Adenosine receptors

Will extreme weather events influence the toxic impacts of caffeine in coastal systems? Comparison between two widely used bioindicator species

In the recent years, marine heatwaves (MHWs) have caused devastating impacts on marine life. The understanding of the combined effects of these extreme events and anthropogenic pollution is a vital challenge. In particular, the combined effect of MHWs on the toxicity of pharmaceuticals to aquatic life remains unclear. To contribute to these issues, the main goal of the present investigation was to evaluate how MHWs may increase caffeine (CAF) toxicity on the clam Ruditapes philippinarum and the mussel Mytilus galloprovincialis. Bioaccumulation levels and changes on oxidative stress, metabolic capacity and neurotoxic status related biomarkers were investigated. The obtained results revealed the absence of CAF accumulation in both species. However, the used contaminant generated in both bivalve species alteration on neurotransmission, detoxification mechanisms induction as well as cellular damage. The increase of antioxidant defence mechanisms was complemented by an increase of metabolic activity and decrease of energy reserves. The obtained results seemed magnified under a simulated MHWs, suggesting to a climate-induced toxicant sensitivities' response. On this perspective, understanding of how toxicological mechanisms interact with climate-induced stressors will provide a solid platform to improve effect assessments for both humans and wildlife.

Caffeine as a contaminant of concern: A review on concentrations and impacts in marine coastal systems

Caffeine has been identified as emerging contaminant of concern due to its widespread occurrence in the aquatic environment and potential to be biologically active. Recently, these concerns have been translated in an increasing research on its occurrence and effects on biota. However, there is still a limited knowledge on seawater matrices and the implications of caffeine presence in coastal and marine ecosystems are not fully known. The present review aims to fill these knowledge gaps, analysing the existing literature regarding the occurrence, effects and potential risks of caffeine residues to coastal ecosystems, contributing to the risk assessment of this psychoactive drug in the aquatic environment. The analysed literature reported caffeine concentrations in the coastal ecosystems, raising high concerns about the potential adverse impacts on the ecological safety and human health. Caffeine has been found in tissues from coastal and marine biota including microalgae, coral reefs, bivalves and fish due to bioaccumulation after chronic, long-term exposures in a contaminated environment. Additionally, caffeine residues had been demonstrated to have adverse impacts on aquatic organisms, at environmentally realistic concentrations, inducing oxidative stress and lipid peroxidation, neurotoxicity, changing energy reserves and metabolic activity, affecting reproduction and development and, in some cases, causing mortality. Considering the increasing adverse impacts of caffeine pollution in the coastal environment, this review highlights the urgent need to minimize the increasing load of caffeine to the aquatic ecosystems; being imperative the implementation of scientific programs and projects to classify effectively the caffeine as a high-priority environmentally hazardous emerging pollutant.

Purinergic transmission in depressive disorders

Purinergic signaling involves the actions of purine nucleotides and nucleosides (such as adenosine) at P1 (adenosine), P2X, and P2Y receptors. Here, we present recent data contributing to a comprehensive overview of the association between purinergic signaling and depression. We start with background information on adenosine production and metabolism, followed by a detailed characterization of P1 and P2 receptors, with an emphasis on their expression and function in the brain as well as on their ligands. We provide data suggestive of altered metabolism of adenosine in depressed patients, which might be regarded as a disease biomarker. We then turn to considerable amount of preclinical/behavioral data obtained with the aid of the forced swim test, tail suspension test, learned helplessness model, or unpredictable chronic mild stress model and genetic activation/inactivation of P1 or P2 receptors as well as nonselective or selective ligands of P1 or P2 receptors. We also aimed to discuss the reason underlying discrepancies observed in such studies.

Focusing on Adenosine Receptors as a Potential Targeted Therapy in Human Diseases

Adenosine is involved in a range of physiological and pathological effects through membrane-bound receptors linked to G proteins. There are four subtypes of adenosine receptors, described as A1AR, A2AAR, A2BAR, and A3AR, which are the center of cAMP signal pathway-based drug development. Several types of agonists, partial agonists or antagonists, and allosteric substances have been synthesized from these receptors as new therapeutic drug candidates. Research efforts surrounding A1AR and A2AAR are perhaps the most enticing because of their concentration and affinity; however, as a consequence of distressing conditions, both A2BAR and A3AR levels might accumulate. This review focuses on the biological features of each adenosine receptor as the basis of ligand production and describes clinical studies of adenosine receptor-associated pharmaceuticals in human diseases.

Dyskinesias and impulse control disorders in Parkinson's disease: From pathogenesis to potential therapeutic approaches

Dopaminergic treatment in Parkinson's disease (PD) reduces the severity of motor symptoms of the disease. However, its chronic use is associated with disabling motor and behavioral side effects, among which levodopa-induced dyskinesias (LID) and impulse control disorders (ICD) are the most common. The underlying mechanisms and pathological substrate of these dopaminergic complications are not fully understood. Recently, the refinement of imaging techniques and the study of the genetics and molecular bases of LID and ICD indicate that, although different, they could share some features. In addition, animal models of parkinsonism with LID have provided important knowledge about mechanisms underlying such complications. In contrast, animal models of parkinsonism and abnormal impulsivity, although useful regarding some aspects of human ICD, do not fully resemble the clinical phenotype of ICD in patients with PD, and until now have provided limited information. Studies on animal models of addiction could complement the previous models and provide some insights into the background of these behavioral complications given that ICD are regarded as behavioral addictions. Here we review the most relevant advances in relation to imaging, genetics, biochemistry and pharmacological interventions to treat LID and ICD in patients with PD and in animal models with a view to better understand the overlapping and unique maladaptations to dopaminergic therapy that are associated with LID and ICD.

New selective A2A agonists and A3 antagonists for human adenosine receptors: synthesis, biological activity and molecular docking studies

Synthesis and pharmacological characterization of a new series of adenosine derivatives on the four human adenosine receptors are reported.

