Complementary role of extracellular ATP and adenosine in ischemic preconditioning in the rat heart

Cell Death Induction and Protection by Activation of Ubiquitously Expressed Anion/Cation Channels. Part 1: Roles of VSOR/VRAC in Cell Volume Regulation, Release of Double-Edged Signals and Apoptotic/Necrotic Cell Death

Cell volume regulation (CVR) is essential for survival and functions of animal cells. Actually, normotonic cell shrinkage and swelling are coupled to apoptotic and necrotic cell death and thus called the apoptotic volume decrease (AVD) and the necrotic volume increase (NVI), respectively. A number of ubiquitously expressed anion and cation channels are involved not only in CVD but also in cell death induction. This series of review articles address the question how cell death is induced or protected with using ubiquitously expressed ion channels such as swelling-activated anion channels, acid-activated anion channels and several types of TRP cation channels including TRPM2 and TRPM7. The Part 1 focuses on the roles of the volume-sensitive outwardly rectifying anion channels (VSOR), also called the volume-regulated anion channel (VRAC), which is activated by cell swelling or reactive oxygen species (ROS) in a manner dependent on intracellular ATP. First we describe phenotypical properties, the molecular identity, and physical pore dimensions of VSOR/VRAC. Second, we highlight the roles of VSOR/VRAC in the release of organic signaling molecules, such as glutamate, glutathione, ATP and cGAMP, that play roles as double-edged swords in cell survival. Third, we discuss how VSOR/VRAC is involved in CVR and cell volume dysregulation as well as in the induction of or protection from apoptosis, necrosis and regulated necrosis under pathophysiological conditions.

Cystic fibrosis transmembrane conductance regulator-dependent bicarbonate entry controls rat cardiomyocyte ATP release via pannexin1 through mitochondrial signalling and caspase activation

AIM

Cystic fibrosis transmembrane conductance regulator (CFTR) is expressed in the heart, but its function there is unclear. CFTR regulates an ATP release pore in many tissues, but the identity and regulatory mechanism of the pore are unknown. We investigated the role of CFTR in ATP release from primary cardiomyocytes and ventricular wall in vivo.

METHODS

Proteins involved in the signalling pathway for ATP release during simulated ischaemia (lactic acid treatment) were investigated using inhibitors and siRNA; colocalization was identified by coimmunofluorescence and proximity ligation assays; changes in near-membrane pH and calcium were identified with total internal reflection microscopy; in vivo ATP release was investigated using interstitial microdialysis of rat heart.

RESULTS

Lactic acid-induced CFTR-dependent ATP release from cultured cardiomyocytes and left ventricle in vivo. Lactic acid entry elevated near-membrane calcium, which involved Na/H- and Na/Ca-exchangers colocalized with CFTR. Calcium entry-induced CFTR activation, which involved cAMP, protein kinase A, FAK, Pyk2 and Src. Removal of extracellular bicarbonate abolished cardiomyocyte ATP release induced by lactic acid or CFTR activators. Bicarbonate stimulated cytochrome c expression, cytochrome c release and ATP release from isolated cardiomyocyte mitochondria. Pannexin 1 (Panx1) colocalized with CFTR. Lactic acid increased cardiomyocyte caspase activity: caspase inhibitors or Panx1 siRNA abolished cardiomyocyte ATP release, while pannexin inhibition abolished cardiac ATP release in vivo.

CONCLUSION

During simulated ischaemia, CFTR-dependent bicarbonate entry stimulated ATP and cytochrome c release from mitochondria; in the cytoplasm, cytochrome c-activated caspase 3, which in turn activated Panx1, and ATP was released through the opened Panx1 channel.

Complementary Role of P2 and Adenosine Receptors in ATP Induced-Anti-Apoptotic Effects Against Hypoxic Injury of HUVECs

BACKGROUND

Vascular endothelial injury during ischemia generates apoptotic cell death and precedes apoptosis of underlying tissues. We aimed at studying the role of extracellular adenosine triphosphate (ATP) on endothelial cells protection against hypoxia injury.

METHODS

In a hypoxic model on endothelial cells, we quantified the extracellular concentration of ATP and adenosine. The expression of mRNA (ectonucleotidases, adenosine, and P2 receptors) was measured. Apoptosis was evaluated by the expression of cleaved caspase 3. The involvement of P2 and adenosine receptors and signaling pathways was investigated using selective inhibitors.

RESULTS

Hypoxic stress induced a significant increase in extracellular ATP and adenosine. After a 2-h hypoxic injury, an increase of cleaved caspase 3 was observed. ATP anti-apoptotic effect was prevented by suramin, pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), and CGS15943, as well as by selective A2A, A2B, and A3 receptor antagonists. P2 receptor-mediated anti-apoptotic effect of ATP involved phosphoinositide 3-kinase (PI3K), extracellular signal-regulated kinases (ERK1/2), mitoK_ATP, and nitric oxide synthase (NOS) pathways whereas adenosine receptor-mediated anti-apoptotic effect involved ERK1/2, protein kinase A (PKA), and NOS.

