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

Purinergic signalling during development and ageing

Extracellular purines and pyrimidines play major roles during embryogenesis, organogenesis, postnatal development and ageing in vertebrates, including humans. Pluripotent stem cells can differentiate into three primary germ layers of the embryo but may also be involved in plasticity and repair of the adult brain. These cells express the molecular components necessary for purinergic signalling, and their developmental fates can be manipulated via this signalling pathway. Functional P1, P2Y and P2X receptor subtypes and ectonucleotidases are involved in the development of different organ systems, including heart, blood vessels, skeletal muscle, urinary bladder, central and peripheral neurons, retina, inner ear, gut, lung and vas deferens. The importance of purinergic signalling in the ageing process is suggested by changes in expression of A1 and A2 receptors in old rat brains and reduction of P2X receptor expression in ageing mouse brain. By contrast, in the periphery, increases in expression of P2X3 and P2X4 receptors are seen in bladder and pancreas.

Purinergic signaling in embryonic and stem cell development

Nucleotides are of crucial importance as carriers of energy in all organisms. However, the concept that in addition to their intracellular roles, nucleotides act as extracellular ligands specifically on receptors of the plasma membrane took longer to be accepted. Purinergic signaling exerted by purines and pyrimidines, principally ATP and adenosine, occurs throughout embryologic development in a wide variety of organisms, including amphibians, birds, and mammals. Cellular signaling, mediated by ATP, is present in development at very early stages, e.g., gastrulation of Xenopus and germ layer definition of chick embryo cells. Purinergic receptor expression and functions have been studied in the development of many organs, including the heart, eye, skeletal muscle and the nervous system. In vitro studies with stem cells revealed that purinergic receptors are involved in the processes of proliferation, differentiation, and phenotype determination of differentiated cells. Thus, nucleotides are able to induce various intracellular signaling pathways via crosstalk with other bioactive molecules acting on growth factor and neurotransmitter receptors. Since normal development is disturbed by dysfunction of purinergic signaling in animal models, further studies are needed to elucidate the functions of purinoceptor subtypes in developmental processes.

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.

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.

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.

Structure Activity Relationships for Derivatives of Adenosine-5'-Triphosphate as Agonists at P(2) Purinoceptors: Heterogeneity Within P(2X) and P(2Y) Subtypes

The structure-activity relationships for a variety of adenine nucleotide analogues at P(2x)- and P(2Y)-purinoceptors were investigated. Compounds formed by structural modifications of the ATP molecule including substitutions of the purine ring (C2, C8, N1, and N(6)-substituents, and a uridine base instead of adenine), the ribose moiety (2' and 3'-positions), and the triphosphate group (lower phosphates, bridging oxygen substitution, and cyclization) were prepared. Pharmacological activity at P(2Y)-purinoceptors was assayed in the guinea pig taenia coli, endothelial cells of the rabbit aorta, smooth muscle of the rabbit mesenteric artery, and turkey erythrocyte membranes. Activity at P(2X)-purinoceptors was assayed in the rabbit saphenous artery and the guinea-pig vas deferens and urinary bladder. Some of the analogues displayed selectivity, or even specificity, for either the P(2X)- or the P(2Y)-purinoceptors. Certain analogues displayed selectivity or specificity within the P(2X)- or P(2Y)-purinoceptor superfamilies, giving hints about possible subclasses. For example, 8-(6-aminohexylamino)ATP and 2',3'-isopropylidene-AMP were selective for endothelial Pzypurinoceptors over P(2Y)-purinoceptors in the guinea pig taenia coli, rabbit aorta, and turkey erythrocytes. These compounds were both inactive at P(2X)-purinoceptors. The potent agonist N(6)-methyl ATP and the somewhat less potent agonist 2'-deoxy-ATP were selective for P(2Y)-purinoceptors in the guinea pig taenia coli, but were inactive at P(2X)-purinoceptors and the vascular P(2Y)-purinoceptors. 3'-Benzylamino-3'-deoxyATP was very potent at the P(2X)-purinoceptors in the guinea pig vas deferens and bladder, but not in the rabbit saphenous artery and was inactive at P(2Y) receptors. These data suggest that specific compounds can be developed that can be utilized to activate putative subtypes of the P(2X)- and P(2Y)-purinoceptor classes.

