2-Methylthioadenosine[beta-32P]diphosphate. An agonist and radioligand for the receptor that inhibits the accumulation of cyclic AMP in intact blood platelets

Characterization of [3H]AZ12464237 as a high affinity, non-nucleotide antagonist radioligand for the P2Y12 receptor

The purinergic receptor P2Y12 (P2Y12R) is a well-recognized target for anti-thrombotic agents. This receptor is also expressed in microglia, the main immune cells of the brain, where it modulates microglial activation states and inflammatory responses. To investigate P2Y12R-mediated actions in the central nervous system (CNS), developing novel brain-penetrant ligands and further in vitro studies on brain tissues are essential. A radiolabeled, easily accessible tool compound would significantly advance such drug discovery efforts. Herein, we describe the 3H-labeling of a non-nucleotide P2Y12R antagonist AZ12464237, and its in vitro binding properties to the receptor in membrane preparations from transfected cells, as well as on mouse brain tissues. The radioligand shows high affinity toward both the human and rat P2Y12R, with Kd values of 3.12 ± 0.70 nM (human) and 16.6 ± 3.40 nM (rat), as determined by saturation binding studies. The binding kinetics of [3H]AZ12464237 are rapid with a short target residence time (∼1 min). We further confirmed the selectivity of the radioligand by performing competitive displacement studies, in which reported P2Y12R ligands and other P2Y receptors ligands were tested for binding against [3H]AZ12464237. Additionally, the radioligand proved valuable for in vitro autoradiography studies on mouse brain tissues, although limited off-target binding was observed in P2Y12R knock-out mouse brain. This could be traced to glycogen synthase kinase 3 α. Considering the growing interest in P2Y12R as a biomarker for anti-inflammatory microglia, [3H]AZ12464237 represents a promising tool for in vitro studies, including screening assays aimed at identifying novel P2Y12R ligands for CNS applications.

The Role and Molecular Mechanism of P2Y12 Receptors in the Pathogenesis of Atherosclerotic Cardiovascular Diseases

The P2Y receptor family is a class of G protein-coupled receptors activated primarily by adenosine triphosphate (ATP), adenosine diphosphate (ADP), uridine triphosphate (UTP) and uridine diphosphate (UDP). The P2Y12 receptor is expressed on platelets which mediates platelet aggregation and morphological changes. At the same time, during the process of vascular remodeling and atherosclerosis, ADP can also promote the migration and proliferation of vascular smooth muscle and endothelial cells through P2Y12 receptor activating. Furthermore, P2Y12 is involved in many signal transductions processes, such as intimal hyperplasia, monocyte infiltration and so on, which play an important role in immune inflammation and brain injury. In order to solve the diseases induced by P2Y12 receptor, inhibitors such as ticagrelor, clopidogrel were widely used for cardiovascular diseases. However, there were some problems, such as limited antithrombotic effect, remain unsolved. This article summarizes the role and molecular mechanism of P2Y12 receptors in the pathogenesis of cardiovascular-related diseases, providing in-depth expounding on the molecular mechanism of P2Y12 receptor inhibitors and contributing to the treatment of diseases based on P2Y12 receptors.

Human erythrocytes release ATP by a novel pathway involving VDAC oligomerization independent of pannexin-1

We previously demonstrated that the translocase protein TSPO2 together with the voltage-dependent anion channel (VDAC) and adenine nucleotide transporter (ANT) were involved in a membrane transport complex in human red blood cells (RBCs). Because VDAC was proposed as a channel mediating ATP release in RBCs, we used TSPO ligands together with VDAC and ANT inhibitors to test this hypothesis. ATP release was activated by TSPO ligands, and blocked by inhibitors of VDAC and ANT, while it was insensitive to pannexin-1 blockers. TSPO ligand increased extracellular ATP (ATPe) concentration by 24–59% over the basal values, displaying an acute increase in [ATPe] to a maximal value, which remained constant thereafter. ATPe kinetics were compatible with VDAC mediating a fast but transient ATP efflux. ATP release was strongly inhibited by PKC and PKA inhibitors as well as by depleting intracellular cAMP or extracellular Ca²⁺, suggesting a mechanism involving protein kinases. TSPO ligands favoured VDAC polymerization yielding significantly higher densities of oligomeric bands than in unstimulated cells. Polymerization was partially inhibited by decreasing Ca²⁺ and cAMP contents. The present results show that TSPO ligands induce polymerization of VDAC, coupled to activation of ATP release by a supramolecular complex involving VDAC, TSPO2 and ANT.

Adenine Nucleotides Attenuate Murine T Cell Activation Induced by Concanavalin A or T Cell Receptor Stimulation

Extracellular ATP and its metabolites affect various cellular immune responses, including T cell function, but there are apparently conflicting reports concerning the effects of adenine nucleotides on T cells. For example, it has been reported that ATP-mediated activation of P2 receptor is involved in T cell activation; activation of adenosine receptors suppresses T cell function; and 1 mM ATP induces T cell death via activation of P2X7 receptor. Therefore, in this work we investigated in detail the effects of 100–250 μM ATP, ADP, or AMP on murine T cell activation. First, an in vitro study showed that pretreatment of murine splenic T cells with 100–250 μM ATP, ADP, or AMP significantly suppressed the concanavalin A (ConA)-induced release of cytokines, including IL-2. This suppression was not due to induction of cell death via the P2X7 receptor or to an immunosuppressive effect of adenosine. ATP attenuated the expression of CD25, and decreased the cell proliferation ability of activated T cells. The release of IL-2 by ConA-stimulated lymphocytes was suppressed by post-treatment with ATP, as well as by pretreatment. These results suggest that exogenous ATP suppresses the activation of T cells. Secondly, we evaluated the effect of ATP in a ConA-treated mice. Treatment with ATP attenuated the increase of IL-2 concentration in the blood. Overall, these results suggest that adenine nucleotides might have potential as supplemental therapeutic agents for T cell-mediated immune diseases, by suppressing T cell activation.

ADP-induced bladder contractility is mediated by P2Y12 receptor and temporally regulated by ectonucleotidases and adenosine signaling

Purinergic signaling comprises one key pathway in modulating bladder smooth muscle (BSM) contractility, disorders of which become highly prevalent with aging. ADP was first observed to modulate BSM contractility >40 yr ago, yet the underlying molecular mechanism still remains unclear. Here, we demonstrate, using myography, that ADP and ADPβS dose-dependently induce mouse BSM contraction, and ADP-induced BSM contraction is blocked by a selective P2Y12 receptor (P2Y12R) antagonist, PSB 0739 (25 μM), but is unaffected by P2Y1 and P2Y13 receptor antagonists. P2Y12R in BSM exhibits distinct pharmacological properties that are different from P2Y12R in platelets. After an immediate contraction, prolonged exposure to ADP causes BSM to become refractory to further ADP-mediated contraction. However, in mice lacking ectonucleotidases Entpd1 (ATP→ADP→AMP) or Nt5e (AMP→adenosine), or by inhibiting adenosine signaling, the refractory response was altered, resulting in repeated BSM contractions in response to repeated ADP (0.1-1 mM) stimulation. Our data indicate that P2Y12R undergoes slow desensitization; ADP-P2Y12 signaling is tightly regulated by Entpd1/Nt5e activity and adenosine receptors; and ADP-adenosine signaling play an important role in modulating P2X-mediated BSM contraction. The identification of P2Y12R in BSM, and the current clinical availability of P2Y12R inhibitors, such as clopidogrel, offers potentially novel treatment strategies for bladder contractility disorders.