CONCLUSIONS

These results suggest a complementary role of P2 and adenosine receptors in ATP-induced protective effects against hypoxia injury of endothelial. This could be considered therapeutic targets to limit the development of ischemic injury of organs such as heart, brain, and kidney.

The organic anion transporter SLCO2A1 constitutes the core component of the Maxi-Cl channel

The maxi-anion channels (MACs) are expressed in cells from mammals to amphibians with ~60% exhibiting a phenotype called Maxi-Cl. Maxi-Cl serves as the most efficient pathway for regulated fluxes of inorganic and organic anions including ATP However, its molecular entity has long been elusive. By subjecting proteins isolated from bleb membranes rich in Maxi-Cl activity to LC-MS/MS combined with targeted siRNA screening, CRISPR/Cas9-mediated knockout, and heterologous overexpression, we identified the organic anion transporter SLCO2A1, known as a prostaglandin transporter (PGT), as a key component of Maxi-Cl. Recombinant SLCO2A1 exhibited Maxi-Cl activity in reconstituted proteoliposomes. When SLCO2A1, but not its two disease-causing mutants, was heterologously expressed in cells which lack endogenous SLCO2A1 expression and Maxi-Cl activity, Maxi-Cl currents became activated. The charge-neutralized mutant became weakly cation-selective with exhibiting a smaller single-channel conductance. Slco2a1 silencing in vitro and in vivo, respectively, suppressed the release of ATP from swollen C127 cells and from Langendorff-perfused mouse hearts subjected to ischemia-reperfusion. These findings indicate that SLCO2A1 is an essential core component of the ATP-conductive Maxi-Cl channel.

Role of Extracellular Adenosine Triphosphate in Human Skin

Background:

The nucleotide adenosine triphosphate (ATP) has long been known to drive and participate in countless intracellular processes. Extracellular ATP and its metabolite adenosine have also been shown to exert a variety of effects on nearly every cell type in human skin. Knowledge of the sources and effects of extracellular ATP in human skin may help shape new therapies for skin injury, inflammation, and numerous other cutaneous disorders.

Objective:

The objective of this review is to introduce the reader to current knowledge regarding the sources and effects of extracellular ATP in human skin and to outline areas in which further research is necessary to clarify the nature and mechanism of these effects.

Conclusion:

Extracellular ATP seems to play a direct role in triggering skin inflammatory, regenerative, and fibrotic responses to mechanical injury, an indirect role in melanocyte proliferation and apoptosis, and a complex role in Langerhans cell-directed adaptive immunity.

Current knowledge on the role of P2Y receptors in cardioprotection against ischemia-reperfusion

During ischemia, numerous effective endogenous extracellular mediators have been identified, particularly, nucleosides such as adenosine as well as purinergic and pyrimidinergic nucleotides. They may play important regulatory roles within the cardiovascular system and notably as cardio-protectants. Indeed, the distribution of the P2Y receptors in mammalian heart includes several cellular constituents relevant for the pathophysiology of myocardial ischemia. Beside the well-known cardioprotective effect of adenosine, the additional protective role of P2Y receptors has emerged. However, interpretation of experimental results may be sometimes perplexing. This is due to the variability of: the experimental models, the endpoints criteria, the chemical structure of agonist and antagonist ligands and their concentrations, the sequences of drug administration with respect to the model used (before and/or during and/or after ischemia). The net effect may be in the opposite direction after a transient or a prolonged stimulation. Nevertheless, the overall reading of published data highlights the beneficial role of the P2Y_2/4 receptor stimulation, the useful and synergistic role of P2Y_6/11 receptor activation and even of the P2Y₁₁ receptor alone in cardioprotection. More, the P2Y₁₁ receptor could be involved in counter-regulation of profibrotic processes. Paradoxically, transient P2X₇ receptor stimulation could contribute to the net cardioprotective effect of ATP. Recently, experimental data have shown that blocking the P2Y₁₂ receptor after ischemia confers cardioprotection independently of platelet antiaggregatory effect. This suggests for P2Y receptors an important role in primary prevention and as a therapeutic target in myocardial protection during ischemia and reperfusion.

