Action Of Adenosine Research Articles

Adenosine A 3 Receptor and Cardioprotection

Ischemic injury to the myocardium is a leading cause of morbidity and mortality around the globe.1 It is worth remembering, however, that this insult to the heart emerged only in the last few centuries, an instant in time from an evolutionary perspective. Thus, any cardioprotective responses to ischemic injury undoubtedly evolved for an entirely different purpose. Beyond pure academic interest, however, there is a profound clinical need to identify new means of protecting the myocardium from disease-related stress. Among the strategies in development is a concerted effort to understand and enhance the body’s innate mechanisms of cardioprotection. Article p 1713 Among these cardioprotective mechanisms is one mediated by the molecule adenosine. This purine nucleoside, derived largely as a metabolite of 5′-AMP, coordinates organ metabolism and blood supply and modulates immune responses. Under conditions of cellular/metabolic stress, activation of the adenosinergic system has beneficial effects, and in heart, adenosine has well-established functions to mitigate myocardial damage from ischemia-reperfusion injury.2 For this reason, there is great interest in adenosinergic signaling as a means of treating ischemic heart disease. However, the biology of adenosinergic signaling in several organ systems has proven complex, manifesting context-dependent pro- and anti-survival actions. A number of mechanisms have been invoked to explain the cardioprotective actions of adenosine. These include preservation of ATP levels, stimulation of glycolysis, and limitation of oxygen demand. Some of these events are mediated by alterations in protein kinase C, phosphoinositide-3 kinase, and mitogen-activated protein kinase signaling pathways. To cite 1 example, A1 adenosine receptor antagonism prevents ischemia-induced sensitization of adenylyl cyclase via a protein kinase C–mediated pathway.3 However, there is considerable disagreement in the literature, possibly resulting from the fact that several commonly used signaling inhibitors, including inhibitors of protein kinase C, significantly limit agonism of adenosine receptors.4 Adenosine …

Reduced ligand affinity leads to an impaired function of the adenosine A2A receptor of human granulocytes in sepsis

The enhanced release of reactive oxygen species by excessively activated polymorphonuclear leucocytes (PMN) is a key step in the pathogenesis of sepsis. Potent action of adenosine in inhibiting cytotoxic PMN functions has been documented. Recent data, however provide evidence that in sepsis a diminished capability of adenosine to inhibit the generation of oxygen radicals by PMN occurs. Here, we investigated the underlying mechanisms in an in vitro sepsis model and in PMN of sepsis patients. We report that lipopolysaccharide (LPS)-incubation of human PMN elicited the same increase in the half-maximal inhibitory concentration (IC50) of adenosine as observed in patients with septic shock. Coupling to adenylyl cyclase was impaired as well, as indicated by a decreased potency of adenosine to stimulate cyclic adenosine monophosphate (cAMP) accumulation. Ligand-binding studies conducted with native, LPS-stimulated PMN, and with PMN of sepsis patients revealed that, despite an increased adenosine A2A receptor (A2AR) expression, the receptor function declines due to a diminished ligand-binding affinity most likely caused by allosteric modulators within the inflammatory environment. A2AR function obviously is highly dependent upon the cellular environment and thus, further functional characterization of A2AR responses in sepsis may be a promising approach to develop new adenosine or A2AR agonists based therapeutic strategies.

