ADP receptor P2Y12 is expressed in vascular smooth muscle cells and stimulates contraction in human blood vessels

Characterization of P2Y receptors mediating ATP induced relaxation in guinea pig airway smooth muscle: involvement of prostaglandins and K+ channels

In airway smooth muscle (ASM), adenosine 5'-triphosphate (ATP) induces a relaxation associated with prostaglandin production. We explored the role of K(+) currents (I (K)) in this relaxation. ATP relaxed the ASM, and this effect was abolished by indomethacin. Removal of airway epithelium slightly diminished the ATP-induced relaxation at lower concentration without modifying the responses to ATP at higher concentrations. ATPγS and UTP induced a concentration-dependent relaxation similar to ATP; α,β-methylene-ATP was inactive from 1 to 100 μM. Suramin or reactive blue 2 (RB2), P2Y receptor antagonists, did not modify the relaxation, but their combination significantly reduced this effect of ATP. The relaxation was also inhibited by N-ethylmaleimide (NEM; which uncouples G proteins). In myocytes, the ATP-induced I (K) increment was not modified by suramin or RB2 but the combination of both drugs abolished it. This increment in the I (K) was also completely nullified by NEM and SQ 22,536. 4-Amynopyridine or iberiotoxin diminished the ATP-induced I (K) increment, and the combination of both substances diminished ATP-induced relaxation. The presence of P2Y(2) and P2Y(4) receptors in smooth muscle was corroborated by Western blot and confocal images. In conclusion, ATP: (1) produces relaxation by inducing the production of bronchodilator prostaglandins in airway smooth muscle, most likely by acting on P2Y(4) and P2Y(2) receptors; (2) induces I (K) increment through activation of the delayed rectifier K(+) channels and the high-conductance Ca(2+)-dependent K(+) channels, therefore both channels are implicated in the ATP-induced relaxation; and (3) this I (K) increment is mediated by prostaglandin production which in turns increase cAMP signaling pathway.

P2 receptors and platelet function

Following vessel wall injury, platelets adhere to the exposed subendothelium, become activated and release mediators such as TXA(2) and nucleotides stored at very high concentration in the so-called dense granules. Released nucleotides and other soluble agents act in a positive feedback mechanism to cause further platelet activation and amplify platelet responses induced by agents such as thrombin or collagen. Adenine nucleotides act on platelets through three distinct P2 receptors: two are G protein-coupled ADP receptors, namely the P2Y(1) and P2Y(12) receptor subtypes, while the P2X(1) receptor ligand-gated cation channel is activated by ATP. The P2Y(1) receptor initiates platelet aggregation but is not sufficient for a full platelet aggregation in response to ADP, while the P2Y(12) receptor is responsible for completion of the aggregation to ADP. The latter receptor, the molecular target of the antithrombotic drugs clopidogrel, prasugrel and ticagrelor, is responsible for most of the potentiating effects of ADP when platelets are stimulated by agents such as thrombin, collagen or immune complexes. The P2X(1) receptor is involved in platelet shape change and in activation by collagen under shear conditions. Each of these receptors is coupled to specific signal transduction pathways in response to ADP or ATP and is differentially involved in all the sequential events involved in platelet function and haemostasis. As such, they represent potential targets for antithrombotic drugs.

A(2A) adenosine receptor-mediated increase in coronary flow in hyperlipidemic APOE-knockout mice

Adenosine-induced coronary vasodilation is predominantly A_2A adenosine receptor (AR)-mediated, whereas A₁ AR is known to negatively modulate the coronary flow (CF). However, the coronary responses to adenosine in hyperlipidemia and atherosclerosis are not well understood. Using hyperlipidemic/atherosclerotic apolipoprotein E (APOE)–knockout mice, CF responses to nonspecific adenosine agonist (5′-N-ethylcarboxamide adenosine, NECA) and specific adenosine agonists (2-chloro-N6-cyclopentyl-adenosine [CCPA, A₁ AR-specific] and CGS-21680, A_2A AR-specific) were assessed using isolated Langendorff hearts. Western blot analysis was performed in the aorta from APOE and their wild-type (WT) control (C57BL/6J). Baseline CF (expressed as mL/min/g heart weight) was not different among WT (13.23 ± 3.58), APOE (13.22 ± 2.78), and APOE on high-fat diet (HFD) for 12 weeks (APOE-HFD, 12.37 ± 4.76). Concentration response curves induced by CGS-21680 were significantly shifted to the left in APOE and APOE-HFD when compared with WT. CCPA induced an increase in CF only at 10⁻⁶ M in all groups and the effect was reversed by the addition of a selective A_2A AR antagonist, SCH-58261 (10⁻⁶ M), and a significant decrease in CF from baseline was observed. Western blot analysis showed a significant upregulation of A_2A AR in the aorta from APOE and APOE-HFD. This study provides the first evidence that CF responses to A_2A AR stimulation were upregulated in hyperlipidemic/atherosclerotic animals. The speculation is that the use of A_2A AR-specific agonist for myocardial perfusion imaging (such as regadenoson) could overestimate the coronary reserve in coronary artery disease patients.

Endothelium-independent vasorelaxation by ticlopidine and clopidogrel in rat caudal artery

OBJECTIVES

Thienopyridines are prodrugs currently used as anti-aggregating agents. The aim of this study was to determine if these compounds might have vascular activity independent of hepatic bioactivation.

METHODS

The direct activity of thienopyridines was studied in rat caudal arterial rings and aortic smooth muscle cells in culture.

KEY FINDINGS

Both compounds (0.01 µm-100 µm) showed a concentration-dependent vasorelaxation in arterial tissues precontracted with phenylephrine, 5-hydroxytryptamine and KCl. The relaxation induced by 100 µm ticlopidine and clopidogrel was greater than 80%. The relaxation by ticlopidine was compared with the activity of acetylcholine. These two agents showed similar potency, although ticlopidine was slightly more active. Pretreatment with the nitric oxide synthase inhibitor L-NAME inhibited the relaxation by acetylcholine but not that by ticlopidine. To further study vasorelaxation by ticlopidine, other pharmacological inhibitors including propranolol, nifedipine and suramin were used. These compounds lacked inhibitory effects on the vasorelaxation by ticlopidine. In vascular smooth muscle cells, 1 µm ticlopidine induced a decrease in cell proliferation, while incubation with both ticlopidine and ADP or 2-methioADP led to an additive effect.

CONCLUSIONS

The data suggest that ticlopidine and clopidogrel cause relaxation of arterial tissues and influence vascular smooth muscle cell proliferation directly without hepatic biotransformation. Furthermore, the arterial relaxation induced in vitro by thienopyridines is endothelium independent, and β-adrenergic and P2 receptors are not involved.

