroatomic compounds. Among xanthine-based compounds, some common features could be pointed out. The recent A(1) AR ligands which show good biological profile, possess long alkyl chains in position 1 and 3 as well as bulky C(8)-substituent, while A(2A) AR antagonists with a high A(2A) AR affinity are C(8)-styryl substituted with N(1)-alkyl/alkynyl moiety or fused tricyclic xanthines possessing heteroatom(s) in the third cycle. The research in the field of heteroaromatic A(1) and A(2A) ARs antagonists impressively has a wide range. Ligands are as well non-fused monocyclic substituted compounds as fused bi- and tricyclic derivatives with the nitrogen, oxygen and sulfur heteroatoms. Most often, adenosine A(1) receptor non-xanthine antagonists are adenine-based, having substituted amino group and variable nitrogen atoms positions in the molecules. A(2A) AR ligands show good affinity when furanyl function, which is crucial for binding, is present in the fused bicyclic and tricyclic analogs. Moreover, tricyclic nitrogen containing antagonists in order to be active, frequently possess long-alkylphenyl moiety.

Adenosine receptors: promising targets for the development of novel therapeutics and diagnostics for asthma.

Interest in the role of adenosine in asthma has escalated considerably since the early observation of its powerful bronchoconstrictor effects in asthmatic but not normal airways. A growing body of evidence has emerged in support of a proinflammatory and immunomodulatory role for the purine nucleoside adenosine in the pathogenic mechanisms of chronic inflammatory disorders of the airways such as asthma. The fact that adenosine enhances mast cell allergen-dependent activation, that elevated levels of adenosine are present in chronically inflamed airways, and that adenosine given by inhalation cause dose-dependent bronchoconstriction in subjects with asthma emphasizes the importance of adenosine in the initiation, persistence and progression of these common inflammatory disorders of the airways. These distinctive features of adenosine have been recently exploited in the clinical and research setting to identify innovative diagnostic applications for asthma. In addition, because adenosine exerts its multiple biological activities by interacting with four adenosine receptor subtypes, selective activation or blockade of these receptors may lead to the development of novel therapies for asthma.

Adenosine receptors in the nervous system: pathophysiological implications.

Adenosine is a ubiquitous homeostatic substance released from most cells, including neurones and glia. Once in the extracellular space, adenosine modifies cell functioning by operating G-protein-coupled receptors (GPCR; A(1), A(2A), A(2B), A(3)) that can inhibit (A(1)) or enhance (A(2)) neuronal communication. Interactions between adenosine receptors and other G-protein-coupled receptors, ionotropic receptors and receptors for neurotrophins also occur, and this might contribute to a fine-tuning of neuronal function. Manipulations of adenosine receptors influence sleep and arousal, cognition and memory, neuronal damage and degeneration, as well as neuronal maturation. These actions might have therapeutic implications for neurodegenerative diseases such as Parkinson's disease, Alzheimer's disease, as well as for other neurological situations such as epilepsy, idiopathic pain or even drug addition. Peripheral side effects associated with adenosine receptor agonists limit their usefulness in therapeutics; in contrast, adenosine receptor antagonists appear to have less side effects as it is the case of the well-known non-selective antagonists theophylline (present in tea) or caffeine (abundant in coffee and tea), and their emerging beneficial actions in Parkinson's disease and Alzheimer's disease are encouraging. A(1) receptor antagonism may also be useful to enhance cognition and facilitate arousal, as well as in the periphery when deficits of neurotransmitter release occur (e.g. myasthenic syndromes). Enhancement of extracellular adenosine levels through drugs that influence its metabolism might prove useful approaches in situations such as neuropathic pain, where enhanced activation of inhibitory adenosine A(1) receptors is beneficial. One might then consider adenosine as a fine-tuning modulator of neuronal activity, which via subtle effects causes harmonic actions on neuronal activity. Whenever this homeostasis is disrupted, pathology may be installed and selective receptor antagonism or agonism required.

