Cloned and transfected P2Y4 receptors: characterization of a suramin and PPADS-insensitive response to UTP

P2Y2-P2X7 receptors cross-talk in primed mesenteric endothelial cells upregulates NF-κB signaling favoring mononuclear cell adhesion in schistosomiasis

Schistosomiasis is an intravascular infectious disease that impacts over 200 million people globally. In its chronic stage, it leads to mesenteric inflammation with significant involvement of monocytes/macrophages. Endothelial cells lining the vessel lumens play a crucial role, and mount of evidence links this disease to a downregulation of endoprotective cell signaling favoring a primed and proinflammatory endothelial cell phenotype and therefore the loss of immunovascular homeostasis. One hallmark of infectious and inflammatory conditions is the release of nucleotides into the extracellular milieu, which, in turn, act as innate messengers, activating purinergic receptors and triggering cell-to-cell communication. ATP influences the progression of various diseases through P2X and P2Y purinergic receptor subtypes. Among these receptors, P2Y2 (P2Y2R) and P2X7 (P2X7R) receptors stand out, known for their roles in inflammation. However, their specific role in schistosomiasis has remained largely unexplored. Therefore, we hypothesized that endothelial P2Y2R and P2X7R could contribute to monocyte adhesion to mesenteric endothelial cells in schistosomiasis. Using a preclinical murine model of schistosomiasis associated with endothelial dysfunction and age-matched control mice, we showed that endothelial P2Y2R and P2X7R activation increased monocyte adhesion to cultured primary endothelial cells in both groups. However, a distinct upregulation of endothelial P2Y2R-driven canonical Ca2+ signaling was observed in the infected group, amplifying adhesion. In the control group, the coactivation of endothelial P2Y2R and P2X7R did not alter the maximal monocyte adhesion induced by each receptor individually. However, in the infected group, this coactivation induced a distinct upregulation of P2Y2R-P2X7R-driven canonical signaling, IL-1β release, and VCAM-1 expression, with underlying mechanisms involving inflammasome and NF-κB signaling. Therefore, current data suggest that schistosomiasis alters endothelial cell P2Y2R/P2X7R signaling during inflammation. These discoveries advance our understanding of schistosomiasis. This intricate interplay, driven by PAMP-triggered endothelial P2Y2R/P2X7R cross-talk, emerges as a potential key player in the mesenteric inflammation during schistosomiasis.

The blockage of downstream P2Y2 receptor signaling inhibits the prostate cancer cell adhesion to endothelial cells

AIMS

Prostate cancer is the second most frequently malignancy in men worldwide. Most deaths are caused by metastasis, and tumor cell dissemination involves the interaction with endothelial cells. However, the endothelial cell signaling involved in such interaction is not entirely understood. The tumor microenvironment contains extracellular ATP, an endogenous agonist of the purinergic P2Y₂ receptor (P2Y₂R). P2Y₂R signaling changes endothelial cell phenotype, which may be relevant to cancer pathophysiology. Therefore, we hypothesized that P2Y₂R activation could favor the metastatic prostate cancer cells adhesion to endothelial cells.

MAIN METHODS

For adhesion assays, confluent endothelial cells EA.hy926 were treated with P2Y₂R agonists before adding and imaging stained DU-145 cells. Alternatively, fluorescent probes and antibodies were used to determine intracellular endothelial Ca²⁺, nitric oxide (NO), and flow cytometry assays.

KEY FINDINGS

Endothelial P2Y₂R activation with ATP, UTP, or the selective agonist 2-thio-UTP increased DU-145 cell adhesion to EA.hy926 cells. This effect required endothelial cell Ca²⁺ mobilization and relied on the endothelial expression of VCAM-1 and ICAM-1. Conversely, inhibiting this proadhesive endothelial phenotype could impair DU-145 cell adhesion. To evaluate this, we chose atorvastatin based on its notable improvement of endothelial cell dysfunction. Atorvastatin blocked UTP-induced DU-145 cell adhesion to endothelial cell monolayer in a NO-dependent manner, unveiling a P2Y₂R and NO signaling crosstalk.

SIGNIFICANCE

Endothelial P2Y₂R signaling contributes to the adhesion of metastatic prostate cancer cells suggesting that the downstream signaling blockade by statins could be a putative mechanism to reduce prostate cancer metastasis.

Autocrine regulation of wound healing by ATP release and P2Y2 receptor activation

AIMS

Application of exogenous nucleotides can modulate wound healing via the activation of purinergic receptors. However, evidence for the release of endogenous nucleotides and the subsequent activation of purinergic receptors in this process has not been well defined. Therefore, the current study aimed to investigate wound-mediated nucleotide release and autocrine purinergic signalling during HaCaT keratinocyte wound closure following scratch injury.

MAIN METHODS

An in vitro scratch wound apparatus was employed to study wound healing over 24-h in the presence of modulators of ATP release, P2 receptors and pathways downstream of P2 receptor activation.

KEY FINDINGS

Adenosine 5'-triphosphate (ATP) was released from scratched cells. The ectonucleotidase apyrase and pharmacological inhibition of the nucleotide release hemichannel, pannexin-1, decreased wound closure over time. The non-selective P2Y receptor antagonist suramin and the selective P2Y₂ receptor antagonist AR-C118925XX, but not other P2 antagonists, decreased wound closure. AR-C118925XX decreased wound closure in a concentration-dependent fashion. However, exogenous P2Y₂ receptor agonists, ATP or uridine 5'-triphosphate, did not enhance wound closure. PCR and immunoblotting confirmed P2Y₂ receptor expression in HaCaT cells. U73122, a phospholipase C antagonist, and 2-aminoethoxydiphenylborate, an inositol 1,4,5-trisphosphate receptor-sensitive Ca²⁺-release channel antagonist, decreased wound closure consistent with P2Y₂ receptor activation. Absence of extracellular or intracellular Ca²⁺ or inhibition of intracellular Ca²⁺-release also impaired wound closure.

SIGNIFICANCE

These data describe a novel autocrine signalling mechanism in which wound-mediated release of endogenous ATP in response to mechanical scratching of HaCaT cells activates P2Y₂ receptors to facilitate wound closure.

P2Y2 and P2X4 Receptors Mediate Ca2+ Mobilization in DH82 Canine Macrophage Cells

Purinergic receptors of the P2 subclass are commonly found in human and rodent macrophages where they can be activated by adenosine 5'-triphosphate (ATP) or uridine 5'-triphosphate (UTP) to mediate Ca²⁺ mobilization, resulting in downstream signalling to promote inflammation and pain. However, little is understood regarding these receptors in canine macrophages. To establish a macrophage model of canine P2 receptor signalling, the expression of these receptors in the DH82 canine macrophage cell line was determined by reverse transcription polymerase chain reaction (RT-PCR) and immunocytochemistry. P2 receptor function in DH82 cells was pharmacologically characterised using nucleotide-induced measurements of Fura-2 AM-bound intracellular Ca²⁺. RT-PCR revealed predominant expression of P2X4 receptors, while immunocytochemistry confirmed predominant expression of P2Y₂ receptors, with low levels of P2X4 receptor expression. ATP and UTP induced robust Ca²⁺ responses in the absence or presence of extracellular Ca²⁺. ATP-induced responses were only partially inhibited by the P2X4 receptor antagonists, 2',3'-O-(2,4,6-trinitrophenyl)-ATP (TNP-ATP), paroxetine and 5-BDBD, but were strongly potentiated by ivermectin. UTP-induced responses were near completely inhibited by the P2Y₂ receptor antagonists, suramin and AR-C118925. P2Y₂ receptor-mediated Ca²⁺ mobilization was inhibited by U-73122 and 2-aminoethoxydiphenyl borate (2-APB), indicating P2Y₂ receptor coupling to the phospholipase C and inositol triphosphate signal transduction pathway. Together this data demonstrates, for the first time, the expression of functional P2 receptors in DH82 canine macrophage cells and identifies a potential cell model for studying macrophage-mediated purinergic signalling in inflammation and pain in dogs.

Profiling of a suramin-derived compound library at recombinant human P2Y receptors identifies NF272 as a competitive but non-selective P2Y2 receptor antagonist

Extracellular nucleotides mediate multiple physiological effects such as proliferation, differentiation, or induction of apoptosis through G protein-coupled P2Y receptors or P2X ion channels. Evaluation of the complete physiological role of nucleotides has long been hampered by a lack of potent and selective ligands for all P2 subtypes. Meanwhile, for most of the P2 receptors, selective ligands are available, but only a few potent and selective P2Y₂ receptor antagonists are described. This limits the understanding of the role of P2Y₂ receptors. The purpose of this study was to search for P2Y₂ receptor antagonists by a combinatorial screening of a library of around 415 suramin-derived compounds. Calcium fluorescence measurements at P2Y₂ receptors recombinantly expressed in human 1321N1 astrocytoma cells identified NF272 [8-(4-methyl-3-(3-phenoxycarbonylimino-benzamido)benzamido)-naphthalene-1,3,5-trisulfonic acid trisodium salt] as a competitive P2Y₂ receptor antagonist with a K_i of 19 μM which is 14-fold more potent than suramin at this receptor subtype. The SCHILD analysis of competitive inhibition resulted in a pA₂ value of 5.03 ± 0.22 (mean ± SEM) with a slope not significantly different from unity. Among uracil-nucleotide-preferring P2Y receptors, NF272 shows a moderate selectivity over P2Y₄ (3.6-fold) and P2Y₆ (5.7-fold). However, NF272 is equipotent at P2Y₁, and even more potent at P2Y₁₁ and P2Y₁₂ receptors. Up to 250 μM, NF272 showed no cytotoxicity in MTT cell viability assays in 1321N1, HEK293, and OVCAR-3 cells. Further, NF272 was able to inhibit the ATP-induced calcium signal in OVCAR-3 cells demonstrated to express P2Y₂ receptors. In conclusion, NF272 is a competitive but non-selective P2Y₂ receptor antagonist with 14-fold higher potency than suramin lacking cytotoxic effects. Therefore, NF272 may serve as a lead structure for further development of P2Y₂ receptor antagonists.

