Action Of Adenosine Research Articles

Adenosine Receptors and Angiogenesis

See related article, pages 1130–1138 As originally hypothesized by Berne,1 adenosine is formed in the heart during hypoxia or ischemia and serves as a negative feedback signal to maintain oxygen delivery within a normal physiological range. Adenosine is formed by the metabolism of ATP and transported into the extracellular space by various nucleoside transport proteins.2 In a more localized manner that provides the opportunity for coordinated signaling with the diverse P2 purinoreceptor family that recognizes ATP and ADP, adenosine is also formed from the extracellular metabolism of adenine nucleotides released from stored granules (sympathetic nerves, platelets, mast cells) or through hemichannels (neutrophils, endothelial cells, epithelial cells) by ecto-apyrases and ecto-nucleotidases.3–5 Adenosine signals through activation of widely distributed cell surface G protein–coupled receptors, of which 4 different subtypes have been identified designated A1, A2A, A2B, and A3.6 By activating A2A receptors that are abundantly expressed in coronary vascular smooth muscle, adenosine generated under hypoxic conditions produces potent vasodilation thereby increasing nutrient and oxygen delivery. Although less appreciated, adenosine also serves to maintain tissue oxygenation in response to chronic ischemic/hypoxic stress by increasing the formation of new blood vessels; that is, by stimulating angiogenesis.7,8 Using adenosine receptor agonists or modulators of adenosine metabolism, an important role for adenosine has been established in in vivo models of angiogenesis.7,8 In cell culture systems, adenosine stimulates endothelial cell migration, proliferation, and tube formation,7–11 actions required for the formation of new capillary networks. This action occurs at concentrations of adenosine that are …

A 1 and A 2A receptor activation by endogenous adenosine is required for VIP enhancement of K +-evoked [ 3H]-GABA release from rat hippocampal nerve terminals

Vasoactive intestinal peptide (VIP) modulates GABA release from hippocampal nerve terminals and enhances hippocampal synaptic transmission through a pathway dependent on GABAergic transmission. Since VIP modulation of hippocampal synaptic transmission is dependent on the tonic actions of adenosine we investigated if endogenous adenosine could influence VIP enhancement of GABA release from isolated hippocampal nerve endings, and which adenosine receptors could be mediating this influence. When extracellular endogenous adenosine was removed using adenosine deaminase (ADA, 1 U/ml), the enhancement (57.2 ± 3.7%) caused by VIP on GABA release was prevented. Blockade of adenosine A 1 receptors with 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 10 nM) or of A 2A receptors with ZM241385 (50 nM) abolished the effect of VIP. In the presence of ADA, selective A 2A receptor-activation with CGS21680 (10 nM) readmitted most of the enhancement caused by VIP on GABA release (50.7 ± 5.3%). Also in the presence of ADA, A 1 receptor activation with N 6-cyclopentyladenosine (CPA, 50 nM) partially readmitted that effect of VIP (32.6 ± 3.8%). In conclusion, the enhancement of GABA release caused by VIP in hippocampal nerve terminals is dependent on the tonic actions of adenosine on both A 1 and A 2A receptors, and this action of adenosine is essential to VIP modulation of GABA release.

Hypoxanthine uptake and release by equilibrative nucleoside transporter 2 (ENT2) of rat microvascular endothelial cells

The cardioprotective actions of adenosine are terminated by its uptake into endothelial cells with subsequent metabolism through hypoxanthine to uric acid. This process involves xanthine oxidase-mediated generation of reactive oxygen species (ROS), which have been implicated in the vascular dysfunction observed in ischemia–reperfusion injury. The equilibrative nucleoside transporter, ENT2, mediates the transfer of hypoxanthine into cells. We hypothesize that ENT2 also mediates the cellular release of hypoxanthine, which would limit the amount of intracellular hypoxanthine available for xanthine oxidase-mediated ROS production. Rat microvascular endothelial cells (MVECs) were isolated from skeletal muscle by lectin-affinity purification. The transport of [ 3H]hypoxanthine was assessed using an oil-stop method, and hypoxanthine metabolites were identified by thin-layer chromatography. MVECs accumulated hypoxanthine with a K m of 300 μM and a V max of 2.8 pmol μl − 1 s − 1 . ATP-depleted cells loaded with [ 3H]hypoxanthine released the radiolabel with kinetics similar to that obtained for [ 3H]hypoxanthine influx. The uptake and release of [ 3H]hypoxanthine were both blocked by ENT2 inhibitors with similar order of potency. Thus, ENT2 mediates both the influx and efflux of hypoxanthine. Inhibition of ENT2 in MVECs might be expected to increase the amount of intracellular hypoxanthine available for metabolism by xanthine oxidase and enhance the intracellular production of ROS.