Importance of measurement of platelet reactivity to ADP in patients with coronary artery disease: an historical account

The pivotal roles of platelets in physiological hemostasis and pathological thrombosis at the site of plaque rupture are well established. The latter roles provide the fundamental basis for the most widely implemented pharmacologic management of coronary artery disease--dual antiplatelet therapy with aspirin to inhibit platelet thromboxane A2 generation, and a P2Y12 receptor inhibitor to prevent adenosine diphosphate (ADP)-induced platelet activation. Although suboptimal pharmacodynamic efficacy, also described as high on-treatment platelet reactivity to ADP, has been associated with greater risk for post-stenting ischemic event occurrence, enhanced responsiveness is associated with higher risk for bleeding in selected patients. In this review article, we aim to provide an historical account of the one and a half century long journey starting with the first description of platelets through the first report of ex vivo measurement of ADP-induced platelet aggregation, the first demonstration of an association between ADP-induced platelet aggregation and post-stenting ischemic event occurrence, and finally to the most recent description of a 'therapeutic window' concept for P2Y12 receptor inhibitor therapy.

Pharmacological characterization of the P2 receptors profile in the podocytes of the freshly isolated rat glomeruli

Calcium flux in the podocytes is critical for normal and pathophysiological regulation of these types of cells, and excessive calcium signaling results in podocytes damage and improper glomeruli function. Purinergic activation of P2 receptors is a powerful and rapid signaling process; however, the exact physiological identity of P2 receptors subtypes in podocytes remains essentially unknown. The goal of this study was to determine the P2 receptor profile in podocytes of the intact Sprague-Dawley rat glomeruli using available pharmacological tools. Glomeruli were isolated by differential sieving and loaded with Fluo-4/Fura Red cell permeable calcium indicators, and the purinergic response in the podocytes was analyzed with ratiometric confocal fluorescence measurements. Various P2 receptors activators were tested and compared with the effect of ATP, specifically, UDP, MRS 2365, bzATP, αβ-methylene, 2-meSADP, MRS 4062, and MRS 2768, were analyzed. Antagonists (MRS 2500, 5-BDBD, A438079, and NF 449) were tested when 10 μM ATP was applied as the EC50 for ATP activation of the calcium influx in the podocytes was determined to be 10.7 ± 1.5 μM. Several agonists including MRS 2365 and 2-meSADP caused calcium flux. Importantly, only the P2Y1-specific antagonist MRS 2500 (1 nM) precluded the effects of ATP concentrations of the physiological range. Immunohistochemical analysis confirmed that P2Y1 receptors are highly expressed in the podocytes. We conclude that P2Y1 receptor signaling is the predominant P2Y purinergic pathway in the glomeruli podocytes and P2Y1 might be involved in the pathogenesis of glomerular injury and could be a target for treatment of kidney diseases.

The platelet P2Y(12) receptor under normal and pathological conditions. Assessment with the radiolabeled selective antagonist [(3)H]PSB-0413

Various radioligands have been used to characterize and quantify the platelet P2Y(12) receptor, which share several weaknesses: (a) they are metabolically unstable and substrates for ectoenzymes, (b) they are agonists, and (c) they do not discriminate between P2Y(1) and P2Y(12). We used the [(3)H]PSB-0413 selective P2Y(12) receptor antagonist radioligand to reevaluate the number of P2Y(12) receptors in intact platelets and in membrane preparations. Studies in humans showed that: (1) [(3)H]PSB-0413 bound to 425 ± 50 sites/platelet (K (D) = 3.3 ± 0.6 nM), (2) 0.5 ± 0.2 pmol [(3)H]PSB-0413 bound to 1 mg protein of platelet membranes (K (D) = 6.5 ± 3.6 nM), and (3) competition studies confirmed the known features of P2Y(12), with the expected rank order of potency: AR-C69931MX > 2MeSADP ≫ ADPβS > ADP, while the P2Y(1) ligand MRS2179 and the P2X(1) ligand α,β-Met-ATP did not displace [(3)H]PSB-0413 binding. Patients with severe P2Y(12) deficiency displayed virtually no binding of [(3)H]PSB-0413 to intact platelets, while a patient with a dysfunctional P2Y(12) receptor had normal binding. Studies in mice showed that: (1) [(3)H]PSB-0413 bound to 634 ± 87 sites/platelet (K (D) = 14 ± 4.5 nM) and (2) 0.7 pmol ± 0.3 [(3)H]PSB-0413 bound to 1 mg protein of platelet membranes (K (D) = 9.1 ± 5.3 nM). Clopidogrel and other thiol reagents like pCMBS or DTT abolished the binding both to intact platelets and membrane preparations. Therefore, [(3)H]PSB-0413 is an accurate and selective tool for radioligand binding studies aimed at quantifying P2Y(12) receptors, to identify patients with P2Y(12) deficiencies or quantify the effect of P2Y(12) targeting drugs.

Reciprocal regulation of platelet responses to P2Y and thromboxane receptor activation

BACKGROUND

Thromboxane A(2) and ADP are two major platelet agonists that stimulate two sets of G protein-coupled receptors to activate platelets. Although aggregation responses to ADP and thromboxane desensitize, there are no reports currently addressing whether activation by one agonist may heterologously desensitize responses to the other.

OBJECTIVES

To demonstrate whether responses to ADP or U46619 may be modulated by prior treatment of platelets with the alternate agonist, revealing a level of cross-desensitization between receptor systems.

RESULTS

Here we show that pretreatment of platelets with either agonist substantially desensitizes aggregation responses to the other agonist. Calcium responses to thromboxane receptor activation are desensitized by preactivation of P2Y(1) but not P2Y(12) receptors. This heterologous desensitization is mediated by a protein kinase C (PKC)-independent mechanism. Reciprocally, calcium responses to ADP are desensitized by pretreatment of platelets with the thromboxane analogue, U46619, and P2Y(12)-mediated inhibition of adenylate cyclase is also desensitized by pretreatment with U46619. In this direction, desensitization is comprised of two components, a true heterologous component that is PKC-independent, and a homologous component that is mediated through stimulated release of dense granule ADP.

CONCLUSIONS

This study reveals cross-desensitization between ADP and thromboxane receptor signaling in human platelets. Cross-desensitization is mediated by protein kinases, involving PKC-dependent and independent pathways, and indicates that alterations in the activation state of one receptor may have effects upon the sensitivity of the other receptor system.

