ATP-activated channels in excitable cells

Purinergic Signaling in Spermatogenesis

Adenosine triphosphate (ATP) serves as the essential source of cellular energy. Over the last two decades, however, ATP has also attracted increasing interest as an extracellular signal that activates purinergic plasma membrane receptors of the P2 family. P2 receptors are divided into two types: ATP-gated nonselective cation channels (P2X) and G protein-coupled receptors (P2Y), the latter being activated by a broad range of purine and pyrimidine nucleotides (ATP, ADP, UTP, and UDP, among others). Purinergic signaling mechanisms are involved in numerous physiological events and pathophysiological conditions. Here, we address the growing body of evidence implicating purinergic signaling in male reproductive system functions. The life-long generation of fertile male germ cells is a highly complex, yet mechanistically poorly understood process. Given the relatively sparse innervation of the testis, spermatogenesis relies on both endocrine control and multi-directional paracrine communication. Therefore, a detailed understanding of such paracrine messengers, including ATP, is crucial to gain mechanistic insight into male reproduction.⁠.

Purinergic signaling in the male reproductive tract

Purinergic receptors are ubiquitously expressed throughout the body and they participate in the autocrine and paracrine regulation of cell function during normal physiological and pathophysiological conditions. Extracellular nucleotides activate several types of plasma membrane purinergic receptors that form three distinct families: P1 receptors are activated by adenosine, P2X receptors are activated by ATP, and P2Y receptors are activated by nucleotides including ATP, ADP, UTP, UDP, and UDP-glucose. These specific pharmacological fingerprints and the distinct intracellular signaling pathways they trigger govern a large variety of cellular responses in an organ-specific manner. As such, purinergic signaling regulates several physiological cell functions, including cell proliferation, differentiation and death, smooth muscle contraction, vasodilatation, and transepithelial transport of water, solute, and protons, as well as pathological pathways such as inflammation. While purinergic signaling was first discovered more than 90 years ago, we are just starting to understand how deleterious signals mediated through purinergic receptors may be involved in male infertility. A large fraction of male infertility remains unexplained illustrating our poor understanding of male reproductive health. Purinergic signaling plays a variety of physiological and pathophysiological roles in the male reproductive system, but our knowledge in this context remains limited. This review focuses on the distribution of purinergic receptors in the testis, epididymis, and vas deferens, and their role in the establishment and maintenance of male fertility.

ATP activation of peritubular cells drives testicular sperm transport

Spermatogenesis, the complex process of male germ cell proliferation, differentiation, and maturation, is the basis of male fertility. In the seminiferous tubules of the testes, spermatozoa are constantly generated from spermatogonial stem cells through a stereotyped sequence of mitotic and meiotic divisions. The basic physiological principles, however, that control both maturation and luminal transport of the still immotile spermatozoa within the seminiferous tubules remain poorly, if at all, defined. Here, we show that coordinated contractions of smooth muscle-like testicular peritubular cells provide the propulsive force for luminal sperm transport toward the rete testis. Using a mouse model for in vivo imaging, we describe and quantify spontaneous tubular contractions and show a causal relationship between peritubular Ca2+ waves and peristaltic transport. Moreover, we identify P2 receptor-dependent purinergic signaling pathways as physiological triggers of tubular contractions both in vitro and in vivo. When challenged with extracellular ATP, transport of luminal content inside the seminiferous tubules displays stage-dependent directionality. We thus suggest that paracrine purinergic signaling coordinates peristaltic recurrent contractions of the mouse seminiferous tubules to propel immotile spermatozoa to the rete testis.

Distinct purinergic signaling pathways in prepubescent mouse spermatogonia

Spermatogenesis ranks among the most complex, yet least understood, developmental processes. The physiological principles that control male germ cell development in mammals are notoriously difficult to unravel, given the intricate anatomy and complex endo- and paracrinology of the testis. Accordingly, we lack a conceptual understanding of the basic signaling mechanisms within the testis, which control the seminiferous epithelial cycle and thus govern spermatogenesis. Here, we address paracrine signal transduction in undifferentiated male germ cells from an electrophysiological perspective. We identify distinct purinergic signaling pathways in prepubescent mouse spermatogonia, both in vitro and in situ. ATP-a dynamic, widespread, and evolutionary conserved mediator of cell to cell communication in various developmental contexts-activates at least two different spermatogonial purinoceptor isoforms. Both receptors operate within nonoverlapping stimulus concentration ranges, display distinct response kinetics and, in the juvenile seminiferous cord, are uniquely expressed in spermatogonia. We further find that spermatogonia express Ca(2+)-activated large-conductance K(+) channels that appear to function as a safeguard against prolonged ATP-dependent depolarization. Quantitative purine measurements additionally suggest testicular ATP-induced ATP release, a mechanism that could increase the paracrine radius of initially localized signaling events. Moreover, we establish a novel seminiferous tubule slice preparation that allows targeted electrophysiological recordings from identified testicular cell types in an intact epithelial environment. This unique approach not only confirms our in vitro findings, but also supports the notion of purinergic signaling during the early stages of spermatogenesis.

Physiological mechanisms for the modulation of pannexin 1 channel activity

It is widely recognized that ATP, along with other nucleotides, subserves important intercellular signalling processes. Among various nucleotide release mechanisms, the relatively recently identified pannexin 1 (Panx1) channel is gaining prominence by virtue of its ability to support nucleotide permeation and release in a variety of different tissues. Here, we review recent advances in our understanding of the factors that control Panx1 channel activity. By using electrophysiological and biochemical approaches, diverse mechanisms that dynamically regulate Panx1 channel function have been identified in various settings; these include, among others, activation by caspase-mediated channel cleavage in apoptotic immune cells, by G protein-coupled receptors in vascular smooth muscle, by low oxygen tension in erythrocytes and neurons, by high extracellular K(+) in various cell types and by stretch/strain in airway epithelia. Delineating the distinct mechanisms of Panx1 modulation that prevail in different physiological contexts provides the possibility that these channels, and ATP release, could ultimately be targeted in a context-dependent manner.

A new function for ATP: activating cardiac sympathetic afferents during myocardial ischemia

Myocardial ischemia activates cardiac sympathetic afferents leading to chest pain and reflex cardiovascular responses. Brief myocardial ischemia leads to ATP release in the interstitial space. Furthermore, exogenous ATP and α,β-methylene ATP (α,β-meATP), a P2X receptor agonist, stimulate cutaneous group III and IV sensory nerve fibers. The present study tested the hypothesis that endogenous ATP excites cardiac afferents during ischemia through activation of P2 receptors. Nerve activity of single unit cardiac sympathetic afferents was recorded from the left sympathetic chain or rami communicates (T(2)-T(5)) in anesthetized cats. Single fields of 45 afferents (conduction velocities = 0.25-4.92 m/s) were identified in the left ventricle with a stimulating electrode. Five minutes of myocardial ischemia stimulated 39 of 45 cardiac afferents (8 Aδ, 37 C fibers). Epicardial application of ATP (1-4 μmol) stimulated six ischemically sensitive cardiac afferents in a dose-dependent manner. Additionally, epicardial ATP (2 μmol), ADP (2 μmol), a P2Y agonist, and α,β-meATP (0.5 μmol) significantly activated eight other ischemically sensitive afferents. Third, pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid, a P2 receptor antagonist, abolished the responses of six afferents to epicardial ATP (2 μmol) and attenuated the ischemia-related increase in activity of seven other afferents by 37%. In the absence of P2 receptor blockade, cardiac afferents responded consistently to repeated application of ATP (n = 6) and to recurrent myocardial ischemia (n = 6). Finally, six ischemia-insensitive cardiac spinal afferents did not respond to epicardial ATP (2-4 μmol), although these afferents did respond to epicardial bradykinin. Taken together, these data indicate that, during ischemia, endogenously released ATP activates ischemia-sensitive, but not ischemia-insensitive, cardiac spinal afferents through stimulation of P2 receptors likely located on the cardiac sensory neurites.

