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Amino-acyl tRNA synthetases generate dinucleotide polyphosphates as second messengers: functional implications. Top Curr Chem (Cham) 2013; 344:189-206. [PMID: 23536246 DOI: 10.1007/128_2013_426] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022]
Abstract
In this chapter we describe aminoacyl-tRNA synthetase (aaRS) production of dinucleotide polyphosphate in response to stimuli, their interaction with various signaling pathways, and the role of diadenosine tetraphosphate and diadenosine triphosphate as second messengers. The primary role of aaRS is to mediate aminoacylation of cognate tRNAs, thereby providing a central role for the decoding of genetic code during protein translation. However, recent studies suggest that during evolution, "moonlighting" or non-canonical roles were acquired through incorporation of additional domains, leading to regulation by aaRSs of a spectrum of important biological processes, including cell cycle control, tissue differentiation, cellular chemotaxis, and inflammation. In addition to aminoacylation of tRNA, most aaRSs can also produce dinucleotide polyphosphates in a variety of physiological conditions. The dinucleotide polyphosphates produced by aaRS are biologically active both extra- and intra-cellularly, and seem to function as important signaling molecules. Recent findings established the role of dinucleotide polyphosphates as second messengers.
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Kummer U, Krajnc B, Pahle J, Green AK, Dixon CJ, Marhl M. Transition from stochastic to deterministic behavior in calcium oscillations. Biophys J 2005; 89:1603-11. [PMID: 15994893 PMCID: PMC1366664 DOI: 10.1529/biophysj.104.057216] [Citation(s) in RCA: 73] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Simulation and modeling is becoming more and more important when studying complex biochemical systems. Most often, ordinary differential equations are employed for this purpose. However, these are only applicable when the numbers of participating molecules in the biochemical systems are large enough to be treated as concentrations. For smaller systems, stochastic simulations on discrete particle basis are more accurate. Unfortunately, there are no general rules for determining which method should be employed for exactly which problem to get the most realistic result. Therefore, we study the transition from stochastic to deterministic behavior in a widely studied system, namely the signal transduction via calcium, especially calcium oscillations. We observe that the transition occurs within a range of particle numbers, which roughly corresponds to the number of receptors and channels in the cell, and depends heavily on the attractive properties of the phase space of the respective systems dynamics. We conclude that the attractive properties of a system, expressed, e.g., by the divergence of the system, are a good measure for determining which simulation algorithm is appropriate in terms of speed and realism.
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Affiliation(s)
- Ursula Kummer
- Bioinformatics and Computational Biochemistry Group, EML Research, D-69118 Heidelberg, Germany.
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Larsen AZ, Olsen LF, Kummer U. On the encoding and decoding of calcium signals in hepatocytes. Biophys Chem 2004; 107:83-99. [PMID: 14871603 DOI: 10.1016/j.bpc.2003.08.010] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/01/2003] [Revised: 08/25/2003] [Accepted: 08/25/2003] [Indexed: 11/24/2022]
Abstract
Many different agonists use calcium as a second messenger. Despite intensive research in intracellular calcium signalling it is an unsolved riddle how the different types of information represented by the different agonists, is encoded using the universal carrier calcium. It is also still not clear how the information encoded is decoded again into the intracellular specific information at the site of enzymes and genes. After the discovery of calcium oscillations, one likely mechanism is that information is encoded in the frequency, amplitude and waveform of the oscillations. This hypothesis has received some experimental support. However, the mechanism of decoding of oscillatory signals is still not known. Here, we study a mechanistic model of calcium oscillations, which is able to reproduce both spiking and bursting calcium oscillations. We use the model to study the decoding of calcium signals on the basis of co-operativity of calcium binding to various proteins. We show that this co-operativity offers a simple way to decode different calcium dynamics into different enzyme activities.