Extracellular ATP triggers two functionally distinct calcium signalling pathways in PC12 cells

We have investigated the effects of extracellular ATP on Ca2+ signalling, and its relationship to secretion in rat pheochromocytoma (PC12) cells. In single cells, extracellular ATP evoked two very distinct subcellular distributions of intracellular calcium concentration ([Ca2+]i), only one of which could be mimicked by the pyrimidine nucleotide UTP, suggesting the involvement of more than one cell surface receptor in mediating the ATP-induced responses. ATP and UTP were equipotent in activating a receptor leading to inositol phosphate production and the mobilisation of intracellular Ca2+. In some cells (19%) this rise in [Ca2+]i initiated at a discrete site and then propagated across the cell in the form of a Ca2+ wave. In addition to mobilising intracellular Ca2+ through a ‘nucleotide’ receptor sensitive to ATP and UTP, the results indicate that ATP also activates divalent cation entry through an independent receptor-operated channel. Firstly, ATP-induced entry of Ca2+ or Mn2+ was independent of Ca2+ mobilisation, as prior treatment of cell populations with UTP abolished the ATP-evoked release of intracellular Ca2+ stores, but left the Ca(2+)- and Mn(2+)-entry components uneffected. Secondly, although UTP and ATP were equally effective in generating inositol phosphates, only ATP stimulated divalent cation entry, indicating that ATP-activated influx was independent of phosphoinositide turnover. Thirdly, single cell experiments revealed a subpopulation of cells that responded to ATP with divalent cation entry without mobilising Ca2+ from intracellular stores. Lastly, the dihydropyridine antagonist, nifedipine, reduced the ATP-induced rise in [Ca2+]i by only 24%, suggesting that Ca2+ entry was largely independent of L-type voltage-operated Ca2+ channels. The Ca2+ signals could also be distinguished at a functional level. Activation of ATP-induced divalent cation influx was absolutely required to evoke transmitter release, because ATP triggered secretion of [3H]dopamine only in the presence of external Ca2+, and UTP was unable to promote secretion, irrespective of the extracellular [Ca2+]. The results suggest that the same extracellular stimulus can deliver different Ca2+ signals into the same cell by activating different Ca2+ signalling pathways, and that these Ca2+ signals can be functionally distinct.

ATP-induced cytoplasmic [Ca2+] increases in isolated cochlear outer hair cells. Involved receptor and channel mechanisms

Outer hair cells (OHC) of the mammalian cochlea are thought to preprocess the sound signal by active movements, which can be induced by electrical or chemical stimulation, e.g. depolarization evoked by high [K+] or increased cytoplasmic [Ca2+]. Extracellular ATP has been found to induce cytoplasmic [Ca2+] increases in OHC but involved mechanisms have not been elucidated. Cytoplasmic [Ca2+] was measured in non-enzymatically isolated single OHC using Fura-2 microspectrometry. Results, using ATP/derivatives and other P2-purinergic receptor (P2R) ligands, as well as Ca(2+)-channel blockers and pertussis toxin, revealed several signal transduction pathways that increase cytoplasmic [Ca2+] in OHC: a P2-purinergic receptor (P2R)--G-protein--effector (phospholipase C or an ion channel) system and a voltage-dependent Ca2+ channel. Agonist potency studies denote a pattern analogous to that found in skeletal muscle, i.e. ATP-alpha-S > ATP = 2-methyl-S-ATP >> ADP > alpha,beta-methylene-ATP, but no activation by ADP beta F or UTP, leaving a choice of P2y or P2zR subtypes. The latter possibility gained strength from calculations showing that up to 8% of ATP may have formed the P2zR agonist ATP4- in the experimental medium. Experiments in Ca(2+)-free medium and with pertussis toxin revealed that the main Ca2+ source was intracellular. Pertussis toxin did not affect [Ca2+] increase induced by carbachol. Acetylcholine, administered a few seconds before ATP, did not affect total cytoplasmic [Ca2+] increases. Induced cytoplasmic [Ca2+] increases were high enough (> 500 nM at 50 microM ATP/derivatives) to hyperpolarize the OHC membrane by opening K(+)-channels and decreased little with time.(ABSTRACT TRUNCATED AT 250 WORDS)