What Animal Models Have Taught us About Adenosine Receptor Function

The development and characterization of transgenic animal models with genetic manipulation over the last decade has significantly broaded our knowledge of adenosine receptor neurobiology, particularly their involvement in various pathological processes of neuropsychiatric disorders. In this review, we summarize the contributions made to the field of adenosine research by transgenic animal models with either target deletion or over-expression of adenosine receptor subtypes and molecules involved in adenosine metabolism pathways. The available data on the various aspects of physiological (e.g. sleep, motor activity, memory, anxiety, agression and depression and vascular function) and pathological (e.g. ischemia, inflammation, neurodegenerative diseases such as Parkinson’s Disease, Huntington’s Disease, multiple sclerosis, seizures and pain) actions of adenosine as revealed by these transgenic animal models are reviewed. The phenotypes revealed by pharmacological, neurochemical and molecular analyses of these transgenic models into the neuromodulatory function of adenosine receptors in the brain and the homeostatic function of adenosine receptors both in physiological and pathological conditions. These analyses, in many cases, also provided compelling evidence for the phenotypes of adenosine receptor transgenic models that differ from adenosine pharmacological studies, indicating complex actions of adenosine receptors. The complex interplay of adenosine receptors and actions in different cell types and subcellular elements is an important aspect of adenosine’s role that requires new transgenic models designed to target specific brain regions, cell types or subcellular elements. Ultimately these studies should prove important in defining strategies for adenosine-based therapeutical approaches for neuropsychiatric disorders. Keywords: Adenosine, adenosine receptor, gene knockout, transgene, mouse model

Mass spectrometric identification of Rab23 phosphorylation as a response to challenge by cytidine 3′,5′‐cyclic monophosphate in mouse brain

While the functions and mechanisms of action of adenosine 3',5'-cyclic monophosphate (cAMP) and guanosine 3',5'-cyclic monophosphate (cGMP) are well established and are the basis of the action of a large number of successful pharmaceuticals, the role of a third naturally occurring cyclic nucleotide, cytidine 3',5'-cyclic monophosphate (cCMP), remains to be elucidated. Immobilized metal affinity chromatography (IMAC) was used to selectively extract proteins phosphorylated in mouse brain in response to challenge by cAMP, cGMP and cCMP, followed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-ToFMS) and liquid chromatography/electrospray ionization tandem mass spectrometry (LC/ESI-MS/MS) of tryptic digests to identify Rab23 as the first protein reported to be phosphorylated only in response to cCMP.

Intracellular signalling pathways in the vasoconstrictor response of mouse afferent arterioles to adenosine

Adenosine causes vasoconstriction of afferent arterioles of the mouse kidney through activation of adenosine A(1) receptors and Gi-mediated stimulation of phospholipase C. In the present study, we further explored the signalling pathways by which adenosine causes arteriolar vasoconstriction. Adenosine (10(-7) M) significantly increased the intracellular calcium concentration in mouse isolated afferent arterioles measured by fura-2 fluorescence. Pre-treatment with thapsigargin (2 microM) blocked the vasoconstrictor action of adenosine (10(-7) M) indicating that release of calcium from the sarcoplasmic reticulum (SR), stimulated presumably by IP(3), is involved in the adenosine contraction mechanism of the afferent arteriole. In agreement with this notion is the observation that 2 aminoethoxydiphenyl borate (100 microM) blocked the adenosine-induced constriction whereas the protein kinase C inhibitor calphostin C had no effect. The calcium-activated chloride channel inhibitor IAA-94 (30 microM) inhibited the adenosine-mediated constriction. Patch clamp experiments showed that adenosine treatment induced a depolarizing current in preglomerular smooth muscle cells which was abolished by IAA-94. Furthermore, the vasoconstriction caused by adenosine was significantly inhibited by 5 microM nifedipine (control 8.3 +/- 0.2 microM, ado 3.6 +/- 0.6 microM, ado + nifedipine 6.8 +/- 0.2 microM) suggesting involvement of voltage-dependent calcium channels. We conclude that adenosine mediates vasoconstriction of afferent arterioles through an increase in intracellular calcium concentration resulting from release of calcium from the SR followed by activation of Ca(2+)-activated chloride channels leading to depolarization and influx of calcium through voltage-dependent calcium channels.

