Effects of adenosine on the functions of circulating polymorphonuclear leukocytes during hyperdynamic endotoxemia

Adenosine and adenosine receptors in the pathogenesis and treatment of rheumatic diseases

Adenosine, a nucleoside derived primarily from the extracellular hydrolysis of adenine nucleotides, is a potent regulator of inflammation. Adenosine mediates its effects on inflammatory cells by engaging one or more cell-surface receptors. The expression and function of adenosine receptors on different cell types change during the course of rheumatic diseases, such as rheumatoid arthritis (RA). Targeting adenosine receptors directly for the treatment of rheumatic diseases is currently under study; however, indirect targeting of adenosine receptors by enhancing adenosine levels at inflamed sites accounts for most of the anti-inflammatory effects of methotrexate, the anchor drug for the treatment of RA. In this Review, we discuss the regulation of extracellular adenosine levels and the role of adenosine in regulating the inflammatory and immune responses in rheumatic diseases such as RA, psoriasis and other types of inflammatory arthritis. In addition, adenosine and its receptors are involved in promoting fibrous matrix production in the skin and other organs, and the role of adenosine in fibrosis and fibrosing diseases is also discussed.

Functional changes in neutrophils and psychoneuroendocrine responses during 105 days of confinement

The innate immune system as one key element of immunity and a prerequisite for an adequate host defense is of emerging interest in space research to ensure crew health and thus mission success. In ground-based studies, spaceflight-associated specifics such as confinement caused altered immune functions paralleled by changes in stress hormone levels. In this study, six men were confined for 105 days to a space module of ∼500 m3mimicking conditions of a long-term space mission. Psychic stress was surveyed by different questionnaires. Blood, saliva, and urine samples were taken before, during, and after confinement to determine quantitative and qualitative immune responses by analyzing enumerative assays and quantifying microbicide and phagocytic functions. Additionally, expression and shedding of L-selectin (CD62L) on granulocytes and different plasma cytokine levels were measured. Cortisol and catecholamine levels were analyzed in saliva and urine. Psychic stress or an activation of the psychoneuroendocrine system could not be testified. White blood cell counts were not significantly altered, but innate immune functions showed increased cytotoxic and reduced microbicide capabilities. Furthermore, a significantly enhanced shedding of CD62L might be a hint at increased migratory capabilities. However, this was observed in the absence of any acute inflammatory state, and no rise in plasma cytokine levels was detected. In summary, confinement for 105 days caused changes in innate immune functions. Whether these changes result from an alert immune state in preparation for further immune challenges or from a normal adaptive process during confinement remains to be clarified in future research.

In vivo hypoxic preconditioning protects from warm liver ischemia-reperfusion injury through the adenosine A2B receptor

BACKGROUND

Liver ischemia-reperfusion injury (IRI) is a known risk factor for the postoperative outcome of patients undergoing liver surgery/transplantation. Attempts to protect from organ damage require multidisciplinary strategies and are of emerging interest in view of patients with higher age and American Society of Anesthesiology status. Ischemic preconditioning has been successfully applied to prevent from IRI during liver resection/transplantation. Because even short periods of ischemia during preconditioning inevitably lead to hypoxia and formation of anti-inflammatory/cytoprotective acting adenosine, we reasoned that short nonischemic hypoxia also protects against hepatic IRI.

METHODS

Mice underwent hypoxic preconditioning (HPC) by breathing 10% oxygen for 10 min followed by 10 min of 21% oxygen before left liver lobe ischemia (45 min) and reperfusion (4 hr). The interactions of hypoxia→adenosine→adenosine receptors were tested by pharmacologic antagonism at adenosine receptor (AR) sites in wild-type mice and in mice with genetic deletions at the A1, A2A, A2B, and A3 ARs. Hepatocellular damage, inflammation, and metabolic effects were quantified by enzyme activities, cytokines, liver myeloperoxidase, blood adenosine, and tissue AMP, respectively.

RESULTS

Hepatoprotection by HPC was significant in wild-type and A1, A2A, and A3 AR knockout mice as quantified by lower alanine aminotransferase serum activities, cytokine levels, histologic damage scores, tissue myeloperoxidase concentrations, and preserved AMP concentrations. Protection by HPC was blunted in mice pretreated with the A2B AR antagonist MRS1754 or in A2B AR knockout mice.

CONCLUSIONS

Because liver protective effects of HPC are negated when the A2B receptor is nonfunctional, the hypoxia→adenosine→A2B receptor pathway plays a critical role in the prevention of warm IRI in vivo. Hypoxic activation of this pathway warrants use of selective A2B AR agonists or even intermittent hypoxia (e.g., in deceased organ donors) to protect from liver IRI.

