Ca(2+)-stores mobilization by diadenosine tetraphosphate, Ap4A, through a putative P2Y purinoceptor in adrenal chromaffin cells

María Teresa Miras Portugal: a pioneer in the study of purinoceptors in chromaffin cells

María Teresa Miras Portugal devoted most of her scientific life to the study of purinergic signalling. In an important part of her work, she used a model system: the chromaffin cells of the adrenal medulla. It was in these cells that she identified diadenosine polyphosphates, from which she proceeded to the study of adrenomedullary purinome: nucleotide synthesis and degradation, adenosine transport, nucleotide uptake into chromaffin granules, exocytotic release of nucleotides and autocrine regulation of chromaffin cell function via purinoceptors. This short review will focus on the current state of knowledge of the purinoceptors of adrenal chromaffin cells, a subject to which María Teresa made seminal contributions and which she continued to study until the end of her scientific life.

Role of the Purinergic P2Y2 Receptor in Pulmonary Hypertension

Pulmonary arterial hypertension (PAH), group 1 pulmonary hypertension (PH), is a fatal disease that is characterized by vasoconstriction, increased pressure in the pulmonary arteries, and right heart failure. PAH can be described by abnormal vascular remodeling, hyperproliferation in the vasculature, endothelial cell dysfunction, and vascular tone dysregulation. The disease pathomechanisms, however, are as yet not fully understood at the molecular level. Purinergic receptors P2Y within the G-protein-coupled receptor family play a major role in fluid shear stress transduction, proliferation, migration, and vascular tone regulation in systemic circulation, but less is known about their contribution in PAH. Hence, studies that focus on purinergic signaling are of great importance for the identification of new therapeutic targets in PAH. Interestingly, the role of P2Y2 receptors has not yet been sufficiently studied in PAH, whereas the relevance of other P2Ys as drug targets for PAH was shown using specific agonists or antagonists. In this review, we will shed light on P2Y receptors and focus more on the P2Y2 receptor as a potential novel player in PAH and as a new therapeutic target for disease management.

That was then, this is now: the development of our knowledge and understanding of P2 receptor subtypes

P2 receptors are present in virtually all tissues and cell types in the human body, and they mediate the physiological and pharmacological actions of extracellular purine and pyrimidine nucleotides. They were first characterised and named by Geoff Burnstock in 1978, then subdivided into P2X and P2Y purinoceptors in 1985 on the basis of pharmacological criteria in functional studies on native receptors. Molecular cloning of receptors in the 1990s revealed P2X receptors to comprise seven different subunits that interact to produce functional homo- and heterotrimeric ligand-gated cation channels. A family of eight P2Y G protein-coupled receptors were also cloned, which can form homo- and heterodimers. Deep insight into the molecular mechanisms of agonist and antagonist action has been provided by more recent determination of the tertiary and quaternary structures of several P2X and P2Y receptor subtypes. Agonists and antagonists that are highly selective for individual subtypes are now available and some are in clinical use. This has all come about because of the intelligence, insight and drive of the force of nature that was Geoff Burnstock.

The P2Y/P2X divide: How it began

Extracellular purine and pyrimidine nucleotides produce their pharmacological effects through P2 receptors. These were first named by Geoff Burnstock in an extensive review in 1978. They were then subdivided into P_2X and P_2Y purinoceptors by Burnstock and Kennedy in 1985, based on applying pharmacological criteria to data generated by functional studies in smooth muscle tissues. Several other P2 subtypes, P_2T, P_2Z, P_2U and P_2D were subsequently identified in the following years, again using pharmacological criteria. The number and identity of subtypes were clarified and simplified by the cloning of seven ATP-sensitive ligand-gated ion channel subunits and eight adenine and/or uracil nucleotide-sensitive G protein-coupled receptors from 1993 onwards. The former were all classified as members of the P2X receptor family and the latter as members of the P2Y receptor family. More recently, high resolution imaging of the tertiary and quaternary structures of several P2X and P2Y receptor subtypes has provided a much greater understanding of how and where agonists and antagonists bind to the receptors and how this leads to changes in receptor conformation and activity. In addition, medicinal chemistry has produced a variety of subtype-selective agonists and antagonists, some of which are now in clinical use. This progress and success is a testimony to the foresight, intelligence, enthusiasm and drive of Geoff Burnstock, who led the field forward throughout his scientific life.

Geoffrey Burnstock, our friend and magister: the diadenosine polyphosphate connection

Development of science needs the cooperation of many creative brains. Sometimes, ideas on a specific area get suddenly exhausted and then it is the time for a privileged mind to think in a different way and reach the turning point to introduce a new paradigm. This happened to Geoffrey Burnstock, a heterodox thinker and nonconformist scientist that has been the paladin of purinergic signalling since 1972, opening neuroscience to the understanding of organs and tissues functioning and development of a new pharmacology. This review summarizes the contribution of our group to the understanding of the role of the diadenosine polyphosphates, Ap_nA, as signalling molecules, describing their tissue and organ distribution, their transport and storage in secretory vesicles and their release and interaction with purinergic receptors. We also have to acknowledge the friendly and kindly support of Professor Burnstock that showed a great interest in the field from our initial findings and actively stimulated our efforts to establish the extracellular roles and biological significance of these dinucleotides.

P2Y Receptors in Immune Response and Inflammation

Metabotropic pyrimidine and purine nucleotide receptors (P2Y receptors) are expressed in virtually all cells with implications in very diverse biological functions, including the well-established platelet aggregation (P2Y12), but also immune regulation and inflammation. The classical P2Y receptors bind nucleotides and are encoded by eight genes with limited sequence homology, while phylogenetically related receptors (e.g., P2Y12-like) recognize lipids and peptides, but also nucleotide derivatives. Growing lines of evidence suggest an important function of P2Y receptors in immune cell differentiation and maturation, migration, and cell apoptosis. Here, we give a perspective on the P2Y receptors' molecular structure and physiological importance in immune cells, as well as the related diseases and P2Y-targeting therapies. Extensive research is being undertaken to find modulators of P2Y receptors and uncover their physiological roles. We anticipate the medical applications of P2Y modulators and their immune relevance.

Purinergic signalling in endocrine organs

There is widespread involvement of purinergic signalling in endocrine biology. Pituitary cells express P1, P2X and P2Y receptor subtypes to mediate hormone release. Adenosine 5'-triphosphate (ATP) regulates insulin release in the pancreas and is involved in the secretion of thyroid hormones. ATP plays a major role in the synthesis, storage and release of catecholamines from the adrenal gland. In the ovary purinoceptors mediate gonadotrophin-induced progesterone secretion, while in the testes, both Sertoli and Leydig cells express purinoceptors that mediate secretion of oestradiol and testosterone, respectively. ATP released as a cotransmitter with noradrenaline is involved in activities of the pineal gland and in the neuroendocrine control of the thymus. In the hypothalamus, ATP and adenosine stimulate or modulate the release of luteinising hormone-releasing hormone, as well as arginine-vasopressin and oxytocin. Functionally active P2X and P2Y receptors have been identified on human placental syncytiotrophoblast cells and on neuroendocrine cells in the lung, skin, prostate and intestine. Adipocytes have been recognised recently to have endocrine function involving purinoceptors.

