Potentiation of postjunctional cholinergic sensitivity of rat diaphragm muscle by high-energy-phosphate adenine nucleotides

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.

The birth and postnatal development of purinergic signalling

The purinergic signalling system is one of the most ancient and arguably the most widespread intercellular signalling system in living tissues. In this review we present a detailed account of the early developments and current status of purinergic signalling. We summarize the current knowledge on purinoceptors, their distribution and role in signal transduction in various tissues in physiological and pathophysiological conditions.

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.

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.

Molecular cloning and characterization of a novel ATP P2X receptor subtype from embryonic chick skeletal muscle

We have cloned a new P2X ligand-gated ion channel receptor from embryonic chick skeletal muscle, which is tentatively named as chick P2X(8) (cP2X(8)) receptor. The cloned cDNA encodes a protein with 402 amino acids. Electrophysiological study of the recombinant cP2X(8) receptor expressed in Xenopus oocytes showed that 10 microm ATP induced a fast inward current followed by rapid and long lasting desensitization in medium containing 1.8 mm Ca(2+). In medium with 0. 3 mm Ca(2+) ATP induced a bi-phasic response as follows: a slower inward current succeeded the initial fast one. 2-Methylthio-ATP, alpha,beta-methylene-ATP, and adenosine 5'-O-(thio)triphosphate were potent agonists, whereas ADP was a very weak agonist. ATP-induced currents were blocked by 100 microm suramin and pyridoxal phosphate 6-azophenyl-2',4'-disulfonic acid. Northern blot analysis and reverse transcription-polymerase chain reaction showed that cP2X(8) RNA transcripts were mainly expressed in skeletal muscle, brain, and heart of Day 10 chick embryos. A moderate level of expression was also detected in gizzard and retina. Whole mount in situ hybridization showed that cP2X(8) RNA transcripts were expressed mainly in neurotube, notochord, and stomach in Day 3 embryos. In Day 4 and Day 6 embryos, the cP2X(8) RNA transcripts were highly expressed in the myotome and premuscle mass. The physiological role of this receptor in the establishment of the skeletal muscle innervation will be studied.

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.

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.

Release and actions of adenosine in the central nervous system

Adenosine is released from active neurons into the extracellular fluid at a concentration of about 1 mumol/l. Neither the precise cellular origin nor the biochemical form of release has been firmly established, though the nucleotide is probably released partly directly, as a result of raised intracellular levels, and partly as nucleotides, which are subsequently hydrolysed. Once in the extracellular medium, adenosine markedly inhibits the release of excitatory neurotransmitters and modulatory peptides and has direct inhibitory effects on postsynaptic excitability via A1 receptors. A population of A2 receptors may mediate depolarization and enhanced transmitter release. Adenosine also modulates neuronal sensitivity to acetylcholine and catecholamines, all these effects probably contributing to the behavioural changes observed in conscious animals. As a result of their many actions, adenosine analogues are being intensively investigated for use as anticonvulsant, anxiolytic, and neuroprotective agents.

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.

Specific binding of ATP to extracellular sites on Torpedo acetylcholine receptor

The beta- and delta-subunits of the nicotinic acetylcholine receptor from Torpedo californica were covalently photolabeled at the synaptic surface with the ATP photoaffinity analogue [alpha-32P]-8-azido-ATP. The specificity of labeling for nucleotide binding sites was demonstrated by the saturation of labeling with increasing concentration of 8-azido-ATP and the inhibition of photolabeling by ATP. Protection studies suggest that the binding sites for the photolabel are unique and are not associated with the cholinergic ligand binding sites.

Suramin reverses non-depolarizing neuromuscular blockade in rat diaphragm

Unexpectedly, it was observed that the P2-purinoceptor antagonist, suramin (10 microM to 1 mM), reversed the muscle paralysis caused by structurally unrelated non-depolarizing relaxants. Suramin competitively reversed the blocking action of pancuronium. Both the pre- and postsynaptic blockade of nicotinic receptors by pancuronium was counteracted, as shown by the action of suramin, using train-of-four stimulation. Suramin did not affect the paralysis caused by the depolarizing relaxant, succinylcholine. The reversal action of suramin was not due to an increase in the acetylcholine concentration in the synaptic cleft, since neither the contraction of preparations partially paralysed by diminished acetylcholine release in the presence of low Ca2+ or high Mg2+ nor acetylcholinesterase activity were affected. Suramin did not affect the reduction in twitch tension caused by adenosine and potentiated the ATP-induced reduction in twitch, indicating that ATP-sensitive receptors are not involved in the reversal action of suramin. Consequently, these results suggest that the action of suramin is due to binding with a site on the acetylcholine receptor also occupied by non-depolarizing relaxants, but different from the site occupied by succinylcholine.

