Selective antagonism by PPADS at P2X-purinoceptors in rabbit isolated blood vessels

Purinergic inhibitory regulation of esophageal smooth muscle is mediated by P2Y receptors and ATP-dependent potassium channels in rats

Purines such as ATP are regulatory transmitters in motility of the gastrointestinal tract. The aims of this study were to propose functional roles of purinergic regulation of esophageal motility. An isolated segment of the rat esophagus was placed in an organ bath, and mechanical responses were recorded using a force transducer. Exogenous application of ATP (10-100 μM) evoked relaxation of the esophageal smooth muscle in a longitudinal direction under the condition of carbachol (1 μM) -induced precontraction. Pretreatment with a non-selective P2 receptor antagonist, suramin (500 μM), and a P2Y receptor antagonist, cibacron blue F3GA (200 μM), inhibited the ATP (100 μM) -induced relaxation, but a P2X receptor antagonist, pyridoxal phosphate-6-azophenyl-2,4-disulfonic acid (50 μM), did not affect it. A blocker of ATP-dependent potassium channels (KATP channels), glibenclamide (200 μM), inhibited the ATP-induced relaxation and application of an opener of KATP channels, nicorandil (50 μM), produced relaxation. The findings suggest that ATP is involved in inhibitory regulation of the longitudinal smooth muscle in the muscularis mucosae of the rat esophagus via activation of P2Y receptors and then opening of KATP channels.

Myography of isolated blood vessels: Considerations for experimental design and combination with supplementary techniques

The study of the mechanisms of regulation of vascular tone is an urgent task of modern science, since diseases of the cardiovascular system remain the main cause of reduction in the quality of life and mortality of the population. Myography (isometric and isobaric) of isolated blood vessels is one of the most physiologically relevant approaches to study the function of cells in the vessel wall. On the one hand, cell-cell interactions as well as mechanical stretch of the vessel wall remain preserved in myography studies, in contrast to studies on isolated cells, e.g., cell culture. On the other hand, in vitro studies in isolated vessels allow control of numerous parameters that are difficult to control in vivo. The aim of this review was to 1) discuss the specifics of experimental design and interpretation of data obtained by myography and 2) highlight the importance of the combined use of myography with various complementary techniques necessary for a deep understanding of vascular physiology.

Exogenous ATP modulates PGE2 release in macrophages through sustained phosphorylation of CDK9 and p38 MAPK

An important mediator of inflammation is prostaglandin E₂ (PGE₂ ), whose levels are determined by the activity of the enzyme cyclooxygenase (COX). Of the two isoforms of the enzyme, COX-2 has been shown to be induced in macrophages during inflammation. Although general COX inhibitors, belonging to the class of nonsteroidal anti-inflammatory drugs, or specific COX-2 inhibitors, called coxibs, are useful in the control of acute inflammation, adverse reactions were seen when used chronically in the treatment of rheumatoid arthritis or neurodegenerative diseases. Extracellular ATP (eATP) has been reported as a damage-associated molecular pattern signal. In this report, we show that eATP synergistically increases the levels of COX-2 enzyme and PGE₂ in LPS-activated RAW264.7 macrophages and human monocytes. Activation of macrophages also occurred when cultured in media obtained from dying neurons that contained higher levels of ATP. We show that eATP increases the levels of COX-2 protein, which is sustained up to 36 h poststimulation. This is in turn due to sustained levels of phosphorylated, or activated, cyclin-dependent kinase 9 and p38 MAPK in ATP-treated cells compared to LPS-stimulated cells. The eATP-dependent increase in COX-2/PGE₂ levels in LPS-activated RAW264.7 cells could be abolished using antagonists for purinergic P2X7 -and P2Y6 receptors. Similarly, the increase in COX-2/PGE₂ levels in the peritoneum of LPS-treated mice could be significantly abolished in mice that were preinjected with the nonspecific P2 receptor antagonist, suramin. P2 receptor antagonists, therefore, should be explored in our search for an ideal anti-inflammatory candidate.

History of Geoff Burnstock's research on P2 receptors

Geoffrey Burnstock is a purinergic signalling legend who's discoveries and conceptualisation created and shaped the field. His scientific achievements were extraordinary and sustained. They included his demonstration that ATP can act as a neurotransmitter and hence extracellular signalling molecule, which he championed despite considerable initial opposition to his proposal that ATP acts outside of its role as an energy source inside cells. He led on purine receptor classification: initially of the P1 and P2 receptor families, then the P2X and P2Y receptor families, and then subtypes of P2X and P2Y receptors. This was achieved across several decades as he conceptualised and made sense of the emerging and growing evidence that there were multiple receptor subtypes for ATP and other nucleotides. He made discoveries about short term and long term/trophic purinergic signalling. He was a leader in the field for over 50 years. He inspired many and was a great colleague and mentor. I had the privilege of spending over 10 years (from 1985) with Geoff at the Department of Anatomy and Developmental Biology, University College London. This review is a personal perspective of some of Geoff's research on P2 receptors carried out during that time. It is a tribute to Geoff who I regarded with enormous respect and admiration.

Regulating quantal size of neurotransmitter release through a GPCR voltage sensor

Current models emphasize that membrane voltage (Vm) depolarization-induced Ca²⁺ influx triggers the fusion of vesicles to the plasma membrane. In sympathetic adrenal chromaffin cells, activation of a variety of G protein coupled receptors (GPCRs) can inhibit quantal size (QS) through the direct interaction of G protein Giβγ subunits with exocytosis fusion proteins. Here we report that, independently from Ca²⁺, Vm (action potential) per se regulates the amount of catecholamine released from each vesicle, the QS. The Vm regulation of QS was through ATP-activated GPCR-P2Y₁₂ receptors. D76 and D127 in P2Y₁₂ were the voltage-sensing sites. Finally, we revealed the relevance of the Vm dependence of QS for tuning autoinhibition and target cell functions. Together, membrane voltage per se increases the quantal size of dense-core vesicle release of catecholamine via Vm → P2Y₁₂(D76/D127) → Giβγ → QS → myocyte contractility, offering a universal Vm-GPCR signaling pathway for its functions in the nervous system and other systems containing GPCRs.

Purinergic Antagonism Prevents Mitochondrial Dysfunction and Behavioral Deficits Associated with Dopaminergic Toxicity Induced by 6-OHDA in Rats

Purinoceptors are present in neurons, microglia and oligodendrocytes and regulate dopamine (DA) release, striatal-related function and striatal neuronal and DA cells damage. Therefore, purinoceptors may be involved in the pathology of Parkinson's disease (PD) and purinergic antagonism may show neuroprotective effect. The study investigated the role of the non-selective purinergic receptor antagonist pyridoxalphosphate-6-azophenyl-2^', 4^'-disulfonic acid (PPADS) and a selective purinergic receptor P2X7 receptor antagonist Brilliant Blue G (BBG) against 6-OHDA induced dopaminergic neurotoxicity in rats; while adenosine triphosphate (ATP) was used as a P2X receptor agonist. Behavioral parameters like spontaneous motor activity, narrow beam walk, footprint, bar catalepsy, grip strength and rotarod tests were performed to evaluate motor deficits in PD. Striatal DA contents were estimated as neurochemical measures of PD. Mitochondrial studies and oxidative status were assessed to investigate the mechanism of purinergic system antagonists. Involvement of purinergic receptors in apoptosis was assessed by expressing cytochrome-C, caspase-9 and caspase-3. Both the antagonists not only attenuated 6-OHDA induced motor deficits but also protected against 6-OHDA induced DA depletion in the striatum. Oxidative stress, mitochondrial integrity and dysfunction were attenuated by purinergic antagonists. Further, they attenuated mitochondrial-linked apoptosis as observed by a decrease in expression of cytochrome-C, caspase-9 and caspase-3. Therefore, purinoceptor antagonism shows neuroprotective effect in 6-OHDA induced dopamine toxicity through preservation of mitochondrial bioenergetics and anti-apoptotic activities.

The Role of Extracellular Adenosine Triphosphate in Ischemic Organ Injury

OBJECTIVES

Ischemic tissue injury contributes to significant morbidity and mortality and is implicated in a range of pathologic conditions, including but not limited to myocardial infarction, ischemic stroke, and acute kidney injury. The associated reperfusion phase is responsible for the activation of the innate and adaptive immune system, further accentuating inflammation. Adenosine triphosphate molecule has been implicated in various ischemic conditions, including stroke and myocardial infarction.

