Guanine Nucleotide-sensitive Interaction of a Radiolabeled Agonist with a Phospholipase C-linked P2y-purinergic Receptor

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.

The diverse series of recombinant P2Y purinoceptors

A cDNA encoding a P2Y purinoceptor was originally cloned from chick brain and the bovine and human homologues have recently been obtained. These are seven-transmembrane-domain polypetides, i.e. G protein-coupled receptors. When activated by agonists, this P2Y receptor mobilizes intracellular Ca2+ and has been shown to be coupled to inositol-1,4,5-trisphosphate formation. Its pharmacology has been established in several expression systems, using both ligand binding and functional responses: 2-methylthioATP has the highest potency of nucleotides and derivatives tested, while UTP and alpha, beta-methylene ATP are inactive. This was hence assigned as a new subtype of the pharmacologically defined P2Y receptors, P2Y1. P2Y1 receptors are exceptionally abundant in the brain. A P2U receptor reported by others can be designated P2Y2. Another P2 receptor subtype, P2Y3, now cloned as a cDNA from the brain and expressed in oocytes and in transfected cells, shows a quite different ligand potency profile to the first two. A fourth subtype is expressed primarily in certain haemopoietic cells and in cardiac muscle. A putative fifth subtype is expressed only in T lymphocytes, upon activation. Yet other P2Y subtypes are indicated by recent cloning studies. The amino acid sequences of all of these P2 receptors, while displaying some homology, are strikingly diverse: they form a separate and unusual new family in the G protein-coupled receptor main superfamily.

[3H]A-317491, a Novel High-Affinity Non-Nucleotide Antagonist That Specifically Labels Human P2X2/3and P2X3Receptors

A-317491 is a potent and selective antagonist of P2X3 and P2X(2/3) receptors. In the present studies, the ability of [3H]A-317491 to label recombinant human P2X(2/3) and P2X(3) receptors was characterized. Using membranes prepared from 1321N1 cells expressing P2X(2/3) receptors, [3H]A-317491 specifically labeled high-affinity (Kd = 0.9 nM) recognition sites. High-affinity [3H]A-317491 binding was not detected in membrane preparations from native 1321N1 cells or cells expressing homomeric P2X1, P2X2, or P2X3 receptors. Specific [3H]A-317491 P2X3 receptors could only be reliably detected following treatment of intact P2X3 receptor-expressing cells with apyrase (1 U/ml) both before and during membrane preparation. Under these conditions, [3H]A-317491 also labeled high-affinity (Kd = 9 nM) binding sites. Lower affinity binding components (Kd values of 87-790 nM) were detected in both assays using higher ligand concentrations that likely represent nonfunctional recognition sites. [3H]A-317491 binding to both P2X(2/3) and P2X3 receptors was reversible, and ligand kinetic studies provided similar estimates of the high-affinity binding constants. Potent P2X3 receptor agonists 2-methylthio-ATP, 2,3-O-(4-benzoylbenzoyl)-ATP, and alpha,beta-methylene adenosine triphosphate also potently inhibited specific [3H]A-317491 binding to both P2X(2/3) and P2X3 receptors. The pharmacological profile for P2X receptor antagonists to inhibit [3H]A-317491 binding to P2X(2/3) and P2X3 receptors was highly correlated (r = 0.98, P < 0.05), and a similar rank order of potency was observed for blockade of P2X(2/3) receptor-mediated calcium influx. These data demonstrate that [3H]A-317491 is the first useful radioligand for the specific labeling of P2X3-containing channels.

Quantitation of the P2Y(1) receptor with a high affinity radiolabeled antagonist

2-Chloro-N(6)-methyl-(N )-methanocarba-2'-deoxyadenosine-3',5'- bisphosphate (MRS2279) was developed previously as a selective high-affinity, non-nucleotide P2Y(1) receptor (P2Y1-R) antagonist (J Med Chem 43:829-842, 2002; Br J Pharmacol 135:2004-2010, 2002). We have taken advantage of the N(6)-methyl substitution in the adenine base to incorporate [(3)H]methylamine into the synthesis of [(3)H]MRS2279 to high (89 Ci/mmol) specific radioactivity and have used this molecule as a radioligand for the P2Y1-R. [(3)H]MRS2279 bound to membranes from Sf9 insect cells expressing recombinant human P2Y1-R but not to membranes from wild-type Sf9 cells or Sf9 cells expressing high levels of recombinant P2Y(2) or P2Y(12) receptors. Equilibrium binding of [(3)H]MRS2279 to P2Y1-R expressed in Sf9 membranes was with a high affinity (K(d) = 8 nM) essentially identical to the apparent affinity of MRS2279 determined previously in studies of P2Y1-R-promoted inositol phosphate accumulation or platelet aggregation. A kinetically derived K(d) calculated from independent determinations of the rate constants of association (7.15 x 10(7) M(-1) min(-1)) and dissociation (0.72 min(-1)) of [(3)H]MRS2279 also was in good agreement with the K(d) derived from equilibrium binding studies. Competition binding assays with [(3)H]MRS2279 and P2Y1-R expressing Sf9 cell membranes revealed K(i) values for the P2Y1-R antagonists MRS2279 (K(i) = 13 nM), N(6)-methyl-2'-deoxyadenosine-3',5'-bisphosphate (MRS2179; K(i) = 84 nM), adenosine-3', 5'-bisphosphate (K(i)=900 nM), and pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (K(i) = 6 microM) that were in good agreement with antagonist activities of these molecules previously determined at the P2Y1-R in intact tissues. Moreover, [(3)H]MRS2279 also bound with high affinity (K(d) = 4-8 nM) to Chinese hamster ovary (CHO) or 1321N1 human astrocytoma cells stably expressing the human P2Y1-R, but specific binding was not observed in wild-type CHO or 1321N1 cells. [(3)H]MRS2279 bound with high affinity (K(d) = 16 nM) to a binding site on out-dated human platelets (5-35 receptors/platelet) and rat brain membranes (210 fmol/mg protein) that fit the expected drug selectivity of a P2Y1-R. Taken together, these results indicate that [(3)H]MRS2279 is the first broadly applicable antagonist radioligand for a P2Y receptor.

Dehydroepiandrosterone activates endothelial cell nitric-oxide synthase by a specific plasma membrane receptor coupled to Galpha(i2,3)

The adrenal steroid dehydroepiandrosterone (DHEA) has no known cellular receptor or unifying mechanism of action, despite evidence suggesting beneficial vascular effects in humans. Based on previous data from our laboratory, we hypothesized that DHEA binds to specific cell-surface receptors to activate intracellular G-proteins and endothelial nitric-oxide synthase (eNOS). We now pharmacologically characterize a putative plasma membrane DHEA receptor and define its associated G-proteins. The [3H]DHEA binding to isolated plasma membranes from bovine aortic endothelial cells was of high affinity (K(d) = 48.7 pm) and saturable (B(max) = 500 fmol/mg protein). Structurally related steroids failed to compete with DHEA for binding. The putative DHEA receptor was functionally coupled to G-proteins, because guanosine 5'-O-(3-thio)triphosphate (GTPgammaS) inhibited [3H]DHEA binding to plasma membranes by 69%, and DHEA increased [35S]GTPgammaS binding by 157%. DHEA stimulated [35S]GTPgammaS binding to Galpha(i2) and Galpha(i3), but not to Galpha(i1) or Galpha(o). Pretreatment of plasma membranes with antibody to Galpha(i2) or Galpha(i3), but not to Galpha(i1), inhibited the DHEA activation of eNOS. Thus, DHEA receptors are expressed on endothelial cell plasma membranes and are coupled to eNOS activity through Galpha(i2) and Galpha(i3). These novel findings should allow us to isolate the putative receptor and reevaluate the physiological role of DHEA activity.

UTP binding and phosphoinositidase C activation in ampulla from frog semicircular canal

Pyrimidine nucleotide-sensitive phosphoinositidase C activity (PLC), previously identified in frog semicircular canal ampulla, was pharmacologically characterized. Binding of [(3)H]UTP and abilities of unlabeled nucleotide analogs to inhibit binding and to stimulate PLC in myo-[(3)H]inositol-loaded ampullas were determined. Specific [(3)H]UTP binding was competitively inhibited by UTP [apparent dissociation binding constant = 0.8 microM; Hill coefficient = 0.7]. Scatchard analysis revealed a minor class of high-affinity binding sites [45 fmol UTP bound/microgram protein; dissociation constant (K(D1)) = 0.4 microM] and a major class of moderate-affinity binding sites (365 fmol UTP bound/microgram protein; K(D2) = 10 microM). The stereospecificity pattern for UTP analog recognition was UMP > UDP >/= ADP = UTP = dTTP > adenosine 5'-O-(3-thiotriphosphate) = ATP = CTP = 2'-and 3'-O-4-(benzoylbenzoyl)-ATP (Bz-ATP) >/= AMP >/= 2-methylthio-ATP = alpha,beta-methylene-ATP > uridine = diadenosine tetraphosphate (Ap(4)A); cAMP and adenosine were inactive. Antagonist recognition pattern was DIDS = pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) = reactive blue 2 > suramin. The rank order of potencies for agonist-induced PLC activation was UDP >/= UTP >/= Ap(4)A >/= UMP = Bz-ATP; uridine was inactive. UTP-stimulated PLC activity was inhibited by DIDS = reactive blue 2 = PPADS > suramin. These results suggest that the population of [(3)H]UTP-labeled binding sites is heterogeneous, with a low number of high-affinity UTP receptors whose function(s) need to be determined and a large number of moderate-affinity receptors triggering PLC activation.

