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

P2 Receptors in Cardiac Myocyte Pathophysiology and Mechanotransduction

ATP is a major energy source in the mammalian cells, but it is an extracellular chemical messenger acting on P2 purinergic receptors. A line of evidence has shown that ATP is released from many different types of cells including neurons, endothelial cells, and muscle cells. In this review, we described the distribution of P2 receptor subtypes in the cardiac cells and their physiological and pathological roles in the heart. So far, the effects of external application of ATP or its analogues, and those of UTP on cardiac contractility and rhythm have been reported. In addition, specific genetic alterations and pharmacological agonists and antagonists have been adopted to discover specific roles of P2 receptor subtypes including P2X4-, P2X7-, P2Y2- and P2Y6-receptors in cardiac cells under physiological and pathological conditions. Accumulated data suggest that P2X4 receptors may play a beneficial role in cardiac muscle function, and that P2Y2- and P2Y6-receptors can induce cardiac fibrosis. Recent evidence further demonstrates P2Y1 receptor and P2X4 receptor as important mechanical signaling molecules to alter membrane potential and Ca²⁺ signaling in atrial myocytes and their uneven expression profile between right and left atrium.

Purinergic receptors in the splanchnic circulation

There is considerable evidence that purines are vasoactive molecules involved in the regulation of blood flow. Adenosine is a well known vasodilator that also acts as a modulator of the response to other vasoactive substances. Adenosine exerts its effects by interacting with adenosine receptors. These are metabotropic G-protein coupled receptors and include four subtypes, A(1), A(2A), A(2B) and A(3). Adenosine triphosphate (ATP) is a co-transmitter in vascular neuroeffector junctions and is known to activate two distinct types of P2 receptors, P2X (ionotropic) and P2Y (metabotropic). ATP can exert either vasoconstrictive or vasorelaxant effects, depending on the P2 receptor subtype involved. Splanchnic vascular beds are of particular interest, as they receive a large fraction of the cardiac output. This review focus on purinergic receptors role in the splanchnic vasomotor control. Here, we give an overview on the distribution and diversity of effects of purinergic receptors in splanchnic vessels. Pre- and post-junctional receptormediated responses are summarized. Attention is also given to the interactions between purinergic receptors and other receptors in the splanchnic circulation.

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.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

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

Activation and desensitization of the recombinant P2X1 receptor at nanomolar ATP concentrations

Activation and desensitization kinetics of the rat P2X1 receptor at nanomolar ATP concentrations were studied in Xenopus oocytes using two-electrode voltage-clamp recording. The solution exchange system used allowed complete and reproducible solution exchange in <0.5 s. Sustained exposure to 1-100 nM ATP led to a profound desensitization of P2X1 receptors. At steady-state, desensitization could be described by the Hill equation with a K1/2 value of 3.2 +/- 0.1 nM. Also, the ATP dependence of peak currents could be described by a Hill equation with an EC50 value of 0.7 microM. Accordingly, ATP dose-effect relationships of activation and desensitization practically do not overlap. Recovery from desensitization could be described by a monoexponential function with the time-constant tau = 11.6 +/-1.0 min. Current transients at 10-100 nM ATP, which elicited 0.1-8.5% of the maximum response, were compatible with a linear three-state model, C-O-D (closed-open-desensitized), with an ATP concentration-dependent activation rate and an ATP concentration-independent (constant) desensitization rate. In the range of 18-300 nM ATP, the total areas under the elicited current transients were equal, suggesting that P2X1 receptor desensitization occurs exclusively via the open conformation. Hence, our results are compatible with a model, according to which P2X1 receptor activation and desensitization follow the same reaction pathway, i.e., without significant C to D transition. We assume that the K1/2 of 3.2 nM for receptor desensitization reflects the nanomolar ATP affinity of the receptor found by others in agonist binding experiments. The high EC50 value of 0.7 microM for receptor activation is a consequence of fast desensitization combined with nonsteady-state conditions during recording of peak currents, which are the basis of the dose-response curve. Our results imply that nanomolar extracellular ATP concentrations can obscure P2X1 receptor responses by driving a significant fraction of the receptor pool into a long-lasting refractory closed state.

Adenosine 5'-triphosphate: a P2-purinergic agonist in the myocardium

ATP, besides an intracellular energy source, is an agonist when applied to a variety of different cells including cardiomyocytes. Sources of ATP in the extracellular milieu are multiple. Extracellular ATP is rapidly degraded by ectonucleotidases. Today ionotropic P2X(1--7) receptors and metabotropic P2Y(1,2,4,6,11) receptors have been cloned and their mRNA found in cardiomyocytes. On a single cardiomyocyte, micromolar ATP induces nonspecific cationic and Cl(-) currents that depolarize the cells. ATP both increases directly via a G(s) protein and decreases Ca(2+) current. ATP activates the inward-rectifying currents (ACh- and ATP-activated K(+) currents) and outward K(+) currents. P2-purinergic stimulation increases cAMP by activating adenylyl cyclase isoform V. It also involves tyrosine kinases to activate phospholipase C-gamma to produce inositol 1,4,5-trisphosphate and Cl(-)/HCO(3)(-) exchange to induce a large transient acidosis. No clear correlation is presently possible between an effect and the activation of a given P2-receptor subtype in cardiomyocytes. ATP itself is generally a positive inotropic agent. Upon rapid application to cells, ATP induces various forms of arrhythmia. At the tissue level, arrhythmia could be due to slowing of electrical spread after both Na(+) current decrease and cell-to-cell uncoupling as well as cell depolarization and Ca(2+) current increase. In as much as the information is available, this review also reports analog effects of UTP and diadenosine polyphosphates.

Do ATP and UTP involve cGMP in positive inotropism on rat atria?

ATP and UTP induced a dual inotropic effect in rat left atria: first a decrease and then an increase in contractile tension were observed. PPADS, an antagonist of P2X receptors, inhibited positive inotropism induced by ATP and alpha,beta-meATP. Chiefly, we investigated intracellular mechanisms responsible for the positive inotropism. We tested cromakalim and glibenclamide, an activator and an inhibitor, respectively, of ATP-sensitive K(+) channels. These compounds did not influence the effects of ATP. IBMX, a phosphodiesterase inhibitor, and H-7, an inhibitor of protein kinase C and cAMP-dependent protein kinase, did not modify the inotropic effects of ATP. Instead, H-8, an inhibitor of cAMP- and cGMP-dependent protein kinases, strongly inhibited the positive effects of both ATP and UTP, suggesting the possible involvement of cGMP in the inotropism. Also, LY 83583, an inhibitor of cGMP production, reduced positive inotropism by alpha,beta-meATP, ATP and UTP. Moreover, 8-Br-cGMP (50 microM), a stable analogue of cGMP, inhibited positive inotropism by all nucleotides. Lastly, we determined intracellular cGMP levels by RIA; the cyclic nucleotide increased during positive inotropism induced by ATP and UTP. The results regarding positive inotropism suggest that: (a) ATP acts through P2X receptors, while UTP may act by P2X, but also through PPADS-insensitive receptors; and (b) changes in intracellular cGMP concentration are involved in this inotropic effect.

