Effect of Liposomes Containing Various Divalent Cations on the Release of Acetylcholine from Synaptosomes

Abstract: 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.