In human and rat lung membranes [35S]GTPgammaS binding is a tool for pharmacological characterization of G protein-coupled dinucleotide receptors

The P2Y receptor family is activated by extracellular nucleotides such as ATP and UTP. P2Y receptors regulate physiological functions in numerous cell types. In lung, the P2Y2 receptor subtype plays a role in controlling Cl- and fluid transport. Besides ATP or UTP, also diadenosine tetraphosphate (Ap4A), a stable nucleotide, seems to be of physiological importance. In membrane preparations from human and rat lung we applied several diadenosine polyphosphates to investigate whether they act as agonists for G protein-coupled receptors. We assessed this by determining the stimulation of [35S]GTPgammaS binding. Stimulation of [35S]GTPgammaS binding to G proteins has already been successfully applied to elucidate agonist binding to various G protein-coupled receptors. Ap(n)A (n = 2 to 6) enhanced [35S]GTPgammaS binding similarly in human and rat lung membranes, an indication of the existence of G protein-coupled receptor binding sites specific for diadenosine polyphosphates. Moreover, in both human and rat lung membranes comparable pharmacological properties were found for a diadenosine polyphosphate ([3H]Ap4A) binding site. The affinity for Ap2A, Ap3A, Ap4A, Ap5A, and Ap6A was also comparable. 8-Diazido-Ap4A and ATP were less potent, whereas the pyrimidine nucleotide UTP showed hardly any affinity. Thus, we present evidence that different diadenosine polyphosphates bind to a common G protein-coupled receptor binding site in membranes derived either from human or rat lung.

Evidence for a G protein-coupled diadenosine-5',5'''-P1,P4-tetraphosphate (Ap4A) receptor binding site in lung membranes from rat

Nucleotide receptors are of considerable importance in the treatment of lung diseases, such as cystic fibrosis. Because diadenosine polyphosphates may also be of significance as signalling molecules in lung, as they are in a variety of tissues, in the present work we investigated the binding sites for [3H]diadenosine-5',5'''-P1,P4-tetraphosphate (Ap4A) in plasma membranes from rat lung and studied their possible coupling to G proteins. We present evidence for a single high-affinity binding site for [3H]Ap4A with similar affinity for other diadenosine polyphosphates ApnA (n = 2 to 6). Displacement studies with different nucleotides revealed that the [3H]Ap4A binding site was different from P2X and P2Y2 receptor binding sites. Pretreatment of lung membranes with GTPgammaS or GTP in the presence of Mg2+ increased the Ki for Ap4A from 91 nM to 5.1 microM, which is indicative of G protein coupling. The putative coupling to G proteins was further confirmed by the enhancement of [35S]GTPgammaS binding (to Galpha proteins) to lung membranes by Ap4A (63% increase over basal) in a concentration-dependent manner. Therefore, our data for the first time provide evidence of a G protein-coupled Ap4A binding site in lung membranes.

Steady-state binding of [3H]ATP to rat liver plasma membranes and competition by various purinergic agonists and antagonists

Steady-state analysis of nucleotide-binding sites on rat liver plasma membranes was carried out using 3H-labelled ATP as radioligand under complete inhibition of ecto-ATPase activity by excess EDTA. Binding of [3H]ATP to the membranes is saturable, reversible and apparently involves one population of specific binding sites with Kd of about 90 nM and binding capacity (Bmax) of 15 pmol/mg protein. A broad spectrum of purinergic agonists and antagonists was examined as potential inhibitors of the measured binding. The displacement studies showed the following rank order of inhibitory potency for [3H]ATP-binding sites (pIC50 values in parentheses): ATPgammaS (7.49)>2-MeSATP (7.18)>ATP (6.91)>ADPbetaS (6.64)>/=ADP (6.56)>>RB2 (6.14)>>suramin (5.40)>>Ap4A (4. 57)>alpha,beta-MeATP (4.19)>/=beta,gamma-MeATP (3.97). AMP, adenosine, Ap5A, PPADS, beta-glycerophosphate as well as non-adenine nucleoside triphosphates GTP, UTP and CTP did not exert any effect on the measured binding at concentration ranges of 10-6-10-4 M. In order to ascertain whether ATP and its analogues are capable of interacting with the same binding domain, 2-MeSATP and ADP were treated as alternative ligands that could compete with unlabelled ATP for its binding sites. A 2-fold increase of Kd value for ATP-receptor interaction was observed in the presence of 2-MeSATP (60 nM) or ADP (250 nM) without any modulation of Bmax value, confirming that inhibitory effects of these compounds are competitive in nature. These studies demonstrate that ATP and its analogues are able to interact with a single binding domain on liver plasma membranes, which may be identified as ligand-binding component of P2 purinoceptors of the P2Y1 subtype.

Pharmacological characterization of ATP receptors in ampulla from frog semicircular canal

Phosphoinositidase C activities sensitive to purine and pyrimidine nucleotides have been identified earlier in ampulla from Rana ridibunda semicircular canal. The aim of this study was to characterize the pharmacological properties of other P2 receptors borne by this structure. A microassay was developed to measure the binding of [35S]adenosine 5'-O-(2-thiodiphosphate) ([35S]ADPbetaS) to a few ampullas microdissected from frog semicircular canals. When determined at 4 degrees C in the absence of divalent cations, [35S]ADPbetaS binding was saturable with incubation time and reversible after elimination of free radioligand. The dissociation kinetics were biphasic and comprised a major component that was rapidly reversible and a minor component that dissociated slowly. [35S]ADPbetaS binding was competitively inhibited by unlabeled ADPbetaS with an apparent dissociation constant of 0.48 +/- 0.09 microM and a Hill coefficient of 0.70 +/- 0.06, and Scatchard analysis revealed a minor class of high-affinity binding sites (RT1 = 52 +/- 11 fmol [35S]ADPbetaS bound/ampulla and Kd1 = 0.15 +/- 0.04 microM) and a major class of low-affinity binding sites (RT2 = 436 +/- 79 fmol [35S]ADPbetaS bound/ampulla and Kd2 = 2.0 +/- 0.8 microM). The pattern of stereospecificity for recognition of unlabeled structural ATP analogs was ADPbetaS >/= alpha, beta-methyleneadenosine 5'-triphosphate = ADP = adenosine 5'-O-(3-thiotriphosphate) > ATP = diadenosine tetraphosphate = AMP > 2'- and 3'-O-(4-benzoylbenzoyl)-adenosine 5'-triphosphate >/= 2-methylthioadenosine 5'-triphosphate > 2-desoxythymidine 5'-triphosphate = guanosine 5'-triphosphate = inosine-5'-triphosphate = xanthosine 5'-triphosphate = cytosine 5'-triphosphate = uridine 5'-triphosphate = uridine-5'-diphosphate, whereas cAMP and adenosine were devoid of activity. For antagonists, suramin revealed competitive inhibitor potencies, whereas reactive blue 2 and DIDS acted as pure noncompetitive inhibitors. Results suggest that the population of labeled receptors is heterogeneous and contains a low number of P2Y-like receptors and a large number of P2X-like receptors whose molecular subtypes and functions in endolymph homeostasis remain to be defined.

Differential characterization of binding sites for adenine and uridine nucleotides in membranes from rat lung as possible tools for studying P2 receptors in lung

Nucleotide receptors (P2 receptors) are involved in stimulating Cl- secretion in airway epithelia. These receptors may play a key role in development of new therapeutic strategies in the treatment of cystic fibrosis. However, the diversity of nucleotide binding sites in lung tissue has not yet been clarified. Here we studied the characteristics of various nucleotide binding sites in rat lung membranes by equilibrium binding analysis of several P2 receptor specific ligands. Displacement studies revealed a recognition site for adenosine 5'-O-(1-thiotriphosphate) ([35S]ATPalphaS; Kd 243 nM). From this site the ligand is readily displaced by adenosine 5'-O-(2-thiodiphosphate) (ADPbetaS), a typical agonist for P2Y1 receptors and also by alpha,beta-methylene adenosine 5'-triphosphate (alpha,beta-MeATP), a typical agonist for P2X receptors. [3H]alpha,beta-MeATP labelled specific binding sites (Kd 56 nM) in rat lung membranes. Analysis of binding of [3H]UTP to lung membranes revealed a high-affinity binding site (Kd 44 nM). Membrane-bound [3H]UTP was not displaced even by high concentrations of ATP, indicating no common binding site for UTP and ATP. Furthermore, specific binding of P-1,P-4-di(adenosine 5')tetraphosphate ([3H]Ap4A; Kd 91 nM) was found in lung membranes. Thus, we demonstrate at least four distinct types of nucleotide binding sites in lung membranes: Two have characteristics comparable to P2X and P2Y1 receptors, while two further sites still have to be identified, one binding Ap4A and the other binding UTP very specifically.