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Affiliation(s)
- Ann Zahle Larsen
- Celcom, Department of Biochemistry and Molecular Biology, Syddansk Universitet, Campusvej 55, Odense M DK-5230, Denmark.
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Baier G, Müller M, Ørsnes H. Excitable Spatio-Temporal Chaos in a Model of Glycolysis. J Phys Chem B 2002. [DOI: 10.1021/jp0138173] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Gerold Baier
- Facultad de Ciencias, Universidad Autónoma del Estado de Morelos, 62210 Cuernavaca, Morelos, México, and Physical Biochemistry Group, Institute of Biochemistry, South Danish University, DK-5230 Odense M, Denmark
| | - Markus Müller
- Facultad de Ciencias, Universidad Autónoma del Estado de Morelos, 62210 Cuernavaca, Morelos, México, and Physical Biochemistry Group, Institute of Biochemistry, South Danish University, DK-5230 Odense M, Denmark
| | - Henrik Ørsnes
- Facultad de Ciencias, Universidad Autónoma del Estado de Morelos, 62210 Cuernavaca, Morelos, México, and Physical Biochemistry Group, Institute of Biochemistry, South Danish University, DK-5230 Odense M, Denmark
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Kummer U, Olsen LF, Dixon CJ, Green AK, Bornberg-Bauer E, Baier G. Switching from simple to complex oscillations in calcium signaling. Biophys J 2000; 79:1188-95. [PMID: 10968983 PMCID: PMC1301015 DOI: 10.1016/s0006-3495(00)76373-9] [Citation(s) in RCA: 116] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022] Open
Abstract
We present a new model for calcium oscillations based on experiments in hepatocytes. The model considers feedback inhibition on the initial agonist receptor complex by calcium and activated phospholipase C, as well as receptor type-dependent self-enhanced behavior of the activated G(alpha) subunit. It is able to show simple periodic oscillations and periodic bursting, and it is the first model to display chaotic bursting in response to agonist stimulations. Moreover, our model offers a possible explanation for the differences in dynamic behavior observed in response to different agonists in hepatocytes.
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Affiliation(s)
- U Kummer
- European Media Laboratory, 69118 Heidelberg, Germany.
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Holden CP, Haughey NJ, Dolhun B, Shepel PN, Nath A, Geiger JD. Diadenosine pentaphosphate increases levels of intracellular calcium in astrocytes by a mechanism involving release from caffeine/ryanodine- and IP3-sensitive stores. J Neurosci Res 2000. [DOI: 10.1002/(sici)1097-4547(20000115)59:2<276::aid-jnr14>3.0.co;2-v] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
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Edgecombe M, McLennan AG, Fisher MJ. Diadenosine polyphosphates and the control of cyclic AMP concentrations in isolated rat liver cells. FEBS Lett 1999; 457:455-8. [PMID: 10471828 DOI: 10.1016/s0014-5793(99)01099-6] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Abstract
Extracellular diadenosine polyphosphates (Ap(n)A), through their interactions with appropriate P(2) receptors, influence a diverse range of intracellular activities. In particular, Ap(4)A stimulates alterations in intracellular calcium homeostasis and subsequent activation of glycogen breakdown in isolated liver cells. Here we show that, like ATP, Ap(4)A and other naturally occurring diadenosine polyphosphates attenuate glucagon-stimulated accumulation of cyclic AMP in isolated rat liver cells. The characteristics of Ap(4)A- and ATP-dependent modulation of glucagon-stimulated cyclic AMP accumulation are similar. These results are discussed in the context of the repertoire of intracellular signalling processes modulated by extracellular nucleotides.