Characterisation of ATP-induced facilitation of transmission in rat hippocampus

Superfusion of rat hippocampal slices with ATP induces a form of facilitation that has been poorly characterised. The present study has confirmed that at low concentrations of ATP (10 microM or less), an initial depression of evoked potential size is followed by a rebound facilitation which is not reproduced by alphabeta-methyleneATP, betagamma-methyleneATP, or the dinucleotide P1,P6-diadenosine hexaphosphate. The post-ATP facilitation could be prevented by the adenosine A1 receptor antagonists 8-phenyltheophylline or 1,3-dipropyl-8-cyclopentyltheophylline (50 nM), or superfusion of adenosine deaminase. The adenosine A2A receptor antagonist 8-(chlorostyryl)-caffeine did not affect the inhibition but prevented the post-ATP facilitation. The NMDA receptor antagonist 2-amino-5-phosphonopentanoic acid prevented the establishment of post-ATP facilitation. The post-ATP facilitation was also blocked by suramin at a concentration (50 microM) that does not block glutamate receptors. Suramin prevented the induction but not the maintenance phase of the post-ATP facilitation. The repeated induction of post-ATP facilitation by bursts of electrical stimulation designed to saturate the normal mechanisms of long-term potentiation prevented the induction of post-ATP facilitation. However, repeated applications of ATP to achieve saturation of its receptor did not prevent the subsequent induction of electrically evoked long-term potentiation. It is concluded that ATP can induce a form of synaptic facilitation which resembles only partially that induced by electrical stimulation and which may require the simultaneous activation of P1 and P2 receptors.

Presynaptic P2 receptors?

Although the emphasis in ATP research has been on postjunctional receptors, there is also evidence for presynaptic receptors regulating transmitter release in the autonomic nervous system. Recent work has attempted to identify similar mechanisms in the central nervous system. Some of the existing results can be explained by the metabolism of nucleotides to adenosine or adenosine 5'-monophosphate (AMP). However, studies of presynaptic effects using sensitive electrophysiological tests such as paired-pulse interactions indicate that nucleotides can act at presynaptic sites, but that their effects may be mediated by a release of adenosine. Results are also described which indicate that, under some conditions, nucleotides can mediate phenomena such as long-term potentiation, which probably involves a significant presynaptic element. In part these effects may involve a nucleotide-induced release of adenosine and the simultaneous activation of P1 and P2 receptors.

Complex hippocampal responses to ATP: fade due to nucleotidase inhibition and P2-receptor-mediated adenosine release

When ATP or the related stable analogue, betagamma-imidoATP, were applied to rat hippocampal slices showing population spikes larger than 5 mV peak-to-peak amplitude, a depression of spike size was obtained, which showed a marked fade during the 10-min period of superfusion. The inhibitory responses were prevented by adenosine deaminase or 8-phenyltheophylline. Adenosine responses showed no fade. alphabeta-MethyleneADP enhanced the fade, while suramin at 50 micrometer prevented the early component of the responses. The results suggest that in slices with large population spikes, inhibitory responses to nucleotides are partly due to their conversion to adenosine, and partly due to the activation of P2 receptors which trigger the release of endogenous adenosine.

Distribution of purinergic P2X receptors in the rat heart

The distribution of P2X purinergic receptor subtypes has been determined in relation to nerve varicosities in the rat heart with immunohistochemistry. Large clusters (about 1 microm diameter) of co-localised and sometimes co-extensive P2X1 and P2X3 receptors were found at sites of tyrosine hydroxylase (TH) positive axon varicosities in the atrium and the ventricle. Varicosities that were labelled with antibodies to the synaptic vesicle epitope SV2 were frequently labelled also with antibodies to P2X3, P2X5 and P2X6 but not always with antibodies to P2X1. Especially prominent were large numbers of small clusters (about 400 nm diameter) of co-localised P2X2 and P2X5 receptors on the sarcolemma unrelated to nerves at all. During development the 1 day-old heart possessed an abundance of co-localised P2X2 and P2X5 small receptor clusters on the sarcolemma. These observations are discussed in relation to the role of purinergic receptors in the mammalian heart.

Analysis of novel P2X subunit-specific antibodies in rat cardiac and smooth muscle

P2X receptors are cation-selective channels gated by extracellular adenosine triphosphate (ATP). There are relatively few known types of ligand-gated receptors. In vertebrates they include acetylcholine (Ach), 5-hydroxytryptamine (5-HT), gamma-aminobutyric acid (GABA), glycine, and glutamate as well as ATP. Ach, 5-HT, GABA and glycine ligand-gated receptors are related in evolutionary terms, while glutamate and ATP receptors form separate groups. There have been seven cloned proteins identified to date as members of the P2X receptor family in a wide range of cells and species. We have carried out hydropathy investigations and sequence comparisons of each of the seven subunits in order to examine the putative transmembrane and cysteine-rich extracellular domains. Probable locations of disulphide bridges are consistent with there being two separate extracellular folding domains. Assessment of the putative surface-accessible regions was used to select small localised amino acid segments in nonglycosylated regions for raising antibodies against each of the P2X receptor subunits. To test the specificity of these novel P2X receptor antibodies and their presence in cardiac and smooth muscle, sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE)/Western blotting was undertaken in homogenised rat heart, bladder, kidney, and vas deferens.

Central P2X4 and P2X6 channel subunits coassemble into a novel heteromeric ATP receptor

Ionotropic ATP receptors are widely expressed in mammalian CNS. Despite extensive functional characterization of neuronal homomeric P2X receptors in heterologous expression systems, the subunit composition of native central P2X ATP-gated channels remains to be elucidated. P2X4 and P2X6 are major central subunits with highly overlapping mRNA distribution at both regional and cellular levels. When expressed alone in Xenopus oocytes, P2X6 subunits do not assemble into surface receptors responsive to ATP applications. On the other hand, P2X4 subunits assemble into bona fide ATP-gated channels, slowly desensitizing and weakly sensitive to the partial agonist alpha,beta-methylene ATP and to noncompetitive antagonists suramin and pyridoxal-5-phosphate-6-azophenyl-2',4'-disulfonic acid. We demonstrate here that the coexpression of P2X4 and P2X6 subunits in Xenopus oocytes leads to the generation of a novel pharmacological phenotype of ionotropic ATP receptors. Heteromeric P2X4+6 receptors are activated by low-micromolar alpha, beta-methylene ATP (EC50 = 12 microM) and are blocked by suramin and by Reactive Blue 2, which has the property, at low concentrations, to potentiate homomeric P2X4 receptors. The assembly of P2X4 with P2X6 subunits results from subunit-dependent interactions, as shown by their specific copurification from HEK-293 cells transiently transfected with various epitope-tagged P2X channel subunits. Our data strongly suggest that the numerous cases of neuronal colocalizations of P2X4 and P2X6 subunits observed in mammalian CNS reflect the native expression of heteromeric P2X4+6 channels with unique functional properties.