Characterization of [35S]-ATP alpha S and [3H]-alpha, beta-MeATP binding sites in rat brain cortical synaptosomes: regulation of ligand binding by divalent cations

1. We made a comparative analysis of the binding characteristics of the radioligands [35S]-ATP alpha S and [3H]-alpha, beta-MeATP in order to test whether these ligands can be used to analyse P2-purinoceptors in synaptosomal membranes from rat brain cortex. 2. Synaptosomes possess sites with high affinity for [35S]-ATP alpha S (Kd = 22.2 +/- 9.1 nM, Bmax = 14.8 pmol mg-1 protein). The rank order of the competition potency of the different compounds (ATP alpha S, ATP, ATP gamma S > ADP beta S, 2-MeSATP > deoxyATP, ADP > > UTP, alpha, beta-MeATP, AMP, Reactive Blue-2, suramin, isoPPADS) is consistent with pharmacological properties of P2Y-purinoceptors. 3. Under identical conditions [35S]-ATP alpha S and [3H]-alpha, beta-MeATP bind to different binding sites at synaptosomal membranes from rat brain cortex. The affinity of the [3H]-alpha, beta-MeATP binding sites (Kd = 13.7 +/- 1.8 nM, Bmax = 6.34 +/- 0.28 pmol mg-1 protein) was 38 fold higher than the potency of alpha, beta-MeATP to displace [35S]-ATP alpha S binding (Ki = 0.52 microM). ATP and ADP beta S competed at both binding sites with different affinities, 60 fold and 175 fold, respectively. The other agonists tested (2-MeSATP, UTP, GTP) did not affect specific [35H]-alpha, beta-MeATP binding at concentrations up to 100 microM. The antagonists (suramin, isoPPADS, Evan's Blue) showed completely different affinities for both binding sites. 4. Binding of [35S]-ATP alpha S on synaptosomes was regulated by GTP, which is indicative for G-protein coupled receptors. The Kd value for the high affinity binding site was reduced in the presence of GTP about 5 fold (from 1.8 nM to 8.6 nM). In the presence of Mg2+ the affinity was increased (Kd 1.8 nM versus 22 nM in the absence of Mg2+). 5. The binding of both radioligands was regulated in an opposite manner by physiological concentrations of Ca2+ and Mg2+. Binding of [3H]-alpha, beta-MeATP to synaptosomal membranes was increased 3 fold by raising the Ca2+ concentration from 10 microM to 1 mM, whereas the addition of Mg2+ in the same concentration range resulted in an 80% reduction of the binding. In contrast, [35S]-ATP alpha S binding was not influenced at the same range of Ca2+ or Mg2+ concentrations (10 microM to 1 mM). The addition of Mg2+ (5 mM) increased the affinity of [35S]-ATP alpha S for the high affinity site 10 fold. 6. Diadenosine polyphosphates had a bimodal effect on [35S]-ATP alpha S binding to synaptosomal membranes. AP5A and Ap6A enhanced binding of [35S]-ATP alpha S 1.6 fold in a concentration range between 0.1 and 50 microM. Ap3A was a weak inhibitor with a Ki value of 7.2 microM. Ap4A, AP5A and Ap6A inhibited with Ki values > 100 microM. These data support the concept that diadenosine polyphosphates do not directly interact with ATP alpha S binding sites. 7. In conclusion, on the basis of present knowledge of the interaction of P2-purinoceptor active compounds with P2x- and/or P2Y-purinoceptors, our data strongly suggest that [35S]-ATP alpha S is a useful tool to study P2Y-purinoceptors. Thus, the [35S]-ATP alpha S binding site might to a large extent represent P2Y-purinoceptors in synaptosomes from rat brain cortex. The nucleotide binding is regulated by G proteins, indicated by the effects of GTP/Mg2+ on binding.

Effects of ATP on phosphoinositide hydrolysis and prostaglandin E2 generation in rabbit astrocytes

Extracellular ATP secreted from stimulated nerves plays a role in neurotransmission. This study examined the effects of extracellular ATP on phospholipase A2 and C signalling pathways in rabbit astrocytes. ATP caused prostaglandin E2 (PGE2) generation and phosphoinositide hydrolysis in a time- and concentration-dependent manner. A P2y purinoceptor-selective agonist, 2-methylthio-ATP also caused phosphoinositide hydrolysis, but not PGE2 generation. A P2x purinoceptor-selective agonist, alpha, beta-methylene-ATP did not cause either phosphoinositide hydrolysis or PGE2 generation. Although pertussis toxin had no effect on 2-methylthio-ATP-induced phosphoinositide hydrolysis, it markedly decreased ATP-induced PGE2 generation, with significant inhibition of phosphoinositide hydrolysis. Dexamethasone and indomethacin which potently inhibited ATP-induced PGE2 generation, caused partial inhibition of phosphoinositide hydrolysis, suggesting that pertussis toxin-sensitive component of ATP-induced phospholipase C activation is mediated by cyclo-oxygenase metabolites of arachidonic acid. These results suggest that a stimulation of P2y receptor results in phospholipase C activation in a pertussis toxin-insensitive manner, and that a P2 receptor other than the P2y or P2x subtypes is involved in ATP-induced phospholipase A2 activation via a pertussis toxin-sensitive G protein.

An examination of deoxyadenosine 5'(alpha-thio)triphosphate as a ligand to define P2Y receptors and its selectivity as a low potency partial agonist of the P2Y1 receptor

1. The functional activity of deoxyadenosine 5'(alpha-thio)triphosphate (dATP alpha S) was assessed at the cloned human P2Y1 receptor stably expressed in 1321N1 human astrocytoma cells and transiently expressed in Cos-7 cells. 2. Cells expressing the receptor responded to adenine nucleotides with an increase in [3H]-inositol phosphate accumulation. Half-maximal responses were obtained at approximately 30 nM for 2-methylthioadenosine-5'-triphosphate (2MeSATP), 300 nM for dATP alpha S, and 1000 nM for adenosine 5'-triphosphate (ATP). dATP alpha S produced a maximal response that was only 37 +/- 4% of that produced by ATP or 2MeSATP. dATP alpha S also competitively antagonized the phospholipase C response to 2MeSATP with a KB of 644 +/- 14 nM. Thus dATP alpha S acts as a low potency partial agonist at P2Y1 receptors. 3. The selectivity of dATP alpha S for P2Y1 receptors was determined by examining its capacity to activate P2Y2, P2Y4 and P2Y6 receptors also stably expressed in 1321N1 cells. Although dATP alpha S was a partial agonist at P2Y1 receptors it was a full agonist at P2Y2 receptors, albeit with a potency that was two orders of magnitude lower than at P2Y1 receptors. No agonist or antagonist activity was observed at P2Y4 and P2Y6 receptors. 4. Although [35S]-dATP alpha S bound to a relatively high density (ca 10 pmol mg-1 protein) of binding sites in membranes from 1321N1 or Cos-7 cells expressing the P2Y1 receptor, no difference in the total density of sites was observed between membranes from wild-type, empty vector-transfected, or P2Y1 receptor-expressing cells. Moreover, adenine nucleotide analogues inhibited [35S]-dATP alpha S binding with an order of potency that differed markedly from that for the accumulation of inositol phosphates in intact transfected P2Y1 receptor-expressing cells. Saturation binding experiments demonstrated multiple affinity states for [35S]-dATP alpha S binding in wild-type Cos-7 cell membranes. These data from 1321N1 and Cos-7 cells suggest that cellular membranes exhibit a large number of high affinity binding sites for [35S]-dATP alpha S that are not related to P2Y receptor subtypes.

The lipid transfer activity of phosphatidylinositol transfer protein is sufficient to account for enhanced phospholipase C activity in turkey erythrocyte ghosts

BACKGROUND

The minor membrane phospholipid phosphatidylinositol 4, 5-bisphosphate (PIP2) has been implicated in the control of a number of cellular processes. Efficient synthesis of this lipid from phosphatidylinositol has been proposed to require the presence of a phosphatidylinositol/phosphatidylcholine transfer protein (PITP), which transfers phosphatidylinositol and phosphatidylcholine between membranes, but the mechanism by which PITP exerts its effects is currently unknown. The simplest hypothesis is that PITP replenishes agonist-sensitive pools of inositol lipids by transferring phosphatidylinositol from its site of synthesis to sites of consumption. Recent cellular studies, however, led to the proposal that PITP may play a more active role as a co-factor which stimulates the activity of phosphoinositide kinases and phospholipase C (PLC) by presenting protein-bound lipid substrates to these enzymes. We have exploited turkey erythrocyte membranes as a model system in which it has proved possible to distinguish between the above hypotheses of PITP function.

