Adenosine 5'-triphosphate increases acetylcholine channel opening frequency in rat skeletal muscle

Modulatory Roles of ATP and Adenosine in Cholinergic Neuromuscular Transmission

A review of the data on the modulatory action of adenosine 5’-triphosphate (ATP), the main co-transmitter with acetylcholine, and adenosine, the final ATP metabolite in the synaptic cleft, on neuromuscular transmission is presented. The effects of these endogenous modulators on pre- and post-synaptic processes are discussed. The contribution of purines to the processes of quantal and non-quantal secretion of acetylcholine into the synaptic cleft, as well as the influence of the postsynaptic effects of ATP and adenosine on the functioning of cholinergic receptors, are evaluated. As usual, the P2-receptor-mediated influence is minimal under physiological conditions, but it becomes very important in some pathophysiological situations such as hypothermia, stress, or ischemia. There are some data demonstrating the same in neuromuscular transmission. It is suggested that the role of endogenous purines is primarily to provide a safety factor for the efficiency of cholinergic neuromuscular transmission.

Postsynaptic Potentiation in Mouse Motor Synapses Induced by ATP Accumulation in Synaptic Cleft

In mouse motor synapses, a non-selective purinoceptor antagonist suramin increased the quantum content of endplate potentials (EPP) without changing the time course of synaptic potentials. An ectonucleotidase inhibitor ARL 67156 had no effect on the amplitude and quantum content of EPP and miniature endplate potentials (mEPP) evoked by single stimuli, but significantly prolonged their duration. Long-term high-frequency stimulation of the nerve in the presence of ARL 67156 persistently increased the amplitude and duration of EPP during the train of impulses, but did not change their quantum content. ATP-γ-S, a non-hydrolyzed ATP analogue, significantly increased the amplitudes and prolonged the rising and falling phases of EPP and mEPP. The ATP-induced postsynaptic potentiation in neuromuscular transmission can result from the increase in ATP content and its longer presence in the synaptic cleft.

Neuronal involvement in muscular atrophy

The innervation of skeletal myofibers exerts a crucial influence on the maintenance of muscle tone and normal operation. Consequently, denervated myofibers manifest atrophy, which is preceded by an increase in sarcolemma permeability. Recently, de novo expression of hemichannels (HCs) formed by connexins (Cxs) and other none selective channels, including P2X7 receptors (P2X7Rs), and transient receptor potential, sub-family V, member 2 (TRPV2) channels was demonstrated in denervated fast skeletal muscles. The denervation-induced atrophy was drastically reduced in denervated muscles deficient in Cxs 43 and 45. Nonetheless, the transduction mechanism by which the nerve represses the expression of the above mentioned non-selective channels remains unknown. The paracrine action of extracellular signaling molecules including ATP, neurotrophic factors (i.e., brain-derived neurotrophic factor (BDNF)), agrin/LDL receptor-related protein 4 (Lrp4)/muscle-specific receptor kinase (MuSK) and acetylcholine (Ach) are among the possible signals for repression for connexin expression. This review discusses the possible role of relevant factors in maintaining the normal functioning of fast skeletal muscles and suppression of connexin hemichannel expression.

Purinergic signalling in the musculoskeletal system

It is now widely recognised that extracellular nucleotides, signalling via purinergic receptors, participate in numerous biological processes in most tissues. It has become evident that extracellular nucleotides have significant regulatory effects in the musculoskeletal system. In early development, ATP released from motor nerves along with acetylcholine acts as a cotransmitter in neuromuscular transmission; in mature animals, ATP functions as a neuromodulator. Purinergic receptors expressed by skeletal muscle and satellite cells play important pathophysiological roles in their development or repair. In many cell types, expression of purinergic receptors is often dependent on differentiation. For example, sequential expression of P2X5, P2Y1 and P2X2 receptors occurs during muscle regeneration in the mdx model of muscular dystrophy. In bone and cartilage cells, the functional effects of purinergic signalling appear to be largely negative. ATP stimulates the formation and activation of osteoclasts, the bone-destroying cells. Another role appears to be as a potent local inhibitor of mineralisation. In osteoblasts, the bone-forming cells, ATP acts via P2 receptors to limit bone mineralisation by inhibiting alkaline phosphatase expression and activity. Extracellular ATP additionally exerts significant effects on mineralisation via its hydrolysis product, pyrophosphate. Evidence now suggests that purinergic signalling is potentially important in several bone and joint disorders including osteoporosis, rheumatoid arthritis and cancers. Strategies for future musculoskeletal therapies might involve modulation of purinergic receptor function or of the ecto-nucleotidases responsible for ATP breakdown or ATP transport inhibitors.