Clopidogrel improves endothelial function and NO bioavailability by sensitizing adenylyl cyclase in rats with congestive heart failure

Clopidogrel treatment in patients with coronary artery disease not only inhibits platelet activation but also improves endothelial function and nitric oxide (NO) bioavailability. Congestive heart failure (CHF) is associated with endothelial dysfunction and increased platelet activation. In rats with CHF following myocardial infarction (MI), we investigated whether treatment with clopidogrel modifies endothelial function. Eight weeks after coronary artery ligation, rats with CHF were randomized to placebo or the P2Y(12) receptor antagonist clopidogrel (5 mg/kg twice daily, given by gavage) for another 2 weeks. Afterwards, endothelial function was assessed in isolated aortic rings in organ bath experiments. Acetylcholine-induced, endothelium-dependent, nitric oxide-mediated vasorelaxation was significantly attenuated in CHF rats compared to sham-operated animals, and was significantly improved by treatment with clopidogrel. Adenosine-induced vasorelaxation via adenylyl cyclase stimulation was attenuated in CHF and significantly improved by clopidogrel. Increased vasoconstriction to phenylephrine was observed in CHF, particularly evident under cyclooxygenase inhibition, but prevented by clopidogrel treatment. Vasoconstriction by the P2Y(12) activator 2MeS-ADP was increased in CHF. Clopidogrel-treated CHF animals displayed enhanced phosphorylation of AKT and eNOS. In conclusion, clopidogrel improved endothelial function and NO bioavailability in heart failure. During CHF, sensitivity to P2Y(12) signaling was increased leading to impaired adenylyl cyclase-mediated signaling. Chronic P2Y(12)-blockade with clopidogrel improved adenylyl cyclase-mediated signaling including increased AKT- and eNOS-phosphorylation contributing to improved NO-mediated vasorelaxation.

P2Y receptors in health and disease

The purine- and pyrimidine-sensitive P2Y receptors belong to the large group of G-protein-coupled receptors that are the target of approximately one-third of the pharmaceutical drugs used in the clinic today. It is therefore not unexpected that the P2Y receptors could be useful targets for drug development. This chapter will discuss P2Y receptor-based therapies currently used, in development and possible future developments. The platelet inhibitors blocking the ADP-receptor P2Y(12) reduce myocardial infarction, stroke, and mortality in patients with cardiovascular disease. Clopidogrel (Plavix) was for many years the second most selling drug in the world. The improved P2Y(12) inhibitors prasugrel, ticagrelor, and elinogrel are now entering the clinic with even more pronounced protective effects. The UTP-activated P2Y(2) receptor stimulates ciliary movement and secretion from epithelial cells. Cystic fibrosis is a monogenetic disease where reduced chloride ion secretion results in a severe lung disease and early death. No specific treatment has been available, but the P2Y(2) agonist Denufosol has been shown to improve lung function and is expected to be introduced as treatment for cystic fibrosis soon. In preclinical studies, there are indications that P2Y receptors can be important for diabetes, osteoporosis, cardiovascular, and atherosclerotic disease. In conclusion, P2Y receptors are important for the health of humans for many diseases, and we can expect even more beneficial drugs targeting P2Y receptors in the future.

Ectonucleotidases as regulators of purinergic signaling in thrombosis, inflammation, and immunity

Evolving studies in models of transplant rejection, inflammatory bowel disease, and cancer, among others, have implicated purinergic signaling in clinical manifestations of vascular injury and thrombophilia, inflammation, and immune disturbance. Within the vasculature, spatial and temporal expression of CD39 nucleoside triphosphate diphosphohydrolase (NTPDase) family members together with CD73 ecto-5'-nucleotidase control platelet activation, thrombus size, and stability. This is achieved by closely regulated phosphohydrolytic activities to scavenge extracellular nucleotides, maintain P2-receptor integrity, and coordinate adenosinergic signaling responses. The CD38/CD157 family of extracellular NADases degrades NAD(+) and generates Ca(2+)-active metabolites, including cyclic ADP ribose and ADP ribose. These mediators regulate leukocyte adhesion and chemotaxis. These mechanisms are crucial in vascular homeostasis, hemostasis, thrombogenesis, and during inflammation. There has been recent interest in ectonucleotidase expression by immune cells. CD39 expression identifies Langerhans-type dendritic cells and efficiently distinguishes T regulatory cells from other resting or activated T cells. CD39, together with CD73 in mice, serves as an integral component of the suppressive machinery of T cells. Purinergic responses also impact generation of T helper-type 17 cells. Further, CD38 and changes in NAD(+) availability modulate ADP ribosylation of the cytolytic P2X7 receptor that deletes T regulatory cells. Expression of CD39, CD73, and CD38 ectonucleotidases on either endothelial or immune cells allows for homeostatic integration and control of vascular inflammatory and immune cell reactions at sites of injury. Ongoing development of therapeutic strategies targeting these and other ectonucleotidases offers promise for the management of vascular thrombosis, disordered inflammation, and aberrant immune reactivity.

Platelets as initiators and mediators of inflammation at the vessel wall

Platelets are dynamic cells with activities that extend beyond thrombosis including an important role in initiating and sustaining vascular inflammation. A role for platelets has been described in many physiologic and pathophysiologic processes such as atherosclerosis, stem cell trafficking, tumor metastasis, and arthritis. Platelet activation at sites of an intact inflamed endothelium contributes to vascular inflammation and vascular wall remodeling. Platelets secrete a wide array of preformed and synthesized inflammatory mediators upon activation that can exert significant local and systemic effects. This review will focus on the role of platelet derived mediators in vascular inflammation and vascular wall remodeling.

Regulation of functionally active P2Y12 ADP receptors by thrombin in human smooth muscle cells and the presence of P2Y12 in carotid artery lesions

OBJECTIVE

The platelet P2Y12 ADP receptor is a well-known target of thienopyridine-type antiplatelet drugs. This study is the first to describe increased transcriptional expression of a functionally active P2Y12 in response to thrombin in human vascular smooth muscle cells (SMC).

METHODS AND RESULTS

On exposure to thrombin, P2Y12 mRNA was transiently increased, whereas total protein and cell surface expression of P2Y12 were markedly increased within 6 hours and remained elevated over 24 hours. This effect was mediated by activation of nuclear factor κB. Preincubation with thrombin significantly enhanced the efficacy of the P2Y receptor agonist 2-methylthio-ADP to induce interleukin 6 expression and SMC mitogenesis. Effects induced by 2-methylthio-ADP were prevented by RNA interference-mediated knockdown of P2Y12 and a selective P2Y12-antagonist R-138727, the active metabolite of prasugrel. In addition, positive P2Y12 immunostaining was shown in SMC of human carotid artery plaques and was found to colocalize with tissue factor, the rate-limiting factor of thrombin formation in vivo.