Characteristics of ATP-activated current in nodose ganglion neurons of rats

The characteristics of ATP-activated currents (I(ATP)) in rat nodose ganglion (NG) neurons have not been fully clarified. Especially, the correlation between I(ATP) phenotype and P2X receptor subunit genotype in rat NG neuron is not clear. By whole-cell patch-clamp and single cell immunocytochemical techniques, we explored the characteristics of the I(ATP) phenotype and its correlation with P2X receptor subunits in acutely isolated NG neuron of rats. The results indicated that I(ATP) of NG neurons can be classified into four types: F type (fast type, 8.1%), I type (intermediate type, 14.8%), S type (slowing type, 37.0%) and vS type (very slowing type, 21.5%). The single immunocytochemical studies have demonstrated that F type cells express P2X1 and P2X3 subunits, I type cells P2X1, P2X3 and P2X4 subunits, S type cells P2X1, P2X2 and P2X3 subunits and vS type cells P2X2, P2X3 and P2X4 subunits. The results reveal that there are four types of I(ATP) in NG neurons, differential expression of distinct P2X subunits may underlie the I(ATP) phenotype.

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.

Activation kinetics of single P2X receptors

After the primary structure of P2X receptors had been identified, their function had to be characterized on the molecular level. Since these ligand-gated ion channels become activated very quickly after binding of ATP, methods with adequate time resolution have to be applied to investigate the early events induced by the agonist. Single-channel recordings were performed to describe conformational changes on P2X(2), P2X(4), and P2X(7) receptors induced by ATP and also by allosteric receptor modifiers. The main results of these studies and the models of P2X receptor kinetics derived from these observations are reviewed here. The investigation of purinoceptors by means of the patch clamp technique following site-directed mutagenesis will probably reveal more details of P2X receptor function at the molecular level.

Estradiol inhibits the effects of extracellular ATP in human sperm by a non genomic mechanism of action

Steroid hormones, beside their classical genomic mechanism of action, exert rapid, non genomic effects in different cell types. These effects are mediated by still poorly characterized plasma membrane receptors that appear to be distinct from the classic intracellular receptors. In the present study we evaluated the non genomic effects of estradiol (17βE₂) in human sperm and its effects on sperm stimulation by extracellular ATP, a potent activator of sperm acrosome reaction. In human sperm 17βE₂ induced a rapid increase of intracellular calcium (Ca²⁺) concentrations dependent on an influx of Ca²⁺ from the extracellular medium. The monitoring of the plasma membrane potential variations induced by 17βE₂ showed that this steroid induces a rapid plasma membrane hyperpolarization that was dependent on the presence of Ca²⁺ in the extracellular medium since it was absent in Ca²⁺ free-medium. When sperm were pre-incubated in the presence of the K⁺ channel inhibitor tetra-ethylammonium, the 17βE₂ induced plasma membrane hyperpolarization was blunted suggesting the involvement of K⁺ channels in the hyperpolarizing effects of 17βE₂. Extracellular ATP induced a rapid plasma membrane depolarization followed by acrosome reaction. Sperm pre-incubation with 17βE₂ inhibited the effects of extracellular ATP on sperm plasma membrane potential variations and acrosome reaction. The effects of 17βE₂ were specific since its inactive steroisomer 17αE₂ was inactive. Furthermore the effects of 17βE₂ were not inhibited by tamoxifen, an antagonist of the classic 17βE₂ intracellular receptor.

Extracellular ATP as a signaling molecule for epithelial cells

The charge of this invited review is to present a convincing case for the fact that cells release their ATP for physiological reasons. Many of our "purinergic" colleagues as well as ourselves have experienced resistance to this concept, because it is teleologically counter-intuitive. This review serves to integrate the three main tenets of extracellular ATP signaling: ATP release from cells, ATP receptors on cells, and ATP receptor-driven signaling within cells to affect cell or tissue physiology. First principles will be discussed in the Introduction concerning extracellular ATP signaling. All possible cellular mechanisms of ATP release will then be presented. Use of nucleotide and nucleoside scavengers as well as broad-specificity purinergic receptor antagonists will be presented as a method of detecting endogenous ATP release affecting a biological endpoint. Innovative methods of detecting released ATP by adapting luciferase detection reagents or by using "biosensors" will be presented. Because our laboratory has been primarily interested in epithelial cell physiology and pathophysiology for several years, the role of extracellular ATP in regulation of epithelial cell function will be the focus of this review. For ATP release to be physiologically relevant, receptors for ATP are required at the cell surface. The families of P2Y G protein-coupled receptors and ATP-gated P2X receptor channels will be introduced. Particular attention will be paid to P2X receptor channels that mediate the fast actions of extracellular ATP signaling, much like neurotransmitter-gated channels versus metabotropic heptahelical neurotransmitter receptors that couple to G proteins. Finally, fascinating biological paradigms in which extracellular ATP signaling has been implicated will be highlighted. It is the goal of this review to convert and attract new scientists into the exploding field of extracellular nucleotide signaling and to convince the reader that extracellular ATP is indeed a signaling molecule.

Extracellular adenosine triphosphate stimulates acrosomal exocytosis in bovine spermatozoa via P2 purinoceptor

The presence of ATP in the genital tract fluid of mammals provokes questions regarding its function in the fertilization process. We investigated the effect of extracellular ATP (ATPe) on the activation of bovine spermatozoa. A signal transduction mechanism for ATP involving the receptor-mediated release of second messengers is described. Treatment of spermatozoa with ATP, uridine triphosphate (UTP), or 2-methylthio-ATP resulted in a concentration-dependent increase of acrosomal exocytosis, whereas treatment with either AMP or adenosine induced little exocytosis. This suggested that the receptor involved is of the P2 and not the P1 type. Several lines of evidence also suggest that the ATP purinoceptor is of the P2y and not the P2x type. First, the acrosome reaction was induced by the P2y-agonists ATP, UTP, or 2-methylthio-ATP, but no effects were shown by the P2x-agonists alpha,beta-methylene-ATP or beta,gamma-methylene-ATP. Second, ATP-induced acrosomal exocytosis was inhibited by the P2y antagonists, but not by the P2x antagonists. Third, enhanced Ca2+ uptake into the cells was observed with ATP and 2-methylthio-ATP, but not with beta,gamma-methylene-ATP. Additionally, ATP induced elevation of intracellular Ca2+ and cAMP, and the effect on cAMP was predominantly enhanced by including Ca2+ and the Ca2+-ionophore A23187 in the incubation medium. Extracellular ATP also activates protein kinase Calpha (PKCalpha), and the acrosome reaction, stimulated by ATPe, is inhibited by a PKC-specific inhibitor. In summary, we suggest that ATPe activates the P2 purinoceptor that elevates [Ca2+]i, which leads to PKCalpha activation and culminates in acrosomal exocytosis.