Hypoxia-induced force increase (HIFI) is a novel mechanism underlying the strengthening of labor contractions, produced by hypoxic stresses

For successful birth, contractions need to become progressively stronger. The underlying mechanisms are unknown, however. We have found that a novel mechanism, hypoxia-induced force increase (HIFI), is switched on selectively, at term, and is essential to strengthening contractions. HIFI is initiated as contractions cyclically reduce blood flow and produce repeated hypoxic stresses, with associated metabolic and transcriptomic changes. The increases in contractility are a long-lasting, oxytocin-independent, intrinsic mechanism present only in the full-term pregnant uterus. HIFI is inhibited by adenosine receptor antagonism and blockade of cyclooxygenase-2 signaling, and partially reproduced by brief episodes of acidic (but not alkalotic) pH. HIFI explains how labor can progress despite paradoxical metabolic challenge, and provides a new mechanistic target for the 1 in 10 women suffering dysfunctional labor because of poor contractions.

Cardiac purinergic signalling in health and disease

This review is a historical account about purinergic signalling in the heart, for readers to see how ideas and understanding have changed as new experimental results were published. Initially, the focus is on the nervous control of the heart by ATP as a cotransmitter in sympathetic, parasympathetic, and sensory nerves, as well as in intracardiac neurons. Control of the heart by centers in the brain and vagal cardiovascular reflexes involving purines are also discussed. The actions of adenine nucleotides and nucleosides on cardiomyocytes, atrioventricular and sinoatrial nodes, cardiac fibroblasts, and coronary blood vessels are described. Cardiac release and degradation of ATP are also described. Finally, the involvement of purinergic signalling and its therapeutic potential in cardiac pathophysiology is reviewed, including acute and chronic heart failure, ischemia, infarction, arrhythmias, cardiomyopathy, syncope, hypertrophy, coronary artery disease, angina, diabetic cardiomyopathy, as well as heart transplantation and coronary bypass grafts.

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

Extracellular NAADP affords cardioprotection against ischemia and reperfusion injury and involves the P2Y11-like receptor

BACKGROUND AND PURPOSE

Extracellular nucleotides may play important regulatory roles within the cardiovascular system and notably in cardioprotection. We aimed to look for a possible pharmacological preconditioning effect of extracellular NAADP ([NAADP]e) against ischemia/reperfusion injury. [NAADP]e has been recently reported to be a full agonist of the P2Y11 receptor. Therefore, we characterized the involvement of the P2Y11-like receptor in mediating ischemic/reperfusion tolerance induced by [NAADP]e.

EXPERIMENTAL APPROACH

The cardioprotective effects of [NAADP]e were evaluated in a model of ischemia/reperfusion carried out on Langendorff perfused rat hearts. This model was also instrumented with a microdialysis probe. Furthermore, using isolated cardiomyocytes, we assessed cAMP, inositol phosphate accumulation and prosurvival protein kinases activation induced by [NAADP]e pretreatement.

RESULTS

Pretreatment with 1μM [NAADP]e induced cardioprotective effects with regards to functional recovery, necrosis and arrhythmogenesis (p<0.05). These effects were completely suppressed with NF157, an antagonist of the P2Y11 receptor. Moreover, global ischemia induced a time-dependent increase in interstitial concentration of adenosine, NAADP and UTP. In cardiomyocyte cultures, NF157 suppressed cAMP and inositol phosphate accumulation induced by [NAADP]e. [NAADP]e induced phosphorylation of ERK 1/2, AKT and its downstream target GSK-3β (p<0.05). These activations were also suppressed by NF157.

CONCLUSIONS

Evidence suggests that NAADP signalling at the P2Y11-like receptor affords significant cardioprotection against ischemia/reperfusion injury. Besides adenosine and UTP, microdialysis study supports a potential endogenous role of [NAADP]e.

The inflammasome promotes adverse cardiac remodeling following acute myocardial infarction in the mouse

Acute myocardial infarction (AMI) initiates an intense inflammatory response that promotes cardiac dysfunction, cell death, and ventricular remodeling. The molecular events underlying this inflammatory response, however, are incompletely understood. In experimental models of sterile inflammation, ATP released from dying cells triggers, through activation of the purinergic P2X7 receptor, the formation of the inflammasome, a multiprotein complex necessary for caspase-1 activation and amplification of the inflammatory response. Here we describe the presence of the inflammasome in the heart in an experimental mouse model of AMI as evidenced by increased caspase-1 activity and cytoplasmic aggregates of the three components of the inflammasome--apoptosis speck-like protein containing a caspase-recruitment domain (ASC), cryopyrin, and caspase-1, localized to the granulation tissue and cardiomyocytes bordering the infarct. Cultured adult murine cardiomyocytes also showed the inducible formation of the inflammasome associated with increased cell death. P2X7 and cryopyrin inhibition (using silencing RNA or a pharmacologic inhibitor) prevented the formation of the inflammasome and limited infarct size and cardiac enlargement after AMI. The formation of the inflammasome in the mouse heart during AMI causes additional loss of functional myocardium, leading to heart failure. Modulation of the inflammasome may therefore represent a unique therapeutic strategy to limit cell death and prevent heart failure after AMI.

