Solubilization and molecular size determination of the P2x purinoceptor from rat vas deferens

History of Geoff Burnstock's research on P2 receptors

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

A crucial sequence for transglutaminase type 2 extracellular trafficking in renal tubular epithelial cells lies in its N-terminal beta-sandwich domain

Transglutaminase type 2 (TG2) catalyzes the formation of an ε-(γ-glutamyl)-lysine isopeptide bond between adjacent peptides or proteins including those of the extracellular matrix (ECM). Elevated extracellular TG2 leads to accelerated ECM deposition and reduced clearance that underlie tissue scarring and fibrosis. The extracellular trafficking of TG2 is crucial to its role in ECM homeostasis; however, the mechanism by which TG2 escapes the cell is unknown as it has no signal leader peptide and therefore cannot be transported classically. Understanding TG2 transport may highlight novel mechanisms to interfere with the extracellular function of TG2 as isoform-specific TG2 inhibitors remain elusive. Mammalian expression vectors were constructed containing domain deletions of TG2. These were transfected into three kidney tubular epithelial cell lines, and TG2 export was assessed to identify critical domains. Point mutation was then used to highlight specific sequences within the domain required for TG2 export. The removal of β-sandwich domain prevented all TG2 export. Mutations of Asp(94) and Asp(97) within the N-terminal β-sandwich domain were identified as crucial for TG2 externalization. These form part of a previously identified fibronectin binding domain ((88)WTATVVDQQDCTLSLQLTT(106)). However, siRNA knockdown of fibronectin failed to affect TG2 export. The sequence (88)WTATVVDQQDCTLSLQLTT(106) within the β-sandwich domain of TG2 is critical to its export in tubular epithelial cell lines. The extracellular trafficking of TG2 is independent of fibronectin.

P2-Purinoceptors: Advances and therapeutic opportunities

The recent cloning of a number of distinct receptors belonging to the P₂-purinoceptor superfamily has provided conclusive evidence for a pivotal role for ATP and other nucleotides as effector molecules involved in cell-to-cell communication and the modulation of many basic aspects of tissue function. ATP itself is being clinically evaluated as a cytotoxic agent for the treatment of cancer and as an adjunct to inhalation anaesthetic use. The pyrimidine nucleotide, UTP, is in clinical trials for the treatment of cystic fibrosis. The stable ATP bioisostere, ARL 67085, is being developed as a novel antithrombotic agent, blocking with a superior safety profile and increased efficacy as compared to other agents. The diversity of P₂ receptors, with eleven having been defined using both pharmacological and molecular cloning criteria, indicates considerable additional potential and subtlety in regard to the effects of ATP on tissue function and pathophysiology.

P2 purinoceptors: historical perspective and classification

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

Structure-activity relationships of novel P2-receptor antagonists structurally related to Reactive Blue 2

P2 membrane receptors for nucleotides represent significant targets for experimental pharmacology and drug research. In earlier publications, we have shown that Reactive Blue 2 (RB 2), one of the most widely used P2-receptor antagonists, displays only moderate affinity and does not discriminate between native P2X- and P2Y-receptor subtypes. In the present study we have pharmacologically evaluated a series of 15 synthesized and re-evaluated four commercially obtained and chromatographically purified RB 2 type anthraquinone derivatives on contractions of the rat vas deferens (RVD) elicited by alpha,beta-methylene ATP (alpha,beta-meATP), mediated by P2X1-receptors, and relaxations of the carbachol-precontracted guinea-pig taenia coli (GPTC) elicited by adenosine 5'-O-(2-thiodiphosphate) (ADPbetaS), mediated by P2Y1-like receptors. Based on the structure-activity relationships (SAR) it is concluded that hydrophobic interactions of aromatic pi-electron systems, hydrogen bonds with nitrogen as donor and acceptor atoms, and, particularly, position, conformational distance and number of anionic sulfonate groups are of great importance for the blockade of the two native P2-receptor subtypes. We have also identified novel, for the most part reversible antagonists that bind with higher affinity and improved subtype selectivity in comparison to RB 2. In particular, 1-amino-4-{4-[4-chloro-6-(2-sulfonatophenylamino)-[1,3,5]triazine-2-ylamino]-2-sulfonatophenylamino}-9,10-dioxo-9,10-dihydroanthracene-2-sulfonic acid trisodium salt (MG 50-3-1) is the most potent antagonist at the P2Y1-like-receptors of the GPTC reported so far (IC50=4.6 nM). It is significantly less potent as reversible antagonist at the P2X1-receptors of the RVD (IC50=2.8 microM). Thus, MG 50-3-1 represents a selective pharmacological tool and may be a lead compound for future investigations.

Crystal structure of coproporphyrinogen III oxidase reveals cofactor geometry of Radical SAM enzymes

'Radical SAM' enzymes generate catalytic radicals by combining a 4Fe-4S cluster and S-adenosylmethionine (SAM) in close proximity. We present the first crystal structure of a Radical SAM enzyme, that of HemN, the Escherichia coli oxygen-independent coproporphyrinogen III oxidase, at 2.07 A resolution. HemN catalyzes the essential conversion of coproporphyrinogen III to protoporphyrinogen IX during heme biosynthesis. HemN binds a 4Fe-4S cluster through three cysteine residues conserved in all Radical SAM enzymes. A juxtaposed SAM coordinates the fourth Fe ion through its amide nitrogen and carboxylate oxygen. The SAM sulfonium sulfur is near both the Fe (3.5 A) and a neighboring sulfur of the cluster (3.6 A), allowing single electron transfer from the 4Fe-4S cluster to the SAM sulfonium. SAM is cleaved yielding a highly oxidizing 5'-deoxyadenosyl radical. HemN, strikingly, binds a second SAM immediately adjacent to the first. It may thus successively catalyze two propionate decarboxylations. The structure of HemN reveals the cofactor geometry required for Radical SAM catalysis and sets the stage for the development of inhibitors with antibacterial function due to the uniquely bacterial occurrence of the enzyme.

Pharmacological and histochemical evidence for P2X receptors in human umbilical vessels

The presence of P2X purinoceptors in human umbilical vessels were studied with organ bath recording, radioligand binding assays, autoradiography, and immunohistochemistry. In isolated umbilical arteries and veins from normal term pregnancy, both ATP and alpha,beta-methylene ATP caused concentration-dependent contractions. ATP-induced responses were blocked by desensitisation with alpha,betamethylene ATP. However, both the ATP- and alpha,beta-methylene ATP-induced responses were not antagonised by suramin. No significant difference in responses was observed in the vessels with or without endothelial cells. Radioligand binding assays using [3H]alpha,beta-methylene ATP showed the presence of a population of high-affinity binding sites in both the arteries and veins. The Kd values of the binding sites were 2.77 + 1.10 nM for the arteries, and 3.23+/-1.22 nM for the veins. The maximum binding site densities were 634+/-237 and 947+/-308 fmol/mg protein for the arteries and the veins, respectively. Autoradiographic localisation with [3H]alpha,beta-methylene ATP demonstrated that the specific binding sites were only distributed over the smooth muscle cells of the vessels. Immunohistochemical studies with specific polyclonal antibodies against P2X1-6 receptors showed that positive immunostaining was also restricted to smooth muscle cells. Antibodies against P2X1 receptors produced the strongest signals, while antibodies against the other five P2X subtypes produced much weaker signals. The results in the present study indicate the existence of P2X purinoceptors in the smooth muscle of human umbilical vessels. Their physiological functions remain to be studied.