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

1. The effects of extracellular adenosine 5'-triphosphate (ATP) on the acetylcholine (ACh) channel opening activity was studied in dissociated rat skeletal muscle cells using patch-clamp recording techniques in cell-attached configuration. 2. With 10 microM-ATP in the recording pipette, the spontaneous opening frequency on the the alpha-bungarotoxin-sensitive ACh channel increased significantly from 0.3 to 4.7 s-1, although the opening frequency was not as high as that activated by very low (0.4 microM) ACh concentrations (64 s-1). 3. Spontaneous ACh channel openings, and ATP-associated and ACh-activated channel openings had similar single-channel conductances, 55, 59 and 56 pS, respectively. 4. ATP-associated events and spontaneous ACh channel opening events had similar mean channel open durations (0.6 ms); however, these values were considerably shorter than the duration of ACh-activated events (2 ms). 5. Pre-treatment with alpha-bungarotoxin (100 nM) blocked spontaneous ACh channel openings, ATP-associated openings and ACh-activated openings. 6. When delivered through a separate drug pipette after the formation of a gigaseal, ATP increased ACh-activated single-channel open probability in a dose-dependent fashion. 7. The increase in channel open probability was due primarily to the increase in channel opening frequency. ATP did not significantly alter the mean channel open duration or the single-channel conductance. 8. The ATP analogue adenosine-5'-O-(3-thiotriphosphate) (ATP-gamma-S) also enhanced ACh-activated channel open probability with relatively less potency. ADP, AMP and adenosine (up to 1 mM) did not significantly increase ACh channel open probability. 9. It is concluded that ATP in the micromolar range facilitates both spontaneous and agonist-activated ACh channel opening. The facilitation is due to ATP itself and not to products of ATP hydrolysis. The facilitatory actions of ATP on ACh channels are manifested by the increase in the channel opening frequency, and they may be mediated by an intracellular second messenger.

Sources of adenosine released during neuromuscular transmission in the rat

1. The levels of adenine nucleotides and adenosine which accumulate in the neuromuscular junction during nerve stimulation of the rat extensor digitorum longus (EDL) muscle were assayed biochemically. The sources were also determined by the use of different inhibitors. 2. ATP and total adenine nucleotide release increased as stimulation frequency increased, consistent with previous evidence indicating ATP release from presynaptic sources. 3. Adenosine levels also increased during nerve stimulation. However, accumulation decreased by 46-58% when muscle activation was blocked by the addition of d-tubocurarine (dTC). Adenosine levels also decreased by 40-59% when adenine nucleotide hydrolysis to adenosine was blocked by the addition of 1 mM-alpha,beta-methyladenosine 5'-diphosphate. Thus, approximately half of the extracellular adenosine is released from activated muscle while the other half is derived from adenine nucleotide hydrolysis. 4. Similar quantities of adenine nucleotide and acetylcholine (ACh) accumulated during nerve stimulation. With adenine nucleotide and ACh hydrolysis blocked by alpha,beta-methyladenosine 5'-diphosphate and eserine, respectively, the calculated amounts of adenine nucleotide and ACh released were 1.2 x 10(-16) and 1.5 x 10(-16) mol (stimulus impulse)-1 endplate-1. 5. AH5183 (vesamicol), which blocks ACh release, reduced extracellular ACh and adenine nucleotide accumulation by 40 and 45%, respectively. It did not affect adenosine release from the activated muscle. 6. Theophylline (100 microM), which blocks adenosine receptors, caused ATP accumulation to increase by 38%; extracellular levels of adenosine derived from adenine nucleotide hydrolysis also increased by 17%. These results are consistent with the presence of adenosine-mediated inhibition of adenine nucleotide release. 7. It is concluded that adenine nucleotides (presumably in the form of ATP) and ACh are released jointly, and that ATP is hydrolysed fairly rapidly to adenosine. Adenosine resulting from ATP hydrolysis accounts for about half of the extracellular adenosine accumulating during nerve stimulation, while the other half is released directly by the underlying muscle.