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Affiliation(s)
- M Edgecombe
- School of Biological Sciences, University of Liverpool, Liverpool, UK
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Ross FM, Brodie MJ, Stone TW. The effects of adenine dinucleotides on epileptiform activity in the CA3 region of rat hippocampal slices. Neuroscience 1998; 85:217-28. [PMID: 9607713 DOI: 10.1016/s0306-4522(97)00619-2] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
Alpha, omega-adenine dinucleotides (Ap(n)A) consist of two adenosine molecules linked at the 5' position by phosphate groups, the number of which is denoted by n and can range from 2 to 6. The aim of this study was to investigate the effect of Ap4A and Ap5A on the rate of epileptiform activity. Hippocampal slices (450 microm), when perfused with a medium containing no added magnesium and 4-aminopyridine (50 microM), generate epileptiform activity of an interictal nature. Ap4A and Ap5A at 1 microM depressed the discharge rate to a significant extent. At this concentration adenosine (1 microM) did not produce any effect. However at 10 microM adenosine, Ap4A and Ap5A all decreased the burst frequency. Adenosine deaminase (0.2 U/ml) totally annulled the inhibition of epileptiform activity produced by 10 microM adenosine or 1 microM Ap4A and Ap5A. Adenosine deaminase did not significantly change the maximum depression of activity produced by 10 microM Ap4A and Ap5A. 8-cyclopentyl-1,3-dimethylxanthine, an A1, receptor antagonist, increased the basal rate of epileptiform activity and prevented the depression of burst discharges by Ap4A. 5'-adenylic acid deaminase converts AMP into IMP which is inactive. 5'-adenylic acid deaminase did not prevent the inhibitory effects of Ap4A. The results suggests that in the CA3 region of the hippocampus, Ap4A and Ap5A act partly by stimulating xanthine-sensitive receptors directly and partly through the formation of the metabolite, adenosine.
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Affiliation(s)
- F M Ross
- Institute of Biomedical and Life Sciences, Division of Neuroscience and Biomedical Systems, University of Glasgow, UK
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Ross FM, Brodie MJ, Stone TW. Adenosine monophosphate as a mediator of ATP effects at P1 purinoceptors. Br J Pharmacol 1998; 124:818-24. [PMID: 9690876 PMCID: PMC1565445 DOI: 10.1038/sj.bjp.0701890] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/08/2023] Open
Abstract
1. When perfused with a medium containing no added magnesium and 4-aminopyridine (4AP) (50 microM) hippocampal slices generated epileptiform bursts of an interictal nature. We have shown in a previous study that adenosine 5'-triphosphate (ATP) depressed epileptiform activity and that this effect was blocked by the adenosine A1 receptor antagonist cyclopentyltheophylline but was not affected by adenosine deaminase. This implied that ATP might act indirectly at P1 receptors or at a xanthine-sensitive P2 receptor. The aim of the present study was to investigate further the action of ATP on epileptiform activity. 2. ATP can be metabolized by ecto-nucleotidases to adenosine 5'-diphosphate (ADP), adenosine 5'-monophosphate (AMP) and adenosine, respectively. Each of these metabolites can activate receptors in its own right: P2 receptors for ADP and P1 receptors for AMP and adenosine. 3. We now show that both AMP and ATP (50 microM) significantly decrease epileptiform discharge rate in a rapid and reversible manner. 5'Adenylic acid deaminase (AMP deaminase, AMPase) (0.2 u ml(-1)), when perfused alone did not significantly alter the discharge rate over the 10 min superfusion period used for drug application. When perfused concurrently with AMP (50 microM), AMP deaminase prevented the depressant effect of AMP on discharge rate. 4. AMP deaminase, at a concentration of 0.2 u ml(-1) which annulled the effect of AMP (50 microM), prevented the inhibitory activity of ATP (50 microM). A higher concentration of ATP (200 microM) depressed the frequency of spontaneous bursts to approximately 30% control and this response was also prevented by AMP deaminase. 5. Superfusion of the slices with 5'-nucleotidase also prevented the inhibitory activity of ATP on epileptiform discharges. 6. The results suggest that AMP mediates the inhibitory effects of ATP on epileptiform activity, a conclusion which can explain the earlier finding that cyclopentyltheophylline but not adenosine deaminase inhibited the effect of ATP. A corollary to this is that, when examining the pharmacology of ATP, care must be taken to inactivate AMP with AMP deaminase, as well as adenosine with adenosine deaminase, before a direct action of ATP on P1 receptors can be postulated. Failure to do so may have led to erroneous conclusions in some previous studies of nucleotide activity on nucleotide receptors.