Activation of purinergic receptors by ATP induces ventricular tachycardia by membrane depolarization and modifications of Ca2+ homeostasis

Cardiac myocytes are continuously exposed to extracellular nucleotides secreted by the myocytes themselves, nerve terminals, or platelets and other blood cells during coronary perfusion, and the concentrations of such extracellular nucleotides are known to increase during cardiac ischemia and hypoxia. The effects of the extracellular nucleotides ATP, ADP, UTP, and adenosine on ventricular arrhythmogenic properties were explored in 36 Langendorff-perfused mouse hearts using monophasic action potential recording. Extracellular nucleotides induced arrhythmic phenomena in form of ectopic activity and ventricular tachycardia in a potency order of ATP (n=7) > ADP (n=5) > UTP (n=3) approximately adenosine (n=3). The purinergic receptor antagonists suramin and pyridoxal phosphate-6-azo(benzene-2,4-disulphonic acid) reduced the incidence of ATP-triggered arrhythmias. In isolated ventricular myocytes, ATP induced sustained increases in diastolic Ca2+ and triggered multiple Ca2+ waves, which were inhibited by suramin but not by the L-type Ca2+ channel antagonist nifedipine. In whole-cell patch clamp experiments, extracellular ATP induced two distinct types of inward currents, which were inhibited by suramin and PPADS, suggesting activation of P2X receptors. ATP also induced delayed after-depolarizations and ectopic action potentials in current clamped ventricular myocytes. In conclusion, extracellular ATP activates purinergic receptors and induces arrhythmic activity through modifications of Ca2+ homeostasis and an activation of depolarizing membrane currents.

Involvement of P2Y receptors in pyridoxal-5'-phosphate-induced cardiac preconditioning

Using an isolated non-working rat heart model, this study investigated the mechanisms of pharmacological pre-conditioning (PC) induced by P2Y receptor stimulation with pyridoxal-5'-phosphate (PLP). After 6-hydroxydopamine pretreatment and a 15-min stabilization period, isolated rat hearts were perfused for 25 min then subjected to 40 min of global ischemia and 30 min of reperfusion (I/R); exposed for 15 min to 0.05 microM PLP bracketed for 25 min with broad-spectrum P2 antagonists (suramin or PPADS) or with more specific P2Y antagonists (AMPalphaS or MRS2578), 1 microM each, followed by a 5-min PLP-free perfusion before I/R; treated during 25 min with either glybenclamide (GLY, 1 microM), 5-hydroxydecanoic acid (5-HD, 100 microM), U73122 (0.5 microM), H89 (1 microM), or KN93 (1 microM), with an infusion starting 5 min before PLP. The main endpoints were the rate-pressure product (RPP), creatine kinase (CK) release and area necrosis. Recovery of RPP, measured 5 min after reperfusion, was rapidly improved by PLP, blocked by the P2 antagonists, and decreased with the different inhibitors. Fifteen minutes after the end of ischemia, CK release reached maximal values in all groups. PLP provided significant protection, whereas the P2 antagonists, 5-HD, a mitochondrial selective K(ATP) antagonist and GLY a non-selective K(ATP) channel blocker, suppressed the protective effect on myocardial injury. The suppression of the cardioprotective effects of PLP by AMPalphaS, the PKA inhibitor (H89), and phospholipase C blocker (U73122) is in agreement with the P2Y11 receptor as a receptor for PLP-induced PC. The suppression of the cardioprotective effects of PLP by MRS2578 and U73122 is in agreement with the P2Y6 receptor as a receptor for PLP-induced PC. Pre-ischemic exposure to nanomolar concentrations of PLP is protective against I/R. P2Y11 and P2Y6 represents the most likely candidate receptors for PLP-induced cardiac PC.

Ischemic preconditioning: from molecular mechanisms to therapeutic opportunities

Ischemia/reperfusion (I/R) injury is a major contributory factor to cardiac dysfunction and infarct size that determines patient prognosis after acute myocardial infarction. Considerable interest exists in harnessing the heart's endogenous capacity to resist I/R injury, known as ischemic preconditioning (IPC). The IPC research has contributed to uncovering the pathophysiology of I/R injury on a molecular and cellular basis and to invent potential therapeutic means to combat such damage. However, the translation of basic research findings learned from IPC into clinical practice has often been inadequate because the majority of basic research findings have stemmed from young and healthy animals. Few if any successful implementations of IPC have occurred in the diseased hearts that are the primary target of viable therapies activating cardioprotective mechanisms to limit cardiac dysfunction and infarct size. Therefore, the first purpose of this review is to facilitate understanding of pathophysiology of I/R injury and the mechanisms of cardioprotection afforded by IPC in the normal heart. Then I focus on the problems and opportunities for successful bench-to-bedside translation of IPC in the diseased hearts.