P2X7 purinoceptor alterations in dystrophic mdx mouse muscles: relationship to pathology and potential target for treatment

Duchenne muscular dystrophy (DMD) is a lethal inherited muscle disorder. Pathological characteristics of DMD skeletal muscles include, among others, abnormal Ca(2+) homeostasis and cell signalling. Here, in the mdx mouse model of DMD, we demonstrate significant P2X7 receptor abnormalities in isolated primary muscle cells and cell lines and in dystrophic muscles in vivo. P2X7 mRNA expression in dystrophic muscles was significantly up-regulated but without alterations of specific splice variant patterns. P2X7 protein was also up-regulated and this was associated with altered function of P2X7 receptors producing increased responsiveness of cytoplasmic Ca(2+) and extracellular signal-regulated kinase (ERK) phosphorylation to purinergic stimulation and altered sensitivity to NAD. Ca(2+) influx and ERK signalling were stimulated by ATP and BzATP, inhibited by specific P2X7 antagonists and insensitive to ivermectin, confirming P2X7 receptor involvement. Despite the presence of pannexin-1, prolonged P2X7 activation did not trigger cell permeabilization to propidium iodide or Lucifer yellow. In dystrophic mice, in vivo treatment with the P2X7 antagonist Coomassie Brilliant Blue reduced the number of degeneration-regeneration cycles in mdx skeletal muscles. Altered P2X7 expression and function is thus an important feature in dystrophic mdx muscle and treatments aiming to inhibit P2X7 receptor might slow the progression of this disease.

Extracellular ATP signaling during differentiation of C2C12 skeletal muscle cells: role in proliferation

Evidence shows that extracellular ATP signals influence myogenesis, regeneration and physiology of skeletal muscle. Present work was aimed at characterizing the extracellular ATP signaling system of skeletal muscle C2C12 cells during differentiation. We show that mechanical and electrical stimulation produces substantial release of ATP from differentiated myotubes, but not from proliferating myoblasts. Extracellular ATP-hydrolyzing activity is low in myoblasts and high in myotubes, consistent with the increased expression of extracellular enzymes during differentiation. Stimulation of cells with extracellular nucleotides produces substantial Ca(2+) transients, whose amplitude and shape changed during differentiation. Consistently, C2C12 cells express several P2X and P2Y receptors, whose level changes along with maturation stages. Supplementation with either ATP or UTP stimulates proliferation of C2C12 myoblasts, whereas excessive doses were cytotoxic. The data indicate that skeletal muscle development is accompanied by major functional changes in extracellular ATP signaling.

Effect of purinergic receptor activation on Na+-K+ pump activity, excitability, and function in depolarized skeletal muscle

Activity-induced elevation of extracellular purines and pyrimidines has been associated with autocrine and paracrine signaling in many tissues. Here we investigate the effect of purinergic signaling for the excitability and contractility of depolarized skeletal muscle. Muscle excitability was experimentally depressed by elevating the extracellular K(+) from 4 to 10 mM, which reduced the tetanic force to 24 +/- 2% of the force at 4 mM K(+). Upon addition of 1 mM ATP, however, the force recovered to 65 +/- 8% of the control force (P < 0.001, n = 5). A similar recovery was seen with ADP, but not with UTP or adenosine. The ATP-induced force recovery could be inhibited by P2Y(1) receptor antagonists (3 muM SCH-202676 or 1 muM MRS-2500). A fourfold increase in M-wave area demonstrated that the ATP-induced force recovery was associated with restoration of muscle excitability (P < 0.05, n = 4). Experiments using (86)Rb(+) as a tracer for K(+) showed that ATP also induced a twofold increase in the activity of muscle Na(+)-K(+) pumps. The force recovery and the stimulation of the Na(+)-K(+) pump activity by ATP were inhibited by 50 muM of the phospholipase C inhibitor U-73122. It is concluded that purinergic signaling can increase the Na(+)-K(+) pump activity and improve force and excitability of depolarized skeletal muscles. This novel purinergic regulation may be important for the maintenance of muscle excitability during intense exercise, where the extracellular K(+) can increase substantially.

Extracellular ATP inhibits chloride channels in mature mammalian skeletal muscle by activating P2Y1 receptors

ATP is released from skeletal muscle during exercise, a discovery dating back to 1969. Surprisingly, few studies have examined the effects of extracellular ATP on mature mammalian skeletal muscle. This electrophysiological study examined the effects of extracellular ATP on fully innervated rat levator auris longus using two intracellular microelectrodes. The effects of ATP were determined by measuring the relative changes of miniature endplate potentials (mEPPs) and voltage responses to step current pulses in individual muscle fibres. Exposure to ATP (20 microm) prolonged the mEPP falling phase by 31 +/- 7.5% (values +/- s.d., n = 3 fibres). Concurrently, the input resistance increased by 31 +/- 2.0% and the time course of the voltage responses increased by 59 +/- 3.0%. Analogous effects were observed using 2 and 5 microm ATP, and on regions distal from the neuromuscular junction, indicating that physiologically relevant levels of ATP enhanced electrical signalling over the entire muscle fibre. The effects of extracellular ATP were blocked by 200 microm anthracene-9-carboxylic acid, a chloride channel inhibitor, and reduced concentrations of extracellular chloride, indicating that ATP inhibited chloride channels. A high affinity agonist for P2Y receptors, 2-methylthioadenosine-5-O-diphosphate (2MeSADP), induced similar effects to ATP with an EC(50) of 160 +/- 30 nm. The effects of 250 nm2MeSADP were blocked by 500 nmMRS2179, a specific P2Y(1) receptor inhibitor, suggesting that ATP acts on P2Y(1) receptors to inhibit chloride channels. The inhibition of chloride channels by extracellular ATP has implications for muscle excitability and fatigue, and the pathophysiology of myotonias.