RESULTS

In turkey erythrocyte ghosts, agonist-stimulated PIP2 hydrolysis is initially rapid, but it declines and reaches a plateau when approximately 15% of the phosphatidylinositol has been consumed. PITP did not affect the initial rate of PIP2 hydrolysis, but greatly prolonged the linear phase of PLC activity until at least 70% of phosphatidylinositol was consumed. PITP did not enhance the initial rate of phosphatidylinositol 4-kinase activity but did increase the unstimulated steady-state levels of both phosphatidylinositol 4-phosphate and PIP2 by a catalytic mechanism, because the amount of polyphosphoinositides synthesized greatly exceeded the molar amount of PITP in the assay. Furthermore, when polyphosphoinositide synthesis was allowed to proceed in the presence of exogenous PITP, after washing ghosts to remove PITP before activation of PLC, enhanced inositol phosphate production was observed, whether or not PITP was present in the subsequent PLC assay.

CONCLUSION

PITP acts by catalytically transferring phosphatidylinositol down a chemical gradient which is created as a result of the depletion of phosphatidylinositol at its site of use by the concerted actions of the phosphoinositide kinases and PLC. PITP is therefore not a co-factor for the phosphoinositide-metabolizing enzymes present in turkey erythrocyte ghosts.

P2U agonists induce chemotaxis and actin polymerization in human neutrophils and differentiated HL60 cells

Human neutrophils or HL60 cells express P2U receptors and respond to micromolar concentrations of ATP, adenosine 5'-O-(thiotriphosphate) (ATPgammaS), or UTP with immediate increases in intracellular Ca2+ through activation of phosphoinositide phospholipase C (Cowen, D. S., Lazarus, H. M., Shurin, S. B., Stoll, S. E., and Dubyak, G. R. (1989) J. Clin. Invest. 83, 1651-1660). P2U agonists reportedly induce limited enzyme secretion and enhance the respiratory burst in response to chemotactic factors. We demonstrate here that P2U agonists are chemotactic for neutrophils or differentiated HL60 cells. Rhodamine phalloidin staining indicates that ATPgammaS treatment induces actin polymerization and shape changes similar to those seen when these cells are treated with chemotactic peptide fMet-Leu-Phe. Although undifferentiated HL60 cells fail to mount a rise in Ca2+ when challenged with fMet-Leu-Phe, they increase Ca2+ in response to P2U agonists. However, functional expression of phospholipase C-coupled receptors is not sufficient for chemotaxis since HL60 cell migration in response to these agonists or to fMet-Leu-Phe occurs only after exposure to differentiating agents such as BT2cAMP. In addition to the well known G protein-linked receptors for lipid or peptide chemotactic factors, neutrophils apparently also can utilize G protein-linked purino/pyrimidino receptors to recognize nucleotides as chemoattractants. High concentrations of ATP and UTP generated at sites of platelet aggregation and tissue injury could thus be important mediators of inflammation.

Evidence that most high-affinity ATP binding sites on aortic endothelial cells and membranes do not correspond to P2 receptors

It has recently been demonstrated that two types of ATP receptors, the P2Y and P2U receptors, are coexpressed on bovine aortic endothelial cells. The aim of the present study was to characterize directly P2Y and P2U subtypes on intact bovine aortic endothelial cells and on membranes prepared from these cells using adenosine 5'-0-(3-thio[35S]triphosphate) ([35S]ATP gamma S), [alpha-32P]ATP and [alpha-32P]UTP as radioligands. [35S]ATP gamma S binding to bovine aortic endothelial cell membranes was saturable and apparently involved a single class of high-affinity binding sites (Kd: 14 +/- 11 nM. Bmax 1.6 +/- 0.7 pmol/mg protein; mean +/- S.D.). A similar class of high-affinity binding sites was identified with [alpha-32P]ATP (Kd: 14 +/- 9 nM; Bmax: 1.7 +/- 1.1 pmol/mg protein; mean +/- S.D.). Competition experiments showed that only one third of these sites bound 2-methylthio-ATP (2-MeSATP) with high affinity (Ki: 21 +/- 5 and 14 +/- 10 nM, mean +/- S.D., for [35S]ATP gamma S and [alpha-32P]ATP, respectively) and might therefore represent the P2Y receptors. UTP did not compete with [35S]ATP gamma S or [alpha-32P]ATP for binding at the remaining sites, indicating that they are not the P2U receptors. No high-affinity UTP binding sites could be detected using [alpha-32P]UTP. [35S]ATP gamma S binding to intact bovine aortic endothelial cells was competed by ATP gamma S (Kd: 1.0 +/- 0.5 microM; mean +/- S.D.), but not by 2-MeSATP and UTP, indicating that these binding sites are neither the P2Y nor the P2U receptors.

Study of the mechanisms involved in adenosine-5'-O-(2-thiodiphosphate) induced relaxation of rat thoracic aorta and pancreatic vascular bed

1. The endothelium-dependent relaxation of blood vessels induced by P2Y-purinoceptor activation has often been shown to involve prostacyclin and/or nitric oxide (NO) release. In this work, we have investigated the mechanisms involved in the relaxant effect of the P2Y agonist, adenosine -5'-O-(2-thiodiphosphate) (ADP beta S) using two complementary preparations: rat pancreatic vascular bed and aortic ring. 2. On the pancreatic vascular bed, ADP beta S (1.5 and 15 microM) infused for 30 min induced a concentration-dependent vasodilatation; it was progressive during the first 10 min (first period) and sustained from 10 to 30 min (second period). Indomethacin (10 microM) delayed ADP beta S-induced vasodilatation (1.5 and 15 microM) by about 6 min. N omega-nitro-L-arginine methyl ester (L-NAME) (200 microM) suppressed the relaxation for about 5 min but thereafter ADP beta S at the two concentrations progressively induced an increase in the flow rate. Even the co-administration of L-NAME and indomethacin did not abolish the ADP beta S-induced vasorelaxation. 3. On 5-hydroxy tryptamine (5-HT) precontracted rings mounted in isometric conditions in organ baths, we observed that ADP beta S induced a concentration-dependent relaxation of rings with a functional endothelium; this effect was stable for 25 min. The ADP beta S relaxant effect was strongly inhibited by Reactive Blue 2 (30 microM) and was suppressed by pretreatment of rings with saponin (0.05 mg ml-1 for 30 min), which also abolished the acetylcholine-induced relaxation. 4. ADP beta S-induced relaxation of 5-HT precontracted rings is largely inhibited by indomethacin (100 or 10 microM) or L-NAME (100 microM). 5. We conclude that: the ADP beta S-induced relaxation is endothelium-dependent, mediated by P2Y-purinoceptors, and at least in part linked to NO and prostacyclin release, depending on the preparation used. Furthermore, on the pancreatic vascular bed, (an)other mechanism(s) than prostacyclin and NO releases may be involved in ADP beta S-induced vasodilatation.

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.

Some P2 purinergic agonists increase cytosolic calcium but not inositol 1,4,5-trisphosphate in isolated rat hepatocytes

Based on the capacity to increase IP3 (inositol 1,4,5-trisphosphate), P2 purinergic agonists can be subdivided into two classes: ATP, ADP, UTP, 2deoxyATP, NAD and GTP significantly increased IP3 levels whereas ADP beta S, 2MeSATP, NADP, alpha, beta MeATP, beta, gamma MeATP and ATP alpha S had only a minor, non-significant effect. Irrespective of their potency to increase IP3, all agonists were full glycogenolytic agonists and they all increased cytosolic calcium. With ATP and NAD, IP3 increasing agonists, and 2MeSATP and ADP beta S, non-IP3 increasing agonists, we found that the initial calcium response appeared to be an 'all or none' phenomenon, small amounts of the agonists being either ineffective or equally effective as high amounts. The minimal amount of an agonist needed to initiate a calcium increase and to promote glycogenolysis was very similar. In the absence of extracellular calcium, both groups of purinergic agonists (tested with ATP and 2MeSATP) were equally able to release calcium from intracellular stores. Cells with emptied intracellular calcium stores rapidly took up extracellular calcium upon treatment with ATP or 2MeSATP, the latter being the most potent. It seems therefore that all nucleotides tested increased cytosolic calcium and activated phosphorylase in a very similar way but some nucleotides had no effect on the levels of IP3.

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.

P2 purinergic receptors for diadenosine polyphosphates in the nervous system

1. The actions of diadenosine polyphosphates, diadenosine tetraphosphate (Ap4A), diadenosine pentaphosphate (Ap5A) and diadenosine hexaphosphate (Ap6A) in the nervous system have been reviewed. 2. In the peripheral nervous system, diadenosine polyphosphates bind to P2-purinergic receptors such as the P2Y in chromaffin cells and Torpedo synaptosomes, P2X in vas deferens and urinary bladder and also Torpedo synaptosomes and P2U in endothelial chromaffin cells. 3. In the central nervous system ApnA compounds can act through P2X-purinoceptors opening cation channels in nodose ganglion neurones. Diadenosine polyphosphates bind to a P2d-purinergic receptor in rat brain synaptic terminals and hippocampus, linked to protein kinase C (PKC) activation. 4. P4-purinoceptors are specific receptors for diadenosine polyphosphates, coupled to the Ca2+ influx, in the central synapses. This purinoceptor is not activated by ATP and synthetic analogs. The P4-purinoceptor could act as a positive modulator of the synaptic transmission, giving even more importance to diadenosine polyphosphates as neurotransmitters.