STUDY SELECTION

Adenosine triphosphate is a well-defined intracellular energy transfer and is commonly referred to as the body's "energy currency." However, Laboratory studies have demonstrated that extracellular adenosine triphosphate has the ability to initiate inflammation and is therefore referred to as a damage-associated molecular pattern. Purinergic receptors-dependent signaling, proinflammatory cytokine release, increased Ca influx into cells, and subsequent apoptosis have been shown to form a common underlying extracellular adenosine triphosphate molecular mechanism in ischemic organ injury.

CONCLUSIONS

In this review, we aim to discuss the molecular mechanisms behind adenosine triphosphate-mediated ischemic tissue injury and evaluate the role of extracellular adenosine triphosphate in ischemic injury in specific organs, in order to provide a greater understanding of the pathophysiology of this complex process. We also appraise potential future therapeutic strategies to limit damage in various organs, including the heart, brain, kidneys, and lungs.

A critical look at the function of the P2Y11 receptor

The P2Y11 receptor is a member of the purinergic receptor family. It has been overlooked, somewhat due to the lack of a P2ry11 gene orthologue in the murine genome, which prevents the generation of knockout mice, which have been so helpful for defining the roles of other P2Y receptors. Furthermore, some of the studies reported to date have methodological shortcomings, making it difficult to determine the function of P2Y11 with certainty. In this review, we discuss the lack of a murine "P2Y11-like receptor" and highlight the limitations of the currently available methods used to investigate the P2Y11 receptor. These methods include protein recognition with antibodies that show very little specificity, gene expression studies that completely overlook the existence of a fusion transcript between the adjacent PPAN gene and P2RY11, and agonists/antagonists reported to be specific for the P2Y11 receptor but which have not been tested for activity on numerous other adenosine 5'-triphosphate (ATP)-binding receptors. We suggest a set of criteria for evaluating whether a dataset describes effects mediated by the P2Y11 receptor. Following these criteria, we conclude that the current evidence suggests a role for P2Y11 in immune activation with cell type-specific effects.

Constitutive and functional expression of runt-related transcription factor-2 by microglial cells

Runt-related transcription factor-2 (Runx2) is the master regulator of osteoblastogenesis with an ability to promote differentiation of mesenchymal stem cells into the osteoblastic lineage. We have previously shown constitutive and functional expression of Runx2 by astroglial cells. In this study, we investigated the possible expression of Runx2 by both murine microglia and microglial cell line BV-2 cells. Runx2 expression was seen in cultured microglia and BV-2 cells, while sustained exposure to 1mM ATP led to a significant but transient increase in mRNA and corresponding protein expression of Runx2 within 24 h. The increase in Runx2 expression was invariably prevented by several chemicals with antagonistic properties for P2X7 purinergic receptor, calmodulin and calcineurin in BV-2 cells, with a P2X7 receptor agonist more than quadrupling Runx2 expression. A significant increase in Runx2 expression was seen in osteoclastic cells, but not in osteoblastic or chondrocytic cells, when exposed to a high concentration of ATP. In BV2-cells with control siRNA, a significant decrease was found in the number of cells with at least one process within 3 h after the exposure to 1mM ATP, followed by an increase up to 24 h. However, Runx2 siRNA significantly deteriorated the property to induce delayed process extension during 6-24 h after exposure to ATP along with drastically decreased Runx2 protein levels. These results suggest that Runx2 is constitutively and functionally expressed by microglial cells with responsiveness to ATP for upregulation in the murine brain.

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.

ATP in the locus coeruleus as a modulator of cardiorespiratory control in unanaesthetized male rats

Locus coeruleus (LC) noradrenergic neurons are chemosensitive to CO2 and pH in mammals and amphibians and are involved in the CO2-related drive to breathe. Purinergic neuromodulation in the LC is of particular interest because ATP acts as a neuromodulator in brainstem regions involved in cardiovascular and respiratory regulation, such as the LC. ATP acting on LC P2 receptors influences the release of noradrenaline. Thus, the goal of the present study was to investigate the role of LC purinergic neuromodulation of ventilatory and cardiovascular responses in normocapnic and hypercapnic conditions in unanaesthetized male Wistar rats. We assessed the purinergic modulation of cardiorespiratory systems by microinjecting an ATP P2X receptor agonist [α,β-methylene ATP (α,β-meATP), 0.5 or 1 nmol in 40 nl] and two non-selective P2 receptor antagonists [pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), 0.5 or 1 nmol in 40 nl; and suramin, 1 nmol in 40 nl] into the LC. Pulmonary ventilation (measured by plethysmography), mean arterial pressure (MAP) and heart rate (HR) were determined before and after unilateral microinjection (40 nl) of α,β-meATP, PPADS, suramin or 0.9% saline (vehicle) into the LC. These measurements were made during a 60 min exposure to normocapnic conditions or a 30 min exposure to 7% CO2. Subsequently, animals undergoing pharmacological treatment were subjected to a 30 min exposure to normocapnic conditions as a recovery period. In normocapnic conditions, α,β-meATP did not affect any parameter, whereas PPADS decreased respiratory frequency and increased MAP and HR. Suramin increased MAP and HR but did not change ventilation. Moreover, hypercapnic conditions induced an increase in ventilation and a decrease in HR in all groups. In hypercapnic conditions, α,β-meATP increased ventilation but did not change cardiovascular parameters, whereas PPADS increased MAP but did not alter ventilation, and suramin increased both ventilation and MAP. Thus, our data suggest that purinergic signalling, specifically through P2 receptors, in the LC plays an important role in cardiorespiratory control in normocapnic and hypercapnic conditions in unanaesthetized rats.

Hyposmotic stress causes ATP release and stimulates Na,K-ATPase activity in porcine lens

Purinergic receptors in lens epithelium suggest lens function can be altered by chemical signals from aqueous humor or the lens itself. Here we show release of ATP by intact porcine lenses exposed to hyposmotic solution (200 mOsm). 18α-glycyrrhetinic acid (AGA) added together with probenecid eliminated the ATP increase. N-ethylmaleimide (200 µM), an exocytotic inhibitor, had no significant effect on ATP increase. Lenses exposed to hyposmotic solution displayed a ~400% increase of propidium iodide (PI) entry into the epithelium. The increased ability of PI (MW 668) to enter the epithelium suggests possible opening of connexin and/or pannexin hemichannels. This is consistent with detection of connexin 43, connexin 50, and pannexin 1 in the epithelium and the ability of AGA + probenecid to prevent ATP release. Na,K-ATPase activity doubled in the epithelium of lenses exposed to hyposmotic solution. The increase of Na,K-ATPase activity did not occur when apyrase was used to prevent extracellular ATP accumulation or when AGA + probenecid prevented ATP release. The increase of Na,K-ATPase activity was inhibited by the purinergic P2 antagonist reactive blue-2 and pertussis toxin, a G-protein inhibitor, but not by the P2X antagonist PPADS. Hyposmotic solution activated Src family kinase (SFK) in the epithelium, judged by Western blot. The SFK inhibitor PP2 abolished both SFK activation and the Na,K-ATPase activity increase. In summary, hyposmotic shock-induced ATP release is sufficient to activate a purinergic receptor- and SFK-dependent mechanism that stimulates Na,K-ATPase activity. The responses might signify an autoregulatory loop initiated by mechanical stress or osmotic swelling.

Negative-feedback regulation of ATP release: ATP release from cardiomyocytes is strictly regulated during ischemia

Extracellular ATP acts as a potent agonist on cardiomyocytes, inducing a broad range of physiological responses via P2 purinoceptors. Its concentration in the interstitial space within the heart is elevated during ischemia or hypoxia due to its release from a number of cell types, including cardiomyocytes. However, the exact mechanism responsible for the release of ATP from cardiomyocytes during ischemia is not known. In this study, we investigated whether and how the release of ATP was strictly regulated during ischemia in cultured neonatal rat cardiomyocytes. Ischemia was mimicked by oxygen-glucose deprivation (OGD). Exposure of cardiomyocytes to OGD resulted in an increase in the concentration of extracellular ATP shortly after the onset of OGD (15 min), and the increase was reversed by treatment with blockers of maxi-anion channels. Unexpectedly, at 1 and 2h after the onset of OGD, the blocking of maxi-anion channels increased the concentration of extracellular ATP, and the increase was significantly suppressed by co-treatment with blockers of hemichannels, suggesting that ATP release via maxi-anion channels was involved in the suppression of ATP release via hemichannels during persistent OGD. Here we show the possibility that the release of ATP from cardiomyocytes was strictly regulated during ischemia by negative-feedback mechanisms; that is, maxi-anion channel-derived ATP-induced suppression of ATP release via hemichannels in cardiomyocytes.