Potentiation of miniature endplate potential frequency by ATP in Xenopus tadpoles

1. Extracellular application of ATP (1 mM), a substance co-stored and co-released with acetylcholine in peripheral nervous systems, potentiated the spontaneous secretion of acetylcholine (ACh) but had no effect on the amplitude and decay time constant of miniature endplate potentials (m.e.p.ps) at neuromuscular synapses in Xenopus tadpoles. 2. alpha,beta-Methylene ATP (0.3 mM) and GTP (1 mM) were also effective in increasing m.e.p.p. frequency. On the other hand, ADP, AMP and adenosine (all at 1 mM) decreased m.e.p.p. frequency. 3. Unlike the transient effect of ATP analogue and GTP on m.e.p.p. frequency, the phorbol ester TPA (2 microM) which is a protein kinase C activator, increased m.e.p.p. frequency consistently and the effects lasted as long as the presence of TPA. 4. Staurosporine (0.5 microM) and H-7 (10 microM), which are protein kinase C inhibitors, each decreased the basal level of m.e.p.p. frequency and markedly inhibited the effects of both ATP and TPA. 5. These results suggest that there is a basal activity of cytosolic protein kinases in the nerve terminals of Xenopus tadpoles and the effect of ATP is probably mediated by the binding of membrane surface purinoceptors which in turn activates cytosolic protein kinases and increases ACh release.

SYNTHESIS AND BIOLOGICAL ACTIVITY OF NOVEL 2-THIO DERIVATIVES OF ATP

2-Alkylthio analogues of adenosine 5'-triphosphate were synthesized and evaluated as P2y purinoceptor agonists. ATP and analogues transiently increased intracellular Ca2+ levels in C6 glioma cells and in skeletal muscle derived myotubes in culture. Most derivatives were resistant to stepwise dephosphorylation by ecto-ATPases.

Stimulation of the P2Y purinergic receptor on type 1 astroglia results in inositol phosphate formation and calcium mobilization

Cultured astroglia express purinergic receptors that initiate phosphoinositide metabolism and calcium mobilization. Experiments were conducted to characterize the purinergic receptor subtype on type 1 astroglia responsible for stimulation these second-messenger systems. Inositol phosphate (IP) accumulation and calcium mobilization were measured after stimulation with ATP or purinergic receptor subtype-selective ATP analogues. ATP (10(-5) M) increased IP accumulation severalfold. Dose-effect assays monitoring astroglial IP accumulation revealed the order of potency that defines the P2Y receptor: 2-methylthioadenosine 5'-triphosphate greater than ATP greater than alpha beta-methyleneadenosine 5'-triphosphate greater than beta gamma-methyleneadenosine 5'-triphosphate. The influence of ATP on intracellular calcium levels in individual type 1 astroglia was examined using the calcium indicator dye, fura-2. Dose-effect experiments indicated that ATP was equally potent for generating inositol phosphates and increasing cellular calcium. The most prevalent response (87% of total responses) to ATP consisted of a rapid increase in calcium to a peak level that was approximately five times greater than the prestimulation level. This peak was followed by a decline to a plateau level that was significantly above baseline. This plateau phase of the calcium increase was maintained for at least 5 min in the presence of ATP and was dependent on external calcium. Many (23%) astroglia exhibited spontaneous calcium oscillations whose frequency and magnitude increased after the addition of 10(-5) M ATP. Immunocytochemical staining indicated that the responses occurred in glial fibrillary acidic protein positive cells. We conclude that type 1 astroglia express the P2Y purinergic receptor which regulates IP production and calcium mobilization.