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

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

The diadenosine polyphosphate receptors: P2D purinoceptors

Diadenosine polyphosphates-Ap4A, Ap5A and Ap6A-are co-stored in neurosecretory vesicles together with ATP and aminergic compounds. They are released from neural cells and synaptic terminals in a Ca(2+)-dependent process. Ligand binding and displacement experiments carried out with [3H]Ap4A on isolated chromaffin cells and synaptosomal preparations result in curvilinear Scatchard plots with Kd values close to 0.1 nM for the high-affinity binding sites. Displacement curves with two steps are obtained for homologous and heterologous nucleotide ligands; the lowest-affinity step exhibits Ki values in the micromolar range for ApnA compounds. The high-affinity binding sites were named P2D purinoceptors on the basis of their binding characteristics. Single-cell studies in neurochromaffin cells indicate the presence of P2X purinoceptors in noradrenergic cells that do not respond to Ap4A and in which noradrenaline secretion can be induced by influx of extracellular Ca2+. P2Y receptors that respond to ATP analogues and ApnAs are present in endothelial cells from adrenal medulla. Those cells that express P2U purinoceptors are unresponsive to ApnAs. Ectodiadenosine polyphosphate hydrolases with Km values of 0.3 to 2 microM are present in both neural and endothelial cells from adrenal medulla. In midbrain synaptic terminals diadenosine polyphosphates induce Ca2+ entry from the extracellular medium. The fact that the synaptic response is not cross-desensitized by ATP and its non-hydrolysable analogues, the non-blocking effect of suramin, and the differential effect of Ca2+ channel blockers, together suggest that there are different receptors for nucleotides and dinucleotides in rat brain synaptosomes, which we have called P4 purinoceptors on the basis of functional studies.

P2 purinoceptors and pyrimidinoceptors of catecholamine-producing cells and immunocytes

ATP is a neuronal (co)transmitter. In addition, both ATP and UTP may exit damaged cells and thereby function as extracellular signal molecules. The targets of signalling may be the P2 (for ATP and UTP) and P1 (for the degradation product adenosine) receptors of, for instance, neurons and immunocytes. UTP may also act at separate pyrimidinoceptors. Catecholamine-producing cells (adrenal chromaffin cells and peripheral and central noradrenergic neurons) possess P2X and P2Y purinoceptors. ATP appears to be a fast excitatory neuro-neuronal transmitter of the noradrenergic coeliac and locus coeruleus neurons. This effect is mediated by P2X purinoceptors. P2Y purinoceptor-mediated slow excitatory synaptic potentials have not yet been demonstrated either in the peripheral or central nervous system. On the other hand, after neuronal injury microglial cells (brain immunocytes) are engaged in a process called 'synaptic stripping', i.e. the displacement of synaptic boutons from the neuronal surface. During this process microglial cells are in direct contact with the (co)transmitter ATP. Activation of P2X, P2Z and P2Y purinoceptors results in an elevated intracellular Ca2+ concentration in microglia and macrophages. Various functions of these cells are regulated by intracellular Ca2+ (e.g. cytokine production, phagocytosis) and may therefore be modulated by nucleotides. Since neuronal damage leads to the transformation of microglial cells to macrophages and, at the same time, to the efflux of nucleotides from the damaged cells, the requirements for a modulatory interaction are fulfilled.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

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

Identification and quantification of diadenosine polyphosphate concentrations in human plasma

OBJECTIVE

Diadenosine polyphosphates have been demonstrated to be involved in the control of vascular tone as well as the growth of vascular smooth muscle cells and hence, possibly, in atherogenesis. In this study we investigated the question of whether diadenosine polyphosphates are present in human plasma and whether a potential source can be identified that may release diadenosine polyphosphates into the circulation.

METHODS AND RESULTS

Plasma diadenosine polyphosphates (ApnA with n=3 to 6) were purified to homogeneity by affinity-, anion exchange-, and reversed phase-chromatography from deproteinized human plasma. Analysis of the homogeneous fractions with matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) revealed molecular masses ([M+H]+) of 757, 837, 917, and 997 d. Comparison of the postsource decay matrix-assisted laser desorption/ionization mass spectrometry mass spectra of these fractions with those of authentic diadenosine polyphosphates revealed that these isolated substances were identical to Ap3A, Ap4A, Ap5A, and Ap6A. Enzymatic analysis showed an interconnection of the phosphate groups with the adenosines in the 5'-positions of the ribose moieties. The mean total plasma diadenosine polyphosphate concentrations (micromol/L; mean +/- SEM) in cubital veins of normotensive subjects amounted to 0.89+/-0.59 for Ap3A, 0.72+/-0.72 for Ap4A, 0.33+/-0.24 for Ap5A, and 0.18+/-0.18 for Ap6A. Cubital venous plasma diadenosine polyphosphate concentrations from normotensives did not differ significantly from those in the hypertensive patients studied. There was no significant difference between arterial and venous diadenosine polyphosphate plasma concentrations in 5 hemodialysis patients, making a significant degradation by capillary endothelial cells unlikely. Free plasma diadenosine polyphosphate concentrations are considerably lower than total plasma concentrations because approximately 95% of the plasma diadenosine polyphosphates were bound to proteins. There were no significant differences in the diadenosine polyphosphate plasma concentrations depending on the method of blood sampling and anticoagulation, suggesting that platelet aggregation does not artificially contribute to plasma diadenosine polyphosphate levels in significant amounts. The ApnA (with n=3 to 6) total plasma concentrations in adrenal veins were significantly higher than the plasma concentrations in both infrarenal and suprarenal vena cava: adrenal veins: Ap3A, 4.05+/-1.63; Ap4A, 6.18+/-2.08; Ap5A, 0.53+/-0.28; Ap6A, 0.59+/-0.31; infrarenal vena cava: Ap3A, 1.25+/-0.66; Ap4A, 0.91+/-0.54; Ap5A, 0.25+/-0.12; Ap6A, 0.11+/-0.06; suprarenal vena cava: Ap3A, 1.40+/-0.91; Ap4A, 1.84+/-1.20; Ap5A, 0.33+/-0.13; Ap6A, 0.11+/-0.07 (micromol/L; mean +/- SEM; each P<0.05 (concentration of adrenal veins versus infrarenal or suprarenal veins, respectively).

CONCLUSIONS

The presence of diadenosine polyphosphates in physiologically relevant concentrations in human plasma was demonstrated. Because in adrenal venous plasma significantly higher diadenosine polyphosphate concentrations were measured than in plasma from the infrarenal and suprarenal vena cava, it can be assumed that, beside platelets, the adrenal medulla may be a source of plasma diadenosine polyphosphates in humans.

Identification of dinucleoside polyphosphates in adrenal glands

Dinucleoside polyphosphates have been characterised as extracellular mediators controlling numerous physiological functions like vascular tone or cell proliferation. Here we describe the isolation and identification of dinucleoside polyphosphates Ap(n)A (with n=2-3), Ap(n)G (with n=2-6) as well as Gp(n)G (with n=2-6) from adrenal glands. These dinucleoside polyphosphates are localised in granules of the adrenal glands. The dinucleoside polyphosphates diadenosine diphosphate (Ap(2)A), diadenosine triphosphate (Ap(3)A), adenosine guanosine polyphosphates (Ap(n)G) and diguanosine polyphosphates (Gp(n)G), both with phosphate group (p) numbers (n) ranging from 2 to 6, were identified by fractionating them to homogeneity by preparative size-exclusion- and affinity-chromatography as well as analytical anion-exchange and reversed-phase-chromatography from deproteinised adrenal glands and by analysis of the homogeneous dinucleoside polyphosphates containing fractions with post-source-decay (PSD) matrix-assisted laser desorption/ionisation mass spectrometry (MALDI-MS). The identity of the dinucleoside polyphosphates was confirmed by retention time comparison with authentic dinucleoside polyphosphates. Enzymatic analysis demonstrated an interconnection of the phosphate groups with the adenosines in the 5(')-positions of the riboses in all dinucleoside polyphosphates purified from adrenal glands. In conclusion, the identification of these dinucleoside polyphosphates in adrenal gland granules emphasises that these dinucleoside polyphosphates can be released from the adrenal glands upon stimulation into the circulation.