P2 purinoceptor-mediated cardioprotection in ischemic-reperfused mouse heart

P2 purinoceptor modulation of injury during ischemia-reperfusion was studied in murine hearts. Effects of P2 agonism or antagonism, and interstitial accumulation of P2 agonists (UTP, ATP, and ADP), were assessed in Langendorff perfused hearts during 20 min of ischemia and 45 min of reperfusion. In control hearts, ventricular pressure development recovered to 68 +/- 4 mm Hg (63 +/- 3% baseline), diastolic pressure remained elevated (23 +/- 2 mm Hg), and 26 +/- 4 U/g lactate dehydrogenase (LDH) was released during reperfusion, evidencing necrosis. Treatment with 250 nM UTP improved pressure development (85 +/- 5 mm Hg, or 77 +/- 2%) and reduced diastolic contracture (by approximately 70%, to 7 +/- 1 mm Hg) and LDH loss (by approximately 60%, to 11 +/- 2 U/g). In contrast, P2Y1 agonism with 50 nM 2-methyl-thio-ATP (2-MeSATP) was ineffective. In the presence of the P2Y antagonist suramin (10 or 200 microM), UTP no longer improved postischemic outcomes. Ischemia also substantially elevated interstitial [UTP], [ATP], and [ADP], potentially activating P2 receptors. This was supported in part by effects of antagonists: 200 microM suramin worsened LDH efflux (53 +/- 9 IU/g) and contractile dysfunction (41 +/- 2 mm Hg diastolic pressure; 28 +/- 3 mm Hg developed pressure), as did P2Y antagonism with either 10 or 100 microM reactive blue 2. However, a 10 microM concentration of suramin failed to alter outcome. P2X antagonism with 10 microM pyridoxal phosphate-6-azo-(benzene-2,4-disulfonic acid and P2X1-selective pyridoxal-alpha5-phosphate-6-phenylazo-4'-carboxylic acid (MRS2159) (30 microM) was ineffective. Data collectively support cardioprotection with low concentrations of UTP, and they are consistent with P2Y2 involvement. Endogenous nucleotides may also play a protective role, as evidenced by effects of P2 antagonists, although this warrants further investigation.

Purinergic signaling and kinase activation for survival in pulmonary oxidative stress and disease

Stimulus-induced release of endogenous ATP into the extracellular milieu has been shown to occur in a variety of cells, tissues, and organs. Extracellular ATP can propagate signals via P2 receptors that are essential for growth and survival of cells. Abundance of P2 receptors, their multiple isoforms, and their ubiquitous distribution indicate that they transmit vital signals. Pulmonary epithelium and endothelium are rich in both P2X and P2Y receptors. ATP release from lung tissue and cells occurs upon stimulation both in vivo and in vitro. Extracellular ATP can activate signaling cascades composed of protein kinases including extracellular signal-regulated kinase (ERK) and phosphatidylinositol-3-kinase (PI3K). Here we summarize progress related to release of endogenous ATP and nucleotide signaling in pulmonary tissues upon exposure to oxidant stress. Hypoxic, hyperoxic, and ozone exposures cause a rapid increase of extracellular ATP in primary pulmonary endothelial and epithelial cells. Extracellular ATP is critical for survival of these cells in high oxygen and ozone concentrations. The released ATP, upon binding to its specific receptors, triggers ERK and PI3K signaling and renders cells resistant to these stresses. Impairment of ATP release and transmission of such signals could limit cellular survival under oxidative stress. This may further contribute to disease pathogenesis or exacerbation.

[Myocardial preconditioning with volatile anesthetics. General anesthesia as protective intervention?]

Reduction of the perioperative cardiovascular risk with pharmacological interventions plays a prominent role in routine anesthesia practice. For example, perioperative beta-blockade is well established in anesthesiological treatment of patients. There is a growing body of evidence supporting the cardioprotective effects of volatile anesthetics known as anesthetic-induced preconditioning. There are numerous and complex data from animal studies. The mechanisms of anesthetic-induced preconditioning have been extensively studied but have still not been clearly identified. Initial clinical data show the cardioprotective effects of volatile agents by looking at parameters of myocardial function and laboratory values and therefore, the question of the relevance of these data for routine clinical practice has been raised. This review gives a summary of the currently available data focusing on the mechanisms of anesthesiological preconditioning and clinical studies.