Adenosine injection prior to cardioplegia enhances preservation of senescent hearts in rat heterotopic heart transplantation

OBJECTIVES

Advanced donor age is one of the risk factors for graft failure and is the leading cause of early death after heart transplantation. Better myocardial preservation methods should reduce graft failure. The purpose of this study was to determine if adenosine, which is known to enhance cardioplegic protection, enhances myocardial preservation during heart transplantation using older donors.

METHODS

We used a rat heterotopic heart transplantation model with Lewis rats that were at least 60 weeks old as donors. We injected saline (control) or adenosine (0.1 or 0.2 mg/kg) before cardioplegia, perfused with cold Celsior and stored the hearts in Celsior for 6 h at 4°C. The grafts were transplanted into syngenic, 12-16-week old recipients, and blood and tissue were collected 3 h after reperfusion.

RESULTS

Bolus injection of adenosine led to faster mechanical arrest after perfusion with Celsior and faster reanimation after reperfusion compared with controls. Adenosine treatment significantly reduced myocardial injury, as indicated by serum troponin I and creatine phosphokinase levels. The mRNAs for inflammatory cytokines were markedly increased in the control grafts, but were less upregulated in the grafts treated with adenosine. The grafts treated with adenosine also exhibited less mitochondrial damage, fewer infiltrating cells and a higher adenosine triphosphate content.

CONCLUSIONS

Adenosine injection prior to perfusion of cardioplegia significantly reduced cold ischaemia/reperfusion injury in cardiac grafts from older donors and improved the stores of cellular energy after reperfusion. This procurement protocol may be clinically feasible and should be considered in the clinical setting, particularly for older donors.

Regulation of neutrophil function by adenosine

Adenosine is an endogenously released purine nucleoside that signals via 4 widely expressed G protein-coupled receptors: A(1), A(2A), A(2B), and A(3). In the setting of inflammation, the generation and release of adenosine is greatly enhanced. Neutrophils play an important role in host defense against invading pathogens and are the cellular hallmark of acute inflammation. Neutrophils both release adenosine and can respond to it via expression of all 4 adenosine receptor subtypes. At low concentrations, adenosine can act via the A(1) and A(3) adenosine receptor subtypes to promote neutrophil chemotaxis and phagocytosis. At higher concentrations, adenosine acts at the lower-affinity A(2A) and A(2B) receptors to inhibit neutrophil trafficking and effector functions such as oxidative burst, inflammatory mediator production, and granule release. Modulation of neutrophil function by adenosine is relevant in a broad array of disease models, including ischemia reperfusion injury, sepsis, and noninfectious acute lung injury. This review will summarize relevant research in order to provide a framework for understanding how adenosine directly regulates various elements of neutrophil function.

Fructose protects murine hepatocytes from tumor necrosis factor-induced apoptosis by modulating JNK signaling

Fructose-induced hepatic ATP depletion prevents TNF-induced apoptosis, whereas it contrarily enhances CD95-induced hepatocyte apoptosis in vitro and in vivo. By contrast, transformed liver cells are not protected against TNF due to metabolic alterations, allowing selective tumor targeting. We analyzed the molecular mechanisms by which fructose modulates cytokine-induced apoptosis. A release of adenosine after fructose-induced ATP depletion, followed by a cAMP response, was demonstrated. Likewise, cAMP and adenosine mimicked per se the modulation by fructose of CD95- and TNF-induced apoptosis. The effects of fructose on cytokine-induced apoptosis were sensitive to inhibition of protein kinase A. Fructose prevented the pro-apoptotic, sustained phase of TNF-induced JNK signaling and thereby blocked bid-mediated activation of the intrinsic mitochondrial apoptosis pathway in a PKA-dependent manner. We explain the dichotomal effects of fructose on CD95- and TNF-induced cell death by the selective requirement of JNK signaling for the latter. These findings provide a mechanistic rationale for the protection of hepatocytes from TNF-induced cell death by pharmacological doses of fructose.

Adenosine and adenosine receptors: Newer therapeutic perspective

Adenosine, a purine nucleoside has been described as a ‘retaliatory metabolite’ by virtue of its ability to function in an autocrine manner and to modify the activity of a range of cell types, following its extracellular accumulation during cell stress or injury. These effects are largely protective and are triggered by binding of adenosine to any of the four adenosine receptor subtypes namely A1, A2a, A2b, A3, which have been cloned in humans, and are expressed in most of the organs. Each is encoded by a separate gene and has different functions, although overlapping. For instance, both A1 and A2a receptors play a role in regulating myocardial oxygen consumption and coronary blood flow. It is a proven fact that adenosine plays pivotal role in different physiological functions, such as induction of sleep, neuroprotection and protection against oxidative stress. Until now adenosine was used for certain conditions like paroxysmal supraventricular tachycardia (PSVT) and Wolff Parkinson White (WPW) syndrome. Now there is a growing evidence that adenosine receptors could be promising therapeutic targets in a wide range of conditions including cardiac, pulmonary, immunological and inflammatory disorders. After more than three decades of research in medicinal chemistry, a number of selective agonists and antagonists of adenosine receptors have been discovered and some have been clinically evaluated, although none has yet received regulatory approval. So this review focuses mainly on the newer potential role of adenosine and its receptors in different clinical conditions.