Genetic inactivation of adenosine A 2A receptors attenuates acute traumatic brain injury in the mouse cortical impact model

The inactivation of the A 2A receptor (A 2AR) has been shown to neuroprotect against brain injury in several animal models of neurological disorders including stroke and Parkinson's disease. However, despite marked elevation of adenosine level, the role of the A 2A in traumatic brain injury (TBI) remains unclear. In the present study, we investigated the effects of genetic inactivation of A 2ARs in the acute stage. The A 2AR knock-out (KO) mice and their wild-type (WT) littermates were subjected to cortical impact injury by a dropping weight. The control group was only craniotomized without TBI. At 24 h post-TBI, the neurological deficit scores of the KO mice were significantly lower than that of WT littermates. Consistent with the behavioral changes, the brain water contents as well as histological changes and the TUNEL-positive cells of the injured cortex of the KO mice were significantly lower than that of WT littermates. Furthermore, the glutamate level in the cerebral spinal fluid (CSF) of the KO mice was also significantly lower than that of WT littermates. In addition, we found that at 12 h post-TBI the mRNA and protein levels of TNF-α and IL-1β were higher in the KO mice than that in the WT littermates. However, at 24 h post-TBI, the level of TNF-α and IL-1β continually increased in the WT mice but largely declined in the KO mice. These results suggest that the genetic inactivation of A 2AR protects against TBI, which is mainly associated with the suppression of glutamate level.

Adenosine A 1 receptors regulate lipolysis and lipogenesis in mouse adipose tissue — Interactions with insulin

Adenosine acting at adenosine A 1 receptors is considered to be one major regulator of adipose tissue physiology. We have examined the role of adenosine and its interactions with insulin in adipose tissue by using A 1R knock out (−/−) mice. Removal of endogenous adenosine with adenosine deaminase caused lipolysis in A 1R (+/+), but not A 1R (−/−) adipocytes. The adenosine analogue, 2-chloroadenosine, inhibited noradrenaline-stimulated lipolysis and cAMP accumulation in A 1R (+/+), but not in A 1R (−/−) adipocytes. Insulin reduces lipolysis and cAMP via another mechanism than adenosine and acted additively, but not synergistically, with adenosine. Plasma levels of free fatty acids, glycerol and triglycerides were significantly lower in A 1R (+/+) than in A 1R (−/−) mice after administration of an adenosine analogue. 2-chloroadenosine induced lipogenesis in presence of insulin in A 1R (+/+), but not in A 1R (−/−) adipocytes. There were no changes in mRNA levels for several genes involved in fat synthesis in adipose tissue between genotypes. Body weight was similar in young A 1R (+/+) and A 1R (−/−) mice, but old male A 1R (−/−) mice were heavier than wild type controls. In conclusion, adenosine inhibits lipolysis via the adenosine A 1 receptor and other adenosine receptors play no significant role. Adenosine and insulin mediate additive but not synergistic antilipolytic effects and 2-chloroadenosine stimulates lipogenesis via adenosine A 1 receptors. Thus deletion of adenosine A 1 receptors should increase lipolysis and decrease lipogenesis, but in fact an increased fat mass was observed, indicating that other actions of adenosine A 1 receptors, possibly outside adipose tissue, are also important.

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.

Activation of the A3Adenosine Receptor Suppresses Superoxide Production and Chemotaxis of Mouse Bone Marrow Neutrophils

Adenosine is formed in injured/ischemic tissues, where it suppresses the actions of essentially all cells of the immune system. Most of the anti-inflammatory actions of adenosine have been attributed to signaling through the G(s) protein-coupled A(2A) adenosine receptor (AR). Here, we report that the A(3)AR is highly expressed in murine neutrophils isolated from bone marrow. Selective activation of the A(3)AR with (2S,3S,4R,5R)-3-amino-5-[6-(2,5-dichlorobenzylamino)purin-9-yl]-4-hydroxytetrahydrofuran-2-carboxylic acid methylamide (CP-532,903) potently inhibited mouse bone marrow neutrophil superoxide generation and chemotaxis induced by various activating agents. The selectivity of CP-532,903 was confirmed in assays using neutrophils obtained from A(2A)AR and A(3)AR gene "knockout" mice. In a model of thioglycollate-induced inflammation, treating mice with CP-532,903 inhibited recruitment of leukocytes into the peritoneum by specifically activating the A(3)AR. Collectively, our findings support the theory that the A(3)AR contributes to the anti-inflammatory actions of adenosine on neutrophils and provide a potential mechanistic explanation for the efficacy of A(3)AR agonists in animal models of inflammation (i.e., inhibition of neutrophil-mediated tissue injury).