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.

The receptor mechanism mediating the contractile response to adenosine on lung parenchymal strips from actively sensitised, allergen-challenged Brown Norway rats.

Parenchymal strips prepared from lungs removed from actively sensitised Brown Norway rats challenged with allergen show hyperresponsiveness to adenosine. The response is mast cell mediated and a preliminary pharmacological analysis suggested the involvement of a receptor (or receptors) that could not be classified as any of the known adenosine receptor subtypes. We present a further analysis of the response. Male Brown Norway (BN) rats, actively sensitised to ovalbumin (OA), were challenged intratracheally with OA and killed 3 h later to provide parenchymal strip preparations. The augmented contractile responses to adenosine were partially blocked by the 5-HT receptor antagonist, methysergide, or the A(1) receptor antagonist, DPCPX, and abolished in the presence of both antagonists. Responses to high concentrations of the A(1) receptor agonist, CPA were, like those to adenosine, augmented on tissues from allergen-challenged animals and blocked by a combination of methysergide and DPCPX. The A(3) receptor agonist, Cl-IB-MECA, did not contract the tissue, but partially blocked the response to adenosine. A combination of Cl-IB-MECA and methysergide induced a similar degree of blockade to that seen with either drug given alone. Combination of Cl-IB-MECA and/or methysergide with DPCPX abolished the response to adenosine. The effects of the A(3) receptor agonist, inosine, were augmented on tissues from allergen-challenged animals and markedly inhibited by disodium cromoglycate, methysergide or Cl-IB-MECA. Responses to adenosine were abolished when parenchymal strips were taken from rats pretreated 48 h previously with pertussis toxin. 8-SPT, CGS 15943, XAC, MRS 1754, DPCPX and theophylline, at concentrations which inhibit the A(1) A(2A) and/or A(2B) receptors but have negligible affinity for the rat A(3) receptor, inhibited responses to adenosine, but high concentrations were required and blockade was incomplete. MRS 1523 and MRS 1191, which are antagonists at the rat A(3) receptor, had no effect on the response to adenosine. The present results support and clarify our earlier conclusion that an atypical receptor mechanism mediates contraction of the parenchymal strip prepared from the lungs of actively sensitised BN rats challenged with allergen to adenosine. The response arises from a combined effect of adenosine on the A(1) receptor and a receptor with similarities to the A(3) receptor, but where Cl-IB-MECA behaves as an antagonist and MRS 1523 and MRS 1191 are inactive at concentrations that substantially exceed their affinities for the rat A(3) receptor.

Localization and characterization of adenosine receptor expression in rat testis.

Adenosine has long been suspected to play an important role in regulating male reproduction. To determine the sites of adenosine action in testis, the distribution of a family of recently cloned adenosine receptors was examined in rats. Northern blot analysis and in situ hybridization studies revealed high levels of testicular A3 adenosine receptor messenger RNA and lower levels of A1 adenosine receptor messenger RNA. Neither A2a nor A2b adenosine receptor gene expression could be detected. In situ hybridization and comparative polymerase chain reaction studies showed high level A3 receptor gene expression in germ cells (spermatocytes and spermatids), whereas high levels of A1 receptor gene expression was seen in Sertoli cells. To test for the presence of functional A1 and A3 receptors, the regulation of adenylyl cyclase by adenosine analogs was examined in seminiferous tubules and spermatozoa. Treatment of seminiferous tubules with forskolin resulted in a 5-fold increase in cAMP levels. The A1/A3 receptor agonist iodo-N6-aminobenzyladenosine inhibited forskolin-stimulated cAMP levels in the presence of the A1 8-cyclopentyl-1,3-dipropylxanthine antagonist 8-cyclopentyl-1,3-dipropylxanthine, showing that functional A3 receptors were present. The A1-selective agonist N6-cyclopentyladenosine also inhibited forskolin-stimulated cAMP levels, showing that functional A1 receptors were present. Treatment of spermatozoa with forskolin resulted in a 2-fold increase in cAMP levels. However, neither iodo-N6-aminobenzyladenosine nor N6-cyclopentyladenosine altered basal or forskolin-stimulated cAMP levels in sperm. These data show that functional inhibitory adenosine receptors are widely distributed in testis, with different patterns of expression.