Extracellular adenosine triphosphate affects neural cell adhesion molecule (NCAM)-mediated cell adhesion and neurite outgrowth

The neural cell adhesion molecule (NCAM) plays an important role in synaptic plasticity in embryonic and adult brain. Recently, it has been demonstrated that NCAM is capable of binding and hydrolyzing extracellular ATP. The purpose of the present study was to evaluate the role of extracellular ATP in NCAM-mediated cellular adhesion and neurite outgrowth. We here show that extracellularly added adenosine triphosphate (ATP) and its structural analogues, adenosine-5'-O-(3-thiothiophosphate), beta, gamma-methylenadenosine-5'-triphosphate, beta, gamma-imidoadenosine-5-triphosphate, and UTP, in varying degrees inhibited aggregation of hippocampal neurons. Rat glial BT4Cn cells are unable to aggregate when grown on agar but acquire this capacity when transfected with NCAM. However, addition of extracellular ATP to NCAM-transfected BT4Cn cells inhibited aggregation. Furthermore, neurite outgrowth from hippocampal neurons in cultures allowing NCAM-homophilic interactions was inhibited by addition of extracellular nucleotides. These findings indicate that NCAM-mediated adhesion may be modulated by extracellular ATP. Moreover, extracellularly added ATP stimulated neurite outgrowth from hippocampal neurons under conditions non-permissive for NCAM-homophilic interactions, and neurite outgrowth stimulated by extracellular ATP could be inhibited by a synthetic peptide corresponding to the so-called cell adhesion molecule homology domain (CHD) of the fibroblast growth factor receptor (FGFR) and by FGFR antibodies binding to this domain. Antibodies against the fibronectin type-III homology modules of NCAM, in which a putative site for ATP binding and hydrolysis is located, also abolished the neurite outgrowth-promoting effect of ATP. The non-hydrolyzable analogues of ATP all strongly inhibited neurite outgrowth. Our results indicate that extracellular ATP may be involved in synaptic plasticity through a modulation of NCAM-mediated adhesion and neurite outgrowth.

Cardiac ionic currents and acute ischemia: from channels to arrhythmias

The aim of this review is to provide basic information on the electrophysiological changes during acute ischemia and reperfusion from the level of ion channels up to the level of multicellular preparations. After an introduction, section II provides a general description of the ion channels and electrogenic transporters present in the heart, more specifically in the plasma membrane, in intracellular organelles of the sarcoplasmic reticulum and mitochondria, and in the gap junctions. The description is restricted to activation and permeation characterisitics, while modulation is incorporated in section III. This section (ischemic syndromes) describes the biochemical (lipids, radicals, hormones, neurotransmitters, metabolites) and ion concentration changes, the mechanisms involved, and the effect on channels and cells. Section IV (electrical changes and arrhythmias) is subdivided in two parts, with first a description of the electrical changes at the cellular and multicellular level, followed by an analysis of arrhythmias during ischemia and reperfusion. The last short section suggests possible developments in the study of ischemia-related phenomena.

P2X3 is expressed by DRG neurons that terminate in inner lamina II

The P2X3 receptor subunit, a member of the P2X family of ATP-gated ion channels, is almost exclusively localized in sensory neurons. In the present study, we sought to gain insight into the role of P2X3 and P2X3-containing neurons in sensory transmission, using immunohistochemical approaches. In rat dorsal root ganglia (DRG), P2X3-immunoreactivity (-ir) was observed in small- and medium-sized neurons. Approximately 40% of DRG neuronal profiles in normal rats contained P2X3-ir. In rats that had received neonatal capsaicin treatment, the number of P2X3-positive neurons was decreased by approximately 70%. Analysis of the colocalization of P2X3-ir with cytochemical markers of DRG neurons indicated that approximately 94% of the P2X3-positive neuronal profiles were labelled by isolectin B4 from Bandeiraea simplicifolia, while only 3% contained substance P-ir, and 7% contained somatostatin-ir. In dorsal horn of rat spinal cord, P2X3-ir was observed in the inner portion of lamina II and was reduced subsequent to dorsal rhizotomy, as well as subsequent to neonatal capsaicin treatment. Finally, P2X3-ir accumulated proximal to the site of sciatic nerve ligation, and was seen in nerve fibres in skin and corneal epithelium. In summary, our results suggest that P2X3 is expressed by a functionally heterogeneous population of BSI-B4-binding sensory neurons, and is transported into both central and peripheral processes of these neurons.

Time course of the initial [Ca2+]i response to extracellular ATP in smooth muscle depends on [Ca2+]e and ATP concentration

In response to extracellular application of 50 microM ATP, all individual porcine aortic smooth muscle cells respond with rapid rises from basal [Ca2+]i to peak [Ca2+]i within 5 s. The time from stimulus to the peak of the [Ca2+]i response increases with decreasing concentration of ATP. At ATP concentrations of 0.5 microM and below, the time to the [Ca2+]i peak varies more significantly from cell to cell than at higher concentrations, and each cell shows complicated initiation and decay kinetics. For any individual cell, the lag phase before a response decreases with increasing concentration of ATP. An increase in lag time with decreasing ATP concentration is also observed in the absence of extracellular Ca2+, but the lag phase is more pronounced, especially at concentrations of ATP below 0.5 microM. Whole-cell patch-clamp electrophysiology shows that in porcine aortic smooth muscle cells, ATP stimulates an inward current carried mainly by Cl- ion efflux with a time course similar to the [Ca2+]i changes and no detectable current from an ATP-gated cation channel. A simple signal cascade initiation kinetics model, starting with nucleotide receptor activation leading to IP3-mediated Ca2+ release from IP3-sensitive internal stores, fits the data and suggests that the kinetics of the Ca2+ response are dominated by upstream signal cascade components.

Immunohistochemical study of the P2X2 and P2X3 receptor subunits in rat and monkey sensory neurons and their central terminals

Of the cloned P2X receptor subunits, six are expressed in sensory neurons, suggesting that the native channels may be heteromultimers with diverse composition. It has been proposed that P2X2 and P2X3 form heteromultimers in sensory neurons. We further tested this hypothesis by examining the relationship of P2X2 and P2X3 immunocytochemically. In rat dorsal root and nodose ganglia, P2X2- and P2X3-immunoreactivity (-ir) were highly colocalized, although single-labeled cells were also present. In dorsal root ganglia (DRG), in some cases P2X2-ir appeared to be present in satellite cells. In dorsal horn of spinal cord, at low magnification the laminar localization of P2X2- and P2X3-ir overlapped, but at high magnification colocalization was rarely observed. In contrast, in the solitary tract and its nucleus (NTS), colocalization of P2X2- and P2X3-ir was seen at low and high magnification. These results suggest that the relationship of P2X2- and P2X3-ir is different in nodose and dorsal root ganglia and might reflect differences in the targeting of P2X receptors in different sensory neurons. In monkey, P2X2-ir was observed in DRG neurons and satellite cells and in dorsal horn of spinal cord. P2X3-ir was also seen in DRG neurons. However, the presence of P2X2-ir in NTS as well as the presence of P2X3-ir in spinal cord and NTS could not be established definitively. These results suggest species differences, although a more extensive study of primate sensory systems is necessary.

