Characteristics of the binding of [3H]-GR32191 to the thromboxane (TP-) receptor of human platelets

Analysis of Extracellular ATP Distribution in the Intervertebral Disc

Progressive loss of proteoglycans (PGs) is the major biochemical change during intervertebral disc (IVD) degeneration. Adenosine triphosphate (ATP) as the primary energy source is not only critical for cell survival but also serves as a building block in PG synthesis. Extracellular ATP can mediate a variety of physiological functions and was shown to promote extracellular matrix (ECM) production in the IVD. Therefore, the objective of this study was to develop a 3D finite element model to predict extracellular ATP distribution in the IVD and evaluate the impact of degeneration on extracellular ATP distribution. A novel 3D finite element model of the IVD was developed by incorporating experimental measurements of ATP metabolism and ATP-PG binding kinetics into the mechano-electrochemical mixture theory. The new model was validated by experimental data of porcine IVD, and then used to analyze the extracellular distribution of ATP in human IVDs. Extracellular ATP was shown to bind specifically with PGs in IVD ECM. It was found that annulus fibrosus cells hydrolyze ATP faster than that of nucleus pulposus (NP) cells whereas NP cells exhibited a higher ATP release. The distribution of extracellular ATP in a porcine model was consistent with experimental data in our previous study. The predictions from a human IVD model showed a high accumulation of extracellular ATP in the NP region, whereas the extracellular ATP level was reduced with tissue degeneration. This study provides an understanding of extracellular ATP metabolism and its potential biological influences on the IVD via purinergic signaling.

A Review of Prostanoid Receptors: Expression, Characterization, Regulation, and Mechanism of Action

Prostaglandin signaling controls a wide range of biological processes from blood pressure homeostasis to inflammation and resolution thereof to the perception of pain to cell survival. Disruption of normal prostanoid signaling is implicated in numerous disease states. Prostaglandin signaling is facilitated by G-protein-coupled, prostanoid-specific receptors and the array of associated G-proteins. This review focuses on the expression, characterization, regulation, and mechanism of action of prostanoid receptors with particular emphasis on human isoforms.

Overview of the detection methods for equilibrium dissociation constant KD of drug-receptor interaction

Drug-receptor interaction plays an important role in a series of biological effects, such as cell proliferation, immune response, tumor metastasis, and drug delivery. Therefore, the research on drug-receptor interaction is growing rapidly. The equilibrium dissociation constant (KD) is the basic parameter to evaluate the binding property of the drug-receptor. Thus, a variety of analytical methods have been established to determine the KD values, including radioligand binding assay, surface plasmon resonance method, fluorescence energy resonance transfer method, affinity chromatography, and isothermal titration calorimetry. With the invention and innovation of new technology and analysis method, there is a deep exploration and comprehension about drug-receptor interaction. This review discusses the different methods of determining the KD values, and analyzes the applicability and the characteristic of each analytical method. Conclusively, the aim is to provide the guidance for researchers to utilize the most appropriate analytical tool to determine the KD values.

Chemical Tools for Studying Lipid-Binding Class A G Protein-Coupled Receptors

Cannabinoid, free fatty acid, lysophosphatidic acid, sphingosine 1-phosphate, prostanoid, leukotriene, bile acid, and platelet-activating factor receptor families are class A G protein-coupled receptors with endogenous lipid ligands. Pharmacological tools are crucial for studying these receptors and addressing the many unanswered questions surrounding expression of these receptors in normal and diseased tissues. An inherent challenge for developing tools for these lipid receptors is balancing the often lipophilic requirements of the receptor-binding pharmacophore with favorable physicochemical properties to optimize highly specific binding. In this study, we review the radioligands, fluorescent ligands, covalent ligands, and antibodies that have been used to study these lipid-binding receptors. For each tool type, the characteristics and design rationale along with in vitro and in vivo applications are detailed.

The Concise Guide to PHARMACOLOGY 2013/14: G protein-coupled receptors

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. G protein-coupled receptors are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

Prostanoid receptor antagonists: development strategies and therapeutic applications

Identification of the primary products of cyclo-oxygenase (COX)/prostaglandin synthase(s), which occurred between 1958 and 1976, was followed by a classification system for prostanoid receptors (DP, EP(1), EP(2) ...) based mainly on the pharmacological actions of natural and synthetic agonists and a few antagonists. The design of potent selective antagonists was rapid for certain prostanoid receptors (EP(1), TP), slow for others (FP, IP) and has yet to be achieved in certain cases (EP(2)). While some antagonists are structurally related to the natural agonist, most recent compounds are 'non-prostanoid' (often acyl-sulphonamides) and have emerged from high-throughput screening of compound libraries, made possible by the development of (functional) assays involving single recombinant prostanoid receptors. Selective antagonists have been crucial to defining the roles of PGD(2) (acting on DP(1) and DP(2) receptors) and PGE(2) (on EP(1) and EP(4) receptors) in various inflammatory conditions; there are clear opportunities for therapeutic intervention. The vast endeavour on TP (thromboxane) antagonists is considered in relation to their limited pharmaceutical success in the cardiovascular area. Correspondingly, the clinical utility of IP (prostacyclin) antagonists is assessed in relation to the cloud hanging over the long-term safety of selective COX-2 inhibitors. Aspirin apart, COX inhibitors broadly suppress all prostanoid pathways, while high selectivity has been a major goal in receptor antagonist development; more targeted therapy may require an intermediate position with defined antagonist selectivity profiles. This review is intended to provide overviews of each antagonist class (including prostamide antagonists), covering major development strategies and current and potential clinical usage.

