Effects of Na+ and other monovalent cations on Ca-efflux from synaptosomes

Effects of monovalent cations on Ca-efflux were examined in rat brain cortex slices, synaptosomes and synaptic plasma membranes. Effluxes of 45Ca from brain slices and synaptosomes were stimulated by Na+ in medium and the effects of Na+ on the 45Ca-effluxes disappeared at low temperature. Furthermore, we observed that Rb+ had more effect than Na+ on 45Ca-efflux from the synaptosomes, while Li+ had less effect. On the other hand, release of 45Ca from the synaptic plasma membranes which had been preincubated with ATP, MG++ and 45Ca was stimulated by Na+ and Li+. But Cs+ and Rb+ were less effective on the release. These results indicate occurrence of Na-dependent Ca-efflux at nerve endings. However, the assumption that ATP-dependent Ca-binding with synaptic plasma membranes may be a partial reaction of Ca-efflux was not supported by these experiments.