Current state of purinoceptor research

It is hoped that this summary of the history and current status of purinoceptors will convince readers that receptors for purines are now established alongside other well-known extracellular messenger systems. These receptors are primitive, widespread and serve many different systems. Receptors of adenosine (P1-purinoceptors) are clearly different from receptors of ATP (P2-purinoceptors). As for other major transmitters such as acetylcholine, GABA, glutamate and 5-HT, receptors of two major families are activated by ATP, one (the P2X-purinoceptor family) mediates fast responses via ligand-gated ion channels, while the other (the P2Y-purinoceptor family) mediates slower responses via G-proteins (see Table 3). Subclasses of these two families have been suggested on the basis of recent molecular biology studies and the development of new selective agonists and antagonists (Abbracchio and Burnstock, 1994). It would indeed be helpful if the work on purinoceptors could be extended to studies of their chemical structure employing crystallography.

Review: Ca(2+)-mobilizing receptors for ATP and UTP

Extracellular nucleotides are potent Ca2+ mobilizing agents. A variety of receptors for extracellular ATP are recognised. Some are involved in fast neuronal transmission and operate as ligand-gated ion channels. Others are involved in the paracrine or autocrine modulation of cell function. Many receptors of this type are coupled to phosphoinositide-specific phospholipase C and, in some cases, other phospholipases. One of these receptors (P2z), however, also appears to operate, at least in part, as a ligand-gated ion channel. Pharmacological data suggest that one nucleotide receptor subtype (currently designated P2U) responds selectively to either a purine nucleotide, ATP, or a pyrimidine nucleotide, UTP. According to an alternative view, ATP and UTP recognise distinct receptors. Because of the diversity of receptors for extracellular nucleotides this may be the case in some cells. Nevertheless, a G-protein coupled receptor that confers both ATP and UTP sensitivity has been cloned, expressed in cultured cell lines and sequenced. This receptor appears to have two ligand binding domains that may partially overlap. The nature of this overlap is discussed and a simple model presented. Activation of the receptor protein via one or other ligand binding domain may underlie some of the more subtle differences between the effects of ATP and UTP.

Signal transduction by G proteins: 1994 edition

Findings from the last two years in signal transduction research, including the elucidation of the crystal structure of alpha1, the uncovering of multiple roles for lipidation, the mimicry of receptor action with peptides, and both the in vitro reconstitution of inhibition of adenylyl cyclase and the in cell reconstitution of receptor-G protein coupling in transient and stable expression studies, are integrated into a "current" view of the receptor --> G protein --> effector pathway. The question is raised whether receptor or betagamma is the nucleotide exchange factor, and the central participation of Mg2+ in G protein activation and the change in affinity of the G protein for Mg2+ during receptor-stimulated activation are stressed.

The increase in the intracellular Ca2+ concentration induced by mechanical stimulation is propagated via release of pyrophosphorylated nucleotides in mammary epithelial cells

Mechanical stimulation of one mammary tumor cell in culture induced an increase in its intracellular calcium concentration which spread to surrounding cells. The increase in calcium can also be induced by addition of a solution in which cultured mammary tumor cells were stimulated by repeated pipetting (solution after pipetting cells, SAPC). The activity of the SAPC was completely abolished by treatment with snake venom phosphodiesterase or pyrophosphatase. Uridine triphosphate (UTP), uridine diphosphate (UDP) and ATP (1 microM each) were detected in the SAPC, whereas 5'-UMP and 5'-AMP were produced by phosphodiesterase digestion. A mixture of UTP, UDP and ATP (1 microM each) elicited a calcium response which was comparable to that induced by SAPC, while UTP, UDP or ATP alone at 1 microM elicited a small increase in calcium concentration in mammary tumor cells. Suramin, a competitive antagonist of P2 purinoceptors, diminished the spreading of the calcium wave induced by mechanical stimulation. It also blocked the responses to SAPC, UTP, UDP and ATP. These findings suggest that the mechanical stimulation results in the release of UTP, UDP and ATP into the extracellular space which mediates induction of the spreading calcium response via P2U-type purinoceptors.

Novel competitive antagonists for P2 purinoceptors

Binding of the radioligand [35S]adenosine 5'-O-(2-thiodiphosphate) (ADP beta 35S) to P2 gamma purinoceptors on turkey erythrocyte membranes was used to determine the affinity of suramin and various suramin congeners belonging to different structure classes (large urea, small urea, dibenzamides and benzamides) for these receptors. Suramin was shown to be a competitive antagonist with a Ki value of 7.3 +/- 2.2 microM. The simple benzamide compound XAMR0721 (8-(3,5-dinitrophenylene carbonylimino)-1,3,5-naphthalene trisulfonate, trisodium salt) displays a high affinity for the P2 gamma purinoceptor (Ki value of 19 +/- 6 microM). Similar to suramin, compound XAMR0721 is a competitive antagonist at P2 gamma purinoceptors. In contrast to suramin, which is a potent inhibitor of the ecto-nucleotidase activity in human blood cells (44 +/- 2% residual activity at 100 microM), compound XAMR0721 is hardly active in this assay (93 +/- 1% residual activity at 100 microM). So XAMR0721, the first competitive antagonist for P2 purinoceptors that is able to discriminate between P2 purinoceptor affinity and ecto-nucleotidase activity, is an interesting pharmacological tool for the characterization of P2 purinoceptor mediated effects.

Sensitivity of G-protein involved in endothelin-1-induced Ca2+ influx to pertussis toxin in porcine endothelial cells in situ

1. We designed a new method to determine quantitatively the intracellular Ca2+ concentration ([Ca2+]i) in endothelial cells in situ, using front-surface fluorometry and fura-2-loaded porcine aortic valvular strips. Using this method, we investigated the characteristics of the G-protein involved in endothelin-1 (ET-1)-induced changes in [Ca2+]i of endothelial cells in situ. 2. Endothelial cells were identified by specific uptake of acetylated-low density lipoprotein labelled with 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate (DiI-Ac-LDL). Double staining with DiI-Ac-LDL and fura-2 showed that the valvular strip was covered with a monolayer of endothelial cells and that the cellular component which contributed to the fura-2 fluorescence, [Ca2+]i signal, was exclusively endothelial cells. 3. ET-1 (10(-7) M) induced an elevation of [Ca2+]i consisting of two components: the first was a rapid and transient elevation to reach a peak, followed by a second, sustained elevation (the second phase). The first phase was composed of extracellular Ca(2+)-independent and -dependent components, while the second phase was exclusively extracellular Ca(2+)-dependent. The extracellular Ca(2+)-independent component of the first phase was due to the release of Ca2+ from intracellular storage sites. The second phase and part of the first phase of [Ca2+]i elevation were attributed to the influx of extracellular Ca2+. The Ca2+ influx component was completely inhibited by 10(-3) M Ni2+ but was not affected by 10(-5) M diltiazem. 4. Pertussis toxin (IAP) markedly inhibited the extracellular Ca2+-dependent elevation of [Ca2+]j, but had no effect on the extracellular Ca2+-independent elevation of [Ca2+], caused by ET-1 (10-7M).5. Bradykinin (10-7 M) or ATP (10- 5M) elevated [Ca2+]i and these responses also consisted of extracellular Ca2+-independent and extracellular Ca2+-dependent components. IAP had no effect on either component of the [Ca2+]i elevation induced by bradykinin or ATP.6. From these findings we conclude that, in porcine endotheliel cells in situ, ET-1 elevates [Ca2+]i as are result of a Ca2+ influx component from the extracellular space and release of intracelluarly stored Ca2+ .The Ca2+ influx is regulated by an IAP-sensitive G-protein, while the release of Ca2+ from the intracellular store is not.