ATP in central respiratory control: A three-part signaling system

The landmark demonstrations in 2005 that ATP released centrally during hypoxia and hypercapnia contributes to the respective ventilatory responses validated a decade-old hypothesis and ignited interest in the potential significance of P2 receptor signaling in central respiratory control. Our objective in this review is to provide a non-specialist overview of ATP signaling from the perspective that it is a three-part system where the net effects are determined by an interaction between the signaling actions of ATP and adenosine at P2 and P1 receptors, respectively, and a family of enzymes (ectonucleotidases) that breakdown ATP into adenosine. We review the rationale for the original interest in P2 signaling in respiratory control, the evolution of this hypothesis, and the mechanisms by which ATP might affect respiratory behaviour. The potential significance of P2 receptor, P1 receptor and ectonucleotidase diversity for the different compartments of the respiratory control system is also considered. We conclude with a look to future questions and technical challenges.

Adenosine Actions are Preserved in Corpus Cavernosum from Obese and Type II Diabetic db/db Mouse

Erectile dysfunction (ED) in diabetes is associated with autonomic neuropathy and endothelial dysfunction. Whereas the nonadrenergic-noncholinergic (NANC)/neurogenic nitric oxide pathway has received great attention in diabetes-associated ED, few studies have addressed sympathetic overactivity. To test the hypothesis that adenosine-induced inhibition of adrenergic-mediated contractile responses in mouse corpus cavernosum is impaired in the presence of diabetes. The db/db (obesity and type II diabetes caused by a leptin receptor mutation) mouse strain was used as a model of obesity and type II diabetes, and standard procedures were performed to evaluate functional cavernosal responses. Increased cavernosal responses to sympathetic stimulation in db/db mice are not associated with impaired prejunctional actions of adenosine. Electrical field stimulation (EFS)-, but not phenylephrine (PE)-, induced contractions are enhanced in cavernosal strips from db/db mice in comparison with those from lean littermates. Direct effects of adenosine, 2-chloro-adenosine, A(1) receptor agonist C-8031 (N6 cyclopentyladenosine), and sodium nitroprusside are similar between the strips from lean and db/db mice, whereas relaxant responses to acetylcholine and NANC stimulation are significantly impaired in the cavernosal strips from db/db mice. 5'-Iodotubercidin (adenosine kinase inhibitor) and dipyridamole (inhibitor of adenosine transport), as well as the A(1) agonist C-8031, significantly and similarly inhibit contractions induced by stimulation of adrenergic nerves in the cavernosal strips from lean and db/db mice. Results from this study suggest that corpora cavernosa from obese and diabetic db/db mice display altered neural-mediated responses that would favor penile detumescence, i.e., increased contractile response to adrenergic nerve stimulation and decreased relaxant responses upon activation of NANC nerves. However, increased cavernosal responses to adrenergic nerve stimulation are not due to impaired negative modulation of sympathetic neurotransmission by adenosine in this diabetic model.

Acute inhibition of the betaine transporter by ATP and adenosine in renal MDCK cells