Dual control of local blood flow by purines

The potent and widespread vascular actions of purine nucleotides and nucleosides have long been recognized. A dual function for ATP in the regulation of vascular tone is considered. ATP acts as an excitatory cotransmitter with noradrenaline from sympathetic perivascular nerves, to cause vasoconstriction via P2X-purinoceptors located on vascular smooth muscle. In contrast, ATP can act via P2Y-purinoceptors located on vascular endothelial cells to release EDRF, which diffuses to the vascular smooth muscle and produces vasodilatation. The main source of intraluminal ATP is likely to be endothelial cells, and its release can be measured during conditions such as changes in flow and hypoxia, in amounts sufficient to activate endothelial P2Y-purinoceptors. In some vessels, ATP acts directly on P2Y-purinoceptors located in the vascular smooth muscle to produce vasodilatation; the possibility that the origins of this ATP are nonsympathetic purinergic or sensory-motor nerves is discussed. ATP can also be released during intravascular platelet aggregation and from intact and damaged vascular smooth muscle cells, and so may play a role in the complex physiological mechanisms controlling local vascular tone under normoxic conditions, during changes in blood flow and during vessel injury.

Regulation of nicotinic acetylcholine receptor function by adenine nucleotides

1. Nicotinic acetylcholine receptors (nAChR)4 from BC3H1 cells (which express a skeletal muscle-type receptor) and from Torpedo californica electric organ were expressed in Xenopus laevis oocytes and studied with a voltage-clamp technique. 2. We found that bath application of ATP in the micromolar to millimolar range increased the ACh-elicited current in both muscle and electrocyte receptors. The effect of ATP increased with successive applications. This "use-dependent" increase in potentiation was Ca2+ dependent, while the potentiation itself was not. 3. Four other nucleotides were tested on muscle nAChR: ADP, AMP, adenosine, and GTP. Of these, only ADP was a potentiator, but its effect was not use dependent. Neither ATP nor ADP affected the resting potential of the oocyte membrane. 4. ADP potentiated the response to suberyldicholine and nicotine, as well as ACh. 5. Finally, ADP reversed the phencyclidine-induced block of ACh currents in oocytes expressing muscle nAChR.

ATP-activated channels in excitable cells

Extracellular ATP is an activator or modulator of ionic channels in a wide variety of excitable cells. There appears to be a class of related cation-permeable ATP-activated channels in skeletal muscle, cardiac muscle, smooth muscle, and neurons; the channels in the different cell types appear to be similar, but not identical, in their ionic selectivity, receptor selectivity, and pharmacology. In all cases, these channels reverse near 0 mV and activation by ATP produces an excitatory effect. Much remains to be learned about these channels, their possible existence and roles in other cell types, and their relation to other types of ligand-gated channels. It will be especially important to develop more specific pharmacological blockers (and activators) in order to distinguish subtypes and to assess their physiological role. Another type of channel, so far described only in cardiac atrial cells, is identical to the channels in cardiac atrial cells activated by ACh receptors; it will be interesting to see if this type of receptor-channel complex is also found in neurons or other cells. In a variety of cells, ATP also acts as a modulator of voltage-dependent channels and of channels activated by other transmitters. It seems very likely that more instances of such modulation will be described in years to come. Possible second-messenger pathways mediating such modulation remain to be elucidated.

Extracellularly applied ATP alters the calcium flux through dihydropyridine-sensitive channels in cultured chick myotubes

Extracellularly applied ATP mediates a biphasic calcium signal in cultured chick myotubes. A rapid and transient increase in cytosolic calcium was independent of extracellular calcium while a second signal, slower in onset and decay, was absent without extracellular calcium. In depolarized myotubes, the cytosolic [Ca2+] was increased more than ten times above baseline level. Addition of ATP to the incubation medium immediately increased the rate of return of cytosolic Ca2+ levels to baseline. The ATP effect was half-maximal at about 10 microM ATP and was mimicked by ATP S. This ATP-sensitive calcium influx was also rapidly stopped by addition of dihydropyridines such as PN 200-110, suggesting that it is the voltage operated Ca2+-channel that was inactivated by ATP.

Modulation of the sensitivity of nicotinic receptors in autonomic ganglia

This article reviews some of the evidence suggesting that a variety of endogenous substances either facilitates or inhibits the sensitivity of nicotinic acetylcholine (ACh) receptors at the subsynaptic membrane of cholinergic synapses. It is noteworthy that 5-hydroxytryptamine and histamine act as competitive antagonists, like curare, presumably changing the affinity of ACh for the specific binding site on the nicotinic receptor. Catecholamine, neuropeptides, prostaglandin and glucocorticoids act as non-competitive antagonists on an allosteric site on the receptor-ionic channel complex. ATP and LH-RH (in a subpopulation of sympathetic neurons) caused a facilitation of the sensitivity of nicotinic receptors. The mode of actions of endogenous substances which modulate the nicotinic receptor-sensitivity is similar to those of pharmacological agents. Therefore, these neurotransmitters and neurohormones have been termed endogenous 'antagonists' or 'sensitizers'.