ATP-gated cation channels (P2X purinoceptors) in trigeminal mesencephalic nucleus neurons of the rat

1. We have investigated whether receptors for ATP exist on neurones of the trigeminal mesencephalic nucleus (MNV) of the rat using whole-cell and outside-out patch-clamp recording in coronal brainstem slices. 2. With whole-cell recording, the batch application of ATP, adenosine 5'-O-[3-thiotriphosphate] (ATP gamma S) and alpha,beta methylene adenosine triphosphate (alpha beta meATP) caused concentration-dependent inward currents in all cells tested (holding potential, -62 mV), with EC50 values of 437, 15 and 254 microM, respectively. All three agonist-evoked currents developed rapidly (rise time, approximately 10-25 s), desensitized slowly (over approximately 20-50 s), cross-desensitized with each other, were associated with an increase in membrane conductance and were attenuated by the application of suramin (30 microM). 3. The inward current evoked by ATP decreased as the membrane potential was made less negative and had a zero current potential of +1.0 +/- 3.7 mV. The current-voltage relationship showed marked inward rectification. 4. Brief flickery single-channel openings could be resolved in response to ATP (3 microM) in outside-out membrane patches. Unitary current at -82 mV was -1.81 +/- 0.2 pA, which corresponds to a unitary conductance of 22 pS. 5. We conclude that proprioceptive MNV neurones contain ATP-gated cation channels. Such P2X purinoceptors may be involved in the processing of proprioceptive information, thus suggesting a potentially important physiological role of ATP.

Purinergic modulation of neural control of cardiac function

1. The purine nucleotide adenosine 5'-triphosphate (ATP) and its related nucleoside, adenosine (Ado), exert pronounced electrophysiologic, inotropic, lusitropic and metabolic effects in the mammalian heart. 2. These effects are the result of direct actions of these compounds on cardiac myocytes and endothelial cells, mediated by cell surface receptors. 3. In addition, ATP and Ado can stimulate neural elements inside and outside the heart and thereby modulate neural control of cardiac function. These latter actions of ATP and Ado are briefly reviewed and their hypothetical physiological role is outlined.

Implication of ATP receptors in brain functions

The possible implication of P2-purinoceptors in brain functions is reviewed. Involvement of P2-purinoceptors in memory and learning (Section 2) is suggested by ATP release from hippocampal slices [Wieraszko, A., Goldsmith, G. and Seyfried, T. N. (1989) Brain Res. 485, 244-250], induction of fast synaptic currents in cultured hippocampal neurons [Inoue, K., Nakazawa, K., Fujimori, W. and Takanaka, A. (1992a) Neurosci. Lett. 134, 294-299] and long-lasting enhancement of the population spikes [Wieraszko, A. and Seyfried, T. N. (1989) Brain Res. 491, 356-359; Nishimura, S., Mohri, M., Okada, Y. and Mori, M. (1990) Brain Res. 525, 165-169; Fujii, S., Kato, H., Furuse, H., Ito, K., Osada, H., Hamaguchi, T. and Kuroda, Y. (1995) Neurosci, Lett. 187, 130-132], as well as ATP release on glutamate stimulation to evoke an increase in intracellular Ca2+ in hippocampal cells [Inoue, K., Koizum, S. and Nakazawa, K. (1995) NeuroReport 6, 437-440]. Moreover, mRNAs for certain types of P2x-purinoceptors are present in the hippocampus [Collo, G., North, R. A., Kawashima, E., Merlo-Pich, E., Neidhart, S., Surprenant, A. and Buell, G. (1996) J. Neurosci. 16, 2495-2507]. It is likely, therefore, that ATP may be involved in modulation of synaptic efficiency in the hippocampus. The implication of ATP in schizophrenia is suggested by the fact that antipsychotic drugs inhibit ATP-evoked responses in PC12 cells [Koizumi, S., Ikeda, M., Nakazawa, K., Inoue, K., Ito, K. and Inoue, K. (1995b) Biochem. Biophys. Res. Commun. 210, 624-630] without blocking the action of dopamine D2 receptors. Involvement of P2-purinoceptors in Sections 4 ("Pain and cognition") and 5 ("Central regulation of the autonomic system") are also discussed.

Substance P potentiates ATP-activated currents in rat primary sensory neurons

The aim of this study was to explore whether substance P could modulate the response mediated by ATP receptor. Experiments were carried out on rat dorsal root ganglion (DRG) neurons isolated acutely with enzymatic and mechanical treatment. The ATP-activated inward currents were recorded using the whole-cell patch-clamp technique. The majority of the neurons examined (82/85, 96.5%) were sensitive to ATP (1-1000 microM). Application of substance P (0.01-100 microM) also caused an inward current. Several differences between these two kinds of currents were observed. 0.01, 0.1 and 1 microM substance P increased the ATP (10 microM)-activated current to 113.7 +/- 3.1%, (n = 8); 127.2 +/- 6.7%, (n = 12) and 154.7 +/- 14.4% (n = 6) (means +/- S.E.M.), respectively. This potentiating effect can be blocked by spantide, an NK1 receptor antagonist, and intracellular application of H7 (which is a potent inhibitor of PKC) could also block this kind of potentiation of SP on ATP-activated current. Since the substance P receptor and ATP receptor can coexist in rat DRG neurons and activation of substance P receptor can modulate the response mediated by ATP receptor, it suggests that they may cooperate with each other in activating peripheral nociceptive endings of sensory neurons, especially during tissue damage and/or inflammation.

Signaling by extracellular nucleotides

ATP and other nucleotides can be released from cells through regulated pathways, or following the loss of plasma membrane integrity. Once outside the cell, these compounds take on new roles as intercellular signaling molecules that elicit a broad spectrum of physiological responses through the activation of numerous cell surface receptor subtypes. This review summarizes recent advances in the molecular characterization of ATP receptors and discusses roles for cloned receptors in established and novel physiological processes.

Molecular genetic approaches to nociceptor development and function

The activation of peripheral nociceptors is the subject of intense scrutiny, because of its significance in pain regulation. Genetic approaches, including homology cloning, difference cloning and transgenic manipulation of mice are providing useful insights into nociceptor function. Recent work suggests that transcriptional regulators (for example, islet-I), which are expressed relatively selectively in sensory neurones, play a crucial role in defining cellular phenotype. Difference cloning has identified genes which encode both ligand-gated and voltage-gated ion channels expressed by small-diameter sensory neurones. The role of inflammatory mediators such as NGF in regulating nociceptor function has been clarified in mis-expression and deletion studies. An understanding of the mechanisms that regulate gene expression in nociceptors should provide new ways to manipulate nociceptor sensitivity, with potential significance for pain therapy.

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.

