Uridine activates fast transmembrane Ca2+ ion fluxes in rat brain homogenates

Excitatory receptors responsive to extracellular pyrimidine nucleotides have been identi®ed [1±6]. Although there is evidence for uridine acting as a CNS depressant [7,8], suggestions for an excitatory role in the CNS have so far not been supported by any direct evidence at the molecular level [9]. Hepatic uridine [10] enters the extracellular space of the brain, and subsequently brain cells, from the blood, where it can be phosphorylated and incorporated into RNA or be catabolized to uracil [11,12]. Since the extracellular concentration of uridine has been found to be increased by high K+ in the rat thalamus [13], it was of interest to study uridineresponsive cation ¯uxes through brain membranes. Here we describe a ¯uorescent tracer method to study uridine-activated Ca2+ and K+ ion translocation in suspensions of resealed plasmalemma fragments and nerve endings on the time scale achievable by stopped-¯ow spectroscopy. Uptake, release and binding of radiolabeled uridine in suspensions of synaptosomes and synaptosomal membranes, respectively, were also measured. Cerebral ±cortical suspensions were controlled by HPLC analyses of endogenous purines and pyrimidines