Pannexin 3 functions as an ER Ca(2+) channel, hemichannel, and gap junction to promote osteoblast differentiation

Pannexin 3 functions as an essential protein for Ca²⁺ and ATP transport and cell–cell communication during osteoblast differentiation

The pannexin proteins represent a new gap junction family. However, the cellular functions of pannexins remain largely unknown. Here, we demonstrate that pannexin 3 (Panx3) promotes differentiation of osteoblasts and ex vivo growth of metatarsals. Panx3 expression was induced during osteogenic differentiation of C2C12 cells and primary calvarial cells, and suppression of this endogenous expression inhibited differentiation. Panx3 functioned as a unique Ca²⁺ channel in the endoplasmic reticulum (ER), which was activated by purinergic receptor/phosphoinositide 3-kinase (PI3K)/Akt signaling, followed by activation of calmodulin signaling for differentiation. Panx3 also formed hemichannels that allowed release of ATP into the extracellular space and activation of purinergic receptors with the subsequent activation of PI3K–Akt signaling. Panx3 also formed gap junctions and propagated Ca²⁺ waves between cells. Blocking the Panx3 Ca²⁺ channel and gap junction activities inhibited osteoblast differentiation. Thus, Panx3 appears to be a new regulator that promotes osteoblast differentiation by functioning as an ER Ca²⁺ channel and a hemichannel, and by forming gap junctions.

The birth and postnatal development of purinergic signalling

The purinergic signalling system is one of the most ancient and arguably the most widespread intercellular signalling system in living tissues. In this review we present a detailed account of the early developments and current status of purinergic signalling. We summarize the current knowledge on purinoceptors, their distribution and role in signal transduction in various tissues in physiological and pathophysiological conditions.

P2 purinoceptors: historical perspective and classification

This article presents an overview that gives some historical perspective to the detailed papers at the cutting edge of P2 purinoceptor research that follow. I consider the proposal, first put forward by Abbracchio & Burnstock (Pharmacol Ther 64:445-475, 1994), that P2 purinoceptors should be regarded as members of two main families: a P2X purinoceptor family consisting of ligand-gated ion channels, and a P2Y purinoceptor family consisting of G protein-coupled receptors. The latest subclasses of these two families (P2X1-4 and P2Y1-5), identified largely on the basis of molecular cloning and expression, are tabled. Finally, I suggest some future directions for P2 purinoceptor research, including studies of the long-term (trophic) actions of purines, the evolution and development of purinoceptors and therapeutic applications.

Contractile activity of ATP and diadenosine tetraphosphate on urinary bladder in the rat: role of A1- and P2X-purinoceptors and nitric oxide

1. Both adenosine-5'-triphosphate (ATP) and diadenosine tetraphosphate (AP4A) produced a dose-dependent contraction of the isolated rat urinary bladder rings. AP(4)A dose-response curve was to the left of that of ATP, and maximum response was greater than that produced by ATP. 2. 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), the A1-purinergic receptor blocker (0.01 mm) significantly inhibited the ATP- and AP4A-induced contractions at the whole dose range. The inhibition was between 31-41%, and 15-25% for ATP and AP4A respectively. 3. Pyridoxal phosphate 6-azophenyl-2',4'-disulphonic acid (PPADS), the P2X-purinoceptor antagonist (0.01 mm) potently inhibited the bladder contractions in response to ATP and AP4A by around 75-80%. 4. The nitric oxide (NO) precursor L-arginine reduced the bladder contractile response to ATP by about 22-41% and that of AP4A to a lesser extent by around 20-32%. 5. The nitric oxide synthase inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME, 0.1 mM), did not produce any significant effect on ATP except for a weak inhibition of about 14% at the lowest dose of ATP. The contractions in response to AP4A were only slightly reduced by L-NAME by about 20%. 6. In conclusion, the contractile response of the bladder to ATP and to the dinucleotide AP4A is mediated mainly through P2X-purinoceptors and A1-purinergic receptors. In the detrusor muscle, NO donation possesses an inhibitory effect on ATP-mediated contractility more than that produced by the dinucleotide AP4A.

Neurally released ATP mediates endothelium-dependent hyperpolarization in the circular smooth muscle cells of chicken anterior mesenteric artery

The object of the present study was to clarify the neurotransmitter(s) controlling membrane responses to electrical field stimulation (EFS) in the circular smooth muscle cells of first-order branches of chicken anterior mesenteric artery.EFS (five pulses at 20 Hz, 1 ms) evoked a hyperpolarization of amplitude--21.6+/-1.2 mV, total duration 21.8+/-1.2 s and latency 641.7+/-81.9 ms. The response was tetrodotoxin-sensitive and nonadrenergic noncholinergic (NANC) in nature. The NANC response was blocked by the nonspecific purinergic antagonist, suramin, indicating that the response is mediated by the neurotransmitter adenosine 5'-triphosphate (ATP). Either desensitization or blockade of P2Y receptor with its putative agonist 2-methylthioATP (1 microM for 30 min) or with its antagonist cibacron blue F3GA (10 microM), respectively, abolished the purinergic hyperpolarization. PPADS at concentrations up to 100 microM had no effect on the EFS-induced response, indicating that this response is mediated through P2Y, but not P2X, receptor. In addition, the response was completely abolished by two specific P2Y1 receptor antagonists, namely, MRS 2179 (300 nM) and A3P5PS (10 microM). Removal of the endothelium abolished the purinergic hyperpolarization, which was converted, in some preparations, to a small depolarization, indicating that the hyperpolarizing response is endothelium-dependent. The present study suggests that in first-order branches of chicken anterior mesenteric artery, ATP released from perivascular nerves may diffuse to the endothelium-activating P2Y1 receptor to induce release of an inhibitory substance that mediates hyperpolarization in the circular smooth muscle.

Term-dependency of P2 receptor-mediated contractile responses of isolated human pregnant uterus

OBJECTIVE

The aim of the study was to test the hypothesis that in the human uterus, the effectiveness of P2 receptor-mediated contractile responses is up-regulated during pregnancy.

STUDY DESIGN

Experiments were performed on myometrial samples obtained from women undergoing caesarean section at 28-30 weeks of pregnancy (3 women, Group 1), 32-34 weeks of pregnancy (6 women, Group 2) and 38-41 weeks of pregnancy (16 women, Group 3). Concentration-response relationships for a non-selective P2 receptor agonist, adenosine 5'-triphosphate (ATP), a selective P2X receptor agonist, alpha,beta-methylene-ATP (alpha,beta-meATP), and a frequency-response relationship for non-adrenergic non-cholinergic (NANC) electrical field stimulation (EFS) were obtained using routine pharmacological organ bath technique. Effects of pyridoxalphosphate azophenyl-2',4'-disulphonic acid (PPADS, 10(-5) M), a P2 receptor antagonist, were also evaluated. Parametric Student's t-test, non-parametric Wilcoxon T-test, Mann-Whitney U-test, two-way analysis of variance (ANOVA) and Krushkal-Wallis tests were used for statistical analysis.

RESULTS

ATP (10(-6) to 3 x 10(-4) M), alpha,beta-meATP (10(-7) to 3 x 10(-5) M) and EFS (2-32 Hz) evoked contractions of isolated pregnant uterus in all three groups. Uterus responses to ATP were not correlated with the term of pregnancy while the amplitude of uterine contractions to alpha,beta-meATP and EFS was higher in full term pregnancy than in earlier pregnancy. PPADS antagonized uterus responses to alpha,beta-meATP and EFS, but not to ATP, in all three groups.

CONCLUSION

P2X receptor-mediated contractions of human pregnant uterus to alpha,beta-meATP and EFS, but not to ATP, are increased with the progression of pregnancy.

Potentiation of uterine effects of prostaglandin F2{alpha} by adenosine 5'-triphosphate

OBJECTIVE

To investigate the interaction of exogenous adenosine 5'-triphosphate (ATP), a P2 receptor agonist, with prostaglandin F(2alpha) (PGF(2alpha)) on pregnant women in labor as well as on isolated human pregnant uterus preparations.