Modification of the ATP-Induced Increase in

BACKGROUND: It is generally accepted that the plasma membrane of mammalian ventricular myocytes regulates the cytosolic concentration of Ca(2+). In this study we investigated the effects of some P2-purinoceptor antagonists and metals such as copper and zinc on the adenosine triphosphate (ATP)-induced increase in intracellular concentration of free Ca(2+) ([Ca(2+)](i)). METHODS AND RESULTS: Cardiomyocytes were isolated from adult male Sprague-Dawley rats loaded with Fura-2, and fluorescence measurements were performed by employing stirred cell suspensions at room temperature. ATP (50 µM) increased [Ca(2+)](i) over the basal value, and 10 µM cibacron blue or verapamil virtually abolished it. The ATP-induced increase in [Ca(2+)](i) was not observed in Ca(2+)- or Mg(2+)-free buffers. Incubation of cells with ZnCl(2) produced a significant depression of the ATP-induced increase in [Ca(2+)](i); 25 µM Zn(2+) decreased the peak response to approximately 50% of the control value. The ATP-induced increase in [Ca(2+)](i), was inhibited by low concentrations (1-5 µM) of Cu(2+) but was markedly augmented by high concentrations (25 µM) of Cu(2+). The increase in the [Ca(2+)](i) response to cron blue, and Zn(2+), but not by ryanodine or caffeine pretreatment. CONCLUSIONS: The ATP-induced increase in [Ca(2+)](i) is dependent on the extracellular concentrations of Ca(2+) as well as Mg(2+) and is antagonized by cibacron blue and Zn(2+). On the other hand, Cu(2+) produced a biphasic response to the ATP-induced increase in [Ca(2+)](i) in cardiomyocytes.

Interaction of lactate dehydrogenase with anthraquinone dyes: characterization of ligands for dye-ligand chromatography

Anthraquinone dyes (ADs), originally developed for the textile industry, are useful nucleotide-specific ligands for the purification of proteins by affinity techniques. Their specific feature is to mimic the adenine nucleotides ATP, ADP, NAD, NADH, which enables them to interact with the nucleotide-binding sites of enzymes such as dehydrogenases, kinases and ATPases. In the present study, the interactions and/or inhibitory effects of seven ADs, including Cibacron Blue F3G-A, Remazol Brilliant Blue R, on the activity of lactate dehydrogenase (LDH) were investigated. The ADs used in this paper could be divided into two groups: (i) AD1-AD3 which do not contain a triazine moiety; (ii) AD4-AD7 which contain the triazine moiety. Enzyme kinetics and zonal affinity chromatography were used for the characterization of the interaction affinity between the dye and LDH. Enzyme kinetic measurements were carried out at three different pH values: 6.5, 7.5 and 8.5. The relationship between physical and chemical properties of ADs (e.g., acid-basic properties, three dimensional structure of the respective dyes) and their interaction efficiency with LDH was studied. LDH activity was inhibited by all ADs, excluding AD1 (precursor of the blue dyes) and inhibition was always competitive. Similarity in the mutual position of the acidic and basic groups in NADH and the respective AD molecule was found to be a crucial factor for influencing the inhibitory action of the substance. The existence of ADs in the protonated form should be considered as another factor, important for the ADs inhibitory action on this enzyme.

Ligand recognition sites on P2X receptors studied by quantitative autoradiography of [3H]alpha,beta-methylene-ATP binding in rat brain

The specificity of alpha, beta-methylene-ATP for P2X receptor binding sites in the CNS has been examined by testing the effects of several ATP analogues and other ATP-related substances on the binding of 10 nM [3H]alpha,beta-methylene-ATP to 20 microns thick sections of fresh-frozen rat brain. The labelling of the putative P2X receptor binding sites by [3H]alpha,beta-methylene-ATP was evaluated by quantitative densitometry. [3H]alpha,beta-methylene-ATP binding was strongly inhibited by two close ATP analogues, 3'-O-(trinitrophenyl)-adenosine-5'-triphosphate and beta,gamma-imido-ATP (IC50 2.5 microM). beta,gamma-Methylene-ATP was, however, less potent (< 50% inhibition at 25 microM). Inosine-5'-triphosphate, guanosine-5'-triphosphate, uridine-5'-triphosphate, and cytidine-5'-triphosphate were practically inactive up to concentrations of 100 microM. Periodate oxidised ATP and 1, N6-etheno-ATP produced < 50% inhibition at 100 and 500 microM concentrations, respectively. Cations (K+, Rb+, Cs+, and Mg2+ at 5 mM and Na+ at 150 mM) reduced [3H]alpha,beta-methylene-ATP binding by no more than 50%. Several agents known to interact with Ca2+- and/or ATP-related cationic channels (Cd2+, glibenclamide, dantrolene, nifedipine, and thapsigargin) had no effect. We conclude that [3H]alpha,beta-methylene-ATP at low nanomolar concentrations binds to a site that has very strict structural requirements and is pharmacologically similar to ATP P2X receptors.

Regional distribution of [35S]2'-deoxy 5'-O-(1-thio) ATP binding sites and the P2Y1 messenger RNA within the chick brain

The distribution of the P2Y1 receptor protein and transcript in the one-day-old chick brain were determined by quantitative in vitro ligand autoradiography and in situ hybridization histochemistry. We have previously used [35S]2'-deoxy 5'-O-(1-thio) ATP as a radioligand for the recombinant P2Y1 receptor transiently expressed in COS-7 cells and have also shown that such sites are present at high density (Bmax: approximately 37 pmol radioligand bound/mg protein) in chick brain membranes. Here we report the macroscopic localization of these [35S]2'-deoxy 5'-O-(1-thio) ATP binding sites within the chick brain. They were found to be widely distributed there (within the range of 0.047 +/- 0.012 to 0.309 +/- 0.035 pmol bound/mg wet tissue). The affinities of P2 agonists and antagonists at these binding sites was comparable to that found previously for the recombinant P2Y1 receptor. In parallel experiments, the regional and cellular localization of the P2Y1 receptor messenger RNA was examined by in situ hybridization. The transcript was also found to be widely distributed throughout the brain. High levels of hybridization were detected in the cortex piriformis, ectostriatum, hippocampus, cerebellum and in a range of discrete nuclei throughout the brain, including the ovoidalis, isthmo-opticus and spiriformis lateralis nuclei. Localization at cellular level indicates that this receptor transcript is expressed in neurons and also at non-neuronal sites. Furthermore, the distribution of the P2Y1 transcript and the [35S]2'-deoxy 5'-O-(1-thio) ATP binding sites matched in a number of the regions and structures mentioned above. The present study clarifies the anatomical distribution of the P2Y1 receptor within the chick brain. Its broad distribution coupled with its neuronal expression suggest an important role for this type of metabotropic nucleotide receptor within the brain.