ATP release via anion channels

ATP serves not only as an energy source for all cell types but as an 'extracellular messenger' for autocrine and paracrine signalling. It is released from the cell via several different purinergic signal efflux pathways. ATP and its Mg(2+) and/or H(+) salts exist in anionic forms at physiological pH and may exit cells via some anion channel if the pore physically permits this. In this review we survey experimental data providing evidence for and against the release of ATP through anion channels. CFTR has long been considered a probable pathway for ATP release in airway epithelium and other types of cells expressing this protein, although non-CFTR ATP currents have also been observed. Volume-sensitive outwardly rectifying (VSOR) chloride channels are found in virtually all cell types and can physically accommodate or even permeate ATP(4-) in certain experimental conditions. However, pharmacological studies are controversial and argue against the actual involvement of the VSOR channel in significant release of ATP. A large-conductance anion channel whose open probability exhibits a bell-shaped voltage dependence is also ubiquitously expressed and represents a putative pathway for ATP release. This channel, called a maxi-anion channel, has a wide nanoscopic pore suitable for nucleotide transport and possesses an ATP-binding site in the middle of the pore lumen to facilitate the passage of the nucleotide. The maxi-anion channel conducts ATP and displays a pharmacological profile similar to that of ATP release in response to osmotic, ischemic, hypoxic and salt stresses. The relation of some other channels and transporters to the regulated release of ATP is also discussed.

Renal interstitial fluid ATP responses to arterial pressure and tubuloglomerular feedback activation during calcium channel blockade

A close relationship between changes in renal interstitial fluid (RIF) ATP concentrations and renal autoregulatory or tubuloglomerular feedback (TGF)-dependent changes in renal vascular resistance (RVR) has been demonstrated, but it has not been determined whether the changes in RIF ATP are a consequence or the cause of the changes in RVR. The present study was performed in anesthetized dogs to assess the changes in RIF ATP following changes in renal arterial pressure (RAP) or stimulation of the TGF mechanism under conditions where changes in RVR were prevented by nifedipine, a calcium channel blocker. RIF ATP levels were measured by using microdialysis probes. Intra-arterial infusion of nifedipine (0.36 microg x kg(-1) x min(-1)) increased renal blood flow (RBF: from 4.49 +/- 0.27 to 5.34 +/- 0.39 ml x min(-1) x g(-1)) and glomerular filtration rate (GFR: from 0.84 +/- 0.07 to 1.09 +/- 0.11 ml x min(-1) x g(-1)). Under conditions of nifedipine infusion, autoregulatory adjustments in RBF, GFR, and RVR were not observed during stepwise reductions in RAP within the autoregulatory range (from 135 +/- 7 to 76 +/- 1 mmHg, n = 7). Furthermore, stimulation of the TGF mechanism with intra-arterial infusion of acetazolamide (100 microg x kg(-1) x min(-1)) did not alter RBF, GFR, and RVR (n = 7). During treatment with nifedipine, RIF ATP levels were significantly decreased in response to reductions in RAP (10.7 +/- 0.7, 5.8 +/- 0.7 and 2.8 +/- 0.3 nmol/l at 135 +/- 7, 101 +/- 4, and 76 +/- 1 mmHg, n = 7) and increased by acetazolamide infusion (from 8.8 +/- 0.8 to 17.0 +/- 1.8 nmol/l, n = 7). These results are similar to those that occurred in dogs not treated with nifedipine and thus demonstrate that the changes in RIF ATP can occur in the absence of autoregulatory or TGF-mediated changes in RVR. The data provide further support to the hypothesis that RIF ATP contributes to adjustments in RVR associated with renal autoregulation and changes in activity of the TGF mechanism.

Role of ATP-conductive anion channel in ATP release from neonatal rat cardiomyocytes in ischaemic or hypoxic conditions

It is known that the level of ATP in the interstitial spaces within the heart during ischaemia or hypoxia is elevated due to its release from a number of cell types, including cardiomyocytes. However, the mechanism by which ATP is released from these myocytes is not known. In this study, we examined a possible involvement of the ATP-conductive maxi-anion channel in ATP release from neonatal rat cardiomyocytes in primary culture upon ischaemic, hypoxic or hypotonic stimulation. Using a luciferin-luciferase assay, it was found that ATP was released into the bulk solution when the cells were subjected to chemical ischaemia, hypoxia or hypotonic stress. The swelling-induced ATP release was inhibited by the carboxylate- and stilbene-derivative anion channel blockers, arachidonic acid and Gd3+, but not by glibenclamide. The local concentration of ATP released near the cell surface of a single cardiomyocyte, measured by a biosensor technique, was found to exceed the micromolar level. Patch-clamp studies showed that ischaemia, hypoxia or hypotonic stimulation induced the activation of single-channel events with a large unitary conductance (approximately 390 pS). The channel was selective to anions and showed significant permeability to ATP4- (PATP/PCl approximately 0.1) and MgATP2- (PATP/PCl approximately 0.16). The channel activity exhibited pharmacological properties essentially identical to those of ATP release. These results indicate that neonatal rat cardiomyocytes respond to ischaemia, hypoxia or hypotonic stimulation with ATP release via maxi-anion channels.