METHODS

For an in vitro study, myometrial samples were obtained from 27 women undergoing elective cesarean delivery at term. Concentration-response relationships for ATP (10(-8) -3 x 10(-4) mol/L), PGF(2alpha) (10(-9) -10(-5) mol/L), and their combination were obtained by using routine pharmacological organ bath technique. An in vivo study was performed with 34 pregnant women with dysfunctional abnormalities of the active stage of labor who were randomly allocated into 2 study groups. The women in the control group (18 patients) received intravenous prostaglandin F(2alpha) at an initial rate of 7.5 mug/min, whereas the women in the ATP group (16 patients) received prostaglandin F(2alpha) concomitantly with ATP (0.45 nmol/min, intravenously).

RESULTS

Adenosine 5'-triphosphate at concentrations of 10(-6) -3 x 10(-4) mol/L and PGF(2alpha) at concentrations of 10(-8) -10(-5) mol/L caused concentration-dependent contractions of isolated smooth muscle preparations of the human pregnant uterus. At concentrations of 10(-6) mol/L and below, ATP had no effects on mechanical activity of the isolated uterus, but at concentrations of 10(-7) mol/L and 10(-6) mol/L, it significantly potentiated the contractile responses of the uterus induced by PGF(2alpha) (P < .05, 2-way analysis of variance). Patients receiving intravenous infusion of ATP as a supplement to PGF(2alpha) treatment, compared with those without ATP, had a significantly shorter interval from the start of the treatment to full cervical dilatation (3.31 +/- 1.49 hours and 4.67 +/- 1.11 hours in ATP and control groups, respectively; P = .014, Wilcoxon Mann-Whitney test). The total dose of prostaglandin received was significantly lower in the ATP group than that of controls (1,489.8 +/- 699.9 mug and 3,394.2 +/- 1,951.9 mug, respectively; P = .003, Wilcoxon Mann-Whitney test). No side effects of ATP treatment were observed during or after infusion.

CONCLUSION

Adenosine 5'-triphosphate potentiates effects of PGF(2alpha) on pregnant human uterus in vitro and in vivo and thus could be a useful supplemental drug to increase uterine contractility at labor.

The influence of hypothermia on P2 receptor-mediated responses of frog skeletal muscle

The contractile responses of isolated Rana ridibunda frog sartorius muscle contractions evoked by electrical field stimulation (EFS) were studied at three temperature conditions of 17, 22 and 27 degrees C. Temperature-dependent increase of muscle contractility was found. ATP (10-100 microM) concentration dependently inhibited the electrical field stimulation-evoked contractions of sartorius muscle at all three temperatures; this effect was significantly more prominent at a temperature of 17 degrees C than at other two temperatures. Adenosine (100 microM) also caused inhibition of electrical field stimulation-evoked contractions which was statistically identical at all three temperature conditions tested. A P2 receptor antagonist, pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 10 microM) reduced the inhibitory effect of ATP at all three temperatures but did not affect inhibitory action of adenosine. In contrast, 8-(p-sulfophenyl)theophylline (8-SPT, 100 microM), a nonselective P1 receptor antagonist, abolished inhibitory effects of adenosine at all three temperature conditions but did not antagonize inhibition caused by ATP. In electrophysiological experiments, ATP (100 microM) and adenosine (100 microM) temperature dependently reduced end-plate currents recorded in sartorius neuromuscular junction by voltage-clamp technique. The inhibitory effects of both agonists were enhanced with the decrease of temperature. 8-SPT (100 microM) abolished the inhibitory effect of adenosine but not ATP on end-plate currents. Suramin (100 microM), a nonselective P2 receptor antagonist, inhibited the action of ATP but not adenosine, while PPADS (10 microM) had no influence on the effects of either ATP or adenosine. It is concluded from this study that the effectiveness of P2 receptor-mediated inhibition of frog skeletal muscle contraction in contrast to that of adenosine is dependent on the temperature conditions.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Temperature dependency of P2 receptor-mediated responses

The P2 receptor-mediated responses of isolated guinea pig urinary bladder and vas deferens (P2X receptors) and taenia caeci (P2Y receptors) were registered at the three temperature conditions of 30, 37 and 42 degrees C. The contractile responses of both urinary bladder and vas deferens to a P2X receptor agonist alpha,beta-methylene ATP (alpha,beta-meATP; 0.01-30 microM) and to electrical field stimulation (1-64 Hz, 0.1 ms, supramaximal voltage) in the presence of atropine (0.1 microM) and phentolamine (1 microM) were markedly more prominent at a temperature of 30 degrees C than at 37 or 42 degrees C. Similarly, relaxation of carbachol-precontracted taenia caeci caused by electrical field stimulation (0.5-8 Hz, 0.1 ms, supramaximal voltage) temperature-dependently increased with decrease of temperature, while relaxation of this tissue by exogenous ATP (1-100 microM) was not affected by the temperature. A P2 receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 1-30 microM) at all three temperature conditions concentration-dependently antagonised contractile responses to alpha,beta-methylene ATP and electrical field stimulation in both urinary bladder and vas deferens. PPADS, even at the highest concentration tested (30 microM), had no effect on the relaxant responses of the taenia caeci either to electrical field stimulation or ATP and its action was not affected by the change of temperature. It is concluded from this study that the effectiveness of P2 receptor-mediated responses in guinea pig urinary bladder, vas deferens and taenia caeci increases by decrease of temperature.

Structure-activity relationships of diadenosine polyphosphates (Ap(n)As), adenosine polyphospho guanosines (Ap(n)Gs) and guanosine polyphospho guanosines (Gp(n)Gs) at P2 receptors in the rat mesenteric arterial bed

1. Vascular effects of diadenosine polyphosphates (Ap(n)As), adenosine polyphospho guanosines (Ap(n)Gs) and guanosine polyphospho guanosines (Gp(n)Gs), novel families of naturally-occurring signalling molecules, were investigated in methoxamine preconstricted rat isolated perfused mesenteric arterial beds. 2. Three different types of response were elicited by Ap(n)As and Ap(n)Gs. Those with a short polyphosphate chain (n=2 - 3) elicited vasorelaxation. Ap(3)A was more potent than Ap(2)A, and both were more potent than the corresponding Ap(n)G. Relaxations to Ap(3)A and Ap(3)G, but not to Ap(2)A and Ap(2)G, were blocked by endothelium removal and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), a P2 receptor antagonist. 3. Longer polyphosphate chain Ap(n)As and Ap(n)Gs (n=4 - 6) elicited dose-dependent vasoconstriction followed by prolonged vasorelaxation, with a potency order for both types of response of Ap(5)A> or =Ap(6)A>Ap(4)A. A similar order and potency was observed for Ap(n)Gs. Contractions and prolonged relaxations were blocked by PPADS and P2X(1) receptor desensitization with alpha,beta-methylene ATP (alpha,beta-meATP), and were largely endothelium-independent. 4. In the presence of alpha,beta-meATP rapid relaxations to contractile Ap(n)As and Ap(n)Gs (n=4 - 6) were revealed. 5. Gp(n)Gs were virtually inactive, except for Gp(2)G which elicited vasoconstriction via PPADS- and alpha,beta-meATP-sensitive smooth muscle P2X(1)-like receptors. 6. These data show that, as with Ap(n)As, the length of the polyphosphate chain (n) is an important determinant of the activity of Ap(n)Gs at P2 receptors in the rat mesenteric arterial bed. When the chain is short (n=2 - 3) the purines elicit rapid vasorelaxation, which for Ap(3)A and Ap(3)G is mediated via endothelial P2Y(1)-like receptors. When the chain is long (n=4 - 6) Ap(n)As and Ap(n)Gs elicit vasoconstriction via P2X(1)-like receptors, followed by prolonged endothelium-independent vasorelaxation. Rapid relaxation to contractile dinucleotides (n=4 - 6) is revealed by block of vasoconstriction. Regarding the purine moiety, one adenine is crucial and sufficient for vasoactivity as Gp(n)Gs were largely inactive, and Ap(n)As and Ap(n)Gs approximately equipotent.