Adenosine 5'-triphosphate activates acetylcholine receptor channels in cultured Xenopus myotomal muscle cells

1. Single-channel currents activated by extracellular adenosine 5'-triphosphate (ATP-induced currents) were recorded in cultured muscle cells of Xenopus laevis using the cell-attached patch clamp technique. 2. The amplitude histogram of the ATP-induced currents had two distinct peaks, corresponding to 60 pS (high-conductance (gamma) channels currents) and 41 pS (low-gamma channel currents). The peak values of the currents were unaltered during 1-6 days in culture. 3. The mean open time of the two types of ATP-induced currents was 0.93 ms for high-gamma and 0.86 ms for low-gamma channel currents at 50 mV hyperpolarization. The reversal potential of the ATP-induced current, estimated from the I-V relationship, ranged between -5 and -15 mV. 4. The open-state probability of currents induced by 10 microM-ATP decreased in the presence of 20 microM-d-tubocurarine. 5. The frequency of ATP-induced current events depended upon the ATP concentration. The current events were first detected at 0.1 microM-ATP and occurred with increasing frequency up to 10 microM-ATP. At concentrations higher than 10 microM, the frequency of current events decreased. 6. When acetylcholine (ACh, 0.1 nM) was applied together with various concentrations of ATP, the frequency of current events increased 2- to 3-fold at the ATP concentration range between 0.1 and 10 microM. At higher concentrations of ATP the frequency decreased again. When ACh (0.1 nM) was applied without ATP, current events were rarely observed. 7. Two types of ATP-induced currents were also observed with adenylylimido 5'-diphosphate (AMP-PNP) at one-hundred micromolar concentrations. Neither AMP (adenosine 5'-monophosphate) nor ADP (adenosine 5'-diphosphate) (1-500 microM) induced channel events. 8. It is concluded that the nicotinic ACh receptor channels in cultured Xenopus skeletal muscle cells are opened by micromolar concentrations of exogenous ATP. The possible physiological significance is discussed.

Purine modulation of cholinomimetic responses in the rat hippocampal slice

Iontophoretic application of carbachol caused excitation of CA1 neurones and decreased the amplitude of antidromic CA1 population spikes recorded extracellularly. Adenosine, adenosine triphosphate (ATP) and the purine analogues N-ethylcarboxamide adenosine (NECA) and R- and S-phenylisopropyladenosine (PIA) reduced the effects of carbachol on single cell firing and on the population spike. Responses to excitatory amino acids were unaffected by adenosine except for a small depression of kainate and N-methyl-D-aspartate responses at high concentrations. The rank order of potency for the purine reduction of carbachol responses was R-PIA = S-PIA = NECA much greater than adenosine greater than ATP. The actions of purines and purine analogues were antagonized by 8-phenyltheophylline (8-PT) and other xanthine antagonists. Application of 8-PT and other xanthines without prior exposure to purines frequently resulted in marked potentiation of carbachol responses. Thus in the hippocampus, responses to the cholinomimetic carbachol are markedly and selectively reduced by purines acting at the P1 purine receptor type and it appears that endogenous levels of adenosine limit the effects of cholinergic agents.

Agonist and antagonist characterization of the P2-purinoceptors in the guinea pig ileum

When adenine nucleotides were administered to isolated guinea pig ileum longitudinal muscle, two immediate effects were observed: a contractile effect and a concomitant inhibition of the responses elicited by transmural nerve stimulation. At concentrations up to 10(-4) M the order of potency for the contractile effect was alpha,beta-MeADP = alpha,beta-MeATP greater than ADP = ATP = AMPPNP = beta, tau-MeATP greater than 2'-deoxy AMP = 2'-deoxy ADP. AMP and adenosine did not show any contractile effect, whereas both compounds dose-dependently and reversibly inhibited the nerve-induced contractile responses. ADP, ATP, beta,tau-MeATP and AMPPNP also inhibited contractile responses to transmural nerve stimulation, whereas 2'-deoxy AMP, 2'-deoxy ADP, alpha,beta-MeADP and alpha,beta-MeATP showed but weak inhibitory effects, 2'-deoxyadenosine, IMP, IDP, ITP, 8-BrATP and TDP lacked significant contractile effects and did not exert a significant inhibition on nerve-induced contractions. p-chloromercuribenzene sulphonic acid (PCMBS) irreversibly antagonized the contractile effects of ADP, ATP and the alpha,beta-methylene derivatives, whereas dantrolene sodium, tetrodotoxin, scopolamine and 8-p-sulphophenyltheophylline were without effect on nucleotide-induced contractions. The contractile effect of ADP or ATP was unaffected by indomethacin, whereas the contractile effect by alpha,beta-methylene derivatives was abolished by indomethacin. ADP, ATP and alpha,beta-MeADP enhanced contractile responses to exogenous acetylcholine, alpha,beta-MeADP being most effective. This enhancement was blocked by indomethacin. We suggest that ADP and ATP contracted the guinea pig ileum by an action at postjunctional P2-purinoceptors with different characteristics from prejunctional P1-purinoceptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of adenine compounds in autonomic neurotransmission

