Synthesis and Release of [¹⁴c]acetylch0line in Synaptosomes

Synaptosomes took up [ 14 C]choline, about half or more of which was converted to [ I4 C]acetylcholine when incubated in an appropriate medium containing 1 to 5 μ M‐[ 14 C] choline and neostigmine. The amount of [ 14 C]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 [ I4 C]choline into synaptosomes was dependent on the concentration of choline in the incubation medium. About 25 per cent of [ 14 C]acetylcholine synthesized in synaptosomes was released rapidly into the medium by increasing the K + concentration in the medium from 5 m m to 35 m m . The change of Na + concentration hardly affected the release of [ 14 C]acetylcholine. The effect of K + on the release of [ 14 C]choline was rather small compared to that on [ 14 C] acetylcholine. Ouabain promoted the release of [ 14 C]acetylcholine.