Effect of liposomes containing various divalent cations on the release of acetylcholine from synaptosomes

The effect of increasing the cytoplasmic levels of various divalent cations on the release of [3H]acetylcholine ([3H]ACh) from synaptosomes was investigated. Synaptosomes prepared from rat brain and prelabeled with [3H]choline were incubated with liposomes containing Mg2+, Ca2+, Mn2+, Co2+, Sr2+, or Ba2+. This treatment allows the transfer of the aqueous contents of the liposomes to the cytoplasm of the synaptosomes. The efflux of radioactivity subsequent to this treatment was measured, and the relative proportions of [3H]ACh and [3H]choline were determined. The release of radioactivity from synaptosomes incubated with liposomes containing Mg2+, Mn2+, or Co2+ was not altered when compared with synaptosomes incubated either without liposomes or with liposomes containing isotonic K+/Na+. Synaptosomes incubated with liposomes containing Ca2+, Sr2+, or Ba2+, however, released significantly more radioactivity than did controls. Moreover, the released radioactivity consisted almost entirely of [3H]ACh. Liposomes containing either Ca2+ or Sr2+ were equally effective in promoting the release of [3H]ACh from synaptosomes, whereas liposomes containing Ba2+ were 2.5 times more effective in promoting the release of [3H]ACh than were liposomes containing either Ca2+ or Sr2+. Since liposomes introduce their aqueous contents into cytoplasm via a mechanism not involving plasma membrane channels, the increased release of [3H]ACh caused by liposomes containing Ca2+, Sr2+, or Ba2+ is attributable to an increase in the intrasynaptosomal concentration of these ions, and not to their passage through calcium channels.