Loss of A(1) adenosine receptor attenuates alpha-naphthylisothiocyanate-induced cholestatic liver injury in mice

Cholestasis has limited therapeutic options and is associated with high morbidity and mortality. The A(1) adenosine receptor (A(1)AR) was postulated to participate in the pathogenesis of hepatic fibrosis induced by experimental extrahepatic cholestasis; however, the contribution of A(1)AR to intrahepatic cholestatic liver injury remains unknown. Here, we found that mice lacking A(1)AR were resistant to alpha-naphthyl isothiocyanate (ANIT)-induced liver injury, as evidenced by lower serum liver enzyme levels and reduced extent of histological necrosis. Bile acid accumulation in liver and serum was markedly diminished in A(1)AR(-/-) mice compared with wild-type (WT) mice. However, biliary and urinary outputs of bile acids were significantly enhanced in A(1)AR(-/-) mice. In the liver, mRNA expression of genes related to bile acid transport (Bsep and Mdr2) and hydroxylation (Cyp3a11) was increased in A(1)AR(-/-) mice. In the kidney, A(1)AR deficiency prevented the decrease of glomerular filtration rate caused by ANIT. Treatment of WT mice with A(1)AR antagonist DPCPX also protected against ANIT hepatotoxicity. Our results indicated that lack of A(1)AR gene protects mice from ANIT-induced cholestasis by enhancing toxic biliary constituents efflux through biliary excretory route and renal elimination system and suggested a potential role of A(1)AR as therapeutic target for the treatment of intrahepatic cholestasis.

Design and characterization of optimized adenosine A₂A/A₁ receptor antagonists for the treatment of Parkinson's disease

The design and characterization of two, dual adenosine A(2A)/A(1) receptor antagonists in several animal models of Parkinson's disease is described. Compound 1 was previously reported as a potential treatment for Parkinson's disease. Further characterization of 1 revealed that it was metabolized to reactive intermediates that caused the genotoxicity of 1 in the Ames and mouse lymphoma L51784 assays. The identification of the metabolites enabled the preparation of two optimized compounds 13 and 14 that were devoid of the metabolic liabilities associated with 1. Compounds 13 and 14 are potent dual A(2A)/A(1) receptor antagonists that have excellent activity, after oral administration, across a number of animal models of Parkinson's disease including mouse and rat models of haloperidol-induced catalepsy, mouse and rat models of reserpine-induced akinesia, and the rat 6-hydroxydopamine (6-OHDA) lesion model of drug-induced rotation.

Adenosine A(2A) Receptor Antagonists and Parkinson's Disease

This Review summarizes and updates the work on adenosine A(2A) receptor antagonists for Parkinson's disease from 2006 to the present. There have been numerous publications, patent applications, and press releases within this time frame that highlight new medicinal chemistry approaches to this attractive and promising target to treat Parkinson's disease. The Review is broken down by scaffold type and will discuss the efforts to optimize particular scaffolds for activity, pharmacokinetics, and other drug discovery parameters. The majority of approaches focus on preparing selective A(2A) antagonists, but a few approaches to dual A(2A)/A(1) antagonists will also be highlighted. The in vivo profiles of compounds will be highlighted and discussed to compare activities across different chemical series. A clinical report and update will be given on compounds that have entered clinical trials.

Neuroadaptations in adenosine receptor signaling following long-term ethanol exposure and withdrawal

Ethanol affects the function of neurotransmitter systems, resulting in neuroadaptations that alter neural excitability. Adenosine is one such receptor system that is changed by ethanol exposure. The current review is focused on the A(1) and the A(2A) receptor subtypes in the context of ethanol-related neuroadaptations and ethanol withdrawal because these subtypes (i) are activated by basal levels of adenosine, (ii) have been most well-studied for their role in neuroprotection and ethanol-related phenomena, and (iii) are the primary site of action for caffeine in the brain, a substance commonly ingested with ethanol. It is clear that alterations in adenosinergic signaling mediate many of the effects of acute ethanol administration, particularly with regard to motor function and sedation. Further, prolonged ethanol exposure has been shown to produce adaptations in the cell surface expression or function of both A(1) and the A(2A) receptor subtypes, effects that likely promote neuronal excitability during ethanol withdrawal. As a whole, these findings demonstrate a significant role for ethanol-induced adaptations in adenosine receptor signaling that likely influence neuronal function, viability, and relapse to ethanol intake following abstinence.

Hyaluronan fragments promote inflammation by down-regulating the anti-inflammatory A2a receptor

The tissue microenvironment plays a critical role in regulating inflammation. Chronic inflammation leads to an influx of inflammatory cells and mediators, extracellular matrix turnover, and increased extracellular adenosine. Low molecular weight (LMW) fragments of hyaluronan (HA), a matrix component, play a critical role in lung inflammation and fibrosis by inducing inflammatory gene expression at the injury site. Adenosine, a crucial negative regulator of inflammation, protects tissues from immune destruction via the adenosine A2a receptor (A2aR). Therefore, these two extracellular products of inflammation play opposing roles in regulating immune responses. As such, we wanted to determine the effect of LMW HA on A2aR function. In this article, we demonstrate that LMW HA causes a rapid, significant, and sustained down-regulation of the A2aR. CD44 was found to be necessary for LMW HA to down-modulate the A2aR as was protein kinase C signaling. We also demonstrate that LMW HA induces A2aR down-regulation during inflammation in vivo, and that this down-regulation can be blocked by treatment with an HA-blocking peptide. Because adenosine plays a critical role in limiting inflammation, our data provide a novel mechanism whereby LMW HA itself may further augment inflammation. By defining the pro- and anti-inflammatory properties of extracellular matrix components, we will be better able to identify specific pharmacologic targets as potential therapies.

Adenosine receptor regulation of coronary blood flow in Ossabaw miniature swine

Adenosine clearly regulates coronary blood flow (CBF); however, contributions of specific adenosine receptor (AR) subtypes (A(1), A(2A), A(2B), A(3)) to CBF in swine have not been determined. ARs generally decrease (A(1), A(3)) or increase (A(2A), A(2B)) cyclic adenosine monophosphate, a major mediator of vasodilation. We hypothesized that A(1) antagonism potentiates coronary vasodilation and coronary stent deployment in dyslipidemic Ossabaw swine elicits impaired vasodilation to adenosine that is associated with increased A(1)/A(2A) expression. The left main coronary artery was accessed with a guiding catheter allowing intracoronary infusions. After placement of a flow wire into the left circumflex coronary artery the responses to bolus infusions of adenosine were obtained. Steady-state infusion of AR-specific agents was achieved by using a small catheter fed over the flow wire in control pigs. CBF was increased by the A(2)-nonselective agonist 2-phenylaminoadenosine (CV1808) in a dose-dependent manner. Baseline CBF was increased by the highly A(1)-selective antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), but not changed by other AR-specific agents. The nonselective A(2) antagonist 3,7-dimethyl-1-propargylxanthine and A(2A)-selective antagonist 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM241385) abolished adenosine-induced CBF, whereas A(2B) and A(3) antagonism had no effect. Dyslipidemia and stenting decreased adenosine-induced CBF ∼70%, whereas A(1), A(2A), and A(2B) mRNA were up-regulated in dyslipidemic versus control >5-fold and there was no change in the ratio of A(1)/A(2A) protein in microvessels distal to the stent. In control Ossabaw swine A(1) antagonism by DPCPX positively regulated basal CBF. Impaired adenosine-induced CBF after stenting in dyslipidemia is most likely caused by the altered balance between A(1) and A(2A) signaling, not receptor expression.