The role of purinergic and adrenergic transmitters of the sympathetic system in the control of arterial blood pressure variability

Variability of mean arterial pressure (MAP) was examined in chronically instrumented, conscious, freely moving rats with pharmacologically altered efferent sympathetic influences on the cardiovascular system. MAP was recorded for 30 min beat-to-beat, using a computer under both control and experimental conditions: after administration of adrenoceptor antagonists (prazosin or phentolamine) or under P2X receptor inactivation produced either by desensitization with alpha, beta-methylene ATP or by PPADS blockade. Inhibition of adrenergic sympathetic effects on the cardiovascular system produced long-lasting and stable decrease in MAP. Prazosin did not modify MAP variability whereas phentolamine enhanced it. Under P2X receptor desensitization MAP decreased, the hypotensive effect being accompanied by a significant increase in MAP variability. A similar increase in MAP variability was observed after PPADS administration, while MAP level was not changed. Administration of PPADS in combination with phentolamine increased MAP variability more significantly than each of the drugs given separately. Changes in MAP variability under the various experimental conditions were not consistently correlated with changes in heart rate variability. We propose that ATP, being a mediator of sympathetic vasoconstriction, participates in baroreceptor-induced stabilization of MAP level.

Sensory presynaptic and widespread somatodendritic immunolocalization of central ionotropic P2X ATP receptors

Recent evidence suggests that extracellular ATP plays a neurotransmitter role in the central nervous system. Its fast ionotropic effects are exerted through a family of P2X ATP-gated channels expressed in brain and spinal cord. To determine the physiological significance of central ATP receptors, we have investigated the localization of a major neuronal P2X receptor at the cellular and subcellular levels using affinity-purified antibodies directed against the C-terminal domain of P2X4 subunit. Subunit-specific anti-P2X4 antibodies detected a single band of 57,000 +/- 3000 mol. wt in transfected HEK-293 cells and in homogenates from adult rat brain. The strongest expression of central P2X receptors was observed in the olfactory bulb, lateral septum, cerebellum and spinal cord. P2X4 immunoreactivity was also evident in widespread areas including the cerebral cortex, hippocampus, thalamus and brainstem. In all regions examined, P2X receptors were associated with perikarya and dendrites where they were concentrated at the level of afferent synaptic junctions, confirming a direct involvement of postsynaptic ATP-gated channels in fast excitatory purinergic transmission. Moreover, P2X4-containing purinoceptors were localized in axon terminals in the olfactory bulb and in the substantia gelatinosa of nucleus caudalis of the medulla and dorsal horn of the spinal cord, demonstrating an important selective presynaptic role of ATP in the modulation of neurotransmitter release in central sensory systems.

Modulation by adenine nucleotides of epileptiform activity in the CA3 region of rat hippocampal slices

1. Hippocampal slices (450 microm) generate epileptiform bursts of an interictal nature when perfused with a zero magnesium medium containing 4-aminopyridine (50 microM). The effect of adenine nucleotides on this activity was investigated. 2. ATP and adenosine depressed this epileptiform activity in a concentration-dependent manner, with both purines being equipotent at concentrations above 10 microM. 3. Adenosine deaminase 0.2 u ml(-1), a concentration that annuls the effect of adenosine (50 microM), did not significantly alter the depression of activity caused by ATP (50 microM). 4. 8-Cyclopentyl-1,3-dimethylxanthine (CPT), an A1 receptor antagonist, enhanced the discharge rate significantly and inhibited the depressant effect of both ATP and adenosine such that the net effect of ATP or adenosine plus CPT was excitatory. 5. Several ATP analogues were also tested: alpha, beta-methyleneATP (alpha, beta-meATP), 2-methylthioATP (2-meSATP) and uridine triphosphate (UTP). Only alpha, beta-meATP (10 microM) produced an increase in the frequency of spontaneous activity which suggests a lack of involvement of P2Y or P2U receptors. 6. Suramin and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), P2 receptor antagonists, failed to inhibit the depression produced by ATP (50 microM). The excitatory effect of alpha, beta-meATP (10 microM) was inhibited by suramin (50 microM) and PPADS (5 microM). 7. ATP therefore depresses epileptiform activity in this model in a manner which is not consistent with the activation of known P1 or P2 receptors, suggesting the involvement of a xanthine-sensitive nucleotide receptor. The results are also indicative of an excitatory P2X receptor existing in the hippocampal CA3 region.

Structural motif and characteristics of the extracellular domain of P2X receptors

Seven sequences of the proteins recently cloned from rat tissues and identified as P2X receptors are examined. Their putative transmembrane and extracellular domains and the most probable locations of disulphide bridges are discussed. Some surface accessible regions are identified and the structure of the putative ATP binding domain is discussed in the light of known selectivities and physiological characteristics of individual P2X subunits. The summarised information and the theoretical considerations regarding the protein structure of P2X receptors have been used to select amino acid sequences for raising antibodies against the P2X1 receptor subunit. Initial results suggest that, in rat brainstem, P2X1 receptor subunits are found on a distinct subpopulation of neuronal perikarya in distributions consistent with distributions of known alpha,beta-methylene-ATP autoradiography and physiological effects of purinoceptor activation.

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.

Functional evidence for multiple purinoceptor subtypes in the rat medial vestibular nucleus

Extracellular recording techniques were used in brain slices to characterize excitatory responses produced by purine nucleotides in the rat medial vestibular nucleus, an area where functional purinoceptors have not previously been described. In the continued presence of the adenosine antagonist 8-cyclopentyl-1,3-dipropylxanthine, which alone caused a small increase in the spontaneous firing rate, the P2 purinoceptor agonists alpha,beta-methyleneadenosine 5'-triphosphate (alphabeta meATP) and adenosine 5'-O-(2-thiodiphosphate) (ADPbetaS) caused concentration-dependent increases in spontaneous firing rate, with EC50 values of 41.8 and 1.7 microM, respectively. Only approximately 35% of all neurons studied displayed excitatory responses to these agents. Responses waned in the continued presence of high concentrations of the latter, but not the former agonist. Furthermore, in the continued presence of a maximal concentration of alphabeta meATP, ADPbetaS produced further increases in the firing rate of these neurons. The P2 antagonist, suramin, ablated responses to alphabeta meATP, but did not affect responses to ADPbetaS, whereas pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid antagonized responses to both agonists. The nucleotide analogue alpha,beta-methyleneadenosine 5'-diphosphate, which displays affinity for putative P2X receptors in brain, also produced concentration-dependent increases in firing frequency, which were also markedly antagonized in the presence of suramin, this agonist being only slightly less potent than alphabeta meATP. In conclusion, a subpopulation of rat medial vestibular neuronal responses mediated by both P2X and P2Y purinoceptors can be distinguished. Comparison of their properties with those of recombinantly expressed P2X and P2Y receptors suggests that these endogenous P2 purinoceptor types differ in several important aspects from heterologously expressed recombinant receptors identified from cloning studies.

P2X-purinoceptors in the heart: actions of ATP and UTP

Positive inotropic effects of ATP and UTP (1 microM - 1mM) were studied in isolated rat and guinea pig cardiac tissues. The potency order obtained was ATP>UTP in both species, suggesting possible interaction with P2X-purinoceptors. Binding studies using [(3)H]alpha,beta-methylene ATP as marker of P2X-purinoceptors revealed two receptor sites: one high-, the other low-affinity, in atria and ventricles from rat and guinea pig. Both ATP and UTP were found to bind high-affinity sites of [(3)H]alpha,beta-methylene ATP. The effects of various calcium inhibitors such as nifedipine, dantrolene, ryanodine and TMB-8 on positive inotropic effects induced by ATP and UTP were also studied. The results suggest that ATP and UTP may increase inotropism by interaction with P2X-purinoceptors by means of a calcium-dependent mechanism.