P2T purinoceptors: ADP receptors on platelets

ADP acts on platelets via the P2T purinoceptor to cause aggregation, but the way in which it does so is not fully understood. Most aggregating agents act via G protein-coupled receptors to stimulate phospholipase C (PLC) and so mobilize Ca2+ via inositol trisphosphate, whereas ADP clearly causes the mobilization of Ca2+ from internal stores but is only a weak activator of PLC. ADP also inhibits adenylate cyclase and it has been suggested that this effect is mediated by a different receptor, although evidence from antagonist studies argues against this. Studies of Ca2+ influx have shown that ADP is unique in causing a rapid influx of Ca2+, and patch-clamp studies have confirmed the activation by ADP of non-selective cation channels. This would imply the existence of two ADP receptors on platelets, a receptor-operated channel responsible for the rapid Ca2+ influx and a G protein-coupled receptor possibly linked to both inhibition of adenylate cyclase and mobilization of Ca2+. In this review the structure-activity relationships for aggregation, inhibition of adenylate cyclase and increases in cytoplasmic Ca2+ are summarized, and the relationship between these effects discussed.

The 'allosteric modulator' SCH-202676 disrupts G protein-coupled receptor function via sulphydryl-sensitive mechanisms

1. Previous studies suggest that the thiadiazole compound SCH-202676 (N-(2,3-diphenyl-1,2,4-thiadiazol-5-(2H)-ylidene)methanamine) acts as an allosteric modulator of a variety of structurally distinct G protein-coupled receptors (GPCRs). It was postulated that SCH-202676 would directly bind a structural motif in the receptor molecule common to divergent members of the GPCR family. The molecular mechanisms of such a promiscuous action, however, remain obscure. 2. To clarify the mechanism of SCH-202676 action, we used the functional approach of [35S]GTPgammaS autoradiography with rat brain cryostat sections together with classical membrane [35S]GTPgammaS binding assays to evaluate how the thiadiazole affects G protein activity mediated by various receptors linked to the Gi-family of G proteins. 3. We found that in the absence of dithiotreitol (DTT), SCH-202676 (10(-7)-10(-5) M) elicits nonspecific effects in the [35S]GTPgammaS-based G protein activation assays, thereby severely compromising interpretations on the compounds ability to allosterically inhibit receptor-mediated G protein activity. Such a nonspecific behaviour was fully reversed upon addition of DTT (1 mM), revealing thiol-based mechanism of action. 4. In routine incubations containing DTT, SCH-202676 had no effect on receptor-driven G protein activity, as assessed for adenosine A1, alpha2-adrenergic, cannabinoid CB1, lysophosphatidic acid LPA1, muscarinic M2/M4, purinergic P2Y12 or sphingosine 1-phosphate receptors, suggesting that the thiadiazole does not act as an allosteric modulator of GPCR function. 5. 1H NMR analysis indicated that SCH-202676 underwent structural changes after incubation with the reducing agent DTT or with brain tissue. 6. We conclude that SCH-202676 modulates GPCRs via thiol modification rather than via true allosteric mechanisms.

Synthesis and preliminary evaluation of [3H]PSB-0413, a selective antagonist radioligand for platelet P2Y12 receptors

The selective antagonist radioligand [(3)H]2-propylthioadenosine-5'-adenylic acid (1,1-dichloro-1-phosphonomethyl-1-phosphonyl) anhydride ([(3)H]PSB-0413) was prepared by catalytic hydrogenation of its propargyl precursor with a high specific radioactivity of 74Ci/mmol. In preliminary saturation binding studies, [(3)H]PSB-0413 showed high affinity for platelet P2Y(12) receptors with a K(D) value of 4.57nM. Human platelets had a high density of P2Y(12) receptors exhibiting a B(max) value of 7.66pmol/mg of protein.

Adenine triphosphate nucleotides are antagonists at the P2Y receptor

The aim of the present study was to characterize the pharmacological profile of the P2Y(12) receptor for several adenine triphosphate nucleotides in view of their possible roles as partial agonists or true antagonists. Two distinct cellular systems were used: P2Y(1) receptor deficient mouse platelets ( platelets) previously shown to express a native and functional P2Y(12) receptor and 1321 N1 astrocytoma cells stably expressing the human P2Y(12) receptor (1321 N1 P2Y(12)). ADP and its structural analogues inhibited cAMP accumulation in a dose-dependent manner in both platelets and 1321 N1 P2Y(12) cells with a similar rank order of potency, 2 methylthio-ADP (2MeSADP) >>ADP - Adenosine 5'-(betathio) diphosphate (AlphaDPbetaS). Commercial ATP, 2 chloro; ATP (2ClATP) and 2 methylthio-ATP (2MeSATP) also inhibited cAMP accumulation in both cell systems. In contrast, after creatine phosphate (CP)/creatine phosphokinase (CPK) regeneration, adenine triphosphate nucleotides lost their agonistic effect on platelets and behaved as antagonists of ADP (0.5 microm)-induced adenylyl cyclase inhibition with IC(50) of 13.5 +/- 4.8, 838 +/- 610, 1280 +/- 1246 microm for 2MeSATP, ATP and 2ClATP, respectively. In 1321 N1 P2Y(12) cells, CP/CPK regenerated ATP and 2ClATP lost their agonistic effect only when CP/CPK was maintained during the cAMP assay. The stable ATP analogue ATPgammaS antagonized ADPbetaS-induced inhibition of cAMP accumulation in both platelets and 1321 N1 P2Y(12) cells. Thus, ATP and its triphosphate analogues are not agonists but rather antagonists at the P2Y(12) receptor expressed in platelets or transfected cells, provided care is taken to remove diphosphate contaminants and to prevent the generation of diphosphate nucleotide derivatives by cell ectonucleotidases.

Agonist concentration-dependent differential responsivity of a human platelet purinergic receptor: pharmacological and kinetic studies of aggregation, deaggregation and shape change responses mediated by the purinergic P2Y1 receptor in vitro

Platelet shape change (SC), aggregation and deaggregation responses are integral components of hemostasis that are elicited and modulated in vivo by the simultaneous activation of several membrane receptors. Selective activation of the purinergic P2Y1 receptor in vivo elicits a sustained SC and a small, transient aggregation response that is reversed rapidly by a robust deaggregation response (Platelets 2003; 14: 89). Using a kinetics-based turbidimetric approach to study the modulation of these concurrent components of human platelet responses, we demonstrate that these P2Y1 receptor-related responses and a number of their kinetic and steady-state characteristics are differentially elicited and modulated. P2Y1 receptor agonist concentrations that elicited aggregation (pEC50 for ADP, 2-MeSADP; 5.88, 6.69) were 10-fold greater than those that elicited SC (7.33, 7.67). The magnitude of the aggregation response was agonist concentration-dependent, saturable and was associated with an agonist concentration-dependent deceleration of the deaggregation response. Gi-coupled receptor (alpha 2A-adrenoceptor, EP3 and P2Y12 receptors) agonists also enhanced aggregation through deceleration of the deaggregation response, and an inhibitor of PI3K activity (wortmannin) inhibited aggregation through acceleration of the deaggregation response. Neither treatment affected the extent or the kinetics of the SC response. The aggregation but not the SC response was rapidly desensitized by P2Y1 receptor activation by ADP. The affinity of the presence of a single P2Y1 receptor subtype. The differential characteristics and modulation of the SC and aggregation responses by a single receptor support the idea that different signaling pathways activated at different occupancy states of the same receptor underlie the two responses. P2Y1 receptor-mediated platelet aggregation and SC responses provide a convenient model for studying the phenomenon of agonist-directed signaling by differential occupancy of the same membrane receptor.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