Extracellular ATP interacts with purinergic P2 receptors to regulate a range of physiological responses, including downregulation of transport activity in the nephron. ATP is released from cells by mechanical stimuli such as cell volume changes, and autocrine signaling by extracellular ATP could occur in renal medullary cells during diuresis. This was tested in Madin-Darby canine kidney (MDCK) cells, a model used frequently to study P1 and P2 receptor activity. ATP was released within 1 min after transfer from 500 to 300 mosmol/kgH2O medium. A 30-min incubation with ATP produced dose-dependent inhibition (0.01-0.10 mM) of the renal betaine/GABA transporter (BGT1) with little effect on other osmolyte transporters. Inhibition was reproduced by specific agonists for P2X (alpha,beta-methylene-ATP) and P2Y (UTP) receptors. Adenosine, the final product of ATP hydrolysis, also inhibited BGT1 but not taurine transport. Inhibition by ATP and adenosine was blocked by pertussis toxin and A73122, suggesting involvement of inhibitory G protein and PLC in postreceptor signaling. Both ATP and adenosine (0.1 mM) produced rapid increases in intracellular Ca2+, due to the mobilization of intracellular Ca2+ stores and Ca2+ influx. Blocking these Ca2+ increases with BAPTA-AM also blocked the action of ATP and adenosine on BGT1 transport. Finally, immunohistochemical studies indicated that inhibition of BGT1 transport may be due to endocytic accumulation of BGT1 proteins from the plasma membrane. We conclude that ATP and adenosine, through stimulation of PLC and intracellular Ca2+, may be rapidly acting regulators of BGT1 transport especially in response to a fall in extracellular osmolarity.

Adenosine A1 receptors inhibit GABAergic transmission in rat tuberomammillary nucleus neurons

The adenosinergic modulation of GABAergic spontaneous miniature inhibitory postsynaptic currents (mIPSCs) was investigated in mechanically dissociated rat tuberomammillary nucleus (TMN) neurons using a conventional whole-cell patch clamp technique. Adenosine (100 microM) reversibly decreased mIPSC frequency without affecting the current amplitude, indicating that adenosine acts presynaptically to decrease the probability of spontaneous GABA release. The adenosine action on GABAergic mIPSC frequency was completely blocked by 1 microM DPCPX, a selective A(1) receptor antagonist, and mimicked by 1 microM CPA, a selective A(1) receptor agonist. This suggests that presynaptic A(1) receptors were responsible for the adenosine-mediated inhibition of GABAergic mIPSC frequency. CPA still decreased GABAergic mIPSC frequency even either in the presence of 200 microM Cd(2+), a general voltage-dependent Ca(2+) channel blocker, or in the Ca(2+)-free external solution. However, the inhibitory effect of CPA on GABAergic mIPSC frequency was completely occluded by 1 mM Ba(2+), a G-protein coupled inwardly rectifying K(+) (GIRK) channel blocker. In addition, the CPA-induced decrease in mIPSC frequency was completely occluded by either 100 microM SQ22536, an adenylyl cyclase (AC) inhibitor, or 1 muM KT5720, a specific protein kinase A (PKA) inhibitor. The results suggest that the activation of presynaptic A(1) receptors decreases spontaneous GABAergic transmission onto TMN neurons via the modulation of GIRK channels as well as the AC/cAMP/PKA signal transduction pathway. This adenosine A(1) receptor-mediated modulation of GABAergic transmission onto TMN neurons may play an important role in the fine modulation of the excitability of TMN histaminergic neurons as well as the regulation of sleep-wakefulness.

Nitric oxide signaling pathway in the anti-convulsant effect of adenosine against pentylenetetrazol-induced seizure threshold in mice