The P2X1 Receptor, an Adenosine Triphosphate–Gated Cation Channel, Is Expressed in Human Platelets but not in Human Blood Leukocytes

Extracellular adenosine triphosphate (ATP) and adenosine diphosphate (ADP) activate multiple types of P2-nucleotide receptors expressed in platelets or leukocytes. Electrophysiological and biochemical studies have indicated expression of the P2X1 receptor, an ATP-gated cation channel, in human and rat platelets, rat basophilic leukemia (RBL) cells, and phorbol myristate acetate (PMA)-differentiated HL-60 myeloid cells. Although these findings suggest that P2X1 receptors are present in both blood leukocytes and blood platelets, the relative levels of P2X1receptor expression and function in human blood leukocytes and platelets have not been directly characterized. On the basis of both immunoblot analysis and functional assays of P2X1receptor-mediated ionic fluxes, we report that there is significant expression of P2X1 receptors in human platelets, but not in neutrophils, monocytes, or blood lymphocytes. Thus, unlike platelets and myeloid progenitor cell lines, fully differentiated human blood leukocytes do not express functionally significant numbers of P2X1 receptors, suggesting the downregulation of P2X1 receptor gene expression during the differentiation of phagocytic leukocytes. By contrast, P2X1 receptor expression is strongly maintained during megakaryocytic differentiation and platelet release. Immunoblot analysis indicated that the platelet P2X1 receptor migrates as an approximately 60-kD protein during SDS-electrophoresis under reducing or nonreducing conditions. Treatment of platelet membranes with endoglycosidase-F causes the P2X1 receptor band to migrate as a 46-kD protein, verifying the highly glycosylated nature of the mature receptor protein. Additional studies of nucleotide-induced changes in Ca2+influx/mobilization demonstrated that the platelet P2X1receptors are pharmacologically distinct from the well-characterized ADP receptors of these cells. This finding suggests a unique role for these ATP-gated ion channels during hemostasis or thrombosis.

The P2X1 Receptor, an Adenosine Triphosphate–Gated Cation Channel, Is Expressed in Human Platelets but not in Human Blood Leukocytes

Extracellular adenosine triphosphate (ATP) and adenosine diphosphate (ADP) activate multiple types of P2-nucleotide receptors expressed in platelets or leukocytes. Electrophysiological and biochemical studies have indicated expression of the P2X1 receptor, an ATP-gated cation channel, in human and rat platelets, rat basophilic leukemia (RBL) cells, and phorbol myristate acetate (PMA)-differentiated HL-60 myeloid cells. Although these findings suggest that P2X1 receptors are present in both blood leukocytes and blood platelets, the relative levels of P2X1receptor expression and function in human blood leukocytes and platelets have not been directly characterized. On the basis of both immunoblot analysis and functional assays of P2X1receptor-mediated ionic fluxes, we report that there is significant expression of P2X1 receptors in human platelets, but not in neutrophils, monocytes, or blood lymphocytes. Thus, unlike platelets and myeloid progenitor cell lines, fully differentiated human blood leukocytes do not express functionally significant numbers of P2X1 receptors, suggesting the downregulation of P2X1 receptor gene expression during the differentiation of phagocytic leukocytes. By contrast, P2X1 receptor expression is strongly maintained during megakaryocytic differentiation and platelet release. Immunoblot analysis indicated that the platelet P2X1 receptor migrates as an approximately 60-kD protein during SDS-electrophoresis under reducing or nonreducing conditions. Treatment of platelet membranes with endoglycosidase-F causes the P2X1 receptor band to migrate as a 46-kD protein, verifying the highly glycosylated nature of the mature receptor protein. Additional studies of nucleotide-induced changes in Ca2+influx/mobilization demonstrated that the platelet P2X1receptors are pharmacologically distinct from the well-characterized ADP receptors of these cells. This finding suggests a unique role for these ATP-gated ion channels during hemostasis or thrombosis.

Nucleotide receptors in the nervous system. An abundant component using diverse transduction mechanisms

Extracellular nucleotides achieve their role as cell-to-cell communicators by acting at cell surface transmembrane receptors-the P2 receptors. Before molecular cloning led to the isolation of any P2-receptor sequence, a small number of receptor types had been proposed on the basis of pharmacological evidence. The application of molecular biology to this field of receptor research has indicated that a great underestimation of the number of receptor subtypes and of their abundance had occurred. There are now known to be seven characterized P2Y (G protein linked) receptors and the same number again of P2X receptors of the transmitter-gated ion channel type. In this review, we discuss the properties of these cloned receptors, their distribution within the nervous system, and their methods of signal transduction.

Differences in nucleotide effects on intracellular pH, Na+/H+ antiport activity, and ATP-binding proteins in endothelial cells

Bovine (BPAEC) and human (HPAEC) pulmonary artery endothelial cell monolayers were incubated with either ATP, ATP analogues, or UTP, followed by measurement of intracellular pH (pHi) and the rate of recovery from acidosis. ATP increased baseline pHi and the rate of acid recovery in BPAEC. This response was inhibited by the amiloride analogue, methyisobutylamiloride, demonstrating that activation of the Na+/H+ antiport was responsible for the increase in baseline pHi and the recovery from acidosis. This response had the features of both a P2Y and P2U purinergic receptor, based on the responses to a series of ATP analogues and UTP. In contrast, none of the nucleotides had any significant effect on pHi and Na+/H+ antiport activity in HPAEC. This difference in the response to extracellular nucleotides was not due to a difference in ATP metabolism between cell types, since the ectonucleotidase-resistant analogue. ATP gamma S, also had no effect on HPAEC. Analogues of cAMP had no effect on pHi or acid recovery in either cell type. Incubation of BPAEC and HPAEC with the photoaffinity ligand [32P] 8-AzATP indicated that both BPAEC and HPAEC possess an ATP-binding protein of 48 kDa. However, BPAEC exhibited an additional binding protein of 87 kDa. Thus, the contrasting response to extracellular ATP between bovine and human pulmonary artery endothelial cells may be related to differences in the signal transduction pathway leading to antiport activation, including different ATP-binding sites on the cell membrane.