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

Identification, characterization, and inhibition of the platelet ADP receptors

Adenosine diphosphate (ADP) plays a crucial role in hemostasis and thrombosis, and its receptors are potential targets for antithrombotic drugs. Two G-protein-coupled P2 receptors contribute to platelet aggregation: the P2Y1 receptor initiates aggregation through mobilization of calcium stores, whereas the P2Y12 receptor coupled to adenylyl cyclase inhibition is essential for a full aggregation response to ADP and the stabilization of aggregates. The latter is defective in certain patients with a selective congenital deficiency of aggregation to ADP. It is also the target of the antithrombotic drug clopidogrel and of adenosine triphosphate analogues and other compounds currently under evaluation. In addition, the P2X1 ionotropic receptor is present in platelets, but its role is not yet completely known. Studies in P2Y1-knockout mice and experimental thrombosis models using selective P2Y1 antagonists have shown that the P2Y1 receptor, like the P2Y12 receptor, is a potential target for new antithrombotic drugs.

P2y(12), a new platelet ADP receptor, target of clopidogrel

The binding characteristics of (33)P-2MeS-ADP, a stable analogue of ADP, were determined on CHO cells transfected with the human P2Y(12) receptor, a novel purinergic receptor. These transfected CHO cells displayed a strong affinity for (33)P-2MeS-ADP, the binding characteristics of which corresponded in all points to those observed on platelets. In particular, this receptor recognised purines with the following order of potency: 2MeS-ADP = 2MeS-ATP > ADP = ATPgammaS = ATP >> UTP, a binding profile which is similar to that obtained in platelets. The binding of (33)P-2MeS-ADP was antagonised by pCMPS but not by MRS2179 and FSBA, antagonists of P2Y(1) and aggregin, respectively. Moreover, the binding of (33)P-2MeS-ADP to these cells was strongly and irreversibly inhibited by the active metabolite of clopidogrel with a potency which was consistent with that observed for this compound on platelets. Like in platelets, 2MeS-ADP induced adenylyl cyclase down-regulation in these P2Y(12) transfected CHO cells, an effect which was absent in the corresponding non-transfected cells. As already shown in platelets, the active metabolite of clopidogrel antagonised 2MeS-ADP-induced inhibition of adenylyl cyclase on transfected cells. Our results confirm that P2Y(12) is the previously called "platelet P2t(AC)" receptor and show that this receptor is antagonised by the active metabolite of clopidogrel.

[(35)S]GTPgammaS autoradiography reveals a wide distribution of G(i/o)-linked ADP receptors in the nervous system: close similarities with the platelet P2Y(ADP) receptor

No G(i)-linked P2Y receptors have been cloned to date but the presence of such receptors is thought to be restricted to platelets and certain clonal cell lines. Using the functional approach of [(35)S]guanosine 5'-[gamma-thio]-triphosphate autoradiography, we uncovered the widespread presence of such receptors in the CNS. Under conditions in which the prominent signal due to tonic adenosine receptor activity is masked, ADP and ATP stimulated G-protein activity in multiple grey and white matter regions. Localization in the grey matter suggests inhibitory auto-/heteroreceptor function. In the white matter, activated G proteins appeared as 'hot spots' (presumed oligodendrocyte progenitors) with scattered distribution along the main fibre tracts. Responses to ATP were diminished under conditions that inhibited degradation, suggesting that prior conversion to ADP explained agonist action. Uracil nucleotides were ineffective but 2-methylthio-ADP activated G proteins approximately 500-fold more potently than ADP, although both were similarly degraded. Throughout the brain, ADP-dependent G-protein activity was reversed by 2-hexylthio-AdoOC(O)Asp(2), a non-phosphate ATP analogue, whereas selective P2Y(1) receptor antagonists proved ineffective. A similar receptor was also disclosed from the adrenal medulla. These data witness a hitherto unrecognized abundance of G(i/o)-linked ADP receptors in the nervous system. Biochemical and pharmacological behaviour suggests striking similarities to the elusive platelet P2Y(ADP) receptor.

Identification of the platelet ADP receptor targeted by antithrombotic drugs

Platelets have a crucial role in the maintenance of normal haemostasis, and perturbations of this system can lead to pathological thrombus formation and vascular occlusion, resulting in stroke, myocardial infarction and unstable angina. ADP released from damaged vessels and red blood cells induces platelet aggregation through activation of the integrin GPIIb-IIIa and subsequent binding of fibrinogen. ADP is also secreted from platelets on activation, providing positive feedback that potentiates the actions of many platelet activators. ADP mediates platelet aggregation through its action on two G-protein-coupled receptor subtypes. The P2Y1 receptor couples to Gq and mobilizes intracellular calcium ions to mediate platelet shape change and aggregation. The second ADP receptor required for aggregation (variously called P2Y(ADP), P2Y(AC), P2Ycyc or P2T(AC)) is coupled to the inhibition of adenylyl cyclase through Gi. The molecular identity of the Gi-linked receptor is still elusive, even though it is the target of efficacious antithrombotic agents, such as ticlopidine and clopidogrel and AR-C66096 (ref. 9). Here we describe the cloning of this receptor, designated P2Y12, and provide evidence that a patient with a bleeding disorder has a defect in this gene. Cloning of the P2Y12 receptor should facilitate the development of better antiplatelet agents to treat cardiovascular diseases.

Platelet P2 receptors: from curiosity to clinical targets

Adenosine 5'-diphosphate (ADP) is a paracrine mediator that activates human blood platelets, causing them to become adhesive and thereby contributing to their role in hemostasis. The actions of ADP were initially thought to be mediated by a unique ADP receptor termed P2(T) found only on platelets and antagonized by ATP, but it appears that at least two P2Y receptor subtypes are involved, a P2Y(1) receptor linked in some way to control of intracellular-free calcium levels and another P2Y receptor linked via an inhibitory G protein to adenylate cyclase. In addition, the presence of excitatory P2X(1) receptors that mediate the influx of monovalent and divalent cations in response to both ADP and ATP has been demonstrated. The precise contribution that each of these P2 receptors make to the overall phenomena associated with platelet aggregation, adhesion and hemostasis is yet to be defined. Antithrombotic agents that interfere with the actions of ADP are marketed, and P2 receptor antagonists are entering clinical trials for acute treatments of thrombosis. This review seeks to summarize the present state of knowledge of platelet P2 receptor pharmacology and therapeutics.