Adenosine infusion attenuates soluble RAGE in endotoxin-induced inflammation in human volunteers

AIM

To evaluate possible anti-inflammatory effects of pre-treatment with adenosine in a human experimental inflammatory model.

METHODS

The study design was double-blind, crossover, placebo-controlled and randomized. In the Intensive Care Unit of a university hospital, 16 healthy male volunteers were treated for 5.5 h with infusions of adenosine 40 microg kg(-1) min(-1) or placebo. Thirty minutes after the start of adenosine or placebo, 2 ng kg(-1)E-Coli endotoxin was administered. Heart rate, body temperature, blood pressure, plasma cytokines (TNF-alpha, IL-6 and IL-10), soluble RAGE and resistin, exhaled nitric oxide and nitrite/nitrate in urine were determined.

RESULTS

Endotoxin elicited the expected clinical signs of an inflammatory reaction (tachycardia, fever) and led to prominent release of the cytokines studied (P < 0.001). Resistin in plasma increased after endotoxin (P < 0.001). After placebo treatment, soluble RAGE (sRAGE) in plasma increased 5 h after the endotoxin challenge (P < 0.001) but not after adenosine. After placebo, orally exhaled NO increased with a peak at 4 h (P < 0.001), although there was no statistically significant difference between the two treatments. Nitrite/nitrate in urine (n = 11) did not differ between adenosine and placebo treatments.

CONCLUSION

In conclusion, adenosine infusion starting before endotoxin challenge in humans attenuated sRAGE significantly but otherwise had no clear anti-inflammatory effect. Adenosine as a potential anti-inflammatory treatment in humans needs further study, including use of higher doses. The mechanism underlying the effect of adenosines on sRAGE remains unknown.

Immature circulating neutrophils in sepsis have impaired phagocytosis and calcium signaling

Patients with sepsis commonly develop leukocytosis, which is presumed to reflect a host response to infection. Effective phagocytosis by neutrophils is crucial in the clearance of invading microbes. However, efficacy of phagocytosis in sepsis is controversial. We hypothesized that host phagocytic capacity in sepsis can be affected by immature neutrophils that are released into the circulation. Circulating neutrophils were evaluated in 16 patients with severe sepsis and 5 healthy donors. Immature neutrophils were identified by the cell morphology. Phagocytosis was evaluated by micromanipulation technique and simultaneous cytosolic-free Ca2+ imaging. Leukocytosis was present in 12 of 16 patients. Nine of the 12 patients with leukocytosis and 3 of 4 patients with normal white blood cell counts had increased circulating immature neutrophils (mean, 39.3% +/- 20.7%; normal <or=5%). Quantification of the phagocytic activity revealed a significantly reduced phagocytic index of immature neutrophils as compared with mature neutrophils from both sepsis patients and healthy donors (25% +/- 5% vs. 69% +/- 8% and 42% +/- 6%; P < 0.05). As compared with mature neutrophils, the number of internalized zymosan particles within immature neutrophils was also significantly lower. Mature neutrophils from patients and healthy donors displayed a single rapid transient Ca signal during phagocytosis in contrast with weak signals from immature neutrophils. Our preliminary results show that phagocytic capacity of immature neutrophils is lower as compared with mature neutrophils. An increase in immature neutrophils in severe sepsis may undermine the overall phagocytic efficacy of a host despite observed leukocytosis.

The adenosine deaminase inhibitor erythro-9-[2-hydroxyl-3-nonyl]-adenine decreases intestinal permeability and protects against experimental sepsis: a prospective, randomised laboratory investigation

INTRODUCTION

The treatment of septic conditions in critically ill patients is still one of medicine's major challenges. Cyclic nucleotides, adenosine and its receptors play a pivotal role in the regulation of inflammatory responses and in limiting inflammatory tissue destruction. The aim of this study was to verify the hypothesis that adenosine deaminase-1 and cyclic guanosine monophosphate-stimulated phosphodiesterase inhibition by erythro-9-[2-hydroxyl-3-nonyl]-adenine could be beneficial in experimental endotoxicosis/sepsis.

METHOD

We used two established animal models for endotoxicosis and sepsis. Twenty-four male Wistar rats that had been given intravenous endotoxin (Escherichia coli lipopolysaccharide) were treated with either erythro-9-[2-hydroxyl-3-nonyl]-adenine infusion or 0.9% saline during a study length of 120 minutes. Sepsis in 84 female C57BL/6 mice was induced by caecal ligation and puncture. Animals were treated with repeated erythro-9-[2-hydroxyl-3-nonyl]-adenine injections after 0, 12 and 24 hours or 4, 12 and 24 hours for delayed treatment.