Autonomic neuromuscular transmission at a varicosity

This review attempts to clarify the definition of what constitutes an autonomic neuromuscular function formed by a varicosity. Ultrastructural studies of serial sections through varicosities, partly or wholly bare of Schwann cell covering, show that areas of close apposition occur between varicosities and muscle cell membrane that vary between 20 and 150 nm, depending on the muscle considered. Consideration of the diffusion of purine transmitters and their receptor kinetics after secretion in a packet show that the number of purinergic receptor channels opened at a site of 150 nm apposition by a varicosity is about 15% of that at a site of 50 nm apposition. These results, together with the analysis of the stochastic fast component and the deterministic slow components of the rising phase of the EJP suggest that the stochastic fast component is due to varicosities that form especially close appositions (20-50 nm), whereas the deterministic slow component is due to the large number of varicosities at distances up to about 150 nm. Varicosities forming appositions of 20-150 nm with muscle cells several hundred micrometers long possess junctional receptor types distinct from extrajunctional receptors. According to this argument, then, there are two different classes of varicosities: one that gives rise to a relatively large junctional current and another that is responsible for a very small junctional current. Present evidence suggests that two subclasses of varicosities can be discerned amongst the varicosities that generate large junctional currents. One of these subclasses of varicosity possesses relatively few post-junctional receptors compared with the amount of transmitter reaching the receptors from the varicosity, so that the junctional current generated is determined by the size of the receptor population; in this case, the size of the transmitter packages released from these varicosities is unknown and the size of the junctional current is relatively constant. The other subclass of varicosity possesses large receptor patches, sufficient to accommodate the largest amounts of transmitter released from the varicosities: in this case, the size of the transmitter packages is shown to be highly non-uniform. These speculations await confirmation by direct labelling of the receptor patches beneath varicosities, a possibility that is likely to be realized in the near future.

Partial characterization of a new nucleotide binding glycoprotein of hepatocyte plasma membrane

Hepatocyte plasma membranes contain a glycosylated 230-kDa Ca(2+) -dependent, Mg(2+)-stimulated ATPase (pgp230), which consists of two subunits, one of 120 kDa and the other of 110 kDa. pgp230 can be enriched by the use of affinity chromatography on Concanavalin A-Sepharose, wheat germ lectin-Sepharose, and 5'-AMP-Sepharose. It has a high-affinity Ca2+ binding site. In the presence of Ca2+, it forms a phosphorylated intermediate by autocatalytic transfer of the terminal phosphate residue from ATP. Maximal Ca(2+)-dependent autophosphorylation is observed at pH 5-6. Photoaffinity labeling using 8-azido-[alpha-32P]ATP or [y-32P]ATP confirms the presence of ATP binding sites. Incubation with [alpha-32P]ATP leads to a rapid but transient labeling of pgp230. Various nucleotides, nucleotide receptor agonists, or antagonists inhibit Ca(2+)-dependent phosphorylation by [y-32P]ATP. The concentrations of half-maximal inhibition range from 10(-7) M to 10(-3) M. The rank order of inhibitory potency is: ATP > alpha,beta-methylene-ATP > CTP = TTP > y-4-amino-phenyl-ATP = 2-methyl-thio-ATP > UTP = GTP > GDP = ADP = beta,y-methylene-ATP = beta, y-methylene-TTP = beta,y-methylene-GTP = adenosine-5'-O-2-thiodiphosphate = CMP = AMP > adenosine > cytidine > guanosine = suramin > Reactive blue 2 > iso-butyl-methyl-xanthine > thymidine > uridine. These data suggest a nucleotide binding capacity of this new hepatocyte membrane glycoprotein. Further investigations should be carried out to reveal its biological function.

The binding characteristics of a human bladder recombinant P2X purinoceptor, labelled with [3H]-alpha beta meATP, [35S]-ATP gamma S or [33P]-ATP

1. The binding of [3H]-alpha beta meATP, [35s]-ATP gamma S and [alpha 33P]-ATP to a human bladder P2X purinoceptor, transiently expressed in CHO-K1 cells using the Semliki Forest Virus (SFV) expression system, was examined. The characteristics of the binding sites were compared with results obtained in rat vas deferens, a tissue in which the radioligands are thought to label P2X purinoceptors and in which the endogenous P2X purinoceptor displays high homology with the human bladder P2X purinoceptor. 2. In non-infected CHO-K1 cells, 100 microM ATP evoked only small inward currents (40 pA) in approximately 30% of the cells when studied by the whole-cell voltage clamp technique. In membranes prepared from either these non-infected cells or cells infected with SFV containing the LacZ gene (SFV-LacZ), [3H]-alpha beta meATP bound with low affinity (pKd = 7.04; Bmax = 8.88 pmol ml-1 protein) and there was only a low density of [35S]-ATP gamma S binding sites (pKd = 8.74; Bmax = 358 fmol ml-1 protein). These binding sites differed from those present in rat vas deferens. Thus, pIC50 values for alpha beta meATP (6.5) and L-beta gamma meATP (4.0) at the [3H]-alpha beta meATP binding sites in non-infected CHO-K1 cells were much lower than the respective pIC50 values of 8.3 and 7.7, determined in rat vas deferens. Similarly, affinity estimates (pIC50 values) for ATP (6.82), 2-meS-ATP (5.43), ATP gamma S (7.06) and alpha beta meATP (4.84) at the [35S]-ATP gamma S binding sites in non-infected CHO-K1 cells were up to 2291 fold lower than the respective values of 9.01, 8.79, 8.73 and 7.57, determined in rat vas deferens. 3. In CHO-K1 cells infected using SFV containing the cDNA for the human bladder P2X purinoceptor (SFV-h.P2X), ATP, 2-meS-ATP and alpha beta meATP evoked large inward currents (2-7 nA) in whole cell voltage clamp studies. In membranes prepared from these SFV-h.P2X infected cells, [3H]-alpha beta meATP binding was increased, compared to that measured in the non infected or SFV-LacZ infected cells, with only high affinity [3H]-alpha beta meATP binding sites being detected (pKd = 9.21; Bmax = 3.54 pmol mg-1 protein). The pIC50 values for alpha beta meATP (8.2) and L-beta gamma meATP (7.2) in competing for these sites were the same or similar to the values determined in rat vas deferens. 4. A high density of [35H]-ATP gamma S binding sites (pKd = 9.09; Bmax = 6.82 pmol mg-1 protein) was also present in the membranes from CHO-K1 cells infected with SFV-h.P2X and affinity estimates (pIC50 values) for ATP (8.93), 2-meS-ATP (8.23), ATP gamma S (8.08), and alpha beta meATP (7.17) at competing for these sites were as much as 631 fold higher than the respective values determined in non-infected CHO-K1 cells but were close to the values determined in rat vas deferens. Similar data were obtained with [alpha 33P]-ATP as radioligand. 5. These data suggest that [3H]-alpha beta meATP, [35S]-ATP gamma S and [33P]-ATP label the human bladder recombinant P2X purinoceptor expressed in CHO-K1 cells following infection with SFV-h.P2X and provide further corroborative evidence to support the contention that the high affinity binding sites for these radioligands in rat vas deferens are P2X purinoceptors.