Basal and ATP-stimulated phosphoinositol metabolism in fusing rat skeletal muscle cells in culture

A considerable rise in inositol phosphates was observed at the beginning of myoblast fusion. Extracellular ATP, through P2-purinergic receptors, induced inositol phosphate accumulation before and after fusion; however, no effect of ATP on phosphoinositol levels could be detected during the period of fusion. The possibility of ATP being a fusion signal is discussed.

A receptor that is highly specific for extracellular ATP in developing chick skeletal muscle in vitro

1. Extracellular adenosine 5'-triphosphate (ATP) activated an early excitatory conductance followed by a late potassium conductance in developing chick skeletal muscle. A series of ATP analogues were tested for their ability to activate these two conductances. All compounds tested were either agonists for both responses or for neither. Furthermore, the potency of agonists was similar for the two responses. 2. The order of potency for agonists was ATP approximately adenosine 5'-O-(3-thiotriphosphate) (ATP-gamma-S) approximately 2-methylthio-ATP (2-CH3S-ATP) greater than 2'-deoxy-ATP approximately 3'-deoxy-ATP greater than adenosine 5'-tetraphosphate (ATP-OPO3) approximately adenosine 5'-diphosphate (ADP). Many other ATP analogues were not agonists. 3. Activation of the excitatory response did not require divalent cations. Furthermore, the concentration-response relation of the excitatory response was similar when ATP was applied as the free anion of ATP (ATP4-) or complexed with a divalent cation (M.ATP2-). 4. Three antagonists of the ATP response were characterized. 8-Br-ATP was a weak antagonist, while 2',3'-dialdehyde-ATP and DIDS (4,4'-diisocyanatostilbene-2,2'-disulphonic acid) were potent irreversible inhibitors. The two conductances were equally affected by these antagonists. 5. These results suggest that both ATP responses are activated through the same receptor type, or two very similar receptors.

Neural factors regulate AChR subunit mRNAs at rat neuromuscular synapses

To elucidate the nature of signals that control the level and spatial distribution of mRNAs encoding acetylcholine receptor (AChR), alpha-, beta-, gamma-, delta- and epsilon-subunits in muscle fibers chronic paralysis was induced in rat leg muscles either by surgical denervation or by different neurotoxins that cause disuse of the muscle or selectively block neuromuscular transmission pre- or postsynaptically and cause an increase of AChRs in muscle membrane. After paralysis, the levels and the spatial distributions of the different subunit-specific mRNAs change discoordinately and seem to follow one of three different patterns depending on the subunit mRNA examined. The level of epsilon-subunit mRNA and its accumulation at the end-plate are largely independent on the presence of the nerve or electrical muscle activity. In contrast, the gamma-subunit mRNA level is tightly coupled to innervation. It is undetectable or low in innervated normally active muscle and in innervated but disused muscle, whereas it is abundant along the whole fiber length in denervated muscle or in muscle in which the neuromuscular contact is intact but the release of transmitter is blocked. The alpha-, beta-, and delta-subunit mRNA levels show a different pattern. Highest amounts are always found at end-plate nuclei irrespective of whether the muscle is innervated, denervated, active, or inactive, whereas in extrasynaptic regions they are tightly controlled by innervation partially through electrical muscle activity. The changes in the levels and distribution of gamma- and epsilon-subunit-specific mRNAs in toxin-paralyzed muscle correlate well with the spatial appearance of functional fetal and adult AChR channel subtypes along the muscle fiber. The results suggest that the focal accumulation at the synaptic region of mRNAs encoding the alpha-, beta-, delta-, and epsilon-subunits, which constitute the adult type end-plate channel, is largely determined by at least two different neural factors that act on AChR subunit gene expression of subsynaptic nuclei.