CONCLUSIONS

These data suggest that the P2Y12 receptor not only is central to ADP-induced platelet activation but also may mediate platelet-independent responses, specifically under conditions of enhanced thrombin formation, such as local vessel injury and atherosclerotic plaque rupture.

Ticagrelor effectively and reversibly blocks murine platelet P2Y12-mediated thrombosis and demonstrates a requirement for sustained P2Y12 inhibition to prevent subsequent neointima

OBJECTIVE

Our goal was to study the effects of ticagrelor on murine platelet function and thrombosis and characterize the time course of P2Y(12) inhibition required to inhibit neointima formation following vascular injury.

METHODS AND RESULTS

Mice were treated with ticagrelor or vehicle. Platelet aggregation and P-selectin expression were assessed over time, and thrombus formation was assessed in laser-injured cremasteric arterioles of P2Y(12)+/+ and P2Y(12)-/- mice. Neointima formation in FeCl(3)-injured carotid artery was assessed in C57BL/6 mice treated with different regimens of ticagrelor. Ticagrelor inhibited platelet aggregation and P-selectin expression in a dose-dependent, reversible manner. Ticagrelor inhibited thrombus formation to the same extent as seen in P2Y(12)-/- mice. Neointima formation was markedly reduced in mice treated with ticagrelor before and 4 hours after injury (neointima area: control, 39 921±22 749 μm(2), versus ticagrelor, 3705±2600 μm(2); P<0.01), whereas administration of ticagrelor either before injury only or from 4 hours postinjury was ineffective.

CONCLUSIONS

Ticagrelor effectively and reversibly inhibits P2Y(12)-mediated platelet function and thrombosis in mice. P2Y(12) inhibition is required both at the time of and after injury to effectively inhibit neointima formation. Additional studies are warranted to evaluate the role of P2Y(12) inhibition in preventing restenosis.

Antiplatelet therapy in acute coronary syndromes: focus on ticagrelor

The use of antiplatelet agents, specifically the thienopyridines, has become a standard of care in the approach to the patient presenting with an acute coronary syndrome. These drugs irreversibly inhibit the platelet by permanently binding to the surface P2Y12 receptor and blocking the downstream fibrinogen cross-linking between platelets, which leads to aggregation and thrombus. However, currently available therapeutic choices are limited by potential interaction with other medications, slow hepatic conversion to active metabolite, genetic resistance, and narrow therapeutic safety margin. In order to overcome these disadvantages, there has been an interest in developing alternatives to thienopyridines. Recent investigations have included ticagrelor, a reversible inhibitor of the P2Y12 platelet receptor, which appears to have overcome several drawbacks of the current thienopyridines. Its unique pharmacokinetic and pharmacodynamic profiles result in an inhibition of platelet aggregation that is rapid, high, consistent, and less susceptible to interpatient variability than currently available P2Y12 inhibitors. In addition, ticagrelor offers a potential mortality advantage not apparent with current agents. Although questions regarding the nature, magnitude, and clinical significance of several observed adverse effects (dyspnea and ventricular pauses) remain unanswered, it appears that ticagrelor may represent a significant advancement over currently available oral antiplatelet agents.

Efficacy and Safety of Ticagrelor: A Reversible P2Y12 Receptor Antagonist

Objective:

To summarize the pharmacokinetic and pharmacodynamic properties of ticagrelor, a selective P2Y12 receptor antagonist, and evaluate its role in the treatment of patients with acute coronary syndromes (ACS).

Data Sources:

A literature search was conducted in MEDLINE (1966–November 2009), International Pharmaceutical Abstracts (1970–November 2009), and EMBASE (1990–November 2009) using the MeSH terms and key words AZD6140, ticagrelor, P2Y12 receptor antagonist, cardiovascular disease, ACS, atherothrombosis, and platelets.

Study Selection And Data Extraction:

Selected studies evaluated the pharmacology, pharmacokinetics, pharmacodynamics, safety, and efficacy of ticagrelor for the treatment of ACS.

Data Synthesis:

Ticagrelor selectively and reversibly blocks the P2Y12 receptor, inhibiting platelet aggregation and preventing amplification of platelet activation. Optimal dosing strategy as determined by ticagrelor's pharmacokinetic and pharmacodynamic profile is a loading dose of 180 mg followed by 90 mg by mouth twice daily. At these doses, greater platelet inhibition is observed with ticagrelor as compared to clopidogrel 75 mg once daily in both clopidogrel-experienced and -naïve patients. Studies in patients experiencing ACS concluded that ticagrelor reduced the rate of cardiovascular death, nonfatal myocardial infarction, stent thrombosis, and overall mortality compared to clopidogrel without increasing major bleeding when administered with standard therapy for ACS. There was no significant difference in the risk of stroke with ticagrelor compared to clopidogrel; however, intracranial bleeding was more common with ticagrelor. Ticagrelor is well tolerated; however, minor bleeding, dyspnea, hypotension, nausea, and ventricular pauses were reported more frequently than with clopidogrel. Reversible inhibition with ticagrelor may allow for more rapid surgical intervention after discontinuation, suggesting greater flexibility in treatment of ACS.

Conclusions:

Ticagrelor's improved pharmacokinetic and pharmacodynamic profile builds upon the limitations of currently available P2Y12 receptor antagonists. Ticagrelor represents a promising approach for the prevention of cardiovascular events in patients with ACS.