High affinity P2x-purinoceptor binding sites for [35S]-adenosine 5'-O-[3-thiotriphosphate] in rat vas deferens membranes

1. The binding sites labelled by [35S]-adenosine 5'-O-[3-thiotriphosphate]([35S]-ATP gamma S) at 4 degrees C in rat vas deferens membranes were studied and compared to the sites labelled by [3H]-alpha,beta-methylene ATP ([3H]-alpha beta meATP) to ascertain whether [35S]-ATP gamma S can be used to label the P2x purinoceptor. 2. In the presence of 4 mM CaCl2, the binding of 0.2 nM [35S]-ATP gamma S to vas deferens membranes was increased 3.4 fold, when compared to studies performed in the absence of calcium. However, binding did not appear to be solely to P2x purinoceptors since [35S]-ATP gamma S labelled a heterogeneous population of sites and about 72% of the sites possessed high affinity (pIC50 = 7.5) for guanosine 5'-O-[3-thiotriphosphate] (GTP gamma S). Even in the presence of 1 microM GTP gamma S, to occlude the sites with high affinity for GTP gamma S, the binding of [35S]-ATP gamma S was heterogeneous and since there was also evidence of extensive metabolism of ATP in the presence of calcium, the binding of [35S]-ATP gamma S under these conditions was not studied further. 3. In the absence of calcium ions, [35S]-ATP gamma S bound to a single population of sites (pKD = 9.23; Bmax = 4270 fmol mg-1 protein). Binding reached steady state within 3 h (t1/2 = 38 min), was stable for a further 4 h and was readily reversible upon addition of 10 microM unlabelled ATP gamma S (t1/2 = 45 min). In competition studies the binding of 0.2 nM [35S]-ATP gamma S was inhibited by a number of P2x purinoceptor agonists and antagonists, but not by adenosine receptor agonists, staurosporine (1 microM) or several ATPase inhibitors. The rank order of agonist affinity estimates (pIC50 values) in competing for the [35S]-ATP gamma S binding sites was: ATP (9.01), 2-methylthio- ATP (8.79), ATP gamma S (8.73), alpha beta meATP (7.57), ADP (7.24), beta, gamma-methylene ATP (7.18), L-beta, gamma-methylene ATP (5.83), alpha, beta-methylene ADP (4.36). 4. Affinity estimates (pIC50 values) for the P2x purinoceptor antagonists, suramin (5.20), pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (4.23), pyridoxal 5-phosphate (3.42), cibacron blue (5.70) and Evan's blue (5.79) were broadly similar to those obtained at the [3H]-alpha beta meATP binding sites in vas deferens. However, ATP, 2-methylthio-ATP, ATP gamma S and ADP displayed 17-512 fold higher affinity for the [35S]-ATP gamma S, than for the [3H]-alpha beta meATP binding sites, whereas alpha beta meATP and L-beta, gamma-methylene ATP displayed 5 and 28 fold, respectively, higher affinity for the [3H]-alpha beta meATP than for the [35S]-ATP gamma S binding sites. 5. The differences in agonist affinity for the [35S]-ATP gamma S and [3H]-alpha beta meATP binding sites probably reflect the fact that the former sites were labelled in the absence of calcium, while the latter sites were labelled in its presence. This could differentially affect ionisation state and/or metabolism of the nucleotides when using the two radioligands. Since affinity estimates for ATP, 2-methylthio-ATP, ATP gamma S, alpha beta meATP and L-beta, gamma-methylene ATP were different when calcium ions were omitted in studies using [3H]-alpha beta meATP but similar to the affinity estimates obtained at the [35S]-ATP gamma S binding sites labelled in the absence of calcium, it is likely that [35S]-ATP gamma S and [3H]-alpha beta meATP label the same sites in rat vas deferens. 6. We conclude that, in the absence of divalent cations, [35S]-ATP gamma S labels P2x purinoceptors in rat vas deferens and as such may represent a new, high specific activity, radioligand for the study of such receptors.

Characterisation of a recombinant P2Y purinoceptor

We have previously cloned a cDNA encoding a G-protein-coupled P2 purinoceptor from chick brain and designated this as a P2Y1 purinoceptor (Webb, T.E., J. Simon, B.J. Krishek, A.N. Bateson, T.G. Smart, B.J. King, G. Bumstock and E.A. Barnard, 1993, FEBS Lett. 324, 219). Here, we describe the further characterisation of this recombinant receptor expressed in both simian kidney endothelial (COS-7) cells and Xenopus oocytes. In transfected COS-7 cell membranes, the recombinant receptor showed a high level of expression (Bmax = 7.9 +/- 2.2. pmol [35S]dATP alpha S bound/mg protein) and affinity (Kd = 6.6 +/- 0.3 nM). In these COS-7 cells, the activation of the implanted purinoceptor induced a suramin-sensitive formation of inositol 1,4,5-triphosphatic (1,4,5InsP3). Upon expression in Xenopus oocytes, ATP was the only natural nucleoside triphosphate to elicit a Ca(2+)-activated chloride current. The P2 purinoceptor antagonists suramin and Reactive Blue-2 were both able to inhibit this evoked current. Utilizing both expression systems, the binding affinity profile and the functional pharmacological profile of the agonists, the common series found was: 2-methylthioATP (2-MeSATP) > or = ATP > ADP beta S > ADP. These two agonist series and the lack of activity of adenosine, alpha, beta-methyleneATP (alpha, beta-meATP), 3'-O-(4-benzoyl) benzoyl-ATP (Bz-ATP) and UTP, together confirmed that this receptor is a specific subtype of the P2Y purinoceptors.

Evidence that [3H]-alpha,beta-methylene ATP may label an endothelial-derived cell line 5'-nucleotidase with high affinity