RESULTS

In endotoxaemic rats, intestinal production of hypoxanthine increased from 9.8 +/- 90.2 micromol/l at baseline to 411.4 +/- 124.6 micromol/l and uric acid formation increased from 1.5 +/- 2.3 mmol/l to 13.1 +/- 2.7 mmol/l after 120 minutes. In endotoxaemic animals treated with erythro-9-[2-hydroxyl-3-nonyl]-adenine, we found no elevation of adenosine metabolites. The lactulose/L-rhamnose ratio (14.3 versus 4.2 in control animals; p = 2.5 x 10(-7)) reflects a highly permeable small intestine and through the application of erythro-9-[2-hydroxyl-3-nonyl]-adenine, intestinal permeability could be re-established. The lipopolysaccharide animals had decreased L-rhamnose/3-O-methyl-D-glucose urine excretion ratios. Erythro-9-[2-hydroxyl-3-nonyl]-adenine reduced this effect. The mucosa damage score of the septic animals was higher compared with control and therapy animals (p < 0.05). Septic shock induction by caecal ligation and puncture resulted in a 160-hour survival rate of about 25%. In contrast, direct adenosine deaminase-1 inhibition resulted in a survival rate of about 75% (p = 0.0018). A protective effect was still present when erythro-9-[2-hydroxyl-3-nonyl]-adenine treatment was delayed for four hours (55%, p = 0.029).

CONCLUSIONS

We present further evidence of the beneficial effects achieved by administering erythro-9-[2-hydroxyl-3-nonyl]-adenine, an adenosine deaminase-1 and cyclic guanosine monophosphate-stimulated phosphodiesterase inhibitor, in an endotoxicosis and sepsis animal model. This suggests a potential therapeutic option in the treatment of septic conditions.

Critical role of hypoxia and A2A adenosine receptors in liver tissue-protecting physiological anti-inflammatory pathway

Whole body exposure of wild type control littermates and A2A adenosine receptor (A2AR) gene deleted mice to low oxygen containing inspired gas mixture allowed the investigation of the mechanism that controls inflammatory liver damage and protects the liver using a mouse model of T cell-mediated viral and autoimmune hepatitis. We tested the hypothesis that the inflammatory tissue damage-associated hypoxia and extracellular adenosine --> A2AR signaling plays an important role in the physiological anti-inflammatory mechanism that limits liver damage during fulminant hepatitis. After induction of T cell-mediated hepatitis, mice were kept in modular chambers either under normoxic (21% oxygen) or hypoxic (10% oxygen) conditions for 8 h. It was shown that the whole body exposure to hypoxic atmosphere caused tissue hypoxia in healthy animals as evidenced by a decrease in the arterial blood oxygen tension and increase of the plasma adenosine concentration (P < 0.05). This "hypoxic" treatment resulted in significantly reduced hepatocellular damage and attenuated levels of serum cytokines in mice with acute liver inflammation. The anti-inflammatory effects of hypoxia were not observed in the absence of A2AR in studies of A2AR gene-deficient mice or when A2AR have been pharmacologically antagonized with synthetic antagonist. The presented data demonstrate that total body hypoxia-triggered pathway provides protection in acute hepatitis and that hypoxia (upstream) and A2AR (downstream) function in the same immunosuppressive and liver tissue-protecting pathway.

EFFECTS OF ADENOSINE ON FUNCTIONS OF POLYMORPHONUCLEAR LEUKOCYTES FROM PATIENTS WITH SEPTIC SHOCK

Inasmuch as polymorphonuclear leukocytes (PMNs) play a major role in antibacterial defense but can also cause substantial tissue injury, drugs are needed which are able to attenuate tissue-toxic PMN reactions without inhibiting bactericidal mechanisms. Adenosine as a retaliatory metabolite is produced in response to metabolically unfavorable conditions like inflammation. However, it is not known whether adenosine can selectively downregulate adverse PMN reactions in sepsis. In this prospective clinical study, we characterized the effects of adenosine ex vivo on PMN functions in patients with septic shock ([SS] n = 33) and healthy volunteers ([HV] n = 33). The PMNs were primed by tumor necrosis factor-alpha (TNF-alpha) and subsequently stimulated with N-formyl methionyl-leucyl-phenylalanine (fMLP) to test for the formation of hydrogen peroxide (H2O2) in response to soluble inflammatory stimuli. The PMNs were also challenged by opsonized zymosan particles to assess adhesion, phagocytosis, and the associated H2O2 production. As compared with HV, PMNs from SS patients showed strongly enhanced tissue-toxic H2O2 production elicited by TNF-alpha/fMLP. Increasing concentrations of adenosine dose-dependently reduced this tissue-toxic H2O2 production in both groups with a half-maximal inhibitory concentration of 25 nmol/L and 114 nmol/L in HV and SS patients, respectively. This 4.6-fold decrease in the adenosine-mediated inhibition of PMNs from patients with septic shock was compensated by a 3-fold increase in the plasma concentrations of the nucleoside (HV, 42.5 +/- 2.9 nmol/L vs. SS, 125.6 +/- 18.2 nmol/L; mean +/- SEM). When the effects of adenosine were tested at a very high A2A receptor saturating concentration of 10 mol/L, neither adhesion, phagocytosis, nor the associated H2O2 production induced by opsonized zymosan was affected in both groups. These results were confirmed by the highly selective A2A agonist, CGS21680.Thus, adenosine or A2A agonists may be useful to selectively inhibit the potentially tissue-toxic H2O2 production elicited by soluble inflammatory mediators in patients with septic shock.