Clopidogrel, independent of the vascular P2Y12 receptor, improves arterial function in small mesenteric arteries from AngII-hypertensive rats

The P2Y12 receptor antagonist clopidogrel blocks platelet aggregation, improves systemic endothelial nitric oxide bioavailability, and has anti-inflammatory effects. Since P2Y12 receptors have been identified in the vasculature, we hypothesized that clopidogrel ameliorates angiotensin II (Ang II) -induced vascular functional changes by blockade of P2Y12 receptors in the vasculature. Male Sprague Dawley rats were infused with Ang II (60 ng.min-1) or vehicle for 14 days. The animals were treated with clopidogrel (10mg*kg-1*day-1) or vehicle. Vascular reactivity was evaluated in second-order mesenteric arteries. Clopidogrel treatment did not change systolic blood pressure [(mmHg) control-vehicle, 117+/-7.1 vs. control- Clopidogrel, 125+/-4.2; AngII-vehicle, 197+/-10.7 vs. AngII-Clopidogrel, 198+/-5.2], but it normalized increased phenylephrine-induced vascular contractions [(%KCl) vehicle-treated, 182.2+/-18 vs. Clopidogrel, 133+/-14%), as well as impaired vasodilation to acetylcholine [(%) vehicle-treated, 71.7+/-2.2 vs. Clopidogrel, 85.3+/-2.8) in Ang II-treated animals. Vascular expression of P2Y12 receptor was determined by western blot. Pharmacological characterization of vascular P2Y12 was performed with the P2Y12 agonist 2-MeS-ADP. Although 2-MeSADP induced endothelium-dependent relaxation [(Emax %) = 71%+/-12), as well as contractile vascular responses (Emax %= 83+/-12) these actions are not mediated by P2Y12 receptor activation. 2-MeS-ADP produced similar vascular responses in control and Ang II rats. These results indicate potential effects of Clopidogrel, such as improvement of hypertension-related vascular functional changes that are not associated with direct actions of clopidogrel in the vasculature, supporting the concept that activated platelets contribute to endothelial dysfunction, possibly via impaired NO bioavailability.

ADP signaling in vascular endothelial cells: ADP-dependent activation of the endothelial isoform of nitric-oxide synthase requires the expression but not the kinase activity of AMP-activated protein kinase

ADP responses underlie therapeutic approaches to many cardiovascular diseases, and ADP receptor antagonists are in widespread clinical use. The role of ADP in platelet biology has been extensively studied, yet ADP signaling pathways in endothelial cells remain incompletely understood. We found that ADP promoted phosphorylation of the endothelial isoform of nitric-oxide synthase (eNOS) at Ser(1179) and Ser(635) and dephosphorylation at Ser(116) in cultured endothelial cells. Although eNOS activity was stimulated by both ADP and ATP, only ADP signaling was significantly inhibited by the P2Y(1) receptor antagonist MRS 2179 or by knockdown of P2Y(1) using small interfering RNA (siRNA). ADP activated the small GTPase Rac1 and promoted endothelial cell migration. siRNA-mediated knockdown of Rac1 blocked ADP-dependent eNOS Ser(1179) and Ser(635) phosphorylation, as well as eNOS activation. We analyzed pathways known to regulate eNOS, including phosphoinositide 3-kinase/Akt, ERK1/2, Src, and calcium/calmodulin-dependent kinase kinase-beta (CaMKKbeta) using the inhibitors wortmannin, PD98059, PP2, and STO-609, respectively. None of these inhibitors altered ADP-modulated eNOS phosphorylation. In contrast, siRNA-mediated knockdown of AMP-activated protein kinase (AMPK) inhibited ADP-dependent eNOS Ser(635) phosphorylation and eNOS activity but did not affect eNOS Ser(1179) phosphorylation. Importantly, the AMPK enzyme inhibitor compound C had no effect on ADP-stimulated eNOS activity, despite completely blocking AMPK activity. CaMKKbeta knockdown suppressed ADP-stimulated eNOS activity, yet inhibition of CaMKKbeta kinase activity using STO-609 failed to affect eNOS activation by ADP. These data suggest that the expression, but not the kinase activity, of AMPK and CaMKKbeta is necessary for ADP signaling to eNOS.

6-Amino-2-mercapto-3H-pyrimidin-4-one derivatives as new candidates for the antagonism at the P2Y12 receptors

P2Y(12) plays an important role in platelet aggregation, which makes it an interesting target for antithrombotic agents. Compounds that antagonize P2Y(12) include the active metabolites of thienopyridines and molecules that are structurally related to ATP, which is an antagonist of P2Y(12). During the last few years, our group has been working on the development of P2Y(12) receptors antagonists that are based on an extremely simple chemical structure, the 6-amino-2-mercapto-3H-pyrimidin-4-one, variously substituted at the sulfur and oxygen functions. This nucleus represents the simplified combination of two known P2Y(12) antagonists: the active metabolite of the thienopyridines and ATP derivatives. The effects of the synthesized compounds were tested on ADP-induced human platelet aggregation, using light transmission aggregometry. None of the tested compounds induced platelet aggregation, while some of them, at concentration of 10(-4)M, partially inhibited platelet aggregation induced by ADP 10(-6)M. The most potent compound, 6b, antagonized the inhibitory effect of 2-methylthio-ADP on the forskolin-induced accumulation of cyclic-AMP in CHO FlpIN cells expressing recombinant human P2Y(12)-receptors. In addition, none of the tested compounds, including 6b, interfered with ligand binding to P1 receptors. Our results suggest that some of the synthesized compounds are specific antagonists of P2 receptors, and in particular of P2Y(12) and suggest that further development of this structurally new series of compounds as P2Y(12) receptors antagonists is recommended.

The reversible oral P2Y12 antagonist AZD6140 inhibits ADP-induced contractions in murine and human vasculature

OBJECTIVES

The platelet ADP P2Y(12) receptor which is a target for the antithrombotic drug clopidogrel is also distributed on vascular smooth muscle cells and stimulate contraction. This study investigates whether AZD6140, in contrast to clopidogrel, can inhibit ADP-mediated arterial contractions.

METHODS

Mice were treated with clopidogrel, 50 mg/kg, 24 and 2 h before experiment. Thoracic aorta ring segments from both clopidogrel-treated (n=5) and untreated (n=4) mice were mounted in myograph baths. Contractions of human left internal mammary arteries (IMA) and small arteries were studied in an identical manner.

RESULTS

Clopidogrel treatment per os did not inhibit contractions by the stable ADP analogue 2-MeSADP (10 microM). However, addition of 1 microM AZD6140 in vitro inhibited ADP contraction (% of maximal contraction by 60 mM K(+)) both in the clopidogrel-treated, from 64% to 32% (P=0.002) and in the untreated group, from 59% to 33% (P=0.015). 2-MeSADP contractions in human IMA and small arteries were inhibited by AZD6140.

CONCLUSIONS

The antiplatelet drug AZD6140 blocks the contractile effects of ADP in both murine and human vasculature. These effects of AZD6140 could be beneficial in the management of conditions in which vasospasm may play a role.