Clearly adenine compounds exert numerous effects throughout the autonomic nervous system. The responses of various peripheral tissues to purines are summarized in Table 2. The evidence supporting a possible excitatory neurotransmitter function for ATP is very good in the vas deferens and good in both the bladder detrusor and certain blood vessels. ATP may also be an excitatory neurotransmitter in the colon, hepatocytes and frog atrium. These responses appear to be mediated by P2x-purinoceptors. There is good evidence supporting a role for ATP as an inhibitory neurotransmitter in the taenia coli and duodenum, and some support in the anal sphincter and possibly the rabbit portal vein; these responses appear to be mediated by P2y-purinoceptors. There is good evidence against ATP being an inhibitory neurotransmitter in the stomach fundic muscle and ileum. ATP (or more likely its metabolite adenosine) may act as an inhibitory neurotransmitter by interacting with postsynaptic P1-purinoceptors in cultured sympathetic neurones and also in the parasympathetic vesicle ganglion of the cat. It seems likely that ATP released from heart, platelets or vascular endothelium could be an endogenous relaxant of blood vessels through its actions on the endothelium. Although the addition of exogenous adenosine affects many tissues, evidence supporting modulatory functions for endogenous extracellular adenosine has only been clearly demonstrated in the ileum, gallbladder, vas deferens, fallopian tubes, kidney, blood vessels, carotid sinus, heart and adipose tissue. Both ATP and adenosine, released during periods of hypoxia or ischemia, could exert negative inotropic, chronotropic and dromotropic actions in the heart. In many cases, the potential sources of extracellular purines have not been established. This is particularly important when attempting to establish a neurotransmitter function for ATP in a tissue. For instance, the one outstanding piece of evidence required to confirm that ATP is an excitatory neurotransmitter released from sympathetic nerves in blood vessels is the unequivocal demonstration that it is, in fact, released from the sympathetic nerves when they are stimulated. To date, only the release of radiolabeled metabolites of ATP, possibly from post- rather than presynaptic sites, has been detected. Studies of the release of ATP are complicated by its rapid degradation extracellularly by ecto-ATPase. Unfortunately, there are no specific inhibitors of ecto-ATPase available at present, but one hopes that a suitable inhibitor will be developed shortly.(ABSTRACT TRUNCATED AT 400 WORDS)

Externally applied adenosine-5'-triphosphate causes inositol triphosphate accumulation in cultured chick myotubes

In striated muscle, adenosine-5'-triphosphate (ATP) potentiates the responses to acetylcholine. The underlying biochemical events are unknown. Here we report that ATP, externally applied to chick myotubes, induces a rapid, dose-dependent accumulation of intracellular inositol triphosphate which is correlated with a decrease in phosphatidyl 4,5-bisphosphate. Adenosine-5'-diphosphate, adenosine-5'-monophosphate and adenosine are less potent while beta, gamma-imido ATP is equipotent motoneurons and/or skeletal muscle controls the activation of a polyphosphoinositide phosphodiesterase via a cell membrane P2-purinoceptor, thus modulating skeletal muscle responses to transmitter release.

The effects of adenosine on receptor sensitivity in the rat vas deferens

In the guinea-pig isolated vas deferens adenosine augments the contractile response to noradrenaline (NA). Adenosine also augments the contractile response to NA and phenylephrine in the prostatic portion of the transversely bisected rat vas deferens. However, adenosine appeared to neither augment nor diminish the responses to carbachol, 5HT or high K+. Repetitive exposure of the vas to various agonists induced a desensitisation specific to the particular agonist used. The rate of return of responsiveness (the rate of resensitization) was investigated and, in particular, the effect of adenosine on that rate. In the rat vas deferens the rate of resensitization of responses to NA was retarded by adenosine and this effect was abolished by 8-phenyltheophylline and by yohimbine, but not by atropine or propranolol. Neither 8-phenyltheophylline, yohimbine, propranolol nor atropine by themselves affected the rate of resensitization of the rat alpha-adrenoceptor. Adenosine had no effect on the rate of resensitization of responses to phenylephrine, 5HT, carbachol or high K+. It is considered that the potentiation of catecholamine responses by adenosine involves solely alpha 1-receptors but that the retardation of resensitization may involve alpha 2-receptors which do not contribute directly to contraction.