POLYMORPHONUCLEAR LEUKOCYTE DYSFUNCTION SYNDROME IN PATIENTS WITH INCREASING SEPSIS SEVERITY

Production of oxygen radicals is required for both microbicidal and tissue-toxic effector functions of granulocytes. Inasmuch as an ambivalent role of polymorphonuclear leukocytes (PMNs) may become apparent during sepsis, we studied levels of hydrogen peroxide (H2O2) production by PMNs depending upon the nature of different particulate and soluble stimuli in patients with increasing sepsis severity. Patients with sepsis (n = 15), severe sepsis (n = 12), or septic shock (n = 33) were prospectively enrolled in the study. Healthy volunteers of comparable age and sex served as controls (n = 50). Unopsonized and opsonized zymosan particles were used to assess adhesion, phagocytosis, and the associated H2O2 production. Zymosan particles are rich in beta-glucans and lectin structures that are known to trigger H2O2 production via two major non-toll-like receptor pathogen recognition receptors, comprising the lectin-binding site in the alpha-chain (CD11b) of the complement receptor type 3 and the more recently identified nonclassical C-type lectin, dectin-1. To determine H2O2 production upon cell activation by soluble stimuli, PMNs were activated by the chemotactic tripeptide (N-formyl-methionyl-leucyl-phenylalanine [fMLP]) alone or after priming of cells by preincubation with tumor necrosis factor alpha. To get insight into the changes of fMLP receptor classical intracellular signaling pathways, PMNs were also incubated with the calcium ionophore A23187 and the phorbol ester phorbol myristate acetate, bypassing receptor-dependent signal transduction to directly activate calcium/calmodulin kinase- and protein kinase C-dependent pathways, respectively. As compared with healthy volunteers, levels of H2O2 production by PMNs from septic patients varied depending upon the nature of the activating signal: reduced (zymosan), unchanged (phorbol myristate acetate, opsonized zymosan), and enhanced (spontaneous, fMLP, fMLP + tumor necrosis factor alpha, A23187), with the changes most pronounced in patients with septic shock. Specifically, phagocytosis of zymosan and the associated H2O2 production were significantly decreased whereas spontaneous and stimulated H2O2 production elicited by soluble stimuli strongly increased. Thus, these findings suggest the development of a PMN dysfunction syndrome in patients with increasing sepsis severity. Moreover, as binding of zymosan particles to the PMNs' surface remained unchanged despite increasingly suppressed phagocytosis and associated H2O2 production, observed effects are likely to reflect defects in signaling by the lectin-binding site of CD11b and/or the beta-glucan receptor dectin-1, respectively.

Adenosine 5'-triphosphate and adenosine as endogenous signaling molecules in immunity and inflammation

Human health is under constant threat of a wide variety of dangers, both self and nonself. The immune system is occupied with protecting the host against such dangers in order to preserve human health. For that purpose, the immune system is equipped with a diverse array of both cellular and non-cellular effectors that are in continuous communication with each other. The naturally occurring nucleotide adenosine 5'-triphosphate (ATP) and its metabolite adenosine (Ado) probably constitute an intrinsic part of this extensive immunological network through purinergic signaling by their cognate receptors, which are widely expressed throughout the body. This review provides a thorough overview of the effects of ATP and Ado on major immune cell types. The overwhelming evidence indicates that ATP and Ado are important endogenous signaling molecules in immunity and inflammation. Although the role of ATP and Ado during the course of inflammatory and immune responses in vivo appears to be extremely complex, we propose that their immunological role is both interdependent and multifaceted, meaning that the nature of their effects may shift from immunostimulatory to immunoregulatory or vice versa depending on extracellular concentrations as well as on expression patterns of purinergic receptors and ecto-enzymes. Purinergic signaling thus contributes to the fine-tuning of inflammatory and immune responses in such a way that the danger to the host is eliminated efficiently with minimal damage to healthy tissues.