Pharmacology of P2Y receptors

P2Y receptors are G-protein-coupled receptors (GPCRs) for extracellular nucleotides. There are eight mammalian P2Y receptor subtypes divided into two subgroups (P2Y₁, P2Y₂, P2Y₄, P2Y₆, and P2Y₁₁) and (P2Y₁₂, P2Y₁₃, and P2Y₁₄). The P2Y receptors are expressed in various cell types and play important roles in physiology and pathophysiology including inflammatory responses and neuropathic pain. The antagonism of P2Y₁₂ receptors is used in pharmacotherapy for the prevention and therapy of cardiovascular events. The nucleoside analogue ticagrelor and active metabolites of the thienopyridine compounds ticlopidine, clopidogrel and prasugrel inhibit platelet P2Y₁₂ receptors and reduce thereby platelet aggregation. The P2Y₂ receptor agonist diquafosol is used for the treatment of the dry eye syndrome. The P2Y receptor subtypes differ in their amino acid sequences, their pharmacological profiles and their signaling transduction pathways. Recently, selective receptor ligands have been developed for all subtypes. The published crystal structures of the human P2Y₁ and P2Y₁₂ receptors as well as receptor models will facilitate the development of novel drugs for pharmacotherapy.

Tinkering with targeting nucleotide signaling for control of intracellular Leishmania parasites

Nucleotides are one of the most primitive extracellular signalling molecules across all phyla and regulate a multitude of responses. The biological effects of extracellular nucleotides/sides are mediated via the specific purinergic receptors present on the cell surface. In mammalian system, adenine nucleotides are the predominant nucleotides found in the extracellular milieu and mediate a constellation of physiological functions. In the context of host-pathogen interaction, extracellular ATP is recognized as a danger signal and potentiates the release of pro-inflammatory mediators from activated immune cells, on the other hand, its breakdown product adenosine exerts potential anti-inflammatory and immunosuppressive actions. Therefore, it is increasingly apparent that the interplay between extracellular ATP/adenosine ratios has a significant role in coordinating the regulation of the immune system in health and diseases. Several pathogens express ectonucleotidases on their surface and exploit the purinergic signalling as one of the mechanisms to modulate the host immune response. Leishmania pathogens are one of the most successful intracellular pathogens which survive within host macrophages and manipulate protective Th1 response into disease promoting Th2 response. In this review, we discuss the regulation of extracellular ATP and adenosine levels, the role of ATP/adenosine counter signalling in regulating the inflammation and immune responses during infection and how Leishmania parasites exploit the purinergic signalling to manipulate host response. We also discuss the challenges and opportunities in targeting purinergic signalling and the future prospects.

Tools and drugs for uracil nucleotide-activated P2Y receptors

P2Y receptors (P2YRs) are a family of G protein-coupled receptors activated by extracellular nucleotides. Physiological P2YR agonists include purine and pyrimidine nucleoside di- and triphosphates, such as ATP, ADP, UTP, UDP, nucleotide sugars, and dinucleotides. Eight subtypes exist, P2Y₁, P2Y₂, P2Y₄, P2Y₆, P2Y₁₁, P2Y₁₂, P2Y₁₃, and P2Y₁₄, which represent current or potential future drug targets. Here we provide a comprehensive overview of ligands for the subgroup of the P2YR family that is activated by uracil nucleotides: P2Y₂ (UTP, also ATP and dinucleotides), P2Y₄ (UTP), P2Y₆ (UDP), and P2Y₁₄ (UDP, UDP-glucose, UDP-galactose). The physiological agonists are metabolically unstable due to their fast hydrolysis by ectonucleotidases. A number of agonists with increased potency, subtype-selectivity and/or enzymatic stability have been developed in recent years. Useful P2Y₂R agonists include MRS2698 (6-01, highly selective) and PSB-1114 (6-05, increased metabolic stability). A potent and selective P2Y₂R antagonist is AR-C118925 (10-01). For studies of the P2Y₄R, MRS4062 (3-15) may be used as a selective agonist, while PSB-16133 (10-06) is a selective antagonist. Several potent P2Y₆R agonists have been developed including 5-methoxyuridine 5'-O-((R_p)α-boranodiphosphate) (6-12), PSB-0474 (3-11), and MRS2693 (3-26). The isocyanate MRS2578 (10-08) is used as a selective P2Y₆R antagonist, although its reactivity and low water-solubility are limiting. With MRS2905 (6-08), a potent and metabolically stable P2Y₁₄R agonist is available, while PPTN (10-14) represents a potent and selective P2Y₁₄R antagonist. The radioligand [³H]UDP can be used to label P2Y₁₄Rs. In addition, several fluorescent probes have been developed. Uracil nucleotide-activated P2YRs show great potential as drug targets, especially in inflammation, cancer, cardiovascular and neurodegenerative diseases.

Drug-like Antagonists of P2Y Receptors-From Lead Identification to Drug Development

P2Y receptors are expressed in virtually all cells and tissue types and mediate an astonishing array of biological functions, including platelet aggregation, smooth muscle cell proliferation, and immune regulation. The P2Y receptors belong to the G protein-coupled receptor superfamily and are composed of eight members encoded by distinct genes that can be subdivided into two groups on the basis of their coupling to specific G-proteins. Extensive research has been undertaken to find modulators of P2Y receptors, although to date only a limited number of small-molecule P2Y receptor antagonists have been approved by drug/medicines agencies. This Perspective reviews the known P2Y receptor antagonists, highlighting oral drug-like receptor antagonists, and considers future opportunities for the development of small molecules for clinical evaluation.

Pharmacology and structure of P2Y receptors

P2Y receptors are G-protein-coupled receptors (GPCRs) for extracellular nucleotides. There are eight mammalian P2Y receptor subtypes (P2Y1, P2Y2, P2Y4, P2Y6, P2Y11, P2Y12, P2Y13, and P2Y14). P2Y receptors are widely expressed and play important roles in physiology and pathophysiology. One important example is the ADP-induced platelet aggregation mediated by P2Y1 and P2Y12 receptors. Active metabolites of the thienopyridine compounds ticlopidine, clopidogrel and prasugrel as well as the nucleoside analogue ticagrelor block P2Y12 receptors and thereby platelet aggregation. These drugs are used for the prevention and therapy of cardiovascular events. Moreover, P2Y receptors play important roles in the nervous system. Adenine nucleotides modulate neuronal activity and neuronal fibre outgrowth by activation of P2Y1 receptors and control migration of microglia by P2Y12 receptors. UDP stimulates microglial phagocytosis through activation of P2Y6 receptors. There is evidence for a role for P2Y2 receptors in Alzheimer's disease pathology. The P2Y receptor subtypes are highly diverse in both their amino acid sequences and their pharmacological profiles. Selective receptor ligands have been developed for the pharmacological characterization of the receptor subtypes. The recently published three-dimensional crystal structures of the human P2Y1 and P2Y12 receptors will facilitate the development of therapeutic agents that selectively target P2Y receptors. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Discovery of potent competitive antagonists and positive modulators of the P2X2 receptor

Evaluation and optimization of anthraquinone derivatives related to Reactive Blue 2 at P2X2 receptors yielded the first potent and selective P2X2 receptor antagonists. The compounds were tested for inhibition of ATP (10 μM) mediated currents in Xenopus oocytes expressing the rat P2X2 receptor. The most potent antagonists were sodium 1-amino-4-[3-(4,6-dichloro[1,3,5]triazine-2-ylamino)phenylamino]-9,10-dioxo-9,10-dihydroanthracene-2-sulfonate (63, PSB-10211, IC(50) 86 nM) and disodium 1-amino-4-[3-(4,6-dichloro[1,3,5]triazine-2-ylamino)-4-sulfophenylamino]-9,10-dioxo-9,10-dihydroanthracene-2-sulfonate (57, PSB-1011, IC(50) 79 nM). Compound 57 exhibited a competitive mechanism of action (pA(2) 7.49). It was >100-fold selective versus P2X4, P2X7, and several investigated P2Y receptor subtypes (P2Y(2,4,6,12)); selectivity versus P2X1 and P2X3 receptors was moderate (>5-fold). Compound 57 was >13-fold more potent at the homomeric P2X2 than at the heteromeric P2X2/3 receptor. Several anthraquinone derivatives were found to act as positive modulators of ATP effects at P2X2 receptors, for example, sodium 1-amino-4-(3-phenoxyphenylamino)-9,10-dioxo-9,10-dihydroanthracene-2-sulfonate (51, PSB-10129, EC(50) 489 nM), which led to about a 3-fold increase in the ATP-elicited current.

Molecular pharmacology, physiology, and structure of the P2Y receptors

The P2Y receptors are a widely expressed group of eight nucleotide-activated G protein-coupled receptors (GPCRs). The P2Y(1)(ADP), P2Y(2)(ATP/UTP), P2Y(4)(UTP), P2Y(6)(UDP), and P2Y(11)(ATP) receptors activate G(q) and therefore robustly promote inositol lipid signaling responses. The P2Y(12)(ADP), P2Y(13)(ADP), and P2Y(14)(UDP/UDP-glucose) receptors activate G(i) leading to inhibition of adenylyl cyclase and to Gβγ-mediated activation of a range of effector proteins including phosphoinositide 3-kinase-γ, inward rectifying K(+) (GIRK) channels, phospholipase C-β2 and -β3, and G protein-receptor kinases 2 and 3. A broad range of physiological responses occur downstream of activation of these receptors ranging from Cl(-) secretion by epithelia to aggregation of platelets to neurotransmission. Useful structural models of the P2Y receptors have evolved from extensive genetic analyses coupled with molecular modeling based on three-dimensional structures obtained for rhodopsin and several other GPCRs. Selective ligands have been synthesized for most of the P2Y receptors with the most prominent successes attained with highly selective agonist and antagonist molecules for the ADP-activated P2Y(1) and P2Y(12) receptors. The widely prescribed drug, clopidogrel, which results in irreversible blockade of the platelet P2Y(12) receptor, is the most important therapeutic agent that targets a P2Y receptor.

