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  • 1
    Electronic Resource
    Electronic Resource
    Characterization of Inhibitor-Sensitive and -Resistant Adenosine Transporters in Cultured Human Fetal Astrocytes (1996)
    Oxford, UK : Blackwell Science Ltd
    Journal of neurochemistry 67 (1996), S. 0 
    Details
    ISSN: 1471-4159
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Abstract: The kinetic characteristics of [3H]adenosine uptake, the extent to which accumulated [3H]adenosine was metabolized, the effects such metabolism had on measurements of apparent Michaelis-Menten kinetic values of KT and Vmax, and the sensitivities with which nucleoside transport inhibitors blocked [3H]adenosine accumulations were determined in cultured human fetal astrocytes. KT and Vmax values for accumulations of [3H]-labeled purines using 15-s incubations in the absence of the adenosine deaminase inhibitor erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA) and the adenosine kinase inhibitor 5′-iodotubercidin (ITU) were 6.2 µM and 0.15 nmol/min/mg of protein for the high-affinity and 2.6 mM and 21 nmol/min/mg of protein for the low-affinity components respectively. In the presence of EHNA and ITU, where 〈4% of accumulated [3H]adenosine was metabolized, transport per se was measured, and kinetic values for KT and Vmax were 179 µM and 5.2 nmol/min/mg of protein, respectively. In the absence of EHNA and ITU, accumulated [3H]adenosine was rapidly metabolized to AMP, ADP, and ATP, and caused an appearance of “concentrative” uptake in that the intracellular levels of [3H]-labeled purines (adenosine plus its metabolites) were 1.4-fold higher than in the medium. No apparent concentrative accumulations of [3H]adenosine were found when assays were conducted using short incubation times in the absence or presence of EHNA and ITU. The nucleoside transport inhibitors dipyridamole (DPR), nitrobenzylthioinosine (NBI), and dilazep biphasically inhibited [3H]adenosine transport; for the inhibitor-sensitive components the IC50 values were 0.7 nM for NBI, 1.3 nM for DPR, and 3.3 nM for dilazep, and for the inhibitor-resistant component the IC50 values were 2.5 µM for NBI, 5.1 µM for dilazep, and 39.0 µM for DPR. These findings, in cultured human fetal astrocytes, represent the first demonstration of inhibitor-sensitive and -resistant adenosine transporters in nontransformed human cells.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1471-4159.1996.67030972.x
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  • 2
    Electronic Resource
    Electronic Resource
    [3H]Adenosine Transport in Synaptoneurosomes of Postmortem Human Brain (1993)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of neurochemistry 60 (1993), S. 0 
    Details
    ISSN: 1471-4159
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Abstract: Abstract: [3H]Adenosine transport was characterized in cerebral cortical synaptoneurosomes prepared from postmortem human brain using an inhibitor-stop/centrifugation method. The adenosine transport inhibitors dipyridamole and dilazep completely and rapidly blocked transmembrane fluxes of [3H]adenosine. For 5-s incubations, two kinetically distinguishable processes were identified, i.e., a high-affinity adenosine transport system with Kt and Vmax values of 89 μM and 0.98 nmol/min/mg of protein, respectively, and a low-affinity adenosine transport system that did not appear to be saturable. For incubations with 1 μM [3H]adenosine as substrate, intrasynaptoneurosomal concentrations of [3H]adenosine were 0.26 μM at 5 s and 1 μM at 600 s. Metabolism of accumulated [3H]adenosine to adenine nucleotides was 15% for 5-s, 23% for 15-s, 34% for 30-s, 43% for 60-s, and 80% for 600-s incubations. The concentrations (μM) of total accumulated 3H-purines ([3H]-adenosine plus metabolites) at these times were 0.3, 0.5, 1.0, 1.3 and 5.6, respectively. These results indicate that in the presence of extensive metabolism, the intrasynaptoneurosomal accumulation of 3H-purines was higher than the initial concentration of 1 μM [3H]adenosine in the reaction medium. For 5-, 15-, 30-, 60-, and 600-s incubations in the presence of the adenosine deaminase inhibitor EHNA and the adenosine kinase inhibitor 5′-iodotubercidin, metabolism of the transported [3H]adenosine was 14, 14, 16, 14, and 38%, respectively. During these times, total 3H-purine accumulation was 0.3, 0.5, 0.5, 0.7, and 1.8 μM, respectively. Thus, the apparently “concentrative'’accumulation of 3H-purines can be prevented by inhibition of adenosine metabolism and, taken together, these results suggest that adenosine transport in at least synaptoneurosomes prepared from postmortem human brain is via a nonconcentrative and equilibrative system.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1471-4159.1993.tb03509.x
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  • 3
    Electronic Resource
    Electronic Resource
    Involvement of Bidirectional Adenosine Transporters in the Release of l-[3H]Adenosine from Rat Brain Synaptosomal Preparations (1995)
    Oxford, UK : Blackwell Science Ltd
    Journal of neurochemistry 64 (1995), S. 0 
    Details
    ISSN: 1471-4159
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Abstract: Adenosine transport inhibitors as enhancers of extracellular levels of endogenous adenosine would, presumably, only be effective if, for example, (1) the inhibitors block influx to a greater degree than efflux (release) of intracellular adenosine or (2) the inhibitors block equally well the influx and efflux of adenosine, but significant amounts of adenosine are formed as a result of dephosphorylation of released adenine nucleotides. Limited information is available regarding the directional symmetry of adenosine transporters in neural cells. Using rat brain crude P2 synaptosomal preparations preloaded with l-[3H]adenosine, our objectives here were to determine (1) if l-[3H]adenosine, a substrate for adenosine transporters that is more metabolically stable than physiological d-adenosine, was being released from synaptosomal preparations, (2) the optimal conditions necessary to observe the release, and (3) the degree