Direct blockade of inflammatory hypernociception by peripheral A1 adenosine receptors: involvement of the NO/cGMP/PKG/KATP signaling pathway

Through activation of the A1 adenosine receptors (A1Rs) at both the central and peripheral level, adenosine produces antinociception in a wide range of tests. However, the mechanisms involved in the peripheral effect are still not fully understood. Therefore, the mechanisms by which peripheral activation of A1Rs reduces inflammatory hypernociception (a decrease in the nociceptive threshold) were addressed in the present study. Immunofluorescence of rat dorsal root ganglion revealed significant expression of A1Rs in primary sensory neurons associated with nociceptive pathways. Functionally, peripheral activation of A1Rs reduced inflammatory hypernociception because intraplantar (i.pl.) administration of an A1R antagonist (DPCPX) enhanced carrageenan-induced hypernociception. On the other hand, local (paw) administration of CPA (a selective A1R agonist) reversed mechanical hypernociception induced by carrageenan or by the directly acting hypernociceptive mediator prostaglandin E(2) (PGE(2)). Down-regulation of A1Rs expression in primary nociceptive neurons by intrathecal treatment with antisense oligodeoxinucleotides significantly reduced peripheral antinociceptive action of CPA. Direct blockade of PGE(2) inflammatory hypernociception by the activation of A1Rs depends on the nitric oxide/cGMP/Protein Kinase G/KATP signaling pathway because the peripheral antinociceptive effect of CPA was prevented by pretreatment with inhibitors of neuronal nitric oxide synthase (N-propyl-l-arginine), guanylyl cyclase (ODQ), and Protein Kinase G (KT5823) as well as with a KATP blocker (glibenclamide). However, this effect of CPA was not reduced by naloxone, excluding the participation of endogenous opioids. These results suggest that the peripheral activation of A1R plays a role in the regulation of inflammatory hypernociception by a mechanism that involves the NO/cGMP/PKG/KATP intracellular signaling pathway.

A kinetic study of the rat liver adenosine kinase reverse reaction

Adenosine kinase is an enzyme catalyzing the reaction: adenosine + ATP --> AMP + ADP. We studied some biochemical properties not hitherto investigated and demonstrated that the reaction can be easily reversed when coupled with adenosine deaminase, which transforms adenosine into inosine and ammonia. The overall reaction is: AMP + ADP --> ATP + inosine + NH(3). The exoergonic ADA reaction shifts the equilibrium and fills the energy gap necessary for synthesis of ATP. This reaction could be used by cells under particular conditions of energy deficiency and, together with myokinase activity, may help to restore physiological ATP levels.

The adenosine a2a receptor inhibits matrix-induced inflammation in a novel fashion

Endogenous mediators within the inflammatory milieu play a critical role in directing the scope, duration, and resolution of inflammation. High-molecular-weight extracellular matrix hyaluronan (HA) helps to maintain homeostasis. During inflammation, hyaluronan is broken down into fragments that induce chemokines and cytokines, thereby augmenting the inflammatory response. Tissue-derived adenosine, released during inflammation, inhibits inflammation via the anti-inflammatory A2 adenosine receptor (A2aR). We demonstrate that adenosine modulates HA-induced gene expression via the A2aR. A2aR stimulation inhibits HA fragment-induced pro-fibrotic genes TNF-alpha, keratinocyte chemoattractant (KC), macrophage inflammatory protein (MIP)-2, and MIP-1alpha while simultaneously synergizing with hyaluronan fragments to up-regulate the TH1 cytokine IL-12. Interestingly, A2aR stimulation mediates these affects via the novel cAMP-activated guanine nucleotide exchange factor EPAC. In addition, A2aR-null mice are more susceptible to bleomycin-induced lung injury, consistent with a role for endogenous adenosine in inhibiting the inflammation that may lead to fibrosis. Indeed, the bleomycin treated A2aR-null mice demonstrate increased lung inflammation, HA accumulation, and histologic damage. Overall, our data elucidate the opposing roles of tissue-derived HA fragments and adenosine in regulating noninfectious lung inflammation and support the pursuit of A2aR agonists as a means of pharmacologically inhibiting inflammation that may lead to fibrosis.

Adenosine modulates excitatory synaptic transmission and suppresses neuronal death induced by ischaemia in rat spinal motoneurones

Although adenosine is an important neuromodulator, its role in modulating motor functions at the level of the spinal cord is poorly understood. In the present study, we investigated the effects of adenosine on excitatory synaptic transmission and neuronal death induced by experimental ischaemia by using whole-cell patch-clamp recordings from lamina IX neurones in spinal cord slices. Adenosine significantly decreased the frequency of miniature excitatory postsynaptic currents (mEPSCs) in almost all neurones examined that could be mimicked by an A(1) receptor agonist, N (6)-cyclopentyladenosine (CPA), and inhibited by an A(1) receptor antagonist, 8-cyclopentyl-1, 3-dipropylxanthine (DPCPX). Interestingly, adenosine increased mEPSC frequency in the presence of DPCPX in a subpopulation of neurones. In these neurones, an A(2A) receptor agonist, 2-[4-(2-carbonylethyl)-phenethylamino]-5'-N-ethylcarboxamidoadenosine (CGS21680), increased mEPSC frequency. Adenosine also induced an outward current that was blocked by the addition of Cs(+) and tetraethylammonium into the patch-pipette solution and inhibited in the presence of Ba(2+). The adenosine-induced outward current was mimicked by CPA, but not CGS21680, and inhibited by DPCPX. Moreover, superfusing with ischaemia simulating medium (ISM) generated an agonal inward current in all of the neurones tested. The latencies of the inward currents induced by ISM were significantly prolonged by adenosine or CPA, but not by CGS21680. These results suggest that adenosine receptors are functionally expressed in both the pre- and postsynaptic sites of lamina IX neurones and that their activation may exert multiple effects on motor function. Moreover, this study has provided a cellular basis for an involvement of A(1) receptors in the neuroprotective actions of adenosine.