A nicotinic acetylcholine receptor-like alpha-bungarotoxin-binding site on outer hair cells

Acetylcholine (ACh) appears to be the major neurotransmitter liberated from olivocochlear efferents terminating on outer hair cells (OHC). Recently, cholinergic receptor epitopes were visualized at the basal pole of the OHCs. To evaluate the ACh receptor type at OHC we performed binding studies with [125I]-labelled alpha-bungarotoxin (alpha-bgtx), a close to irreversibly acting blocker of the nicotinic acetylcholine receptor (nAChR) of skeletal muscle and of electrocytes of Torpedo and Electrophorus. An irreversible and saturable binding (80 nM) of the radiolabelled compound to OHCs was observed. The number of alpha-bgtx sensitive binding sites present on each OHC was calculated to be about 2 X 10(-17) mol/OHC, which would amount to about 10(7) binding sites/cell. Preincubation with the reversibly acting cholinergic ligands, carbamylcholine (1 mM), nicotine (0.1 mM) and d-tubocurarine (1-100 microM) was found to inhibit alpha-bgtx binding to a varying degree. Atropine (0.05 mM), a muscarinic antagonist, had no influence on the binding of alpha-bgtx to OHCs. [3H]-QNB, a specific marker and antagonist for muscarinic AChR, and [125I]-kappa-toxin, known to react with neuronal and ganglionic nAChR, showed no specific binding to OHCs. The data indicate that a peripheral type nAChR is present on OHCs mediating ACh-induced modulation of the biomechanics of the cochlea by influencing OHC motility.

ATP potentiates spontaneous transmitter release at developing neuromuscular synapses

Extracellular application of ATP, a substance co-stored and co-released with acetylcholine in peripheral nervous systems, potentiates the spontaneous secretion of acetylcholine at developing neuromuscular synapses in Xenopus cell culture, as shown by a marked increase in the frequency of spontaneous synaptic currents recorded in the postsynaptic muscle cell. The effect of ATP is apparently mediated by the activation of cytosolic protein kinases and requires the influx of Ca2+ through the plasma membrane. Since spontaneous acetylcholine release is known to regulate the development of contractile properties of the postsynaptic muscle cell, extracellular ATP may serve as a positive trophic factor at developing neuromuscular synapses.

Receptor-triggered polyphosphoinositide turnover produces less cytosolic free calcium in cultured dysgenic myotubes than in normal myotubes

Myotubes prepared from mice with muscular dysgenesis (mdg) were used to further elucidate the putative role of inositol triphosphate (InsP3) in excitation-contraction (E-C) coupling of skeletal muscle. The mdg mutation is characterized by an uncoupling of the E-C coupling. InsP3 production in normal and mdg/mdg myotube cultures and its relation to the levels of cytosolic free calcium were analyzed. Basal and ATP-stimulated levels of InsP3 were equal in normal and mdg/mdg myotube cultures. In contrast, the transient increases of cytosolic free calcium in mdg/mdg myotubes in culture were generally much lower than those in normal ones. This suggests that the defect in dysgenic myotubes does not rest on the InsP3 formation but on the InsP3-triggered transduction of excitation and/or the induction of calcium release from internal stores.

