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  • 1
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Journal of neurochemistry 60 (1993), S. 0 
    ISSN: 1471-4159
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Abstract: Adenosine transport in cultured chromaffin cells was inhibited by purinergic P2y-receptor agonists without significant changes in the affinity constant, the values being between 1 ± 0.4 and 1.6 ± 0.6 μM. The Vmax parameter was modified significantly, being 40 ± 1.0, 26 ± 5.0, 32 ± 3.0, and 22 ± 4.7 pmol/106 cells/min for control, adenosine-5′-O-(2-thiodiphosphate), 5′-adenylylimidodiphosphate, and P1,P4-di(adenosine-5′-) tetraphosphate (Ap4A) (100 μM for every effector), respectively. Ap4A, a physiological ligand for P2y receptors in chromaffin cells, showed the highest inhibitory effect (45%). This transport inhibition is explained by an increase in the cytosolic Ca2+ concentration ([Ca2+]i) and the activation of protein kinase C (PKC). Experiments of [Ca2+]i measurement with the fura-2 technique showed that P2y agonists, as well as bradykinin, were able to increase [Ca2+]i, this effect being independent of the presence of extracellular Ca2+. The peptide bradykinin, determined to be coupled to phosphatidylinositol hydrolysis and internal Ca2+ mobilization in chromaffin cells, exhibited a behavior similar to that of P2y agonists in adenosine transport inhibition (39%). P2y agonists and bradykinin increased PKC activity associated with the membrane fraction (about 50% increase in particulate PKC activity with respect to controls). The present studies suggest that adenosine transport is regulated by P2y-purinergic receptors mediated via Ca2+ mobilization and PKC activation.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Journal of neurochemistry 54 (1990), S. 0 
    ISSN: 1471-4159
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Abstract: Extracellular adenosine is transported into chromaffin cells by a high-affinity transport system. The action of adenosine receptor ligands was studied in this cellular model 5-(N-Ethylcarboxamido)adenosine (NECA), an agonist receptors, activated adenosine transport. Km values for adenosine were 4.6 ± 1.0 (n = 5) and 10.2 ± 3.0 μM (n = 5) for controls and 100 nM NECA, respectively. The Vmax values were 66.7 ± 23.5 and 170.2 ± 30 pmol/106 cells/min for control and 100 nM NECA, respectively. The A1 agonist N6-cyclohexyladenosine, the A1 antagonist 8-cyclopentyl-1, 3-dipropylxanthine, and the A1-A2 antagonist 1,3-dipropyl-8-{4-[(2-aminoethyl)amino]-carbonylmethyloxyphenyl}-xanthine did not significantly modify the adenosine transport in this system. Binding studies done with [3H]dipyridamole, a nucleoside transporter ligand, did not show changes in either the number or affinity of transporter sites after NECA treatment. This ligand can enter cells and quantifies the total number of transporters. The binding studies with [3H]-nitrobenzylthioinosine, which quantifies the plasma membrane transporters, showed a Bmax of 19,200 ± 800 and 23,200 ± 700 transporters/cell for controls and 100 nM NECA, repectively. No changes in the KD were obtained. The effects of NECA were not mediated through adenylate cyclase activation, because its action was not imitated by forskolin.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Oxford UK : Blackwell Science Ltd.
    Journal of neurochemistry 75 (2000), S. 0 
    ISSN: 1471-4159
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Abstract: We have examined the expression of mRNA for several P2Y nucleotide receptors by northern blot analysis in purified type 1 cerebellar astrocyte cultures. These results suggest that different P2Y subtypes could be responsible for ATP metabotropic calcium responses in single type 1 astrocytes. To identify these subtypes we have studied the pharmacological profile of ATP calcium responses using fura-2 microfluorimetry. All tested astrocytes responded to ATP and UTP stimulations evoking similar calcium transients. Most astrocytes also responded to 2-methylthioATP and ADP challenges. The agonist potency order was 2-methylthioATP 〉 ADP 〉 ATP = UTP. Cross-desensitization experiments carried out with ATP, UTP, and 2-methylthioATP showed that 2-methylthioATP and UTP interact with different receptors, P2Y1 and P2Y2 or P2Y4. In a subpopulation of type 1 astrocytes, ATP prestimulation did not block UTP responses, and UDP elicited clear intracellular Ca2+ concentration responses at very low concentrations. 2-MethylthioATP and UTP calcium responses exhibited different sensitivity to pertussis toxin and different inhibition patterns in response to P2 antagonists. The P2Y1-specific antagonist N6-methyl-2′-deoxyadenosine 3′,5′-bisphosphate (MRS 2179) specifically blocked the 2-methylthio-ATP responses. We can conclude that all single astrocytes coexpressed at least two types of P2Y metabotropic receptors: P2Y1 and either P2Y2 or P2Y4 receptors. Moreover, 30-40% of astrocytes also coexpressed specific pyrimidine receptors of the P2Y6 subtype, highly selective for UDP coupled to pertussis-toxin insensitive G protein.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Biochemistry 32 (1993), S. 14203-14209 
    ISSN: 1520-4995
    Source: ACS Legacy Archives
    Topics: Biology , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Science Ltd
    Journal of neurochemistry 83 (2002), S. 0 
    ISSN: 1471-4159
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: In this study, we show specific intracellular responses evoked by the stimulation of astrocytes with the P1,P5-di(adenosine-5′)pentaphosphate, Ap5A. The stimulation of astrocytes with micromolar concentrations of the dinucleotide elicited rapid increases in intracellular calcium concentration ([Ca2+]i), showing an EC50 value of 15.27 ± 0.61 µm. Moreover, the stimulation of cells with nanomolar concentrations of Ap5A, unable to induce calcium responses, increased the phosphorylated forms of extracellular-signal regulated kinase 1/2 (ERK) with an EC50 value of 9.8 ± 2.4 nm. The maximal activation was observed at 100 nm Ap5A, which was similar to that produced by epidermal growth factor (EGF) under the same experimental conditions. The present data reported here indicate that Ap5A mediated these effects by interacting with a specific receptor, not yet identified, which was different from the P2Y1 and P2Y2/P2Y4 receptors present in all individual astrocytes.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Springer
    ISSN: 1573-6881
    Keywords: (Ca2+ + Mg2+)-ATPase ; sarcoplasmic reticulum ; membrane fluidity ; enzyme kinetics
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology , Physics
    Notes: Abstract (Ca2+ + Mg2+)-ATPase from sarcoplasmic reticulum has been reconstituted with dipalmitoylphosphatidylcholine, and the activating effect of ATP and Ca2+ on this enzyme has been studied at different temperatures. It has been found that two kinetic forms of the enzyme are interconverted at about 31°C, and this is possibly related to a phase change in the phospholipid which is more directly associated with the protein. Above 31°C the enzyme is less dependent on ATP activation at high ATP concentrations but shows positive cooperativity for Ca2+ activation. On the other hand, below 31°C, the reconstituted enzyme is more dependent on ATP for activation at high ATP concentrations than the purified ATPase and does not show cooperativity for Ca2+ activation.
    Type of Medium: Electronic Resource
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