Depression of neuron responses to acetylcholine by combined application of norepinephrine and substrates of the tricarboxylic acid cycle

The possible relationship between the function of nicotinic acetylcholine receptors in Lymnaea stagnalis neurons and energy metabolism was studied. Oxidative phosphorylation was activated by treatment of neurons with substrates of the tricarboxylic acid cycle and norepinephrine. Transmembrane currents induced by acetylcholine in isolated neurons were measured by voltage clamp. Succinate dehydrogenase activity was determined histochemically in the same neurons. Cyclic adenosine monophosphate concentration in ganglia were assayed by the protein saturation method of Gilman (1970). When used alone, succinate depressed the responses of about 50% of neurons to acetylcholine. Norepinephrine did not affect the acetylcholine-induced currents but almost doubled the inhibitory action of succinate. The mixture of norepinephrine and isocitrate also diminished the responses to acetylcholine but to a lesser extent than norepinephrine with succinate. A short-term exposure of the ganglia to succinate with norepinephrine led to the activation of succinate dehydrogenase in neurons and a threefold increase in cyclic adenosine monophosphate concentrations in ganglia. When used alone, norepinephrine doubled the cyclic adenosine monophosphate concentration. The results obtained suggest energy-dependent regulation of acetylcholine receptors.

Acetylcholine, ATP, and proteoglycan are common to synaptic vesicles isolated from the electric organs of electric eel and electric catfish as well as from rat diaphragm

Cholinergic synaptic vesicles were isolated from the electric organs of the electric eel (Electrophorus electricus) and the electric catfish (Malapterurus electricus) as well as from the diaphragm of the rat by density gradient centrifugation followed by column chromatography on Sephacryl-1000. This was verified by both biochemical and electron microscopic criteria. Differences in size between synaptic vesicles from the various tissue sources were reflected by their elution pattern from the Sephacryl column. Specific activities of acetylcholine (ACh; in nmol/mg of protein) of chromatography-purified vesicle fractions were 36 (electric eel), 2 (electric catfish), and 1 (rat diaphragm). Synaptic vesicles from all three sources contained ATP in addition to ACh (molar ratios of ACh/ATP, 9-12) as well as binding activity for an antibody raised against Torpedo cholinergic synaptic vesicle proteoglycan. Synaptic vesicles from rat diaphragm contained binding activity for the monoclonal antibody asv 48 raised against a rat brain 65-kilodalton synaptic vesicle protein. Antibody asv 48 binding was absent from electric eel and electric catfish synaptic vesicles. These antibody binding results, which were obtained by a dot blot assay on isolated vesicles, directly correspond to the immunocytochemical results demonstrating fluorescein isothiocyanate staining in the respective nerve terminals. Our results imply that ACh, ATP, and proteoglycan are common molecular constituents of motor nerve terminal-derived synaptic vesicles from Torpedo to rat. In addition to ACh, both ATP and proteoglycan may play a specific role in the process of cholinergic signal transmission.

Ionic channels: II. Voltage- and agonist-gated and agonist-modified channel properties and structure

This article reviews the different forms of ionic channels: voltage-gated, agonist-gated, and agonist- and second messenger-modified channels. The recent advances in our knowledge of the amino acid sequence of the sodium channel and the nicotinic acetylcholine receptor and the relationship of the primary structure to the channels' quarternary structure and function are discussed.

Potentiating synergism between adenosine diphosphate or triphosphate and acetylcholine on insulin secretion

The interaction between adenosine di- or triphosphate (ADP or ATP) and acetylcholine (ACh) was studied on insulin secretion. The experiments were performed on the isolated perfused rat pancreas, in the presence of a physiological nonstimulating glucose concentration (5.5 mM). ADP or ATP (1.65 microM) and ACh (0.05 microM) elicited a comparable peak of insulin secretion. When either ATP or ADP was simultaneously infused with ACh, insulin secretory response was significantly higher than the sum of the responses of each agonist applied separately. Similar effects were obtained with stable structural analogues of ATP and ADP (adenylimidodiphosphate, AMP-PNP, and alpha, beta-methyleneadenosine 5'-diphosphate, alpha, beta-methylene ADP) whether they acted alone or in combination with ACh. In contrast, adenosine was ineffective. Furthermore, the study of combined half doses of ATP (or ADP) and ACh allowed us to establish a potentiating synergism between both agonists. These data indicate a potentiating synergism on the beta-cell between ACh and ATP or ADP, the substances acting, respectively, via muscarinic cholinergic receptors and purinergic P2 receptors. So, ATP and ADP by activating P2 receptors could be involved in the parasympathetic control of insulin secretion.