Ticagrelor: the first reversibly binding oral P2Y12 receptor antagonist

Ticagrelor (AZD6140) is the first reversibly binding oral P2Y(12) receptor antagonist that blocks ADP-induced platelet aggregation. Unlike thienopyridines, which irreversibly bind to the P2Y(12) receptor for the lifetime of the platelet, ticagrelor binds reversibly to the receptor and exhibits rapid onset and offset of effect, which closely follow drug exposure levels. Animal models indicate greater separation between antithrombotic effects and bleeding effects with ticagrelor than with thienopyridines. Unlike the thienopyridines, ticagrelor does not require metabolic activation. It is quickly absorbed and exhibits a rapid antiplatelet effect, with higher and more consistent levels of inhibition of platelet aggregation (IPA) being maintained across the dosing interval than with clopidogrel. IPA levels decline with plasma drug levels after discontinuation of dosing. In the phase II DISPERSE-2 trial of 990 patients with non-ST-elevation acute coronary syndromes (ACS), ticagrelor treatment with 90 mg and 180 mg twice daily showed comparable rates of major and minor bleeding compared with clopidogrel 75 mg while there were numerically fewer myocardial infarctions. Ticagrelor resulted in greater IPA in clopidogrel-naïve patients and produced substantial additional reductions in platelet aggregation activity in patients pretreated with clopidogrel. Ticagrelor treatment was well tolerated in DISPERSE-2, and discontinuation rates were comparable to those observed for clopidogrel. An increased risk of mild to moderate dyspnea and mostly asymptomatic ventricular pauses were observed in phase II studies. The mechanisms for these effects are currently being investigated. The efficacy and safety of ticagrelor are being further evaluated in the phase III PLATO trial, involving approximately 18,000 patients with ACS, including both ST-elevation and non-ST-elevation ACS.

Platelet P2Y(12) receptor influences the vessel wall response to arterial injury and thrombosis

BACKGROUND

Platelets are believed to play an important role in atherogenesis and the vessel response to vascular injury. The P2Y(12) receptor (P2Y(12)) plays a central role in amplifying platelet aggregation, dense granule and alpha-granule secretion, P-selectin expression, microparticle formation, and procoagulant membrane changes, regardless of the activating stimulus. We hypothesized that P2Y(12) deficiency might reduce the vessel wall response to vascular injury as well as thrombosis in murine vascular injury models.

METHODS AND RESULTS

P2Y(12)-deficient (-/-) mice and littermate controls (+/+) were bred on a C57 BL/6 background. In vivo murine models of arterial injury were employed alone and in combination with bone marrow transplantation to investigate the role of P2Y(12) in the vessel wall response to arterial injury and thrombosis. At 21 days after ferric chloride injury, neointima formation in P2Y(12)(-/-) arteries was significantly less than that observed in control strain arteries (P<0.025). In agreement with this, the intima-media ratio was significantly greater in femoral wire-injured arteries from P2Y(12)(+/+) compared with P2Y(12)(-/-) animals (P<0.05). Bone marrow transplantation was used to examine the importance of vessel wall P2Y(12) versus platelet P2Y(12). Analysis of arterial sections from chimeric animals at 21 days after injury revealed a smaller intima-media ratio in -/- to +/+ animals than in the positive (+/+ to +/+) control group (P<0.01).

CONCLUSIONS

These data demonstrate a role for platelet P2Y(12) in the vessel wall response to arterial injury and thrombosis. This illustrates the manner in which platelets may contribute to atherogenesis and restenosis.

Serotonin induces vasoconstriction of smooth muscle cell-rich neointima through 5-hydroxytryptamine2A receptor in rabbit femoral arteries

BACKGROUND

Smooth muscle cell (SMC)-rich intima is a morphological feature of atherosclerotic lesions that is observed in eroded plaque and spastic arteries. Arteries with SMC-rich intima are susceptible to vasoconstriction or vasospasm against some vasoactive agents.

OBJECTIVE

The present study evaluates the contribution of SMC-rich intima to thrombogenic vasoconstriction.

METHODS

We established SMC-rich neointima by damaging rabbit femoral arteries using balloons and then measured the isometric tension of the femoral strips against 5-hydroxytryptamine (5-HT), adenosine diphosphate, adenosine triphosphate and thrombin.

RESULTS

Among these agents, only 5-HT induced a hypercontractile response of the injured arteries with SMC-rich neointima, compared with non-injured arteries. Smooth muscle cells of both the neointima and media expressed 5-HT(2A) receptor, and sarpogrelate, a selective 5-HT(2A) receptor antagonist significantly inhibited the hypercontraction. Furthermore, 5-HT induced contraction of separated neointima and hypercontraction of separated media compared with non-injured media. Sarpogrelate and fasudil, a specific Rho-kinase inhibitor, significantly suppressed such contraction of both the neointima and media of injured arteries.

CONCLUSIONS

These results suggest that 5-HT plays a crucial role in thrombogenic vasoconstriction, and that SMC-rich intima as well as media directly contributes to the hypercontractile response of atherosclerotic vessels through the 5-HT(2A) receptor and the Rho-kinase pathway.

Platelet receptor P2RY12 haplotypes predict restenosis after percutaneous coronary interventions

The platelet receptor P2Y12 (gene symbol P2RY12) is involved in several processes that contribute to restenosis after percutaneous coronary interventions (PCI). Therefore, common variation in the P2Y12 gene may serve as a useful marker for risk stratification. We studied whether common variation in the platelet receptor P2Y12 gene affects the risk of restenosis after PCI. Comprehensive coverage of common variation in the P2Y12 gene was obtained by genotyping five haplotype-tagging SNPs (ht-SNPs) in 2,062 PCI-treated patients who received a stent and participated in the GENetic DEterminants of Restenosis (GENDER) Study. Haplotypes were inferred and their association with target vessel revascularization (TVR) was studied. Seven P2Y12 haplotypes were identified with an allelic frequency above 5% (designated here H1 to H7) of which two (H5 and H7) were associated with a higher risk of TVR (hazard ratios [HR]=1.4, 95% confidence interval [CI]=1.0-2.0; and HR=1.6, 95% CI=1.2-2.0, respectively) than the reference P2Y12 haplotype (H1), which contains the common alleles of all five P2Y12 ht-SNPs. Our study shows that common variation in the P2Y12 gene predicts restenosis in PCI-treated patients.