Oxygent as a top load to colloid and hyperoxia is more effective in resuscitation from hemorrhagic shock than colloid and hyperoxia alone

Perfluorocarbon (PFC) emulsions are intravascular oxygen therapeutics that temporarily enhance tissue oxygenation in dilutional anemia. However, PFC emulsions are not resuscitation fluids because PFCs only work optimally in the presence of high O2 partial pressure (hyperoxia); moreover, because they have no oncotic potential, dosing limitations prevent their use to permanently replace large hemorrhage volumes. Our objective was to clarify whether in the presence of hyperoxia a conventional colloid therapy supplemented by PFC is more efficacious than colloid alone. To answer this question, 22 anesthetized, ventilated dogs were hemorrhaged to a mean arterial pressure of 45 mmHg and were kept at this level until a metabolic O2 debt of 120 mL kg(-1) body weight had evolved. Hyperoxia was established and dogs were randomly allocated to receive colloid (6% HES, Hydroxy Ethyl Starch shed blood volume) or colloid together with Oxygent (perflubron emulsion, 60%, w/v; Alliance Pharmaceutical Corp., San Diego, CA; single dose, 4.5 mL kg(-1); i.e., 2.7 g PFC kg body weight) in a blinded fashion. Hemodynamic and O2 transport parameters, intestinal mucosal blood flow (microspheres), and O2 partial pressure (MDO-Electrode; Eschweiler, Kiel, Germany) were measured at baseline, in shock, and during 3 h post-therapy. In the presence of hyperoxia, Oxygent improved the amount of physically dissolved O2 in plasma and increased the contribution of physically dissolved O2 to global O2 delivery (P < 0.05) and thus whole body O2 consumption when compared with colloid alone (P < 0.05). As a result, Oxygent reduced intestinal mucosal hypoxia and global O2 debt within the first hour post-therapy (P < 0.05). We conclude that under hyperoxic conditions, fluid resuscitation supplemented by Oxygent was more efficacious than colloid and hyperoxia alone. PFC temporarily enhanced intestinal mucosal tissue oxygenation during resuscitation.

Oxygenation inhibits the physiological tissue-protecting mechanism and thereby exacerbates acute inflammatory lung injury

Acute respiratory distress syndrome (ARDS) usually requires symptomatic supportive therapy by intubation and mechanical ventilation with the supplemental use of high oxygen concentrations. Although oxygen therapy represents a life-saving measure, the recent discovery of a critical tissue-protecting mechanism predicts that administration of oxygen to ARDS patients with uncontrolled pulmonary inflammation also may have dangerous side effects. Oxygenation may weaken the local tissue hypoxia-driven and adenosine A2A receptor (A2AR)-mediated anti-inflammatory mechanism and thereby further exacerbate lung injury. Here we report experiments with wild-type and adenosine A2AR-deficient mice that confirm the predicted effects of oxygen. These results also suggest the possibility of iatrogenic exacerbation of acute lung injury upon oxygen administration due to the oxygenation-associated elimination of A2AR-mediated lung tissue-protecting pathway. We show that this potential complication of clinically widely used oxygenation procedures could be completely prevented by intratracheal injection of a selective A2AR agonist to compensate for the oxygenation-related loss of the lung tissue-protecting endogenous adenosine. The identification of a major iatrogenic complication of oxygen therapy in conditions of acute lung inflammation attracts attention to the need for clinical and epidemiological studies of ARDS patients who require oxygen therapy. It is proposed that oxygen therapy in patients with ARDS and other causes of lung inflammation should be combined with anti-inflammatory measures, e.g., with inhalative application of A2AR agonists. The reported observations may also answer the long-standing question as to why the lungs are the most susceptible to inflammatory injury and why lung failure usually precedes multiple organ failure.

PhysiologicalControl ofImmuneResponse andInflammatoryTissueDamage byHypoxia-InducibleFactors andAdenosineA2AReceptors

Immune cell-mediated destruction of pathogens may result in excessive collateral damage to normal tissues, and the failure to control activated immune cells may cause immunopathologies. The search for physiological mechanisms that downregulate activated immune cells has revealed a critical role for extracellular adenosine and for immunosuppressive A2A adenosine receptors in protecting tissue from inflammatory damage. Tissue damage-associated deep hypoxia, hypoxia-inducible factors, and hypoxia-induced accumulation of adenosine may represent one of the most fundamental and immediate tissue-protecting mechanisms, with adenosine A2A receptors triggering "OFF" signals in activated immune cells. In these regulatory mechanisms, oxygen deprivation and extracellular adenosine accumulation serve as "reporters," while A2A adenosine receptors serve as "sensors" of excessive tissue damage. The A2A receptor-triggered generation of intracellular cAMP then inhibits activated immune cells in a delayed negative feedback manner to prevent additional tissue damage. Targeting A2A adenosine receptors may have important clinical applications.

Role of adenosine in immunomodulation: Review of the literature

OBJECTIVE

Advances in the understanding of sepsis have failed to deliver satisfactory new treatments aimed at attenuating inflammatory-mediated organ dysfunction. Phagocytic cells play a pivotal role in driving the inflammatory response and causing direct tissue injury. Adenoreceptor stimulation may attenuate such inflammatory-mediated damage by down-regulating phagocytic activity and preventing excessive respiratory burst activation.