Development of potent and selective inhibitors of ecto-5'-nucleotidase based on an anthraquinone scaffold

ecto-5'-Nucleotidase (eN, CD73) plays a major role in controlling extracellular adenosine levels. eN inhibitors have potential as novel drugs, for example, for the treatment of cancer. In the present study, we synthesized and investigated a series of 55 anthraquinone derivatives as potential inhibitors of eN, 11 of which are novel compounds and another 11 of which had previously been described but have now been synthesized by an improved method. We identified several potent inhibitors of rat eN. The most potent compounds were 1-amino-4-[4-fluoro-2-carboxyphenylamino]-9,10-dioxo-9,10-dihydroanthracene-2-sulfonate (45, PSB-0952, K(i) = 260 nM) and 1-amino-4-[2-anthracenylamino]-9,10-dioxo-9,10-dihydroanthracene-2-sulfonate (52, PSB-0963, 150 nM), with 52 being the most potent eN inhibitor described to date. Selected compounds were further characterized and found to exhibit a competitive mechanism of inhibition. Investigations of ecto-nucleoside triphosphate diphosphohydrolases (NTPDases) and the P2Y receptor subtypes P2Y(2), P2Y(4), P2Y(6), and P2Y(12) showed that compound 45 exhibited the highest degree of selectivity (>150-fold).

P2Y2 receptors mediate ATP-induced resensitization of TRPV1 expressed by kidney projecting sensory neurons

The transient receptor potential vanilloid type 1 (TRPV1) channel is a ligand-gated cation channel expressed by sensory nerves. P2Y receptors are G protein-coupled receptors that are also expressed by TRPV1-positive sensory neurons. Therefore, we studied interactions between P2Y receptors and TRPV1 function on kidney projecting sensory neurons. Application of Fast Blue (FB) to nerves surrounding the renal artery retrogradely labeled neurons in dorsal root ganglia of rats. Whole cell recording was performed on FB-labeled neurons maintained in primary culture. Capsaicin was used to activate TRPV1. Four types of kidney projecting neurons were identified based on capsaicin responses: 1) desensitizing (35%), 2) nondesensitizing (29%), 3) silent (3%), and 4) insensitive (30%). Silent neurons responded to capsaicin only after ATP (100 microM) pretreatment. ATP reversed desensitization in desensitizing neurons. Insensitive neurons never responded to capsaicin. UTP, a P2Y purinoceptor 2 (P2Y(2))/P2Y(4) receptor agonist, reversed capsaicin-induced TRPV1 desensitization. 2-methyl-thio-ATP (2-Me-S-ATP), a P2Y(1) receptor agonist, did not change desensitization. MRS 2179 and pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), drugs that block P2Y(1) receptors, did not block ATP-induced resensitization of TRPV1. Suramin, a P2Y(2) receptor antagonist, blocked resensitization caused by UTP. Immunocytochemical studies showed that FB-labeled neurons coexpressed P2Y(2) receptors and TRPV1. We conclude that P2Y(2) receptor activation can maintain TRPV1 function perhaps during sustained episodes of activity of kidney projecting sensory neurons.

Characterization of Calcium-Mediated Intracellular and Intercellular Signaling in the rMC-1 Glial Cell Line

Retinal Müller glial cells, in addition to providing homeostatic support to retinal neurons, have been shown to engage in modulation of neuronal activity and regulate vasomotor responses in the retina, among other functions. Calcium-mediated signaling in Müller cells has been implicated to play a significant role in the intracellular and intercellular interactions necessary to carry out these functions. Although the basic molecular mechanisms of calcium signaling in Müller cells have been described, the dynamics of calcium responses in Müller cells have not been fully explored. Here, we provide a quantitative characterization of calcium signaling in an in vitro model of Müller cell signaling using the rMC-1 cell line, a well-established line developed from rat Müller cells. rMC-1 cells displayed robust intracellular calcium transients and the capacity to support calcium transient-mediated intercellular calcium waves with signaling dynamics similar to that reported for Müller cells in in situ retinal preparations. Furthermore, pharmacological perturbations of intracellular calcium transients with thapsigargin and intercellular calcium waves with purinergic receptor antagonists and gap junction blockers (PPADS and FFA, respectively) suggest that the molecular mechanisms that underlie calcium signaling in rMC-1 cells has been conserved with those of Müller cells. This model provides a robust in vitro system for investigating specific mechanistic hypotheses of intra- and intercellular calcium signaling in Müller cells.

Calcium oscillations induced by ATP in human umbilical cord smooth muscle cells

Arterial smooth muscle cells exhibit vasomotion, related to oscillations in intracellular Ca(2+) concentration, but the origin and function of these has not yet been fully determined. We measured intracellular Ca(2+) using conventional fluorescent methods in primary cultured, human umbilical cord artery smooth muscle cells (HUCASMC). Spontaneous oscillations in Ca(2+) was found in only 1% of all cells but exogenous, micromolar concentrations of ATP could induce Ca(2+) oscillations in 70% of cells with the most common pattern being one of regular amplitude and frequency with a return to basal levels between each peak. The P2Y agonist, UTP, but not the P2X agonist alphabeta-methylene ATP, could also induce Ca(2+) oscillations. Once induced, these oscillations could not be blocked by G-protein, PLC, VGCC or TRP channel antagonists applied individually, but could be prevented when antagonists were applied together. In the presence of EGTA, micromolar concentrations of ATP induced an elevation in intracellular Ca(2+) but did not induce Ca(2+) oscillations. The oscillation frequency induced by ATP was affected by bath Ca(2+) concentration. Taken together, these data suggest that external Ca(2+) entry maintains the Ca(2+) oscillation induced by activation of P2Y receptors. Once induced, multiple mechanisms are involved to maintain the oscillation and the oscillation frequency is determined by the speed of Ca(2+) refilling. Chronic hypoxia enhanced the Ca(2+) response and altered the oscillation frequency. We suggest that these oscillations may play a role in the maintenance of umbilical blood flow during situations in which GPCR are activated.

Stimulation of rat erythrocyte P2X7 receptor induces the release of epoxyeicosatrienoic acids

BACKGROUND AND PURPOSE

Red blood cells (RBCs) are reservoirs of vasodilatory, antiaggregatory, and antiinflammatory lipid mediators-epoxyeicosatrienoic acids (EETs). This study addresses the formation and release of erythrocyte-derived EETs in response to ATP receptor stimulation that may represent an important mechanism regarding circulatory regulation.

EXPERIMENTAL APPROACH

Erythrocyte EET formation and release were investigated by incubating rat RBCs in physiological salt solution with agents that effected ATP release via P2 receptor stimulation of phospholipase A2 and epoxygenase-like activities with activation of the ATP secretory mechanism. EETs were analyzed by gas and liquid chromatography-mass spectrometry.

KEY RESULTS

EETs were released from rat RBCs: 14,15-, 11,12-, 8,9- and 5,6-EETs in a ratio of 1.2:1.0:0.9:0.8. EETs were produced by epoxidation of arachidonic acid catalyzed by hemoglobin. Spontaneous release of EETs, 0.66+/-0.14 ng per 10(9) RBCs, was dose-dependently increased by an ATP analog, BzATP, and inhibited by P2X(7) receptor antagonists. 5 microM ATP increased release of EETs over 20% to 0.83+/-0.15 ng per 10(9) RBCs; 10 microM BzATP tripled the amount of EET release to 1.87+/-0.20 ng per 10(9) RBCs. EET release by ATP or BzATP was not associated with hemolysis. Carbenoxolone, a gap junction inhibitor that inhibits ATP release, and glibenclamide, an inhibitor of the cystic fibrosis transmembrane conductance regulator (CFTR), which is required for ATP release, inhibited the spontaneous and stimulated EET release from RBCs.

CONCLUSIONS AND IMPLICATIONS

EETs are produced and released from RBCs via a mechanism that is mediated by ATP stimulation of P2X(7) receptors coupled to ATP transporters, pannexin-1 and CFTR.

Purinergic signalling and intercellular Ca2+ wave propagation in the organ of Corti

Extracellular ATP is a key neuromodulator of visual and auditory sensory epithelia. In the rat cochlea, pharmacological dissection indicates that ATP, acting through a highly sensitive purinergic/IP(3)-mediated signaling pathway with (little or) no involvement of ryanodine receptors, is the principal paracrine mediator implicated in the propagation of calcium waves through supporting and epithelial cells. Measurement of sensitivity to UTP and other purinergic agonists implicate P2Y(2) and P2Y(4) as the main P2Y receptor isoforms involved in these responses. Ca2+ waves, elicited under highly reproducible conditions by carefully controlling dose (1 microM) and timing of focal agonist application (0.2s), extended over radial distance greater than 160 microm from the source, identical to those activated by damaging single outer hair cells. Altogether, these results indicate that intercellular calcium waves are a robust phenomenon that confers a significant ability for cell-cell communication in the mammalian cochlea. Further ongoing research will reveal the roles that such Ca2+ waves play in the inner ear.

Nucleotide regulation of the voltage-dependent nonselective cation conductance in murine colonic myocytes

ATP is proposed to be a major inhibitory neurotransmitter in the gastrointestinal (GI) tract, causing hyperpolarization and smooth muscle relaxation. ATP activates small-conductance Ca(2+)-activated K(+) channels that are involved in setting the resting membrane potential and causing inhibitory junction potentials. No reports are available examining the effects of ATP on voltage-dependent inward currents in GI smooth muscle cells. We previously reported two types of voltage-dependent inward currents in murine proximal colonic myocytes: a low-threshold voltage-activated, nonselective cation current (I(VNSCC)) and a relatively high-threshold voltage-activated (L-type) Ca(2+) current (I(L)). Here we have investigated the effects of ATP on these currents. External application of ATP (1 mM) did not affect I(VNSCC) or I(L) in dialyzed cells. ATP (1 mM) increased I(VNSCC) and decreased I(L) in the perforated whole-cell configuration. UTP and UDP (1 mM) were more potent than ATP on I(VNSCC). ADP decreased I(L) but had no effect on I(VNSCC). The order of effectiveness was UTP = UDP > ATP > ADP. These effects were not blocked by pyridoxal phosphate-6-azo(benzene-2,4-disulfonic acid) (PPADS), but the phospholipase C inhibitor U-73122 reversed the effects of ATP on I(VNSCC). ATP stimulation of I(VNSCC) was also reversed by protein kinase C (PKC) inhibitors chelerythrine chloride or bisindolylmaleimide I. Phorbol 12,13-dibutyrate mimicked the effects of ATP. RT-PCR showed that P2Y(4) is expressed by murine colonic myocytes, and this receptor is relatively insensitive to PPADS. Our data suggest that ATP activates I(VNSCC) and depresses I(L) via binding of P2Y(4) receptors and stimulation of the phospholipase C/PKC pathway.