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.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

Increased susceptibility to ATP via alteration of P2X receptor function in dystrophic mdx mouse muscle cells

Pathological cellular hallmarks of Duchenne muscular dystrophy (DMD) include, among others, abnormal calcium homeostasis. Changes in the expression of specific receptors for extracellular ATP in dystrophic muscle have been recently documented: here, we demonstrate that at the earliest, myoblast stage of developing dystrophic muscle a purinergic dystrophic phenotype arises. In myoblasts of a dystrophin-negative muscle cell line established from the mdx mouse model of DMD but not in normal myoblasts, exposure to extracellular ATP triggered a strong increase in cytoplasmic Ca2+ concentrations. Influx of extracellular Ca2+ was stimulated by ATP and BzATP and inhibited by zinc, Coomassie Brilliant Blue-G, and KN-62, demonstrating activation of P2X7 receptors. Significant expression of P2X4 and P2X7 proteins was immunodetected in dystrophic myoblasts. Therefore, full-length dystrophin appears, surprisingly, to play an important role in myoblasts in controlling responses to ATP. Our results suggest that altered function of P2X receptors may be an important contributor to pathogenic Ca2+ entry in dystrophic mouse muscle and may have implications for the pathogenesis of muscular dystrophies. Treatments aiming at inhibition of specific ATP receptors could be of a potential therapeutic benefit.

The T-tubule membrane ATP-operated P2X4 receptor influences contractility of skeletal muscle

Evidence indicates that extracellular ATP may have relevant functions in skeletal muscle, even though the physiological role and distribution of specific signaling pathway elements are not well known. The present work shows that P2X4 receptor, an extracellular ATP-regulated cell membrane channel permeable to Ca2+, is expressed in several tissues of the rat, including skeletal muscle. A specific antibody detected a protein band of approximately 60 kDa. Immunofluorescence demonstrated that P2X4 has an intracellular localization, and confocal analysis revealed that the receptor colocalizes with the T-tubule membrane DHP receptor. Considering that the natural agonist of P2X4 is ATP, we explored if changes of extracellular ATP levels could occur in contracting skeletal muscle to regulate the channel. In vitro experiments showed that substantial ATP is released and rapidly hydrolyzed after electrical stimulation of rat muscle fibers. Results show that the presence of ATP-degrading enzymes (hexokinase/apyrase), inhibitors of P2X receptors or Ca2+-free conditions, all abolished the progressive twitch tension potentiation produced in soleus muscle by low-frequency (0.05 Hz) stimulation. These data reveal that ATP-mediated Ca2+ entry, most likely through P2X4 receptor, may play an important role in modulating the contractility of skeletal muscle.

ATP potentiates agrin-induced AChR aggregation in cultured myotubes: activation of RhoA in P2Y1 nucleotide receptor signaling at vertebrate neuromuscular junctions

At vertebrate neuromuscular junctions, ATP is known to stabilize acetylcholine in the synaptic vesicles and to be co-released with it. We have shown previously that a nucleotide receptor, P2Y(1) receptor, is localized at the nmjs, and we propose that this mediates a trophic role for synaptic ATP there. In cultured myotubes, the activation of P2Y(1) receptors modulated agrin-induced acetylcholine receptor (AChR) aggregation in a potentiation manner. This potentiation effect in agrin-induced AChR aggregation was reduced by antagonizing the P2Y(1) receptors. The guanosine triphosphatase RhoA was shown to be responsible for this P2Y(1)-potentiated effect. The localization of RhoA in rat and chicken skeletal muscles was restricted at the neuromuscular junctions. Application of P2Y(1) agonists in cultured myotubes induced RhoA activation, which showed an additive effect with agrin-induced RhoA activation. Over-expression of dominant-negative mutant of RhoA in cultured myotubes diminished the agrin-induced AChR aggregation, as well as the potentiation effect of P2Y(1)-specific agonist. Application of UTP in the cultures also triggered similar responses as did 2-methylthioadenosine 5'-diphosphate, suggesting the involvement of other subtypes of P2Y receptors. These results demonstrate that RhoA could serve as a downstream mediator of signaling mediated by P2Y(1) receptor and agrin, which therefore synergizes the effects of the two neuron-derived trophic factors in modulating the formation and/or maintenance of post-synaptic apparatus at the neuromuscular junctions.

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.

Expression of the P2Y1 nucleotide receptor in chick muscle: its functional role in the regulation of acetylcholinesterase and acetylcholine receptor

In vertebrate neuromuscular junctions, ATP is stored at the motor nerve terminals and is co-released with acetylcholine during neural stimulation. Here, we provide several lines of evidence that the synaptic ATP can act as a synapse-organizing factor to induce the expression of acetylcholinesterase (AChE) and acetylcholine receptor (AChR) in muscles, mediated by a metabotropic ATP receptor subtype, the P2Y(1) receptor. The activation of the P2Y(1) receptor by adenine nucleotides stimulated the accumulation of inositol phosphates and intracellular Ca(2+) mobilization in cultured chick myotubes. P2Y(1) receptor mRNA in chicken muscle is very abundant before hatching and again increases in the adult. The P2Y(1) receptor protein is shown to be restricted to the neuromuscular junctions and colocalized with AChRs in adult muscle (chicken, Xenopus, and rat) but not in the chick embryo. In chicks after hatching, this P2Y(1) localization develops over approximately 3 weeks. Denervation or crush of the motor nerve (in chicken or rat) caused up to 90% decrease in the muscle P2Y(1) transcript, which was restored on regeneration, whereas the AChR mRNA greatly increased. Last, mRNAs encoding the AChE catalytic subunit and the AChR alpha-subunit were induced when the P2Y(1) receptors were activated by specific agonists or by overexpression of P2Y(1) receptors in cultured myotubes; those agonists likewise induced the activity in the myotubes of promoter-reporter gene constructs for those subunits, actions that were blocked by a P2Y(1)-specific antagonist. These results provide evidence for a novel function of ATP in regulating the gene expression of those two postsynaptic effectors.