Auto-inhibition of rat parallel fibre-Purkinje cell synapses by activity-dependent adenosine release.

Adenosine is an important signalling molecule involved in a large number of physiological functions. In the brain these processes are as diverse as sleep, memory, locomotion and neuroprotection during episodes of ischaemia and hypoxia. Although the actions of adenosine, through cell surface G-protein-coupled receptors, are well characterized, in many cases the sources of adenosine and mechanisms of release have not been defined. Here we demonstrate the activity-dependent release of adenosine in the cerebellum using a combination of electrophysiology and biosensors. Short trains of electrical stimuli delivered to the molecular layer in vitro, release adenosine via a process that is both TTX and Ca2+ sensitive. As ATP release cannot be detected, adenosine must either be released directly or rapidly produced by highly localized and efficient extracellular ATP breakdown. Since adenosine release can be modulated by receptors that act on parallel fibre–Purkinje cell synapses, we suggest that the parallel fibres release adenosine. This activity-dependent adenosine release exerts feedback inhibition of parallel fibre–Purkinje cell transmission. Spike-mediated adenosine release from parallel fibres will thus powerfully regulate cerebellar circuit output.

G protein‐independent neuromodulatory action of adenosine on metabotropic glutamate signalling in mouse cerebellar Purkinje cells

Adenosine receptors (ARs) are G protein-coupled receptors (GPCRs) mediating the neuromodulatory actions of adenosine that influence emotional, cognitive, motor, and other functions in the central nervous system (CNS). Previous studies show complex formation between ARs and metabotropic glutamate receptors (mGluRs) in heterologous systems and close colocalization of ARs and mGluRs in several central neurons. Here we explored the possibility of intimate functional interplay between G(i/o) protein-coupled A(1)-subtype AR (A1R) and type-1 mGluR (mGluR1) naturally occurring in cerebellar Purkinje cells. Using a perforated-patch voltage-clamp technique, we found that both synthetic and endogenous agonists for A1R induced continuous depression of a mGluR1-coupled inward current. A1R agonists also depressed mGluR1-coupled intracellular Ca(2+) mobilization monitored by fluorometry. A1R indeed mediated this depression because genetic depletion of A1R abolished it. Surprisingly, A1R agonist-induced depression persisted after blockade of G(i/o) protein. The depression appeared to involve neither the cAMP-protein kinase A cascade downstream of the alpha subunits of G(i/o) and G(s) proteins, nor cytoplasmic Ca(2+) that is suggested to be regulated by the beta-gamma subunit complex of G(i/o) protein. Moreover, A1R did not appear to affect G(q) protein which mediates the mGluR1-coupled responses. These findings suggest that A1R modulates mGluR1 signalling without the aid of the major G proteins. In this respect, the A1R-mediated depression of mGluR1 signalling shown here is clearly distinguished from the A1R-mediated neuronal responses described so far. These findings demonstrate a novel neuromodulatory action of adenosine in central neurons.

Eliminating the antilipolytic adenosine A1 receptor does not lead to compensatory changes in the antilipolytic actions of PGE2 and nicotinic acid