P2Y2 receptor up-regulation induced by guanosine or UTP in rat brain cultured astrocytes

Among P2 metabotropic ATP receptors, P2Y2 subtype seems to be peculiar as its upregulation triggers important biological events in different cells types. In non-stimulated cells including astrocytes, P2Y2 receptors are usually expressed at levels lower than P2Y1 sites, however the promoter region of the P2Y2 receptors has not yet been studied and little is known about the mechanisms underlying the regulation of the expression of this ATP receptor. We showed that not only UTP and ATP are the most potent and naturally occurring agonist for P2Y2 sites, but also guanosine induced an up-regulation of astrocyte P2Y2 receptor mRNA evaluated by Northern blot analysis. We also focused our attention on this nucleoside since in our previous studies it was reported to be released by cultured astrocytes and to exert different neuroprotective effects. UTP and guanosine-evoked P2Y2 receptor up-regulation in rat brain cultured astrocytes was linked to an increased P2Y2-mediated intracellular calcium response, thus suggesting an increased P2Y2 activity. Actinomycin D, a RNA polymerase inhibitor, abrogated both UTP and guanosine-mediated P2Y2 up-regulation, thus indicating that de novo transcription was required. The effect of UTP and guanosine was also evaluated in astrocytes pretreated with different inhibitors of signal transduction pathways including ERK, PKC and PKA reported to be involved in the regulation of other cell surface receptor mRNAs. The results show that ERK1-2/MAPK pathway play a key role in the P2Y2 receptor up-regulation mediated by either UTP or guanosine. Moreover, our data suggest that PKA is also involved in guanosine-induced transcriptional activation of P2Y2 mRNA and that increased intracellular calcium levels and PKC activation may also mediate P2Y2 receptor up-regulation triggered by UTP. The extracellular release of ATP under physiological and pathological conditions has been widely studied. On the contrary, little is known about the release of pyrimidines and in particular of UTP. Here we show that astrocytes are able to release UTP, either at rest or during and following hypoxia/hypoglycemia obtained by submitting the cells to glucose-oxygen deprivation (OGD). Interestingly, also P2Y2 receptor mRNA increased by about two-fold the control values when the cultures were submitted to OGD. It has been recently reported that P2Y2 receptors can play a protective role in astrocytes, thus either guanosine administration or increased extracellular concentrations of guanosine and UTP reached locally following CNS injury may increase P2Y2-mediated biological events aimed at promoting a protective astrocyte response.

ATP stimulates interleukin-6 production via P2Y receptors in human HaCaT keratinocytes

We evaluated the role of ATP in functions of human HaCaT keratinocytes. ATP was released from HaCaT cells by changing the culture medium. Reverse transcription-polymerase chain reaction analysis revealed that HaCaT cells expressed multiple P2 purinergic receptor mRNAs. UTP was the most potent agonist to increase the intracellular Ca2+ concentration ([Ca2+]i). UTP and ATP caused the accumulation of [3H]inositol phosphates, suggesting that UTP binds to the Gq/11-coupled P2Y receptor. UTP increased IL-6 mRNA and protein levels, and the increases were inhibited by a P2 purinergic receptor antagonist (suramin, 300 microM). While a protein kinase C inhibitor (GF109203X, 10 microM) was without effect, an intracellular free Ca2+ chelator (BAPTA-AM, 50 microM) suppressed UTP-mediated IL-6 induction. These results suggest that 1) ATP is released from HaCaT cells upon physical stimulation and may act as an autocrine molecule, and 2) the stimulation of P2Y receptors causes IL-6 production via mRNA expression through [Ca2+]i elevation.

Extracellular ATP activates ERK1/ERK2 via a metabotropic P2Y1 receptor in a Ca2+ independent manner in differentiated human skeletal muscle cells

ATP is released at the neuromuscular junction to regulate development and proliferation. The sequential expression of P2X and P2Y receptors has been correlated to these effects in many species and cell lines. We have therefore investigated ATP mediated signalling in differentiated primary human skeletal muscle cells. ATP was capable to trigger Ca2+ transients in these cells via P2Y receptors which were not attributable to Ca2+ influx via P2X receptors. Instead, ATP propagated the formation of inositol phosphate (IP) with an EC50 of 21.3 microM. The Ca2+ transient provoked by ATP was abrogated roughly 75% by the phospholipase C (PLC) inhibitor, U73122. Interestingly, the ryanodine sensitive Ca2+ pool was not involved in ATP triggered Ca2+ release. On mRNA level and by a pharmacological approach we confirmed the presence of the P2Y1, P2Y2, P2Y4 and P2Y6 receptors. Substantially, ATP activated IP formation via a P2Y1 receptor. In addition, ATP elicited extracellular signal regulated kinase (ERK)1/2 phosphorylation in a time and concentration dependent manner, again mainly via P2Y1 receptors. The ATP mediated ERK1/2 phosphorylation was strictly dependent on phospholipase C and PI3 kinase activity. Importantly, ATP mediated ERK1/2 phosphorylation was Ca2+ independent. This observation was corroborated by the finding that conventional protein kinase C inhibitors did not suppress ATP triggered ERK1/2 phosphorylation. Taken together, these observations highlight the importance of ATP as a co-neurotransmitter at the neuromuscular junction via dual signalling, i.e. IP3 receptor mediated Ca2+ transients and Ca2+ insensitive phosphorylation of ERK1/2.

Apical membrane P2Y4 purinergic receptor controls K+ secretion by strial marginal cell epithelium

Background

It was previously shown that K⁺ secretion by strial marginal cell epithelium is under the control of G-protein coupled receptors of the P2Y family in the apical membrane. Receptor activation by uracil nucleotides (P2Y₂, P2Y₄ or P2Y₆) leads to a decrease in the electrogenic K⁺ secretion. The present study was conducted to determine the subtype of the functional purinergic receptor in gerbil stria vascularis, to test if receptor activation leads to elevation of intracellular [Ca²⁺] and to test if the response to these receptors undergoes desensitization.

Results

The transepithelial short circuit current (Isc) represents electrogenic K⁺ secretion and was found to be decreased by uridine 5'-triphosphate (UTP), adenosine 5'-triphosphate (ATP) and diadenosine tetraphosphate (Ap4A) but not uridine 5'-diphosphate (UDP) at the apical membrane of marginal cells of the gerbil stria vascularis. The potencies of these agonists were consistent with rodent P2Y₄ and P2Y₂ but not P2Y₆ receptors. Activation caused a biphasic increase in intracellular [Ca²⁺] that could be partially blocked by 2-aminoethoxy-diphenyl borate (2-APB), an inhibitor of the IP3 receptor and store-operated channels. Suramin (100 μM) did not inhibit the effect of UTP (1 μM). The ineffectiveness of suramin at the concentration used was consistent with P2Y₄ but not P2Y₂. Transcripts for both P2Y₂ and P2Y₄ were found in the stria vascularis. Sustained exposure to ATP or UTP for 15 min caused a depression of Isc that appeared to have two components but with apparently no chronic desensitization.

Conclusion

The results support the conclusion that regulation of K⁺ secretion across strial marginal cell epithelium occurs by P2Y₄ receptors at the apical membrane. The apparent lack of desensitization of the response is consistent with two processes: a rapid-onset phosphorylation of KCNE1 channel subunit and a slower-onset of regulation by depletion of plasma membrane PIP₂.

Pharmacological profiles of cloned mammalian P2Y-receptor subtypes

Membrane-bound P2-receptors mediate the actions of extracellular nucleotides in cell-to-cell signalling. P2X-receptors are ligand-gated ion channels, whereas P2Y-receptors belong to the superfamily of G-protein-coupled receptors (GPCRs). So far, the P2Y family is composed out of 8 human subtypes that have been cloned and functionally defined; species orthologues have been found in many vertebrates. P2Y1-, P2Y2-, P2Y4-, P2Y6-, and P2Y11-receptors all couple to stimulation of phospholipase C. The P2Y11-receptor mediates in addition a stimulation of adenylate cyclase. In contrast, activation of the P2Y12-, P2Y13-, and P2Y14-receptors causes an inhibition of adenylate cyclase activity. The expression of P2Y1-receptors is widespread. The receptor is involved in blood platelet aggregation, vasodilatation and neuromodulation. It is activated by ADP and ADP analogues including 2-methylthio-ADP (2-MeSADP). 2'-Deoxy-N6-methyladenosine-3',5'-bisphosphate (MRS2179) and 2-chloro-N6-methyl-(N)-methanocarba-2'-deoxyadenosine 3',5'-bisphosphate (MRS2279) are potent and selective antagonists. P2Y2 transcripts are abundantly distributed. One important example for its functional role is the control of chloride ion fluxes in airway epithelia. The P2Y2-receptor is activated by UTP and ATP and blocked by suramin. The P2Y2-agonist diquafosol is used for the treatment of the dry eye disease. P2Y4-receptors are expressed in the placenta and in epithelia. The human P2Y4-receptor has a strong preference for UTP as agonist, whereas the rat P2Y4-receptor is activated about equally by UTP and ATP. The P2Y4-receptor is not blocked by suramin. The P2Y6-receptor has a widespread distribution including heart, blood vessels, and brain. The receptor prefers UDP as agonist and is selectively blocked by 1,2-di-(4-isothiocyanatophenyl)ethane (MRS2567). The P2Y11-receptor may play a role in the differentiation of immunocytes. The human P2Y11-receptor is activated by ATP as naturally occurring agonist and it is blocked by suramin and reactive blue 2 (RB2). The P2Y12-receptor plays a crucial role in platelet aggregation as well as in inhibition of neuronal cells. It is activated by ADP and very potently by 2-methylthio-ADP. Nucleotide antagonists including N6-(2-methylthioethyl)-2-(3,3,3-trifluoropropylthio)-beta,gamma-dichloromethylene-ATP (=cangrelor; AR-C69931MX), the nucleoside analogue AZD6140, as well as active metabolites of the thienopyridine compounds clopidogrel and prasugrel block the receptor. These P2Y12-antagonists are used in pharmacotherapy to inhibit platelet aggregation. The P2Y13-receptor is expressed in immunocytes and neuronal cells and is again activated by ADP and 2-methylthio-ADP. The 2-chloro-5-nitro pyridoxal-phosphate analogue 6-(2'-chloro-5'-nitro-azophenyl)-pyridoxal-alpha5-phosphate (MRS2211) is a selective antagonist. mRNA encoding for the human P2Y14-receptor is found in many tissues. However, a physiological role of the receptor has not yet been established. UDP-glucose and related analogues act as agonists; antagonists are not known. Finally, UDP has been reported to act on receptors for cysteinyl leukotrienes as an additional agonist--indicating a dual agonist specificity of these receptors.