Expression of the P2Y1 nucleotide receptor in chick muscle: its functional role in the regulation of acetylcholinesterase and acetylcholine receptor

In vertebrate neuromuscular junctions, ATP is stored at the motor nerve terminals and is co-released with acetylcholine during neural stimulation. Here, we provide several lines of evidence that the synaptic ATP can act as a synapse-organizing factor to induce the expression of acetylcholinesterase (AChE) and acetylcholine receptor (AChR) in muscles, mediated by a metabotropic ATP receptor subtype, the P2Y(1) receptor. The activation of the P2Y(1) receptor by adenine nucleotides stimulated the accumulation of inositol phosphates and intracellular Ca(2+) mobilization in cultured chick myotubes. P2Y(1) receptor mRNA in chicken muscle is very abundant before hatching and again increases in the adult. The P2Y(1) receptor protein is shown to be restricted to the neuromuscular junctions and colocalized with AChRs in adult muscle (chicken, Xenopus, and rat) but not in the chick embryo. In chicks after hatching, this P2Y(1) localization develops over approximately 3 weeks. Denervation or crush of the motor nerve (in chicken or rat) caused up to 90% decrease in the muscle P2Y(1) transcript, which was restored on regeneration, whereas the AChR mRNA greatly increased. Last, mRNAs encoding the AChE catalytic subunit and the AChR alpha-subunit were induced when the P2Y(1) receptors were activated by specific agonists or by overexpression of P2Y(1) receptors in cultured myotubes; those agonists likewise induced the activity in the myotubes of promoter-reporter gene constructs for those subunits, actions that were blocked by a P2Y(1)-specific antagonist. These results provide evidence for a novel function of ATP in regulating the gene expression of those two postsynaptic effectors.

Ligand-gated purinergic receptors are differentially expressed in the adult rat vestibular periphery

To further characterize the pattern of expression of the ligand-gated purinergic P2X receptors in the peripheral vestibular system, we conducted reverse transcription-polymerase chain reaction amplification of P2X1 and P2X2 messenger RNA extracted from adult rat vestibular ganglia (Scarpa's ganglia) and vestibular end organs. Transcripts encoding P2X1 were found in both Scarpa's ganglia and the end organs, but transcripts encoding P2X2 were found only in the vestibular end organs. These results support previous electrophysiological data, and they provide a more complete understanding of the specific role of purinergic (adenosine-5'-triphosphate) transmission in the vestibular periphery.

Facilitation by P(2) receptor activation of acetylcholine release from rat motor nerve terminals: interaction with presynaptic nicotinic receptors

ATP is released from motor nerve endings together with acetylcholine. Released adenine nucleotides can be extracellularly metabolized into adenosine, which is a presynaptic neuromodulator at neuromuscular junctions, but it is not known if P(2) receptor activation also modulates acetylcholine release from mature motor nerve endings. We now tested the effect of a stable ATP analogue, beta,gamma-imido ATP on the nerve-evoked release of acetylcholine from adult rat hemidiaphragm preparations. beta,gamma-Imido ATP (10-100 microM) facilitated in a concentration-dependent manner evoked acetylcholine release, and 30 microM beta,gamma-imido ATP caused a 125% facilitation of evoked acetylcholine release. This facilitatory effect of beta,gamma-imido ATP (30 microM) was abolished by the P(2) receptor antagonists, suramin (100 microM) and pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS, 10 microM), but not by the A(1) or A(2A) adenosine receptor antagonists, 1,3-dipropyl-8-cyclopentylxanthine (50 nM) and ZM 241385 (50 nM), respectively. The facilitation of acetylcholine release by beta, gamma-imido ATP (30 microM) was also prevented by the nicotinic acetylcholine receptor antagonist, D-tubocurarine (1 microM) and the facilitatory effect (40%) of the nicotinic acetylcholine receptor agonist, 1,1-dimethyl-4-phenylpiperazinium (1 microM) was abolished by PPADS (10 microM). These results demonstrate a presynaptic facilitatory effect of P(2) receptor activation at the rat phrenic nerve endings, which is tightly coupled with the presynaptic nicotinic autofacilitatory system.

