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Parameters of [3H]uridine binding to synaptic membranes isolated from rat brain cortex (KD=71±4 nM, Bmax=1.37±0.13 pmol/mg protein) were obtained. Pyrimidine and purine analogues displayed different rank order of potency in displacement of specifically bound [3H]uridine (uridine>5-F-uridine>5-Br-uridine∼adenosine⪢5-ethyl-uridine∼suramin>theophylline) and in the inhibition of [14C]uridine uptake (adenosine>uridine>5-Br-uridine∼5-F-uridine∼5-ethyl-uridine) into purified cerebrocortical synaptosomes. Furthermore, the effective ligand concentration for the inhibition of [14C]uridine uptake was about two order of magnitude higher than that for the displacement of specifically bound [3H]uridine. Adenosine evoked the transmembrane Na+ ion influx, whereas uridine the transmembrane Ca2+ ion influx much more effectively. Also, uridine was shown to increase free intracellular Ca2+ ion levels in hippocampal slices by measuring Calcium-Green fluorescence. Uridine analogues were found to be ineffective in displacing radioligands that were bound to various glutamate and adenosine—recognition and modulatory—binding sites, however, increased [35S]GTPgammaS binding to membranes isolated from the rat cerebral cortex. These findings provide evidence for a rather specific, G-protein-coupled site of excitatory action for uridine in the brain.