Release of [14C]adenosine derivatives from superfused synaptosome preparations. Effects of depolarizing agents and metabolic inhibitors.

Adenine 5'-mononucleotides of synaptosome beds from guinea pig neocortex were labelled by incubation with [8-(14)C]adenosine, and excess adenosine was then removed with precursor-free medium. During continuous superfusion, labelled adenosine derivatives were released at a rate of about 0.5% of the synaptosome 14C/min and this rate was increased 2.5-fold by depolarization with 33 mM K+. Some depolarizing agents and metabolic inhibitors were examined for action on the release of [14C)-adenosine derivatives from the synaptosome preparations, and also on the rate of lactate production by these preparations, both before and during K + depolarization. The synaptosome content of adenine 5'-mononucleotides following exposure of the synaptosome beds to these compounds was also estimated. Ouabain, 0.1 mM, brought about an increase in 14C-labelled compounds output, but it caused little alteration in lactate output and some decrease in the adenylate energy charge of the synaptosomes. Veratridine, 80 microM, increased markedly both the output of radioactivity and the lactate production. Tetrodotoxin, 1 microM, when added together with veratridine, completely abolished the effect of veratridine on the efflux of [14C]adenosine derivatives, but this was not associated with a complete blockade of the output of lactate. Sodium cyanide, 2.5 mM, FCCP, 6 muM together with iodoacetate, 2.5 mM, caused an increase in the output of 14C-labelled compounds, and a decrease in the adenylate energy charge. The production of lactate was also increased by sodium cyanide and FCCP, but it was completely inhibited by iodoactate. Oligomycin, 4.65 microM, and amytal, 1 mM, added in the incubation medium before labelling the synaptosome beds with [8-(14)C]adenosine, did not affect very much the output of [14C]adenosine derivatives, while the output of lactate increased independently of the depolarization with 33 mM K+. The release of adenosine derivatives from superfused synaptosome beds induced by depolarizing agents or metabolic inhibitors did not seem to be due either to an effect of membrane permeability changes that follow a decrease of ATP supply, or to an increased metabolic rate occurring during nerve ending stimulation. It is concluded that this release of adenosine derivatives appeared to be a process triggered primarily by the influx of Na+ and the increased intrasynaptosomal calcium, and closely related to the process of neurotransmission.