Modulation of endothelial cell migration by extracellular nucleotides: involvement of focal adhesion kinase and phosphatidylinositol 3-kinase-mediated pathways

Extracellular nucleotides bind to type-2 purinergic/pyrimidinergic (P2) receptors that mediate various responses, such as cell activation, proliferation and apoptosis, implicated in inflammatory processes. The role of P2 receptors and their associated signal transduction pathways in endothelial cell responses has not been fully investigated. Here, it is shown that stimulation of human umbilical vein endothelial cells (HUVEC) with extracellular ATP or UTP increased intracellular free calcium ion concentrations ([Ca(2+)](i)), induced phosphorylation of focal adhesion kinase (FAK), p130(cas) and paxillin, and caused cytoskeletal rearrangements with consequent cell migration. Furthermore, UTP increased migration of HUVEC in a phosphatidylinositol 3-kinase (PI3-K)-dependent manner. BAPTA or thapsigargin inhibited the extracellular nucleotide-induced increase in [Ca(2+)](i), a response crucial for both FAK phosphorylation and cell migration. Furthermore, long-term exposure of HUVEC to ATP and UTP, agonists of the G protein-coupled P2Y2 and P2Y4 receptor subtypes, caused upregulation of alpha(v) integrin expression, a cell adhesion molecule known to directly interact with P2Y2 receptors. Our results suggest that extracellular nucleotides modulate signaling pathways in HUVEC influencing cell functions, such as cytoskeletal changes, cellular adhesion and motility, typically associated with integrin-activation and the action of growth factors. We propose that P2Y2 and possibly P2Y4 receptors mediate those responses that are important in vascular inflammation, atherosclerosis and angiogenesis.

Modulation of endothelial cell migration by extracellular nucleotides: involvement of focal adhesion kinase and phosphatidylinositol 3-kinase-mediated pathways

Extracellular nucleotides bind to type-2 purinergic/pyrimidinergic (P2) receptors that mediate various responses, such as cell activation, proliferation and apoptosis, implicated in inflammatory processes. The role of P2 receptors and their associated signal transduction pathways in endothelial cell responses has not been fully investigated. Here, it is shown that stimulation of human umbilical vein endothelial cells (HUVEC) with extracellular ATP or UTP increased intracellular free calcium ion concentrations ([Ca(2+)](i)), induced phosphorylation of focal adhesion kinase (FAK), p130(cas) and paxillin, and caused cytoskeletal rearrangements with consequent cell migration. Furthermore, UTP increased migration of HUVEC in a phosphatidylinositol 3-kinase (PI3-K)-dependent manner. BAPTA or thapsigargin inhibited the extracellular nucleotide-induced increase in [Ca(2+)](i), a response crucial for both FAK phosphorylation and cell migration. Furthermore, long-term exposure of HUVEC to ATP and UTP, agonists of the G protein-coupled P2Y2 and P2Y4 receptor subtypes, caused upregulation of alpha(v) integrin expression, a cell adhesion molecule known to directly interact with P2Y2 receptors. Our results suggest that extracellular nucleotides modulate signaling pathways in HUVEC influencing cell functions, such as cytoskeletal changes, cellular adhesion and motility, typically associated with integrin-activation and the action of growth factors. We propose that P2Y2 and possibly P2Y4 receptors mediate those responses that are important in vascular inflammation, atherosclerosis and angiogenesis.

Activation of a P2Y4-like purinoceptor triggers an increase in cytosolic [Ca2+] in the red blood cells of the lizard Ameiva ameiva (Squamata, Teiidae)

An increasing number of pathophysiological roles for purinoceptors are emerging, some of which have therapeutic potential. Erythrocytes are an important source of purines, which can be released under physiological and physiopathological conditions, acting on purinergic receptors associated with the same cell or with neighboring cells. Few studies have been conducted on lizards, and have been limited to ATP agonist itself. We have previously shown that the red blood cells (RBCs) of the lizard Ameiva ameiva store Ca2+ in the endoplasmic reticulum (ER) and that the purinergic agonist ATP triggers a rapid and transient increase of [Ca2+]c by mobilization of the cation from internal stores. We also reported the ability of the second messenger IP3 to discharge the ER calcium pool of the ER. Here we characterize the purinoceptor present in the cytoplasmic membrane of the RBCs of the lizard Ameiva ameiva by the selective use of ATP analogues and pyrimidine nucleotides. The nucleotides UTP, UDP, GTP, and ATPgammaS triggered a dose-dependent response, while interestingly 2MeSATP, 2ClATP, alpha, ss-ATP, and ADP failed to do so in a 1- to 200-microm con- centration. The EC50 obtained for the compounds tested was 41.77 microM for UTP, 48.11 microM for GTP, 53.11 microM for UDP, and 30.78 microM for ATPgammaS. The present data indicate that the receptor within the RBCs of Ameiva ameiva is a P2Y4-like receptor due to its pharmacological similarity to the mammalian P2Y4 receptor.

P2X7-like receptor subunits enhance excitatory synaptic transmission at central synapses by presynaptic mechanisms

Recent studies demonstrate that P2X7 receptor subunits (P2X7RS) are present at central and peripheral synapses and suggest that P2X7RS can regulate transmitter release. In brainstem slices from 15 to 26 day old pentobarbitone-anesthetized mice, we examined the effect of P2X7RS activation on excitatory postsynaptic currents (EPSCs) recorded from hypoglossal motoneurons using whole-cell patch clamp techniques. After blockade of most P2X receptors with suramin (which is inactive at P2X7RS) and of adenosine receptors with 8-phenyltheophylline (8PT), bath application of the P2X receptor agonist 3'-0-(4-benzoyl)ATP (BzATP) elicited a 40.5+/-16.0% (mean+/-S.E.M., n = 8, P = 0.039) increase in evoked EPSC amplitude and significantly reduced paired pulse facilitation of evoked EPSCs. This response to BzATP (with suramin and 8PT present) was completely blocked by prior application of Brilliant Blue G (200 nM or 2 microM), a P2X7RS antagonist. In contrast, BzATP application with suramin and 8PT present did not alter miniature EPSC frequency or amplitude when action potentials were blocked with tetrodotoxin. These electrophysiological results suggest that P2X7RS activation increases central excitatory transmitter release via presynaptic mechanisms, confirming previous indirect measures of enhanced transmitter release. We suggest that possible presynaptic mechanisms underlying enhancement of evoked transmitter release by P2X7RS activation are modulation of action potential width or an increase in presynaptic terminal excitability, due to subthreshold membrane depolarization which increases the number of terminals releasing transmitter in response to stimulation.

Identification of the P2Y(12) receptor in nucleotide inhibition of exocytosis from bovine chromaffin cells

Nucleotides are released from bovine chromaffin cells and take part in a feedback loop to inhibit further exocytosis. To identify the nucleotide receptors involved, we measured the effects of a range of exogenous nucleotides and related antagonists on voltage-operated calcium currents (I(Ca)), intracellular calcium concentration ([Ca(2+)](i)), and membrane capacitance changes. In comparative parallel studies, we also cloned the bovine P2Y(12) receptor from chromaffin cells and determined its properties by coexpression in Xenopus laevis oocytes with inward-rectifier potassium channels made up of Kir3.1 and Kir3.4. In both systems, the agonist order of potency was essentially identical (2-methylthio-ATP approximately 2-methylthio-ADP >> ATP approximately ADP > UDP). alphabeta-Methylene-ATP and adenosine were inactive. UTP inhibited I(Ca) in chromaffin cells (pEC(50) = 4.89 +/- 0.11) but was essentially inactive at the cloned P2Y(12) receptor. The relatively nonselective P2 antagonist pyridoxal-phosphate-6-azophenyl-2',4' disulfonic acid blocked nucleotide responses in both chromaffin cells and X. laevis oocytes, whereas the P2Y(12)- and P2Y(13)-selective antagonist N(6)-(2-methylthioethyl)-2-(3,3,3-trifluoropropylthio)-beta,gamma-dichloromethylene ATP (ARC69931MX) blocked responses to ATP in both chromaffin cells and X. laevis oocytes but not to UTP in chromaffin cells. These results identify the P2Y(12) purine receptor as a key component of the nucleotide inhibitory pathway and also demonstrate the involvement of a UTP-sensitive G(i/o) -coupled pyrimidine receptor.