DATA

A Medline database was used to perform a literature search for all articles relating to the use of adenosine as an immunomodulatory agent.

CONCLUSION

There is convincing evidence to suggest that adenoreceptor modulation can prevent tissue injury through a variety of pathways. The use of adenosine modulation in ischemia/reperfusion injury has been the subject of considerable investigation, although experience with its use in sepsis is limited.

Attenuation by intravenous 2-chloroadenosine of acute lung injury induced by live escherichia coli or latex particles added to endotoxin in the neutropenic state

Although neutrophil depletion can reduce the level of acute lung injury (ALI) induced by Escherichia coli endotoxin, that induced by live E coli cannot be attenuated even in neutropenia. This suggests that live E coli cause ALI by way of an mechanism independent of circulating neutrophil. Tumor necrosis factor-alpha (TNF-alpha), which is released from monocytes and macrophages, is a proinflammatory cytokine that is recognized as a central mediator of several forms of inflammation. In this controlled experimental study, we examined the effects of an adenosine-receptor agonist, 2-chloroadenosine (2CA), that has suppressive effects on various cell types and TNF-alpha, on endotoxin plus latex particles, and on ALI induced by live E coli in the neutropenic state. We studied 42 guinea pigs rendered neutropenic by means of intraperitoneal cyclophosphamide administration. Experimental groups consisted of (1) a saline-solution control group; (2) an endotoxin (0.2 mg/kg)-treated group; (3) a group treated with endotoxin plus 2CA (10 micro g/kg); (4) a group treated with latex (2 x 10(9)/kg); (5) a group exposed to endotoxin and latex; (6) a group exposed to endotoxin, latex, and 2CA; (7) a group exposed to E coli (2 x 10(9)/kg); and (8) a group exposed to E coli and 2CA. The injection of endotoxin alone in neutropenic animals did not increase the indexes of ALI (lung tissue/plasma ratio [T/P] and lung wet weight/dry weight ratio [W/D], calculated with the use of iodine 125-labeled albumin). In contrast, these indexes were increased in the endotoxin-and-latex groups compared with those of the control group. ALI in the endotoxin-and-latex group was attenuated by intravenous 2CA. The intravenous injection of live E coli also caused increases in T/P, W/D, and plasma TNF-alpha, but thse were limited by 2CA. In summary, ALI induced by latex particles added to endotoxin and live E coli in the neutropenic state was attenuated by 2CA, suggesting a partial contribution of various cell types or humoral mediators as a neutrophil-independent pathway in its pathogenesis.

Adenosine treatment attenuates cytokine interleukin-6 responses to endotoxin challenge in healthy volunteers

Anti-inflammatory effects of adenosine were evaluated in two randomized, double-blind, placebo-controlled studies. In one study healthy male volunteers received no endotoxin (adenosine study, n = 10), in the other intravenous endotoxin (4 ng/kg, endotoxin study n = 11) was given. All subjects were treated with adenosine infusion (40 microg/kg/min) and placebo (saline) infusion in a crossover design. Heart rate, body temperature, blood pressure and plasma cytokines (tumor necrosis factor [TNF]-alpha, interleukin [IL]-6, IL-10, and soluble TNF receptors I and II), nitric oxide oxidation products, nitrite and nitrate, as well as superoxide anions were determined. There were no significant changes of any measured parameter after adenosine treatment alone. Endotoxin elicited clinical signs of an inflammatory reaction, prominent release of all cytokines and O2- synthesis by neutrophils (N-formyl-methionin-leucyl-phenylalanin-stimulated cells measured by cytochrome C reduction). The plasma IL-6 response to endotoxin was attenuated by adenosine, as IL-6 increased from 0.9 (0.8-1.6) to 1345 (743-1906) pg/mL (median; 25-75th percentiles) with adenosine infusion, and from 0.8 (0.5-1) to 1,959 (1,344-2,505) pg/mL with placebo (P = 0.0065). There was no significant influence of adenosine infusion on the other variables examined. In conclusion, systemic adenosine infusion counteracts the release of IL-6 in healthy volunteers, indicating an anti-inflammatory effect of adenosine which should be further explored.

The critical role of adenosine A2A receptors in downregulation of inflammation and immunity in the pathogenesis of infectious diseases

Adenosine can be described as a retaliatory metabolite, the production and release of which is usually enhanced under adverse environmental conditions. Binding via specific receptors, adenosine activates endogenous protective mechanisms aiming at the restoration of tissue homeostasis. While adenosinergic downregulation of tissue damage is beneficial in acute inflammation, chronic suppression of the immune system by adenosine may account for immunoparalysis in long-term septic patients.

