Probing for calcium at presynaptic nerve terminals.

The nonmitochondrial ATP-dependent calcium sequestration site within "pinched-off" presynaptic nerve terminals (synaptosomes) was localized by morphological techniques. The terminals contain mitochondria, smooth endoplasmic reticulum (SER), synaptic vesicles, and occasional coated vesicles. Three-dimensional reconstructions of serial sections of synaptosomes reveal that the SER consists, in part, of flattened sacs or cisterns often situated adjacent to mitochondria. Synaptosomes with leaky plasma membranes (induced by saponin treatment) were incubated in physiological salt solutions containing Ca, ATP, and oxalate to promote Ca sequestration. After incubation in these solutions, synaptosomes contained electron-dense deposits, presumably calcium oxalate, localized within mitochondria, SER cisterns, and vesicular profiles. Electron probe microanalyses of these electron-dense deposits confirmed the presence of calcium. When mitochondrial poisons were included in the incubation media, electron-dense deposits were still observed in the SER; however, under these conditions, mitochondria very rarely contained such deposits. When A23187 or EGTA was included in the incubation solutions, electron-dense deposits rarely were observed in any organelles within the terminals. When oxalate was omitted from the incubation media, no electron-dense deposits were found in the synaptosomes. These results show that the nerve terminal SER is capable of sequestering Ca. The data are consistent with biochemical and physiological evidence that the SER plays a significant role in intraterminal Ca buffering during neuronal activity.