ADP-induced platelet aggregation and inhibition of adenylyl cyclase activity stimulated by prostaglandins: signal transduction mechanisms

ADP is the oldest and one of the most important agonists of platelet activation. ADP induces platelet shape change, exposure of fibrinogen binding sites, aggregation, and influx and intracellular mobilization of Ca2+. ADP-induced platelet aggregation is important for maintaining normal hemostasis, but aberrant platelet aggregation manifests itself pathophysiologically in myocardial ischemia, stroke, and atherosclerosis. Another important aspect of ADP-induced platelet activation is the ability of ADP to antagonize adenylyl cyclase activated by prostaglandins. ADP-induced inhibition of the stimulated adenylyl cyclase activity does not appear to play a role in ADP-induced platelet aggregation in vitro or in vivo. It is believed that a single ADP receptor mediates the above two ADP-induced platelet responses in platelets. The ADP receptor mediating ADP-induced platelet aggregation and inhibition of the stimulated adenylyl cyclase activity has not been purified. Therefore, the nature of molecular mechanisms underlying the two seemingly unrelated ADP-induced platelet responses remains either unclear or less well understood. The purpose of this commentary is to examine and make suggestions concerning the role of phospholipases and G-proteins in the molecular mechanisms of signal transduction underlying the two ADP-induced platelet responses. It is hoped that such discussion would stimulate thinking and invite future debates on this subject, and energize investigators in their efforts to advance our knowledge of the details of the molecular mechanisms of ADP-induced platelet activation.

Relaxant effect of 2-methyl-thio-adenosine diphosphate on rat thoracic aorta: effect of clopidogrel

The main aim of this study was to determine the functional effect of 2-methyl-thio-adenosine diphosphate (2MeS-ADP) on vascular purinoceptors, in comparison with that of a characterised agonist of the P2Y1 receptor, 2-methyl-thio-adenosine triphosphate (2MeS-ATP), and of the P2Y2 receptor, uridine triphosphate (UTP). On phenylephrine-precontracted rat aortic rings, mounted isometrically in organ baths, we found that 2MeS-ADP (10(-9) to 10(-6) M) induced concentration-dependent relaxation of rings with a functional endothelium. Mechanical removal of the endothelium abolished the relaxant effect of 2MeS-ADP. The 2MeS-ADP-induced relaxation of phenylephrine-precontracted rings was inhibited by Nomega-nitro-L-arginine methyl ester (L-NAME) (100 microM) but not by indomethacin (100 microM) or aspirin (1 mM), indicating that the 2MeS-ADP-induced relaxation was nitric oxide (NO) synthase-mediated but not cyclooxygenase-dependent. Repeated stimulation with 2MeS-ADP resulted in desensitisation of the receptor. Under these conditions, the relaxant effect of 2MeS-ATP was abolished. On the contrary, UTP-induced relaxation was not affected, showing that 2MeS-ADP and 2MeS-ATP but not UTP shared the same receptor. Suramin (100 microM), a non-specific P2 inhibitor, abolished the effect of 2MeS-ADP, 2MeS-ATP and UTP. In contrast, pyridoxal-phosphate-6-azophenyl-2'-4'-disulphonic acid (PPADS) and adenosine-3'-phosphate-5'-phosphosulphate (A3P5PS) abolished only the vasodilator responses to 2MeS-ADP and 2MeS-ATP and did not affect the relaxant effect of UTP, showing that 2MeS-ADP acted through the P2Y1 receptor. Clopidogrel, a potent platelet ADP receptor antagonist, at a dose that strongly inhibited ADP-induced platelet aggregation ex vivo, did not modify the relaxant responses to 2MeS-ADP or 2MeS-ATP. In conclusion, these results showed that 2MeS-ADP induces endothelium-dependent, NO-mediated relaxation of rat aortic rings. This effect, resistant to clopidogrel treatment, occurred through activation of the P2Y1 receptor.

ADP-induced platelet activation

Platelet activation is central to the pathogenesis of hemostasis and arterial thrombosis. Platelet aggregation plays a major role in acute coronary artery diseases, myocardial infarction, unstable angina, and stroke. ADP is the first known and an important agonist for platelet aggregation. ADP not only causes primary aggregation of platelets but is also responsible for the secondary aggregation induced by ADP and other agonists. ADP also induces platelet shape change, secretion from storage granules, influx and intracellular mobilization of Ca2+, and inhibition of stimulated adenylyl cyclase activity. The ADP-receptor protein mediating ADP-induced platelet responses has neither been purified nor cloned. Therefore, signal transduction mechanisms underlying ADP-induced platelet responses either remain uncertain or less well understood. Recent contributions from chemists, biochemists, cell biologists, pharmacologists, molecular biologists, and clinical investigators have added considerably to and enhanced our knowledge of ADP-induced platelet responses. Although considerable efforts have been directed toward identifying and cloning the ADP-receptor, these have not been completely successful or without controversy. Considerable progress has been made toward understanding the mechanisms of ADP-induced platelet responses but disagreements persist. New drugs that do not mimic ADP have been found to inhibit fairly selectively ADP-induced platelet activation ex vivo. Drugs that mimic ADP and selectively act at the platelet ADP-receptor have been designed, synthesized, and evaluated for their therapeutic efficacy to block selectively ADP-induced platelet responses. This review examines in detail the developments that have taken place to identify the ADP-receptor protein and to better understand mechanisms underlying ADP-induced platelet responses to develop strategies for designing innovative drugs that block ADP-induced platelet responses by acting selectively at the ADP-receptor and/or by selectively interfering with components of ADP-induced platelet activation mechanisms.

Slow-phase kinetics of nucleotide binding to the uncoupling protein from brown adipose tissue mitochondria

The kinetics of nucleotide binding to the uncoupling protein (UCP) from brown adipose tissue mitochondria were studied with a filter binding method. Fast and slow phases of binding were observed, corresponding to the two-stage binding model based on equilibrium binding studies (Huang, S. G., and Klingenberg, M. (1996) Biochemistry 35, 7846-7854) (Reaction 1). [reaction: see text] Although this method determines total binding, only the slow phase can be resolved. The fast unresolved phase represents the formation of the initial loose UCP-nucleotide complex (UN; Kd approximately 2 microM), whereas the slow phase reflects the tight binding (U*N) associated with a conformational change induced by the bound nucleotide. Best fits of the binding data yielded, for the slow phase, k+1 values of 3.0 x 10(-3) s-1 for GTP, 4.8 x 10(-3) s-1 for ATP, 0.13 s-1 for GDP, and >0.7 s-1 for ADP and dissociation rate constants (k-1) of 0.10 x 10(-3) s-1 for GTP, 0.58 x 10(-3) s-1 for ATP, 8.8 x 10(-3) s-1 for GDP, and >0.3 s-1 for ADP at pH 6.7 and 4 degrees C. The rates were fairly pH- and temperature-dependent. The distribution constant Kc' (=k+1/k-1) between the tight and loose complexes ranged between 2 and 30, suggesting formation of 71-97% of the tight complex at equilibrium. The Kc' decreases with increasing pH, indicating a progressively less tight complex population. Anions (SO42-) form a loose complex with UCP, thus affecting the initial association step, but not the subsequent transition step. While the kinetic constants were verified by dilution and chase experiments as well as in mass action plots, they were further corroborated with data obtained by fluorescence competition measurements. Taken together, our results show that nucleotide binding to UCP occurs via a two-stage mechanism in which the initial loose complex rearranges slowly into a tight complex.