Adenosine reduces the reverse mode of the Na+/Ca(2+) exchanger in ferret cardiac fibres

The aim of this study was to investigate the effects of adenosine on reverse mode Na+/Ca(2+) exchange. In intact ferret cardiac trabeculae, Na+-free contractures were investigated after treating preparations with ryanodine, a sarcoplasmic reticulum Ca(2+) -channel inhibitor, and thapsigargin, a sarcoplasmic reticulum Ca(2+) -pump inhibitor added to suppress the sarcoplasmic reticulum function. The effects of adenosine (50-100 nmol/L), adenosine deaminase (ADA, 0.1-0.5 U/L), the A1 and A2A receptor agonists CCPA (3-100 nmol/L) and CGS 21680 (25-100 nmol/L), and the A1 and A2A receptor antagonists DPCPX (25 nmol/L) and ZM 241385 (25 nmol/L) were tested on Na+-free contractures. The application of adenosine (50-100 nmol/L) had no significant effect on the characteristics of the Na+-free contractures. However, the results show that treatment with ADA (0.3 U/L), adenosine (> or =50 nmol/L) and CCPA, a specific A1 receptor agonist (3-100 nmol/L), all reduced the Na+-free contracture amplitude. In the presence of ADA, the effects of adenosine and CCPA were also reduced by a specific antagonist of A1 receptors (DPCPX, 25 nmol/L). Furthermore, adenosine, ADA, and CCPA did not affect the properties of the contractile apparatus in Triton-skinned fibres. It is therefore proposed that endogenous adenosine reduced the reverse mode of the Na+/Ca(2+) exchanger by acting on A1 receptors present in the sarcolemmal membrane.

Renal interstitial adenosine is increased in angiotensin II-induced hypertensive rats

Since marked renal vasoconstriction is observed in angiotensin II (ANG II)-mediated hypertensive rats, we studied the possible interaction between ANG II and adenosine in this model. ANG II was infused into male Wistar rats through osmotic minipumps (435 ng·kg−1·min−1) for 14 days. In sham and ANG II groups, renal tissue and interstitial adenosine were measured; both increased to a similar twofold extent in the ANG II-treated rats (31.40 ± 4 vs. 62.0 ± 8.4 nM, sham vs. ANG II, interstitial adenosine; P< 0.001). The latter decreased by 47% with the specific blockade of 5′-nucleotidase. Glomerular hemodynamics demonstrated marked renal vasoconstriction in the angiotensin-treated group, which was reverted by an adenosine A1-receptor antagonist (8-cyclopentyl-1,3-dipropylxanthine, 10 μg·kg−1·min−1). 5′-Nucleotidase and adenosine deaminase (ADA) activities were measured in the cytosolic and membrane fractions. Only the membrane ADA activity decreased from 1,202 ± 80 to 900 ± 50 mU/mg protein in the ANG II-treated rats ( P< 0.05), as well as in their protein and mRNA expression. Despite the adenosine elevation, A1and A2breceptor protein did not change; in contrast, downregulation was observed in A2areceptor and upregulation in A3receptor. A similar pattern was found in the cortex and in the medulla; mRNA significantly decreased only in the A3receptor in both segments. These results suggest that the elevation of renal tissue and interstitial adenosine contributes to the renal vasoconstriction observed in the ANG II-induced hypertension and that it is mediated by a decrease in the activity and expression of ADA, increased production of adenosine, and an induced imbalance in adenosine receptors.

Reduced basal and lipopolysaccharide-stimulated adenosine A1 receptor expression in the brain of nuclear factor-kappaB p50-/- mice

Adenosine promotes cytoprotection under conditions of infection, ischemic preconditioning and oxidative stress. Previous studies from our laboratory indicate that the expression of the adenosine A1 receptor (A1AR) is induced by oxidative stress via activation of nuclear factor (NF)-kappaB. The prototypic transcription factor is composed of homo- or heterodimers of p50 and p65 subunits. To determine the role of NF-kappaB in the regulation of the A1AR in vivo, we compared the A1AR RNA and protein levels in the brains of mice lacking the p50 subunit of NF-kappaB (p50-/- mice) and age-matched B6129PF2/J (F2) controls. Radioligand binding assays in the cortex revealed a significantly lower number of A(1)AR (maximal binding capacity, Bmax) in the cortex of p50-/- mice (151+/-62 fmol/mg protein) versus 479+/-181 fmol/mg protein in the F2 (N=5 per strain, P<0.05), but no change in the equilibrium dissociation constant. Similar reductions in A1AR were measured in the hippocampus, brain stem and hypothalamus and in peripheral tissues, such as the adrenal gland, kidney and spleen. Estimation of the A1AR following purification by antibody affinity columns also indicated reduced A1AR in the p50-/- mice cortex, as compared with the F2 mice. A1AR immunocytochemistry indicates distinct neuronal labeling in the F2 cortex, which was substantially reduced in similar sections obtained from p50-/- mice. The p50-/- mice expressed lower levels of A1AR mRNA than F2 mice, as determined by real time PCR. Quantitation of the A1AR transducing G proteins by Western blotting show significantly less Galphai3, no change in Galphai1, but higher levels of Galphao and Gbeta in the cortices of p50-/-, as compared with F2 mice. Administration of bacterial lipopolysaccharide (LPS), an activator of NF-kappaB, increased A1AR expression in the cortices of F2 mice but not p50-/- mice. Cortical neurons cultures prepared from p50-/- mice showed a greater degree of apoptosis, compared with neurons from F2 mice. Activation of the A1AR reduced apoptosis with greater efficacy in cultures from F2 than p50-/- mice. Taken together, these data support a role for NF-kappaB in determining both the basal and LPS-stimulated A1AR expression in vivo which could contribute to neuronal survival.