We examined whether compensatory changes after adenosine A(1) receptor knockout [A(1)R (-/-)] eliminate the antilipolytic actions mediated by this receptor. Lipolysis experiments were performed on adipocytes prepared from the wild type A(1)R (+/+), A(1)R (-/-) and heterozygous mice. Gene expression was assayed with cDNA microarray technique and real-time PCR; protein expression with immunoblotting. The A(1)R was the only adenosine receptor involved in lipolysis. The effects of adenosine deaminase and 2-chloroadenosine were abolished in A(1)R (-/-) mice. The IC(50) value of 2-chloroadenosine doubled from 16.6 to 33.6 nm when half of the A(1)Rs were eliminated. Adrenergic alpha(2) agonists had no effects on lipolysis. Prostaglandin E(2) (PGE(2)) inhibited lipolysis with an IC(50) value of 5.8 nm (4.7-7.2 nm) in the A(1)R (+/+) mice and 10.6 nm (9.0-12.6 nm) in the A(1)R (-/-) mice. Nicotinic acid inhibited lipolysis with an IC(50) value of 0.30 microm (0.19-0.46 microm) in the A(1)R (+/+) mice and 0.24 microm (0.16-0.37 microm) in the A(1)R (-/-) mice. G(i)alpha(1) mRNA was significantly up-regulated in adipose tissue from A(1)R (-/-) mice. However, immunoblotting showed that G(ialpha1) was not up-regulated at the protein level. The A(1)R mediates the antilipolytic actions of adenosine. Deletion of the A(1)R in mice does not result in compensatory increases in G-protein-mediated antilipolytic actions of PGE(2) or nicotinic acid.

Involvement of guanylate cyclase in the cardiovascular response induced by adenosine A 2B receptor stimulation in the posterior hypothalamus of the anesthetized rats

Cardiovascular inhibitory effects induced by the posterior hypothalamic adenosine A 2 receptors were suggested by our previous reports. In this experiment, we examined the influence of the posterior hypothalamic adenosine A 2B receptors on central cardiovascular regulation of blood pressure (BP) and heart rate (HR). Posterior hypothalamic injection of drugs was performed in anesthetized, artificially ventilated male Sprague–Dawley rats. Injection of 5′-( N-cyclopropyl)-carboxamidoadenosine (CPCA; 2 nmol), an adenosine A 2 receptor agonist, showed the decrease of arterial blood pressure and heart rate, and the alloxazine, an adenosine A 2B receptor antagonist, partially blocked the depressor and bradycardiac effects of CPCA (2 nmol). To examine the role of adenosine A 2B receptors among the adenosine A 2 subtypes, we applied the 5′- N-Ethylcarboxamidoadenosine (NECA), an adenosine A 2B receptor agonist, to the posterior hypothalamus. Injection of NECA (1, 4 and 8 nmol) produced a dose-dependent decrease of arterial blood pressure and HR. Pretreatment with alloxazine (5 nmol) partially blocked the depressor and bradycardiac effects of NECA (4 nmol). Also, pretreatment with LY-83,583 (5 nmol), a soluble guanylate cyclase inhibitor, attenuated the depressor and bradycardiac effects of NECA (4 nmol). However, pretreatment with MDL-12,330 (10 nmol), an adenylate cyclase inhibitor, did not affect these effects of NECA (4 nmol). These results suggest that adenosine A 2B receptor in the posterior hypothalamus plays an inhibitory role in central cardiovascular regulation, and that guanylate cyclase mediates the depressor and bradycardiac actions of adenosine A 2B receptors.

Effects of airway anesthesia on dyspnea and ventilatory response to intravenous injection of adenosine in healthy human subjects

We have recently shown that intravenous injection of adenosine causes dyspnea and hyperventilation in man, and we suggested that stimulation of vagal C-fibers in the airways and lungs is involved. To test this hypothesis further, the present study was performed in healthy subjects ( n = 12 ; age 32.4±10.2 yrs, 7 females) to determine if the effect of adenosine could be attenuated by blocking the airway sensory receptors by inhalation of aerosolized lidocaine, a local anesthetic. In each subject, the effects of intravenous injection of adenosine (10 mg) on dyspneic sensation, minute ventilation, airway resistance and heart rate were measured after the subject inhaled lidocaine or placebo aerosol on two separate days. After a latency of ∼20 s, adenosine injection evoked a distinct dyspneic sensation, increase in minute ventilation ( V ˙ E ) , and transient bradycardia followed by tachycardia in all subjects. The increase in V ˙ E resulted primarily from a significant increase in tidal volume. The intensity of adenosine-induced dyspnea was markedly reduced after the lidocaine pretreatment compared to placebo. In a sharp contrast, the V ˙ E and heart rate responses to adenosine were not affected by lidocaine. These results lend further support to our previous studies indicating that the origin of the dyspnogenic action of intravenous adenosine is most likely vagal bronchopulmonary C-fiber sensory nerves.