Selective enhancement by an adenosine A1 receptor agonist of agents inducing contraction of the rat vas deferens

The adenosine analogue N6-cyclopentyladenosine (CPA), acting via postjunctional A1 receptors, has been shown to enhance contractions of the rat vas deferens induced by adenosine 5'-triphosphate (ATP), the sympathetic cotransmitter in this tissue. The aim of the present study was to examine the ability of CPA to enhance contractions induced by other contractile agents. CPA (0.01-0.3 microM) enhanced contractions induced by exogenous ATP (10 microM), 5-hydroxytryptamine (5-HT) (3 microM), tyramine (10 microM), 2-methyl-5-hydroxytryptamine (2-Me-5-HT) (10 microM) and KCl (35 mM) and this enhancement was blocked by an A1-selective concentration (3 nM) of 1, 3-dipropyl-8-cyclopentylxanthine (DPCPX). CPA failed to enhance contractions induced by exogenous noradrenaline (NA) (1 microM or 10 microM), bradykinin (0.1 microM), phenylephrine (3 microM) or carbachol (10 microM). The contractions induced by ATP (10 microM), 5-HT (3 microM), 2-Me-5-HT (10 microM) and KCl (35 mM) were unaffected by tetrodotoxin (1 microM) as well as by desensitisation of the P2x-purinoceptors with the ATP analogue adenosine 5'-(alpha, beta-methylene) triphosphonate. The contractions induced by tyramine (10 microM) and 2-Me-5-HT (10 microM) were blocked by prazosin (100 nM) or by imipramine (1 microM). Ketanserin (10 nM) antagonised the response to 5-HT giving a dose-ratio of 12.9 corresponding to an apparent pA2 of 9.1. In conclusion, the A1-mediated effect was clearly selective for certain contractile agents and not due to a non-specific increase in contractility of the tissue. CPA enhanced contractions induced by both ATP and indirect sympathomimetics which release endogenous NA, and this enhancement of the two sympathetic cotransmitters may have a functional significance, and demonstrates the complexity of the neuromodulatory effects of adenosine in the rat vas deferens.

Potentiation by cadmium ion of ATP-evoked dopamine release in rat phaeochromocytoma cells

1. The effects of cadmium ion (Cd2+) on release of dopamine and on an inward current evoked by extracellular ATP were investigated in rat phaeochromocytoma PC12 cells. 2. Cd2+ (100 microM-3 mM) potentiated the dopamine release evoked by 30 microM ATP from the cells. Cd2+ (100 microM) shifted the concentration-response curve of ATP-evoked dopamine release to the left without affecting the maximal response. 3. Suramin (30 microM) completely abolished the dopamine release evoked by 30 microM ATP but only partially inhibited the release evoked by 100 microM ATP consistent with its role as a competitive antagonist. The response evoked by 30 microM ATP in the presence of Cd2+ (300 microM) was comparable to that observed with 100 microM ATP alone; however, only the former was almost completely inhibited by suramin. 4. Cd2+ (100 microM) potentiated an inward current activated by 30 microM ATP alone. A higher concentration of Cd2+ (300 microM) had a smaller effect on amplitude potentiation but significantly prolonged the duration of the current. 5. The time-course of the ATP-evoked dopamine release was investigated using a real-time monitoring system for dopamine release. Although Cd2+ (300 microM) had little effect on the time-course of activation the ATP-evoked dopamine release, it produced a long-lasting dopamine release which slowly returned to the baseline. 6. Taken together, these observations suggest that Cd2+ enhances ATP-evoked dopamine release by affecting P2-purinoceptor/channels. The enhancement may be attributed to a Cd(2+)-dependent increase in sensitivity to ATP.

Mechanism of action of ATP on canine pulmonary vagal C fibre nerve terminals

1. The effects of extracellular adenosine 5'-triphosphate (ATP) on pulmonary vagal afferent fibres (n = 46) was studied in a canine model in vivo (n = 38). 2. ATP (3-6 mumol kg-1), administered as a rapid bolus into the right atrium, elicited a transient burst of action potentials in cervical vagal fibres, which was not affected by either blockade of ganglionic transmission (hexamethonium) or a drop in arterial blood pressure (nitroglycerine). 3. The fibres with ATP-sensitive terminals were otherwise quiescent with no activity related to either cardiac or respiratory cycles and their conduction velocity was 0.85 +/- 0.13 m s-1 (n = 7). 4. Inflation of the lungs to 2-3 times the tidal volume triggered brief bursts of action potentials in these fibres. 5. Capsaicin (10 micrograms kg-1), given as a rapid bolus into the right atrium, elicited a burst of action potentials in these ATP-sensitive fibres. 6. Smaller amounts of ATP and capsaicin (0.5-3 mumol kg-1 and 1-5 micrograms kg-1, respectively) had similar effects when the two compounds were given into the right pulmonary artery. 7. Adenosine, adenosine 5'-monophosphate, or adenosine 5'-diphosphate did not excite these fibres (n = 30). 8. The non-degradable analogue of ATP alpha,beta-methylene ATP (alpha,beta-mATP) was tenfold more potent than ATP while beta,gamma-methylene ATP (beta,gamma-mATP) was in active. 9. The selective P2x-purinoceptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid markedly attenuated the effect of ATP but not of capsaicin. The P2Y-purinoceptor antagonist Reactive Blue 2 was without effect. 10. Pretreatment with pertussis toxin (PTX) did not affect this action of ATP. 11. In the canine lungs ATP activates vagal C fibre nerve terminals. This action is mediated by P2X-purinoceptors and is independent of a PTX-sensitive guanine nucleotide binding protein (G protein).

A P2X purinoceptor expressed by a subset of sensory neurons

ATP is known to depolarize sensory neurons, and may play a role in nociceptor activation when released from damaged tissue. Here we report the molecular cloning and characterization of a new member of the P2X receptor family, P2X3, expressed by these cells. The channel transcript was present in a subset of rat dorsal-root-ganglion sensory neurons, some of which express nociceptor-associated markers; it was absent in other tissues that were tested, including sympathetic, enteric and central nervous system neurons. Moreover, when expressed in Xenopus oocytes, the channel showed an ATP-dependent cation flux. P2X3 is the only ligand-gated channel known to be expressed exclusively by a subset of sensory neurons. The remarkable selectivity of expression of the channel coupled with its sensory neuron-like pharmacology suggests that this channel may transduce ATP-evoked nociceptor activation.

Rapid refilling of Ca2+ stores in macrophages stimulated by ATP involves the sequential activation of phospholipase D and protein kinase C

Ca2+ movements between intracellular stores, the cytoplasm and external solution were analysed in murine peritoneal macrophages stimulated by various agonists. The Ca2+ content of intracellular stores was estimated from the amplitude of Ca(2+)-transients elicited by ionomycin applied in Ca(2+)-free solution. Both uridine 5'-triphosphate (UTP) and platelet-activating factor (PAF) triggered the release of Ca2+ followed by a sustained influx, during which intracellular stores remained totally empty. In contrast, in the continuous presence of adenosine 5'-triphosphate (ATP), Ca2+ was initially released and then rapidly sequestered again by the stores. ATP-induced store refilling was not related to cell depolarization or to an increase in the intracellular Na+ concentration (two specific consequences of ATP stimulation which are not induced by PAF and UTP). Store refilling was not caused by a signal that ATP would fail to induce (e.g. as a result of receptor desensitization), but was positively controlled by ATP, even in the simultaneous presence of a concentration of PAF which, on its own, would have caused a persistent store depletion. The hypothesis that the signal delivered by ATP involves the sequential activation of phospholipase D and protein kinase C is consistent with the present pharmacological evidence. However, although we found conditions in which Ca2+ stores did not refill in the presence of ATP, this maintained store depletion was not accompanied by a sustained Ca2+ response similar to that elicited by PAF or UTP, suggesting that store depletion is a condition which is necessary, but not sufficient, for inducing Ca2+ influx.