Inosine stimulates chemotaxis, Ca2+-transients and actin polymerization in immature human dendritic cells via a pertussis toxin-sensitive mechanism independent of adenosine receptors

Inosine is an endogenous purine nucleoside, which is formed by adenosine deaminidase during adenosine breakdown and is released into the extracellular space from the sympathetic nervous system or injured cells. Here, we studied the biological activity of inosine on human dendritic cells (DC), which are specialized antigen presenting cells characterized by their ability to migrate from the blood to peripheral tissues, and then to secondary lymphoid organs where they initiate adaptive immune responses. In immature DC, inosine concentration-dependently stimulated Ca(2+)-transients, actin polymerization, and chemotaxis. Experiments with adenosine receptor antagonists and pertussis toxin (PTX) as well as desensitization studies suggested that the activity of inosine was mediated by a G protein-coupled receptor pathway independent of adenosine receptors. DC, induced to mature by lipopolysaccharide, lost their ability to respond towards inosine with these activities. Moreover, inosine did neither influence membrane expression of CD54, CD80, CD83, CD86, HLA-DR, and MHC class I molecules nor modulated secretion of interleukin (IL)-12, IL-10, and tumor necrosis factor alpha in immature and lipopolysaccharide-matured DC. In aggregate, our study indicates that inosine may be involved in the trafficking control system of immature DC, and mediates its chemotactic activity by a PTX-sensitive mechanism independent of adenosine receptors.

Combined pharmacological preconditioning with a G-protein-coupled receptor agonist, a mitochondrial KATP channel opener and a nitric oxide donor mimics ischaemic preconditioning

1. Although pharmacological preconditioning (PPC) has emerged as an alternative to ischaemic preconditioning (IPC) in cardioprotection, the efficacy of PPC compared with IPC has not been investigated. Because IPC is mediated by complex signalling cascades arising from multiple triggers, we have hypothesized that combined PPC is necessary to mimic IPC. 2. Isolated and perfused rat hearts underwent IPC by three cycles of 5 min ischaemia and 5 min reperfusion before 30 min global ischaemia followed by 120 min reperfusion. Adenosine (30 micromol/L), diazoxide (50 micromol/L) and s-nitroso-N-acetylpenicillamine (SNAP; 50 micromol/L) were added for 25 min just before (pretreatment modality) or 45 min before (PPC modality) the index ischaemia. 3. Ischaemic preconditioning significantly improved isovolumic left ventricular (LV) function and reduced infarct size. Although pretreatment with adenosine, diazoxide or SNAP alone was capable of reducing infarct size, PPC with each drug alone or in a combination of two drugs except for diazoxide plus SNAP failed to reduce infarct size. In contrast, PPC in combination with adenosine, diazoxide and SNAP (triple combination PPC) conferred significant improvement of LV function and reduction of infarct size that was as effective as IPC. 4. Cardioprotection afforded by triple combination PPC was abolished by the Gi/o-protein inhibitor pertussis toxin, the mitochondiral KATP channel inhibitor 5-hydroxydecanoate or the nitric oxide (NO) scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethyl imidazoline-1-oxyl 3-oxide (carboxy-PTIO). 5. Protein kinase C (PKC)-epsilon in the particulate fraction was activated throughout preconditioning ischaemia and reperfusion. Although PKC-epsilon was activated during treatment with adenosine, diazoxide or SNAP alone, it was inactivated after washout. In contrast, PKC-epsilon remained activated after triple combination PPC. The PKC inhibitor chelerythrine abolished activation of PKC-epsilon and cardioprotection afforded by IPC and triple combination PPC. 6. These results demonstrate that combined PPC with a G-protein-coupled receptor agonist, a mitochondrial KATP channel opener and an NO donor is necessary to mimic IPC and such synergistic cardioprotection is associated with enhanced and sustained activation of PKC-epsilon.

Integrated pharmacological preconditioning in combination with adenosine, a mitochondrial KATP channel opener and a nitric oxide donor

BACKGROUND

Mitochondrial K(ATP) channel activation is an essential component of ischemic preconditioning. These channels are selectively opened by diazoxide and may be up-regulated by adenosine and nitric oxide. Therefore, pharmacological preconditioning with diazoxide in combination with adenosine and a nitric oxide donor (triple-combination pharmacological preconditioning) may enhance cardioprotection.

METHODS AND RESULTS

Isolated and perfused rat hearts underwent ischemic preconditioning with 3 cycles of 5 minutes of ischemia and 5 minutes of reperfusion before 5 minutes of oxygenated potassium cardioplegia and 35 minutes of ischemia. Pharmacological preconditioning was performed by adding adenosine, diazoxide, and a nitric oxide donor S-nitroso-N-acetyl-penicillamine each alone or in combinations for 25 minutes followed by 10 minutes washout before cardioplegic arrest. Only triple-combination pharmacological preconditioning conferred significant cardioprotection as documented by highly improved left ventricular function and limited creatine kinase release during reperfusion that was comparable to that afforded by ischemic preconditioning. Mitochondrial K(ATP) channel activity assessed by flavoprotein oxidation was increased by diazoxide, but no further increase in flavoprotein oxidation was obtained by ischemic preconditioning and triple-combination pharmacological preconditioning. Significant activation of protein kinase C-epsilon was observed in only ischemic preconditioning and triple-combination pharmacological preconditioning. Pretreatment with the mitochondrial K(ATP) channel inhibitor 5-hydroxydecanoate or the protein kinase C inhibitor chelerythrine abrogated activation of protein kinase C-epsilon and cardioprotection afforded by ischemic preconditioning and triple-combination pharmacological preconditioning.