Extracellular nucleotide signaling along the renal epithelium

During the past two decades, several cell membrane receptors, which preferentially bind extracellular nucleotides, and their analogs have been identified. These receptors, collectively known as nucleotide receptors or "purinergic" receptors, have been characterized and classified on the basis of their biological actions, their pharmacology, their molecular biology, and their tissue and cell distribution. For these receptors to have biological and physiological relevance, nucleotides must be released from cells. The field of extracellular ATP release and signaling is exploding, as assays to detect this biological process increase in number and ingenuity. Studies of ATP release have revealed a myriad of roles in local regulatory (autocrine or paracrine) processes in almost every tissue in the body. The regulatory mechanisms that these receptors control or modulate have physiological and pathophysiological roles and potential therapeutic applications. Only recently, however, have ATP release and nucleotide receptors been identified along the renal epithelium of the nephron. This work has set the stage for the study of their physiological and pathophysiological roles in the kidney. This review provides a comprehensive presentation of these issues, with a focus on the renal epithelium.

Physiological evidence for a P2Y receptor responsive to diadenosine polyphosphates in human lung via Ca(2+) release studies in bronchial epithelial cells

P2Y(2) receptors that are activated by the extracellular nucleotides ATP or UTP mediate Cl(-) secretion via an increase in [Ca(2+)](i) (intracellular calcium concentration). Therefore, in the lung of patients suffering from cystic fibrosis, inhalation of aerosolized UTP offers a way to circumvent the defect in Cl(-) secretion by the cystic fibrosis transmembrane conductance regulator. A possible alternative for the relatively unstable UTP in inhalation therapy is the more resistant diadenosine tetraphosphate (Ap(4)A). In human and rat lung membranes, Ap(4)A binds to P2 receptor sites coupled to G proteins. Here, we showed that Ap(4)A caused an increase in [Ca(2+)](i) with an EC(50) of 17 microM in human bronchial epithelial cells (HBE1). The [Ca(2+)](i) rise evoked by ATP and UTP was completely, but that induced by Ap(4)A only partially, caused by release of Ca(2+) from internal stores. Moreover, the potency of Ap(4)A to mobilize Ca(2+) was lower than that of ATP and UTP (EC(50) 1.5 and 1.8 microM, respectively), and the maximal increase in [Ca(2+)](i) was considerably smaller than that after ATP or UTP. In accordance with our previous results providing evidence for a common binding site for various diadenosine polyphosphates in lung membranes, all Ap(n)A analogues tested (n = 3 to 6) caused a comparable [Ca(2+)](i) increase. Homologous or heterologous prestimulation largely diminished the increase in [Ca(2+)](i) found after a second pulse of either UTP or Ap(4)A. Although specific binding characteristics and functional responses of Ap(4)A on lung cells are in favor of a distinct receptor for Ap(4)A, the cross-talk between UTP and Ap(4)A in HBE1 cells and the only slight differences in Ca(2+) mobilization by ATP or UTP and Ap(4)A render it impossible at this point to state unequivocally whether there exists a distinct P2Y receptor specific for diadenosine polyphosphates in lung epithelia or whether Ap(4)A activates one of the nucleotide receptors already described.

The discovery and development of P2 receptor subtypes

Extracellular purine and pyrimidine nucleotides modulate cellular activity by acting at P2 receptors. The first receptor to be identified was the P(2)-purinoceptor, which was characterised and named in 1978. In the 1980s this site was subdivided into P(2X) and P(2Y) purinoceptors on the basis of pharmacological criteria in functional studies on native receptors. Subsequently, a similar approach led to the characterisation of the P(2T), P(2Z), P(2U) and P(2D) purinoceptors. In the 1990s a molecular biological approach has led to the cloning and functional expression of at least 12 mammalian P2 receptor subtypes. The challenge now is to relate these recombinant receptors to native receptors present within a wide range of tissues.

Characterization of a diadenosine tetraphosphate-receptor distinct from the ATP-purinoceptor in human tracheal gland cells

Human submucosal tracheal glands are now believed to play a major role in the physiopathology of cystic fibrosis, a genetic disease in which ATP is used as a therapeutic agent. However, actions of ATP on tracheal gland cells are not well known. ATP binds to P2 receptors and induced secretory leucocyte protease inhibitor (SLPI) secretion through formation of cyclic adenosine monophosphate and mobilization of intracellular [Ca(2+)]. Since diadenosine polyphosphates (ApnA) are also endogenous effectors of P2 receptors, we investigated their effects in a cell line (MM39) of human tracheal gland cells. Diadenosine tetraphosphates (Ap4A) induced significant stimulation (+50+/-12%) of SLPI secretion and to a similar extent to that of ATP (+65+/-10%). No significant effects were observed with diadenosine triphosphate (Ap3A), diadenosine pentaphosphate (Ap5A), ADP and 2-methylthio-adenosine triphosphate (2-MeS-ATP). Since Ap4A was weakly hydrolyzed (<2% of total), and the hydrolysis product was only inosine which is ineffective on cells, this Ap4A effect was not due to Ap4A hydrolysis in ATP and adenosine monophosphate (AMP). A mixture of Ap4A and ATP elicited only partial additive effects on SLPI secretion. ADP was shown to be a potent antagonist of ATP and Ap4A receptors, with IC(50)s of 0.8 and 2 microM, respectively. 2-MeS-ATP also showed antagonistic properties with IC(50)s of 20 and 30 microM for ATP- and Ap4A-receptors, respectively. Single cell intracellular calcium ([Ca(2+)](i)) measurements showed similar transient increases of [Ca(2+)](i) after ATP or Ap4A challenges. ATP desensitized the cell [Ca(2+)](i) responses to ATP and Ap4A, and Ap4A also desensitized the cell response to Ap4A. Nevertheless, Ap4A did not desensitize the cell [Ca(2+)](i) responses to ATP. In conclusion, both P2Y2-ATP-receptors and Ap4A-P2D-receptors seem to be present in tracheal gland cells. Ap4A may only bind to P2D-receptors whilst ATP may bind to both Ap4A- and ATP-receptors.

Characterization of P1,P4-diadenosine 5'-tetraphosphate binding on bovine aortic endothelial cells

In recent years it has become increasingly clear that alpha, omega-dinucleotides act as extracellular modulators of various biological processes. P1,P4-diadenosine 5'-tetraphosphate (Ap4A) is the best characterized alpha,omega-dinucleotides and acts as an extracellular signal molecule by inducing the release of nitric oxide (NO) from bovine aortic endothelial cells (BAEC) (R. H. Hilderman, and E. F. Christensen (1998) FEBS Lett. 407, 320-324). However, the characteristics of Ap4A binding to endothelial cells have not been determined. In this report we demonstrate that Ap4A binds to a heterogeneous population of receptors on BAEC. Competition ligand-binding studies using various adenosine dinucleotides, guanosine dinucleotides, adenosine/guanosine dinucleotides, and synthetic P2 purinoceptor agonists and antagonists demonstrate that Ap4A binds to a receptor on BAEC that has a high affinity for some of the adenosine dinucleotides. The apparent IC50 values for Ap4A, Ap2A, and Ap3A are between 12 and 15 microM, while the apparent IC50 values for Ap5A and Ap6A are greater than 500 microM. Evidence is also presented which suggests that this receptor can be classified as a putative P4 purinoceptor. Competition studies also demonstrate that Ap4A binds at a lower affinity to a second class of binding sites.