The present study was performed to examine the involvement of nitric oxide (NO) signaling pathway in the anti-convulsant effect of adenosine against pentylenetetrazol seizure threshold in mice. Minimal dose of pentylenetetrazol (i.v., mg/kg) needed to induce different phases (myoclonic jerks, generalized clonus and tonic extension) of convulsions was recorded as an index of seizure threshold. Adenosine (100 or 200 mg/kg i.p.) produced a significant increase in the seizure threshold for convulsions induced by pentylenetetrazol i.v. infusion. The anti-convulsant effect of adenosine (100 mg/kg i.p.) was prevented by either l-arginine (50 mg/kg i.p.) [substrate for nitric oxide synthase (NOS)] or sodium nitroprusside (3 mg/kg i.p.) [a NO donor]. On the other hand, N(G)-nitro- l-arginine methyl ester (L-NAME, 2.5 mg/kg i.p.) [a non-selective NOS inhibitor] or 7-nitroindazole (7-NI) (25 mg/kg i.p.) [a specific neuronal nitric oxide synthase (nNOS) inhibitor] potentiated the anti-convulsant action of sub-effective dose of adenosine (50 mg/kg i.p.). Aminoguanidine (100 mg/kg i.p.) [a specific inducible NOS (iNOS) inhibitor] pre-treatment was not effective in inducing anti-convulsant effect with sub-effective dose of adenosine (50 mg/kg i.p.). Furthermore, the increase in seizure threshold elicited by adenosine (100 mg/kg i.p.) was also inhibited by concomitant administration with sildenafil (5 mg/kg i.p.) [phosphodiesterase 5 inhibitor]. In contrast, treatment of mice with methylene blue (1 mg/kg i.p.) [a direct inhibitor of both nitric oxide synthase (NOS) and soluble guanylate cyclase (sGC)] failed to induce anti-convulsant action with adenosine (50 mg/kg i.p.) against pentylenetetrazol i.v. infusion. The results demonstrated that the anti-convulsant action of adenosine in the pentylenetetrazol i.v. seizure threshold paradigm may possibly involve an interaction with the l-arginine-NO-cGMP pathway which may be secondary to the activation of adenosine receptors.

Chronic Dietary l‐Arginine Down‐Regulates Adenosine Receptor and Nitric Oxide Synthase Expression in Rat Heart

L-Arginine increases myocardial nitric oxide production. Nitric oxide mediates many of the cardiovascular actions of adenosine and modulates adenosine metabolism. In this study, we examined the effect of chronic L-arginine (5%) intake on cardiac nitric oxide synthase (NOS) and adenosine receptor expression and cardiac function in rat Langendorff-isolated perfused hearts. Our results show that 4-week chronic l-arginine ingestion increases the weight of rat hearts by 17.6% (P < 0.05). L-Arginine treatment decreased the expression of all the cardiac adenosine receptors, with reductions in adenosine A(1) (20-fold), A(2A) (7.7-fold), A(2B) (76-fold) and A(3) (25.6-fold) mRNA (P < 0.05). NOS expression was variably affected with no change in the expression of NOS(1) and 4.2-fold down-regulation of NOS(3) expression with chronic L-arginine treatment (P < 0.05). NOS(2) was expressed in control tissues; however, in L-arginine-treated hearts the amount of NOS(2) mRNA was reduced to non-detectable levels. Following chronic L-arginine treatment, an increase in coronary perfusion pressure was observed (P < 0.05). Purine efflux was used as an indicator of metabolic efficiency. L-Arginine did not alter catecholamine-induced purine efflux (P > 0.05); however, noradrenaline-mediated increases in contractility and myocardial oxygen consumption were reduced. Vasodilator responses to 5'-N-ethylcarboxamidoadenosine (NECA) were reduced in hearts from l-arginine-treated rats and the NOS inhibitor N omega-nitro-L-arginine methyl ester (3 microM) did not inhibit responses to NECA. In conclusion, 4-week dietary supplementation of L-arginine reduced the expression of cardiac adenosine receptors and NOSs with a subsequent decrease in noradrenaline-stimulated cardiac function and adenosine receptor-mediated coronary vasodilation.

Sleep homeostasis: A role for adenosine in humans?