Caffeine, mood and mental performance in everyday life

Summary Caffeine is the most popular drug in the world. After consumption, it is rapidly distributed throughout the body where it acts by blocking the action of endogenous adenosine at adenosine A1 and A2a receptors, resulting in a variety of physiological effects. Although it is valued as a useful psychostimulant, recent evidence suggests that actually little or no acute benefit is gained from regular caffeine consumption. This is because withdrawal of caffeine (e.g. overnight) lowers alertness and mood and degrades performance, and while consumption of some more caffeine reverses these effects, it does not boost functioning to above ‘normal’ levels. It also tends to increase anxiety, particularly in susceptible individuals. In contrast, caffeine consumption may separately lower the risk of cognitive decline in older age, perhaps owing to effects involving neuroprotective functions of the adenosine system. Currently, however, not enough is known to do a full risk assessment on these or other potential health effects of dietary caffeine.

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

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. The expression of ICAM-1 was examined by flow cytometry. IL-12, IFN-gamma and TNF-alpha were determined by ELISA assay. 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. Adenosine differentially regulates IL-18-induced adhesion molecule expression and cytokine production through several subtypes of its receptors.

Effect of adenosine receptor subtypes stimulation on mixed lymphocyte reaction

The cell-to-cell interaction through binding of intercellular adhesion molecule (ICAM)-1 on monocytes to their ligands lymphocyte function-associated antigen (LFA)-1 on T-cells plays important roles in cytokine production and T-cell proliferation. Interleukin (IL)-18, which plasma levels are elevated in patients during acute rejection following organ transplantation, induces the expression of ICAM-1 on monocytes, production of interferon (IFN)-γ and IL-12 and lymphocyte proliferation during human mixed lymphocyte reaction. Activation of the adenosine A2A receptor on during reperfusion of various tissues has been found to markedly reduce ischemia–reperfusion injury. In the present study, we examined the effect of adenosine at increasing concentrations ranging from 0.1 to 100 μM on the IL-18-enhanced expression of ICAM-1, production of IFN-γ and IL-12 and lymphocyte proliferation during human mixed lymphocyte reaction. Adenosine inhibited the IL-18-initiated immune responses. The IC50 values of adenosine for inhibition of the IL-18-enhanced ICAM-1 expression, IFN-γ production and lymphocyte proliferation were 20 μM, respectively. The actions of adenosine depended on the stimulation of adenosine A2A receptor. An inhibitor of protein kinase A (PKA) at 100 μM inhibited the actions of adenosine, suggesting that PKA might be involved in the actions of adenosine. On the other hand, the stimulation of adenosine A1 and A3 receptor blocked the actions of adenosine A2A receptor stimulation. These results suggest that adenosine inhibits the immune responses during mixed lymphocyte reaction via adenosine A2A receptor.

Increased mechanoreceptor/metaboreceptor stimulation explains the exaggerated exercise pressor reflex seen in heart failure