Extracellular nucleotides and adenosine independently activate AMP-activated protein kinase in endothelial cells: involvement of P2 receptors and adenosine transporters

AMP-activated protein kinase (AMPK) plays a key role in the regulation of energy homeostasis and is activated in response to cellular stress, including hypoxia/ischemia and hyperglycemia. The stress events are accompanied by rapid release of extracellular nucleotides from damaged tissues or activated endothelial cells (EC) and platelets. We demonstrate that extracellular nucleotides (ATP, ADP, and UTP, but not UDP) and adenosine independently induce phosphorylation and activation of AMPK in human umbilical vein EC (HUVEC) by the mechanism that is not linked to changes in AMP:ATP ratio. HUVEC express NTPDases, as well as 5'-nucleotidase; hence, nucleotides can be metabolized to adenosine. However, inhibition of 5'-nucleotidase had no effect on ATP/ADP/UTP-induced phospho- rylation of AMPK, indicating that AMPK activation occurred as a direct response to nucleotides. Nucleotide-evoked phosphorylation of AMPK in HUVEC was mediated by P2Y1, P2Y2, and/or P2Y4 receptors, whereas P2Y6, P2Y11, and P2X receptors were not involved. The nucleotide-induced phosphorylation of AMPK was affected by changes in the concentration of intracellular Ca2+ and by Ca2+/calmodulin-dependent kinase kinase (CaMKK), although most likely it was not dependent on LKB1 kinase. Adenosine-induced phosphorylation of AMPK was not mediated by P1 receptors but required adenosine uptake by equilibrative nucleoside transporters followed by its (intracellular) metabolism to AMP. Moreover, adenosine effect was Ca2+ and CaMKK independent, although probably associated with upstream LKB1. We hypothesize that P2 receptors and adenosine transporters could be novel targets for the pharmacological regulation of AMPK activity and its downstream effects on EC function.

The Effects of Plasma Insulin and Glucose on Myocardial Blood Flow in Patients With Type 1 Diabetes Mellitus

OBJECTIVES

The objective of this study was to determine the impact of insulin and glucose on myocardial vasodilator function in patients with type 1 diabetes mellitus (T1DM).

BACKGROUND

The relative importance of plasma insulin and glucose levels on the abnormal vasodilator function observed in T1DM is unknown.

METHODS

Twenty T1DM patients underwent positron emission tomography studies to measure myocardial blood flow (MBF) (in ml/g/min) at rest (MBFr) and during adenosine (MBFa), both under baseline metabolic conditions and then during either hyperinsulinemic-euglycemic clamp (HE) (n = 10; 40 +/- 9 years, 8 female subjects, hemoglobin A1c [HbA1c] 7.8 +/- 1.1%) or hyperinsulinemic-hyperglycemic clamp (HH) (n = 10; 44 +/- 12 years, 8 female subjects, hemoglobin A1c 7.7 +/- 0.6%).

RESULTS

Both groups showed similar MBFr and MBFa under baseline metabolic conditions (p = NS). Compared with baseline conditions, MBFr increased in the HH group (p < 0.005), whereas it did not change in the HE group. Compared with baseline conditions, MBFa decreased in the HH group (p < 0.05) but did not change in the HE group. Myocardial perfusion reserve (MPR) (MBFa/MBFr) was similar between the HE and HH groups at baseline (p = NS). During clamp, MPR tended to decrease in the HH group (p < 0.1) but did not change in the HE group (p = NS) when compared with baseline conditions. However, during the clamp MPR was significantly lower in the HH group when compared with the HE group (p < 0.0001).

CONCLUSIONS

In the short term, hyperglycemia has a deleterious effect on myocardial vasodilator function, which outweighs the beneficial effect of hyperinsulinemia.

Novel mitogenic effect of adenosine on coronary artery smooth muscle cells: role for the A1 adenosine receptor

Adenosine is a vascular endothelial cell mitogen, but anti-mitogenic for aortic smooth muscle cells and fibroblasts when acting via the A2B adenosine receptor. However, we show that adenosine increases porcine coronary artery smooth muscle cell (CASMC) number, cellular DNA content, protein synthesis, and PCNA staining. RT-PCR analysis indicates that porcine CASMC express A1, A2A, A3, and barely detectable levels of A2B receptor mRNAs. The mitogenic effect of adenosine is mimicked by NECA, CCPA, and R-PIA, but not by CGS21680and 2-Cl-IB-MECA, and is inhibited by DPCPX, indicating a prominent role for the A1 receptor. This interpretation is supported by the finding that adenosine- and CCPA-induced DNA synthesis is significantly inhibited by pertussis toxin, but substantially potentiated by PD81723, an allosteric enhancer of the A1 receptor. When a cDNA encoding the porcine A1 receptor was cloned and expressed in COS-1 cells, A1 receptor pharmacology is confirmed. Anti-sense oligonucleotides to the cloned sequence dramatically suppress the mitogenic effect of adenosine and CCPA. Conversely, over-expression of the cloned A1 receptor in CASMC increases adenosine- and CCPA-induced DNA synthesis. Furthermore, stimulation with adenosine or CCPA of intact coronary arteries in an organ culture model of vascular disease increases cellular DNA synthesis, which was abolished by DPCPX. We conclude that adenosine acts as a novel mitogen in porcine CASMC that express the A1 adenosine receptor, possibly contributing to the development of coronary artery disease.

The therapeutic potential of agents acting via purine receptors

A host of physiological processes associated with the cardiovascular (CV) system, central nervous system (CNS), and a variety of other organ systems and tissues are regulated by agents, primarily adenosine (ado) and adenosine triphosphate (ATP), that act via cell-surface purine receptors. These receptors have therefore been the focus of a variety of programmes directed at the discovery and development of new therapeutic agents, most notably for the treatment of disorders of the CV system. Currently, only a handful of agents, including ado, theophylline, dipyridamole, and ticlopidine, are approved for clinical use. A variety of new agents intended for use in CV disease, disorders of the CNS, such as Parkinson's disease, treatment of pain, inflammatory disorders, and diverse other pathophysiological conditions are in clinical development. Historically, ado receptors have been the primary target. Recent research efforts have begun to examine alternative strategies including agents that modulate endogenous levels of extracellular ado and agents that act via P(2) receptors.

Adenosine kinase inhibitor attenuates the expression of inducible nitric oxide synthase in glial cells

The present study demonstrates the anti-inflammatory effect of adenosine kinase inhibitor (ADKI) in glial cells. Treatment of glial cells with IC51, an ADKI, stimulated the extracellular adenosine release and reduced the LPS/IFNgamma-mediated production of NO, and induction of iNOS and TNF-alpha gene expression. The recovery of IC51-mediated inhibition of iNOS expression by adenosine transport inhibitor, S-(4-nitrobenzyl)-6-thioinosine (NBTI), and the inhibition of LPS/IFNgamma-induced iNOS gene expression by exogenous adenosine indicate a role for adenosine release in IC51-mediated iNOS expression. The rescue of IC51-mediated inhibition of iNOS expression by adenosine receptor antagonist for A2A, 8-(3-chlorostyryl)caffeine (CSC) and alloxazine for A2B, further supports a role for interaction of adenosine and its receptors in anti-inflammatory activity. The IC51-mediated induction of cAMP levels, downstream target of A2A and A2B, and inhibition of LPS/IFNgamma-induced expression of iNOS by forskolin, a cAMP activator, document a role for cAMP mediated pathway in anti-inflammatory activity of IC51. Taken together, these studies document that IC51-mediated inhibition of iNOS expression is through activation of adenosine receptors, which activates A2A and A2B resulting in increased cAMP levels following LPS/IFNgamma stimulation. Moreover, the lack of effect of IC51 or adenosine on NFkappaB DNA binding activity and its transactivity indicates that the inhibition of iNOS expression mediated by IC51 may be through an NFkappaB independent pathway.