Sleep is not the mere absence of wakefulness, but an active state which is finely regulated. The homeostatic facet of sleep–wake regulation is keeping track of changes in ‘sleep propensity’ (or ‘sleep need’), which increases during wakefulness and decreases during sleep. Increased sleep propensity following extended prior wakefulness (sleep deprivation) is counteracted by prolonged sleep duration, but also by enhanced non-rapid-eye-movement (nonREM) sleep intensity as measured by electroencephalographic (EEG) slow-wave activity (SWA, power within ∼1–4Hz). This highly reliable regulatory feature of nonREM sleep may be the most important aspect of sleep in relation to its function. The neurochemical mechanisms underlying nonREM sleep homeostasis are poorly understood. Here we provide compelling and convergent evidence that adenosinergic neurotransmission plays a role in nonREM sleep homeostasis in humans. Specifically, a functional polymorphism in the adenosine metabolizing enzyme, adenosine deaminase, contributes to the high inter-individual variability in deep slow-wave sleep duration and intensity. Moreover, the adenosine receptor antagonist, caffeine, potently attenuates the EEG markers of nonREM sleep homeostasis during sleep, as well as during wakefulness. Finally, adenosinergic mechanisms modulate individual vulnerability to the detrimental effects of sleep deprivation on neurobehavioral performance, and EEG indices of disturbed sleep after caffeine consumption. While these convergent findings strongly support an important contribution of adenosine and adenosine receptors to nonREM sleep homeostasis, further research is needed to elucidate the underlying mechanisms that mediate the actions of adenosine on sleep and the sleep EEG.

Effect of genetic A2B adenosine receptor ablation on proinflammatory actions of adenosine in mast cells

We have previously shown that A2B adenosine receptors (AR) play a pro‐inflammatory/pro‐angiogenic role, upregulating production of Th2 cytokines and angiogenic factors in human HMC‐1 mast cells.1,2 However, the pro‐inflammatory role of A2bAR in mast cells was recently questioned by a study that found an enhanced antigen‐induced degranulation of A2bAR knockout (A2BKO) mouse bone marrow‐derived mast cells (BMMCs).3 The authors interpreted this as evidence of anti‐inflammatory actions of A2BAR in BMMCs and suggested that A2BAR antagonists with inverse agonist activity could promote activation of mast cells. Here, we revisited these conclusions by studying the effect of genetic A2BAR ablation on adenosine‐dependent regulation of BMMCs. Our results show that genetic ablation of A2BAR had no effect on adenosine‐dependent potentiation of antigen‐induced degranulation, but abrogated adenosine‐dependent stimulation of IL‐13 and VEGF secretion. A2BAR antagonists with inverse agonist activity inhibited A2BAR‐dependent secretion, but did not mimic the enhanced antigen‐induced degranulation observed in A2BKO BMMCs. Thus, our study confirmed the pro‐inflammatory role of mast cell A2BAR and demonstrated anti‐inflammatory actions of A2BAR antagonists.Supported by NIH.

Role of the cAMP sensor Epac as a determinant of KATP channel ATP sensitivity in human pancreatic β‐cells and rat INS‐1 cells

Protein kinase A (PKA)-independent actions of adenosine 3',5'-cyclic monophosphate (cAMP) are mediated by Epac, a cAMP sensor expressed in pancreatic beta-cells. Evidence that Epac might mediate the cAMP-dependent inhibition of beta-cell ATP-sensitive K(+) channels (K(ATP)) was provided by one prior study of human beta-cells and a rat insulin-secreting cell line (INS-1 cells) in which it was demonstrated that an Epac-selective cAMP analogue (ESCA) inhibited a sulphonylurea-sensitive K(+) current measured under conditions of whole-cell recording. Using excised patches of plasma membrane derived from human beta-cells and rat INS-1 cells, we now report that 2'-O-Me-cAMP, an ESCA that activates Epac but not PKA, sensitizes single K(ATP) channels to the inhibitory effect of ATP, thereby reducing channel activity. In the presence of 2'-O-Me-cAMP (50 microM), the dose-response relationship describing ATP-dependent inhibition of K(ATP) channel activity (NP(o)) is left-shifted such that the concentration of ATP producing 50% inhibition (IC(50)) is reduced from 22 microM to 1 microM for human beta-cells, and from 14 microM to 4 microM for rat INS-1 cells. Conversely, when patches are exposed to a fixed concentration of ATP (10 microM), the administration of 2'-O-Me-cAMP inhibits channel activity in a dose-dependent and reversible manner (IC(50) 12 microM for both cell types). A cyclic nucleotide phosphodiesterase-resistant ESCA (Sp-8-pCPT-2'-O-Me-cAMPS) also inhibits K(ATP) channel activity, thereby demonstrating that the inhibitory actions of ESCAs reported here are unlikely to arise as a consequence of their hydrolysis to bioactive derivatives of adenosine. On the basis of such findings it is concluded that there exists in human beta-cells and rat INS-1 cells a novel form of ion channel modulation in which the ATP sensitivity of K(ATP) channels is regulated by Epac.