Binding and internalization of p1,p4-diadenosine 5'-tetraphosphate by bovine aortic endothelial cells

p1,p4-Diadenosine 5'-tetraphosphate (Ap4A) has been implicated as a modulator of blood vessel tone. We have recently demonstrated that the infusion of Ap4A into swine induces vasodilation (Hilderman et al., Am. J. Hypertension 10 (1997) 94A) and that Ap4A induces the release of nitric oxide (NO) from bovine aortic endothelial cells (BAEC) (Hilderman and Christensen, FEBS Lett. 427 (1998) 320-324). However, the interaction of Ap4A with endothelial cells is incompletely understood. Therefore, we determined the characteristics of [3H]-Ap4A binding to BAEC in normal and ATP-depleted cells. These binding studies demonstrate that the interaction of Ap4A with BAEC involves two distinct steps: an ATP independent step and a second ATP dependent step leading to internalization of Ap4A. The initial interaction of Ap4A with BAEC is not affected by either EGTA or iodoacetate; however, both agents block the second step. These data suggest that calcium ions and sulfhydryl groups are required for Ap4A internalization but not for an initial binding event.

Characterization of ATP and P2 agonists binding to the cardiac plasma membrane P1,P4-diadenosine 5'-tetraphosphate receptor

P1,P4-Diadenosine 5'-tetraphosphate (Ap4A) acts as an extracellular modulator through its interaction with purinoceptors. Our laboratory has demonstrated the presence of an Ap4A receptor in cardiac tissue [1,2]. Due to the rapid hydrolysis of ATP by cardiac membranes the relationship of ATP and Ap4A binding to purinoceptors on cardiac membranes has not been characterized. In this communication we used two approaches to determine the relationship of ATP to the Ap4A receptor. Radioligand binding carried out with [alpha-32P]Ap4A and adenosine 5'-O-¿3-thiotriphosphate¿ ([gamma-35S]ATPgammaS) demonstrates the presence of a single high affinity binding site for Ap4A and the presence of two binding sites for ATPgammaS. The second approach utilized immunoaffinity purified Ap4A receptor that was shown to be free of ATPase and Ap4Aase activities. Non-radiolabeled Ap4A and ATPgammaS effectively inhibited photocrosslinking of [alpha-32P]8-N3Ap4A to the receptor polypeptide while ATP was a much less effective inhibitor. Furthermore, on plasma membranes [alpha-32P]8-N3Ap4A photocrosslinked to only a 50 kDa polypeptide. These data are consistent with Ap4A interacting with a homogeneous population of receptors on cardiac plasma membranes but with ATP having a low affinity for the receptor.

Purinergic signalling: pathophysiological roles

In this review, after a summary of the history and current status of the receptors involved in purinergic signalling, we focus on the distribution and physiological roles of purines and pyrimidines in both short-term events such as neurotransmission, exocrine and endocrine secretion and regulation of immune cell function, and long-term events such as cell growth, differentiation and proliferation in development and regeneration. Finally, the protective roles of nucleosides and nucleotides in events such as cancer, ischemia, wound healing, drug toxicity, inflammation and pain are explored and some suggestions made for future developments in this rapidly expanding field, with particular emphasis on the involvement of selective agonists and antagonists for purinergic receptor subtypes in therapeutic strategies.

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.

ATP acting on P2Y receptors triggers calcium mobilization in Schwann cells at the neuroelectrocyte junction in skate

Schwann cells are integral cellular components of the dense cholinergic presynaptic plexus (nerve plate) which innervates each electrocyte in skate electric organ. Using the Ca2+-sensitive dye fura-2, we have followed the response in these cells to various chemical challenges. In K+ depolarized nerve plates nerve terminals consistently responded with a rapid and sustained Ca2+ signal. Schwann cell responses to depolarization were rarely seen but, when observed, were always delayed in onset when compared to nerve terminal response (6-10 s later). The possibility that these responses were triggered by mediators released from nerve terminals was tested by direct application of candidate substances. Schwann cells were found to respond to adenosine triphosphate and adenosine diphosphate with a biphasic increase of intracellular Ca2+ concentration, a rapid peak response being followed in the majority of cells by a sustained plateau phase. In the absence of external Ca2+ only the transient peak response was observed. Depletion of internal Ca2+ stores with thapsigargin completely inhibited the adenosine triphosphate-stimulated rise in Schwann cell Ca2+. The response to adenosine triphosphate was concentration-dependent (EC50 2.8 microM) and was reversibly blocked by two antagonists of P2 purinoceptors: suramin and reactive blue 2. Adenosine diphosphate and 2-methylthio-adenosine triphosphate were equipotent with adenosine triphosphate and at high concentrations (100 microM) diadenosine tetraphosphate produced responses comparable to low concentrations of adenosine triphosphate. Adenosine, adenosine monophosphate, the alpha beta-methylene analogues of adenosine triphosphate and adenosine diphosphate, uridine triphosphate, cytidine triphosphate and guanosine triphosphate were without significant effect. These results show that, in skate electric organ Schwann cells, the release of Ca2+ from intracellular stores is triggered by adenosine triphosphate acting on P(2gamma) receptors and suggest that Schwann cells may be targets for synaptically-released adenosine triphosphate in the electric organ model of the neuromuscular junction.

The interaction of diadenosine polyphosphates with P2x-receptors in the guinea-pig isolated vas deferens

1. The site(s) at which diadenosine 5',5"'-P1,P4-tetraphosphate (AP4A) and diadenosine 5', 5"'-P1,P5-pentaphosphate (AP5A) act to evoke contraction of the guinea-pig isolated vas deferens was studied by use of a series of P2-receptor antagonists and the ecto-ATPase inhibitor 6-N,N-diethyl-D-beta,gamma-dibromomethyleneATP (ARL 67156). 2. Pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) (300 nM - 30 microM), suramin (3-100 microM) and pyridoxal-5'-phosphate (P-5-P) (3-1000 microM) inhibited contractions evoked by equi-effective concentrations of AP5A (3 microM), AP4A (30 microM) and alpha,beta-methyleneATP (alpha,beta-meATP) (1 microM), in a concentration-dependent manner and abolished them at the highest concentrations used. 3. PPADS was more potent than suramin, which in turn was more potent than P-5-P. PPADS inhibited AP5A, AP4A and alpha,beta-meATP with similar IC50 values. No significant difference was found between IC50 values for suramin against alpha,beta-meATP and AP5A or alpha,beta-meATP and AP4A, but suramin was more than 2.5 times more potent against AP4A than AP5A. P-5-P showed the same pattern of antagonism. 4. Desensitization of the P2xi-receptor by alpha,beta-meATP abolished contractions evoked by AP5A (3 microM) and AP4A (30 microM), but had no effect on those elicited by noradrenaline (100 microM). 5. ARL 67156 (100 microM) reversibly potentiated contractions evoked by AP4A (30 microM) by 61%, but caused a small, significant decrease in the mean response to AP5A (3 microM). 6. It is concluded that AP4A and AP5A act at the P2xi-receptor, or a site similar to the P2xi-receptor, to evoke contraction of the guinea-pig isolated vas deferens. Furthermore, the potency of AP4A, but not AP5A, appears to be inhibited by an ecto-enzyme which is sensitive to ARL 67156.