Synthesis and biological activity of new potential agonists for the human adenosine A2A receptor

New adenosine derivatives have been synthesized and tested as putative agonists of adenosine receptors. Compounds 2-6 derive from the introduction of several types of substituents (electron donating, electron withdrawing, and halogens) in the para-position of the phenyl ring of the parent compound 1, and compound 7 lacks the hydroxyl group of amino alcohol 1. In radioligand binding assays using recombinant human A(1), A(2A), A(2B), and A(3) receptors, all compounds showed very low or negligible affinity for A(1) and A(2B) receptors but compounds 3, 5, and 7 displayed a remarkably potent affinity for the A(2A) receptor with K(i) values of 1-5 nM. Bromo derivative 3 displayed a selectivity A(1)/A(2A) = 62 and A(3)/A(2A) = 16 whereas the presence of a hydroxyl group (compound 5) improved the selectivity of A(1)/A(2A) and A(3)/A(2A) to 120- and 28-fold, respectively. When the methoxy derivative 4 lacks the hydroxyl group on the side chain (compound 7), the binding affinity for A(2A) is increased to 1 nM, improving selectivity ratios to 356- and 100-fold against A(1) and A(3), respectively. In Chinese hamster ovary cells transfected with human A(2A) and A(2B) receptors, most compounds showed a remarkable activity for the A(2A) receptor, except chloro derivative 2, with EC(50) values ranging from 1.4 to 8.8 nM. The compounds behaved as good A(2A) agonists, and all were more selective than 5'-(N-ethylcarboxamino)adenosine (NECA), with A(2B)/A(2A) ratios of cAMP accumulation ranging from 48 for compound 2 to 666 for compound 7 while the corresponding A(2B)/A(2A) ratio for NECA was only 9. Compounds 1, 3, 5, and 7 also displayed higher selectivities than NECA up to 100-fold in isolated aortas of rat and guinea pig. In guinea pig tracheal rings precontracted by carbachol, compounds 2 and 4 were more potent than adenosine (100-fold) and NECA (10-fold), whereas compounds 1 and 7 displayed similar effects to NECA. Pretreatment of the tracheal rings with A(2), A(2A), and A(2B) receptor antagonists 3,7-dimethyl-l-propargylxanthine, 8-(3-chlorostyryl)caffeine, and alloxazine produced a marked inhibition of the tracheal relaxations induced by compounds 1, 2, and 4, but none of the compounds showed selectivity toward any of the adenosine receptors.

Adenosine A2A receptor occupancy stimulates expression of proteins involved in reverse cholesterol transport and inhibits foam cell formation in macrophages

Transport of cholesterol out of macrophages is critical for prevention of foam cell formation, the first step in the pathogenesis of atherosclerosis. Proteins involved in this process include cholesterol 27-hydroxylase and adenosine 5'-triphosphate-binding cassette transporter A1 (ABCA1). Proinflammatory cytokines and immune complexes (IC) down-regulate cholesterol 27-hydroxylase and impede cholesterol efflux from macrophages, leading to foam cell formation. Prior studies have suggested occupancy of the anti-inflammatory adenosine A2A receptor (A2AR) minimizes early atherosclerotic changes in arteries following injury. We therefore asked whether A2AR occupancy affects macrophage foam cell formation in response to IC and the cytokine interferon-gamma. We found that the selective A2AR agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamido-adenosine (CGS-21680) inhibited foam cell formation in stimulated THP-1 human macrophages, and the effects of CGS-21680 were reversed by the selective A2AR antagonist 4-(2-[7-amino-2-(2-furyl) [1, 2, 4]triazolo[2,3-a] [1, 3, 5]triazin-5-ylamino]ethyl)phenol. In confirmation of the role of A2AR in prevention of foam cell formation, CGS-21680 also inhibited foam cell formation in cultured murine peritoneal macrophages but did not affect foam cell formation in A2AR-deficient mice. Agents that increase foam cell formation also down-regulate cholesterol 27-hydroxylase and ABCA1 expression. Therefore, we determined the effect of A2AR occupancy on expression of these reverse cholesterol transport (RCT) proteins and found that A2AR occupancy stimulates expression of message for both proteins. These results indicate that one mechanism for the antiatherogenic effects of adenosine is stimulation of the expression of proteins involved in RCT. These findings suggest a novel approach to the development of agents that prevent progression of atherosclerosis.

Development of a Sensitive and HTS-Compatible Reporter Gene Assay for Functional Analysis of Human Adenosine A2a Receptors in CHO-K1 Cells

Adenosine A2a receptor, a member of the G protein-coupled receptor superfamily, has been demonstrated to be an important pharmacological target. It couples to stimulatory G protein and activates adenylate cyclase upon agonist stimulation. Here we attempted to stably transfect Chinese hamster ovary (CHO-K1) cells, which lack any known subtypes of adenosine receptors, with recombinant human adenosine A2a receptors (hA2aR). Rapid down-regulation of hA2aR in a clonal cell line, CHOA2a-2, was observed over a short period of time in culture. This is consistent with other groups' findings of low expression and poor G protein coupling of this receptor in several cell systems. To facilitate pharmacological profiling for hA2aR ligand, we introduced a cyclic AMP response element (CRE)-linked beta-galactosidase reporter gene into CHOA2a-2 cells to generate a stable cell line, CHOA2a-2CREbetagal#26. Robust cyclic AMP signal amplification was obtained using a colorimetric assay measuring beta-galactosidase activity. The EC(50) of 5'-N-ethylcarboxamidoadenosine (NECA), a potent A2a agonist, for inducing beta-galactosidase activity was 23.3 +/- 3.5 nM, similar to 22.7 +/- 3.9 nM, which was the NECA EC(50) in the direct measurement of cyclic AMP of CHOA2a-2 cells in early culture. Subsequently we validated this assay for high throughput screening for hA2aR agonists. The Z' factor for robotic assay performance was 0.79 +/- 0.03, the ratio of signal/noise was 157 +/- 36, and the ratio of signal/background was 10.6 +/- 1.2, demonstrating that this assay is well suitable for quality high throughput screening. High throughput screening of Johnson & Johnson libraries uncovered a couple of distinct series of nonadenosine small molecules, in addition to adenosine analogues, as potential hA2aR agonists with EC(50) values of 2-6 microM. Preliminary characterization of those compounds was presented.