Adenosine Prompts the Heart to Recruit Endothelial Progenitors

See related article, pages 356–363 Adenosine has a long history of involvement in cardiac function. It was first purified from mammalian heart homogenates in 1929 by Drury and Szent-Gyorgyi,1 who reported a “suppressive” action of adenosine on the heart, including decreased heart beat rate and perfusion pressure,1 now termed bradycardia and coronary artery vasodilation, respectively. Based on a variety of experimental results from several laboratories, Berne et al proposed in the early 1960s that adenosine plays an important role in the adjustment of blood flow to the metabolic requirements of organs, including the heart, brain, and skeletal muscle.2 Numerous studies now support the hypothesis that adenosine can be directly released or generated from released ATP/ADP in the heart during ischemia, causing increased oxygen and nutrition supply by inducing coronary artery relaxation.3 Although adenosine exhibits reuptake via specific adenosine transporters, many, if not all, of the biological effects of adenosine are believed to be mediated by adenosine receptors (ARs), of which 4 subtypes (A1R, A2AR, A2BR, and A3R) have been cloned and pharmacologically characterized.4 The diverse physiological functions mediated by the different AR subtypes, particularly in the modulation of the cardiovascular system, have been confirmed by genetically engineered mice.4 Null mice have been generated for each of the AR subtypes, and none of the 4 ARs plays a critical role during development.4,5 Although each AR protein had been thought to exist in the cell membrane as a monomer, recent studies with recombinant expression systems have revealed that some of the AR subtypes heterodimerize with other G-protein coupled receptors. For example, A1R has been shown to heterodimerize with …

Actions of Adenosine A2A Receptors on Synaptic Connections of Spiny Projection Neurons in the Neostriatal Inhibitory Network

There is growing evidence that adenosine plays a crucial role in basal ganglia function, particularly in the modulation of voluntary movement. An adenosine-based treatment for Parkinson's disease shows promise in recent clinical studies. Adenosine A(2A) receptors, the receptors involved in this treatment, are highly expressed in the neostriatum. Previous studies have suggested opposing actions of these receptors on synaptic transmission at striatal and pallidal terminals of the same spiny projection neurons, but the cells of origin of the intrastriatal terminals mediating these actions have not been identified. We used dual whole cell recordings to record simultaneously from pairs of striatal cells; this enabled definitive identification of the presynaptic and postsynaptic cells mediating the effects of A(2A) receptors. We found that A(2A) receptors facilitate GABAergic synaptic transmission by intrastriatal collaterals of the spiny projection neurons, consistent with their previously reported actions on synaptic transmission at pallidal terminals. This neuromodulatory action on lateral inhibition in the striatum may underlie, in part, the therapeutic efficacy of adenosine-based treatments for Parkinson's disease.

