EFFECTS OF Na⁺ AND OTHER MONOVALENT CATIONS ON Ca-EFFLUX FROM SYNAPTOSOMES

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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.