Characterization of alpha, beta-methylene ATP binding sites in mouse crude synaptic membranes

Since ATP has been reported to be a potent excitatory transmitter in the mammalian central nervous system (CNS), we studied the neurochemical characters of the binding sites of alpha,beta-methylene ATP, an agonist of P2x receptors, in mouse crude synaptic membranes. ATP and its related compounds inhibited [3H] alpha,beta-methylene ATP binding in a concentration-dependent manner. The potency order in the inhibition of the binding was as follows; alpha,beta-methylene ATP = ADP beta S > ATP gamma S > ATP > or = ADP > beta,gamma-methylene ATP >> UTP > 2-methylthio ATP. And adenosine did not affect the binding. The order was different from those reported in peripheral tissues. And Sr2+, Ca2+, Mg2+, and Cd2+ enhanced the binding. These results suggest that alpha,beta-methylene ATP binding sites in CNS have different characters from those in peripheral tissues.

Alteration in cardiac sarcolemmal ATP receptors by oxyradicals

We previously demonstrated that cardiac sarcolemmal membranes bind [35S]ATP gamma S at both low and high affinity binding sites. In this study we examined the effects of some P2-purinoceptor antagonists as well as of two oxidants (H2O2 and HOCl) on the high affinity ATP-binding sites under in vitro conditions. It was found that putative P2-purinoceptor antagonists such as Cibacron blue, suramin, and 4,4'-diisothiocyanatostilbene 2-2 acid markedly inhibited specific ATP-binding with sarcolemmal membrane. H2O2 produced a biphasic effect (first increase and then decrease) on the specific ATP-binding with cardiac sarcolemma in a time- and concentration-dependent manner; these effects were prevented by catalase. On the other hand, HOCl markedly inhibited ATP-binding; this inhibition was prevented by l-methionine. These results suggest that the high affinity ATP-binding sites in cardiac sarcolemma may represent the P2-purinoceptors, which are susceptible to modification by oxidative stress under pathophysiological conditions including myocardial ischemia-reperfusion injury.

A comparison of the binding characteristics of recombinant P2X1 and P2X2 purinoceptors

1. We have recently provided evidence that [35S]-adenosine 5'-O-[3-thiotriphosphate] ([35S]-ATP gamma S) can label the human bladder recombinant P2X1 purinoceptor (human P2X1 purinoceptor). In this study we have characterized the binding of [35S]-ATP gamma S to a second P2X purinoceptor subtype, the rat PC12 phaeochromocytoma cell recombinant P2X2 purinoceptor (rat P2X2 purinoceptor), and compared its binding properties with those of both endogenous and recombinant P2X1 purinoceptors. 2. Infection of CHO-K1 cells with the rat P2X2 purinoceptor using Semliki forest virus (SFV) resulted in the expression of high affinity (pKd = 9.3; Bmax = 18.1 pmol mg-1 protein) binding sites for [35S]-ATP gamma S but not for [3H]-alpha, beta-methylene ATP ([3H]-alpha beta meATP). Since functional P2X purinoceptors could be detected electrophysiologically in these cells, but not in non-infected or CHO-K1 cells infected with SFV containing the LacZ gene, these results suggest that the rat P2X2 purinoceptor can be labelled using [35S]-ATP gamma S. 3. The binding characteristics of the rat P2X2 purinoceptor were compared with those of the human P2X1 purinoceptor, which was also expressed in the CHO-K1 cells using SFV. A major difference between the two recombinant P2X purinoceptor types was in the binding characteristics of alpha, beta-methylene ATP (alpha beta meATP). Thus, in the absence of divalent cations, alpha beta meATP possessed low affinity for both the human P2X1 purinoceptor (pIC50 = 7.2) and rat P2X2 purinoceptor (pIC50 = 7.1) labelled using [35S]-ATP gamma S. However, when the recombinant P2X purinoceptors were labelled with [3H]-alpha beta meATP in the presence of 4 mM CaCl2, the affinity of alpha beta meATP for the human P2X1 purinoceptor increased (pIC50 for alpha beta meATP = 8.2), while the affinity of the rat P2X2 purinoceptor for alpha beta meATP did not change (pIC50 for alpha beta meATP = 6.8). 4. Affinity estimates of 15 other nucleotide analogues for the [35S]-ATP gamma S binding sites on the two recombinant P2X purinoceptor subtypes were surprisingly similar (less than 5 fold difference), the only exception being 2'-deoxy ATP which possessed 8 fold higher affinity for rat P2X2 than for human P2X1 purinoceptors. In contrast dextran sulphate and the P2 purinoceptor antagonists, pyridoxalphosphate-6-azophenyl-2', 4'-disulphonic acid and 4,4'-diisothiocyanatostilbene-2,2' disulphonic acid, possessed 7 to 33 fold higher affinity for the human P2X1 than for the rat P2X2 purinoceptor. These data provide a correlation coefficient (r) of 0.894. 5. There was some evidence for species differences in the P2X1 purinoceptor. Thus, most nucleotides possessed slightly greater (up to 9-10 fold), while the P2 purinoceptor antagonists possessed slightly lower (up to 7-16 fold), affinity for the endogenous rat vas deferens and rat bladder P2X1 purinoceptors than for the human recombinant P2X1 purinoceptor. These differences were reflected in a slightly lower correlation coefficient, when comparing across species between the human recombinant P2X1 purinoceptor and the endogenous P2X1 purinoceptors labelled in either the rat deferens (r = 0.915) or the rat bladder (r = 0.932), than when comparing within species between the endogenous rat vas deferens and rat bladder P2X1 purinoceptors (r = 0.995). 6. In summary, [35S]-ATP gamma S can be used to label the recombinant P2X1 and P2X2 purinoceptors. Despite the marked differences reported between these two forms of P2X purinoceptor in functional studies, the differences in binding studies were more limited. However, a number of antagonists could discriminate between the P2X purinoceptor subtypes in the binding studies raising expectations that selective antagonists for these receptors can be developed.