to which this release was mediated by efflux through bidirectional nucleoside transporters. l-[3H]Adenosine release was found to be concentration and time dependent, temperature sensitive, and linear with synaptosomal protein. l-[3H]Adenosine release was inhibited dose-dependently by dipyridamole, nitrobenzylthioinosine, and dilazep; at concentrations of 100 µM inhibition was at least 40% for dipyridamole, 52% for nitrobenzylthioinosine, and 49% for dilazep. After loading with l-[3H]adenosine alone or l-[3H]adenosine plus unlabeled l-adenosine, d-adenosine, or uridine, l-[3H]-adenosine release was inhibited 42% by l-adenosine, 69% by uridine, and 81% by d-adenosine. The inhibition of l-[3H]adenosine release from the synaptosomal preparations by substrates for or inhibitors of nucleoside transporters suggests that a portion of the release was mediated by nucleoside transporters. This experimental system may prove useful for evaluating the effects of pharmacological agents on bidirectional transport of adenosine.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1471-4159.1995.64052105.x
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  • 4
    Electronic Resource
    Electronic Resource
    Transport and Metabolism of D-[3H]Adenosine and L-[3H]Adenosine in Rat Cerebral Cortical Synaptoneurosomes (1992)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of neurochemistry 58 (1992), S. 0 
    Details
    ISSN: 1471-4159
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Abstract: The relationship between transport and metabolism in synaptoneurosomes was examined to determine the metabolic stability of rapidly accumulated D-[3H]adenosine and L-[3H]adenosine and the degree to which metabolism of the accumulated purines affected measurements of apparent KT and Vmax values for adenosine transport. For D-[3H]adenosine, high- and low-affinity accumulation processes were present. For the high-affinity system an inverse relationship was found between transport reaction times and KT and Vmax values. For incubations of 5, 15, and 600 s, which corresponded to 24, 32, and 76% phosphorylation of accumulated D-[3H]adenosine to nucleotides, apparent KT values were 9.4, 8.4, and 4.5 μM, respectively, and Vmax values were 850, 70, and 12 pmol/min/mg of protein, respectively. Pretreatment with 10 μM erythro-9-(2-hydroxy-3-nonyl)adenine, an adenosine deaminase inhibitor, and 5′-iodotubercidin, an adenosine kinase inhibitor, decreased the phosphorylation of accumulated D-[3H]adenosine to 6% with 5-s and 9% with 15-s incubations. This resulted in significantly higher KT values: 36 μM at 5 s and 44 μM at 5 s. At 10-min incubations in the presence of these inhibitors, metabolism of accumulated D-[3H]adenosine was 32%, and apparent KT and Vmax values at this time were not significantly different from those obtained without inhibitors. For L-[3H]adenosine, apparent KT and Vmax values for 20-s incubations were 38.7 μM and 330 pmol/min/mg of protein, respectively. Metabolism (mainly phosphorylation) of accumulated L-[3H]adenosine was observed only at incubations of 〉30 s. Taken together, these results demonstrate that adenosine transport is significantly faster than subsequent metabolism; that accumulated D-adenosine is rapidly incorporated into and trapped intracellularly as adenine nucleotides, thereby affecting measured kinetic parameters for adenosine transport and giving an “appearance” of concentrative accumulations; and that the apparent KTT value of 39 μM for D-adenosine transport conducted in the presence of the enzyme inhibitors was the same as the apparent KT value for L-adenosine transport.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1471-4159.1992.tb10043.x
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  • 5
    Electronic Resource
    Electronic Resource
    L-[3H]Adenosine, a New Metabolically Stable Enantiomeric Probe for Adenosine Transport Systems in Rat Brain Synaptoneurosomes (1991)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of neurochemistry 56 (1991), S. 0 
    Details
    ISSN: 1471-4159
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Abstract: The stereoenantiomers D-[3H]adenosine and L-[3H]adenosine were used to study adenosine accumulation in rat cerebral cortical synaptoneurosomes. L-Adenosine very weakly inhibited rat brain adenosine deaminase (ADA) activity with a Ki value of 385 μM. It did not inhibit rat brain adenosine kinase (AK) activity, nor was it utilized as a substrate for either ADA or AK. The rate constants (fmol/mg of protein/s) for L-[3H]adenosine accumulation measured in assays where transport was stopped either with inhibitor-stop centrifugation or with rapid filtration methods were 82 ± 14 and 75 ± 10, respectively. Using the filtration method, the rates of L-[3H]adenosine accumulation were not significantly different from the value of 105 ± 15 fmol/mg of protein/s measured for D-[3H]adenosine transport. Unlabeled D-adenosine and nitrobenzylthioinosine, both at a concentration of 100 μM, reduced the levels and rates of L-[3H]adenosine accumulation by 〉44%. These findings suggest that L-adenosine, a metabolically stable enantiomeric analog, and the naturally occurring D-adenosine are both taken up by rat brain synaptoneurosomes by similar processes, and as such L-adenosine may represent an important new probe with which adenosine uptake may be studied.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1471-4159.1991.tb08184.x
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  • 6
    Electronic Resource
    Electronic Resource
    Synaptic strengthening through activation of Ca 2+ -permeable AMPA receptors (1996)
    [s.l.] : Nature Publishing Group
    Nature 381 (1996), S. 793-796 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Fast application of a hyperosmotic solution to dorsal horn neurons in culture caused a large increase in the probability of transmitter release, as indicated by an enhancement in the frequency of miniature excitatory postsynaptic currents (mEPSCs; Fig. la) and shown previously at several types of ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/381793a0
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