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Affiliation(s)
- F M Ross
- Institute of Biomedical and Life Sciences, Division of Neuroscience and Biomedical Systems, Glasgow, Scotland
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Kisselev LL, Justesen J, Wolfson AD, Frolova LY. Diadenosine oligophosphates (Ap(n)A), a novel class of signalling molecules? FEBS Lett 1998; 427:157-63. [PMID: 9607303 DOI: 10.1016/s0014-5793(98)00420-7] [Citation(s) in RCA: 153] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
The diadenosine oligophosphates (Ap(n)A) were discovered in the mid-sixties in the course of studies on aminoacyl-tRNA synthetases (aaRS). Now, more than 30 years later, about 300 papers have been published around these substances in attempt to decipher their role in cells. Recently, Ap(n)A have emerged as intracellular and extracellular signalling molecules implicated in the maintenance and regulation of vital cellular functions and become considered as second messengers. Great variety of physiological and pathological effects in mammalian cells was found to be associated with alterations of Ap(n)A levels (n from 2 to 6) and Ap3A/Ap4A ratio. Cell differentiation and apoptosis have substantial and opposite effects on Ap3A/Ap4A ratio in cultured cells. A human Ap3A hydrolase, Fhit, appeared to be involved in protection of cells against tumourigenesis. Ap3A is synthesised by mammalian u synthetase (TrpRS) which in contrast to most other aaRS is unable to synthesise Ap4A and is an interferon-inducible protein. Moreover, Ap3A appeared to be a preferred substrate for 2-5A synthetase, also interferon-inducible, priming the synthesis of 2' adenylated derivatives of Ap3A, which in turn may serve as substrates of Fhit. Tumour suppressor activity of Fhit is assumed to be associated with involvement of the Fhit.Ap3A complex in cytokine signalling pathway(s) controlling cell proliferation. The Ap(n)A family is potentially a novel class of signal-transducing molecules whose functions are yet to be determined.
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Gasmi L, McLennan AG, Edwards SW. Diadenosine polyphosphates induce intracellular Ca2+ mobilization in human neutrophils via a pertussis toxin sensitive G-protein. Immunol Suppl 1997; 90:154-9. [PMID: 9038726 PMCID: PMC1456712 DOI: 10.1046/j.1365-2567.1997.00123.x] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Abstract
The diadenosine polyphosphates diadenosine 5',5"'-P1,P3-triphosphate (Ap3A), diadenosine 5',5"'-P1,P4-tetraphosphate (Ap4A), diadenosine 5',5"'-P1,P5-pentaphosphate (Ap5A) and diadenosine 5',5"'-P1,P6-hexaphosphate (Ap6A) all stimulated increases in intracellular Ca2+ in human neutrophils. Maximal increases in intracellular Ca2+ of 650 nM were obtained at dinucleotide concentrations of 500-700 microM. These increases in intracellular, Ca2+ were completely abolished by pre-treatment of the neutrophils with pertussis toxin and were hardly affected when the extracellular buffer was devoid of Ca2+. On the other hand, adenosine triphosphate (ATP) could stimulate much greater increases in intracellular Ca2+ (up to 1.1 microM) at much lower concentrations (half maximal responses obtained at around 5 microM ATP). Receptor de-sensitization experiments indicate that human neutrophils may possess two types of P2-purinoceptors. The first of these may bind ATP (but not the dinucleotides) with high affinity whilst the second may bind the dinucleotides with lower affinity and also bind ATP.