Comparative antiplatelet effects of aspirin, vapiprost and GR144053, a GPIIb/IIIa antagonist, with a special reference to the role of platelet microaggregates

Microthrombi produced have a potential to form larger thrombi, leading to vascular occlusions. Recently, a new device to easily detect microaggregates using laser-light scattering (LS) has been developed. We adopted this device to comparatively evaluate the inhibitory effects of aspirin (1,3 or 10 mg kg(-1)), vapiprost (0.3, 1 or 3 mg kg(-1)) or GR144053 (0.1, 0.3 or 1 mg kg(-1)) on ex vivo aggregation of hamster platelets in relation to their in vivo antithrombotic effects. A transluminal thrombus was produced in the hamster femoral artery by the photochemical reaction. Each compound was injected i.v. as a bolus 10 min prior to the reaction, showing a dose-dependent antithrombotic effect, i.e. they prolonged the time before the artery occluded. At that time cyclic flow reductions occurred more marked when aspirin or vapiprost was given. At the end of experiments, blood was collected to evaluate the platelet aggregation using both the new LS device and the conventional optical density (OD) method. Many more small aggregates were still formed when the highest dose of aspirin or vapiprost was used as compared with that of GR144053, although suppression of the platelet aggregation using the OD method, prolongation of the occlusion time and the bleeding time were quite similar. In conclusion, a GPIIb/IIIa antagonist markedly suppressed the microthrombi and reduced the cyclic flow reduction. This further indicates the importance of small aggregates as triggers of thrombosis and shows that prevention of their formation may result in improved vascular patency after thrombotic insult.

Platelet prostanoid receptors

Prostaglandins (PGs) and thromboxanes are important modulators of platelet activation, and there is strong evidence to support the existence of distinct thromboxane, prostacyclin, PGD2 and PGE2 receptors on the platelet plasma membrane. In this review, each of these platelet prostanoid receptors is discussed in detail, with respect to their receptor pharmacology, molecular biology and signal transduction, and as to any therapeutic implications of the development of specific agonists and/or antagonists. In addition, it considers the possibility that there are separate vascular receptors for 8-epi PGF2 alpha, which are not present on the platelet.

Thromboxane A2-mediated shape change: independent of Gq-phospholipase C--Ca2+ pathway in rabbit platelets

1. Thromboxane A2 (TXA2) receptor-mediated signal transduction was investigated in washed rabbit platelets to clarify the mechanisms of induction of shape change and aggregation. 2. The TXA2 agonist, U46619 (1 nM to 10 microM) caused shape change and aggregation in a concentration-dependent manner. A forty-times higher concentration of U46619 was needed for aggregation (EC50 of 0.58 microM) than shape change (EC50 of 0.013 microM). The aggregation occurred only when external 1 mM Ca2+ was present, but the shape change could occur in the absence of Ca2+. 3. SQ29548 at 30 nM and GR32191B at 0.3 microM (TXA2 receptor antagonists) competitively inhibited U46619-induced shape change and aggregation with similar potency, showing that both aggregation and shape change induced by U46619 were TXA2 receptor-mediated events. However, ONO NT-126 at 1 nM, another TXA2 receptor antagonist, inhibited U46619-induced aggregation much more potently than the shape change, suggesting the possible existence of TXA2 receptor subtypes. 4. ONO NT-126 (2 nM to 3 microM) by itself caused a shape change without aggregation in a concentration-dependent manner, independent of external Ca2+. Therefore, ONO NT-126 is a partial agonist at the TXA2 receptor in rabbit platelets. 5. U46619 (10 nM to 10 microM) increased internal Ca2+ concentration ([Ca2+]i) and activated phosphoinositide (PI) hydrolysis in a concentration-dependent manner with a similar concentration-dependency. 6. U46619 (3 nM to 10 microM) also activated GTPase concentration-dependently in the membranes derived from platelets. U46619-induced activation of GTPase was partly inhibited by treatment of membranes with QL, an antibody against Gq/11. 7. The EC50 values of U46619 in Ca2+ mobilization (0.15 microM), PI hydrolysis (0.20 microM) and increase in GTPase activity (0.12 microM) were similar, but different from the EC50 value in shape change (0.013 microM), suggesting that activation of TXA2 receptors might cause shape change via an unknown mechanism. 8. U46619-induced shape change was unaffected by W-7 (30 microM), a calmodulin antagonist or ML-7 (30 microM), a myosin light-chain kinase inhibitor, indicating that an increase in [Ca2+]i might not be involved in the shape change. In fact, U46619 (10 nM) could cause shape change without affecting [Ca2+]i level, determined by simultaneous recordings. 9. [3H]-SQ29548 and [3H]-U46619 bound to platelets at a single site with a Kd value of 14.88 nM and Bmax of 106.1 fmol/10(8) platelets and a Kd value of 129.8 nM and Bmax of 170.4 fmol/10(8) platelets, respectively. The inhibitory constant Ki value for U46619 as an inhibitor of 3H-ligand binding was similar to the EC50 value of U46619 in GTPase activity, phosphoinositide hydrolysis and Ca2+ mobilization, but significantly different (P < 0.001 by Student's t test) from the effect on shape change. 10. Neither U46619 nor ONO NT-126 affected the adenosine 3',5'-cyclic monophosphate (cyclic AMP) level in the presence or absence of external Ca2+ and/or isobutyl methylxanthine. 11. The results indicate that TXA2 receptor stimulation causes phospholipase C activation and increase in [Ca2+]i via a G protein of the Gq/11 family leading to aggregation in the presence of external Ca2+, and that shape change induced by TXA2 receptor stimulation might occur without involvement of the Gq-phospholipase C-Ca2+ pathway.