Regulation of adenosine receptor subtypes during cultivation of human monocytes: role of receptors in preventing lipopolysaccharide-triggered respiratory burst

Adenosine is a potent anti-inflammatory agent that modulates the function of cells involved in the inflammatory response. Here we show that it inhibits lipopolysaccharide (LPS)-induced formation of reactive oxygen intermediates (ROI) in both freshly isolated and cultured human monocytes. Blocking of adenosine uptake and inactivation of the adenosine-degrading enzyme adenosine deaminase enhanced the inhibitory action of adenosine, indicating that both pathways regulate the extracellular adenosine concentration. Adenosine-mediated inhibition could be reversed by XAC (xanthine amine congener), an antagonist of the adenosine receptor A(2A), and MRS 1220 [N-9-chloro-2-(2-furanyl)[1, 2, 4]-triazolo[1,5-c]quinazolin-5-benzeneacetamide], an A(3) receptor antagonist, in both cell populations, while DPCPX (1,3-dipropyl-8-cyclopentylxanthine), an A(1) receptor antagonist, had no effect. Similar to what was seen with adenosine, CGS 21680, an A(2A) and A(3) receptor agonist, and IB-MECA, a nonselective A(1) and A(3) receptor agonist, dose dependently prevented ROI formation, indicating the involvement of A(3) and probably also A(2A) in the suppressive effect of adenosine. Pretreatment of monocytes with adenosine did not lead to changes in the LPS-induced increase in intracellular calcium levels ([Ca(2+)](i)). Thus, participation of [Ca(2+)](i) in the action of adenosine seems unlikely. The adenosine-mediated suppression of ROI production was found to be more pronounced when monocytes were cultured for 18 h, a time point at which changes in the mRNA expression of adenosine receptors were observed. Most prominent was the increase in the A(2A) receptor mRNA. These data demonstrate that cultivation of monocytes is accompanied by changes in the inhibitory action of adenosine mediated by A(3) and probably also the A(2A) receptor and that regulation of adenosine receptors is an integral part of the monocyte differentiation program.

Regulation of adenosine receptor engagement by ecto-adenosine deaminase

Adenosine deaminase (ADA) can localize to the cell surface through its interaction with CD26. Using CD26-transfected cells, we demonstrate that cell surface ADA (ecto-ADA) can regulate adenosine receptor engagement by degrading extracellular adenosine (Ado) to inosine. This ability was dependent upon CD26 expression, the extent of CD26 saturation with ecto-ADA, and the kinetics of the cAMP response. Thus, the cAMP response was markedly decreased when CD26-transfected cells were incubated with an exogenous source of ADA to increase ecto-ADA expression. The ability of ecto-ADA to inhibit the cAMP response was demonstrated by treatment with the specific ADA inhibitor 2'-deoxycoformycin. This inhibited the ability of ecto-ADA to degrade Ado and increased the cAMP response. Although CD26 expression on human thymocytes was low compared with that of CD26-transfected cells, it was saturated with ecto-ADA. When thymocytes incubated at high densities (to mimic the situation in tissues) were exposed to exogenous adenosine, the cAMP response was dramatically decreased by ecto-ADA. We conclude that ecto-ADA has the potential to regulate adenosine receptor-mediated cAMP responses in vivo in tissues with CD26+ cells and sufficient cell death caused by apoptosis or inflammation to provide a source of ADA to bind to CD26.

Characterisation of an uridine-specific binding site in rat cerebrocortical homogenates

Parameters of [3H]uridine binding to synaptic membranes isolated from rat brain cortex (K(D)=71+/-4 nM, B(max)=1.37+/-0.13 pmol/mg protein) were obtained. Pyrimidine and purine analogues displayed different rank order of potency in displacement of specifically bound [3H]uridine (uridine>5-F-uridine>5-Br-uridine approximately adenosine>>5-ethyl-uridine approximately suramin>theophylline) and in the inhibition of [14C]uridine uptake (adenosine>uridine>5-Br-uridine approximately 5-F-uridine approximately 5-ethyl-uridine) into purified cerebrocortical synaptosomes. Furthermore, the effective ligand concentration for the inhibition of [14C]uridine uptake was about two order of magnitude higher than that for the displacement of specifically bound [3H]uridine. Adenosine evoked the transmembrane Na(+) ion influx, whereas uridine the transmembrane Ca(2+) ion influx much more effectively. Also, uridine was shown to increase free intracellular Ca(2+) ion levels in hippocampal slices by measuring Calcium-Green fluorescence. Uridine analogues were found to be ineffective in displacing radioligands that were bound to various glutamate and adenosine-recognition and modulatory-binding sites, however, increased [35S]GTPgammaS binding to membranes isolated from the rat cerebral cortex. These findings provide evidence for a rather specific, G-protein-coupled site of excitatory action for uridine in the brain.

Analysis of DNase I-hypersensitive sites in the chromatin of the chicken adenosine receptor 2B gene reveals multiple cell-type-specific cis-regulatory elements

We have cloned and sequenced the gene encoding the chicken A2B adenosine receptor, a member of the family of G-protein-coupled seven-transmembrane-domain receptors which is activated by extracellular adenosine. The gene occupies about 10 kb of genomic sequences and contains two exons. The promoter of the gene is associated with a CpG-rich island and lacks a canonical TATA-box. Previous work has shown that the chicken A2B adenosine receptor is a direct target gene of the transcription factors v-Myb and c-Myb and that its expression in the hematopoietic system is specific for myelomonocytic as well as erythroid cells. To understand the molecular basis of this cell type specificity we have searched for cis-regulatory DNA elements that direct the expression of the gene to specific hematopoietic lineages. As a first step towards this aim we have mapped DNase I-hypersensitive sites in the chromatin of the A2B adenosine receptor gene. Our studies reveal multiple nuclease-sensitive sites which are located in the intron of the gene and in the upstream region. A number of these sites are cell-type-specific suggesting that they correspond to cell-type-specific cis-regulatory DNA elements. To characterize the possible function of these elements we have performed reporter gene studies. Our results show that several of the nuclease-sensitive regions act as myelomonocytic-specific cis-acting stimulatory elements. Taken together, our data suggest that the expression of the A2B adenosine receptor gene in myelomonocytic cells is controlled by multiple cell type-specific cis-acting sequences located upstream and within the intron of the gene.