ATP-induced cation influx in myotubes is additive to cholinergic agonist action

Using biochemical methods, ATP was shown to induce an inward flux of 86Rb into cultured chick myotubes. A biphasic dose-response curve was observed, the first part of which was saturable and had an EC50 value of approximately 10 microM. beta, gamma-imido ATP inhibited the ATP-induced uptake. ADP, AMP and adenosine were less potent than ATP in producing influx. ATP-induced influx of 86Rb was found to be additive to carbachol-induced influx and, in contrast to the latter, not blocked by the nicotinic acetylcholine receptor antagonist alpha-bungarotoxin. The results suggest the presence of an ATP recognition-site on myotubes, triggering ion-permeation.

Effect of adenosine triphosphate on the sensitivity of the nicotinic acetylcholine-receptor in the bullfrog sympathetic ganglion cell

The effects of adenosine triphosphate (ATP) and related compounds on the sensitivity of the nicotinic acetylcholine (ACh)-receptor of bullfrog sympathetic ganglion cells were analysed electro-physiologically. ATP in concentrations between 0.05 and 2 mM increased the amplitudes of the potentials and currents induced by ACh, and carbachol-induced currents. Compared with ATP, ADP was less potent in producing augmentation of the carbachol-induced current by one order of magnitude. AMP, cyclic AMP and adenosine had no appreciable effect. Analysis of this ATP effect, based on Michaelis-Menten type kinetics, revealed that ATP increased the maximum response (Vmax) of the dose-response curve of ACh currents without an appreciable effect on the affinity (Km) of ACh for its receptor. It is suggested that ATP increased the receptor sensitivity by acting on an allosteric site of the nicotinic ACh receptor-ionic channel complex which, thus, may be linked to an ATP receptor, probably of the P2-receptor type (Burnstock, 1981).

Actions of ATP on the soma of bullfrog primary afferent neurons and its modulating action on the GABA-induced response

Adenosine 5'-triphosphate (ATP) produced a long-lasting depolarization in bullfrog spinal ganglion cells. Since the ATP-induced slow depolarization was associated with an increase in membrane resistance and a reverse in polarity (about--90 mV) which was most likely brought about by an inactivation of membrane potassium conductance. In some cells, a rapid and transient depolarization followed by the long-lasting depolarization was produced by ATP and it was markedly reduced in sodium-free solution. ATP reversibly augmented the GABA-induced depolarization which was caused by ionophoresis of GABA. These observations were confirmed using a voltage clamp method. Dose-response analysis of the action of ATP on the GABA-induced response suggests that the facilitatory action of ATP on the GABA response is effected on the GABA receptor channel complexes without changing the GABA affinity.

Characteristics of neuronal release of ATP

Recent studies have described a transmitter-like release of ATP in brain. Once released, extraneuronal ATP is rapidly metabolized to adenosine by ecto-ATPase and nucleotidase. Adenosine, through actions at specific receptors, inhibits neuronal firing in the brain. ATP shares these inhibitory actions, presumably by forming adenosine extraneuronally. Caffeine and theophylline probably exert CNS stimulation by antagonizing adenosine's inhibitory actions in the brain. Extracellular ATP occasionally excites quiescent neurons in the cortex. A possible role for ATP as a sensory neurotransmitter is suggested by its excitatory actions on a subpopulation of dorsal horn cells. ATP release has also been described from sensory nerves in the periphery, motor nerves, nerves of the myenteric plexus, bladder, vas deferens, and from adrenal chromaffin cells and platelets. The possibility that ATP might function as a transmitter, co-transmitter or modulator in the peripheral nervous system is discussed.