Single-cell fura-2 microfluorometry reveals different purinoceptor subtypes coupled to Ca2+ influx and intracellular Ca2+ release in bovine adrenal chromaffin and endothelial cells

ATP and adenosine(5')tetraphospho(5')adenosine (Ap4A), released from adrenal chromaffin cells, are potent stimulators of endothelial cell function. Using single-cell fura-2 fluorescence recording techniques to measure free cytosolic Ca2+ concentration ([Ca2+]i), we have investigated the role of purinoceptor subtypes in the activation of cocultured chromaffin and endothelial cells. ATP evoked concentration-dependent [Ca2+]i rises (EC50 = 3.8 microM) in a subpopulation of chromaffin cells. Both ATP-sensitive and -insensitive cells were potently activated by nicotine, bradykinin and muscarine. Reducing extracellular free Ca2+ concentration to around 100 nM suppressed the [Ca2+]i transient evoked by ATP but not the [Ca2+]i response to bradykinin. ATP-sensitive chromaffin cells were also potently stimulated by 2-methylthioadenosine triphosphate (2MeSATP; EC50 = 12.5 microM) and UTP, but did not respond to either adenosine 5'-[beta-thio]diphosphate (ADP[beta S]), a P2Y receptor agonist, adenosine 5'-[alpha,beta-methylene]triphosphate (pp-[CH2]pA), a P2X agonist or AMP. Adrenal endothelial cells displayed concentration-dependent [Ca2+]i responses when stimulated with ATP (EC50 = 0.86 microM), UTP (EC50 = 1.6 microM) and 2MeSATP (EC50 = 0.38 microM). 2MeSATP behaved as a partial agonist. Ap4A and ADP[beta S] also raised the [Ca2+]i in endothelial cells, whereas AMP and pp[CH2]pA were ineffective. Lowering extracellular free Ca2+ to around 100 nM did not affect the peak ATP-evoked [Ca2+]i rise in these cells. It is concluded that different purinoceptor subtypes are heterogeneously distributed among the major cell types of the adrenal medulla. An intracellular Ca(2+)-releasing P2U-type purinoceptor is specifically localized to adrenal endothelial cells, while a subpopulation of chromaffin cells expresses a non-P2X, non-P2Y subtype exclusively coupled to Ca2+ influx.

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.

Extracellular ATP triggers two functionally distinct calcium signalling pathways in PC12 cells

We have investigated the effects of extracellular ATP on Ca2+ signalling, and its relationship to secretion in rat pheochromocytoma (PC12) cells. In single cells, extracellular ATP evoked two very distinct subcellular distributions of intracellular calcium concentration ([Ca2+]i), only one of which could be mimicked by the pyrimidine nucleotide UTP, suggesting the involvement of more than one cell surface receptor in mediating the ATP-induced responses. ATP and UTP were equipotent in activating a receptor leading to inositol phosphate production and the mobilisation of intracellular Ca2+. In some cells (19%) this rise in [Ca2+]i initiated at a discrete site and then propagated across the cell in the form of a Ca2+ wave. In addition to mobilising intracellular Ca2+ through a ‘nucleotide’ receptor sensitive to ATP and UTP, the results indicate that ATP also activates divalent cation entry through an independent receptor-operated channel. Firstly, ATP-induced entry of Ca2+ or Mn2+ was independent of Ca2+ mobilisation, as prior treatment of cell populations with UTP abolished the ATP-evoked release of intracellular Ca2+ stores, but left the Ca(2+)- and Mn(2+)-entry components uneffected. Secondly, although UTP and ATP were equally effective in generating inositol phosphates, only ATP stimulated divalent cation entry, indicating that ATP-activated influx was independent of phosphoinositide turnover. Thirdly, single cell experiments revealed a subpopulation of cells that responded to ATP with divalent cation entry without mobilising Ca2+ from intracellular stores. Lastly, the dihydropyridine antagonist, nifedipine, reduced the ATP-induced rise in [Ca2+]i by only 24%, suggesting that Ca2+ entry was largely independent of L-type voltage-operated Ca2+ channels. The Ca2+ signals could also be distinguished at a functional level. Activation of ATP-induced divalent cation influx was absolutely required to evoke transmitter release, because ATP triggered secretion of [3H]dopamine only in the presence of external Ca2+, and UTP was unable to promote secretion, irrespective of the extracellular [Ca2+]. The results suggest that the same extracellular stimulus can deliver different Ca2+ signals into the same cell by activating different Ca2+ signalling pathways, and that these Ca2+ signals can be functionally distinct.

Purinoceptors: are there families of P2X and P2Y purinoceptors?

There has been an exponential growth in interest in purinoceptors since the potent effects of purines were first reported in 1929 and purinoceptors defined in 1978. A distinction between P1 (adenosine) and P2 (ATP/ADP) purinoceptors was recognized at that time and later, A1 and A2, as well as P2x and P2y subclasses of P1 and P2 purinoceptors were also defined. However, in recent years, many new subclasses have been claimed, particularly for the receptors to nucleotides, including P2t, P2z, P2u(n) and P2D, and there is some confusion now about how to incorporate additional discoveries concerning the responses of different tissues to purines. The studies beginning to appear defining the molecular structure of P2-purinoceptor subtypes are clearly going to be important in resolving this problem, as well as the introduction of new compounds that can discriminate pharmacologically between subtypes. Thus, in this review, on the basis of this new data and after a detailed analysis of the literature, we propose that: (1) P2X(ligand-gated) and P2Y(G-protein-coupled) purinoceptor families are established; (2) four subclasses of P2X-purinoceptor can be identified (P2X1-P2X4) to date; (3) the variously named P2-purinoceptors that are G-protein-coupled should be incorporated into numbered subclasses of the P2Y family. Thus: P2Y1 represents the recently cloned P2Y receptor (clone 803) from chick brain; P2Y2 represents the recently cloned P2u (or P2n) receptor from neuroblastoma, human epithelial and rat heart cells; P2Y3 represents the recently cloned P2Y receptor (clone 103) from chick brain that resembles the former P2t receptor; P2Y4-P2Y6 represent subclasses based on agonist potencies of newly synthesised analogues; P2Y7 represents the former P2D receptor for dinucleotides. This new framework for P2 purinoceptors would be fully consistent with what is emerging for the receptors to other major transmitters, such as acetylcholine, gamma-aminobutyric acid, glutamate and serotonin, where two main receptor families have been recognised, one mediating fast receptor responses directly linked to an ion channel, the other mediating slower responses through G-proteins. We fully expect discussion on the numbering of the different receptor subtypes within the P2X and P2Y families, but believe that this new way of defining receptors for nucleotides, based on agonist potency order, transduction mechanisms and molecular structure, will give a more ordered and logical approach to accommodating new findings. Moreover, based on the extensive literature analysis that led to this proposal, we suggest that the development of selective antagonists for the different P2-purinoceptor subtypes is now highly desirable, particularly for therapeutic purposes.

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)

Blockade of ATP binding site of P2 purinoceptors in rat parotid acinar cells by isothiocyanate compounds

Extracellular ATP activates a P2Z-type purinergic receptor (purinoceptor) in rat parotid acinar cells that increases the intracellular free Ca2+ concentration via the entry of extracellular Ca2+ through an ATP-sensitive cation channel (Soltoff et al., Am J Physiol 262: C934-C940, 1992). To learn more about the ATP binding site of the purinoceptor, we examined the effects of several stilbene isothiocyanate analogs of DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid), which block the binding of [32P]ATP to intact parotid cells (McMillian et al., Biochem J 255:291-300, 1988) and blocked the activation of the P2Z purinoceptor. The ATP-stimulated 45Ca2+ uptake was blocked by DIDS, H2DIDS (dihydro-DIDS; 4,4'-diisothiocyanatodihydrostilbene-2,2'-disulfonic acid), and SITS (4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid), but not by DNDS (4,4'-dinitrostilbene-2,2'-disulfonic acid), a stilbene disulfonate compound lacking isothiocyanate (SCN-) groups, or by KSCN. The potency of the stilbene disulfonates was related to the number of isothiocyanate groups on each compound. Under the experimental conditions, the IC50 value of DIDS (approximately 35 microM), which has two SCN-groups, was much lower than that of SITS (approximately 125 microM), which has only one SCN-group. The inhibitory effects of DIDS appeared to be much more potent than those of SITS due to the kinetics of their binding to the purinoceptors. Eosin-5-isothiocyanate (EITC) and fluorescein-5-isothiocyanate (FITC), non-stilbene isothiocyanate compounds with single SCN-groups, also blocked the response to ATP and were less potent than DIDS. Trinitrophenyl-ATP (TNP-ATP), an ATP derivative that is not an effective agonist of the parotid P2Z receptor, blocked the covalent binding of DIDS to the plasma membrane, suggesting that ATP and DIDS bind to the same site. Reactive Blue 2 (Cibacron Blue 3GA), an anthraquinone-sulfonic acid derivative that is a noncovalent purinergic antagonist, also blocked the covalent binding of DIDS to the plasma membrane. These results suggest that isothiocyanate compounds interact with the ATP binding site of this P2 purinoceptor, and that isothiocyanate groups make an important contribution in determining the effectiveness of the stilbene disulfonate compounds in blocking the binding of nucleotide agonists to this purinoceptor.