A new neuromodulatory pathway with a glial contribution mediated via A(2a) adenosine receptors

A low concentration (10 nM) of adenosine potentiated hippocampal neuronal activity via A(2a) adenosine receptors without affecting presynaptic glutamate release or postsynaptic glutamatergic conductance. Adenosine inhibited glutamate uptake through the glial glutamate transporter, GLT-1, via A(2a) adenosine receptors. In addition, adenosine stimulated GLT-1-independent glutamate release from astrocytes, possibly in response to a rise in intracellular Ca(2+), via A(2a) adenosine receptors involving PKA activation. Those adenosine actions could lead to an increase in synaptic glutamate concentrations responsible for the potentiation of hippocampal neuronal activity. The results of the present study thus represent a novel neuromodulatory pathway with a glial contribution, bearing both inhibition of GLT-1 function and stimulation of glial glutamate release, as mediated via A(2a) adenosine receptors.

Cisplatin up-regulates the adenosine A(1) receptor in the rat kidney

Cisplatin, a widely used anticancer drug, produces significant oto- and nephrotoxicity. Previous data from our laboratory, using cultured cell lines, indicated that cisplatin increases the expression of the adenosine A(1) receptor subtype through generation of reactive oxygen species and activation of nuclear factor-kappa B (NF-kappa B). Since the adenosine A(1) receptor plays an important role in normal renal physiology, this study was performed to determine whether cisplatin modulates adenosine A(1) receptor expression in vivo and whether these receptors play a role in the nephrotoxicity. Male Sprague-Dawley rats, treated with cisplatin (8 mg/kg), developed nephrotoxicity within 3 days, as demonstrated by increased serum creatinine and blood urea nitrogen. Cisplatin also produced a significant increase in malondialdehyde, apoptosis and necrosis in the kidney. The above changes were associated with a time-dependent increase in the expression of adenosine A(1) receptor, as determined by radioligand binding assays, Western blotting and immunocytochemistry, and an increase in adenosine A(1) receptor transcripts. Administration of selective and nonselective antagonists of the adenosine A(1) receptor produced either no change or exacerbated the nephrotoxicity produced by cisplatin. These data indicate that cisplatin can regulate the adenosine A(1) receptor in the kidney and suggest a cytoprotective role of this receptor subtype against cisplatin-induced nephrotoxicity.

Signal transduction mechanism via adenosine A1 receptor in the cat esophageal smooth muscle cells

We investigated what adenosine receptor type exists and the signaling pathways on the contraction of circular muscle cells isolated by enzymatic digestion from the cat esophagus. Adenosine or the selective A1 receptor agonist R-PIA causes a concentration-dependent contraction. After pretreatment with A1 receptor antagonist, DPCPX, adenosine-mediated contraction was abolished. Adenosine-induced contraction was significantly increased when A1 receptors were preserved by pretreatment with DPCPX followed by inactivation of all unprotected receptors with N-ethylmaleimide. Adenosine- or R-PIA-induced contraction was significantly augmented in the preserved cells and the increase was abolished in the presence of the A1 receptor antagonist DPCPX. PTX abolished contraction induced by adenosine or R-PIA, implying that contraction activated by A1 receptor was coupled to a pertussis toxin (PTX)-sensitive G(i) protein. After permeabilization, contraction was inhibited by G(i2), but not by G(i1) and G(i3), antibodies. These data suggest that adenosine-induced contraction of esophagus depends on PTX-sensitive G(i2.) Adenosine- or R-PIA-induced contraction of esophageal smooth muscle cells was not affected by the phospholipase D (PLD) inhibitor rho-chloromercuribenzoic acid (rhoCMB), phospholipase A(2) (PLA(2)) inhibitor DEDA or PKC antagonist chelerythrine, but was significantly abolished by phospholipase C (PLC) inhibitor, neomycin. PLC-beta3 antibody inhibited R-PIA-induced contraction. R-PIA-induced contraction of esophageal muscle cells was inhibited by IP(3) receptor antagonist heparin, which suggests that the contraction of esophageal smooth muscle cells is dependent on phosphatidylinositol-specific phospholipase (PI-PLC) and IP(3). In conclusion, adenosine- and R-PIA-induced contraction in cat esophageal smooth muscle cell was mediated by A1 receptor. A1 receptor is coupled to PTX-sensitive G protein G(i2), which results in the activation of PI-PLC-beta3. PI hydrolysis by PI-PLC forms IP(3), which binds to IP(3) receptor on endoplasmic reticulum, resulting in the release of intracellular Ca(2+).

Synthesis of 2-alkyl-substituted-N6-methyladenine derivatives as potential adenosine receptor ligand

2-(1-Hexynyl), 2-((E)-1-hexenyl) and 2-(n-hexyl)-N6-methyladenines were synthesized, starting from 2-amino-6-chloropurine using palladium-catalyzed coupling as a key step as potential adenosine receptor ligand.

The effects of adenosine on isolated right atrial preparations from streptozotocin-diabetic rats

1. The aim of the present study was to investigate the inhibitory effects of adenosine on the contractile force and chronotropic action of isolated right atrial preparations from streptozotocin (STZ)-diabetic rats. 2. The rats were anaesthetized with diethyl ether and STZ (65 mg kg(-1)) was injected intravenously via the tail vein. 3. Adenosine produced concentration-dependent decreases in the force of contraction and a negative chronotropic action of atria both in control and diabetic groups. The inhibition responses to adenosine were significantly higher in diabetic rat atria than control. 4. Dypiridamole incubation caused a significant potentiation of the inhibitory effect of adenosine on contractile force and chronotropic action of atria in the control group, but not in the diabetic group. In the presence of dipyridamole, the inhibitory effects of adenosine on measured parameters in diabetic rats were not significantly different from those in control rats. 5. These results suggested that atria from 6 weeks STZ-diabetic rats exhibited a supersensitivity to the negative inotropic and chronotropic effects of adenosine compared with atria from control rats because of an impairment in adenosine uptake mechanism. Altered sensitivity to effects of adenosine might reflect relatively early changes in the course of diabetes.