Role of thromboxane A2 in bradykinin-induced human isolated small bronchi contraction

We previously demonstrated that the bradykinin-induced contraction of human isolated small bronchi is inhibited by indomethacin, capsaicin (N-methyl-N-6-nonenamide) and ruthenium red but not by tachykinin receptor antagonists. The thromboxane A2 receptor (TP receptor) antagonist GR32191 ((1R-(1 alpha(Z),2 beta,3 beta,5 alpha))-(+)-7-(5-(((1,1'-biphenyl)-4-yl)- methoxy)-3-hydroxy-2-(1-piperidinyl)cyclopentyl)-4-heptenoic acid, hydrochloride) (10(-10) to 10(-8) M) dose dependently inhibited the effect of bradykinin, suggesting the mediation of the TP receptor in the action of bradykinin. With higher concentrations of GR32191 (10(-7) and 10(-6) M) bradykinin induced a relaxation which was inhibited by indomethacin and by the bradykinin B2 receptor antagonist Hoe 140 (D-Arg0[Hyp3,Thi-5,D-Tic7,Oic8]bradykinin). The thromboxane A2 synthase inhibitor dazoxiben (4-(-2-(1H-imidazol-1-yl)ethoxy) benzoic acid hydrochloride) 10(-6) M inhibited the bradykinin-induced contraction, suggesting that thromboxane A2 was involved in TP receptor stimulation. The thromboxane A2 mimetic U-46619 (9,11-dideoxy-11 alpha,9 alpha-epoxy-methano-prostaglandin F2 alpha)-induced contraction of human distal bronchi was not inhibited by capsaicin and ruthenium red. Our data suggest that bradykinin contracts human isolated small bronchi through thromboxane A2 release. The inhibitory effect of ruthenium red and capsaicin on the bradykinin response may be due to inhibition of thromboxane A2 release or arachidonic mobilisation.

Aspects of the thromboxane receptor system

1. The aim of this review is to establish what is known about the thromboxane (TP) receptor, and to identify where future research is headed. In addition, the impact of the recent advances at the molecular level on resolving pharmacological controversies, such as possible subtypes of the TP receptor, is discussed and what molecular information is known about the TP receptor presented. 2. The clinical status of TP receptor antagonists is considered particularly in relation to the potential role of epi prostaglandins. 3. Basic information about TP agonists, antagonists and signal transduction pathways is also given.

Reduction in the number of thromboxane receptors on human platelets after exposure to GR32191

1. Exposure of human resuspended platelets in vitro for 30 min to the potent thromboxane A2 (TP)-receptor blocking drug GR32191, followed by its removal by dilution-dissociation, reduced the degree of subsequent binding to 2 nM [3H]-GR32191 by almost 50%. Exposure for longer periods (60 min) led to a further reduction. However, no change in the Kd of the radioligand was observed. 2. This effect of GR32191 could not be explained by persistent binding of drug to platelets since a dilution-dissociation stage, designed to remove all drug, was included prior to measurement of binding. 3. Using an alternative TP-receptor radioligand, [3H]-SQ29,548, to monitor receptor number, a reduction in Bmax was observed after GR32191 pre-treatment; the Kd value of the radioligand remained unchanged. 4. The effect was not a common property of TP-receptor blocking drugs since pre-exposure of platelets in vitro for 30 min to BM13.177 or SQ29,548 did not produce a fall in subsequent Bmax to [3H]-SQ29,548. 5. While the mechanism behind this apparent down-regulation of platelet TP-receptor is unknown, it may explain the long duration of action of GR32191 upon platelets in man which persists in the absence of detectable drug in the plasma.