1. In membranes prepared from a permanent cell line of endothelial origin (WEC cells), [3H]-alpha, beta-methylene ATP ([3H]-alpha, beta-meATP) labelled high (pKd = 9.5; Bmax = 3.75 pmol mg-1 protein) and low (pKd = 7.2; Bmax = 23.3 pmol mg-1 protein) affinity binding sites. The high affinity [3H]-alpha, beta-meATP binding sites in the WEC cell membranes could be selectively labelled with a low concentration of the radioligand (1 nM). In competition studies performed at a radioligand concentration of 1 nM, 88.6% of the sites possessed high affinity (pIC50 = 8.26) for alpha, beta-meATP. 2. The high affinity [3H]-alpha, beta-meATP binding sites appeared heterogeneous since in competition studies a number of nucleotide analogues (alpha, beta-meADP, ATP, ADP, AMP, GTP, GppNHp, GMP) and adenosine identified two populations of the sites labelled by 1 nM [3H]-alpha, beta-meATP. The proportion of sites with high affinity for these compounds was found to vary between 42 and 69%. 3. Approximately 60-69% of the binding sites labelled with 1 nM [3H]-alpha, beta-meATP possessed high affinity for alpha, beta-meADP (pIC50 = 8.87), AMP (pIC50 = 7.12), GMP (pIC50 = 7.34), UTP (pIC50 = 6.12), GTP (pIC50 = 7.59), GppNHp (pIC50 = 7.35) and adenosine (pIC50 = 5.45). The sites at which these compounds possessed high affinity were probably the same, since, in the presence of GMP at a concentration (10 microM) sufficient to inhibit selectively the binding of [3H]-alpha,beta-meATP, the [3H]-alpha,beta-meATP binding sites with high affinity for AMP, UTP, alpha, beta-meADP, GTP, GppNHp and adenosine were also occluded.4. WEC cell membranes were able to metabolize a trace concentration (6 nM) of [3H]-AMP to [3H]-adenosine under the conditions of the binding assay. The pIC50 values of adenosine (5.99), GMP (7.55)and the substrate AMP (7.19) for inhibiting this [3H]-AMPase activity were almost identical to their high affinity pIC50 estimates obtained in the binding assay. Although alpha, beta-meADP, alpha, beta-meATP, beta,upsilon-meATP,ATP, ADP and GppNHp identified heterogeneity in the [3H]-AMPase activity of the WEC cells, theirpIC50 values for inhibiting the major portion of the [3H]-AMPase activity were similar to their respective high affinity pIC50 values in the binding assay. It thus seems likely that WEC cells express a form of 5'-nucleotidase that possesses high affinity for both alpha,beta-meADP and alpha,beta-meATP and that this enzyme can be labelled by [3H]-alpha,beta-meATP.5. In the presence of 10 microM GMP, the affinity estimates for alpha,beta-meADP, AMP, GMP, GTP, GppNHp,ADP and adenosine at the high affinity [3H]-alpha,beta4-meATP binding sites that remained available, were lowa nd similar to their affinity estimates at the high affinity [3H]-alpha,beta-meATP binding sites of rat vas deferens. Since the high affinity [3H]-alpha,beta-meATP binding sites in rat vas deferens are thought to be P2x purinoceptors it is possible that the high affinity [3H]-alpha,beta-meATP binding sites in the WEC which possess low affinity for alpha,beta-meADP are also P2x purinoceptors.

Binding of [35S]adenosine 5'-O-(2-thiodiphosphate) to endothelial cells in culture

We have investigated the binding of [35S]adenosine 5'-O-(2-thiodiphosphate) ([35S]ADP beta S) to intact cultured bovine aortic endothelial cells which have been previously shown to co-express P2y and P2u purinoceptors and to bovine adrenal medulla endothelial cells which solely possess P2u purinoceptors. ADP beta S has been shown to stimulate phospholipase C activity in these cells via the P2y purinoceptor and does not interact with the P2u purinoceptor. We describe a simple equilibrium binding procedure designed for the study of low affinity agonists and compare these results with those obtained by separation of bound and free by filtration. Saturation analysis of equilibrium binding data revealed two sites for ADP beta S binding; one with KD = 3.3 x 10(-8) M, Bmax = 32 pmol/mg protein; and the other with KD = 4.3 x 10(-6) and Bmax = 2155 pmol/mg protein. Use of filtration did not significantly alter the KD of either of these sites, nor the Bmax of the high affinity site, but reduced the Bmax of the low affinity site by more than 95%. The rank order of agonist potency for competing for [35S]ADP beta S binding indicated that most of this was to non-P2y purinoceptor sites as beta,gamma-methylene ATP, a P2x purinoceptor agonist, was more potent than 2-methylthio ATP, a P2y purinoceptor agonist. Binding was also carried out in the presence of beta,gamma-methylene ATP, in an attempt to reduce non-P2y purinoceptor binding and produced similar results. Specific [35S]ADP beta S binding sites were also found in bovine adrenal medulla endothelial cells which do not possess P2y purinoceptors. These results indicate that [35S]ADP beta S was able to bind to endothelial cells from different parts of the vasculature but that the ligand can only be considered suitable for investigation of P2y purinoceptors on mammalian cells when specific conditions are designed to reduce the large amount of non-receptor binding.

P2X receptors bring new structure to ligand-gated ion channels

P2X receptors are cation-selective ion channels that open on binding to extracellular ATP; they play a role in fast synaptic transmission between neurones, and from autonomic nerves to smooth muscles. Isolation of cDNAs that encode P2X receptors in the smooth muscle of vas deferens and in phaeochromocytoma cells indicates that the receptors are not related to other ligand-gated ion channels. Their overall structure resembles more closely that of epithelial Na+ channels and the proteins that are thought to form mechanosensitive channels in Caenorhabditis elegans. The type of P2X RNA that is found in vas deferens is expressed preferentially by apoptotic thymocytes, and the type of P2X RNA that is found in PC12 cells is abundant in the pituitary gland, suggesting hitherto unsuspected roles for ATP-gated channels in endocrine and immune function.

Characterization of a P2Y purinoceptor in the brain

Little has been known of the abundance in the brain of any of the G protein coupled P2 purinoceptors nor their pharmacology. Here we show that [35S]dATP alpha S is a suitable radioligand for investigating these receptors and hence that they are exceptionally abundant both in one-day-old chick (Bmax: 37 pmol agonist sites/mg protein) and adult rat brain membranes (Bmax: 39 pmol/mg protein). [35S]dATP alpha S (which is selective for P2Y over the P2X types of purinoceptor) binds with high affinity to these sites in the chick (Kd: 13.3 nM) and in the rat brain membranes (Kd: 9.1 nM). The rank order of potency of purinoceptor-active agonists and antagonists displacing [35S]dATP alpha S binding is: dATP alpha S > (3'-deoxyATP, 2-methylthioATP, ATP alpha S, ATP) > 2'-deoxyATP > 2-methylthioADP > ADP >> suramin, Reactive Blue-2 >> UTP, L-beta,gamma-methyleneATP, adenosine; this defines these binding sites as P2Y subtypes of the P2 purinoceptors. This pharmacological profile of purinergic ligands is in excellent agreement with the potency order established for the recombinant P2Y1 purinoceptor from chick brain, identifying the great majority of the brain P2 purinoceptors as identical or very similar to the native P2Y1 receptor.