Diadenosine polyphosphates regulate cytosolic calcium in human fibroblast cells by interaction with P2x purinoceptors coupled to phospholipase C

The effects of diadenosine pentaphosphate (AP5A), and diadenosine hexaphosphate (AP6A) on the cytosolic-free Ca2+ concentration ([Ca2+]i) were evaluated in cultured human fibroblast cells (HF cells) using the fluorescent dye technique. AP5A, and AP6A concentration-dependently increased [Ca2+]i in HF cells. The addition of 10 mumol/1 AP5A and AP6A significantly increased [Ca2+]i in HF cells from 71 +/- 3 nmol/1 (n = 184) to 241 +/- 39 nmol/1 (n = 11; P < 0.001 compared to resting value) and to 227 +/- 26 nmol/1 (n = 23; P < 0.001), respectively. The purinoceptor P2 blockers, suramin and pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), inhibited the diadenosine polyphophate-induced [Ca2+]i increase, whereas the P2y purinoceptor blocker, reactive blue, had no effect. Adenosinetriphosphate (ATP) and the P2x agonist, alpha 1 beta-methylene-ATP also significantly increased [Ca2+]i in HF cells, whereas the P2y agonist methylthio-ATP showed only a small [Ca2+]i response. Diadenosine polyphosphates mainly induced transplasmamembrane Ca2+ influx as was confirmed by experiments in the absence of extracellular Ca2+ or by manganese quenching studies. Organic (verapamil) and inorganic Ca2+ channel blockers (NiCI2) significantly reduced the AP6A induced transplasmamembrane Ca2+ influx. The inhibitor of phosphatidylcholine-specific phospholipase C, D609, significantly reduced the effect of diadenosine polyphosphates on [Ca2+]i in HF cells. It is concluded that diadenosine polyphosphates regulate transplasmamembrane Ca2+ influx after occupation of P2x receptors via activation of phosphatidylcholine-specific phospholipase C and hence of voltage-operated Ca2+ channels.

Brain synaptosomes display a diadenosine tetraphosphate (Ap4A)-mediated Ca2+ influx distinct from ATP-mediated influx

Studies undertaken to compare the effects of Ap4A and ATP on altering intrasynaptosomal Ca2+ levels from deermouse brain reveal that both ligands induce a rapid influx of extracellular Ca2+. The Ca2+ profile elicited by 167 microM Ap4A is "spike-like" (half-time for decline to baseline, 19.1 +/- 1.2 sec), in contrast to the gradual decline observed with ATP (104.0 +/- 7.4 sec). DIDS (4-4'-diisothiocyano-2,2'-disulfonic acid stilbene) and suramin preincubation alter only the ATP-induced Ca2+ profile. Cross-desensitization studies indicate that prior application of ATP does not significantly affect the Ca2+ influx elicited by Ap4A, and that prior application of Ap4A does not affect the Ca2+ influx elicited by ATP. These results demonstrate that extracellular Ap4A and ATP elicit distinct intrasynaptosomal Ca2+ influx profiles, and suggest that these two nucleotides may be interacting with distinct purinoceptor subclasses or purinoceptor-effector complexes. Subjecting the synaptosomes simultaneously to depolarization and Ap4A, or to depolarization and ATP, induces an additive effect on Ca2+ influx. Preincubation with verapamil negates the effects of depolarization without modifying the ligand-elicited Ca2+ fluxes. These results indicate the presence of Ap4A and ATP ligand-gated channels that may function as modulators of neuronal activity.

Evidence for two distinct P2-purinoceptors subserving contraction of the rat anococcygeus smooth muscle

1. The effects of the P2-purinoceptor agonists, adenosine 5'-triphosphate (ATP), alpha, beta-methylene ATP (alpha, beta-MeATP), beta, gamma-methylene ATP (beta, gamma-MeATP), L-beta, gamma-methylene ATP (L-beta, gamma-MeATP), adenosine-5'-O-(2-thiodiphosphate) (ADP beta S), and 2-methylthio ATP (2-MeSATP) were investigated on the isometric tension of the rat anococcygeus muscle. 2. Non-cumulative additions of ATP (100-1500 microM), alpha, beta-MeATP (1-300 microM), beta, gamma-MeATP (10-300 microM), L-beta, gamma-MeATP (3-100 microM) and ADP beta S (1-100 microM) produced concentration-dependent contractions, whereas 2-MeSATP (1-100 microM) had no effect. The rank order of potency was alpha, beta-MeATP > L-beta, gamma-MeATP > or = ADP beta S > beta, gamma-MeATP > > ATP > 2-MeSATP. 3. Contractions to cumulative additions of ATP, alpha, beta-MeATP, beta, gamma-MeATP and L-beta, gamma-MeATP were subject to desensitization whilst those to ADP beta S were unaffected. 4. Contractions to ATP, alpha, beta-MeATP, beta, gamma-MeATP and ADP beta S were abolished by the non-selective P2X/. P2Y-purinoceptor antagonist, suramin (100 microM). In contrast, contractions to ATP, alpha, beta-MeATP and beta, gamma-MeATP were not affected by the non-selective P1-purinoceptor antagonist 8-(p-sulphophenyl)-theophylline (8SPT, 30 microM). Blockade of P2X-purinoceptors with the selective P2X-purinoceptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 10 microM) or desensitization with L-beta, gamma-MeATP (10 microM) abolished contractions to alpha, beta-MeATP, but enhanced those to ADP beta S. The P2Y-purinoceptor antagonist, reactive blue 2 (RB2, 100 microM) enhanced contractions to ATP and alpha, beta-MeATP but abolished those to ADP beta S. 5. Simultaneous addition of alpha, beta-MeATP and ADP beta S produced an additive contraction. 6. The findings suggest that in the rat anococcygeus, smooth muscle cells are endowed with two distinct P2-purinoceptors which subserve contractions: a P2X-purinoceptor activated by ATP and its analogues, and another type of P2-purinoceptor activated by ADP beta S.

Characterization of the binding of diadenosine 5',5'''-P1,P4-tetraphosphate (Ap4A) to rat liver cell membranes

Diadenosine polyphosphates present in the extracellular environment can, through interaction with appropriate purinoceptors, influence a range of cellular activities. Here we have investigated the nature of the ligand:receptor interactions involved in diadenosine 5',5'''-P1, P4-tetraphosphate (Ap4A)-mediated stimulation of glycogen breakdown in isolated rat liver cells. [2-3H]Ap4A showed specific binding to both intact isolated liver cells and plasma membrane fractions prepared from isolated liver cells. HPLC analysis confirmed that binding was mediated by intact Ap4A and not by potential breakdown products (e.g. ATP, adenosine, etc.). Binding of [2-3H]Ap4A, to isolated liver cell plasma membrane preparations, was successfully displaced by a range of both naturally occurring and synthetic diadenosine polyphospates with the rank order potency Ap4A > or = Ap5A > Ap6A > Ap3A > Ap2A. [2-3H]Ap4A binding was not displaced by P1 effectors but was successfully displaced by a range of P2 effectors with the rank order potency 2-methylthio-ATP > ATP > ATP > or = adenosine 5'-[alpha beta-methylene]triphosphate > adenosine 5'-[beta gamma-methylene]triphospate. These findings are consistent with the interaction of Ap4A with a P2y-like subclass of purinoceptor and are discussed in relation to (1) the known purinoceptor populations in liver cell plasma membranes and (2) observations concerning the binding of diadenosine polyphosphates to purinoceptors in other tissues.