Actions of a Series of PPADS Analogs at P2X1 and P2X3 Receptors

[Table: see text] Seven PPADS (Pyridoxal-5'-Phosphate 6-Azophenyl 2',4'-DiSulfonate) analogs were investigated at Group 1 P2X receptors expressed in Xenopus oocytes. All seven analogs potently inhibited P2X1 (IC50 range, 5-32 nM) and P2X3 (IC50 range, 22-345 nM), the two Group I P2X receptor subtypes. Analogs showed greater inhibitory activity where the pyridoxal moiety of PPADS contained a 5'-phosphonate group, rather than a 5'-phosphate group. Analogs also showed greater potency where disulfonate groups were removed from, or substituted at, the azophenyl moiety. The most active analog was MRS 2257 (pyridoxal-5'-phosphonate 6-azophenyl 3',5'-bismethylenephosphonate) at P2X1 (IC50, 5 nM) and P2X3 (IC50, 22 nM) receptors, being 14-fold and 10-fold more potent than PPADS itself. MRS 2257 produced a nonsurmountable inhibition when tested against a range of ATP concentrations, although blockade was reversed by about 85% after 20 minutes of washout. TNP-ATP and Ip5I were equipotent with MRS 2257 at P2X1 receptors, whereas TNP-ATP was 64-fold more potent than MRS 2257 at P2X3 receptors. In conclusion, the PPADS template can be altered at the pyridoxal and phenyl moieties to produce P2X1 and P2X3 receptor antagonists showing higher potency and greater degree of reversibility than the parent compound at these Group I P2X receptors.

Glucose deprivation and chemical hypoxia: neuroprotection by P2 receptor antagonists

In this work we investigate cell survival after glucose deprivation and/or chemical hypoxia and we analyse the neuroprotective properties of selected antagonists of P2 ATP receptors. We find that in rat cerebellar granule neurones, the antagonist basilen blue prevents neuronal death under hypoglycaemia. Basilen blue acts through a wide temporal range and it retains its efficacy under chemically induced hypoxic conditions, in the presence of the respiratory inhibitors of mitochondria electron transport chain complexes II (3-nitropropionic acid) and III (antimycin A). In spite of the presence of these compounds, basilen blue maintains normal intracellular ATP levels. It furthermore prevents neuronal death caused by agents blocking the mitochondrial calcium uptake (ruthenium red) or discharging the mitochondrial membrane potential (carbonyl cyanide m-chlorophenylhydrazone). Inhibition of poly (ADP-ribose) polymerase, modulation of the enzyme GAPDH and mitochondrial transport of mono-carboxylic acids are not conceivable targets for the action of basilen blue. Survival is sustained by basilen blue also in CNS primary cultures from hippocampus and in PNS sympathetic-like neurones. Partial neuroprotection is furthermore provided by three additional P2 receptor antagonists: suramin, pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid 4-sodium and 4,4'-diisothiocyanatostilbene-2,2'disulphonic acid. Our data suggest the exploitation of selected P2 receptor antagonists as potential neuroprotective agents.

Structure-activity relationships of pyridoxal phosphate derivatives as potent and selective antagonists of P2X1 receptors

Novel analogues of the P2 receptor antagonist pyridoxal-5'-phosphate 6-azophenyl-2',5'-disulfonate (2) were synthesized and studied as antagonists in functional assays at recombinant rat P2X1, P2X2, and P2X3 receptors expressed in Xenopus oocytes (ion flux stimulation) and at turkey erythrocyte P2Y1 receptors (phospholipase C activation). Selected compounds were also evaluated as antagonists of ion flux and the opening of a large pore at the recombinant human P2X7 receptor. Modifications were made in the 4-aldehyde and 5'-phosphate groups of the pyridoxal moiety: i.e. a CH2OH group at the 4-position in pyridoxine was either condensed as a cyclic phosphate or phosphorylated separately to form a bisphosphate, which reduced potency at P2 receptors. 5-Methylphosphonate substitution, anticipated to increase stability to hydrolysis, preserved P2 receptor potency. At the 6-position, halo, carboxylate, sulfonate, and phosphonate variations made on the phenylazo ring modulated potency at P2 receptors. The p-carboxyphenylazo analogue, 4, of phosphate 2 displayed an IC50 value of 9 nM at recombinant P2X1 receptors and was 1300-, 16-, and > 10,000-fold selective for P2X1 versus P2X2, P2X3, and P2Y1 subtypes, respectively. The corresponding 5-methylphosphonate was equipotent at P2X1 receptors. The 5-methylphosphonate analogue containing a 6-[3,5-bis(methylphosphonate)]phenylazo moiety, 9, had IC50 values of 11 and 25 nM at recombinant P2X1 and P2X3 receptors, respectively. The analogue containing a phenylazo 4-phosphonate group, 11, was also very potent at both P2X1 and P2X3 receptors. However, the corresponding 2,5-disulfonate analogue, 10, was 28-fold selective for P2X1 versus P2X3 receptors. None of the analogues were more potent at P2X7 and P2Y1 receptors than 2, which acted in the micromolar range at these two subtypes.

Effect of diadenosine tetraphosphate (AP4A) on coronary arterial microvessels in the beating canine heart

Diadenosine tetraphosphate (AP4A) can be released from activated platelets and the present study examined its effect on coronary arterial microvessels. The role of purinoceptors in the coronary microcirculation in vivo was also investigated. In open chest dogs, coronary arterioles were observed using a microscope with a floating objective. In Protocol 1, AP4A (1, 10, 100 and 1,000 micromol/L) was superfused onto the heart surface before and during the superfusion of 10 micromol/L of 8-phenyltheophylline (8-PT), a P1 purinoceptor blocker. In Protocol 2, AP4A (0.1, 1, 10, and 100 nmol x kg(-1) x min(-1)) was infused into the left anterior descending coronary artery before and during the superfusion of 10 micromol/L of 8-PT. In addition to 8-PT, 30 micromol/L of pyridoxalphosphate-6-azophenyl 2',4'-disulphonic acid (PPADS), a P2X purinoceptor blocker in Protocol 3, or 300 micromol/L of N(omega)-nitro-L-arginine (LNNA) in Protocol 4, was continuously superfused, and 4 doses of AP4A were cumulatively superfused as in Protocol 1. In Protocol 5, 10 micromol/L of alpha,beta-methylene ATP, an agonist of P2X purinoceptors, was superfused for 60 min. Superfused AP4A dilated arterioles in a dose-dependent manner. The magnitude of dilatation was greater in smaller arterioles (small vessel < or = 150 microm: 24.5+/-2.2% vs large vessel > 150 microm: 10.6+/-1.5% at a dose of 1,000 micromol/L, p<0.001). On the other hand, intraluminally applied AP4A also dilated arterioles, but no size dependency was shown. In the presence of 8-PT, vasodilatory responses to superfused and intraluminally applied AP4A were attenuated and the lower doses of AP4A constricted arterioles. This vasoconstrictor effect was not affected by PPADS. The vasodilatory effect of the higher doses of AP4A was almost abolished in the presence of LNNA. Alpha,beta-methylene ATP had no effect on coronary microvascular diameters. AP4A has bidirectional effects on coronary arterial microvessels: vasodilatory effects mediated by P1 purinoceptors and NO, which might be mediated by P2Y purinoceptors, and a vasoconstrictor effect, which is not mediated by P2X purinoceptors.

Mechanisms of alpha,beta-methylene atp-induced inhibition in rat ileal smooth muscle: involvement of intracellular Ca2+ stores in purinergic inhibition

