Calcium accumulation by isolated nerve ending particles from brain. II. Factors influencing calcium movements

A synaptosomal preparation from rat brain accumulated 45 Ca by a temperature‐sensitive process requiring ATPd̊ and Mg ++ . The uptake of 45 Ca was correlated with a net increase in total calcium, and after brief incubations the concentration of 45 Ca in the particles was several hundred times that remaining in the medium. Calcium accumulation was associated with a corresponding stoichiometric accumulation of phosphate. Synaptosomes that had accumulated 45 Ca during a prior incubation lost the radioactivity during a subsequent incubation in the absence of ATP. NaCl increased the efflux of accumulated 45 Ca, a process distinguishable from the non‐specific inhibition of 45 Ca accumulation by various solutes corresponding with their effects on intra‐synaptosomal volume. The Na + ‐induced efflux of 45 Ca was not affected by ouabain. Since the intrasynaptosomal mitochondria have been identified as the site of 45 Ca accumulation in synaptosomes, the data suggest that these mitochondria may function as reservoirs of releasable Ca ++ within the nerve ending, possibly participating in the Ca ++ ‐dependent process of neurotransmitter release.