Nucleotide-mediated mucin secretion from differentiated human bronchial epithelial cells

Most current cell-based models for examining the regulation of mucin secretion demonstrate low signal-to-noise ratios, making experimental manipulation and data interpretation difficult. Using adenosine triphosphate (ATP) as a mucin secretagogue, we have developed a model of agonist-induced mucin secretion in differentiated human bronchial epithelial cells. Mucin secretory signals were estimated using enzyme-linked lectin assay, and typical signals of 300-400% of baseline were observed in response to a 30-min exposure to ATP (100 microM). ATP and uridine triphosphate equipotently stimulated mucin secretion consistent with mediation via P2Y2 receptor activation. Suramin and AR-C118925XX, a competitive P2Y2 receptor antagonist, inhibited adenosine 5'-o-(3-thiotriphosphate) (ATP-gammaS)-induced mucin secretion. A selective Gq G-protein antagonist (GP-ANT)-2A completely abrogated ATP-gammaS-induced mucin secretion. Pertussis toxin and the G(i/o)-specific, GP-ANT-2, had no effect. The phospholipase C inhibitor, D609, and the protein kinase C inhibitor, calphostin C, substantially inhibited ATP-gammaS-induced mucin secretion. Phorbol myristate acetate also stimulated mucin secretion in a calphostin C-sensitive manner. ATP-gammaS-induced mucin secretion was inhibited by the Ca2+ chelator, 1,2-bis(o-aminophenoxy) ethane-N,N,N',N'-tetra-acetic acid tetra (acetoxymethyl) ester. Ionomycin and thapsigargin both stimulated mucin secretion. Our data are broadly consistent with known G-protein-coupling and downstream signaling events associated with the P2Y2 receptor. The exceptional signal-to-noise ratios obtained using this model have permitted clear evaluation of the involvement of these mechanisms in agonist-induced mucin secretion from differentiated human bronchial epithelial cells.

Purinergic receptors coupled to intracellular Ca2+ signals and exocytosis in rat prostate neuroendocrine cells

Rat prostate neuroendocrine cells (RPNECs) display a variety of ion channels and exhibit alpha-adrenergic regulation of cytosolic Ca(2+) concentration ([Ca(2+)])(c). In this study, purinergic regulation of [Ca(2+)](c) and exocytosis was investigated in freshly isolated single RPNECs showing chromogranin A immunoreactivity. The presence of P2X and P2Y receptors in RPNECs was verified by the transient activation of Ca(2+)-permeable cationic channels and the release of Ca(2+) from intracellular stores by extracellular ATP, respectively. The transient inward cationic current was effectively activated by alpha,beta-methyleneadenosine 5'-triphosphate (alpha,beta-MeATP) and blocked by 2',3'-O-(2,4,6-trinitrophenyl)adenosine 5'-triphosphate, suggesting the presence of a P2X(1) or P2X(3) subtype. For the release of stored Ca(2+), ATP and UTP were equally potent, indicating the functional expression of the P2Y(2) or P2Y(4) subtype. The mRNAs for P2X(1) and P2Y(2) were confirmed from reverse transcription-PCR analysis of RPNECs. The application of alpha,beta-MeATP induced large and transient increases in [Ca(2+)](c), which were not attenuated by the blockers of voltage-activated Ca(2+) channels or by depleting intracellular Ca(2+) stores, but were abolished by omitting extracellular Ca(2+). The application of UTP increased [Ca(2+)](c) to 55% of the peak Delta[Ca(2+)](c) induced by alpha,beta-MeATP. The application of alpha,beta-MeATP induced exocytotic responses of RPNECs as monitored by carbon fiber amperometry and capacitance measurements. To our interest, the application of UTP did not induce amperometric currents, but reduced the membrane capacitance, indicating a net endocytosis. From these results, we postulate that a sharp rise in [Ca(2+)](c) by the P2X-mediated Ca(2+) influx is required for exocytosis, whereas the relatively slow release of stored Ca(2+) induces endocytosis in RPNECs.

ATP and UTP responses of cultured rat aortic smooth muscle cells revisited: dominance of P2Y2 receptors

1. It has previously been shown that ATP and UTP stimulate P2Y receptors in vascular smooth muscle cells (VSMCs), but the nature of these receptors, in particular the contribution of P2Y2 and P2Y4 subtypes, has not been firmly established. Here we undertake a further pharmacological analysis of [3H]inositol polyphosphate responses to nucleotides in cultured rat VSMCs. 2. ATP generated a response that was partial compared to UTP, as reported earlier. 3. In the presence of a creatine phosphokinase (CPK) system for regenerating nucleoside triphosphates, the response to ATP was increased, the response to UTP was unchanged, and the difference between UTP and ATP concentration-response curves disappeared. Chromatographic analysis showed that ATP was degraded slightly faster than UTP. 4. The response to UDP was always smaller than that to UTP, but with a shallow slope and a high potency component. In the presence of hexokinase (which prevents the accumulation of ATP/UTP from ADP/UDP), the maximum response to UDP was reduced and the high-potency component of the curve was retained. By contrast, the response to ADP was weaker throughout in the presence of hexokinase. 5. ATP gamma S was an effective agonist with a similar EC50 to UTP, but with a lower maximum. ITP was a weak agonist compared with UTP. 6. Suramin was an effective antagonist of the response to UTP (pA2=4.48), but not when ATP was the agonist. However, suramin was an effective antagonist (pA2=4.45) when stimulation with ATP was in the presence of the CPK regenerating system. 7. Taken together with the results of others, these findings indicate that the response of cultured rat VSMCs to UTP and to ATP is predominantly at the P2Y2 receptor, and that there is also a response to UDP at the P2Y6 receptor.

ATP modulates noradrenaline release by activation of inhibitory P2Y receptors and facilitatory P2X receptors in the rat vas deferens

The role of ATP on the modulation of noradrenaline release elicited by electrical stimulation (100 pulses/8 Hz) was studied in the prostatic portion of rat vas deferens preincubated with [3H]noradrenaline. In the presence of P1 antagonists, the nucleotides 2-methylthioadenosine-5'-triphosphate (2-MeSATP), 2-methylthioadenosine 5'-diphosphate (2-MeSADP), ADP, and ATP decreased electrically evoked tritium overflow up to 44%, with the following order of potency: 2-MeSATP > 2-MeSADP > ADP > or = ATP. The P2Y antagonists reactive blue 2 (RB2) and 2-methylthioadenosine 5'-monophosphate (2-MeSAMP) increased, whereas the P2X antagonist pyridoxal-5'-phosphate-6-(2'-naphthylazo-6'-nitro-4',8'-disulfonate) (PPNDS) decreased evoked tritium overflow. The inhibitory effect of 2-MeSATP was antagonized by RB2 (10 microM) and by 2-MeSAMP (10 microM) but not by the selective P2Y1 receptor antagonist 2'-deoxy-N6-methyladenosine 3',5'-bisphosphate (MRS 2179; 10 microM). When, besides P1 receptors, inhibitory P2Y receptors were blocked with RB2, alpha,beta-methyleneadenosine 5'-triphosphate (alpha,beta-meATP), beta,gamma-imidoadenosine 5'-triphosphate (beta,gamma-imidoATP), beta,gamma-methyleneadenosine 5'-triphosphate (beta,gamma-meATP), 2-MeSATP, and ATP enhanced tritium overflow up to 140%, with the following order of potency: alpha,beta-meATP > 2-MeSATP = ATP = beta,gamma-meATP > or = beta,gamma-imidoATP. The facilitatory effects of alpha,beta-MeATP and beta,gamma-imidoATP were prevented by PPNDS. Under the same conditions, apyrase attenuated, whereas the ectonucleotidase inhibitor 6-N,N-diethyl-D-beta,gamma-dibromomethylene 5'-triphosphate enhanced tritium overflow, an effect that was prevented by PPNDS. In the prostatic portion of the rat vas deferens, endogenous ATP exerts a dual and opposite modulation of noradrenaline release: an inhibition through activation of P2Y receptors with a pharmacological profile similar to that of the P2Y12 and P2Y13 receptors and a facilitation through activation of P2X receptors with a pharmacological profile similar to that of P2X1 and P2X3, or PX2/P2X3 receptors.

Extended pharmacological profiles of rat P2Y2 and rat P2Y4 receptors and their sensitivity to extracellular H+ and Zn2+ ions

1. Two molecularly distinct rat P2Y receptors activated equally by adenosine-5'-triphosphate (ATP) and uridine-5'-triphosphate (UTP) (rP2Y2 and rP2Y4 receptors) were expressed in Xenopus oocytes and studied extensively to find ways to pharmacologically distinguish one from the other. 2. Both P2Y subtypes were activated fully by a number of nucleotides. Tested nucleotides were equipotent at rP2Y4 (ATP=UTP=CTP=GTP=ITP), but not at rP2Y2 (ATP=UTP>CTP>GTP>ITP). For dinucleotides (ApnA, n=2-6), rP2Y4 was only fully activated by Ap4A, which was as potent as ATP. All tested dinucleotides, except for Ap2A, fully activated rP2Y2, but none were as potent as ATP. ATP gamma S and BzATP fully activated rP2Y2, whereas ATP gamma S was a weak agonist and BzATP was inactive (as an agonist) at rP2Y4 receptors. 3. Each P2Y subtype showed different sensitivities to known P2 receptor antagonists. For rP2Y2, the potency order was suramin>>PPADS= RB-2>TNP-ATP and suramin was a competitive antagonist (pA2, 5.40). For rP2Y4, the order was RB-2>>suramin>PPADS> TNP-ATP and RB-2 was a competitive antagonist (pA2, 6.43). Also, BzATP was an antagonist at rP2Y4 receptors. 4. Extracellular acidification (from pH 8.0 to pH 5.5) enhanced the potency of ATP and UTP by 8-10-fold at rP2Y4 but did not affect agonist responses at rP2Y2 receptors. 5. Extracellular Zn2+ ions (0.1-300 microM) coapplied with ATP inhibited agonist responses at rP2Y4 but not at rP2Y2 receptors. 6. These two P2Y receptors differ significantly in terms of agonist and antagonist profiles, and the modulatory activities of extracellular H+ and Zn2+ ions. These pharmacological differences will help to distinguish between rP2Y2 and rP2Y4 receptors, in vivo.