Postjunctional synergism of noradrenaline and adenosine 5'-triphosphate in the mesenteric arterial bed of the rat

The ability of purines to modify responses to exogenous noradrenaline (NA) was investigated using the isolated perfused rat mesenteric bed. ATP, at subthreshold doses and above-threshold doses, produced a potentiation of vasoconstrictor responses to NA while adenosine was without effect. The stable analogue of ATP, alpha,beta-methylene ATP alpha,beta-meATP, at subthreshold and above-threshold doses also enhanced pressor responses to NA (to a greater extent than ATP). This potentiation caused a shift to the left of the dose-response curve, with no increase in the maximum response. Pressor responses to 5-hydroxytryptamine (5-HT) and to potassium chloride (KCl), however, were not affected by alpha,beta-meATP. Conversely, suprathreshold doses of NA potentiated contractions evoked by alpha,beta-meATP, but no potentiation was observed using subthreshold doses of NA. These results demonstrate a postjunctional synergistic action between NA and ATP which appeared to be specific for the alpha 1-adrenoceptors and the P2x-purinergic receptors since: (i) the potentiation of the contractile response to NA by ATP was mimicked by alpha,beta-meATP but not by adenosine and (ii) pressor responses to 5-HT or to KCl were not affected by alpha,beta-meATP. Possible mechanisms for this postjunctional synergism are discussed.

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.

Multiple actions of adenosine 5'-triphosphate on chick skeletal muscle

1. Extracellularly applied adenosine 5'-triphosphate (ATP) is known to have an excitatory action on chick skeletal muscle. By making intracellular recordings from cultured chick myotubes bathed with blockers of several types of voltage-dependent channels, the direct action of ATP could be observed. 2. When muscle cells were studied near their resting potential, ATP usually produced a biphasic response. There was a rapid initial depolarization, followed by a slower repolarization. The repolarization could drive cells negative to their initial resting potential, indicating that it was not due simply to desensitization of the process that produced the depolarization. Thus there are at least two distinct responses to ATP. 3. At room temperature the early response to ATP activated within 20 ms, and the second response activated with a latency of approximately 1 s. In our standard blocking solution, the average reversal potential of the early response was -17 mV, while the late response had a reversal potential that was negative to -70 mV. In a few cells the second response appeared to be absent. 4. The amplitude and time course of the late response were substantially decreased by low temperature (12 degrees C) and increased by high temperature (37 degrees C). In contrast, temperature had much smaller effects on the early response. Both the time course and temperature dependence of the late response suggest that an intracellular second messenger system may be involved in its activation. 5. Ion-substitution experiments were performed to determine the type of conductance changes that evoke each response. These indicated that the early response was due to an increased membrane permeability to sodium, potassium and chloride, but not to large cations or anions, and that the late response was due to an increased permeability to potassium. 6. Measurement of the responses of muscle cells to acetylcholine supported the conclusion that both anions and cations are permeable during the early ATP response. Under conditions in which there was a large negative reversal potential for all cations, and a large positive reversal potential for all anions, the early ATP response reversed approximately 50 mV positive to the acetylcholine response. 8. The possibility that the early ATP response is due to a channel selective for size, but not charge, is discussed.

P2-purinoceptor-stimulated phosphoinositide turnover in chick myotubes. Calcium mobilization and the role of guanyl nucleotide-binding proteins

ATP, a trigger of P2-purinoceptor-mediated polyphosphoinositide (PI) turnover in cultured myotubes, increased cytosolic calcium levels in a time- and dose-dependent manner (quin2 fluorescence). The calcium was released from intracellular stores, as acute addition of 5 mM EGTA was without significant effect. Adenosine 5'-(3-thiotriphosphate) and 5'-adenylyl imidodiphosphate also increased intracellular levels of inositol phosphates (InsP) and cytosolic calcium levels. Treatment with cholera or pertussis toxin of myotube cultures did not affect the P2-purinoceptor-mediated InsP increase although PI turnover in permeabilized myotubes was stimulated by guanosine 5'-(3-thiotriphosphate). The results suggest that myotube P2-purinoceptors trigger PI turnover and increase intracellular free calcium levels, via a mechanism insensitive to ADP-ribosylation, by cholera or pertussis toxin of guanyl nucleotide-binding (G) proteins. However, the presence of a phospholipase C-coupled G-protein was otherwise demonstrated.