1. In order to investigate purinergic effects on rat ileal smooth muscle, we used alpha,beta-methylene ATP (alpha,beta-MeATP), ATP, ADP and UTP. alpha,beta-Methylene ATP and ATP were the only agonists that caused a concentration-dependent inhibition of carbachol-precontracted smooth muscle. The inhibitory effect of alpha,beta-MeATP was completely blocked by pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (3 x 10(-5) mol/L), a selective antagonist of the P2X > > P2Y receptor. 2. Using reverse transcription-polymerase chain reaction we demonstrated the presence of both, P2X and P2Y receptor mRNA within the rat ileal longitudinal muscle/myenteric plexus layer preparation. 3. The alpha,beta-MeATP-induced inhibition was blocked in a concentration-dependent manner in the presence of the K+ channel blocker apamin, but was unaffected by other K+ channel blockers, such as charybdotoxin (10(-7) mol/L), 4-aminopyridine (10(-4)mol/L), glibenclamide (10(-5) mol/L) and tetraethylammonium (10(-3) mol/L). 4. The alpha,beta-MeATP-induced inhibition was unaffected by pretreatment with atropine (10(-6) mol/L), phentolamine (10(-6) mol/L), propranolol (10(-6) mol/L), nitrendipine (10(-7) mol/L), pertussis toxin (10(-6) mol/L) NG-nitro-L-arginine (3 x 10(-4) mol/L) and tetrodotoxin (10(-6) mol/L), excluding an involvement of adrenergic, cholinergic, neural, nitrinergic or G-protein involvement in purinergic-mediated inhibition. 5. In order to investigate whether the internal Ca2+ stores participated in the inhibitory effect observed, we depleted internal Ca2+ stores with cyclopiazonic acid, a specific Ca2+-ATPase inhibitor. The inhibitory effect of alpha,beta-MeATP was completely abolished after depletion of the intracellular Ca2+ stores. 6. This is in contrast with the effects seen for neurotensin, where neurotensin-induced inhibition was unchanged after depletion of intracellular Ca2+ stores, suggesting at least two different pathways of apamin-sensitive non-adrenergic, non-cholinergic inhibition in rat ileal smooth muscle. 7. According to our results, the inhibitory effect of alpha,beta-MeATP in rat ileum longitudinal smooth muscle is mediated via a P2 purinoceptor, most likely a P2X receptor, involves G-protein-independent activation of an apamin-sensitive K+ channel and requires filled intracellular Ca+ stores.

Recombinant P2Y receptors: the UCL experience

The beginning of the last decade heralded three important and sequential developments in our understanding of cell-to-cell signalling by extracellular ATP via its cell surface receptors, the P2 purinoceptors. One major development in ATP signalling culminated in a timely review in 1991, when it was established in the clearest of terms that ATP receptors exploited discrete signal transduction pathways (Dubyak, G.R., 1991. Signal transduction by P2-purinergic receptors for extracellular ATP. Am. J. Respir. Cell. Mol. Biol. 4, 295-300; and later in Dubyak, G.R., El-Moatassim, C., 1993. Signal transduction via P2-purinergic receptors for extracellular ATP and other nucleotides. Am. J. Physiol. 265, C577-C606). Henceforth, it was universally acknowledged that some P2 purinoceptors interacted with heterotrimeric G-proteins to activate intracellular signalling cascades (metabotropic ATP receptors), whereas others contained intrinsic ion-channels (ionotropic ATP receptors). A second key development can be traced to 1992, from the discovery that ATP receptors were involved in excitatory neurotransmission in the CNS and PNS (Edwards, F.A., Gibb, A.J., Colquhoun, D., 1992. ATP receptor-mediated synaptic currents in the central nervous system. Nature 359, 144-147; Evans, R.J., Derkach, V., Surprenant, A., 1992. ATP mediates fast synaptic transmission in mammalian neurons. Nature 357, 503-505; Silinsky, E.M., Gerzanich, V., Vanner, S.M., 1992. ATP mediates excitatory synaptic transmission in mammalian neurones. Br. J. Pharmacol., 106, 762-763). Thereafter, it was accepted that ATP could play a neurotransmitter and/or modulatory role throughout the entire nervous system. The third key development stemmed from the isolation of a cDNA, from chick brain, encoding a metabotropic ATP receptor (Webb, T.E., Simon, J., Krishek, B.J., Bateson, A.N., Smart, T.G., King, B.F., Burnstock, G., Barnard, E.A., 1993. Cloning and functional expression of a brain G-protein-coupled ATP receptor. FEBS Lett. 324, 219-225). The cloning of a membrane protein serving as an ATP receptor ignited a widespread international interest in purinergic signalling. Investigators at University College London (UCL) - colleagues and associates of Geoffrey Burnstock - were at the forefront of this rapid phase of discovery. In this review, we highlight the UCL experience when the fields of molecular biology, physiology and cell biology converged to help advance our understanding of ATP as an extracellular signalling molecule.

Neuroprotective effects of modulators of P2 receptors in primary culture of CNS neurones

In previous studies (Volonté and Merlo, 1996. J. Neurosci. Res. 45, 183-193) basilen blue was shown to be a P2 receptor antagonist which abrogated glutamate-mediated cytotoxicity in cerebellar neurones in primary culture. Our work has now been extended to evaluate the neuroprotective action of the compound in additional neuronal systems, as well as in a different paradigm of cell death. We show that basilen blue prevents L-glutamate-mediated neurotoxicity in rat cerebellar (90-100% inhibition), cortical (60-70%) and hippocampal (50%) neurones. Similarly, glutamate-dependent progressive darkening of cell bodies, loss of phase-brightness and rapid cellular swelling are inhibited. Basilen blue is significantly less toxic and more effective at blocking L-glutamate toxicity in mixed cortical/glial cultures, compared to its structural analogue cibacron blue. Moreover, its neuroprotective effect is correlated with the time of incubation with granule neurones. Other purinoceptor ligands, including 2,2'-pyridylisatogen, but not pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid 4-sodium, are also effective in preventing glutamate toxicity. Furthermore, basilen blue prevents serum deprivation- and low potassium-induced apoptotic cell death in cerebellar granule neurones. In summary, our data extend and reinforce the possibility of a potential therapeutic use of P2 receptor modulators as neuroprotective agents for the central nervous system.

ATP is involved in myocardial and vascular effects of exogenous bradykinin in ejecting guinea pig heart

It has recently been reported that bradykinin induces selective left ventricular (LV) relaxation in isolated guinea pig hearts via the release of nitric oxide. Exogenous bradykinin also induces vasodilation, which is only partly due to nitric oxide release. In the present study we investigated the role of adenyl purines on these bradykinin-induced effects. Isolated ejecting guinea pig hearts were studied. LV pressure was monitored by a 2-Fr micromanometer-tipped catheter. ATP concentrations were measured using a luciferin-luciferase assay. Bradykinin (1 and 100 nM) caused a progressive acceleration of LV relaxation together with a transient increase in coronary flow. These effects were inhibited by the nonselective P(2) purinoceptor antagonist suramin (1 microM, n = 6) but were unaffected by the selective P(2x) purinoceptor antagonist pyridoxal phosphate 6-azophenyl-2',4'-disulfonic acid (1 microM, n = 6). These myocardial and vascular effects of bradykinin were associated with increased ATP levels in coronary effluent. These data suggest that the selective enhancement of LV relaxation and rise in coronary flow induced by exogenous bradykinin involve endogenous ATP and the subsequent stimulation of P(2) purinoceptors.

Frequency characteristics of blood pressure oscillations evoked by sympathetic transmitters, noradrenaline and adenosine triphosphate

Mean arterial pressure (MAP) was recorded beat-to-beat in chronically instrumented, conscious, unrestrained rats under control conditions and after pharmacological inhibition of vascular sympathetic influences by means of: (1) ganglion blockade with chlorisondamine; (2) alpha-adrenoceptor antagonist phentolamine; (3) P2 receptor blockade with pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS). Angiotensin II was continuously infused to prevent drastic MAP decrease during chlorisondamine and phentolamine administration. Overall MAP variability increased after ganglion blockade and combined blockade of adreno- and purinoceptors. It increased also after inhibition of purinergic influences, but was not significantly changed after vascular adrenergic blockade. Spectral analysis of spontaneous MAP fluctuations in intact rats revealed a peak centered at 0.4-0.5 Hz. Ganglion blockade suppressed MAP fluctuations with frequencies from 0.1 to 0.8 Hz. After blockade of alpha-adrenoceptors, MAP spectral density was suppressed only within the 0.1-0.45-Hz band, but increased in the 0.45-0.8-Hz band. In the latter case, sympathetically-induced peak of MAP spectrum was centered on 0.6 Hz, being evoked, presumably, by adenosine triphosphate (ATP). Blockade of P2 receptors by PPADS enhanced MAP fluctuations in the 0.1-0.45-Hz frequency band, i.e. the noradrenaline-induced peak was centered on 0.2 Hz. No peaks were observed in the 0.1-0.8-Hz frequency band during combined blockade of adreno- and purinoceptors. The present study supports the concept that sympathetic purinergic co-transmission is essential for stabilization of MAP level. MAP fluctuations evoked by noradrenaline and ATP can be distinguished by their frequency characteristics.