Purinergic receptors contribute to early mesangial cell transformation and renal vessel hypertrophy during angiotensin II-induced hypertension

Chronic ANG II infusions lead to increases in intrarenal ANG II levels, hypertension, and tissue injury. Increased blood pressure also elicits increases in renal interstitial fluid (RIF) ATP concentrations that stimulate cell proliferation. We evaluated the contribution of purinergic receptor activation to ANG II-induced renal injury in rats by treating with clopidogrel, a P2Y12 receptor blocker, or with PPADS, a nonselective P2 receptor blocker. alpha-Actin expression in mesangial cells, afferent arteriolar wall thickness (AAWT), cortical cell proliferation, and macrophage infiltration were used as early markers of renal injury. Clopidogrel and PPADS did not alter blood pressure, renin or kidney ANG II content. alpha-Actin expression increased from control of 0.6 +/- 0.4% of mesangial area to 6.3 +/- 1.9% in ANG II-infused rats and this response was prevented by clopidogrel (0.4 +/- 0.2%) and PPADS. The increase in AAWT from 4.7 +/- 0.1 to 6.0 +/- 0.1 mm in ANG II rats was also prevented by clopidogrel (4.8 +/- 0.1 mm) and PPADS. ANG II infusion led to interstitial macrophage infiltration (105 +/- 16 vs. 62 +/- 4 cell/mm(2)) and tubular proliferation (71 +/- 15 vs. 20 +/- 4 cell/mm(2)) and these effects were prevented by clopidogrel (52 +/- 4 and 36 +/- 3 cell/mm(2)) and PPADS. RIF ATP levels were higher in ANG II-infused rats than in control rats (11.8 +/- 1.9 vs. 5.6 +/- 0.6 nmol/l, P < 0.05). The results suggest that activation of vascular and glomerular purinergic P2 receptors may contribute to the mesangial cell transformation, renal inflammation, and vascular hypertrophy observed in ANG II-dependent hypertension.

The effect of binge fetal alcohol exposure on the number of vasoactive intestinal peptide-producing neurons in fetal sheep brain

Previously we demonstrated that fetal alcohol exposure attenuates hypoxic cerebral vasodilation in fetal and neonatal sheep. One mechanism may be altered expression of brain vasoactive substances. We hypothesized that early fetal alcohol exposure alters the number of fetal neurons expressing vasoactive intestinal peptide (VIP), a potent cerebral vasodilator. Thirteen pregnant ewes received daily i.v. infusions of alcohol (1.5 g/kg) or saline on days 30-54 of gestation (term = 145 days). Fourteen fetal brains (6 alcohol-exposed, 8 saline control) were obtained on gestational day 126. Using unbiased stereology, we counted immunohistochemically-labeled VIP neurons in one half of each forebrain with an optical fractionator. The total NeuN-labeled neurons were similarly counted. Alcohol-exposed fetal sheep brains had fewer VIP-immunopositive neurons per hemisphere, 14.6 x 10(6), compared to saline controls, 19.8 x 10(6). The total neuron number was not different, 1.19 x 10(9) versus 1.23 x 10(9) respectively, indicating a selective decrease in VIP neurons as a result of alcohol exposure. In sheep, alcohol exposure early in gestation is associated with fewer VIP-producing neurons later in gestation compared to saline controls; therefore, alcohol-related changes in the number of VIP-expressing neurons may be responsible in part for the attenuated hypoxic cerebral vasodilation described in fetal and neonatal sheep exposed to alcohol earlier in gestation.

P2 receptors in cardiovascular regulation and disease

The role of ATP as an extracellular signalling molecule is now well established and evidence is accumulating that ATP and other nucleotides (ADP, UTP and UDP) play important roles in cardiovascular physiology and pathophysiology, acting via P2X (ion channel) and P2Y (G protein-coupled) receptors. In this article we consider the dual role of ATP in regulation of vascular tone, released as a cotransmitter from sympathetic nerves or released in the vascular lumen in response to changes in blood flow and hypoxia. Further, purinergic long-term trophic and inflammatory signalling is described in cell proliferation, differentiation, migration and death in angiogenesis, vascular remodelling, restenosis and atherosclerosis. The effects on haemostasis and cardiac regulation is reviewed. The involvement of ATP in vascular diseases such as thrombosis, hypertension and diabetes will also be discussed, as well as various heart conditions. The purinergic system may be of similar importance as the sympathetic and renin-angiotensin-aldosterone systems in cardiovascular regulation and pathophysiology. The extracellular nucleotides and their cardiovascular P2 receptors are now entering the phase of clinical development.

Transitioning patients from cangrelor to clopidogrel: pharmacodynamic evidence of a competitive effect

BACKGROUND

Cangrelor is a direct, parenteral, and reversible inhibitor of the platelet P2Y12 receptor currently undergoing Phase III testing. As many individuals treated acutely with cangrelor will often be treated long-term with a thienopyridine, it is important to determine the effects of concurrent cangrelor and clopidogrel administration.

METHODS AND RESULTS

Ten healthy volunteers received a 600 mg oral loading dose of clopidogrel and then underwent serial platelet function monitoring for 6 h. Two weeks later these same individuals received a 600 mg clopidogrel loading dose simultaneously with a cangrelor IV bolus (30 microg/kg) and a 2-hour infusion (4 microg/kg/min). A separate group of ten volunteers received a 600 mg clopidogrel loading dose after administration of a cangrelor bolus and a 1-hour infusion. The effects on ADP-induced platelet activation and aggregation were evaluated by flow cytometry, whole-blood electrical impedance, and light-transmittance aggregometry. Cangrelor and clopidogrel alone achieved the expected levels of platelet inhibition. However, the sustained platelet inhibition anticipated for clopidogrel treatment did not occur when cangrelor was initiated simultaneously. No such effect was found when clopidogrel was started upon completion of the cangrelor infusion.

CONCLUSION

To achieve sustained platelet P2Y12 inhibition in patients treated with cangrelor, clopidogrel administration should be started when the cangrelor infusion is terminated.

Nicotine upregulates the expression of P2Y12 on vascular cells and megakaryoblasts

BACKGROUND

P2Y12 is the major platelet receptor that mediates ADP-induced aggregation. P2Y12 is also expressed by vascular cells. The factors that regulate P2Y12 expression have not been determined. Since nicotine (NIC) has effects on platelet activation and vascular function, and because nicotinic and purinerigic receptors may interact, we determined whether nicotine altered P2Y12 expression.

METHODS

Four cell lines (human coronary artery endothelial cells, HCAEC; human umbilical vein endothelial cells, HUVEC; human aortic smooth muscle cells, HASMC; and human megakaryoblastic cells, MEG-01) were cultured in the absence or presence of nicotine. Immunoblotting for P2Y12, P2Y2, and actin was performed.

RESULTS

Nicotine, at concentrations of 0.1-1.0 microM, induced P2Y12 (but not P2Y2) expression in all the four cell lines. HASMC exhibited the greatest induction with a sixfold mean increase in P2Y12 expression in response to 0.25 microM nicotine. The induction was inhibited by nicotinic acetylcholine receptor antagonists. Healthy smokers were observed to have higher P2Y12 expression in platelet lysates compared to non-smokers.