Role of phospholipase D in agonist-stimulated lysophosphatidic acid synthesis by ovarian cancer cells

Lysophosphatidic acid (LPA) is a receptor-active lipid mediator with a broad range of biological effects. Ovarian cancer cells synthesize LPA, which promotes their motility, growth, and survival. We show that a murine homolog of a human protein previously reported to hydrolyze LPA is a highly selective detergent-stimulated LPA phosphatase that can be used to detect and quantitate LPA. Use of this protein in novel enzymatic assay demonstrates that SK-OV-3 ovarian cancer cells release physiologically relevant levels of biologically active LPA into the extracellular space. LPA release is markedly increased by nucleotide agonists acting through a P2Y4 purinergic receptor. Promotion of LPA formation by nucleotides is accompanied by stimulation of phospholipase D (PLD) activity. Overexpression of both PLD1 and PLD2 in SK-OV-3 cells produces active enzymes, but only overexpression of PLD2 results in significant amplification of both nucleotide-stimulated PLD activity and LPA production. SK-OV-3 cells express and secrete a phospholipase A2 activity that can generate LPA from the lipid product of PLD, phosphatidic acid. Our results identify a novel role for nucleotides in the regulation of ovarian cancer cells and suggest an indirect but critical function for PLD2 in agonist-stimulated LPA production.

The P2Y(1) receptor as a target for new antithrombotic drugs: a review of the P2Y(1) antagonist MRS-2179

MRS-2179 is a selective P2Y(1) receptor antagonist, a strong inhibitor of ADP-induced platelet aggregation in vitro and ex vivo. By i.v. administration to mice MRS-2179 increases resistance to thromboembolism induced by a mixture of collagen and epinephrine or by a tissue factor. Likewise, it significantly increases the time to thrombus formation in a ferric chloride-induced model of localized arterial thrombosis. MRS-2179 also confers resistance to localized venous thrombosis, which is dependent on thrombin generation and in which platelets play a relatively minor role as compared to stasis or activation of coagulation. These data provide considerable encouragement for the development of new P2Y(1) receptor antagonists. Nevertheless, the properties of MRS-2179 indicate that new compounds should be optimized in order to increase the half-life of the molecule in vivo and its selectivity and potency at the P2Y(1) receptor. Further directions include the synthesis of molecules with modifications of the nucleotide structure which replace the fragile moiety by a stable bond and should lead to a non-hydrolysable structure. In conclusion, P2Y(1) antagonists have been shown to be efficient antithrombotic agents. MRS-2179 is the first P2Y(1) antagonist with antithrombotic action. Its effectiveness demonstrates that the P2Y(1) receptor is a potentially promising target for drugs designed to treat thrombotic syndromes.

P2Y receptor regulation of cultured rat cerebral cortical cells: calcium responses and mRNA expression in neurons and glia

1 We have investigated increases in cytosolic Ca(2+) in response to nucleotides in mixed rat cerebrocortical cultures (neurons and glia in similar numbers) and in essentially neuron-free glial cultures. 2 In both cultures, the agonist-response profile was 2-methylthioADP(2MeSADP)>2-methylthioATP(2MeSATP)>ADP>ATP>adenosine 5'-O-(3-thiotriphosphate), consistent with a P2Y(1) receptor. The maximal responses to 2MeSADP, 2MeSATP and ADP were identical, but that to ATP was higher. 3 Suramin, pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid, reactive blue 2 (RB2), and adenosine biphosphate (A3P5P) were antagonists with apparent pA(2) values of 5.5 for suramin, 6.4 for RB2, and 4.7 for A3P5P. 4 Single cell imaging divided the cells from the mixed neuronal-glial cultures into two populations: responsive (neurons) and unresponsive (glial cells) to high [K(+)]. The response of cells to nucleotides was almost exclusively limited to those not responsive to high K(+). 5 In the presence of extracellular Mn(2+), the response of the mixed cultures to 30 mM K(+) and 20 micro M Bay K 8644 was attenuated. However, when 2MeSADP was added there was no reduction in response in cultures previously loaded with Mn(2+). This further indicated that the 2MeSADP response was not in the neurons. 6 Reverse transcriptase-polymerase chain reaction studies detected transcripts for P2Y(1), P2Y(4) and P2Y(6) in RNA preparations from embryonic rat cortex, and from both mixed and glial cultures. P2Y(2) transcripts were not detected in the embryonic cortex. 7 Based on this and previous work, it is proposed that the principal P2Y influences in the brain are on cytosolic Ca(2+) in glial cells and presynaptic sites on neurons.

Acute downregulation of Cx43 alters P2Y receptor expression levels in mouse spinal cord astrocytes

Propagation of intercellular calcium waves (ICW) between astrocytes depends on the diffusion of signaling molecules through gap junction channels and diffusion through the extracellular space of neuroactive substances acting on plasmalemmal receptors. The relative contributions of these two pathways vary in different brain regions and under certain pathological conditions. We have previously shown that in wild-type spinal cord astrocytes, ICW are primarily gap junction-dependent, but that deletion of the main gap junction protein (Cx43) by homologous recombination results in a switch in mode of ICW propagation to a purinoceptor-dependent mechanism. Such a compensatory mechanism for ICW propagation was related to changes in the pharmacological profile of P2Y receptors, from an adenine-sensitive P2Y(1), in wild-type, to a uridine-sensitive P2U receptor subtype, in Cx43 knockout (KO) astrocytes. Using oligonucleotide antisense to Cx43 mRNA for acute downregulation of connexin43 expression levels, we provide evidence for the molecular nature of such compensatory mechanism. Pharmacological studies and Western blot analysis indicate that there is a reciprocal regulation of P2Y(1) and P2Y(4) expression levels, such that downregulation of Cx43 leads to decreased expression of the adenine-sensitive P2Y(1) receptor and increased expression of the uridine-sensitive P2Y(4) receptor. This change in functional expression of the P2Y receptor subtype population in acutely downregulated Cx43 was paralleled by changes in the mode of ICW propagation, similar to that previously observed for Cx43 KO spinal cord astrocytes. On the basis of these results, we propose that Cx43 regulates both modes of ICW by altering P2Y receptor subtype expression in addition to providing intercellular coupling.

Pharmacological characterization of P2Y receptor subtypes on isolated tiger salamander Müller cells

Müller cells express a variety of neurotransmitter receptors that permit them to "sense" the extracellular environment within the retina. We have used a battery of agonists and antagonists to characterize the purinergic receptor subtypes expressed on isolated tiger salamander Müller cells. Changes in intracellular calcium ion concentration ([Ca(2+)](i)) in Müller cells were measured using the Ca(2+) indicator dye Fura-2 and digital imaging microscopy. ATP, 2-methylthio-ATP, 2-methylthio-ADP, ADP, UTP, UDP, deoxyATP, and 3'-O-(4-benzoyl)benzoyl ATP evoked increases in [Ca(2+)](i) in both the presence and absence of extracellular Ca(2+). Therefore, the increases we observed were likely due to intracellular Ca(2+) release mediated by G-protein-coupled P2Y receptor activation, rather than Ca(2+) influx via P2X receptor channels. The P2Y(1) receptor agonists 2-methylthio-ATP, 2-methylthio-ADP, and ADP evoked increases in [Ca(2+)](i) that were inhibited by the P2Y(1) receptor antagonists adenosine 3'-phosphate 5'-phosphosulfate and 2'-deoxy-N(6)-methyleneadenosine-3',5'-bisphosphate. Responses to ADP were not completely inhibited by the P2Y(1) receptor antagonists. The residual response to ADP could be mediated by P2Y(13) receptors. UTP evoked an increase in [Ca(2+)](i) that was partially inhibited by suramin, suggesting that Müller cells express P2Y(2) and P2Y(4) receptors. The P2Y(6) receptor agonist UDP, and the P2Y(11) receptor agonists deoxyATP, and 3'-O-(4-benzoyl)benzoyl ATP, evoked increases in [Ca(2+)](i) in Müller cells. We conclude that isolated tiger salamander Müller cells express P2Y(1), P2Y(2), P2Y(6), P2Y(11), and possibly P2Y(4) and P2Y(13) receptors. Therefore, the physiological release of ATP, ADP, UTP, and UDP and/or their accumulation in the retina under pathological conditions could stimulate increases in [Ca(2+)](i) in Müller cells.

Regional variation in P2 receptor expression in the rat pulmonary arterial circulation

The P2 receptors that mediate contraction of the rat isolated small (SPA, 200-500 micro m i.d.) and large (LPA, 1-1.5 mM i.d.) intrapulmonary arteries were characterized. 2 In endothelium-denuded vessels the contractile order of potency was alpha,beta-methyleneATP (alpha,beta-meATP)>>UDP=UTP=ATP=2-methylthioATP>ADP in the SPA and alpha,beta-meATP=UTP>or=UDP>2-methylthioATP, ATP>>ADP in the LPA. alpha,beta-meATP, 2-methylthioATP and ATP had significantly greater effects in the SPA than the LPA (P<0.001), but there was no difference in the potency of UTP or UDP between the vessels. 3 In the SPA, P2X1 receptor desensitisation by alpha,beta-meATP (100 microM) inhibited contractions to alpha,beta-meATP (10 nM-300 microM), but not those to UTP or UDP (100 nM-300 microM). In the LPA, prolonged exposure to alpha,beta-meATP (100 microM) did not desensitize P2X receptors. 4 Pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), suramin and reactive blue 2 (RB2) (30-300 microM) inhibited contractions evoked by alpha,beta-meATP. UTP and UDP were potentiated by PPADS, unaffected by RB2 and inhibited, but not abolished by suramin. 1 and 3 mM suramin produced no further inhibition, indicating suramin-resistant components in the responses to UTP and UDP. 5 Thus, both P2X and P2Y receptors mediate contraction of rat large and small intrapulmonary arteries. P2Y agonist potency and sensitivity to antagonists were similar in small and large vessels, but P2X agonists were more potent in small arteries. This indicates differential expression of P2X, but not P2Y receptors along the pulmonary arterial tree.