Autoradiographic labelling of P2 purinoceptors in the guinea-pig cochlea

Two different radioligands were used to identify extracellular ATP binding sites specific to P2 purinoceptors in guinea-pig cochlear tissue. Deoxyadenosine 5'-(alpha-[35S]thio)triphosphate ([35S]dATP alpha S; 10 nM) provided a high activity probe for the P2y purinoceptor subtype on the basis of selective block by 2-methylthio-ATP (2MeSATP; 100 microM). [3H]alpha, beta-methylene-ATP (10 nM), a high affinity probe for a P2x purinoceptor subtype was selectively blocked by inclusion of the related compound beta, gamma-methylene-ATP (100 microM). Both probes labelled the organ of Corti, stria vascularis and spiral prominence regions. The P2x purinoceptor probe also bound to lateral wall tissue below the spiral prominence and insertion point of the basilar membrane within the scala tympani compartment, a region which failed to show significant binding using [35S]dATP alpha S. Frozen sections of whole cochlea permitted analysis of radioligand binding to the cell body region (spiral ganglion in Rosenthal's canal) of the primary auditory afferents and the auditory nerve itself, which lies within the central region of the modiolus of the cochlea. Both these regions exhibited 2MeSATP blockable [35S]dATP alpha S binding whereas specific [3H]alpha, beta-methylene-ATP binding was absent from spiral ganglion and minimal in the auditory nerve region. These results demonstrate a mixed P2 purinoceptor distribution in cochlear tissues and suggest that complex purine-mediated neurohumoral mechanisms may influence cochlear function at a number of sites.

Effects of suramin on contractions of the guinea-pig vas deferens induced by analogues of adenosine 5'-triphosphate

1. Adenosine 5'-triphosphate (ATP) and some of its analogues contract the guinea-pig vas deferens, acting via receptors which have been classified as P2X-purinoceptors. We have recently shown, however, that the effects of ATP are enhanced, rather than inhibited, by the non-selective P2 antagonist, suramin, and that this enhancement could not easily be explained in terms of inhibition by suramin of the breakdown of ATP. We therefore investigated the effects of suramin on contractions induced by ATP analogues, to define the structure-activity relationships of the suramin-resistant response. 2. In the absence of suramin, the order of potency for ATP analogues was adenosine 5'-(alpha,beta-methylene)triphosphonate (AMPCPP) = P1,P5-diadenosine pentaphosphate (Ap5A) = adenosine 5'-tetraphosphate (Ap4) > adenosine 5'-O-(3-thiotriphosphate) (ATP gamma S) = adenylyl 5'-(beta,gamma-methylene) diphosphonate (AMPPCP) > P1,P5-diadenosine tetraphosphate (Ap4A) > adenosine 5'-O-(2- thiodiphosphate) (ADP beta S) > 2-methylthioadenosine 5'-triphosphate (MeSATP) > or = ATP > adenosine 5'-diphosphate (ADP). This is generally in agreement with previously reported structure-activity relationships in this tissue. 3. In the presence of suramin (1 mM), responses to Ap5A, Ap4A, AMPPCP, ADP beta S and ADP were abolished or greatly reduced, and contractions induced by AMPCPP, Ap4 and ATP gamma S were inhibited. Contractions induced by MeSATP however, like those induced by ATP itself, were not reduced, but at concentrations above 100 microM were enhanced. In the presence of suramin (1 mM) the order of potency of analogues was therefore AMPCPP = Ap4> ATP = MeSATP> ATP gamma S, with all other analogues tested being essentially inactive at concentrations up to 500 microM.4. Contractile responses of the vas deferens to transmural nerve stimulation (1-50 Hz) in the presence of the alpha-adrenoceptor antagonist, phentolamine (10 microM), were abolished by suramin (1 mM). This is in agreement with previous reports that suramin inhibits the excitatory junction potential, a response thought to be mediated by P2 purinoceptors. It is however hard to reconcile the evidence implicating ATP as the non-adrenergic transmitter responsible for this response with the failure of suramin to inhibit the contractions induced by ATP itself while abolishing nerve-mediated contractions.5. In conclusion, these results confirm our previous findings of a suramin-resistant component to the ATP-induced contraction in the guinea-pig vas deferens, and show that the structure-activity relationships of this response are not identical to those of any known P2-purinoceptor subclass. Although the inhibition by suramin of the breakdown of ATP may contribute to the suramin-resistance of some of the ATP analogues, it does not appear to provide the full explanation.

Nucleotides as extracellular signalling molecules

There is now wide acceptance that ATP and other nucleotides are ubiquitous extracellular chemical messengers. ATP and diadenosine polyphosphates can be released from synaptosomes. They act on a large and diverse family of P2 purinoceptors, four of which have been cloned. This receptor family can be divided into two distinct classes: ligand-gated ion channels for P2X receptors and G protein-coupled receptors for P2Y, P2U, P2T and P2D receptors. The P2Y, P2U and P2D receptors have a fairly wide tissue distribution, while the P2X receptor is mainly found in neurons and muscles and the P2T and P2Z receptors confined to platelets and immune cells, respectively. Inositol phosphate and calcium signalling appear to be the predominant mechanisms for transducing the G-protein linked P2 receptor signals. Multiple P2 receptors are expressed by neurons and glia in the CNS and also in neuroendocrine cells. ATP and other nucleotides may therefore have important roles not only as a neurotransmitter but also as a neuroendocrine regulatory messenger.

Ecto-ATPases: identities and functions

Ecto-ATPases are ubiquitous in eukaryotic cells. They hydrolyze extracellular nucleoside tri- and/or diphosphates, and, when isolated, they exhibit E-type ATPase activity, (that is, the activity is dependent on Ca2+ or Mg2+, and it is insensitive to specific inhibitors of P-type, F-type, and V-type ATPases; in addition, several nucleotide tri- and/or diphosphates are hydrolysed, but nucleoside monophosphates and nonnucleoside phosphates are not substrates). Ecto-ATPases are glycoproteins; they do not form a phosphorylated intermediate during the catalytic cycle; they seem to have an extremely high turnover number; and they present specific experimental problems during solubilization and purification. The T-tubule Mg2+-ATPase belongs to this group of enzymes, which may serve at least two major roles: they terminate ATP/ADP-induced signal transduction and participate in adenosine recycling. Several other functions have been discussed and identity to certain cell adhesion molecules and the bile acid transport protein was suggested on the basis of cDNA clone isolation and immunological work.

A new type of Ca(2+)-dependent, Mg(2+)-stimulated ATPase of rat liver plasma membrane