Ap4A and ADP-beta-S binding to P2 purinoceptors present on rat brain synaptic terminals

1. Diadenosine tetraphosphate (Ap4A) a dinucleotide stored and released from rat brain synaptic terminals presents two types of affinity binding sites in synaptosomes. When [3H]-Ap4A was used for binding studies a Kd value of 0.10 +/- 0.014 nM and a Bmax value of 16.6 +/- 1.2 fmol mg-1 protein were obtained for the high affinity binding site from the Scatchard analysis. The second binding site, obtained by displacement studies, showed a Ki value of 0.57 +/- 0.09 microM. 2. Displacement of [3H]-Ap4A by non-labelled Ap4A and P2-purinoceptor ligands showed a displacement order of Ap4A > adenosine 5'-O-(2-thiodiphosphate) (ADP-beta-S) > 5'-adenylyl-imidodiphosphate (AMP-PNP) > alpha,beta-methylene adenosine 5'-triphosphate (alpha,beta-MeATP) in both sites revealed by the Ki values of 0.017 nM, 0.030 nM, 0.058 nM and 0.147 nM respectively for the high affinity binding site and values of 0.57 microM, 0.87 microM, 2.20 microM and 4.28 microM respectively for the second binding site. 3. Studies of the P2-purinoceptors present in synaptosomes were also performed with [35S]-ADP-beta-S. This radioligand showed two binding sites the first with Kd and Bmax values of 0.11 +/- 0.022 nM and 3.9 +/- 2.1 fmol mg-1 of protein respectively for the high affinity binding site obtained from the Scatchard plot. The second binding site showed a Ki of 0.018 +/- 0.0035 microM obtained from displacement curves. 4. Competition studies with diadenosine polyphosphates of [35S]-ADP-beta-S binding showed a displacement order of Ap4A > Ap5A > Ap6A in the high affinity binding site and Ki values of 0.023 nM, 0.081 nM and 5.72 nM respectively. The second binding site potency order was Ap5A> Ap4A > Ap6A,with Ki values of 0.28 microM, 0.53 microM and 5.32 microM respectively.5. Displacement studies of [35S]-ADP-beta-S with P2-purinoceptor agonists showed the following potency pattern: ADP-beta-S > AMP-PNP >alpha,beta-MeATP with Ki values of 0.021 nM, 0.029 nM 0.215 nM respectively in the high affinity binding site. 2-Methylthio-adenosine 5'-triphosphate (2MeSATP) was unable to displace [35S]-ADP-beta-S in this binding site. The second binding site showed a profile of ADP-beta-S> a,beta-MeATP> AMP-PNP > 2MeSATP and Ki values of 0.0 18 microM, 0.212 microM, 0.481 microM and 18.04 microM respectively.6. These studies suggest the presence of a new P2-purinoceptor in rat brain synaptosomes with high affinity for diadenosine polyphosphates which we tentatively designate as P2d.

The complex interaction of ATP and UTP with isolated hepatocytes. How many receptors?

1. ATP exerts multiple receptor-mediated effects on isolated hepatocytes: glycogenolysis through the activation of glycogen phosphorylase (cAMP-independent, IP3/calcium-mediated), inactivation of glycogen synthase, inhibition of the glucagon effect on cAMP, activation of phospholipase D. The fact that some of these effects can be selectively altered and that they are not, or differently, reproduced by some other analogues of ATP, suggests the presence of more than one receptor. (i) Pertussis toxin abolishes the anti-glucagon effect of ATP without affecting its glycogenolytic effect. (ii) Single cell calcium measurements reveal major differences between ATP and ADP, (iii) 2MeSATP and ADP beta S, in clear contrast to ATP, barely increase the levels of IP3 and their glycogenolytic effects is completely blocked by phorbol ester treatment of hepatocytes. (iv) 2MeSATP differs from ADP beta S since it has no anti-glucagon effect. 2. Effects of UTP on isolated hepatocytes so far do not show any difference with effects of ATP, suggesting interaction with the same receptor(s). 3. It is proposed that liver plasma membranes contain (at least) three different receptors mediating (a) the activation of phospholipase C, (b) the activation of phospholipase D and (c) the inhibition of adenylate cyclase.

Characterization of the effects of adenosine 5'-[beta-thio]-diphosphate in rat liver

1. In rat liver cells micromolar concentrations of adenosine 5'-[beta-thio]diphosphate (ADP beta S), activate glycogen phosphorylase by an adenosine 3':5'-cyclic monophosphate (cyclic AMP)- independent mechanism. 2. As with adenosine 5'-triphosphate (ATP), ADP beta S also inhibits the rise in cyclic AMP after glucagon. 3. Cytosolic Ca2+ measured in single cells is rapidly increased with a pattern similar for ADP beta S and for ATP. 4. At variance with ATP, ADP beta S hardly increases inositol 1,4,5-trisphosphate (IP3) levels. 5. Phorbol myristic acetate, which inhibits only slightly the glycogenolytic effect of ATP, almost completely abolishes this effect of ADP beta S. 6. With adenosine 5'-[beta-[35S]thio]diphosphate (ADP beta[35S]) as radioligand, we detected specific purinoceptors on rat liver plasma membranes. Binding consists of a major binding component with KD = 0.7 microM and Bmax = 51 pmol mg-1 of protein, probably mediating the activation of glycogen phosphorylase, and a minor high affinity, low capacity binding component with no obvious function. 7. It is concluded that the differences in biological effects between ATP and ADP beta S may involve different receptors and/or different transduction mechanisms and that ADP beta[35S] can be used to detect the specific binding sites for ADP beta S.

Binding of the radioligand [35S]adenosine 5'-O-(2-thiodiphosphate) and intracellular calcium response in rat liver parenchymal cells

The use of the radioligand [35S]adenosine 5'-O-(2-thiodiphosphate) (ADP beta 35S) for the determination of P2y-purinoceptors on turkey erythrocyte membranes has recently been described. In the present study, we were able to demonstrate specific binding of this radioligand in intact rat liver parenchymal cells. Within 10 min a thermodynamic equilibrium was obtained which lasted for 25 min with a subsequent decline. Displacement studies with several nucleotides were performed yielding Ki values of 1.5 +/- 0.47 microM for UTP, 1.8 +/- 0.35 microM for adenosine 5'-O-(2-thiodiphosphate) (ADP beta S), 31 +/- 6.2 microM for ATP and 35 +/- 6.1 microM for GTP. In addition, we showed that ADP beta 35S is highly resistant to degradation by ecto-nucleotidases, with only 14.5 +/- 1.4% of total ADP beta 35S present being degraded after 1 hr, and that the binding of ADP beta 35S to its binding sites was modulated by EDTA. The Ki value of ATP shifted to 8.1 +/- 1.2 microM upon the addition of 1 mM EDTA to the incubation medium. In these rat liver parenchymal cells all nucleotides promoted calcium entry in a dose-dependent manner with EC50 values of 3.5 +/- 0.22 microM for UTP, 20.7 +/- 3.1 microM for ATP, 38.3 +/- 6.4 microM for ADP beta S and 73.6 +/- 13.7 microM for GTP, with GTP being a partial agonist. Based on the data derived from the present study we discuss the possible correlation between binding and functional experiments and conclude that the described receptor resembles most closely the P2u-purinoceptor and/or "nucleotide receptor", in that UTP is at least as active as ATP.

Effect of P2Y agonists on adenosine transport in cultured chromaffin cells

Adenosine transport in cultured chromaffin cells was inhibited by purinergic P2y-receptor agonists without significant changes in the affinity constant, the values being between 1 +/- 0.4 and 1.6 +/- 0.6 microM. The Vmax parameter was modified significantly, being 40 +/- 1.0, 26 +/- 5.0, 32 +/- 3.0, and 22 +/- 4.7 pmol/10(6) cells/min for control, adenosine-5'-O-(2-thiodiphosphate), 5'-adenylylimidodiphosphate, and P1,P4-di(adenosine-5'-) tetraphosphate (Ap4A) (100 microM for every effector), respectively. Ap4A, a physiological ligand for P2y receptors in chromaffin cells, showed the highest inhibitory effect (45%). This transport inhibition is explained by an increase in the cytosolic Ca2+ concentration ([Ca2+]i) and the activation of protein kinase C (PKC). Experiments of [Ca2+]i measurement with the fura-2 technique showed that P2y agonists, as well as bradykinin, were able to increase [Ca2+]i, this effect being independent of the presence of extracellular Ca2+. The peptide bradykinin, determined to be coupled to phosphatidylinositol hydrolysis and internal Ca2+ mobilization in chromaffin cells, exhibited a behavior similar to that of P2y agonists in adenosine transport inhibition (39%). P2y agonists and bradykinin increased PKC activity associated with the membrane fraction (about 50% increase in particulate PKC activity with respect to controls). The present studies suggest that adenosine transport is regulated by P2y-purinergic receptors mediated via Ca2+ mobilization and PKC activation.

SYNTHESIS AND BIOLOGICAL ACTIVITY OF NOVEL 2-THIO DERIVATIVES OF ATP

2-Alkylthio analogues of adenosine 5'-triphosphate were synthesized and evaluated as P2y purinoceptor agonists. ATP and analogues transiently increased intracellular Ca2+ levels in C6 glioma cells and in skeletal muscle derived myotubes in culture. Most derivatives were resistant to stepwise dephosphorylation by ecto-ATPases.