A simple and accurate mathematical method for calculation of the EC50

A simple, accurate, and speedy noncomputational technique for the calculation of the EC50 or any other concentration-related parameter of concentration-effect curves is presented. It avoids the necessity for graph construction or computational curve-fitting programs and allows accurate calculation of the EC50, where the value falls between two known concentrations The technique has been applied to a concentration-response curve constructed to hepatic arterial (HA) vasoconstrictor responses to HA injections of noradrenaline in an isolated dual-perfused rat liver preparation. EC50 values calculated by the new technique were compared to those calculated by conventional, established, noncomputational techniques. The new technique is faster, more accurate, and simpler to perform than other established noncomputational techniques used for the calculation of the EC50 and can be widely applied to many other pharmacological investigations.

Involvement of P2 purinoceptors in the regulation of DNA synthesis in the neural retina of chick embryo

The activation of P2 purinoceptors induces Ca2+ mobilization in the early embryonic chick neural retina. This purinergic Ca2+ response declines parallel with the decrease in mitotic activity during retinal development. To investigate the role of P2 purinoceptors in the regulation of retinal cell proliferation, we studied the effects of the P2 purinoceptor antagonists suramin and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), and of the agonist ATP on DNA synthesis in retinal organ cultures from embryonic day 3 (E3) chick. Suramin inhibited [3H]-thymidine incorporation in a dose-dependent manner (IC50: approximately 70 microM). PPADS also reduced [3H]-thymidine incorporation with maximum inhibition of 46% at 100 microM. Exogenous ATP enhanced [3H]-thymidine incorporation in a dose-dependent manner to maximally 200% of control (EC50: approximately 70 microM). In dissociated retinal cultures from E7 chick, both antagonists showed similar inhibitory effects on [3H]-thymidine incorporation without affecting cell viability. In line with these observations, the presence of extracellular ATP was demonstrated both in vitro and in vivo. In the medium of E3 retinal organ cultures, the concentration of ATP increased 25-fold within 1 h of incubation and this concentration was kept for at least 24 h. In the chick amniotic fluid, the ATP concentration was nearly 3 microM at E3 and declined to 0.15 microM at E7. The results indicate that P2 purinoceptors activated by autocrine or paracrine release of ATP are involved in the regulation of DNA synthesis in the neural retina at early embryonic stages.

Presence of dinucleotide and ATP receptors in human cerebrocortical synaptic terminals

Human cerebrocortical synaptic terminals elicited concentration-dependent Ca2+ transients after Ap5A (diadenosine pentaphosphate) and ATP stimulation, with EC50 values of 23.44 +/- 3.70 microM and 11.48 +/- 2.12 microM, respectively. The lack of cross-desensitisation and the selective antagonism by Ip5I (diinosine pentaphosphate), suggests the activation of a dinucleotide receptor by Ap5A, and a P2X receptor by ATP. Ap5A Ca2+ transients were partially abolished by omega-conotoxin GVI-A (53%), suggesting the participation of a N-type Ca2+ channel in the dinucleotide response. ATP effect on Ca2+ entry was abolished by nicardipine (44%) and by omega-conotoxin GVI-A (52%), suggesting the participation of L- and N-type Ca2+ channels. These data suggest that Ap5A and ATP activate dinucleotide and P2X receptors, respectively, in human brain synaptic terminals.

Metabotropic receptors for ATP and UTP: exploring the correspondence between native and recombinant nucleotide receptors

In the past five years, an extended series (P2Y1-n) of metabotropic nucleotide (P2) receptors has been cloned from vertebrate tissues; these receptors are activated by either ATP or UTP, or both nucleotides. While certain cloned P2Y receptors appear to correspond functionally to particular native P2 receptor phenotypes, such pharmacological phenotypes could be explained by either a combination of several members of the P2Y1-n series being coexpressed in the same tissue or the existence of novel, uncloned P2Y subtypes. Here, Brian King, Andrea Townsend-Nicholson and Geoffrey Burnstock review recent findings on the matter of pharmacological relationships between native P2 and cloned P2Y receptors.

Synthesis and Structure-Activity Relationships of Pyridoxal-6-arylazo-5'-phosphate and Phosphonate Derivatives as P2 Receptor Antagonists

Novel analogs of the P2 receptor antagonist pyridoxal-5'-phosphate-6-phenylazo-2',4'-disulfonate (PPADS) were synthesized. Modifications were made through functional group substitution on the sulfophenyl ring and at the phosphate moiety through the inclusion of phosphonates, demonstrating that a phosphate linkage is not required for P2 receptor antagonism. Substituted 6-phenylazo and 6-naphthylazo derivatives were also evaluated. Among the 6-phenylazo derivatives, 5'-methyl, ethyl, propyl, vinyl, and allyl phosphonates were included. The compounds were tested as antagonists at turkey erythrocyte and guinea-pig taenia coli P2Y(1) receptors, in guinea-pig vas deferens and bladder P2X(1) receptors, and in ion flux experiments by using recombinant rat P2X(2) receptors expressed in Xenopus oocytes. Competitive binding assay at human P2X(1) receptors in differentiated HL-60 cell membranes was carried out by using [(35)S]ATP-γ-S. A 2'-chloro-5'-sulfo analog of PPADS (C(14)H(12)O(9)N(3)ClPSNa), a vinyl phosphonate derivative (C(15)H(12)O(11)N(3)PS(2)Na(3)), and a naphthylazo derivative (C(18)H(14)O(12)N(3)PS(2)Na(2)), were particularly potent in binding to human P2X(1) receptors. The potencies of phosphate derivatives at P2Y(1) receptors were generally similar to PPADS itself, except for the p-carboxyphenylazo phosphate derivative C(15)H(13)O(8)N(3)PNa and its m-chloro analog C(15)H(12)O(8)N(3)ClPNa, which were selective for P2X vs. P2Y(1) receptors. C(15)H(12)O(8)N(3)ClPNa was very potent at rat P2X(2) receptors with an IC(50) value of 0.82 μM. Among the phosphonate derivatives, [4-formyl-3-hydroxy-2-methyl-6-(2-chloro-5-sulfonylphenylazo)-pyrid-5-yl]methylphosphonic acid (C(14)H(12)-O(8)N(3)ClPSNa) showed high potency at P2Y(1) receptors with an IC(50) of 7.23 μM. The corresponding 2,5-disulfonylphenyl derivative was nearly inactive at turkey erythrocyte P2Y(1) receptors, whereas at recombinant P2X(2) receptors had an IC(50) value of 1.1 μM. An ethyl phosphonate derivative (C(15)H(15)O(11)N(3)PS(2)Na(3)), whereas inactive at turkey erythrocyte P2Y(1) receptors, was particularly potent at recombinant P2X(2) receptors.

Two types of relaxation-mediating P2 receptors in rat gastric circular muscle

Effects of purinoceptor antagonists on the relaxant responses to adenine nucleotides were examined to characterize the subtypes of P2-receptor in rat gastric circular muscle. In tissues contracted by acetylcholine, a P2-receptor antagonist, suramin (100 microM), inhibited the relaxant responses to ATP, adenosine 5'-O-(2-thiodiphosphate) (ADPbetaS) and alpha,beta-methylene ATP but not that to adenosine, while a P1-receptor antagonist, 8-phenyltheophylline (3 microM) did vice versa. The inhibitory effect of suramin was more potent for the relaxant responses to alpha,beta-methylene ATP than those to ATP or ADPbetaS. Pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) (3-30 microM) and 4,4'-diisothiocyanatostilbene-2,2'-disulphonate (DIDS) (30 and 100 microM) inhibited the relaxation caused by alpha,beta-methylene ATP but not by ATP, ADPbetaS or adenosine. These results suggest that ATP and ADPbetaS cause relaxation via the classical P2Y receptors resistant to PPADS and DIDS. In addition, alpha,beta-methylene ATP causes relaxation via the distinct P2 receptors sensitive to PPADS and DIDS in rat gastric circular muscle.