Incubation of a glycoprotein fraction obtained from rat liver plasma membrane which has been previously well characterized using [gamma-32P]ATP results in the phosphorylation of a 230-kDa glycoprotein (pgp230). It is composed of a 120-kDa subunit (pgp120) and a 110-kDa subunit (pgp110) linked by interchain disulfide bonds. Peptide maps of pgp120 and pgp110 suggest extensive similarity in their polypeptide chains. Glycan analysis reveals between four and six hybrid-type oligosaccharide chains for both phosphoproteins. Immunoblotting using monoclonal antibodies and endoglycosidase digestion exclude an identity of pgp120 or pgp110 with the hepatocyte plasma membrane glycoproteins dipeptidylpeptidase IV or the taurocholate transport protein, which co-purify and co-migrate in SDS/PAGE. Protein phosphorylation is Ca(2+)-dependent (K0.5(Ca2+) = 0.35 microM, in the absence of Mg2+). In the presence of Mg2+, the glycoprotein undergoes rapid cycles of phosphorylation and dephosphorylation, resulting in ATPase activity. Analysis of phosphorylated amino acids identifies phosphothreonine as the major one. Photoaffinity labeling with 8-azido-[alpha-32P]ATP demonstrates the presence of one or more ATP binding site(s). Preincubation of pgp230 with various purine or pyrimidine nucleotides (ATP, UTP, TTP, ADP, GDP, AMP, CMP) or known P2-purinoceptor agonists or antagonists (adenosine 5'-[alpha,beta-methylene]triphosphate, 2-methyl-thio-adenosine 5'-triphosphate, suramin) inhibits its phosphorylation by [gamma-32P]ATP. The biological function of pgp230 is unknown at present. Several findings of the present study are compatible with the idea that pgp230 may be involved in a P2-purinoceptor function of the hepatocyte. Following this concept, a mechanism is discussed where a cytosolically exposed high-affinity Ca(2+)-binding site of pgp230 would allow for receptor feedback control, via phosphorylation and dephosphorylation, by sensing changes in cytosolic Ca2+ concentration.

Pharmacological profile of the ATP-mediated increase in L-type calcium current amplitude and activation of a non-specific cationic current in rat ventricular cells

1. The pharmacological profile of the ATP-induced increase in ICa amplitude and of ATP activation of a non-specific cationic current, IATP, was investigated in rat ventricular cells. 2. The EC50 values for ICa increase and IATP activation were 0.36 microM and 0.76 microM respectively. Suramin (10 microM) and cibacron blue (1 microM) competitively antagonized both effects of ATP. 3. The rank order of efficacy and potency of ATP analogues in increasing ICa amplitude was 2-methylthio-ATP approximately ATP approximately ATP gamma S. The derivatives alpha,beta-methylene-ATP, beta,gamma-methylene-ATP and beta,gamma-imido-ATP up to 500 microM had no significant effects. 4. The rank order of efficacy of ATP analogues in activating a non-specific cationic current, IATP, was 2-methylthio-ATP > ATP >> ATP gamma S. The rank order of potency was 2-methylthio-ATP approximately ATP. The EC50 of ATP gamma S could not be determined owing to its very low efficacy. 5. The ATP analogues alpha,beta-methylene-ATP, beta,gamma-methylene-ATP and beta,gamma-imido-ATP at 500 microM did not activate IATP but acted as antagonists of activation of IATP by ATP. 6. The results suggest that the increase in ICa amplitude induced by external ATP is due to activation of P2Y-purinoceptors. 7. The mechanism of IATP activation remains to be determined before the receptor subtype involved can be deduced.

A new class of ligand-gated ion channel defined by P2x receptor for extracellular ATP

Extracellular ATP exerts its effects through P2 purinoceptors: these are ligand-gated ion channels (P2x) or G-protein-coupled receptors (P2Y, P2U). ATP at P2x receptors mediates synaptic transmission between neurons and from neurons to smooth muscle, being responsible, for example, for sympathetic vasoconstriction in small arteries and arterioles. We have now cloned a complementary DNA encoding the P2x receptor from rat vas deferens and expressed it in Xenopus oocytes and mammalian cells. ATP activates a cation-selective ion channel with relatively high calcium permeability. Structural predictions suggest that the protein (399 amino acids long) is mostly extracellular and contains only two transmembrane domains plus a pore-forming motif which resembles that of potassium channels. The P2x receptor thus defines a new family of ligand-gated ion channels.

Estimates of antagonist affinities at P2X purinoceptors in rat vas deferens

In functional studies pyridoxalphosphate-6-azophenyl-2',5'-disulphonic acid (iso-PPADS), suramin, GR200282 (4,4'-[carbonyl-bis(imino-3- benzoylimino)]-bis[5-hydroxy-naphthalene-2,7-disulfonic acid] tetrapotassium salt), cibacron blue, trypan blue and congo red, each produced specific antagonism of the contractile responses of isolated rat vas deferens, induced by alpha,beta-methylene ATP (alpha,beta-meATP), with antagonist pKB estimates of 6.6 +/- 0.3, 5.5 +/- 0.2, 5.1 +/- 0.3, 5.8 +/- 0.2, 4.7 +/- 0.2 and 4.6 +/- 0.2, respectively. In radioligand binding studies, iso-PPADS, suramin, cibacron blue, GR200282, trypan blue and congo red competed for the high affinity [3H]alpha,beta-meATP binding sites in rat vas deferens membranes with pKi estimates of 5.6 +/- 0.04, 5.5 +/- 0.08, 5.6 +/- 0.15, 5.6 +/- 0.04, 4.3 +/- 0.06 and 4.9 +/- 0.10, respectively. Comparison of pKB and pKi estimates revealed a good agreement between the two approaches for estimating measures of affinity for the putative antagonists, except in the case of iso-PPADS. However, we found that two populations of [3H]alpha,beta-meATP binding sites can be identified by iso-PPADS, 26.4% of these having low affinity (pKi of 4.4 +/- 0.2), and 73.6% having high affinity (pKi of 6.5 +/- 0.02) for iso-PPADS. The pKi of 6.5 obtained at the high affinity sites identified by iso-PPADS was close to the equivalent pKB value of 6.6 from functional studies. These studies therefore show a good agreement between pKB and pKi estimates for several antagonists, and suggest that the high affinity binding sites labelled with [3H]alpha,beta-meATP in rat vas deferens represents binding to functional P2X purinoceptors.

Structure Activity Relationships for Derivatives of Adenosine-5'-Triphosphate as Agonists at P(2) Purinoceptors: Heterogeneity Within P(2X) and P(2Y) Subtypes

The structure-activity relationships for a variety of adenine nucleotide analogues at P(2x)- and P(2Y)-purinoceptors were investigated. Compounds formed by structural modifications of the ATP molecule including substitutions of the purine ring (C2, C8, N1, and N(6)-substituents, and a uridine base instead of adenine), the ribose moiety (2' and 3'-positions), and the triphosphate group (lower phosphates, bridging oxygen substitution, and cyclization) were prepared. Pharmacological activity at P(2Y)-purinoceptors was assayed in the guinea pig taenia coli, endothelial cells of the rabbit aorta, smooth muscle of the rabbit mesenteric artery, and turkey erythrocyte membranes. Activity at P(2X)-purinoceptors was assayed in the rabbit saphenous artery and the guinea-pig vas deferens and urinary bladder. Some of the analogues displayed selectivity, or even specificity, for either the P(2X)- or the P(2Y)-purinoceptors. Certain analogues displayed selectivity or specificity within the P(2X)- or P(2Y)-purinoceptor superfamilies, giving hints about possible subclasses. For example, 8-(6-aminohexylamino)ATP and 2',3'-isopropylidene-AMP were selective for endothelial Pzypurinoceptors over P(2Y)-purinoceptors in the guinea pig taenia coli, rabbit aorta, and turkey erythrocytes. These compounds were both inactive at P(2X)-purinoceptors. The potent agonist N(6)-methyl ATP and the somewhat less potent agonist 2'-deoxy-ATP were selective for P(2Y)-purinoceptors in the guinea pig taenia coli, but were inactive at P(2X)-purinoceptors and the vascular P(2Y)-purinoceptors. 3'-Benzylamino-3'-deoxyATP was very potent at the P(2X)-purinoceptors in the guinea pig vas deferens and bladder, but not in the rabbit saphenous artery and was inactive at P(2Y) receptors. These data suggest that specific compounds can be developed that can be utilized to activate putative subtypes of the P(2X)- and P(2Y)-purinoceptor classes.