Stimulation-dependent release, breakdown, and action of endogenous ATP in mouse hemidiaphragm preparation: the possible role of ATP in neuromuscular transmission

In this study the in vitro mouse phrenic nerve- hemidiaphragm preparation was utilized to study the release and extracellular catabolism of endogenous ATP and its action on the postsynaptic site, i.e. on the contraction force evoked by nerve stimulation. ATP, measured by the luciferin-luciferase assay, was released stimulation-dependently from the mouse hemidiaphragm in response to electrical field stimulation at 10 Hz. Blockade of the Na(+) channel activity by tetrodotoxin inhibited the majority of the release of ATP in response to stimulation, showing that it is related to neuronal activity. The nicotinic receptor antagonists d-tubocurarine, and alpha-bungarotoxin and cooling the bath temperature to 7 degrees C also reduced stimulation-induced ATP outflow, suggesting that nicotinic receptors are responsible for the part of the release of ATP that is released from postsynaptic sites in a carrier-mediated manner. Exogenous ATP (20-500 microM) added to the bath was degraded to ADP and AMP by the action of ectoATPase and ectoATPdiphosphohydrolase; the K(m) and v(max) values of these enzymes were 185.8 microM and 55.16 nmol/min.g respectively. However, the total amount of nucleotides ([ATP+ADP+AMP]) was increased after the addition of ATP, indicating that ATP itself promoted further adenine nucleotide release. Twitch contractions of the rat hemidiaphragm preparation evoked by low frequency electrical stimulation was blocked concentration-dependently by the non-depolarizing muscle relaxants d-tubocurarine and pancuronium. Suramin (100 microM-1 mM) reversed neuromuscular blockade by d-tubocurarine and pancuronium; i.e., it shifted their concentration-response curves to the right Taken together our data, that endogenous ATP is released by stimulation and subsequently catabolized in the hemidiaphragm preparation and that suramin inhibits ecto-ATPase activity could be interpreted as meaning that suramin prolongs the action of endogenous ATP to elicit twitch contraction, which points to a new, undefined role of ATP in neuromuscular transmission. The source of ATP is partly postsynaptic, released from the muscle in response to activation of nicotinic ACh receptors expressed on the muscle.

Ecto-ATPase activity of alpha-sarcoglycan (adhalin)

alpha-Sarcoglycan is a component of the sarcoglycan complex of dystrophin-associated proteins. Mutations of any of the sarcoglycan genes cause specific forms of muscular dystrophies, collectively termed sarcoglycanopathies. Importantly, a deficiency of any specific sarcoglycan affects the expression of the others. Thus, it appears that the lack of sarcoglycans deprives the muscle cell of an essential, yet unknown function. In the present study, we provide evidence for an ecto-ATPase activity of alpha-sarcoglycan. alpha-Sarcoglycan binds ATP in a Mg2+-dependent and Ca2+-independent manner. The binding is inhibited by 3'-O-(4-benzoyl)benzoyl ATP and ADP. Sequence analysis reveals the existence of a consensus site for nucleotide binding in the extracellular domain of the protein. An antibody against this sequence inhibits the binding of ATP. A dystrophin.dystrophin-associated protein preparation demonstrates a Mg-ATPase activity that is inhibited by the antibody but not by inhibitors of endo-ATPases. In addition, we demonstrate the presence in the sarcolemmal membrane of a P2X-type purinergic receptor. These data suggest that alpha-sarcoglycan may modulate the activity of P2X receptors by buffering the extracellular ATP concentration. The absence of alpha-sarcoglycan in sarcoglycanopathies leaves elevated the concentration of extracellular ATP and the persistent activation of P2X receptors, leading to intracellular Ca2+ overload and muscle fiber death.

Non-additive interaction between nicotinic cholinergic and P2X purine receptors in guinea-pig enteric neurons in culture

1. Acetylcholine (ACh)-activated currents and their interaction with ATP-activated currents were studied in primary cultures of myenteric neurons from guinea-pig small intestine using patch clamp techniques. Peak currents caused by co-application of ACh (1 mM) and ATP (300 microM) were 78 +/- 2 % of the sum of currents activated by each agonist alone (P < 0.05, n = 29). Reversal potentials measured during co-application of ACh and ATP did not differ from those measured during application of ACh or ATP alone. Addition of BAPTA (10 mM) to the pipette solution or replacement of extracellular Ca2+ with Na+ did not prevent occlusion. 2. Responses caused by co-application of 5-HT (300 microM), acting at 5-HT3 receptors, and ACh (3 mM) or ATP (1 mM) were additive (94 +/- 3 or 96 +/- 4 %, respectively, of the sum of currents activated by 5-HT and ACh or ATP alone; P > 0.05). Currents caused by GABA (1 mM), acting at GABAA receptors, and ACh (3 mM) or ATP (1 mM) were also additive (105 +/- 4 or 100 +/- 3 %, respectively, of the sum of currents activated by GABA and ACh or GABA and ATP applied separately; P > 0. 05). 3. Single channel currents caused by ACh and ATP in the same outside-out patches were less than additive (85 +/- 10 % of the predicted sum, P < 0.05). 4. P2X receptors and nicotinic cholinergic receptors (nAChRs) are linked in a mutually inhibitory manner in guinea-pig myenteric neurons. The functional interaction does not involve ligand binding sites, Ca2+-dependent mechanisms, a change in the driving force for Na+ or cytoplasmic signalling mechanisms.