Inhibition by purines of the inotropic action of isoprenaline in rat atria

The effects of a series of adenosine derivatives were examined on the catecholamine-stimulated electrically-driven rat left atrium in vitro. All the purines tested reduced the positive inotropic action of isoprenaline, 0.1 microM, with the potency order: L-N6-phenlylisopropyladenosine (L-PIA) greater than 5'-N-ethylcarboxamide adenosine (NECA) greater than D-PIA greater than 2-chloroadenosine greater than adenosine. Dipyridamole did not change the IC50 of adenosine. The adenosine deaminase inhibitor, 2'deoxycoformycin, produced a small but nonsignificant shift to the left of the adenosine concentration-response curve. The cardiac depressant effects of these purines were reversed by theophylline and the IC50 values were unchanged in the presence of atropine or in atria taken from reserpine-treated rats. It is concluded that the purine receptor mediating these effects should not be classified on the A1/A2 system. The relationship between functionally characterized purine receptors and those originally defined as modulating adenylate cyclase is discussed.

Transmitter-like action of ATP on patched membranes of cultured myoblasts and myotubes

The concept of purinergic neurotransmission, first proposed by Burnstock, has been confirmed in various cell types. We show here, by the patch-clamp method, that external ATP in micromolar concentrations (1-100 microM) activates cation channels in the membranes of fusion-competent myoblasts and myotubes. In cell-attached membrane patches of myoblasts and myotubes the mean number of simultaneously activated channels increases with time after external ATP application. In myoblasts only one population of channels having a mean single-channel conductance of gamma=43 pS was found, while in myotubes two populations with gamma 1=48 pS and gamma 2=20 pS were observed. Treatment of myotube membranes with acetylcholine (ACh) or carbachol resulted in two populations of channels which had conductance values and voltage-dependent mean channel lifetimes similar to those produced in response to ATP. The results show that embryonic skeletal muscle cells contain cation channels sensitive to ATP and provide evidence for a neurotransmitter-like action of ATP on these cells.

Purine receptors in the rat anococcygeus muscle

Adenosine triphosphate (ATP) caused contraction of the resting isolated rat anococcygeus muscle. Non-phosphorylated purines did not cause contraction of the resting muscle but did so in muscles in which the tone was raised by carbachol or guanethidine. Adenosine, (-)N6-phenylisopropyladenosine (PIA) and 5'-N-ethyl-carboxamide adenosine (NECA) were approximately equipotent, and these responses were not prevented by theophylline, quinidine, 2,2'-pyridylisatogen tosylate, phentolamine, methysergide, dipyridamole, hexobendine or indomethacin. The contractions became smaller as muscle tone progressively declined, and it is suggested that this effect may explain the apparent blockade of ATP responses by indomethacin reported previously. Adenosine, 2-chloroadenosine, ATP, PIA and NECA inhibited contractile responses of the anococcygeus to field stimulation of the excitatory adrenergic innervation. This inhibitory action was blocked by theophylline, and as PIA was easily the most potent purine tested, it may involve activation of an A1/Ri receptor. It is also argued, however, that the A/R scheme of classification may be inappropriate for the description of responses of intact tissues. As response to noradrenaline were not changed by the purines, the inhibitory effect on stimulation-evoked contractions is probably mediated at a presynaptic site. None of the purines tested had any effect on the neurally mediated inhibition of the anococcygeus which is seen when intrinsic tone is raised and the excitatory adrenergic nerves are blocked.

Cell-membrane receptors for purines. Review

Purines are involved in many aspects of cell chemistry - intermediary metabolism, nucleic acid synthesis, and the supply of high-energy phosphates to various active transport systems. In addition, however, there appear to be specific receptor molecules located within the plasma membrane of some cells, which mediate changes of cell function in response to purines present in the extracellular fluid. It is the purpose of this review to summarize the kind of functions subserved by those receptors as well as the basic structural requirements for their activation.

Increase of acetylcholine-receptor sensitivity by adenosine triphosphate: a novel action of ATP on ACh-sensitivity

1. The sensitivity of the nicotinic acetylcholine (ACh)-receptor, measured as the amplitude of ACh-current induced by iontophoretic application of ACh to the frog skeletal muscle endplate, was increased by the action of adenosine triphosphate (ATP). 2. This potentiation was not due to the effect of ATP on ACh-esterase, since the increase of the sensitivity could also be demonstrated by use of carbachol (CCh). 3. Kinetic analysis of the effect of ATP on the dose-response curve of CCh-current suggests that ATP increases the ACh-sensitivity by acting on the allosteric site of receptor-ionic channel complex without changing the affinity of ACh for its recognition site. 4. The equilibrium potential and the life-time of the endplate current (e.p.c.) are not altered by the presence of ATP. 5. These results suggest that ATP increases the ACh-sensitivity by increasing either the conductance of unit channels or the total number of available channels.