POINT-COUNTERPOINT COMMENTSIncreased mechanoreceptor/metaboreceptor stimulation explains the exaggerated exercise pressor reflex seen in heart failureCatherine F. Notarius, and John S. FlorasCatherine F. Notarius, and John S. FlorasPublished Online:01 Feb 2007https://doi.org/10.1152/japplphysiol.01330.2006MoreSectionsPDF (27 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInWeChat The following letter is in response to the Point:Counterpoint “Increased mechanoreceptor/metaboreceptor stimulation explains the exaggerated exercise pressor reflex seen in heart failure”.To the Editor: Debate, by its nature, serves to polarize rather than seek truth or consensus (3, 6). The mechanoreceptor reflex contributes importantly to, but cannot fully explain, the exaggerated sympathoneural response to forearm exercise observed in human heart failure, for if it did, muscle sympathetic burst frequency would increase within the first minute of isotonic handgrip, but does not and would not remain elevated during posthandgrip ischemia, yet it does—two observations that depend critically on the severity of heart failure as assessed by peak oxygen uptake, not ejection fraction (4). Middlekauff et al. (2) do not report burst frequency; they opine that arousal may also stimulate the neural response to their passive exercise. The actions of adenosine, a metabolic by-product of muscle contraction known to stimulate type III and IV muscle afferents, can be blocked by caffeine infusion at low dose (4 mg/kg). The hypothesis that such adenosine receptor antagonism affects preferentially the metaboreflex, not the mechanoreflex, of heart failure patients is supported by experiment. Increases in sympathetic burst frequency from rest were still significant within the second minute of handgrip, but caffeine abolished the sympathoneural excitation elicited by posthandgrip ischemia in the control state (5). Thus, in both experimental canine (1) and in human heart failure (2, 4, 5), both metaboreceptors and mechanoreceptors are tonically active, stimulated by ischemic or nonischemic exercise at a lower threshold, and elicit greater increases in muscle sympathetic burst frequency than in healthy controls. Discordance of published findings may reflect differences in the people or experimental models studied.REFERENCES1 Hammond RL, Augustyniak RA, Rossi NF, Churchill PC, Lapanowski K, O'Leary DS. Heart failure alters the strength and mechanisms of the muscle metaboreflex. Am J Physiol Heart Circ Physiol 278: H818–H828, 2000.Link | ISI | Google Scholar2 Middlekauff HR, Chiu J, Hamilton MA, Fonarow GC, MacLellan WR, Hage A, Moriguchi J, Patel J. Muscle mechanoreceptor sensitivity in heart failure. Am J Physiol Heart Circ Physiol 287: H1937–H1943, 2004.Link | ISI | Google Scholar3 Middlekauff HR, Sinoway LI. Point: Increased mechanoreceptor stimulation explains the exaggerated exercise pressor reflex seen in heart failure. J Appl Physiol. In press.Google Scholar4 Notarius CF, Atchison DJ, Floras JS. Impact of heart failure and exercise capacity on sympathetic response to handgrip exercise. Am J Physiol Heart Circ Physiol 280: H969–H976, 2001.Link | ISI | Google Scholar5 Notarius CF, Atchison DJ, Rongen GA, Floras JS. Effect of adenosine receptor blockade with caffeine on sympathetic response to handgrip exercise in heart failure. Am J Physiol Heart Circ Physiol 281: H1312–H1318, 2001.Link | ISI | Google Scholar6 Piepoli M, Coats AJS. Counterpoint: Increased metaboreceptor stimulation explains the exaggerated exercise pressor reflex seen in heart failure. J Appl Physiol. 102: 494–496, 2007.Link | ISI | Google Scholar Download PDF Previous Back to Top Next FiguresReferencesRelatedInformationCited BySympathetic neural responses in heart failure during exercise and after exercise training26 February 2021 | Clinical Science, Vol. 135, No. 4Muscle sympathetic activity in resting and exercising humans with and without heart failureApplied Physiology, Nutrition, and Metabolism, Vol. 40, No. 11Divergent muscle sympathetic responses to dynamic leg exercise in heart failure and age-matched healthy subjects15 December 2014 | The Journal of Physiology, Vol. 593, No. 3Increased metaboreflex activity is related to exercise intolerance in heart transplant patientsAnne Houssiere, Marko Gujic, Gael Deboeck, Agnieszka Ciarka, Robert Naeije, and Philippe van de Borne1 December 2007 | American Journal of Physiology-Heart and Circulatory Physiology, Vol. 293, No. 6 More from this issue > Volume 102Issue 2February 2007Pages 824-824 Copyright & PermissionsCopyright © 2007 the American Physiological Societyhttps://doi.org/10.1152/japplphysiol.01330.2006PubMed17284659History Published online 1 February 2007 Published in print 1 February 2007 Metrics

Intracellularly transported adenosine induces apoptosis in HuH-7 human hepatoma cells by downregulating c-FLIP expression causing caspase-3/-8 activation

Extracellular adenosine induced apoptosis of HuH-7 cells, a Fas-deficient human hepatoma cell line. The adenosine action was inhibited by dipyridamole, an adenosine transporter inhibitor, or 5′-amino-5′-deoxyadenosine, an inhibitor of adenosine kinase to convert from adenosine to AMP, but it was not affected by inhibitors for adenosine A 1, A 2a, A 2b, and A 3 adenosine receptors. Adenosine activated caspase-3 and -8, but not caspase-9, in HuH-7 cells, and the activation was abolished by dipyridamole. In the real-time RT-PCR and Western blot analysis, extracellular adenosine downregulated mRNA and protein levels for c-FLIP, and the effect was suppressed by dipyridamole. Furthermore, overexpression of c-FLIP short in HuH-7 cells inhibited adenosine-induced caspase-8 activity. Taken together, these results suggest that intracellularly transported adenosine, perhaps converted AMP as the ensuing event, activates caspase-8 and the downstream effector caspase caspase-3 by neutralizing caspase-8 inhibition due to c-FLIP as a consequence of decreased c-FLIP expression, leading to apoptosis. This extends our understanding of adenosine-induced molecular apoptotic pathways.