Isolation of a tripeptide from a random phage peptide library that inhibits P1,P4-diadenosine 5'-tetraphosphate binding to its receptor

Extracellular P1,P4-diadenosine 5'-tetraphosphate (Ap4A) has been implicated as a modulator of cell stress. We have previously demonstrated specific receptors for Ap4A at the surface of cardiac myocytes (Walker et al., 1993a). In addition, we have isolated a monoclonal antibody (mAb TL4) that recognized the Ap4A receptor and inhibited binding of Ap4A to its receptor (Walker & Hilderman, 1993). As part of our effort to characterize the Ap4A receptor building domain, we screened a random phage peptide library with mAb TL4. After affinity purification of specifically bound phage, we isolated 38 individual phage clones. Twenty-eight of these clones bound mAb TL4 in ELISA and dot blot analyses. Twenty-two of the twenty-eight individual clones contained inserts with an RGS tripeptide sequence. Synthetic RGS peptide specifically inhibits the binding of mAb TL4 to its membrane receptor. Furthermore, the RGS peptide also inhibits [3H]Ap4A binding to its receptor. These data are consistent with the RGS peptide mimicking part of the mAb TL4 recognition site on the Ap4A receptor. The The RGS peptide may be used to help characterize the Ap4A receptor binding domain and to help determine the physiological significance of the interaction between Ap4A and its receptor.

Pre- and postjunctional effects of diadenosine polyphosphates in the guinea-pig vas deferens

The pre- and postjunctional activities of a number of diadenosine polyphosphates were examined in the guinea-pig isolated vas deferens at the level of the membrane potential, using a modified sucrose-gap technique. P1,P3-Di(adenosine 5')triphosphate (Ap3A), P1,P4-di(adenosine 5')tetraphosphate (Ap4A) and P1,P5-di(adenosine 5')pentaphosphate (Ap5A) all caused concentration-dependent depolarization of the smooth muscle membrane. The potency order was: Ap5A > Ap4A > or = Ap3A. P1,P2-Di(adenosine 5')pyrophosphate (Ap2A) did not evoke depolarization even at the highest concentration tested (1 mM). All the dinucleotides caused a reduction in the amplitude of evoked excitatory junction potentials (e.j.ps). The potency order was: Ap5A = Ap4A > Ap3A > Ap2A. The depolarizations evoked by the dinucleotides were markedly reduced by the selective P2X-purinoceptor antagonist, pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 10 microM), as was the amplitude of the fully facilitated e.j.p. The inhibition of the e.j.p. evoked by Ap3A and Ap2A was reduced by the P1-purinoceptor antagonist, 8-p-sulphophenyltheophylline (8-pSPT, 50 microM), but that evoked by Ap5A and Ap4A was not. Thus, Ap3A, Ap4A and Ap5A evoke depolarization of the guinea-pig vas deferens via P2X-purinoceptors, and additionally Ap2A and Ap3A exert a prejunctional effect via P1-purinoceptors. The prejunctional activity of Ap4A and Ap5A is mediated via an undefined purinoceptor, which is neither P1 nor P2X.

Activation of NO:cGMP pathway by acetylcholine in bovine chromaffin cells. Possible role of Ca2+ in the down-regulation of cGMP signaling

The production of cyclic GMP (cGMP) induced by acetylcholine and other stimuli was studied in bovine chromaffin cells. Acetylcholine increased intracellular cGMP in a transitory (peak at 2 min) and concentration-dependent manner (estimated half maximal increase, EC50 = 61 +/- 5 microM). NG-nitro-L-arginine methyl ester (NAME) inhibited such a rise in cGMP with a half maximal inhibitory concentration (IC50) of 231 +/- 55 microM. The acetylcholine-induced increase in cGMP was also inhibited by a calmodulin antagonist (calmidazolium, 30 microM) and by the absence of extracellular calcium. Other agents that strongly increased cytosolic calcium concentration ([Ca2+]i) as acetylcholine did, such as the nicotinic-agonist, 1,1-dimethyl-4-phenylpiperazinium (DMPP), high-KCl (50 mM), and ionomycin, also caused a rise in cGMP in cultured bovine chromaffin cells. Veratridine, an activator of sodium channels, produced a slowly developing calcium increase and no significant cGMP production. The muscarinic-agonist, muscarine, failed to increase cytosolic calcium, and was the weakest stimulator of cGMP production. cGMP formation, induced by sodium nitroprusside (SNP, 100 microM) and by C-type natriuretic peptide (CNP, 100 nM), was inhibited by 30-40% by increasing [Ca2+]i with ionomycin. This inhibition was abolished by calmidazolium (30 microM) and by the absence of calcium in the extracellular medium. In conclusion, bovine chromaffin cells synthesize nitric oxide (NO) to activate guanylate cyclase in response to several stimuli, which increase [Ca2+]i. Moreover, the increase in [Ca2+]i also stimulates a Ca2+/calmodulin phosphodiesterase, which could down-regulate the levels of cGMP in these cells.

Pharmacological selectivity of the cloned human P2U-purinoceptor: potent activation by diadenosine tetraphosphate

1. The human P2U-purinoceptor was stably expressed in 1321N1 human astrocytoma cells and the pharmacological selectivity of the expressed receptor was studied by measurement of inositol lipid hydrolysis. 2. High basal levels of inositol phosphates occurred in P2U-purinoceptor-expressing cells. This phenomenon was shown to be due to release of large amounts of ATP from 1321N1 cells, and could be circumvented by adoption of an assay protocol that did not involve medium changes. 3. UTP, ATP and ATP gamma S were full and potent agonists for activation of phospholipase C with EC50 values of 140 nM, 230 nM, and 1.72 microM, respectively. 5BrUTP, 2C1ATP and 8BrATP were also full agonists although less potent than their natural congeners. Little or no effect was observed with the selective P2Y-, P2X-, and P2T-purinoceptor agonists, 2MeSATP, alpha,beta-MeATP, and 2MeSADP, respectively. 4. Diadenosine tetraphosphate, Ap4A, was a surprisingly potent agonist at the expressed P2U-purinoceptor with an EC50 (720 nM) in the range of the most potent P2U-purinoceptor agonists. Ap4A may be a physiologically important activator of P2U-purinoceptors.

Cytosolic calcium responses to extracellular adenosine 5',5" '-P1,P4-tetraphosphate in PC12 cells

Binding of adenosine 5',5" '-P1,P4-tetraphosphate (Ap4A) to a purinoceptor on nerve growth factor-differentiated (NGF) pheochromacytoma (PC12) cells modulated cytosolic Ca2+ levels. Both Ap4A and ATP elicited an influx of extracellular Ca2+, but both the sensitivity of the response and the flux profile were different. Preincubation of the PC12 cells with the compounds adenosine 5'-0-(2-thio)diphosphate (ADP-beta-S) and periodate-oxidized ATP had differential effects upon the Ap4A and ATP-induced response. These results indicate that Ap4A and ATP were either interacting with distinct purinoceptor subclasses or with the same purinoceptor with differing affinities. Simultaneous depolarization and application of either Ap4A or ATP to the PC12 cells induced an additive effect on the calcium flux. Preincubation with verapamil negated the effects of depolarization without significantly modifying the ligand-elicited Ca2+ fluxes, suggesting the presence of Ap4A ligand-gated channels that may function as modulators of PC12 cell function.