CONCLUSIONS

Integrated pharmacological preconditioning is not simply mediated by enhanced mitochondrial K(ATP) channel activation, but is presumably mediated through amplified protein kinase C signaling promoted by coordinated interaction of adenosine, mitochondrial K(ATP) channel activation, and nitric oxide.

Adenosine affects expression of membrane molecules, cytokine and chemokine release, and the T-cell stimulatory capacity of human dendritic cells

Dendritic cells (DCs) express functional purinergic type 1 receptors, but the effects of adenosine in these antigen-presenting cells have been only marginally investigated. Here, we further characterized the biologic activity of adenosine in immature DCs (iDCs) and lipopolysaccharide (LPS)-matured DCs (mDCs). Chronic stimulation with adenosine enhanced the macropinocytotic activity and the membrane expression of CD80, CD86, major histocompatibility complex (MHC) class I, and HLA-DR molecules on iDCs. Adenosine also increased LPS-induced CD54, CD80, MHC class I, and HLA-DR molecule expression in mDCs. In addition, adenosine dose-dependently inhibited tumor necrosis factor alpha and interleukin-12 (IL-12) release, whereas it enhanced the secretion of IL-10 from mDCs. The use of selective receptor agonists revealed that the modulation of the cytokine and cell-surface marker profile was due to activation of A(2) adenosine receptor. Functionally, adenosine reduced the allostimulatory capacity of iDCs, but not of mDCs. More important, DCs matured in the presence of adenosine had a reduced capacity to induce T helper 1 (Th1) polarization of naive CD4(+) T lymphocytes. Finally, adenosine augmented the release of the chemokine CCL17 and inhibited CXCL10 production by mDCs. In aggregate, the results provide initial evidence that adenosine diminishes the capacity of DCs to initiate and amplify Th1 immune responses.

Enhanced IPC by activation of pertussis toxin-sensitive and -insensitive G protein-coupled purinoceptors

Extracellular ATP plays an important role in ischemic preconditioning (IPC) through the activation of P(2y) purinoceptors. This study examined whether ATP-stimulated P(2y) purinoceptors are coupled to pertussis toxin (PTX)-insensitive G protein and whether activation of this pathway enhances myocardial protection afforded by IPC. The rat was treated with PTX for 48 h, and the heart was then isolated and buffer perfused. The heart underwent IPC by three cycles of 5-min ischemia and 5-min reperfusion before 25 min of global ischemia. Isovolumic left ventricular function was measured, and functional recovery at 30 min after reperfusion was taken as an end point of myocardial protection. PTX pretreatment partially inhibited functional protection by IPC. Treatment with 100 microM 8-(p-sulfophenyl) theophylline (SPT) during IPC had no further effect on PTX-induced inhibition of functional protection by IPC, whereas suramin (300 microM) or reactive blue (RB) (10 microM) completely abolished myocardial protection in the preconditioned heart pretreated with PTX. Supplementation with adenosine (30 microM), ATP (30 microM), or UTP (50 microM) significantly enhanced IPC-induced functional protection, although preconditioning with these nucleotides without IPC had no protective effect. Adenosine-enhanced IPC was inhibited by pretreatment with PTX and SPT but not by suramin or RB, whereas ATP-enhanced IPC was inhibited by suramin or RB in combination with PTX pretreatment. On the other hand, UTP-enhanced IPC was not affected by PTX pretreatment but was inhibited by suramin or RB. Adenosine supplemented IPC without PTX pretreatment and ATP supplemented IPC with PTX pretreatment were not affected by nitric oxide synthase inhibitor N(omega)-nitro-L-arginine methyl ester (100 microM). Although the protein kinase C inhibitor Ro318425 (0.3 microM) or tyrosine kinase inhibitor genistein (50 microM) had no significant effect on the functional protection afforded by adenosine-supplemented IPC without PTX pretreatment and ATP-supplemented IPC with PTX pretreatment, the combination of Ro318425 and genistein attenuated functional protection afforded by both the purinoceptor agonist-supplemented IPC. These results suggest the crucial involvement of PTX-sensitive and -insensitive G protein coupled purinoceptors in enhanced IPC by supplementation with adenosine, ATP, and UTP.