Adenosine A2A-Receptor Stimulation Inhibits Lipopolysaccharide-Induced Interleukin-18 Production in Monocytes

Adenosine inhibited interleukin (IL)-18 production in lipopolysaccharide (LPS)-stimulated monocytes. The action of adenosine was antagonized by an adenosine A2A-receptor (A2AR) antagonist and was mimicked by an A2AR agonist, suggesting that the stimulation of A2AR may be involved in the actions of adenosine. On the other hand, the stimulation of A1R and A3R inhibited the actions of A2AR stimulation, whereas the stimulation of A2BR had no effect on them. Activation of A2AR is known to increase cyclic adenosine monophosphate (cAMP) levels and to activate protein kinase A (PKA). A PKA inhibitor prevented the actions of A2AR stimulation, indicating that the action mechanism of A2AR stimulation may be via the activation of the cAMP/PKA pathway.

Effect of Adenosine on the Growth of Human T-Lymphocyte Leukemia Cell Line MOLT-4

Adenosine has been observed to suppress the growth of MOLT-4 human leukemia cells in vitro. Changes in the cell cycle, especially increased percentage of cells in S phase, prolonged generation time, and induction of apoptosis at higher adenosine concentrations have been found to be responsible for the growth suppression. Dipyridamole, a drug inhibiting the cellular uptake of adenosine, reversed partially but significantly the adenosine-induced growth suppression. It follows from these results that the action of adenosine on the MOLT-4 cells comprises its cellular uptake and intracellular operation. These findings present new data on anticancer efficacy of adenosine.

Intracellular mediators in regulation of leptin secretion from adipocytes

Leptin is a hormone primarily secreted by adipocytes and participating in the regulation of food intake and energy expenditure. Its blood levels usually correlate with adiposity. The secretion of this hormone is affected, among others, by food consumption, insulin, fasting and cold exposure. Regulation of leptin secretion depends on many intracellular events. It is known that the activation of mTOR (the mammalian target of rapamycin) as well as increase in ATP and malonyl-CoA content in adipocytes enhance secretion of leptin. The rise in intracellular cAMP and fatty acids is thought to evoke the opposite effect. Moreover, the undisturbed action of endogenous adenosine in adipocytes and the proper intracellular Ca(2+) concentration in these cells were also found to have an important function in leptin release. The role of mTOR, ATP, cAMP, fatty acids, malonyl-CoA, adenosine and Ca(2+) in the regulation of leptin secretion from adipocytes is discussed.

A Novel Skeletal Muscle Cytoprotective Action of Adenosine A3 Receptors

Adenosine (Ado) can protect skeletal muscle from ischemic injury, in addition to its cardio- and vasculo-protective effect. However, identity of the Ado receptor and the mechanism mediating skeletal muscle protection is unknown. Using a quantitative mouse hindlimb ischemia-reperfusion injury model, the objective here was to assess the protective role of Ado receptor subtypes. Ado A3 receptor-selective agonist Cl-IBMECA reduced skeletal muscle injury, an effect that was blocked by the A3 receptor antagonist MRS1191 (% injury, vehicle: 28±5.9% vs. Cl-IBMECA: 5.4±3.8%, Cl-IBMECA & MRS1191: 21.5±3.5%, mean±SE, 7–22 mice, ANOVA & comparisons, Cl-IBMECA vs. all others, P<0.05). The A1 receptor agonist CCPA was also protective and and was selectively antagonized by the A1 receptor blocker DPCPX (CCPA: 10.8±1.8%, DPCPX &CCPA: 22±3.3%, 7–12 mice, P<0.05). The protection induced by Cl-IBMECA and another A3 receptor agonist MRS3558 was completely abrogated in phospholipase C β2/3-null mice while that caused by CCPA remained unaffected in these animals (8–10 mice). The study demonstrated a novel potent protective effect of A3 receptors in skeletal muscle and implicates phospholipase C β in mediating this protection. The data add to the cyto-protective actions of adenosine and suggest that the A3 receptor is a novel therapeutic target in ameliorating skeletal muscle injury.