Differential distribution of two ATP-gated channels (P2X receptors) determined by immunocytochemistry

Several P2X receptor subunits were recently cloned; of these, one was cloned from the rat vas deferens (P2X1) and another from pheochromocytoma (PC12) cells differentiated with nerve growth factor (P2X2). Peptides corresponding to the C-terminal portions of the predicted receptor proteins (P2X1 391-399 and P2X2 460-472) were used to generate antisera in rabbits. The specificities of antisera were determined by staining human embryonic kidney cells stably transfected with either P2X1 or P2X2 receptors and by absorption controls with the cognate peptides. In the vas deferens and the ileal submucosa, P2X1 immunoreactivity (ir) was restricted to smooth muscle, whereas P2X2-ir was restricted to neurons and their processes. Chromaffin cells of the adrenal medulla and PC12 cells contained both P2X1- and P2X2-ir. P2X1-ir was also found in smooth muscle cells of the bladder, cardiac myocytes, and nerve fibers and terminals in the superficial dorsal horn of the spinal cord. In contrast, P2X2-ir was observed in scattered cells of the anterior pituitary, neurons in the hypothalamic arcuate and paraventricular nuclei, and catecholaminergic neurons in the olfactory bulb, the substantia nigra, ventral tegmental area, and locus coeruleus. A plexus of nerve fibers and terminals in the nucleus of the solitary tract contained P2X2-ir. This staining disappeared after nodose ganglionectomy, consistent with a presynaptic function. The location of the P2X1 subunit in smooth muscle is consistent with its role as a postjunctional receptor in autonomic transmission, while in neurons, these receptors appear in both postsynaptic and presynaptic locations.

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.

A P2X purinoceptor cDNA conferring a novel pharmacological profile

We have cloned P2X4, a member of the P2-purinoceptor family, which has a new pharmacological profile. Rat P2X4 is distantly related to P2X1, P2X2 and P2X3 and is expressed in brain, spinal cord, lung, thymus, bladder, adrenal, testis and vas deferens. This ligand gated ion channel is activated by ATP and analogs with the potency order of ATP > ATP gamma S > 2-methylthio ATP > ADP approximately alpha beta-methylene ATP. However, none of the currently used P2X purinoceptor antagonists suramin, reactive blue 2 and PPADS blocked ATP evoked currents; in contrast their application resulted in potentiation of the agonist response. Due to lack of any known antagonist for P2X4 it is unlikely that native P2X4 has previously been recognized as a P2X purinoceptor.

Effect of P2-purinoceptor antagonists on glutamatergic transmission in the rat hippocampus

1. A study has been made of the effects of P2-purinoceptor antagonists on the evoked excitatory postsynaptic currents (e.p.s.cs) generated in CA1 pyramidal cells on stimulation of Schaffer collaterals and in CA3 pyramidal cells on stimulation of mossy fibres. The effects of these antagonists on currents generated in the cells on application of glutamate has also been determined. 2. Suramin blocked the evoked e.p.s.cs with an 50% inhibition (ID50) of 62 +/- 8 microM (mean +/- s.e.mean, n = 17), spontaneous miniature e.p.s.cs and the currents induced by application of 100 microM glutamate with an ID50 = 121 +/- 36 microM (n = 15) in all the cells studied. 3. Reactive Blue 2 (RB-2) in a concentration of 200 microM decreased the e.p.s.cs by 80 +/- 10% (n = 6) and the glutamate-activated currents by 83 +/- 3% (n = 6). 4. Pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) in the concentration-range of 40-500 microM decreased the amplitude of the e.p.s.cs in 12 out of 13 cells studied. PPADS at 200 microM reduced the amplitude of the e.p.s.cs by 60 +/- 10% (n = 3). PPADS did not affect the glutamate-induced currents in 4 cells and produced potentiation of the current amplitude by 60 +/- 10% in 4 other cells. 5. These results suggest that both presynaptic and postsynaptic P2-purinoceptors in the hippocampus can modulate the release and action of endogenous glutamate.

The purinoceptors of the guinea-pig isolated taenia caeci

The guinea-pig taenia caeci contains both P1 and P2 purinoceptors mediating relaxation. The P2 purinoceptors have been further characterized using an experimental approach designed to minimise complicating factors. In the presence of the adenosine uptake inhibitor S-(4-nitrobenzyl)-6-thioinosine (NBTI, 300 nM) and a pA100 concentration of the P1 purinoceptor antagonist 8-sulphophenyltheophylline (140 microM), the potency order of agonists was: 2-methylthio-ATP >> adenosine 5'-triphosphate (ATP) = alpha, beta-methylene ATP > beta, gamma-methylene ATP >> uridine 5'-triphosphate. Suramin antagonized ATP (pA2 = 5.52 +/- 0.17, Schild plot slope = 0.67 +/- 0.08) and 2-methylthio-ATP (pA2 = 5.78 +/- 0.30, Schild plot slope = 1.37 +/- 0.39) while responses to 5'-N-ethylcarboxamidoadenosine (NECA) were unaffected. The findings suggest that suramin, while it is selective for P2 relative to P1 purinoceptors, is not a true competitive antagonist. Pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) antagonized ATP in isolated guinea-pig vas deferens, but had no effect on responses to ATP in guinea-pig taenia caeci indicating it is selective for P2X relative to 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.

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.

Autoradiography of P2x ATP receptors in the rat brain

1. Binding of a P2x receptor specific radioligand, [3H]-alpha,beta-methylene adenosine triphosphate ([3H]-alpha,beta-MeATP) to sections of rat brain was reversible and association/dissociation parameters indicated that it consisted of two saturable components. Non-specific binding was very low (< 7% at 10 nM ligand concentration). 2. The binding was completely inhibited by suramin (IC50 approximately 14-26 microM) but none of the ligands specific for P2y receptors such as 2-methylthio-adenosine triphosphate (2-methyl-S-ATP) and 2-chloro-adenosine triphosphate (2-C1-ATP) nor 2-methylthio-adenosine diphosphate (2-methyl-S-ADP) a ligand for the P2 receptor on blood platelets ('P2T' type) produced strong inhibitions except for P1,P4-di(adenosine-5')tetraphosphate (Ap4A). 3. Inhibitors of Na+,K(+)-dependent adenosine triphosphatase (ATPase) ouabain, P1-ligand adenosine and an inhibitor of transport of, respectively, adenosine and cyclic nucleotides, dilazep, had no effect. 4. The highest density of P2x binding sites was found to be in the cerebellar cortex but the binding sites were present in all major brain regions, especially in areas known to receive strong excitatory innervation.

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.

Evidence for the presence of both pre- and postjunctional P2-purinoceptor subtypes in human isolated urinary bladder