Pharmacological characterization of a UTP-sensitive P2Y nucleotide receptor in organ cultured coronary arteries

Our lab has previously demonstrated that organ cultured coronary smooth muscle cells express a nucleotide receptor that is dramatically more responsive to UTP than non-organ cultured cells. Thus, the purpose of this study was to pharmacologically characterize this UTP-sensitive nucleotide receptor. Porcine coronary arteries were organ cultured (serum-free media, 37 degrees C) for 4 days, and fura-2 imaging of single cells was used to measure myoplasmic Ca2+ (Cam) in response to several nucleotide agonists. A concentration-response relationship (0.01-100 microM) was generated to the nucleotide receptor agonists, UTP, UDP, ATP, ADP, and 2-MeSATP. The potency order was UTP >> UDP = ATP = ADP = 2-MeSATP, thus, this nucleotide receptor is predominantly UTP-sensitive. The Cam response to 10 microM UTP was attenuated approximately 50% by the nucleotide receptor antagonists (10 and 100 microM), suramin, reactive blue 2, and pyridoxalphosphate-6-azophenyl-2',4'-disulphonoic acid (PPADS). Depletion of the sarcoplasmic reticulum Ca2+ store with thapsigargin completely abolished the UTP-induced Cam response. In addition, the peak UTP-induced Cam increase was almost two-fold higher in a 2-mM Ca2+ solution than a 0-mM Ca2+ solution. This suggests that the UTP-induced Cam response is comprised of both Ca2+ influx and the mobilization of intracellular Ca2+ stores. Pertussis toxin reduced the UTP-induced Cam response 50%, thus, the UTP-induced increase in Cam is mediated, in part, via Gi/o. These data suggest this UTP-sensitive receptor belongs to the P2Y nucleotide receptor family; however, it does not possess pharmacological characteristics associated with any known P2Y receptor subtype.

Nucleotide receptors involved in UTP-induced rat arterial smooth muscle cell migration

Many factors have been shown to be involved in the development of hyperplasic lesions of vessels, but the role of extracellular nucleotides remains largely unknown. The presence of P2Y and P2X nucleotide receptors on arterial endothelial and smooth muscle cells suggests a potential role for nucleotides in the vessel pathophysiology. Although the role of P2X in physiology of vessels is well documented, that of P2Y is not completely understood. We recently demonstrated that extracellular nucleotides, and particularly UTP, induced migration of cultured arterial smooth muscle cells (ASMCs). This migration is dependent on osteopontin expression and involves the Rho and mitogen-activated protein (MAP) kinase pathways. An important question is to determine the specific role of the different P2Y receptors of rat ASMCs in the UTP-induced migration process. Therefore, we first quantified mRNA levels of P2Y(2), P2Y(4), and P2Y(6) nucleotide receptors in cultured rat ASMCs by a competitive RT-PCR approach and demonstrated that P2Y(2) is the most highly expressed among these receptors potentially involved in the UTP-mediated response. In addition to UTP, UDP also induced ASMC migration even when UTP regeneration was inhibited, suggesting the involvement of UDP receptor P2Y(6). Moreover, suramin, a specific antagonist of rat P2Y(2) receptor, acted as an inhibitor of UTP-induced migration. Taken together, these results suggest a prominent role for the UTP receptor, P2Y(2), and for the UDP receptor, P2Y(6), in UTP-induced rat ASMC migration.

Cold storage of rabbit thoracic aorta in University of Wisconsin solution attenuates P2Y(2) purine receptors

Post-transplantation thrombosis may occur in donor segments of iliac arteries and livers following surgical removal and storage in University of Wisconsin (UW) solution for transplantation. We have previously suggested that purine receptors are vulnerable to denaturation after UW storage. The aims of the present study were to determine what particular subtypes of purine P2Y receptors in rabbit thoracic aorta deteriorate after 8 days of UW storage by studying vascular reactivity to acetylcholine, ATP, 2MeSATP and UTP. Ring segments of aortae from male New Zealand White rabbits were mounted upon fine-wire myographs and vasodilatation to the above agents tested on fresh tissue, and after 8 days of UW storage. Vasodilatation to ATP was attenuated by 100 microM L-NAME in fresh tissue suggesting that the relaxant response was, in part, due to nitric oxide (NO). P2Y-mediated relaxation to ATP was significantly attenuated by UW storage and cholinergic responses were not. This attenuated relaxation to ATP was not further attenuated by L-NAME, suggesting a loss of the NO-dependent mechanism. De-endothelialisation indicated that UTP-mediated vasorelaxation, via P2Y(2) receptors, was endothelium-dependent. Any residual endothelium-independent relaxation to UTP was abolished by UW storage and endothelium-dependent UTP relaxation was reduced to the same level as that seen in fresh, de-endothelialised tissue. In contrast responses to 2MeSATP, via P2Y(1) receptors, were predominantly endothelium-independent and were only partially attenuated by UW storage. Responses to pyridoxalphosphate-6-azophenyl-2('),4(')-disulphonic acid (PPADS) and L-NAME suggested that vasorelaxation to 2MeSATP and UTP was mediated by P2Y(1) and P2Y(2) receptors, respectively. It is therefore concluded that UW storage predominantly decreases P2Y(2) receptor-mediated vascular reactivity.

Nucleotide-mediated relaxation in guinea-pig aorta: selective inhibition by MRS2179

The vasodilatory effects of nucleotides in the guinea-pig thoracic aorta were examined to determine the relationship between molecular expression and function of P2Y receptors. In aortic rings precontracted with norepinephrine, vasodilatory responses to purine nucleotides exhibited a rank-order of potency of 2-methylthio-ATP>ADP>ATP. Responses to UTP, but not UDP suggested a functional role for P2Y4 but not P2Y6 receptors. Aortic endothelial cells express at least four P2Y receptors; P2Y1, P2Y2, P2Y4 and P2Y6. In primary culture, these cells exhibit desensitizing transient calcium responses characteristic of P2Y1, P2Y2 and P2Y4, but not P2Y6 receptors. UDP had no effect on endothelial cell calcium. The pyrimidinergic receptor agonist UTP is capable of eliciting robust vasodilation in aortic rings and causing calcium responses in cultured guineapig aortic endothelial cells. These responses are equivalent to the maximum responses observed to ATP and ADP. Measurement of intracellular calcium release in response to ATP and 2-methylthio-ATP were similar, however only the 2-methylthio-ATP response was sensitive to the P2Y1 antagonist N(6)-methyl-2'-deoxyadenosine-3',5'-bisphosphate (MRS2179). In aortic rings, vasodilatory responses to 2-methylthio-ATP, ATP and ADP were all blocked by pre-incubation of tissues with MRS2179. MRS2179 pretreatment had no effect of the ability of UTP to cause relaxation of norepinephrine responses in aortic rings or the ability of UTP to cause calcium release in aortic endothelial cells. We demonstrate robust effects of purine and pyrimidine nucleotides in guineapig aorta and provide functional and biochemical evidence that MRS2179 is a selective P2Y1 antagonist.

Apical P2Y4 purinergic receptor controls K+ secretion by vestibular dark cell epithelium

It was previously shown that K+ secretion by vestibular dark cell epithelium is under control of G protein-coupled receptors of the P2Y family in the apical membrane that are activated by both purine and uridine nucleotides (P2Y2, P2Y4, or P2Y6). The present study was conducted to determine the subtype of purinergic receptor and to test whether these receptors undergo desensitization. The transepithelial short-circuit current represents electrogenic K+ secretion and was found to be reduced by UTP, ATP, and diadenosine tetraphosphate, but not UDP. Neither pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS, 30 microM) nor suramin (100 microM) inhibited the effect of UTP. The potencies of the agonists were consistent with rodent P2Y4 and P2Y2, but not P2Y6, receptors. The ineffectiveness of suramin was consistent with P2Y4, but not P2Y2. Transcripts for both P2Y2 and P2Y4 were found in vestibular labyrinth. Sustained exposure to ATP or UTP for 15 min caused a constant depression of short-circuit current with no apparent desensitization. The results support the conclusion that regulation of K+ secretion across vestibular dark cell epithelium occurs by P2Y4 receptors without desensitization of the response.

Molecular cloning and characterization of the mouse P2Y4 nucleotide receptor

To isolate the mouse P2Y4 receptor gene, a mouse genomic library was screened with a human P2Y4 probe. An open reading frame encoding a protein of 361 amino acids was isolated. This protein showed 82% and 95% amino acid identity with the human and rat P2Y4 receptors, respectively. By reverse transcription and polymerase chain reaction (RT-PCR), the P2Y4 messenger RNA was detected in mouse liver, intestine, stomach, bladder and lung among the 16 mouse tissues tested. In 1321N1 transfected cells, the mouse P2Y4 receptor was equally activated by UTP and ATP, and was antagonized by pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) and Reactive Blue 2, and not by suramin. Moreover, when expressed in 1321N1 cells, the rat P2Y4 is also antagonized by PPADS. Thus, when compared in the same expression system, the mouse P2Y4 is closer to the rat ortholog in terms of agonist stimulation, while in terms of antagonist profile, the three P2Y4 receptor orthologs are similar.

Methyl orange antagonizes uridine 5' triphosphate and not alpha,beta-methylene-adenosine 5' triphosphate-evoked depolarization of rat superior cervical ganglia

1. Compared with the effects of adenosine 5' triphosphate (ATP) on the nervous system, the actions of pyrimidine nucleosides and their 5'-nucleotides, such as uridine 5' triphosphate (UTP), have received less attention. In part, this is because there is a need for a selective antagonist for responses mediated by UTP-activated receptors. The objective of this study was to discover such an antagonist. 2. Superior cervical ganglia isolated from male rats were superfused with a physiological salt solution. Responses to alpha,beta-methylene-ATP (alpha,beta-Me-ATP), potassium, adenosine and UTP were determined before and in the presence of 1-300 microM methyl orange. 3. Methyl orange at 1-100 microM did not alter resting potential or depolarizing responses to alpha,beta-Me-ATP, potassium, or adenosine-evoked hyperpolarizations, but at 10 and 100 microM methyl orange significantly antagonized UTP-evoked depolarizations (P < 0.05). 4. Although the antagonistic effects of methyl orange were not dramatic, this is the first report of a putative pyrimidinoceptor antagonist. These observations also support the idea of distinct receptors for UTP and ATP on rat superior cervical ganglia.