Involvement of ETA receptors in the facilitation by endothelin-1 of non-adrenergic non-cholinergic transmission in the rat urinary bladder

1. Endothelin-1 (ET-1; 3-10 nM) raised the tone of rat bladders bathed in buffer containing atropine (1 microM) plus guanethidine (3.4 microM). In addition, ET-1 potentiated, in a concentration-dependent fashion (1-10 nM), the contractions evoked by both transmural nerve stimulation and applications of exogenous adenosine 5'-triphosphate (ATP). 2. The threshold concentration of ET-1 required to facilitate non-adrenergic non-cholinergic (NANC) transmission and potentiate ATP-induced contractions, was about 10 fold lower than that required to increase the bladder tone (3 nM). 3. The ET-1-induced increase in basal tension reached its maximal effect within 60-90 s. In contrast, the 7.8 microM ATP-induced contractions increased by 50% within the first minute following incubation with 10 nM ET-1 but required about 5 min to develop the maximal effect. 4. The ET-1-induced potentiation of NANC or ATP responses was long-lasting and persisted in spite of extensive washing. The recovery of the bladder excitability depended on the concentration of ET-1. Following the application of 3 nM ET-1, recovery required 30 min; applications of 10 nM ET-1 required at least 60 min for full recovery. 5. The ET-1-induced potentiation of responses was selective for ATP and related structural analogues. ET-1 did not modify the contractions induced by acetylcholine, 5-hydroxytryptamine, prostaglandin F2 alpha or bradykinin. 6. The potency of ET-2 was similar to that of ET-1. ET-3 and ET-C-terminal hexapeptide were inactive up to 100 M. Sarafotoxin S6b was 2 to 3 fold less potent than ET-1 whereas sarafotoxin S6c (100 nM) was inactive. AGETB-9 and AGETB-89, two ETB receptor agonists, were also inactive (up to 100 nM). 7. Removal of one or both disulphide bonds in ET-1 and tryptophan-21 formylation of ET-1, resulted in inactive peptides (up to 100 nM). 8. The ET-1 receptor antagonists, BE-18257B and FR 139317, blocked both the ET-1-induced rise in tone and the potentiation of ATP responses in a concentration-dependent fashion. FR 139317 was at least 30 fold more potent than BE-18257B. Both antagonists blocked at lower concentrations the ET-1 increase in bladder tone as compared to the ATP potentiation. The antagonism was slowly reversible. 9. Results are consistent with the presence of ETA receptors in the rat bladder, which mediate both actions of ET-1. The interaction of ET-1 with purinergic mechanisms is discussed.

Distribution and characterisation of [3H]alpha,beta-methylene ATP binding sites in the rat

Radioligand binding studies have been performed to study the distribution of the binding sites for the P2x purinoceptor selective agonist radioligand, [3H]alpha,beta-methylene ATP ([3H]alpha beta-meATP), in membranes prepared from various peripheral organs and several brain regions of the rat. In agreement with previous studies in the rat vas deferens, [3H]alpha beta-meATP labelled two populations of sites. One site exhibited high affinity for the ligand (Kd = 0.7 nM; Bmax = 1012 fmol.mg-1 protein) while the other site exhibited lower affinity (Kd = 70.8 nM) and higher capacity (Bmax) = 7470 fmol.mg-1 protein). In competition studies, using a low concentration of radioligand (1 nM), the high affinity alpha beta-meATP binding sites in vas deferens membranes could be preferentially labelled (84-91%). Under these conditions, the P2x purinoceptor agonists, alpha beta-meATP and beta, gamma-methylene ATP, had the highest affinity with pIC50 values of 8.3 and 7.3 respectively. The P2y purinoceptor agonist, 2-methyl-thio-ATP (2-me-S-ATP), had lower affinity (pIC50 = 6.7), while uridine triphosphate, adenosine diphosphate and adenosine, agonists at the P2u, P2t and P1 purinoceptors, respectively, possessed low affinity (pIC50 values < 5.6). In addition, the P2 purinoceptor antagonists, cibacron blue and suramin, inhibited binding over the same concentration range at which they behave as functional antagonists at the P2x purinoceptor. High and low affinity binding sites for [3H]alpha beta-meATP were also identified in a range of other peripheral tissues (spleen, heart and liver) and in several brain regions (striatum, cerebral cortex, hippocampus). In the spleen, heart, cerebral cortex and liver the Kd values at both the high affinity binding sites (Kd = 1-1.2 nM) and the low affinity binding sites (Kd = 98-158 nM) were similar to the respective Kd values at the high and low affinity binding sites in the vas deferens. In competition studies performed using a low concentration of radioligand (1 nM) these sites exhibited a similar pharmacological profile to that seen in the vas deferens. Detailed analysis of competition curves to several of the ATP analogues in each of the tissues revealed that the binding profile of the radioligand was complex since several compounds, and in particular ATP and 2-me-S-ATP, identified a lower proportion of sites with high affinity than did alpha beta-meATP.(ABSTRACT TRUNCATED AT 400 WORDS)

Signal transduction via P2-purinergic receptors for extracellular ATP and other nucleotides

Extracellular ATP, at micromolar concentrations, induces significant functional changes in a wide variety of cells and tissues. ATP can be released from the cytosol of damaged cells or from exocytotic vesicles and/or granules contained in many types of secretory cells. There are also efficient extracellular mechanisms for the rapid metabolism of released nucleotides by ecto-ATPases and 5'-nucleotidases. The diverse biological responses to ATP are mediated by a variety of cell surface receptors that are activated when ATP or other nucleotides are bound. The functionally identified nucleotide or P2-purinergic receptors include 1) ATP receptors that stimulate G protein-coupled effector enzymes and signaling cascades, including inositol phospholipid hydrolysis and the mobilization of intracellular Ca2+ stores; 2) ATP receptors that directly activate ligand-gated cation channels in the plasma membranes of many excitable cell types; 3) ATP receptors that, via the rapid induction of surface membrane channels and/or pores permeable to ions and endogenous metabolites, produce cytotoxic or activation responses in macrophages and other immune effector cells; and 4) ADP receptors that trigger rapid ion fluxes and aggregation responses in platelets. Current research in this area is directed toward the identification and structural characterization of these receptors by biochemical and molecular biological approaches.