Fig. 4.

Continuous exocytosis is stimulated by a low Ca²⁺ concentration and inhibited by a high Ca²⁺ concentration. Cells were continuously superfused with salines containing 10 μm ionomycin. The fluorescence intensity produced by 20 μm FM4–64 is plotted by the upper set of traces; the intracellular Ca²⁺ concentration is plotted by thelower set of traces. Thick traces in this and the following figures were measured in the terminal; thin traces were measured in the soma. Continuous exocytosis was measured as C_x = ρv_x/s Δt. Salines containing 100 nm, 50 μm, and 600 μm extracellular Ca²⁺ were applied as indicated by the dashed vertical lines. Ca²⁺ concentrations below 10 μm were reported by fluo-3; concentrations above 10 μm were reported by mag-fura-5. Increasing the intracellular Ca²⁺ from 0.09 to 1.7 μm increasedC_x to 0.33 MEq min⁻¹. A further rise in the intracellular Ca²⁺ to between 50 and 140 μm reduced C_x to 0.06 MEq min⁻¹.