Effect of Na+ on Ca-uptake of the synaptic plasma membrane

Synaptic plasma membranes prepared from rat brain synaptosomes bound ⁴⁵Ca by an ATP-dependent process. The amount of ⁴⁵Ca taken up by the membranes was increased by addition of K⁺ or Rb⁺ but decreased by addition of Na⁺. The inhibitory effect of Na⁺ was more remarkable in presence of KCI than its absence. Furthermore, rapid release of ⁴⁵Ca was observed on addition of Na⁺ (100 meq) to membranes which had been incubated with ⁴⁵Ca and ATP and more than half of the membrane bound ⁴⁵Ca was released within 10 sec under our experimental conditions. The physiological significance of this finding is discussed.