Evidence for three additional P2X2 purinoceptor isoforms produced by alternative splicing in the adult rat vestibular end-organs

P2X2 receptors are ligand-gated ion channels that are activated by extracellular ATP. To characterize the expression of P2X2 purinoceptor in the adult rat vestibular periphery, reverse transcription-polymerase chain reaction (RT-PCR) was used. No transcript for P2X2 receptor was found in the vestibular primary afferent neurons (Scarpa's ganglia); however, partial cDNAs encoding four splice variants of the P2X2 receptor were isolated from vestibular end-organs. In all four cDNAs, the deletions were of different lengths but started at the same position on the P2X2 gene (Val-370 codon) located toward the intracellular carboxyl terminus. One of these receptor isoforms was identical in sequence to the recently published P2X2(b) receptor (Simon et al., 1997, Mol. Pharmacol. 52, 237-248) (also known as P2X2-2, in the nomenclature of Brändle et al., 1997, FEBS Lett. 404, 294-298). The remaining three novel splice variants of the P2X2 receptor were designated P2X2(e), P2X2(f) and P2X2(g) (GenBank accession numbers AF028603, AF028604 and AF028605, respectively). The functional significance of these three splice variants remains to be determined. Pituitary and cerebellum were used as survey tissues and only the P2X2(b) receptor cDNA was found.

Evaluation of intrinsic modulation of synaptic transmission by ATP in mouse fast twitch muscle

This study aims to evaluate whether endogenous ATP or adenosine modulates the neurotransmission and contractile function of mouse phrenic nerve-diaphragm. Bath application of ATP (1 mM) and alpha, beta-methylene ATP (m-ATP, 0.1 mM) elevated muscle tones, depressed contractions (approximately 12%), and depolarized muscle membranes (approximately 20 mV). Adenosine (1 mM) or low concentrations of ATP (0.1 mM) had no effect. In a low Ca2+ media, ATP caused prolonged inhibitions of endplate potentials (EPPs), whereas m-ATP augmented EPPs while both agents produced slight effects in normal Tyrode solution. When applied by puff ejection, ATP and m-ATP additionally elicited fast transient suppressions of EPPs in association with inhibitions of high K+-evoked releases of miniature EPPs. Blockades of P2 purinoceptors with suramin antagonized all the effects of ATP and m-ATP except the prolonged inhibitions of EPPs induced by ATP, which were antagonized instead by 8-cyclopentyl-1,3-dipropylxanthine (CPDPX), an A1 adenosine receptor antagonist. Suramin and CPDPX did not change contractions nor alter EPPs evoked by a low- or high-frequency nerve stimulation. The results indicate that exogenously applied ATP and m-ATP, via activations of distinct pre- and postsynaptic purinoceptors, exert inhibitory and facilitatory pharmacological modulations on the mature neuromuscular junction. However, because of intrinsic high efficiency of the synaptic transmission under physiological conditions, endogenously released ATP and its degradation product-adenosine-do not build up to concentrations high enough to alter motor functions.

Mutual occlusion of P2X ATP receptors and nicotinic receptors on sympathetic neurons of the guinea-pig

1. The interaction of ion channels activated by nicotinic receptor agonists with ion channels gated by extracellular ATP (i.e. P2X receptors) was studied on sympathetic neurons acutely dissociated from coeliac ganglia of the guinea-pig. Patch clamp methods were used to measure the inward current generated through these non-selective cationic channels under voltage clamp. 2. At the whole cell level, the specific nicotinic receptor agonists nicotine (5-100 microM) or cytisine (50-75 microM) and the P2X receptor agonists ATP (0.1-7 microM) or alpha,beta-methylene ATP (6 microM) were examined separately and in the presence of the other receptor activator. When a nicotinic and P2X receptor agonist were applied together, mutually occlusive effects were generally observed. This occurred even with concentrations of agonists that in themselves generated little to no inward current. 3. The occlusive effects of nicotinic agonists on ATP-gated currents were blocked by the nicotinic receptor/ion channel blocker hexamethonium (150 microM). The occlusive effects of ATP analogues on inward currents generated by nicotinic agonists were blocked by the P2X receptor antagonist suramin (100 microM). 4. Mutual occlusion of the effects of nicotinic agonists and ATP analogues were also observed when currents through single channels were studied in excised (outside-out) patches. 5. The results suggest that nicotinic receptors and P2X ATP receptors do not act independently in these sympathetic neurons.

Mapping of a binding site for ATP within the extracellular region of the Torpedo nicotinic acetylcholine receptor beta-subunit

Using 2,8,5'-[3H]ATP as a direct photoaffinity label for membrane-bound nicotinic acetylcholine receptor (nAChR) from Torpedo marmorata, we have identified a binding site for ATP in the extracellular region of the beta-subunit of the receptor. Photolabeling was completely inhibited in the presence of saturating concentrations of nonradioactive ATP, whereas neither the purinoreceptor antagonists suramin, theophyllin, and caffeine nor the nAChR antagonists alpha-bungarotoxin and d-tubocurarine affected the labeling reaction. Competitive and noncompetitive nicotinic agonists and Ca2+ increased the yield of the photoreaction by up to 50%, suggesting that the respective binding sites are allosterically linked with the ATP site. The dissociation constant KD of binding of ATP to the identified site on the nAChR was of the order of 10(-4) M. Sites of labeling were found in the sequence regions Leu11-Pro17 and Asp152-His163 of the nAChR beta-subunit. These regions may represent parts of a single binding site for ATP, which is discontinuously distributed within the primary structure of the N-terminal extracellular domain. The existence of an extracellular binding site for ATP confirms, on the molecular level, that this nucleotide can directly act on nicotinic receptors, as has been suggested from previous electrophysiological and biochemical studies.