1. In order to characterize P2-purinoceptor(s) in human urinary bladder the contractile effects of ATP and its slowly-hydrolyzable analogues alpha, beta-methylene ATP (alpha, beta-MeATP) and beta, gamma-methylene ATP (beta, gamma-MeATP) were investigated on human detrusor strips taken from patients undergoing cystectomy for bladder carcinoma. 2. Serial concentration-response curves (SCRC) for ATP, alpha, beta-MeATP and beta, gamma-MeATP were constructed with an interval of 25 min between two successive doses to avoid tachyphylaxis. ATP (10 microM-10 mM) induced a phasic contraction, which was very rapid in onset. The dose-response curve to ATP appeared not to be monophasic: at the lower concentrations (10-300 microM) the curve was shallow, whilst at high concentrations (1-10 mM) the curve was steeper. The magnitude of the response obtained at the highest concentration tested (10 mM) was only 21.1 +/- 2.8% (mean +/- s.e. mean; n = 4) of the KCl (100 mM)-induced contraction. 3. alpha, beta-MeATP (0.3 microM-1 mM) and beta, gamma-MeATP (10 microM-1 mM) elicited a phasic contraction with a time course similar to that exhibited by ATP. The magnitude of the response obtained at the highest concentration tested (1 mM) was 70.3 +/- 6.3% for alpha, beta-MeATP (n = 10) and 27.9 +/- 4.5% for beta, gamma-MeATP (n = 8) of KCl (100 mM)-induced contraction. The rank order of potency was alpha, beta-MeATP > beta, gamma-MeATP > ATP. A plateau of response could not be achieved by any of these agonists. 4. The P2-purinoceptor antagonist, suramin (10-300 microM), dose-dependently antagonized only the lower part of alpha,beta-MeATP dose-response curve. Data were analysed in terms of dose-ratio estimated at two levels of response (10% and 35% of KC1 100 mM-induced contraction). At 10% of KCl response the Schild plot slope was 0.98 and the estimated pKB was 5.85, whereas using the dose-ratio at the 35% level of the KCl response, the Schild plot was not linear suggesting an interaction of alpha,beta-MeATP with a heterogeneous receptor population.5. The putative P2-purinoceptor antagonist, Coomassie Brilliant Blue G (CB-G) at 0.3 and 1 l micro M(n = 5), shifted to the left the alpha,beta-MeATP SCRC. The response at the highest concentration of agonist was potentiated, being equal to 78.8 +/- 11.7% of the KCl (100 mM) response (n = 5). CB-G at 0.3 microM also shifted to the left the beta,upsilon-MeATP SCRC and significantly potentiated the response at 1 mM up to 46.3 +/- 5.6% of KCl 100 mM response (n = 4).6. Pretreatment with terodotoxin (TTX) at 1 microM shifted to the left the alpha,beta-MeATP SCRC but the response to the highest concentration of the agonist was not potentiated, being 73.6 +/- 9.9% of the KCl(100 mM) response (n = 5). TTX (1 micro M) shifted to the left the beta,upsilon-MeATP SCRC and significantly potentiated the response at 1 mM (61.6 +/- 3.1% of KCl response; n = 4).7. The NO synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME) at 100 micro M did not modify the SCRC to either alpha, beta or beta,upsilon-MeATP.8. We conclude that in human detrusor muscle there is a heterogeneity of purinoceptors. The complex antagonism exhibited by suramin suggests the presence not only of Ph-purinoceptors but also of another contractile P2-purinoceptor subtype insensitive to suramin. Moreover, the activity of CB-G and TTX seems to support the existence of a prejunctional P2-purinoceptor subtype inducing the release of one or more inhibitor neurotransmitters.

Pharmacological analysis of ecto-ATPase inhibition: evidence for combined enzyme inhibition and receptor antagonism in P2X-purinoceptor ligands

1. Previous studies have shown that suramin and FPL 66301 are competitive antagonists at the P2X-purinoceptor in the rabbit ear artery. Those studies employed alpha,beta-methylene ATP, a poorly hydrolysable ATP analogue, as the agonist. In this study these compounds have been tested using ATP as the agonist. 2. Suramin, in the concentration range 30-1000 microM, potentiated the contractile effects of ATP, producing a 3-fold leftward shift of the ATP E/[A] curves. FPL 66301, in the concentration range 100-1000 microM, produced a significant but small (approximately 3-fold) rightward shift of the ATP curves. These results are in marked contrast with previous studies using alpha,beta-methylene ATP in which 30-fold rightward shifts were achieved using the same concentration ranges of suramin and FPL 66301. 3. Suramin and FPL 66301 were tested as ecto-ATPase inhibitors in a human blood cell assay. Suramin inhibited the enzyme with a pIC50 of 4.3, FPL 66301 with a pIC50 of 3.3. 4. The pharmacological data were analysed using a theoretical model describing the action of a compound with dual enzyme inhibitory and receptor antagonistic properties on the effects of an agonist susceptible to enzymatic degradation. The model was found to fit the data well using the known pKB estimates for suramin and FPL 66301 and similar relative (but not absolute) pK1 estimates to those obtained for the compounds in the enzyme assay. 5. From this analysis it was concluded that the limited shifts of ATP E/[A] curves produced by suramin and FPL 66301 were the result of 'self-cancellation' of the potentiating (enzyme inhibitory) and rightward-shifting (receptor antagonistic) properties.6. The analysis also indicated that the presence of ecto-ATPase activity in the rabbit ear artery preparation has a marked effect on the apparent potency of ATP. The experimental p[A50] was 3.4,whereas the 'true' value, that is the value which would be obtained in the absence of ecto-ATPase activity, was 6.0, some 400-fold higher.7 Two conclusions are drawn from this study. Firstly, caution must be exercised in the use of suramin and FPL 66301 as tools for receptor classification. Absence of overt antagonism by these compounds when metabolically unstable agonists are used could lead to erroneous claims for receptor subtypes.Secondly, the agonist potency order currently used to designate P2X- purinoceptors may require modification.

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.

Evidence for the presence of two types of P2 purinoceptor in the guinea-pig ileal longitudinal smooth muscle preparation

The effects of some agonists acting at P2 purinoceptors on guinea-pig isolated ileum longitudinal smooth muscle have been examined. The preparation contracted in response to ATP, alpha,beta-methylene ATP and 2-methylthio ATP, but not UTP. In this respect, alpha,beta-methylene ATP and 2-methylthio ATP were approximately equipotent and both were 10-50 times more active than ATP. Responses to alpha,beta-methylene ATP, but not 2-methylthio ATP or ATP, were antagonised by atropine and tetrodotoxin, suggesting that alpha,beta-methylene ATP activates cholinergic nerves in the ileum, whilst the other two compounds act on the smooth muscle. Two other purine nucleotide analogues, beta,gamma-methylene ATP and beta,gamma-imido ATP, did not cause contraction. However, both compounds antagonised responses to alpha,beta-methylene ATP, but not those to 2-methylthio ATP. Suramin antagonised responses to both alpha,beta-methylene ATP and 2-methylthio ATP, whilst Cibacron blue was without effect on responses to either agonist. We conclude that the purinoceptor on cholinergic nerves has some of the characteristics of the P2x purinoceptor, whilst the purinoceptor on ileal smooth muscle has some of the characteristics of the P2Y purinoceptor. However, further work will be necessary before definitive classification is possible.