Monospecific antiserum against 5'-nucleotidase from Torpedo electric organ: immunocytochemical distribution of the enzyme and its association with Schwann cell membranes

The cellular and subcellular distribution of 5'-nucleotidase in tissues of the electric ray Torpedo marmorata has been investigated by means of an antiserum raised against the native enzyme purified from the electric organ. As revealed by immunohistochemistry the enzyme is associated with the surface of the axons of the electric nerves and of spinal nerves. Using the post-embedding colloidal gold technique at the electron-microscopical level 5'-nucleotidase could be located at the plasma membrane of the Schwann cells including the myelin and the fine processes covering the terminal axon ramifications. Also the perineurial sheath of the axons inside the electric organ is 5'-nucleotidase positive. The plasma membrane of the axon and the terminal axon region or the postsynaptic membrane do not contain 5'-nucleotidase. Immunoprecipitation studies using polyacrylamide beads suggest that the ecto-Ca2+- or -Mg2+-adenosine 5'-triphosphatase previously ascribed to synaptosomes of the Torpedo electric organ is not associated with the same membranes as 5'-nucleotidase. Within the electric organ the dorsal plasma membrane of the electroplaque cell, blood capillaries and the connective tissue layer surrounding the columns of electroplaque cells also bind the antibodies. In central nervous tissue solely blood vessels show immunofluorescence. Within the electric lobe both the surface of the electromotor neurons as well as the myelinated axons giving rise to the electric nerve are negative. This also applies to the axons of the optic nerve suggesting that the antiserum is Schwann cell specific, and does not bind to a potential oligodendroglial 5'-nucleotidase. In peripheral tissue the surface of skeletal muscle fibres as well as that of individual myofibrils bind the anti-5'-nucleotidase antibodies. Our results demonstrate that the Schwann cell plasma membrane, including myelin, contains 5'-nucleotidase and that one can distinguish by means of a specific antiserum between Schwann cell and oligodendroglia plasma membranes. The functional significance of the association of 5'-nucleotidase with Schwann cells along the entire surface of axons including the synaptic region as well as with other parts of the electric tissue is discussed regarding its catalytic activity and also the possibility that this surface glycoprotein may be involved in mediating cellular interactions.

Two ATP-activated conductances in bullfrog atrial cells

Currents activated by extracellular ATP were studied in single voltage-clamped bullfrog atrial cells. Rapid application of ATP elicited currents carried through two different conductance pathways: a rapidly desensitizing conductance reversing near -10 mV, and a maintained, inwardly rectifying conductance reversing near -85 mV. ATP activated the desensitizing component of current with a K 1/2 of approximately 50 microM and the maintained component with a K 1/2 of approximately 10 microM. Both types of current were activated by ATP but not by adenosine, AMP, or ADP. The desensitizing current was selectively inhibited by alpha, beta-methylene ATP, and the maintained, inwardly rectifying current was selectively suppressed by extracellular Cs. The desensitizing component of current was greatly reduced when extracellular Na was replaced by N-methylglucamine, but was slightly augmented when Na was replaced by Cs. GTP, ITP, and UTP were all ineffective in activating the desensitizing current, and of a variety of ATP analogues, only ATP-gamma-S was effective. Addition of EGTA or BAPTA to the intracellular solution did not obviously affect the desensitizing current. Fluctuation analysis of currents through the desensitizing conductance suggested that current is carried through ionic channels with a small (less than pS) unitary conductance.

[Cholinergic nerve endings: cellular function and molecular structure]

Progress made in the study of the cellular and molecular function of cholinergic nerve terminals suggests that the textbook schemes need to be extended. Vesicles contained in cholinergic nerve terminals not only store acetylcholine but also ATP and Ca2+ as well as peptides. These substances when co-released by exocytosis may modulate transmission both pre- and postsynaptically.