Characterization of the P2 receptors on the human umbilical vein endothelial cell line ECV304

1. To characterize the P2 receptors present on the human umbilical vein endothelial-derived cell line, ECV304, cytosolic Ca2+, ([Ca2+]c), responses were recorded in single cells and in cell suspensions to a series of nucleotides and nucleotide agonists. 2. Concentration response curves were obtained in fura-2-loaded ECV304 cell suspensions, with EC50 values of 4.2 microM for ATP, 2.5 microM for UTP and 14 microM for adenosine-5'-O-(3-thio)triphosphate (ATPgammaS). EC50 values for 2-methylthioATP, ADP, adenosine-5'-O-(2-thio)diphosphate (ADPbetaS) and AMP were 0.5 microM, 3.5 microM, 15 microM and 4.7 microM respectively, but maximal [Ca2+]c responses were less than those produced by a maximal addition of ATP/UTP. ECV304 cells were unresponsive to UDP and beta,gamma,methyleneATP. 3. Cross-desensitization studies on ECV304 cells suggested that ATP and UTP recognized the same receptor. However, ADP recognized a receptor distinct from the UTP-sensitive receptor and AMP recognized a third distinct receptor. 4. ECV304 [Ca2+]c responses to 2-methylthioATP were inhibited in the presence of 30 microM pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), whereas [Ca2+]c responses to UTP were unaffected by this treatment. 5. ECV304 cells responded to the diadenosine polyphosphate Ap3A with rises in [Ca2+]c. Apparent responses to Ap4A, Ap5A and Ap6A, were shown to be due to a minor nucleotide contaminant that could be removed by pre-treatment of the diadenosine samples with either alkaline phosphatase or apyrase. 6. ECV304 cells display a pharmacology consistent with the presence of at least two P2 receptors; a P2Y2 receptor insensitive to the diadenosine polyphosphates and a P2Y1 receptor sensitive to Ap3A. In addition, ECV304 cells respond to AMP with increases in [Ca2+]c via an as yet uncharacterized receptor.

Receptors involved in nerve-mediated vasoconstriction in small arteries of the rat hepatic mesentery

1. We have investigated the neurotransmitters and receptor subtypes involved in nerve-mediated vasoconstriction in small arteries of the rat hepatic mesentery. 2. A dense sympathetic innervation was demonstrated using catecholamine histochemistry and antibodies against the synaptic vesicle protein synaptophysin. 3. Reverse transcription-polymerase chain reaction (RT-PCR) demonstrated very strong expression of the alpha1A-adrenergic, neuropeptide Y (NPY) Y1, P2X1- and P2X4-purinergic receptors, moderate expression of the alpha2B-adrenergic receptor and the purinergic P2X5- and P2X7-receptors and weak expression of the alpha1B-, alpha1D-, alpha2A- and alpha2C-adrenergic receptors and the P2X2- and P2X3-purinergic receptors. NPY2 and P2X6 receptor expression was absent. 4. Electrical field stimulation (10 Hz, 10 s) produced contractions which were abolished by tetrodotoxin (10(-6) M) and/or guanethidine (GE, 5 x 10(-6) M) and a combination of benextramine (10(-5) M) and alpha,beta-methylene ATP, (alpha,beta-mATP, 3 x 10(-6) M) or PPADS (10(-5) M). Selective alpah1-adrenergic receptor antagonists showed the potency order of prazosin > WB-4101 > 5-methyl-urapidil > BMY 7378. Yohimbine (10(-8) M, 10(-7) M), alpha,beta-mATP (3 x 10(-6) M) and PPADS (10(-5) M) each enhanced the response to nerve stimulation. 5. Some experiments demonstrated a slow neurogenic contraction which was abolished by GE or the selective NPY1 receptor antagonist 1229U91 (6 x 10(-7) M). 6. We conclude that nerve-mediated vasoconstriction results from the activation of postsynaptic alpha,beta-adrenergic and P2X-purinergic receptors and under some conditions, NPY1 receptors. Neurotransmitter release is modulated by presynaptic alpha2-adrenergic receptors and possibly also P2X-purinoceptors. The major postsynaptic subtypes involved were well predicted by mRNA expression as measured by RT-PCR, suggesting that this technique may be a useful adjunct to studies aimed at identifying functional receptor subtypes.

Extracellular ATP raises cytosolic calcium and activates basolateral chloride conductance in Necturus proximal tubule

1. Extracellular nucleotides modulate ionic transport mechanisms in various epithelia. In the present study, we investigated the effects of extracellular ATP on the intracellular free Ca+2 concentration ([Ca2+]i) and electrophysiological properties of Necturus maculosus proximal convoluted tubule (PCT). 2. ATP raised [Ca2+]i in microdissected fura-2-loaded PCTs (half-maximal effect, approximately mumol 1(-1) ATP). The initial ATP-induced changes in [Ca2+]i were not blunted by the removal of external Ca2+ nor by the presence of Ca2+ channel blockers, but were abolished by thapsigargin and suramin. The sequence for the potency of various agonists on [Ca2+]i was 2-methylthioATP (2MeSATP) = ADP = ATP >> UTP, 2',3',-O-(4-benzoilbenzoil) ATP (BzATP), alpha, beta-methylene ATP (AMPCPP), adenosine. 3. In vivo electrophysiological measurements showed that 100 mumol 1(-1) peritubular ATP added to a Ringer solution reduced the basolateral cell membrane potential (Vm) and increased the cell membrane input conductance. In a low Cl- solution, this ATP-induced depolarization was enhanced. These effects were inhibited by 1 mmol l-1 SITS, consistent with the activation of a basolateral Cl- conductance. 4. The ATP-induced change in Vm was reproduced by ADP but not by UTP or adenosine, and was prevented by suramin. 5. The ATP-induced membrane depolarization was not influenced by thapsigargin, BAPTA AM, or staurosporine and was not reproduced by manoeuvres increasing [Ca2+]i or intracellular cAMP content. 6. We conclude that, in Necturus PCT, a P2y receptor mobilizes Ca2+ mainly from intracellular pools and increases a basolateral Cl- conductance, GCl. The activation of GCl occurs by a mechanism which is not related either to an increase in [Ca2+]i or cAMP content, or to PKC activation.

A pyridoxine cyclic phosphate and its 6-azoaryl derivative selectively potentiate and antagonize activation of P2X1 receptors

Analogues of the P2 receptor antagonists pyridoxal-5'-phosphate and the 6-azophenyl-2',4'-disulfonate derivative (PPADS), in which the phosphate group was cyclized by esterification to a CH2OH group at the 4-position, were synthesized. The cyclic pyridoxine-alpha4, 5-monophosphate, compound 2 (MRS 2219), was found to be a selective potentiator of ATP-evoked responses at rat P2X1 receptors with an EC50 value of 5.9 +/- 1.8 microM, while the corresponding 6-azophenyl-2',5'-disulfonate derivative, compound 3 (MRS 2220), was a selective antagonist. The potency of compound 3 at the recombinant P2X1 receptor (IC50 10.2 +/- 2.6 microM) was lower than PPADS (IC50 98.5 +/- 5.5 nM) or iso-PPADS (IC50 42.5 +/- 17.5 nM), although unlike PPADS its effect was reversible with washout and surmountable. Compound 3 showed weak antagonistic activity at the rat P2X3 receptor (IC50 58.3 +/- 0.1 microM), while at recombinant rat P2X2 and P2X4 receptors no enhancing or antagonistic properties were evident. Compounds 2 and 3 were found to be inactive as either agonists or antagonists at the phospholipase C-coupled P2Y1 receptor of turkey erythrocytes, at recombinant human P2Y2 and P2Y4 receptors, and at recombinant rat P2Y6 receptors. Similarly, compounds 2 and 3 did not have measurable affinity at adenosine A1, A2A, or A3 receptors. The lack of an aldehyde group in these derivatives indicates that Schiff's base formation with the P2X1 receptor is not necessarily required for recognition of pyridoxal phosphate derivatives. Thus, compounds 2 and 3 are relatively selective pharmacological probes of P2X1 receptors, filling a long-standing need in the P2 receptor field, and are also important lead compounds for future studies.

Comparison of the actions of ATP and UTP and P(2X1) receptors in smooth muscle of the rat tail artery

The actions of ATP and uridine 5'-triphosphate (UTP) were compared at P2X1 receptors in acutely dissociated smooth muscles cells of the rat tail artery. ATP (30 nM-100 microM) and UTP (1 microM-1 mM) elicited concentration-dependent inward currents. ATP was approximately 100-fold more potent than UTP. In both cases, currents were activated within 3 ms of agonist application and had similar time-courses of activation and inactivation. The decay of responses for both agonists was concentration-dependent and in most cells could be fitted by two exponentials. The P2X receptor antagonists suramin (100 microM) and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 5 microM) inhibited responses to both ATP and UTP. An action of UTP at P2X1 receptors has not previously been reported. However, since the responses to ATP and UTP had similar time-courses and as PPADS and suramin inhibited both agonists, it is concluded that ATP and UTP are acting at the same site in these cells, the P2X1 receptor.