Effects of diadenosine polyphosphates, ATP and angiotensin II on cytosolic Ca2+ activity and contraction of rat mesangial cells

Diadenosine polyphosphates (Apn A) are known to influence cellular Ca2+ activity ([Ca2+]i) in several cells. Their vasoactive potency has been described in various systems including the kidney. We examined the effects of diadenosine polyphosphates, adenosine 5'-triphosphate (ATP) and angiotensin II (Ang II) on cytosolic Ca2+ activity of mesangial cells (MC) in culture obtained from normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats. [Ca2+]i was measured as a fluorescence ratio F340/F380 with the fura-2 technique using three excitation wavelengths (340 nm, 360 nm and 380 nm) and a photon counting tube. Resting [Ca2+]i was not significantly different in MC from WKY and SHR rats and was measured as 132 +/- 9 nmol/l (n = 65) and 114 +/- 12 nmol/l (n = 36), respectively. Diadenosine polyphosphates (Ap3A-Ap6A) increased [Ca2+]i transiently with an initial peak and a secondary plateau phase comparable to the effects of ATP or Ang II. Increases in [Ca2+]i induced by all these agonists were not significantly different between MC of WKY and SHR rats. ATP, Ap3A, Ap4A, Ap5A, Ap6A (each 5 micromol/l) increased the fura-2 fluorescence ratio initially by 0.66 +/- 0.09 (n = 33), 0.52 +/- 0.08 (n = 18), 0.25 +/- 0.05 (n = 16), 0.09 +/- 0.06 (n = 7), 0.09 +/- 0.04 (n = 11), respectively. A half-maximal initial increase in the fura-2 fluorescence ratio was reached at 22 nmol/l, 0.9 micromol/l, 2.0 micromol/l and 4.0 micromol/l with Ang II, Ap3A, ATP and Ap4A, respectively. Ap4A (100 micromol/l, n = 18) led to a reversible contraction of MC. Diadenosine polyphosphates increase [Ca2+]i in rat MC, in a similar manner to ATP or Ang II and lead to a contraction of MC, suggesting that these nucleotides are also involved in the control of glomerular haemodynamics.

Cytosolic free Ca2+ oscillations induced by diadenosine 5',5"'-P1,P3-triphosphate and diadenosine 5',5"'-P1,P4-tetraphosphate in single rat hepatocytes are indistinguishable from those induced by ADP and ATP respectively

Diadenosine 5',5"'-P1,P3-triphosphate (Ap3A) and diadenosine 5',5"'-P1,P4-tetraphosphate (Ap4A) induce distinctive patterns of [Ca2+]i oscillations in single rat hepatocytes. We show here that [Ca2+]i oscillations induced by Ap3A and ADP are indistinguishable and that [Ca2+]i oscillations induced by Ap4A closely resemble those induced by ATP. These similarities embrace the following: (1) ADP and Ap3A invariably induce [Ca2+]i transients of short duration (approx. 9 s). Ap4A, like ATP, can induce, depending upon the individual cell, either transients of short duration (approx. 9 s), transients of much longer duration or a mixture of short and long transients within a single response. We show here that the pattern of oscillations induced by Ap4A is similar to that induced by ATP in the same hepatocyte. (2) Elevated intracellular cyclic AMP concentration modulates Ap3A-induced transients, like ADP-induced transients, through an increase in both the peak [Ca2+]i and the frequency of the transients. In contrast, Ap4A-induced transients, like ATP-induced transients, develop an increased duration or a sustained rise in [Ca2+]i, with no rise in peak [Ca2+]i. (3) Ap3A-induced transients, like ADP-induced transients, are abolished by low concentrations of the phorbol ester 4 beta-phorbol 12,13-dibutyrate (PDB; 5-10 nM), whereas long Ap4A-induced transients, like long ATP-induced transients, are refractory to high concentrations of PDB (100 nM). We propose that the [Ca2+]i oscillations induced in rat hepatocytes by Ap3A are mediated by the same purinoceptor that mediates the effects of ADP, whereas the oscillations induced by Ap4A are mediated by the same purinoceptor(s) that mediate the effects of ATP.

Extracellular ATP inhibits agonist-induced mobilization of internal calcium in human platelets

Our previous studies have demonstrated that platelets possess ATP purinergic receptors in addition to the ADP, P2T, receptor. Occupancy of the P2 receptor by ATP inhibited agonist-induced platelet aggregation. This study demonstrated that the mechanism of inhibition may involve ATP inhibition of agonist-induced mobilization of internal calcium. Within the cardiovascular system, the ATP inhibition of calcium mobilization is unique to platelets. All other cell types in the cardiovascular system, where calcium mobilization is affected by extracellular ATP, responded with an increased mobilization as opposed to inhibition. The platelet inhibitory response to ATP was enhanced by the addition of an ATP generating system, creatine phosphate/phosphocreatine kinase. ATP and ATP analogues were found to inhibit calcium mobilization with a rank order of alpha beta-methylene ATP, beta gamma-methylene ATP approximately ATP > benzoyl ATP > 2 methylthio ATP which is a characteristic of P2x-like receptors. The inhibitory effect of ATP could be abrogated by prolonged treatment of platelets with the P2x desensitizing agent, alpha beta-methylene ATP. Also, UTP and CTP were approximately as effective inhibitors as ATP while GTP was not. ATP competition with ADP for the P2T receptor was excluded in studies with platelets derived from an aspirin-treated individual which were essentially insensitive to ADP. The agonist-induced calcium mobilization and inhibition by ATP occurred with the thromboxane A2 mimetic, U46619, collagen and thrombin; however, the kinetics of mobilization varied somewhat with the different agonists. The responses to extracellular ATP were independent of extracellular Ca2+, where 1 mM calcium or 0.3 mM EGTA was added to the reaction mixture. The inhibition of calcium mobilization coupled to inhibition of platelet aggregation by extracellular ATP may serve an important physiologic role. ATP, released from activated platelets at localized sites of vascular injury, may help to limit the size of the platelet plug-clot that, if left unregulated, could occlude the injured blood vessel.

Cultured chick sympathetic neurons: modulation of electrically evoked noradrenaline release by P2-purinoceptors

The present study investigates the pharmacological profile of P2-purinoceptors modulating noradrenaline release from cultured chick sympathetic neurons. ATP (30 microM-3 mM) and 2-methylthio-ATP (3-100 microM), but not alpha, beta-methylene-ATP (up to 100 microM), caused a significant facilitation of electrically evoked [3H]-noradrenaline release when added 2 min before depolarization. The facilitation declined with time of exposure suggesting receptor desensitization. The facilitatory effect was markedly diminished by the P2-purinoceptor antagonists reactive blue 2 (3 microM) and suramin (300 microM), but not changed by mecamylamine (10 microM), a nicotinic receptor antagonist. At 1 mM and higher concentrations, ATP added for 12 min, inhibited noradrenaline release; release was virtually abolished by 6 mM ATP. The inhibitory effect of ATP was slightly diminished by suramin but not affected by reactive blue 2. Electrically evoked [3H]-noradrenaline release remained unaffected in the presence of the adenosine (P1)-receptor agonists R(-)N6-(2-phenylisopropyl)adenosine (R-PIA), 2-[p-(2-carboxyethyl) phenylethylamino]-5'-N-ethylcarboxamidoadenosine (NECA), and N6-2-(4-aminophenyl)ethyladenosine (APNEA), used up to 1 microM. The present results confirm the existence of two P2-purinoceptors affecting noradrenaline release: 1) a facilitatory receptor which is activated by 2-methylthio-ATP as well as ATP, and blocked by suramin as well as reactive blue 2, and 2) an inhibitory receptor which is activated by ATP, only slightly affected by suramin but not at all by reactive blue 2 and does not belong to the established P2-purinoceptor subtypes.