Purinoceptors in neuromuscular transmission

At the neuromuscular junction, P2-purinoceptors mediate the actions of the co-transmitter ATP and P1-purinoceptors, those of its degradation product adenosine. The classification of the subtypes of P1- and P2-purinoceptors and their signal transduction routes is presented. Purinoceptor-mediated effects on the prejunctional release of acetylcholine and the postjunctional desensitization and expression of nicotinic receptors are discussed in depth. An additional section on the reversal action of the P2-purinoceptor antagonist suramin on neuromuscular block underscores the importance of testing purinoceptor-targeted drugs once they will be marketed, to avoid adverse effects in patients.

Calcium flux through predominantly independent purinergic ATP and nicotinic acetylcholine receptors

Ligand-gated nicotinic acetylcholine receptors (nAChRs) and purinergic ATP receptors are often expressed in the same peripheral and central neurons, and ATP and acetylcholine (ACh) are stored together in some synaptic vesicles. Evidence has suggested that nAChRs and ATP receptors are not independent and that some agonists strongly cross-activate and desensitize both receptor types. Rat sympathetic neurons and nAChRs expressed in Xenopus oocytes were studied to determine the significance of the interactions caused by the two agonist types. Current amplitudes included with separate or combined applications of ATP and nicotine are > 90% additive and independent. Half of all neurons tested responded to either ATP or nicotine but not to both, indicating differences in the expression of the two receptors. In neurons that expressed both receptors types, the nAChRs were inhibited by the activity-dependent open-channel blocker chlorisondamine. If the purinergic and nicotinic receptors were significantly dependent and coactivated, then blocking the ion channels opened by a nicotinic agonist should diminish the current activated by a purinergic agonist. That result was not seen; rather, complete open-channel block of nAChRS with chlorisondamine did not significantly alter the amplitude or kinetics of ATP-induced currents in the same neurons. Finally, when cloned nAChR subunits were expressed in oocytes, ATP activated only very small currents compared with the current activated by Ach. For the 13 different nAChR subunit combinations that were studied, ATP (50-500 microM) activated a current that ranged from 0 to 4% of the size of the current activated by 100 microM ACh. In summary, we find that there is little cross reactivity, and nAChRs and purinergic ATP receptors are predominantly independent, acting with separable physiological characteristics. Therefore the quantitative Ca2+ flux could be separately determined for nAChRs and ATP receptors. The fraction of total current that is carried by Ca2+ was quantitatively determined by simultaneously measuring the whole cell current and the associated change in intracellular Ca2+ with fura-2. For sympathetic neurons bathed in 2.5 mM Ca2+ at a holding potential of -50 mV, Ca2+ carries 4.8 +/- 0.3% (mean +/- SE) of the inward current through neuronal nAChRs and 6.5 +/- 0.1% of the current through purinergic ATP receptors. In conclusion, activity-dependent Ca2+ influx through predominately independent populations of nAChRS and ATP neurons can produce different intracellular signals at purinergic and cholinergic synapses.

Molecular cloning of a novel P2 purinoceptor from human erythroleukemia cells

Screening of a human erythroleukemia cell cDNA library with radiolabeled chicken P2Y3 cDNA at low stringency revealed a cDNA clone encoding a novel G protein-coupled receptor with homology to P2 purinoceptors. This receptor, designated P2Y7, has 352 amino acids and shares 23-30% amino acid identity with the P2Y1-P2Y6 purinoceptors. The P2Y7 cDNA was transiently expressed in COS-7 cells: binding studies thereon showed a very high affinity for ATP (37 +/- 6 nM), much less for UTP and ADP (approximately 1300 nM), and a novel rank order of affinities in the binding series studied of 8 nucleotides and suramin. The P2Y7 receptor sequence appears to denote a different subfamily from that of all the other known P2Y purinoceptors, with only a few of their characteristic sequence motifs shared. The P2Y7 receptor mRNA is abundantly present in the human heart and the skeletal muscle, moderately in the brain and liver, but not in the other tissues tested. The P2Y7 receptor mRNA was also abundantly present in the rat heart and cultured neonatal rat cardiomyocytes. The P2Y7 receptor is functionally coupled to phospholipase C in COS-7 cells transiently expressing this receptor. The P2Y7 gene was shown to be localized to human chromosome 14. We have thus cloned a unique member of the P2Y purinoceptor family which probably plays a role in the regulation of cardiac muscle contraction.

Reduction of acetylcholine-activated current by low concentrations of extracellular adenosine 5'-triphosphate

Effects of adenosine 5'-triphosphate (ATP) on ionic currents activated by acetylcholine (ACh) were investigated using rat pheochromocytoma PC12 cells and Xenopus oocytes expressing nicotinic receptors. In PC12 cells, ATP (10 nM to 1 microM) inhibited an inward current activated by ACh in not all but about 60% of cell batches. The ACh-activated current was also inhibited by ATP in Xenopus oocytes and, with a subunit combination of alpha 3 plus beta 4, the inhibition was observed at concentration as low as 100 fM. Uridine 5'-triphosphate (UTP) induced a similar inhibition of the ACh-activated current both in PC12 cells and Xenopus oocytes. These and other properties suggest that the current reduction by ATP is not mediated through conventional P2-purinoceptors.