ATP activates P2x-contracting and P2y-relaxing purinoceptors in the smooth muscle of mouse vas deferens

1. The mechanism for the low potency of exogenous ATP in producing contraction at the P2x-purinoceptors in the smooth muscle of the mouse vas deferens (VD) was examined. 2. The measure of the breakdown of ATP in contact with the VD showed that its degradation was limited and did not account for its weak contractile effect. 3. Externally applied, ATP induced a small and transient contraction but a marked and prolonged increase of the cytosolic Ca2+ concentration ([Ca2+]i), which suggests an efficient binding to the P2x-purinoceptors. Such a calcium-force dissociation was not observed with beta, gamma-methylene ATP (beta, gamma-Me-ATP), a structural ATP analogue. 4. The force response of precontracted VD to ATP was biphasic, consisting of a small initial contraction followed by a sustained marked relaxation. In contrast, beta, gamma-Me-ATP elicited a pronounced contraction without ensuing relaxation. 5. ATP was more potent than adenosine in producing relaxation, and the relaxation was not antagonized by 8-phenyltheophylline, suggesting the activation of P2-purinoceptors. 6. For this relaxation, the rank order of potency was 2-methyl-thio-ATP (2-MeSATP) > ATP > beta, gamma-Me-ATP, which is characteristic for the P2y-purinoceptors. 7. Reactive Blue 2, a P2y-purinoceptor antagonist, was found to reduce the relaxation mediated by ATP. 8. These results indicate that ATP acts in VD not only on contracting but also on relaxing P2-purinoceptors, eliciting thereby overlapping opposite effects. In VD, the classical low potency of ATP or contraction is thus not explained by its low bioavailability or its low binding, but rather by its low specificity for the contracting P2x-purinoceptors, leading to the activation of the relaxing P2y-purinoceptors.

Extracellular nucleotides elevate [Ca2+]i in rat osteoblastic cells by interaction with two receptor subtypes

Extracellular nucleotides, through interaction with specific cell-surface receptors, mediate a variety of biological responses, including elevation of cytosolic free Ca2+ concentration ([Ca2+]i) in a number of cell types. The effects of nucleotides on [Ca2+]i in the rat osteoblastic cell line UMR-106 were studied by fluorescence spectrophotometry of indo-1-loaded cells. In response to ATP (100 microM), [Ca2+]i rose to peaks 228 +/- 16 nM (n = 59) above baseline (85 +/- 3 nM) before returning to near basal levels. Half-maximal elevation of [Ca2+]i was observed at an ATP concentration of 3 +/- 1 microM, consistent with a high-affinity interaction. The response arose primarily by release of Ca2+ from internal stores. UTP, ADP, and 2-methylthioadenosine 5'-triphosphate also induced Ca2+ transients, whereas adenosine, AMP, CTP, and TTP did not, demonstrating specificity. Responsiveness to adenosine 5'-O-(3-thiotriphosphate) and inhibition by Mg2+ of the response to ATP indicated that signaling was not dependent on nucleotide hydrolysis. Ca2+ responses to ADP, ATP, and UTP, added sequentially or simultaneously, were consistent with the presence of two distinct P2-purinoceptor subtypes, both linked to Ca2+ mobilization. ADP appeared to interact selectively with one receptor, whereas ATP and UTP interacted selectively with the other. After maximal stimulation with ATP, subsequent responses to ATP were abolished. However, removal of ATP from the extracellular medium rapidly restored responsiveness, suggesting that, with continued receptor occupation, there is time-dependent inactivation of the Ca2+ signaling pathway. Our findings indicate that extracellular nucleotides elevate [Ca2+]i in osteoblastic cells through interaction with two receptor subtypes.(ABSTRACT TRUNCATED AT 250 WORDS)

Interleukin-1 beta enhances the response of human articular chondrocytes to extracellular ATP

It has been observed that both interleukin-1 (IL-1) and extracellular ATP stimulate the production of prostaglandin E (PGE) by human articular chondrocytes in monolayer culture. The combined effects of recombinant human IL-1 beta and ATP were therefore studied using these cells. IL-1 beta rapidly enhanced the response to a maximally effective concentration of ATP (100 microM). On continuous exposure of the cells to the cytokine, its effect was greatest after approx. 24 h and tended to decline thereafter. The enhancement of the response to 100 microM ATP by IL-1 beta was dose-dependent. Removal of IL-1 beta prior to treating the cells with 100 microM ATP did not affect the degree of enhancement of the response. The effect of the cytokine on the response to suboptimal concentrations of extracellular ATP was also tested. IL-1 beta lowered the minimum concentration of ATP required to elicit an increase in the production of PGE by human articular chondrocytes. These findings are of interest, since they indicate a synergistic interaction between a cytokine and a purinergic agonist. Moreover, since both the sensitivity of the cells to extracellular ATP and the maximum response to this agent were enhanced, it is possible that IL-1 modulates more than one step in the process of P2-purinoceptor-mediated stimulation of PGE production. These observations may be relevant to the pathogenesis of some forms of arthritis.

ATP activates a cation-permeable pathway in rat parotid acinar cells

Effects of several purinergic receptor agonists were examined on rat parotid acinar cells. Extracellular ATP stimulated 45Ca2+ uptake into isolated rat parotid acinar cells in a concentration-dependent fashion (EC50 approximately 125 microM ATP) at a maximum rate of approximately 6 nmol.mg protein-1.min-1. In the absence of extracellular Na+, ATP increased the uptake rate by greater than 100%. Increasing concentrations of extracellular Na+ reduced the ATP-stimulated rate of 45Ca2+ entry in a graded fashion (IC50 16.6 mM), suggesting that Ca2+ and Na+ compete for entry. Uptake rate was not reduced when intracellular Ca2+ was buffered with 1,2-bis(2-aminophenoxy)ethane-N,N,N'N'-tetraacetic acid, indicating that the effects of ATP were not initiated by an elevation in intracellular free Ca2+ concentration. 3-O-(4'-benzoyl)benzoyl-ATP was much more potent (EC50 approximately 4 microM) and stimulated Ca2+ influx at a greater rate (approximately 12 nmol.mg protein-1.min-1) than ATP. Other nucleotide analogues, including adenosine 5'-O-(3-thiotriphosphate), 2-methylthio-ATP, and 5'-adenylylimidodiphosphate, were much less effective than ATP. ATP produced a biphasic effect on membrane potential: an initial hyperpolarization was followed by a rapid depolarization. The depolarization was greatly reduced in the absence of extracellular Na+, but not in the absence of extracellular Ca2+, indicating that the majority of the depolarizing current was due to Na+ entry. Effects of ATP on the membrane potential were distinguishable from those of the Ca2+ ionophore ionomycin and the muscarinic agonist carbachol. Depolarization of the cells by gramicidin or K+ did not produce an increase in 45Ca2+ uptake.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms by which extracellular ATP and UTP stimulate the release of prostacyclin from bovine pulmonary artery endothelial cells

Extracellular ATP and UTP caused increases in the concentration of cytoplasmic free calcium ([Ca2+]i) and the intracellular level of inositol 1,4,5-trisphosphate (IP3), a second messenger for calcium mobilization, prior to the release of prostacyclin (PGI2) from cultured bovine pulmonary artery endothelial (BPAE) cells. The agonist specificity and dose-dependence were similar for nucleotide-mediated increases in IP3 levels, [Ca2+]i and PGI2 release. An increase in [Ca2+]; and PGI2 release was observed after addition of ionomycin, a calcium ionophore, to BPAE cells incubated in a calcium-free medium. The addition of ATP to the ionomycin-treated cells caused no further increase in [Ca2+]i or PGI2 release. The inability of ATP to cause an increase in [Ca2+]i or PGI2 release in ionomycin-treated cells was apparently due to the ionomycin-dependent depletion of intracellular calcium stores since the subsequent addition of extracellular calcium caused a significant increase in both [Ca2+]i and PGI2 release. Introduction of BAPTA, a calcium buffer, into BPAE cells inhibited ATP-mediated increases in [Ca2+]i and PGI2 release, further evidence that PGI2 release is dependent upon an increase in [Ca2+]i. The increase in [Ca2+]i elicited by ATP apparently caused the activation of a calmodulin-dependent phospholipase A2 since trifluoperazine, an inhibitor of calmodulin, and quinacrine, an inhibitor of phospholipase A2, prevented the stimulation of PGI2 release by ATP. Furthermore, ATP caused the specific hydrolysis of [14C]arachidonyl-labeled phosphatidylcholine and the generation of free arachidonic acid, the rate-limiting substrate for PGI2 synthesis, prior to the release of PGI2 from BPAE cells. These findings suggest that the increase in PGI2 release elicited by ATP and UTP is at least partially dependent upon a phospholipase C-mediated increase in [Ca2+]i and the subsequent activation of a phosphatidylcholine-specific phospholipase A2. ATP analogs modified in the adenine base or phosphate moiety caused PGI2 release with a rank order of agonist potency of adenosine 5'-O-(2-thiodiphosphate) (ADP beta S) greater than 2-methylthioATP (2-MeSATP) greater than ATP, whereas alpha, beta methyleneATP and beta, gamma methyleneATP had no effect on PGI2 release.