Nonselective cationic currents elicited by extracellular ATP in human B-lymphocytes

Adenosine 5'-triphosphate-(ATP)-induced whole-cell currents were studied in human B-lymphocytes, transformed by the Epstein-Barr virus, by means of the tight-seal voltage-clamp technique. During bath application of ATP, the membrane conductance was increased. The change of membrane conductance occurred within milliseconds. The dose response relationship for the ATP(4-)-elicited membrane current (Ip) was fitted by the Hill function with a Hill coefficient of 1 and a KD value of 0.2 mmol/l. Adenosine, as well as the Mg(2+)-bound form of ATP, did not effect the membrane conductance. Ip did not desensitize within 1 min and could be evoked repeatedly up to 100 times in 1 cell in the presence of the G-protein blocker Guanosine 5'-o-(2-thiodiphosphate) (GDP [beta S]). Therefore, it seems that ion channels in form of P2Z-purinoceptors are involved in the observed effects. The permeability (P) sequence for cations carrying Ip was PCa:PK:PCs:PNa:PTRIS = 35:2:1.2:1:0.1. The reversal potential of IP was not changed by substitution of intracellular Cl- for aspartate, indicating that anions are not involved in the purinoceptor-dependent conductance. A single-channel conductance of P2Z-receptor-dependent ion channels of about 3 pS was determined by noise analysis of Ip.

Selective and competitive antagonism by suramin of ATP-stimulated catecholamine-secretion from PC12 phaeochromocytoma cells

1. Suramin, a putative P2-antagonist, (10 to 300 microM) inhibited the adenosine 5'-triphosphate (ATP)-stimulated secretion of [3H]-noradrenaline or endogenous dopamine from phaeochromocytoma PC12 cells in a concentration-dependent manner. Suramin (300 microM) did not affect the dopamine-secretion stimulated by high K+ or nicotine. 2. Suramin shifted the concentration-response curve for ATP to the right. The antagonism was competitive with a pA2 value of 4.52. 3. ATP also stimulated an increase in intracellular Ca2+ concentration as determined by fura-2 methods. Suramin antagonized this effect over the same concentration range that antagonized the ATP-stimulated catecholamine secretion. 4. These results suggest that suramin can be used as a selective and competitive antagonist of ATP in experiments concerning mechanisms of catecholamine-secretion.

Improved micro-perfusion chamber for multiple and rapid solution exchange in adherent single cells

A new method for fast perfusion of single adherent cells during whole-cell and single-channel patch-clamp measurements is described. The main advantages over previous methods are: (1) the solution surrounding a single cell in a Petri dish (or cover slip) can be exchanged without contamination of the entire cell population, and (2) only small quantities of test solution (50 microliters) are required. The method consists of inserting a patch pipette into a micro-perfusion chamber (MPC) where whole-cell and single-channel currents can be studied. A small volume (2-3 microliters) surrounding adherent single cells in a Petri dish is separated by enclosing it in a ring of Silicone rubber which is then pressed to the bottom of the dish. A drug-containing test solution (50 microliters) is supplied to a funnel at the chamber inflow. The MPC volume isolated from the main solution in the Petri dish is rapidly changed when suction is applied to the chamber outflow. The speed of solution exchange in the MPC (16 +/- 5 ms, n = 12) was estimated by observing changes in the tip potential of 1-M omega patch pipettes during rapid chamber perfusion with solutions of different ionic composition. The seal of the MPC with the surface of a Petri dish was approximated by measuring the electrical resistance between the MPC interior and a reference electrode placed in the Petri dish outside the MPC (range 300-500 M omega). Additionally, a radioactive calcium channel ligand [3H]isradipine was added to the MPC and the appearance of radioactivity in the Petri dish (outside the MPC) subsequently measured.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of P2-purinoceptors in the smooth muscle of the rat tail artery: a comparison between contractile and electrophysiological responses

1. The electrophysiological actions of the P2-purinoceptor agonists, adenosine 5'-triphosphate (ATP), 2-methylthioATP (2-meSATP), alpha, beta-methyleneATP (alpha, beta-meATP) and uridine 5'-triphosphate (UTP) were studied under concentration and voltage-clamp conditions in acutely dissociated rat tail artery smooth muscle cells. For comparison, their actions as vasoconstrictors were studied in intact ring preparations. 2. Rapid application of ATP (100 nM-1 microM) via a U-tube superfusion system activated concentration-dependent inward currents with a latency to onset of less than 3 ms. The inward current decayed by more than 95% during a 2 s application of 300 nM and 1 microM ATP. 3. 2-meSATP (100 mM-1 microM) and alpha, beta-meATP (100 nM-1 microM) also evoked transient inward currents. The agonist order of potency was ATP = 2-meSATP > or = alpha, beta-meATP. UTP (300 nM-1 microM) did not produce a change in the holding current. 4. A second application of ATP (300 nM and 1 microM) 10 min after the first, evoked currents which were one third of the initial amplitude. This decline was dependent upon activation of the P2-purinoceptor. Similar results were seen with 2-meSATP and alpha, beta-meATP (both 300 nM and 1 microM). Cross-desensitization was seen between ATP and 2-meSATP or alpha, beta-meATP. 5. Inward currents evoked by ATP, 2-meSATP and alpha, beta-meATP (all 1 microM) were abolished by the P2-purinoceptor antagonist suramin (100 microM). 6. Alpha, beta-meATP (100 nM-30 micro M), 2-meSATP (3 micro M- 100 micro M), ATP (3 micro M-I mM) and UTP (3 ELM-I mM)produced concentration-dependent contractions of rat tail artery rings. When measured at a level equal to 50% of the maximum response to noradrenaline, the rank order of agonist potency was alpha,beta-meATP>>2-meSATP >UTP >ATP.7. This study shows that the rank order of agonist potency at the P2X-purinoceptor which mediates contractions of the rat isolated tail artery is very different from the potency order for evoking the inward current which initiates the contractions. It is concluded that this difference may be due to the relative absence of breakdown of some of the agonists in the single cell system compared with artery rings.

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

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

Facilitation by 5-hydroxytryptamine of ATP-activated current in rat pheochromocytoma cells

The effects of 5-hydroxytryptamine (5-HT) on an inward current activated by extracellular ATP were investigated in rat pheochromocytoma PC12 cells. Under whole-cell voltage-clamp conditions 5-HT (10 microM) reversibly enhanced the amplitude of the current activated by 30 microM ATP. The enhancement may not be due to an increase in the number of functional channels because the current activated by 300 microM ATP was not remarkably augmented compared with the current activated by 30 microM ATP. The current enhancement by 100 microM 5-HT was less obvious than that by 10 microM 5-HT. When the current kinetics were compared, activation of the ATP-evoked current was accelerated to the same extent by either 10 or 100 microM 5-HT, whereas deactivation was largely more accelerated by 100 microM 5-HT. Propranolol (10 microM), a 5-HT1 receptor antagonist, or LY53857 (10 microM), a 5-HT2 receptor antagonist, exerted an agonistic effect: the ATP-activated current was facilitated by these compounds. Metoclopramide (10 microM), a 5-HT3 receptor antagonist, neither facilitated the ATP-activated current, nor blocked the current facilitation by 5-HT. Guanosine 5'-O-(2-thiodiphosphate) (GDP[beta S]) (2 mM), the non-hydrolysable analog of guanosine 5'-triphosphate (GTP), or K-252a (2 microM), a protein kinase inhibitor, did not affect the facilitation by 5-HT of the ATP-activated current when they were included in the intracellular solution. The ATP-activated current pre-facilitated by 10 microM dopamine was not enhanced by 10 microM 5-HT. Similarly, the pre-facilitation by 5-HT attenuated the current enhancement by dopamine.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of the inhibitory action of ATP on acetylcholine-activated current by protein phosphorylation in rat sympathetic neurons