CONCLUSION

Nicotine induces the expression of P2Y12 in vascular cells and megakaryoblasts, and is mediated by nicotinic acetylcholine receptors. Smokers exhibit higher platelet P2Y12, possibly mediated via nicotine. These results may contribute to a better understanding of the effects of cigarette smoking on platelet activation and the vessel wall.

CONDENSED ABSTRACT

The factors that regulate the expression of P2Y12, the platelet ADP receptor, have not been determined. Four cell lines (human coronary artery endothelial cells, HCAEC; human umbilical vein endothelial cells, HUVEC; human aortic smooth muscle cells, HASMC; and human megakaryoblastic cells, MEG-01) were cultured in the absence or presence of nicotine. Nicotine, at concentrations of 0.1-1.0 microM, induced P2Y12 expression in all the four cell lines. HASMC exhibited the greatest induction with a sixfold mean increase in P2Y12 expression in response to 0.25 microM nicotine. The induction was inhibited by nicotinic acetylcholine receptor antagonists. Healthy smokers were observed to have higher P2Y12 expression in platelet lysates compared to non-smokers. These results may contribute to a better understanding of the effects of cigarette smoking on platelet activation and the vessel wall.

International Union of Pharmacology LVIII: update on the P2Y G protein-coupled nucleotide receptors: from molecular mechanisms and pathophysiology to therapy

There have been many advances in our knowledge about different aspects of P2Y receptor signaling since the last review published by our International Union of Pharmacology subcommittee. More receptor subtypes have been cloned and characterized and most orphan receptors de-orphanized, so that it is now possible to provide a basis for a future subdivision of P2Y receptor subtypes. More is known about the functional elements of the P2Y receptor molecules and the signaling pathways involved, including interactions with ion channels. There have been substantial developments in the design of selective agonists and antagonists to some of the P2Y receptor subtypes. There are new findings about the mechanisms underlying nucleotide release and ectoenzymatic nucleotide breakdown. Interactions between P2Y receptors and receptors to other signaling molecules have been explored as well as P2Y-mediated control of gene transcription. The distribution and roles of P2Y receptor subtypes in many different cell types are better understood and P2Y receptor-related compounds are being explored for therapeutic purposes. These and other advances are discussed in the present review.

Shaping immune responses through the activation of dendritic cells' P2 receptors

Dendritic cells (DCs) activate and shape the adaptive immune response by capturing antigens, migrating to peripheral lymphoid organs where naïve T cells reside, expressing high levels of MHC and costimulatory molecules and secreting cytokines and chemokines. DCs are endowed with a high degree of functional plasticity and their functions are tightly regulated. Besides initiating adaptive immune responses, DCs play a key role in maintaining peripheral tolerance toward self-antigens. On the basis of the information gathered from the tissue where they reside, DCs adjust their functional activity to ensure that protective immunity is favoured while unwanted or exaggerated immune responses are prevented. A wide variety of signals from neighbouring cells affecting DC functional activity have been described. Here we will discuss the complex role of extracellular nucleotides in the regulation of DC function and the role of P2 receptors as possible tools to manipulate immune responses.

The E-NTPDase family of ectonucleotidases: Structure function relationships and pathophysiological significance

Ectonucleotidases are ectoenzymes that hydrolyze extracellular nucleotides to the respective nucleosides. Within the past decade, ectonucleotidases belonging to several enzyme families have been discovered, cloned and characterized. In this article, we specifically address the cell surface-located members of the ecto-nucleoside triphosphate diphosphohydrolase (E-NTPDase/CD39) family (NTPDase1,2,3, and 8). The molecular identification of individual NTPDase subtypes, genetic engineering, mutational analyses, and the generation of subtype-specific antibodies have resulted in considerable insights into enzyme structure and function. These advances also allow definition of physiological and patho-physiological implications of NTPDases in a considerable variety of tissues. Biological actions of NTPDases are a consequence (at least in part) of the regulated phosphohydrolytic activity on extracellular nucleotides and consequent effects on P2-receptor signaling. It further appears that the spatial and temporal expression of NTPDases by various cell types within the vasculature, the nervous tissues and other tissues impacts on several patho-physiological processes. Examples include acute effects on cellular metabolism, adhesion, activation and migration with other protracted impacts upon developmental responses, inclusive of cellular proliferation, differentiation and apoptosis, as seen with atherosclerosis, degenerative neurological diseases and immune rejection of transplanted organs and cells. Future clinical applications are expected to involve the development of new therapeutic strategies for transplantation and various inflammatory cardiovascular, gastrointestinal and neurological diseases.

Optimal Timing for the Initiation of Pre-Treatment With 300 mg Clopidogrel Before Percutaneous Coronary Intervention

OBJECTIVES

This study sought to determine the optimal timing of a 300-mg clopidogrel loading dose before percutaneous coronary intervention (PCI) in patients enrolled in the Clopidogrel for the Reduction of Events During Observation (CREDO) trial.

BACKGROUND

A loading dose of clopidogrel before a PCI has become relatively commonplace, although the data supporting this practice are limited and sometimes conflicting.

METHODS

Patients were randomized to receive either 300 mg clopidogrel or a matching placebo administered a minimum of 3 h and a maximum of 24 h before PCI. The primary 28-day combined end point was death, myocardial infarction, or urgent target vessel revascularization. Linear splines were used to summarize the effect of the time of pre-treatment as a continuous variable.

RESULTS

A total of 1,762 patients were evaluated. For patients randomized to placebo, there was no relationship between the duration of pre-treatment and the occurrence of the primary end point, whereas longer durations of pre-treatment in patients randomized to clopidogrel were associated with improved outcomes. The event rates diverged maximally at 24 h. The difference in outcomes between placebo and clopidogrel pre-treated patients was not significant until > or =15 h pre-treatment, with a 58.8% (p = 0.028) reduction in the primary end point in patients pre-treated with clopidogrel > or =15 h compared with placebo.

CONCLUSIONS

When a 300-mg loading dose of clopidogrel is used, little benefit is obtained compared with just 75 mg at the time of the PCI when the treatment duration is <12 h. In patients pre-treated for longer durations, the optimal duration seems to approach 24 h.