Purinoceptors and platelet aggregation

1. Adenosine 5'-diphosphate (ADP) is a physiologically important mediator, being released from damaged cells and from aggregating platelets. It acts on platelets to cause aggregation via a purinoceptor ('P2T-purinoceptor') at which adenosine 5'-triphosphate (ATP) is a competitive antagonist; however, the way in which it does so is not fully understood. 2. ADP activates a G12 protein, is a weak activator of phospholipase C but causes calcium mobilization from internal stores, and also inhibits adenylate cyclase. It seems likely that these effects are mediated by a single receptor but this is still unclear. 3. ADP also causes a rapid calcium influx which has the characteristics of a receptor-operated channel, and it has been suggested that this is due to a P2X1 receptor. This suggests the presence of at least two types of receptor responding to ADP, one G protein coupled and one a cation channel, and raises questions about the role of ATP in platelet function. 4. Adenosine acts via an A2a receptor to stimulate adenylate cyclase in platelets, and this nonselectively inhibits platelet activation. As ADP released from platelets is broken down to adenosine by ectonucleotidases on endothelial cells, this may provide an important mechanism for limiting inappropriate platelet aggregation in an intact blood vessel.

Ultrastructural studies of platelet aggregates from human subjects receiving clopidogrel and from a patient with an inherited defect of an ADP-dependent pathway of platelet activation

Our study investigated the effect of the antithrombotic drug clopidogrel (75 mg/d for 7 days) on the ultrastructure of platelet aggregates induced by ADP or 2-methylthio-ADP (2-MeS-ADP) in citrated platelet-rich plasma and examined the activation state of the GP IIb/IIIa complexes. Results were compared with those obtained for patient M.L., who has a congenital disorder characterized by a reduced and reversible platelet response to ADP. When untreated normal platelets were stimulated with high-dose ADP, electron microscopy revealed large and stable aggregates often surrounded by a layer of what appeared to be degranulated platelets. The reversible aggregates of platelets from subjects receiving clopidogrel or from patient M.L. did not show this layer. Electron microscopy showed that in both situations, the aggregates were composed of loosely bound platelets with few contact points. Immunogold labeling of ultrathin sections of Lowicryl-embedded aggregates formed by ADP or 2-MeS-ADP showed a much decreased platelet surface staining by (1) a polyclonal anti-fibrinogen antibody and (2) AP-6, a murine anti-ligand-induced binding site monoclonal antibody specific for GP IIb/IIIa complexes occupied with fibrinogen. Similar findings were seen after disaggregation, when many single platelets were present that showed no signs of secretion. Flow cytometry confirmed that the number of ligand-occupied GP IIb/IIIa complexes was much lower on platelets stimulated with ADP or 2-MeS-ADP after clopidogrel treatment. As expected from previous studies, ADP-induced platelet shape change and Ca2+ influx were unaffected by clopidogrel. These results agree with the hypothesis that platelet activation by ADP is biphasic and highlight a receptor-induced activation pathway affected by clopidogrel (or congenitally impaired in patient M.L.) that is necessary for the full activation of GP IIb/IIIa and the formation of stable macroaggregates.

Characterization of P2 purinergic receptors on human erythro leukemia cells

We have investigated the nature of the nucleotide receptors on human erythro leukemia (HEL) cells, a cell line with some megakaryocytic properties, using a combination of pharmacological, photoaffinity labeling, and molecular biological techniques. Fura-2 loaded HEL cells responded to 2-methylthio ATP, ATP, 2-methylthio ADP, ADP and UTP with an increase in intracellular calcium. 2 Methylthio ADP was the most potent agonist. When external calcium was chelated with EDTA, calcium responses were observed indicating the mobilization of intracellular stores. These responses showed evidence of both homologous and heterologous receptor desensitization. In photoaffinity labeling experiments, beta-[32P]-AzPET-ADP was incorporated into three protein species with mobilities corresponding to M(r) approximately 55 kDa (doublet) and approximately 43 kDa. Labeling of approximately 55 kDa proteins was specifically inhibited by ADP, while that of the approximately 43 kDa was inhibited specifically by UTP. Nucleotide sequence analysis of the positive clones obtained by screening the HEL cell cDNA library with mouse P2U cDNA revealed that the P2U receptor from HEL cells is identical to the previously cloned human P2U receptor. These experiments suggest that the HEL cells contain a P2Y purinoceptor responding to ADP, in addition to a P2U receptor and possibly also a third P2 purinoceptor with a unique agonist profile.

Platelet activation and thrombosis: studies in a patient with essential thrombocythemia

Recent advances permit the detection of activated platelets using specific monoclonal antibodies and flow cytometry. Nevertheless, there are few reports in which activated platelets have been studied over a period of time in patients at risk for thrombosis. Our patient S.D. has essential thrombocythemia and a prothrombotic state manifested in two major thrombotic episodes involving the portal vein and a mesenteric artery. Investigation revealed both spontaneous aggregation and hyperaggregability in response to ADP and the presence of activated platelets in platelet-rich plasma as revealed by flow cytometry. Interestingly, the activated platelets were recognized by an anti-RIBS ("receptor-induced binding site") monoclonal antibody that recognized bound fibrinogen but not by antibodies reactive with antigens whose presence on the platelet surface was secretion dependent. Treatment with aspirin inhibited spontaneous platelet aggregation but had little effect on the activated platelet profile. A change of therapy to ticlopidine suppressed expression of platelet activation markers. Treatment with ticlopidine has continued for 1 year so far without further thrombotic complications.