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

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

Regulation of postsynaptic responses by calcitonin gene related peptide and ATP at developing neuromuscular junctions

Neuronal factors co-released with neurotransmitters may play an important role in synapse development and function. Calcitonin gene related peptide (CGRP) and adenosine 5'-triphosphate (ATP), two principal neuromodulators present in the motor nerve terminals, were studied for their roles and mechanisms during early development of neuromuscular synapses in Xenopus nerve--muscle co-cultures. CGRP treatment increased the decay time and amplitude of spontaneous synaptic currents (SSCs) recorded from innervated myocytes, without affecting SSC frequency, suggesting a postsynaptic mechanism. ATP also increased the SSC amplitude and decay time. In addition, ATP was shown to potentiate the responses of isolated myocytes to iontophoretically applied acetylcholine (ACh). Single-channel recording from isolate myocytes showed that both CGRP and ATP specifically increased the open time of embryonic-type, low-conductance ACh channels. Pharmacological experiments suggest that the CGRP actions were mediated by cAMP-dependent protein kinase (PKA), while ATP exerted its effects by binding to P2 purinoceptors and thereby activating protein kinase C (PKC). Moreover, the effects of CGRP and ATP on ACh channel activity were restricted to immature myocytes. Taken together, these results suggest that endogenous CGRP and ATP co-released with ACh from the nerve terminal may promote synaptic development by potentiating postsynaptic ACh channel activity during the early phase of synaptogenesis.

Potentiation by ATP of the postsynaptic acetylcholine response at developing neuromuscular synapses in Xenopus cell cultures

1. Extracellular application of ATP to developing Xenopus neuromuscular synapses in culture resulted in a marked increase in the amplitude and frequency of spontaneous synaptic currents, using whole-cell recording. 2. The postsynaptic action of ATP was examined by studying the response of isolated muscle cells to iontophoretically applied acetylcholine (ACh). ATP enhanced the responses of the muscle membrane to ACh. The order of potency for various nucleotides (ATP = ADP >> AMP, adenosine, GTP) suggests that ATP acts through P2-purinoceptors. The effect of ATP on whole-cell currents was also abolished by the protein kinase inhibitor H-7. 3. Single-channel measurements indicate that ATP increased the mean open time of low-conductance ACh channels. No change in the conductance of ACh channels was observed. 4. Local application of ATP to one region of the elongated myocyte surface resulted in potentiated ACh responses only at the ATP-treated region, suggesting that the cytosolic second messengers were effectively confined within the muscle cytoplasm. 5. The results of the present study suggest that ATP released from the nerve terminals may potentiate the ACh response of developing muscle cells during the early phase of synaptogenesis, and that the action of ATP can be restricted to the subsynaptic region exposed to the secreted ATP.

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.

Identification of purine binding sites on Torpedo acetylcholine receptor

Electrophysiological studies from this and other laboratories have suggested a direct action of ATP on nicotinic acetylcholine receptors (nAChR). To determine the site of binding of this purine derivative, we have covalently modified the nAChR from Torpedo marmorata electrocytes employing 2-[3H]-8-azido-ATP as a photoactivable affinity label. Covalently attached radioactivity was predominantly found in the beta-polypeptide of the receptor. Based on the results of protection studies with several nAChR ligands whose target sites at the receptor are known, we conclude that the purine site(s) differ from those of acetylcholine and of physostigmine, galanthamine and related ligands, and those of local anesthetics.

ATP regulates synaptic transmission by pre- and postsynaptic mechanisms in guinea-pig myenteric neurons

Intracellular recordings were made from myenteric neurons of the guinea-pig ileum in vitro; they were classified into S and AH neurons according to electrophysiological criteria. ATP (10 nM-100 microM) inhibited excitatory synaptic potentials in the myenteric plexus; fast excitatory postsynaptic potentials and slow excitatory postsynaptic potentials of S neurons and slow excitatory postsynaptic potentials in AH neurons. This inhibitory action was reversible and dose-dependent, and was usually followed by a transient augmentation of the synaptic potentials after washing of ATP. The actions of ATP on the synaptic potentials were prevented by pretreatment with theophylline, caffeine, quinidine and 8-phenyl theophylline. The ATP analogues, ATP-gamma-s (100 nM-100 microM) and alpha-beta-methylene ATP (100 nM-100 microM) also depressed the synaptic potentials recorded from both types of neurons. The inhibitory effect of adenosine on the synaptic potentials was 10 times weaker than that of ATP. Thus, it seems clear that the presynaptic inhibition is not occurring through adenosine A1 or A2 receptors. Furthermore, ATP at high concentrations ( > or = 1 microM) augmented nicotinic fast depolarizations of S neurons produced by extracellular acetylcholine. However, ATP at the same concentrations inhibited the slow depolarizations of S and AH neurons caused by exogenous acetylcholine (muscarinic) and substance P. It is concluded that ATP regulates synaptic transmission in the myenteric plexus of the guinea-pig ileum and the sites of ATP actions are pre- and postsynaptic.

ATP activates junctional and extrajunctional acetylcholine receptor channels in isolated adult rat muscle fibres

Single-channel recordings in the cell-attached configuration were made on freshly dissociated and cultured rat muscle fibers. We demonstrate that ATP (50-100 microM) elicits single channel currents in freshly isolated fibres. These ATP-activated channels have a slope conductance similar to that determined for ACh-activated channels but exhibit a shorter open time. We also show that ATP activates extrajunctional AChR-channels in the membrane of long-term cultured muscle fibres. Here the ATP activated channel has a slope conductance and open time characteristics similar to those of the ACh-activated channel.