Pharmacological characterization of nucleotide P2Y receptors on endothelial cells of the mouse aorta

Nucleotides regulate various effects including vascular tone. This study was aimed to characterize P2Y receptors on endothelial cells of the aorta of C57BL6 mice. Five adjacent segments (width 2 mm) of the thoracic aorta were mounted in organ baths to measure isometric force development. Nucleotides evoked complete (adenosine 5' triphosphate (ATP), uridine 5' triphosphate (UTP), uridine 5' diphosphate (UDP); >90%) or partial (adenosine 5' diphosphate (ADP)) relaxation of phenylephrine precontracted thoracic aortic rings of C57BL6 mice. Relaxation was abolished by removal of the endothelium and was strongly suppressed (>90%) by inhibitors of nitric oxide synthesis. The rank order of potency was: UDP approximately UTP approximately ADP>adenosine 5'-[gamma-thio] triphosphate (ATPgammaS)>ATP, with respective pD2 values of 6.31, 6.24, 6.22, 5.82 and 5.40. These results are compatible with the presence of P2Y1 (ADP>ATP), P2Y2 or P2Y4 (ATP and UTP) and P2Y6 (UDP) receptors. P2Y4 receptors were not involved, since P2Y4-deficient mice displayed unaltered responses to ATP and UTP. The purinergic receptor antagonist suramin exerted surmountable antagonism for all agonists. Its apparent pKb for ATP (4.53+/-0.07) was compatible with literature, but the pKb for UTP (5.19+/-0.03) was significantly higher. This discrepancy suggests that UTP activates supplementary non-P2Y2 receptor subtype(s). Further, pyridoxal-phosphate-6-azophenyl-2'-4'-disulphonic acid (PPADS) showed surmountable (UTP, UDP), nonsurmountable (ADP) or no antagonism (ATP). Finally, 2'-deoxy-N6-methyladenosine3',5'-bisphosphate (MRS2179) inhibited ADP-evoked relaxation only. Taken together, these results point to the presence of functional P2Y1 (ADP), P2Y2 (ATP, UTP) and P2Y6 (UDP) receptors on murine aorta endothelial cells. The identity of the receptor(s) mediating the action of UTP is not fully clear and other P2Y subtypes might be involved in UTP-evoked vasodilatation.

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.

Multiple P2Y receptors couple to calcium-dependent, chloride channels in smooth muscle cells of the rat pulmonary artery

BACKGROUND

Uridine 5'-triphosphate (UTP) and uridine 5'-diphosphate (UDP) act via P2Y receptors to evoke contraction of rat pulmonary arteries, whilst adenosine 5'-triphosphate (ATP) acts via P2X and P2Y receptors. Pharmacological characterisation of these receptors in intact arteries is complicated by release and extracellular metabolism of nucleotides, so the aim of this study was to characterise the P2Y receptors under conditions that minimise these problems.

METHODS

The perforated-patch clamp technique was used to record the Ca2+-dependent, Cl- current (I(Cl,Ca)) activated by P2Y receptor agonists in acutely dissociated smooth muscle cells of rat small (SPA) and large (LPA) intrapulmonary arteries, held at -50 mV. Contractions to ATP were measured in isolated muscle rings. Data were compared by Student's t test or one way ANOVA.

RESULTS

ATP, UTP and UDP (10(-4) M) evoked oscillating, inward currents (peak = 13-727 pA) in 71-93% of cells. The first current was usually the largest and in the SPA the response to ATP was significantly greater than those to UTP or UDP (P < 0.05). Subsequent currents tended to decrease in amplitude, with a variable time-course, to a level that was significantly smaller for ATP (P < 0.05), UTP (P < 0.001) and UDP (P < 0.05) in the SPA. The frequency of oscillations was similar for each agonist (mean approximately to 6-11.min(-1)) and changed little during agonist application. The non-selective P2 receptor antagonist suramin (10(-4) M) abolished currents evoked by ATP in SPA (n = 4) and LPA (n = 4), but pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) (10(-4) M), also a non-selective P2 antagonist, had no effect (n = 4, 5 respectively). Currents elicited by UTP (n = 37) or UDP (n = 14) were unaffected by either antagonist. Contractions of SPA evoked by ATP were partially inhibited by PPADS (n = 4) and abolished by suramin (n = 5). Both antagonists abolished the contractions in LPA.

CONCLUSION

At least two P2Y subtypes couple to ICl,Ca in smooth muscle cells of rat SPA and LPA, with no apparent regional variation in their distribution. The suramin-sensitive, PPADS-resistant site activated by ATP most resembles the P2Y11 receptor. However, the suramin- and PPADS-insensitive receptor activated by UTP and UDP does not correspond to any of the known P2Y subtypes. These receptors likely play a significant role in nucleotide-induced vasoconstriction.

Activation of human eosinophils via P2 receptors: novel findings and future perspectives

A growing body of information indicates that release of intracellular nucleotides represents an important way to modulate several cell pathways in physiological or pathological conditions. Nucleotides released as a consequence of cell damage, cell stress, bacterial infection, or other noxious stimuli signal at a class of plasma membrane receptors--P2 receptors--activating diverse intracellular pathways in many tissues and organs. For example, nucleotides secreted in the airway system control chloride/liquid secretion, goblet cell degranulation, and ciliary beat frequency. Several studies indicate that nucleotides play a role in airway diseases through their action on multiple cell types, including mast cells, dendritic cells, neurons, and eosinophils. Recent work by us and other groups led to the identification and characterization of P2 receptors expressed by human eosinophils. In this review, we will summarize recent developments in this field and put forward a hypothesis about the role of P2 receptors in pathophysiological conditions where eosinophils are major players.

Changes in purinergic signalling in developing and ageing rat tail artery: importance for temperature control

This study aimed to examine the expression and function of P2 receptors of the rat tail and mesenteric arteries during maturation and ageing (4, 6 and 12 weeks, 8 and 24 months). Functional studies and receptor expression by immunohistochemistry revealed a heterogeneous phenotype of P2 receptor subtypes depending on artery age. The purinergic component of nerve-mediated responses in the tail artery was greater in younger animals; similarly responses to ATP and alpha,beta-meATP and the expression of P2X1 receptors decreased with age. Contractile responses to 2-MeSADP decreased with age, and were absent at 8 and 24 months; P2Y1 receptor expression followed this pattern. UTP-induced contractions and P2Y2 receptor expression also decreased with age. The mesenteric artery contracted to UTP, responses at 4 and 6 weeks were larger than at other ages although P2Y2 receptor expression did not significantly differ with age. 2-MeSADP induced relaxation of the mesenteric artery, responses being greatest at 6 weeks and decreased thereafter, which was mimicked by the P2Y1 receptor immunostaining. We speculate that the dramatic changes in expression of P2 receptors in the rat tail artery, compared to the mesenteric artery, during development and ageing are related to the role of the tail artery in temperature regulation.