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Abstract— Synaptosomes took up [14C]choline, about half or more of which was converted to [I4C]acetylcholine when incubated in an appropriate medium containing 1 to 5 μM-[14C] choline and neostigmine. The amount of [14C]acetylcholine synthesized in synaptosomes increased in parallel with the increase of Na+ concentration in the incubation medium. The effect of Na+ on the uptake of [I4C]choline into synaptosomes was dependent on the concentration of choline in the incubation medium.

About 25 per cent of [14C]acetylcholine synthesized in synaptosomes was released rapidly into the medium by increasing the K+ concentration in the medium from 5 mm to 35 mm. The change of Na+ concentration hardly affected the release of [14C]acetylcholine. The effect of K+ on the release of [14C]choline was rather small compared to that on [14C] acetylcholine. Ouabain promoted the release of [14C]acetylcholine.