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Affiliation(s)
- L Gasmi
- Department of Biochemistry, University of Liverpool, UK
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Pintor J, King BF, Miras-Portugal MT, Burnstock G. Selectivity and activity of adenine dinucleotides at recombinant P2X2 and P2Y1 purinoceptors. Br J Pharmacol 1996; 119:1006-12. [PMID: 8922753 PMCID: PMC1915941 DOI: 10.1111/j.1476-5381.1996.tb15771.x] [Citation(s) in RCA: 61] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023] Open
Abstract
1. Adenine dinucleotides (Ap3A, x = 2-6) are naturally-occurring polyphosphated nucleotidic substances which are found in the CNS and are known to be released in a calcium-dependent manner from storage vesicles in brain synaptosomes. The selectivity and activity of adenine dinucleotides for neuronally-derived recombinant P2 purinoceptors were studied using P2X2 and P2Y1 subtypes expressed in Xenopus oocytes. 2. For the P2Y1 subtype derived from chick brain, Ap3A was equipotent and as active as ATP (EC50 values: 375 +/- 86 nM and 334 +/- 25 nM, respectively). Ap4A was a weak partial agonist and other dinucleotides were inactive as agonists. None of the inactive dinucleotides were antagonists nor modulated the activity of Ap3A and ATP. 3. For the P2X2 subtype derived from rat PC12 cells, Ap4A was as active as ATP but less potent (EC50 values: 15.2 +/- 1 microM and 3.7 +/- 0.7 microM, respectively). Other adenosine dinucleotides were inactive as either agonists or antagonists. 4. Ap5A (1-100 nM) potentiated ATP-responses at the P2X2 subtype, showing an EC50 of 2.95 +/- 0.7 nM for this modulatory effect. Ap5A (10 nM) shifted the concentration-response curves for ATP to the left by one-half log10 unit but did not alter the Hill co-efficient for ATP (nH = 2.1 +/- 0.1). Ap5A (10 nM) failed to potentiate Ap4A-responses but did enhance the efficacy of the P2 purinoceptor antagonist, suramin, by 12 fold at the P2X2 subtype. 5. In conclusion, the results show that ionotropic (P2X2) and metabotropic (P2Y1) ATP receptors which occur in the CNS are activated selectively by naturally-occurring adenine dinucleotides which are known to be released with nucleotides from storage vesicles. The observed potentiation of P2X2-responses by Ap5A, where co-released with ATP by brain synaptosomes, may have a functional bearing in purinergic signalling in the CNS.
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Affiliation(s)
- J Pintor
- Department of Anatomy and Developmental Biology, University College London
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Keppens S. Effects of diadenosine triphosphate and diadenosine tetraphosphate on rat liver cells. Differences and similarities with ADP and ATP. Biochem Pharmacol 1996; 52:441-5. [PMID: 8687498 DOI: 10.1016/0006-2952(96)00246-8] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Abstract
Liver cells possess multiple types of purinoceptors that mediate the effects of extracellular nucleotides. Like ADP and ATP, the dinucleotides diadenosine triphosphate (Ap3A) and diadenosine tetraphosphate (Ap4A) fully activated glycogen phosphorylase, with ED50 values of 0.31 microM and 1.3 microM, respectively. At variance with ATP, neither the dinucleotides nor ADP significantly increased the levels of IP3.Ap4A (and also ADP) moderately increased IP3 (+/- 72%) whereas Ap3A was completely ineffective. Like ATP, Ap3A, Ap4A, and ADP inhibited the cAMP increase after glucagon. Phorbol-12-myristate-13-acetate (PMA) pretreatment of the hepatocytes clearly inhibited the glycogenolytic potency of Ap3A and ADP, but had only a minor effect on the potency of Ap4A or ATP. It is concluded that, depending upon the effect studied (glycogenolytic effect with or without PMA, increasing IP3 potency, or inhibition of cAMP increase), different analogies between the agonists studied emerged, indicating the complexity of the interaction of ATP and its analogues with liver purinoceptors and/or of the transduction mechanism(s) initiated by the different nucleotides.