Carbamazepine inhibits the potentiation by adenosine analogues of agonist induced inositolphosphate formation in hippocampal astrocyte cultures.

Carbamazepine (CBZ) resembles lithium in its beneficial effects in therapy and prophylaxis of affective disorders. Since lithium is presumed to act via an attenuation of the inositolphosphate/Ca(2+)-second messenger system, it is of particular interest whether or not CBZ might also have inhibitory effects on this type of signal transduction. CBZ is an antagonist of adenosine A1-receptor subtypes. We show here that activation of adenosine A1-receptors potentiates the phenylephrine induced formation of inositolphosphates in hippocampal astrocytes and that this potentiating effect is inhibited by CBZ at a therapeutically relevant concentration. These results indicate that CBZ can by antagonism of adenosine A1-receptors inhibit the inositolphosphate/Ca(2+)-signalling in neural pathways regulated by adenosine.

Pyrazolo-triazolo-pyrimidine derivatives as adenosine receptor antagonists: a possible template for adenosine receptor subtypes?

Adenosine, a widely distributed modulator, regulates many physiological functions through specific cell membrane G-protein-coupled receptors classified as A(1), A(2A), A(2B) and A(3). An intense medicinal chemistry effort made over the last 20 years has led to a variety of selective adenosine receptor agonists and antagonists. In particular, the pyrazolo-triazolo-pyrimidine nucleus has been strongly investigated in the last years by our group. All the modifications performed and a tentative of structure-activity-relationship is reported. In fact, the combination of different substitutions at the N(7), N(8) and N(5) positions afford compounds which showed good affinity and selectivity for the different adenosine receptor subtypes. The data herein summarized, permit to speculate on the use of this nucleus as possible template for the adenosine receptor subtypes.

Adenosine receptor-mediated modulation of acetylcholine release from rat striatal synaptosomes.

1. The effects of A1 and A2a adenosine receptor agonists on the veratridine-evoked release of [3H]-acetylcholine ([3H]-ACh) from rat striatal synaptosomes was investigated by use of the A1-selective agonist, R-PIA and the 185 fold selective A2a agonist, CGS 21680. The effects of NECA, which is equipotent at both receptor subtypes, were also studied. 2. The evoked release of [3H]-ACh was significantly enhanced by the A2a agonist CGS 21680 but decreased by the A1 agonist, R-PIA. The effects of NECA were dependent on the concentration used, with high concentrations inhibiting and low concentrations enhancing the evoked release of [3H]-ACh. In the absence of any antagonists, the rank order of potency for these three drugs on increasing [3H]-ACh release was CGS 21680 > NECA > R-PIA. 3. The stimulatory effects of CGS 21680 and low NECA concentrations on evoked [3H]-ACh release were antagonized by the A2a receptor antagonists, CP66,713 (300 nM) and CGS 15943A (50 nM) whilst the inhibitory effects of R-PIA were reversed by the selective A1 antagonist, DPCPX (4 nM). In the presence of DPCPX, NECA greatly enhanced the evoked release of [3H]-ACh at all concentrations studied when, during such A1 receptor blockade, the rank order of potency was NECA >> CGS 21680 > R-PIA. 4. These results demonstrate that both A1 and A2a adenosine receptors modulate the veratridine-evoked release of [3H]-ACh from rat striatal synaptosomes.

Electrophysiological subtypes of inhibitory P1 purinoceptors on myenteric neurones of guinea-pig small bowel.