Comparative studies on the affinities of ATP derivatives for P2x-purinoceptors in rat urinary bladder

1. Radioligand binding assays have been used to determine the affinities of a series of ATP derivatives with modifications of the polyphosphate chain, adenine and ribose moieties of the ATP molecule for [H]-alpha,beta-methylene ATP ([3H]-alpha,beta-MeATP) binding sites in rat urinary bladder. 2. The replacement of the bridging oxygen in the triphosphate chain of ATP (pIC50 = 5.58) with a methylene or imido group markedly increased the affinity (691 fold in IC50 values for beta,gamma-imidoATP, 15 fold for beta,gamma-methylene ATP), and the replacement of an ionized oxygen on the gamma-phosphate with a sulphur (ATP gamma S) also led to increased affinity (5623 fold in IC50 values). 3. Modifications at N6, N1, and C-8 positions on the purine base usually reduced the affinity of ATP (a decrease of 2.8 fold in IC50 values for N6-methylATP and 8.9 fold for 8-bromo ATP), while the attachment of an alkylthio group to the C-2 position greatly increased the affinity for P2x-purinoceptors (from 3.5 to 98 fold increase in IC50 values). 4. Replacement of the 3'-hydroxyl group on the ribose with substituted amino or acylamino groups produced more potent P2x-purinoceptor agonists (an increase of 447 fold in IC50 values for 3'-deoxy-3'-benzylamino ATP and 28 fold for 3'-deoxy-3'-(4-hydroxyphenylpropionyl)amino ATP. 5. Diadenosine polyphosphates (Ap[n]A) were also shown to displace the [3H]-alpha,beta-MeATP binding. The rank order of potency was Ap6A > Ap5A > Ap4A >> Ap3A >> Ap2A. 6. Suramin, PPADS, and reactive blue 2 could competitively displace the binding of [3H]-alpha,beta-MeATP toP2X-purinoceptors, with pIC50 values of 6.26, 5.35, and 6.22, respectively.

P2 purinoceptor antagonist properties of pyridoxal-5-phosphate

The antagonist properties of pyridoxal-5-phosphate, a synthesis precursor of pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid, were investigated on P2 purinoceptor-mediated responses of the rat isolated vagus nerve and vas deferens. In addition, the effect of this agent was studied on high affinity tritiated alpha,beta-methylene adenosine triphosphate (alpha,beta-meATP) binding to rat vas deferens membranes, thought to represent binding to functional P2x purinoceptors. In the rat vagus nerve, pyridoxal-5-phosphate (10(-5)-10(-4) M) produced concentration-related antagonism of depolarisation responses induced by alpha,beta-meATP, measured using an extracellular recording technique. In contrast, depolarisation responses to 5-hydroxytryptamine (5-HT) were unaffected by pyridoxal-5-phosphate. In the rat vas deferens, pyridoxal-5-phosphate (10(-5)-10(-4) M) antagonised contractile responses produced by alpha,beta-meATP while contractions to phenylephrine were unaffected. However, responses of the vagus nerve and the vas deferens to alpha,beta-meATP were not antagonised by pyridoxal hydrochloride (10(-4) M). Pyridoxal-5-phosphate competed for high affinity binding of [3H]alpha,beta-meATP to homogenised membranes of the rat vas deferens with a pKi estimate of 4.91 +/- 0.12 and a Hill slope of 0.80 +/- 0.03. Pyridoxal hydrochloride only competed for binding at concentrations in excess of 10(-4) M, yielding a pKi estimate of 3.21 +/- 0.04 and a Hill slope of 1.82 +/- 0.12. These findings indicate that pyridoxal-5-phosphate acts as a specific antagonist of P2 purinoceptors in the vagus nerve and vas deferens of the rat and that the phosphate moiety is required for activity.(ABSTRACT TRUNCATED AT 250 WORDS)

The use of antagonists to characterize the receptors mediating depolarization of the rat isolated vagus nerve by alpha, beta-methylene adenosine 5'-triphosphate

1. We have previously found that the P2x-purinoceptor agonist, alpha, beta-methylene adenosine 5'-triphosphate (alpha, beta-methylene ATP), depolarizes the rat cervical vagus nerve, measured with a 'grease-gap' extracellular recording technique. This effect was attenuated by the P2 purinoceptor antagonist, suramin. In the present study we have investigated in more detail the antagonism produced by suramin and have also investigated the actions of two other putative P2 purinoceptor antagonists, cibacron blue and pyridoxal-phosphate-6-azophenyl-2', 5'-disulphonic acid (iso-PPADS). Furthermore, we have studied the interactions between suramin and cibacron blue or iso-PPADS in an attempt to determine whether these antagonists act at a common receptor site. 2. Suramin (1 x 10(-5)-1 x 10(-4) M) produced reversible, concentration-related rightward displacements of the concentration-effect curve to alpha, beta-methylene ATP. Schild analysis of this antagonism yielded a pA2 value of 5.90 with a slope value of 0.47. 3. Cibacron blue (3 x 10(-5)-1 x 10(-4) M) also antagonized depolarizations induced by alpha, beta-methylene ATP. The antagonistic effects of cibacron blue were slow to reach equilibrium but could be readily reversed on washout. At low concentrations for antagonism, cibacron blue (1 x 10(-5) M and 3 x 10(-5) M) produced enhancement of the maximal response to alpha, beta-methylene ATP. At the highest concentration tested (1 x 10(-4) M) the concentration-effect curve to alpha, beta-methylene ATP was shifted to the right in a parallel manner, yielding a pKB estimate of 4.96. 4. Iso-PPADS (1 X 10-6 1 X 10-5- M) produced a concentration-related depression in the maxima ofthe concentration-effect curves to alpha,beta-methylene ATP. Analysis of these data by a double reciprocal plot yielded a pKB estimate of 6.02. This profile of insurmountable antagonism could not be attributed to irreversible binding of iso-PPADS to the receptor since the effect of iso-PPADS could be reversed on washing, albeit slowly.5. In the presence of suramin (1 x 10-4 M), cibacron blue (1 x 10-4 M) produced no further rightward displacement of the alpha,beta-methylene ATP concentration-effect curve. The mean agonist concentration ratios in the presence of suramin or cibacron blue alone (11.7 and 10.3, respectively) were not significantly different from the mean concentration-ratio in the presence of both antagonists (11.8). This finding suggests that high concentrations of alpha,beta-methylene ATP activate a receptor population which is resistant to blockade by either antagonist.6. The antagonistic effect of iso-PPADS (1 x 10-5 M) was partially attenuated by suramin (1I x 10-4 M).It is possible that this interaction reflects a slow dissociation of iso-PPADS from the receptor with which suramin and alpha,beta-methylene ATP interact.7. Suramin, cibacron blue or iso-PPADS had no marked effect on depolarization produced by 5-hydroxytryptamine (5-HT, 1 x 10-7-3 x 10-5 M), indicating their specificity in antagonizing responses to alpha, beta-methylene ATP.8. Responses to alpha,beta-methylene ATP were not antagonized by 8-para-sulphophenyltheophylline (3 x 10-5M), ondansetron (1 x 10-7 M), bicuculline (1 x I0-5 M), phentolamine (1 X 10-6 M) or hexamethonium(1 X 10-4 M), which are antagonists at P1-purinoceptors, 5-HT3 receptors, GABAA receptors, a-adrenoceptors and nicotinic cholinoceptors, respectively, thereby excluding the involvement of these receptors.Indomethacin (3 X 10-6 M) had no effect on responses to alpha,beta-methylene ATP.9. The results obtained with three purinoceptor antagonists confirm and extend our original supposition that alpha,beta-methylene ATP-induced depolarization of the rat vagus nerve is mediated predominantly via P2 purinoceptors, thought to be of the P2,X subtype. The finding that responses induced by high concentrations of agonist were resistant to blockade by suramin and cibacron blue, but could be attenuated by iso-PPADS, adds further weight to our speculation that the purinoceptor population in the rat vagus nerve is heterogeneous.