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Affiliation(s)
- S Keppens
- Afdeling Biochemie, Faculteit Geneeskunde, Katholieke Universiteit Leuven, Belgium.
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Pintor J, King BF, Ziganshin AU, Miras-Portugal MT, Burnstock G. Diadenosine polyphosphate-activated inward and outward currents in follicular oocytes of Xenopus laevis. Life Sci 1996; 59:PL179-84. [PMID: 8809216 DOI: 10.1016/0024-3205(96)00409-2] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023]
Abstract
Ionic currents evoked by alpha, omega-adenine dinucleotides (ApXA; X = 2-6) in follicular oocytes of Xenopus laevis were studied under voltage-clamp conditions. Dinucleotides evoked inward and outward currents in Xenopus oocytes by activating native P1 and P2 purinoceptors known to be present on the follicle cell monolayer enveloping oocytes. Inward currents were mediated by a suramin-sensitive P2 purinoceptor which showed an agonist potency order (at 10 microM): Ap4A > ATP > Ap3A > > Ap5A, while Ap2A and Ap6A were inactive. Outward currents were mediated by a novel theophylline-sensitive P1 purinoceptor which showed an agonist potency order (at 10 microM): Ap2A > ATP > > Ap4A = Ap5A = Ap6A > Ap3A. Chromatographic analysis confirmed ectonucleotidase activity at the follicle cell layer of oocytes but at a very low rate of dinucleotide cleavage, indicating that currents evoked by dinucleotides resulted from a direct activation of oocyte P1 and P2 purinoceptors and not through their breakdown to ATP, ADP and AMP. There was no evidence for specific receptors (i.e., P4 purinoceptors) for diadenosine polyphosphates in Xenopus oocytes.
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Affiliation(s)
- J Pintor
- Department of Anatomy and Developmental Biology, University College London, UK
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Pivorun EB, Nordone A. Brain synaptosomes display a diadenosine tetraphosphate (Ap4A)-mediated Ca2+ influx distinct from ATP-mediated influx. J Neurosci Res 1996; 44:478-89. [PMID: 8776669 DOI: 10.1002/(sici)1097-4547(19960601)44:5<478::aid-jnr8>3.0.co;2-c] [Citation(s) in RCA: 21] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023]
Abstract
Studies undertaken to compare the effects of Ap4A and ATP on altering intrasynaptosomal Ca2+ levels from deermouse brain reveal that both ligands induce a rapid influx of extracellular Ca2+. The Ca2+ profile elicited by 167 microM Ap4A is "spike-like" (half-time for decline to baseline, 19.1 +/- 1.2 sec), in contrast to the gradual decline observed with ATP (104.0 +/- 7.4 sec). DIDS (4-4'-diisothiocyano-2,2'-disulfonic acid stilbene) and suramin preincubation alter only the ATP-induced Ca2+ profile. Cross-desensitization studies indicate that prior application of ATP does not significantly affect the Ca2+ influx elicited by Ap4A, and that prior application of Ap4A does not affect the Ca2+ influx elicited by ATP. These results demonstrate that extracellular Ap4A and ATP elicit distinct intrasynaptosomal Ca2+ influx profiles, and suggest that these two nucleotides may be interacting with distinct purinoceptor subclasses or purinoceptor-effector complexes. Subjecting the synaptosomes simultaneously to depolarization and Ap4A, or to depolarization and ATP, induces an additive effect on Ca2+ influx. Preincubation with verapamil negates the effects of depolarization without modifying the ligand-elicited Ca2+ fluxes. These results indicate the presence of Ap4A and ATP ligand-gated channels that may function as modulators of neuronal activity.
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Affiliation(s)
- E B Pivorun
- Department of Biological Sciences, Clemson University, South Carolina 29634-1903, USA
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