Modulation by protein phosphorylation of the relation between acetylcholine (ACh)-activated current (IACh) and adenosine triphosphate-(ATP)-activated current (IATP) was investigated with the whole-cell voltage-clamp technique in rat sympathetic neurons. During simultaneous activation by 100 microM ATP of an inward current, the current evoked by 100 microM ACh was reduced to 60-70% of that in the absence of ATP. Effects of compounds that are known to modulate protein phosphorylation were tested by including them in the intracellular solution. The reduction of IACh by ATP was not observed when K252a (1 microM), a non-selective protein kinase inhibitor, adenosine 5'-O-(3-thiotriphosphate) (ATP[gamma S], 1 mM) or alpha, beta-methylene ATP (1 mM) were included in the intracellular solution. Activators of protein kinases, adenosine 3',5'-cyclic monophosphate (cAMP, 100 microM), guanosine 3',5'-cyclic monophosphate (cGMP, 100 microM), phorbol 12-myristate 13-acetate (PMA, 1 microM), also abolished the reduction by ATP of IACh. The effects of okadaic acid, a protein phosphatase inhibitor, were paradoxical: okadaic acid (2 microM) itself abolished the reduction by ATP of IACh but it "antagonized" the abolishment by cAMP or cGMP of the reduction of IACh. Okadaic acid did not affect the disappearance of the reduction of IACh by ATP in the presence of intracellular PMA. The results suggest that the interaction between IACh and IATP is regulated by protein phosphorylation/dephosphorylation. Possible mechanisms underlying the effects of these modulators of protein phosphorylation are discussed.

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

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

Characterization and possible function of adenosine 5'-triphosphate receptors in activated rat microglia

1. Purinoceptor agonist-induced currents in untreated (proliferating) and lipopolysaccharide (LPS; 100 ng ml-1)-treated (non-proliferating) rat microglial cells in culture were recorded by the whole-cell patch-clamp technique. These cells have two preferred resting membrane potentials, one at -35 mV and another one at -70 mV. 2. Most experiments were carried out in non-proliferating cells. ATP, ATP-gamma-S and alpha,beta-MeATP (1-1000 microM in all cases) evoked an inward current at a holding potential of -70 mV, followed, in some experiments, by an outward current. At -70 mV 2-methylthio ATP (1-1000 microM) evoked an inward current, whereas at -35 mV it produced an outward current only. 3. When K+ was replaced in the pipette solution by an equimolar concentration of Cs+ (150 mM), the main outward component of the ATP-gamma-S (10 microM) induced response disappeared. Instead, an inward current was obtained. Replacement of K+ by Cs+ did not affect the inward current evoked by 2-methylthio ATP (300 microM). 4-Aminopyridine (1-10 mM), however, almost abolished this current and unmasked a smaller outward current. 4. The rank order of agonist potency was 2-methylthio ATP > ATP > alpha,beta-MeATP. Adenosine and UTP were inactive. Suramin (300 microM) and reactive blue 2 (50 microM) antagonized the effect of 2-methylthio ATP (300 microM). 5. I-V relations were determined by delivering fast voltage ramps before and during the application of 2-methylthio ATP (300 microM). In the presence of extra- (1 mM) and intracellular (150 mM) Cs+, the 2-methylthio ATP-evoked current crossed the zero current level near 0 mV. When both Cs+ (1 mm) and 4-aminopyridine (1 mM) were present in the bath medium, the intersection of the 2-methylthio ATP current with the zero current level was near - 75 mV.6. 2-Methylthio ATP (1-1I000 MicroM) induced the same inward current both in proliferating and nonproliferating microglia. However, the depolarizing response to 2-methylthio ATP (300 MicroM) was larger and longer-lasting in the proliferating cells. When the free Ca2+ concentration in the pipettes was increased from the standard 0.01 to 1 MicroM, the amplitude and duration of this depolarization was increased in non-proliferating cells. 4-Aminopyridine (1 mM) enhanced the duration, but not the amplitude of responses.7. ATP and its structural analogues stimulate microglial purinoceptors of the P2Y-type. This leads to the opening of non-selective cationic channels and potassium channels. Depending on the resting membrane potential, depolarization or hyperpolarization prevails. Although the inward current produced by 2-methylthio ATP is of similar amplitude in proliferating and non-proliferating microglia, the resulting depolarization is smaller in the latter cell type because of the presence of voltage-sensitive, outwardly rectifying potassium channels.

Zn2+ potentiates ATP-activated currents in rat sympathetic neurons

The ATP-activated inward current (IATP) in cultured rat superior cervical ganglion neurons and its modulation by extracellular Zn2+ were examined. ATP activated a non-specific cation conductance and caused a transient rise in intracellular Ca2+. The current response was specifically activated by ATP and was blocked by the P2-purinoceptor antagonist, suramin. Low concentrations of extracellular Zn2+ rapidly and reversibly potentiated both IATP and the intracellular Ca2+ rise. The potentiation by 10 microM Zn2+ was dependent on the concentration of agonist; Zn2+ increased the sensitivity of activation without potentiating the maximum response. Higher concentrations of Zn2+ reduced and prolonged the current, consistent with open-channel block. We hypothesize that there exist two sites of action for Zn2+: a positively acting allosteric site that enhances current amplitude and a site, possibly within the pore, that blocks conductance through the channel.

Effects of ATP on cultured smooth muscle cells from rat aorta

1. Membrane ionic currents provoked by externally applied ATP were studied by patch-clamp techniques in cultured aortic smooth muscle cells of the rat. 2. Using standard bath and pipette solutions and whole-cell voltage-clamp, ATP evoked an inward current when the cell membrane potential was held at -50 mV and an outward current when the potential was held at 30 mV, with a reversal potential near -10 mV. 3. Application of ATP gamma S gave results similar to those obtained with ATP, while adenosine, AMP and alpha,beta-methylene ATP were ineffective. The ATP-activated current was inhibited by suramin, 100 microM. 4. ATP also induced a biphasic rise in internal free Ca levels as shown directly by Fura-2 measurements and by the increase in Ca-dependent K single-channel activity in cell-attached patches. 5. With outward current through K channels blocked by internal Cs and TEA, modification of the ionic composition of bath and pipette solutions revealed that the reversal potential for the ATP-induced whole-cell current closely followed ECl, the chloride equilibrium potential, and was insensitive to manipulations of the monovalent cation gradient. 6. These results indicate that in rat cultured aortic smooth muscle cells, ATP binding to P2-purinoceptors produces increases of internal free Ca levels and subsequent activation of both Ca-dependent K and Cl currents.

Intracellular signalling by nucleotide receptors in PC12 pheochromocytoma cells

The effect of extracellular ATP was studied in PC12 cells, a neurosecretory line that releases ATP. The addition of micromolar concentrations of ATP to PC12 cells evoked a transient increase in the cytosolic free Ca2+ concentration ([Ca2+]i), as measured with the Ca(2+)-sensitive dye fura 2. AMP and adenosine were without effect, ruling out the involvement of P1 receptors in mediating this response. The increase in [Ca2+]i was reduced in calcium-free media and virtually eliminated by the addition of EGTA, suggesting that calcium influx was the primary response initiated by extracellular ATP. Nucleotide triphosphates such as UTP and, to a lesser degree, ITP also evoked an increase in [Ca2+]i while GTP and CTP had little effect. In order to identify the receptor subtype mediating this response, the efficacy of ATP and ATP cogeners was assessed. The rank order potency was ATP > adenosine 5'-[gamma-thio]triphosphate > ADP > 2-methylthioadenosine triphosphate (2-MeSATP) approximately adenosine 5'-[beta-thio]diphosphate >> adenosine 5'-[alpha beta-methylene] triphosphate, adenosine 5'[beta gamma-imido]triphosphate. This profile is not characteristic of either the P2X or the conventional P2Y receptors. The Ca2+ response exhibited desensitization to ATP that was dependent on the extracellular metabolism of ATP. UTP was equally effective in desensitizing the response. ATP, UTP, ITP, and to a much lesser extent 2MeSATP increased inositol phosphate production in a dose-dependent manner, suggesting receptor coupling to phosphatidylinositol-specific phospholipase C. These data are consistent with the view that PC12 cells express a class of non-P2Y nucleotide receptors (P2N) that mediate calcium influx and the accumulation of inositol phosphates.