Inhibition of ADP-induced platelet responses by covalent modification of aggregin, a putative ADP receptor, by 8-(4-bromo-2,3-dioxobutylthio)ADP

ADP is an important platelet agonist which initiates platelet shape change, aggregation, exposure of fibrinogen receptors, and calcium mobilization. Because of the limitations of previously used affinity analogs and photo-labeling studies as well as controversies surrounding the identity of an ADP receptor on platelets, we have used an affinity label capable of alkylating a putative exofacial receptor on platelets. We now report that 8-(4-bromo-2,3-dioxobutylthio)adenosine-5'-diphosphate (8-BDB-TADP), which is an analog of the natural ligand ADP, blocked ADP-induced platelet shape change, aggregation, exposure of fibrinogen-binding sites, secretion, and calcium mobilization. Following modification by 8-BDB-TADP, the rates of aggregation of platelets induced by thrombin, a calcium ionophore (A23187) or a stimulator of protein kinase C (phorbol myristate acetate) were minimally affected. However, the 8-BDB-TADP-modified platelets exhibited decreased rates of aggregation in response to ADP, as well as collagen and a thromboxane mimetic (U46619), both of which partially require ADP. Autoradiograms of the gels obtained by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of solubilized platelets modified by either [beta-32P]8-BDB-TADP, or 8-BDB-TADP and NaB[3H]4 showed the presence of a single radiolabeled protein band at 100 kDa. The intensity of this band was reduced when platelets were preincubated with ADP, ATP, and 8-bromo-ADP prior to labeling by the radioactive 8-BDB-TADP. The results show that 8-BDB-TADP selectively and covalently labeled aggregin (100 kDa), a putative ADP receptor, resulting in a loss of ADP-induced platelet responses.

Purinoceptors on blood platelets: further pharmacological and clinical evidence to suggest the presence of two ADP receptors

Platelet aggregation by ADP plays a major role in the development and extension of arterial thrombosis. The antithrombotic thienopyridine compounds ticlopidine and clopidogrel have proved useful tools to investigate the mechanisms of ADP-induced platelet activation. In essence, although clopidogrel has been shown to completely and selectively block ADP-induced platelet aggregation, G protein activation and inhibition of adenylyl cyclase, this drug does not affect shape change and Ca2+ influx. Binding studies, using the non-hydrolysable ligand [33P]2MeSADP, have shown that human platelets contain about 600 high-affinity binding sites for 2MeSADP (Kd approximately 5 nM). These sites present pharmacological characteristics of a P2T receptor. Clopidogrel treatment reduces the number of sites by 70% on rat platelets (from 1200 to 450) and leaves the residual binding sites resistant to clopidogrel. Moreover, patients with congenital impairment of ADP-induced platelet aggregation but normal shape change display very low levels of [33P]2MeSADP binding sites. The current data thus strongly suggest the presence of two ADP receptors, one responsible for shape change and rapid Ca2+ influx and the other a Gi protein-coupled receptor responsible for Ca2+ mobilization from internal stores, inhibition of adenylyl cyclase and platelet aggregation.

Platelet activation

This review article describes the different receptors, second-messengers and mechanisms involved in platelet activation. Several platelet agonists have well-defined receptors at the platelet membrane of which a number are single polypeptides with 7 hydrophobic transmembrane domains. These receptors are connected, via GTP regulatory proteins, with cytoplasmic second-messenger-generating enzymes. Phospholipase C and adenylate cyclase are the two best-known enzymes, generating inositol triphosphate (IP3) and diacyl glycerol from phosphatidylinositol biphosphate and cyclic AMP from ATP respectively. The intraplatelet free calcium level, which is critical for the activation status of the platelet, is increased by IP3 and is lowered in the presence of rising cyclic AMP concentrations. Shape-change occurs with small increases in intraplatelet calcium, while aggregation and secretion of granules take place at higher calcium, levels. The role of myosin and actin filaments and of transmembrane glycoproteins is further discussed.

The role of platelets in essential hypertension

Recent research is helping us understand the complex interactions that occur between platelets and their environment. The several intracellular events that occur during platelet activation are being identified as ar their effects on other platelets, the endothelium and coagulation factors. Heightened platelet activation is seen early in essential hypertension and probably plays an important role in the initiation and progression of atherosclerosis and the disorders associated with it. This review identifies some of the changes in platelet structure and function in essential hypertension and their role in the pathogenesis of hypertensive vascular disease.

An inherited bleeding disorder linked to a defective interaction between ADP and its receptor on platelets. Its influence on glycoprotein IIb-IIIa complex function

Much discussion has concerned the central role of ADP in platelet aggregation. We now describe a patient (M.L.) with an inherited bleeding disorder whose specific feature is that ADP induces a limited and rapidly reversible platelet aggregation even at high doses. Platelet shape change and other hemostatic parameters were unmodified. A receptor defect was indicated, for, while epinephrine normally lowered cAMP levels of PGE1-treated (M.L.) platelets, ADP was without effect. The binding of [3H]2-methylthio-ADP decreased from 836 +/- 126 molecules/platelet for normals to 30 +/- 17 molecules/platelet for the patient. Flow cytometry confirmed that ADP induced a much lower fibrinogen binding to (M.L.) platelets. Nonetheless, the binding in whole blood of activation-dependent monoclonal antibodies showed that some activation of GP IIb-IIIa complexes by ADP was occurring. Platelets of a patient with type I Glanzmann's thrombasthenia bound [3H]2-methylthio-ADP and responded normally to ADP in the presence of PGE1. Electron microscopy showed that ADP-induced aggregates of (M. L.) platelets were composed of loosely bound shape-changed platelets with few contact points. Thus this receptor defect has a direct influence on the capacity of platelets to bind to each other in response to ADP.

Evidence for the existence of two different ADP-binding sites on rat platelets

[3H]-2-Methylthio-ADP ([3H]-2-MeS-ADP), a stable analogue of ADP bound to one type of specific binding sites on rat platelets (KD = 0.77 +/- 0.07 nM, Bmax = 160 +/- 11 fmol/10(8) cells). 2-MeS-ADP and ADP antagonized [3H]-2-MeS-ADP binding, showing respective Ki values of 1.4 +/- 0.1 nM and 486 +/- 78 nM. Clopidogrel, a potent and specific inhibitor of ADP-induced platelet aggregation partially inhibited (approximately 70% inhibition) the binding of [3H]-2-MeS-ADP at the same time it abrogated 2-MeS-ADP- and ADP-induced adenylyl cyclase inhibition and aggregation. A population of clopidogrel-resistant [3H]-2-MeS-ADP binding sites was detected on platelets from treated animals. These receptor sites (KD = 0.9 +/- 0.2 nM, Bmax = 47 +/- 5 fmol/10(8) platelets) which showed high affinity for both ADP and 2-MeS-ADP (Ki values in the nanomolar range) might be involved in the ADP-induced shape change, a clopidogrel-resistant ADP-induced event. Using clopidogrel which acts via a direct and irreversible inhibition of ADP binding to its adenylyl cyclase-coupled receptor sites on platelets, we were able to discriminate between two types of ADP receptor sites. The former which was clopidogrel-sensitive represented about 70% of the total [3H]-2-MeS-ADP receptors and was responsible for ADP-induced platelet aggregation and adenylyl cyclase inhibition. The latter which was not affected by clopidogrel might be involved in ADP-induced shape-change.