1. Conventional intracellular microelectrode techniques were used to subclassify P1 purinoceptors linked to reduction of cell input resistance, steady-state hyperpolarization of the membrane potential, or inhibition of fast e.p.s.ps, in neurones of microdissected myenteric plexus preparations from guinea-pig ileum. The potencies of P1 purinoceptor agonists were estimated in neurones that were current clamped to a fixed membrane potential. 2. In AH/Type 2 neurones, the A2 agonist, CGS 21680, the A1 agonist, CCPA or the mixed A1-A2 agonist, NECA, suppressed excitability by reducing input resistance (40-50% max.) and causing hyperpolarization (20-25 mV max.). CGS 21680 (0.1-1 microM) enhanced the after-hyperpolarizing potential. 3. From cumulative dose-response data, the potency order for reducing input resistance was CCPA (IC50 = 5.1 +/- 2.2 nM) >>> CGS 21680 (IC50 = 5.6 +/- 2.5 microM). This effect was reversed by the A1 antagonist, CPT (EC50 = 65 +/- 11 nM). 4. In contrast, the potency order for membrane hyperpolarization was CCPA (IC50 = 61 +/- 23 nM) = CGS 21680 (IC50 = 290 +/- 90 nM) > or = NECA (IC50 = 450 +/- 100 nM). Hyperpolarization elicited by CCPA was sensitive to the A1-A2 antagonist, DPSPX. 5. Agonists suppressed fast e.p.s.ps, but not DMPP responses, with an order of CCPA (IC50 = 8.1 +/- 3.0 nM) >>> CGS 21680 (IC30 = 10 +/- 2.9 microM). 6. In conclusion, the excitability of AH/Type 2 neurones is suppressed by activation of high affinity A l receptors that may be linked to a cyclic AMP-dependent pathway, leading to increase in calcium dependent potassium conductance and enhancement of the after-hyperpolarizing potential. Activation of lower affinity non A1 receptors linked to a cyclic AMP-independent pathway reduces excitability and leads mainly to a steady-state hyperpolarization. Adenosine also suppresses nicotinic cholinergic transmission by activating presynaptic high affinity Al receptors.

Pharmacological classification of adenosine receptors in the sinoatrial and atrioventricular nodes of the guinea-pig.

1. The effects of seven agonist and three antagonist adenosine receptor ligands were compared on the guinea-pig sinoatrial (SA) node (isolated right atrium) and atrioventricular (AV) node (perfused whole heart). Single agonist concentration-effect curves were obtained to 5'-N-ethylcarboxamidoadenosine (NECA), R(-)-N6-(2-phenylisopropyl)adenosine (R-PIA), N6-cyclohexyladenosine (CHA), 2-chloroadenosine (CADO),),S(+)-N6-(2-phenylisopropyl)adenosine (L-PIA), 2-phenylaminoadenosine (CV 1808) and N6-aminoadenosine (MeAdo). Adenosine and/or NECA curves were obtained in the absence and presence of the antagonists 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), 9-chloro-2 (2-furanyl)-5,6-dihydro-1,2,4-triazolo[1,5-c]quinazolin-5-imine (CGS15943) and N6-(endonorbornan-2-yl)-9-methyladenine (N-0861). 2. A formal comparison of the agonist and antagonist potency data was made by fitting the data to a straight line using a least squares procedure based on principal components analysis to account for the variance on both axes. The antagonist affinity estimates made on the two assays did not deviate significantly from the line of identity. 3. The agonist p[A]50 data obtained on the two assays did not deviate from the line of identity, indicating that there were no significant differences in potencies between the two assays. The p[A]50 ratio of R-PIA and S-PIA was 1.24+/-0.09 in the SA node and 1.36+/-0.11 in the AV node, indicating no difference in the stereoselectivity of the PIA isomers between the two tissues. 4. The agonist potency and antagonist affinity data obtained are consistent with previous findings showing that the AV and SA node data are pharmacologically indistinguishable and belong to the adenosine A1-receptor class. No evidence for the reported A3-receptor was found.