Effects of synaptic plasma membranes on release of acetylcholine from synaptic vesicles

The influences of synaptic plasma membranes on release of acetylcholine (ACh) from synaptic vesicles isolated from rat brain were examined. In the presence of ATP, Mg++ and Ca++ but absence of cytoplasm from the nerve endings, the synaptic plasma membranes did not increase ACh release indicating absence of a stimulating factor which is known to be present in the cytoplasm. In presence of ATP, Mg++, Ca++ and the cytoplasm, the synaptic plasma membranes inhibited ACh release from the synaptic vesicles in high K+ medium, though not in high Na+ medium. Binding of Ca++ by the synaptic plasma membranes was dependent on ATP, inhibited by Na+ and stimulated by K+. Thus, the synaptic plasma membranes may inhibit ACh release in high K+ medium due to reduction in the concentration of free Ca++.