Effects of adenosine on extravascular lung water content in endotoxemic pigs

OBJECTIVE

To investigate whether adenosine protects against endotoxin-induced increments in extravascular lung water content.

DESIGN

Prospective, randomized, animal study.

SETTING

University research laboratory.

SUBJECTS

Twenty-one anesthetized juvenile pigs.

INTERVENTIONS

The animals were divided into two groups subjected to endotoxin infusion: Endotoxin alone (n = 7), or endotoxin combined with adenosine infusion (n = 7) administered during the whole experimental period. Two other groups were exposed to anesthesia alone (n = 4) or adenosine infusion alone (n = 3), respectively.

MEASUREMENTS AND MAIN RESULTS

Central hemodynamic variables and extravascular lung water, as assessed by the thermal dye dilution double indicator technique, were monitored. Plasma endothelin-1 concentrations were measured hourly. Extravascular lung water increased significantly in response to endotoxemia (p <.001) along with an increase in pulmonary microvascular pressure (P(mv) [p <.01]). Although the Pmv increased less in endotoxemic animals exposed to adenosine infusion, no intergroup difference was found. From 4 through 6 hrs, adenosine-treated pigs displayed only half of the extravascular lung water content of nontreated animals (p <.01). The latter did not differ from that of anesthetized controls receiving anesthesia or adenosine alone. Adenosine administered alone had no effect on P(mv). In pigs receiving adenosine alone, extravascular lung water content reached nadir after 3 hrs. In both endotoxin groups, plasma endothelin-1 concentration increased two-fold, peaking 4-6 hrs after the start of endotoxin infusion (p <.001).

CONCLUSIONS

The endotoxin-induced increase in lung extravascular water was hampered by intravenously infused adenosine in the presence of a nonsignificantly reduced microvascular pressure. This leaves reduced microvascular permeability the most likely reason for the beneficial effect of adenosine.

Effects of adenosine on cardiopulmonary functions and oxygen-derived variables during endotoxemia

OBJECTIVES

To determine the effects of a prophylactic intravenous infusion of adenosine on cardiopulmonary functions and oxygen-derived variables in a porcine model of endotoxemia.

DESIGN

Prospective, randomized, placebo-controlled, unblinded study.

SETTING

University research laboratory.

SUBJECTS

Thirty country bred pigs, aged 6 to 7 wks, weighing 24.9 +/- 0.65 (SEM) kg body weight.

INTERVENTIONS

Pigs were anesthetized by i.v. pentobarbital and fentanyl, intratracheally intubated, and mechanically normoventilated with a gas mixture of nitrous oxide/oxygen = 1:1. Intravascular catheters were inserted to allow for determination of arterial, central venous blood pressure, pulmonary artery occlusion pressure, cardiac output, and sampling of blood for gas analyses. Group 1 (n = 10) received a 330-min intravenous infusion of Salmonella abortus equi endotoxin (5 microg/kg body weight x hr). Group 2 (n = 10) received an additional intravenous infusion of adenosine (150 microg/kg body weight x min), started 30 mins before the infusion of endotoxin. Control groups 3 and 4 (n = 5 for both groups) received adenosine or physiologic saline, respectively.

MEASUREMENTS AND MAIN RESULTS

Parameters of cardiopulmonary function and oxygen-derived variables were calculated from pulmonary artery catheter measurements and blood gas analyses using standard formula. Plasma concentrations of purine compounds (adenosine, inosine, hypoxanthine) were determined by high-performance liquid chromatography. Since tumor necrosis factor-alpha plays a central role in the development of endotoxic shock, concentrations of this cytokine were determined in serum by enzyme-linked immunosorbent assay. Infusion of adenosine before the beginning of the infusion of endotoxin increased plasma concentrations of the nucleoside from 193 +/- 72 to 553 +/- 65 nmol/L and decreased the systemic vascular resistance by 50%. Although acting as a potent vasodilator under control conditions, adenosine did not aggravate the arterial hypotension elicited by endotoxemia but significantly increased cardiac output by a comparably small decrease in systemic vascular resistance, prevention of pulmonary vasoconstriction, and improvement of left ventricular performance. Despite significant pulmonary vasodilation, gas exchange was not worsened but slightly improved by adenosine. With the increase in cardiac output and arterial oxygenation, systemic oxygen delivery almost doubled. This adenosine-induced oxygen flux was not a surplus but was most likely utilized by tissues, as suggested by the much earlier beginning of the increase in the systemic oxygen consumption and the attenuation of the decrease in the gastric mucosal pHi. No effects of adenosine were observed on the endotoxin-induced increase in serum concentrations of tumor necrosis factor-alpha.

CONCLUSIONS

Infusion of adenosine might be useful to improve flow-dependent oxygen delivery and tissue oxygenation during endotoxic shock without the induction of adverse cardiopulmonary side effects. The beneficial hemodynamic effects of adenosine appear not to be mediated by the inhibition of the release of tumor necrosis factor-alpha.