Block by calcium of ATP-activated channels in pheochromocytoma cells

We have investigated the effects of Ca2+ on Na+ influx through ATP-activated channels in pheochromocytoma PC12 cells using single channel current recordings. Under cell-attached patch-clamp conditions with 150 mM Na+ and 2 mM Ca2+ in the pipette, the unitary current activity showed an open level of about -4.3 pA at -150 mV. The channel opening was interrupted by flickery noise as well as occasional transition to a subconducting state of about -1.7 pA at -150 mV. The open level was decreased with increased external Ca2+, suggesting that external Ca2+ blocks Na+ permeation. We assessed the block by Ca2+ as the mean amplitude obtained with heavy filtration according to Pietrobon et al. (Pietrobon, D., B. Prod'hom, and P. Hess, 1989. J. Gen. Physiol. 94:1-21). The block was concentration dependent with a Hill coefficient of 1 and a half-maximal concentration of approximately 6 mM. A similar block was observed with other divalent cations, and the order of potency was Cd2+ > Mn2+ > Mg2+ not equal to Ca2+ > Ba2+. High Ca2+, Mg2+ and Ba2+ did not block completely, probably because they can carry current in the channel. The block by external Ca2+ did not exhibit voltage dependence between -100 and -210 mV. In the inside-out patch-clamp configuration, the amplitude of inward channel current obtained with 150 mM external Na+ was reduced by increased internal Ca2+. The reduction was observed at lower concentrations than that by external Ca2+. Internal Ba2+ and Cd2+ induced similar reduction in current amplitude. This inhibitory effect of internal Ca2+ was voltage dependent; the inhibition was relieved with hyperpolarization. The results suggest that both external and internal Ca2+ can block Na+ influx through the ATP-activated channel. A simple one-binding site model with symmetric energy barriers is not sufficient to explain the Ca2+ block from both sides.

Mechanisms underlying facilitation by dopamine of ATP-activated currents in rat pheochromocytoma cells

Mechanisms underlying facilitation by dopamine of extracellular adenosine 5'-triphosphate (ATP)-activated current were investigated in rat pheochromocytoma PC12 cells using the whole-cell voltage-clamp techniques. Dopamine (10 and 100 microM) augmented the peak amplitude of an inward current elicited by ATP (3-100 microM). The activation time course of the ATP-evoked current was accelerated by dopamine; the presence of 10 microM dopamine shifted the dependence of activation rate constants on the concentration of ATP toward a lower concentration range two fold. Dopamine also accelerated the inactivation and the deactivation, which was determined from the current decay upon washout of ATP. Intracellular mediators responsible for the dopamine-induced facilitation was estimated by loading various compounds in patch pipettes. Facilitation was not observed when K-252a (1 microM), a protein kinase inhibitor, was included in the intracellular solution. In addition, facilitation was also attenuated by intracellular adenosine 5'-O-(thiotriphosphate)tetralithium salt (ATP gamma S (1 mM) or alpha-beta-methylene ATP (1 mM). Inclusion of adenosine 3',5'-cyclic monophosphate sodium salt (cAMP, 100 microM), guanosine 3',5'-cyclic monophosphate sodium salt (cGMP, 100 microM), 12-O-tetradecanoylphorbol-13-acetate (TPA, 1 microM) or phorbol-12,13-dibutyrate (1 microM) in the intracellular solution did not affect the facilitation. Guanosine 5'-O-(thiotriphosphate)tetralithium salt (GTP gamma S, 500 microM) or guanosine 5'-O(2-thiodiphosphate)-trilithium salt (GDP beta S, 500 microM) did not modify the facilitation either.(ABSTRACT TRUNCATED AT 250 WORDS)

Channels in mitochondrial membranes: knowns, unknowns, and prospects for the future

Rapid diffusion of hydrophilic molecules across the outer membrane of mitochondria has been related to the presence of a protein of 29 to 37 kDa, called voltage-dependent anion channel (VDAC), able to generate large aqueous pores when integrated in planar lipid bilayers. Functional properties of VDAC from different origins appear highly conserved in artificial membranes: at low transmembrane potentials, the channel is in a highly conducting state, but a raise of the potential (both positive and negative) reduces drastically the current and changes the ionic selectivity from slightly anionic to cationic. It has thus been suggested that VDAC is not a mere molecular sieve but that it may control mitochondrial physiology by restricting the access of metabolites of different valence in response to voltage and/or by interacting with a soluble protein of the intermembrane space. The latest application of the patch clamp and tip-dip techniques, however, has indicated both a different electric behavior of the outer membrane and that other proteins may play a role in the permeation of molecules. Biochemical studies, use of site-directed mutants, and electron microscopy of two-dimensional crystal arrays of VDAC have contributed to propose a monomeric beta barrel as the structural model of the channel. An important insight into the physiology of the inner membrane of mammalian mitochondria has come from the direct observation of the membrane with the patch clamp. A slightly anionic, voltage-dependent conductance of 107 pS and one of 9.7 pS, K(+)-selective and ATP-sensitive, are the best characterized at the single channel level. Under certain conditions, however, the inner membrane can also show unselective nS peak transitions, possibly arising from a cooperative assembly of multiple substrates.

Localization of cholinergic and purinergic receptors on outer hair cells isolated from the guinea-pig cochlea

Acetylcholine (ACh) and adenosine 5'-triphosphate (ATP) are shown to act in opposing fashion on guinea-pig cochlear outer hair cells (OHCS) via receptors localized within different fluid compartments of the organ of Corti. The cholinergic (efferent) receptors localized at the basal (perilymphatic) region of these cells activated a rapidly desensitizing hyperpolarizing K+ current. In contrast, purinergic (ATP) receptors were localized at the apical (endolymphatic) surface of OHCS and activated a depolarizing nonselective cation current which exhibited inward rectification and lacked desensitization. Localization of the receptors was determined by using whole-cell patch-clamp, by recording onset latencies and response amplitudes to pulses of either ACh or ATP pressure-applied at selected sites along the length of isolated OHCS. Under voltage-clamp at -60 mV, the largest ACh-induced (outward) currents were recorded when ACh was directed at the basal region of the cells. Conversely, the maximum (inward) ATP currents were obtained when ATP was directed toward the apical surface of these cells. Onset latencies increased rapidly from a minimum of approximately 10 ms for either ACh or ATP as the drug pipette was moved away from these optimal sites. The ATP response was antagonized by amiloride in a dose-dependent manner with a KD of approximately 400 microM. The localization of P2-type purinoceptors to the endolymphatic surface of OHCS suggests that ATP mediates a humoral modulation of the mechano-electrical transduction process.