STIMULATION OF ADENOSINE A2A RECEPTOR INHIBITS LPS-INDUCED EXPRESSION OF INTERCELLULAR ADHESION MOLECULE 1 AND PRODUCTION OF TNF-α IN HUMAN PERIPHERAL BLOOD MONONUCLEAR CELLS

LPS stimulates CD14/Toll-like receptor (TLR) 4, leading to induce TNF-alpha production. Cell-to-cell interaction through the engagement between intercellular adhesion molecule (ICAM) 1 on monocytes and its ligand on T cells has been suggested to play a role in the TNF-alpha production by LPS-treated human peripheral blood mononuclear cells (PBMCs). Adenosine is reported to inhibit LPS-induced TNF-alpha production. However, little is known about the mechanism of the inhibitory effects induced by adenosine on the LPS-induced immune responses. We found that adenosine inhibited the expression of ICAM-1 and the production of TNF-alpha by human PBMC via adenosine A2A receptor in the presence of LPS. However, the stimulation of A1R or A3R enhanced the actions of adenosine. Adenosine had no effect on the expression of CD14 and TLR-4, suggesting that the inhibitory effects of adenosine on the LPS actions might be independent of the expression of CD14 and TLR-4. Thus, adenosine differentially regulates the expression of ICAM-1 and the production of TNF-alpha through plural subtypes of receptors.

Cyclic AMP in Rat Ileum: Evidence for the Presence of an Extracellular Cyclic AMP-Adenosine Pathway

Extracellular adenosine plays a relevant role in regulating intestinal motility and preventing inflammatory processes. Adenosine 3',5'-cyclic monophosphate (cAMP) extruded from cells may be converted to adenosine monophosphate and then to adenosine by ecto-phosphodiesterase and CD73/ecto-5'nucleotidase, respectively, thus representing a source of adenosine. Our purpose was to assess the existence of a functional extracellular cAMP-adenosine pathway in intestinal tissue, obtaining evidence for CD73 expression and evaluating the effect of cAMP on ileum motility. The formation of cAMP metabolites in rat ileum strips incubated with exogenous cAMP or [(3)H]cAMP was monitored by high-performance liquid chromatography. CD73 was detected by immunoprecipitation and immunofluorescence. The functional activity of exogenous cAMP on ileum strips was recorded by measuring tension changes. In ileum strips, the generation of cAMP-derived adenosine monophosphate, adenosine, and inosine was time and concentration dependent and was blocked by phosphodiesterase or CD73 inhibitors in a manner consistent with exogenous cAMP being processed through the extracellular cAMP-adenosine pathway. Accordingly, [(3)H]cAMP uptake in ileum strips was negligible. Immunofluorescence revealed CD73 surface expression on intestinal smooth muscle cells and intact smooth muscle. Exogenous cAMP concentration-dependently increased ileum muscle tension partially inhibited by adenosine inactivation or receptor blockade. Forskolin-stimulated endogenous cAMP induced concentration-dependent ileum relaxations. A functioning extracellular cAMP-adenosine pathway featuring CD73 expression is present in rat ileum and affects intestinal motility. Extracellular cAMP may therefore act on intestinal muscle both directly by binding to specific smooth muscle cell membrane sites and indirectly through its degradation products.

Cytometric Analysis of the Cytotoxic Action of Adenosine 5'-Monophosphate on Rat Thymocytes

The cascade of adenosine 5'-monophosphate-activated protein kinase (AMPK) is known to be a sensor of cellular energy charge. In this context, a paradigm for obesity treatment via the activation of AMPK has attracted the suppliers of complementary medicines and supplements. It is possible that products that increase the concentration of adenosine 5'-monophosphate (AMP) in the body would be efficacious in reducing body weight. However, since there is currently little information concerning the toxicity of AMP, the cytotoxic action of AMP on rat thymocytes was examined by flow cytometry. Incubation of cells with AMP at a concentration of 30 μM or more for 24 hr significantly increased the populations of dead cells, shrunken cells, and cells containing hypodiploidal DNA in a concentration-dependent manner. Z-VAD-FMK, a pan-inhibitor of caspases, attenuated the AMP-induced changes in cell populations. It is concluded that AMP at a concentration of 30 μM or more exerts a cytotoxic action, which is dependent on the activation of caspases.