Cyclic GMP-dependent protein kinase activation mediates inhibition of catecholamines secretion and Ca2+ influx in bovine chromaffin cells

The effects of the membrane-permeable cGMP analogue, 8-bromoguanosine 3':5'-cyclic monophosphate on acetylcholine-evoked catecholamine secretion and cytosolic calcium increases were studied in chromaffin cells from the bovine adrenal gland. Preincubation with 100 microM 8-bromoguanosine 3':5'-cyclic monophosphate during 10 and 30 min decreased the acetylcholine-evoked catecholamine release by 16 +/- 3% and 27 /+- 5%, respectively. The cytosolic calcium increases triggered by acetylcholine and 30 mM KCl were also inhibited by 30 min of preincubation with this compound by 27 +/- 4 and 34 /+- 12%, respectively. Changes in membrane potential induced by acetylcholine and KCl were not affected by preincubation with 8-bromoguanosine 3':5'-cyclic monophosphate. The cyclic GMP-dependent protein kinase inhibitor N-[2-(methylamino)ethyl]-5-isoquinoline sulfonamide dihydrochloride-at l micron abolished the inhibitory effect of 8-bromoguanosine 3':5'-cyclic monophosphate on acetylcholine-evoked calcium increase. By contrast, a potent and selective inhibitor against cyclic AMP-dependent protein kinase, N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide did not block the 8-bromoguanosine 3':5'-cyclic monophosphate effect. Additionally, 8-bromoguanosine 3':5'-cyclic monophosphate stimulated histone F2b phosphorylation by a partial purified cGMP-dependent protein kinase from chromaffin cells. The extent of histone phosphorylation was reduced by N-[2-(methylamino)ethyl]-5-isoquinolinesulfonamide dihydrochloride and 8-(4-chlorophenylthio)-guanosine 3':5'-cyclic monophosphorothioate, Rp-isomer, a specific inhibitor against cyclic GMP-dependent protein kinase, whereas it was not modified by N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinoline sulfonamide. The results suggest that the inhibitory effects of 8-bromoguanosine 3':5' cyclic monophosphate on chromaffin cells are mediated through the activation of cGMP-dependent protein kinase.(ABSTRACT TRUNCATED AT 250 WORDS)

Contribution of P2-purinoceptors to neurogenic contraction of rat urinary bladder smooth muscle

1. The contribution of P2-purinoceptors to neurogenic contraction was investigated in rat urinary bladder smooth muscle by measurement of isotonic tension. 2. Contraction of rat urinary bladder smooth muscle induced by electrical stimulation was decreased to 84.19 +/- 3.90% of the control (n = 16) in the presence of atropine (1 microM), which was further decreased to 38.80 +/- 2.75% of the control (n = 49) in the presence of both atropine and 10 microM alpha, beta-methylene adenosine 5'-triphosphate (alpha, beta-Me ATP). 3. The contractile response induced by electrical stimulation in the presence of atropine and alpha, beta-Me ATP was decreased to 27.81 +/- 4.07% (n = 23) and 26.63 +/- 5.01% (n = 15) of the control, by the addition of 100 microM cibacron blue 3GA and 100 microM suramin, respectively. The application of 100 microM adenosine 5'-o-2-thiodiphosphate (ADP beta S) in the presence of atropine and alpha, beta-Me ATP decreased the contractile response induced by electrical stimulations to 17.15 +/- 3.71% (n = 15) of the control. 4. Pretreatment of muscle strips with 100 microM ADP beta S significantly reduced the response to either 200 microM alpha, beta-methylene adenosine 5'-diphosphate or 200 microM ADP beta S. 5. Uridine 5'-triphosphate (100 microM to 1 mM) concentration-dependently contracted muscle strips, and this contraction was significantly antagonized by desensitization of P2-receptors with alpha, beta-Me ATP (10 microM), and completely antagonized by pretreatment of muscle strips with both alpha, beta-Me ATP and ADP beta S (100 microM). 6. Di(adenosine-5') tetraphosphate (30 and 100 microM) contracted muscle strips, whereas it failed to contract after desensitization of P2-receptors.7. It is suggested that about 20% of the neurogenic contraction of rat urinary bladder smooth muscle is mediated via ADP beta S-sensitive purinoceptors.

Characterisation of an ATP receptor mediating mitogenesis in vascular smooth muscle cells

Adenosine triphosphate (ATP), a co-transmitter in sympathetic nerves and released from platelets, has recently been shown to stimulate growth of vascular smooth muscle cells. It might therefore contribute to the development of vascular hypertrophy seen in hypertension and atherosclerosis. We aimed at characterising the receptor mediating this mitogenic effect in rat aorta smooth muscle cells. The potency of agonists indicates a P2 purinoceptor since ATP > or = ADP >> AMP, adenosine. The P2x-receptor subtype, which is responsible for ATP induced vasoconstriction in rat aorta, does not mediate the mitogenic effect since alpha, beta-methyleneATP had no effect and beta, gamma-methyleneATP had lower potency than ATP. The P2Y-receptor subtype was excluded since the selective agonist 2-methylthioATP had weak effect with lower potency than ATP. When we studied the involvement of other nucleotides similar effects were seen of the purines ATP, GTP and ITP; also the pyrimidine UTP had powerful mitogenic effects (Emax = 52% of ATP) with similar potency. Nucleotides with fewer phosphate groups showed a stepwise fall in mitogenic effect. This indicates involvement of a nucleotide-receptor (P2U). Ap4A were of equal potency and effect as ATP. There was strong correlation between the mitogenic effects of the nucleotides and analogues with both 45Ca(2+)-influx and inositol phosphate (IP) production, indicating that they may participate in mediating the mitogenic response. This is the first study describing the potencies for the mitogenic effects of the selective ATP-analogues and other nucleotides in vascular smooth muscle cells. The receptor characterisation indicates a nucleotide-receptor similar to the receptor which stimulates 45Ca(2+)-influx and inositol phosphate-formation in rat aorta smooth muscle cells. Substances related to ATP such as GTP, ITP, UTP and Ap4A which also can be released extracellularly in vivo stimulate mitogenesis of rat aorta smooth muscle cells through the same receptor.

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.

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.

Modulation of the dihydropyridine-insensitive Ca2+ influx by 8-bromo-guanosine-3':5'-monophosphate, cyclic (8-Br-cGMP) in bovine adrenal chromaffin cells

Pretreatment of chromaffin cells with the permeable analogue of cGMP, 8-Br-cGMP (100 microM), leads to a reduction (35%) of depolarization-evoked intracellular calcium concentration ([Ca2+]i) increases. There is evidence that bovine adrenal chromaffin cells are provided with both dihydropyridine-sensitive and -resistant voltage-sensitive Ca2+ influx pathways. Combined incubations with nifedipine 10 microM and 8-Br-cGMP reduced KCl-evoked intracellular Ca2+ concentration to a greater extent that each compound separately. Moreover, 8-Br-cGMP failed to affect the [Ca2+]i transient induced by the L-type Ca2+ channel agonist Bay K 8644 (1 microM) under conditions of low depolarization. Neomycin (0.2 mM) and omega-AgaToxin-IVA (AgTx) (1 microM) inhibited the calcium transient to a similar extent, and this inhibition was not enhanced by the presence of 8-Br-cGMP. It is concluded that 8-Br-cGMP modulated the dihydropyridine-insensitive Ca2+ influx pathway in the chromaffin cell.