Effects of extracellular divalent cations on responses of human blood platelets to adenosine 5'-diphosphate

The effects of extracellular divalent cations on the responses of human platelets to adenosine 5'-diphosphate (ADP) and on its inhibition by the competitive antagonist adenosine 5'-triphosphate (ATP) were investigated. Two responses were studied, shape change and the inhibition of prostaglandin E1 (PGE1)-stimulated adenylate cyclase, and experiments were carried out in the presence of divalent cations (Ca2+ and Mg2+, 1 mM) or in their absence. For each response there was a small leftward shift of the concentration-response curve to ADP in the absence of divalent cations compared to that in their presence, and this leftward shift disappeared when the results were plotted in terms of ADP3- rather than total ADP concentration. The shape change results were, however, complicated by a reduction in the maximal response to ADP in the absence of divalent cations. For each response there was also a marked increase in the pA2 value of ATP in the absence of divalent cations compared to that in their presence, and this difference disappeared if the results were calculated in terms of ATP4- instead of total ATP. These results suggest that the human platelet ADP receptor, in common with other receptors for adenine nucleotides, recognises predominantly the uncomplexed forms of ADP and ATP as ligands.

Characterization of specific binding sites for [3H]2-MeS-ADP on megakaryocytoblastic cell lines in culture

Binding of [3H]2-methyl thio-adenosine 5' diphosphate ([3H]2-MeS-ADP), a stable analogue of adenosine 5' diphosphate (ADP) to DAMI and Meg-01, two megakaryocytoblastic cell lines, was time-dependent, reversible and saturable. Scatchard analysis of the saturation binding data indicated that [3H]2-MeS-ADP bound to one class of specific binding sites with high affinity (dissociation constants = 45.3 +/- 13.4 and 48.2 +/- 17.7 nM, and maximum binding capacities = 341.2 +/- 31.1 and 903 +/- 98 fmole/10(6) cells for DAMI and Meg-01, respectively) (N = 3). Unlabelled 2-MeS-ADP competitively and selectively inhibited the specific binding of [3H]2-MeS-ADP on DAMI and Meg-01 with inhibitory constant values of 118 +/- 11 and 38 +/- 11 nM, respectively (N = 3). ADP was 3 to 10 times less potent than 2-Mes-ADP in displacing [3H]2-MeS-ADP from its binding sites on DAMI and Meg-01, whereas other ADP analogues, such as AMP, GDP, UDP, adenosine or FSBA, did not interfere with the binding of [3H]2-MeS-ADP, suggesting that DAMI and Meg-01 contain ADP-specific receptors.

Receptors for ADP on human blood platelets

It is well established that ADP causes aggregation of human blood platelets, and indeed it was the first aggregating agent to be studied, but the ways in which platelets respond to ADP are still relatively obscure. Although it is apparent that increases in intracellular Ca2+ concentrations are of major importance in activating platelets, it is not clearly understood how ADP causes these increases and what other signal transduction mechanisms it uses. It is not even clear whether ADP causes its effects by interacting with only one receptor, or whether multiple receptors for ADP exist on platelets. In this review, Susanna Hourani and David Hall examine some of the conflicting evidence in this field, and draw some tentative conclusions about the number and nature of receptors for ADP on human platelets.

Effects of suramin on increases in cytosolic calcium and on inhibition of adenylate cyclase induced by adenosine 5'-diphosphate in human platelets

The effects of the P2-purinoceptor antagonist, suramin, on ADP-induced increases in human platelet cytosolic calcium concentration ([Ca2+]i) and inhibition of prostaglandin E1 (PGE1)-stimulated adenylate cyclase activity were investigated. Suramin (50-200 microM) acted as an antagonist of ADP-induced increases in [Ca2+]i, causing parallel, rightward shifts of the log concentration-response curve to ADP with no apparent depression of the maximal response. However, the slope of the Schild plot was 2.3 +/- 0.3, similar to that obtained in previous studies on aggregation, indicating that the antagonism was not simply competitive. The apparent pA2 for suramin, taken from the Schild plot, was 4.63, similar to that for suramin's inhibition of aggregation, which suggests that these two effects are closely related. Suramin was not specific for the ADP receptor, however, as it was also able to inhibit, non-competitively, increases in [Ca2+]i induced by 5-hydroxytryptamine. Suramin (50-400 microM) also inhibited the effect of ADP on PGE1-stimulated accumulation of cyclic AMP, causing parallel shifts of the log concentration-response curve to ADP, with a Schild plot slope of 1.00 +/- 0.10, suggesting competitive antagonism, and a pA2 value of 5.09. Suramin (400 microM) did not reduce the inhibition of cyclic AMP accumulation by adrenaline, although it was able to inhibit the accumulation of cyclic AMP caused by PGE1, again showing that suramin has some non-specific effects. These data suggest that suramin is an antagonist at the platelet ADP receptor mediating increases in [Ca2+]i and inhibition of adenylate cyclase, but that it also shows non-specific effects and can depress platelet responses to other agonists. In addition, the similar pA2 value of suramin for the two effects of ADP does not support suggestion that they are mediated by two different receptors on human platelets.

Identification of a receptor for ADP on blood platelets by photoaffinity labelling

The synthesis of a new analogue of ADP, 2-(p-azidophenyl)-ethythioadenosine 5'-diphosphate (AzPET-ADP), is described. This compound contains a photolabile phenylazide group attached to the ADP molecule by a thioether link at the purine 2 position. It has been prepared in radioactive form with 32P in the beta-phosphate at a specific radioactivity of 100 mCi/mumol. The reagent activated platelets, causing shape change and aggregation, with somewhat lower affinity than ADP. On photolysis the affinity was increased. The reagent also inhibited platelet adenylate cyclase stimulation by prostaglandin E1, with considerably higher affinity than ADP. On photolysis the affinity was decreased. AzPET-ADP competitively inhibited the binding of 2-methylthio[beta-32P]ADP, a ligand for the receptor by which ADP causes inhibition of adenylate cyclase. In the dark, AzPET-[beta-32P]ADP bound reversibly and with high affinity to a single population of sites similar in number to the sites that bind 2-methylthio[beta-32P]ADP. Binding was inhibited by ADP and by ATP and by p-chloromercuribenzenesulphonic acid (pCMBS). On exposure to u.v. light in the presence of platelets, AzPET-[beta-32P]ADP was incorporated covalently but non-specifically into several platelet proteins, although prominent intracellular proteins were not labelled. Specific labelling was confined to a single region of SDS/polyacrylamide gels, overlying but not comigrating with actin. Incorporation of radioactivity into this region was inhibited by ADP and by ATP as well as by ADP beta S, ATP alpha S and pCMBS, but not by adenosine, GDP or AMP. Inhibition of AzPET-[beta-32P]ADP incorporation was closely correlated with inhibition of equilibrium binding of 2-methylthio[beta-32P]ADP. These results suggests that the labelled protein, which migrates with an apparent molecular mass of 43 kDa in reduced gels, is the receptor through which ADP inhibits adenylate cyclase.