Mitochondrial calcium sequestration buffers [Ca²⁺]_cyto. A, The [Ca²⁺]_cyto transient (using the same stimulation protocol as Fig. 1) was significantly slowed by mitochondrial depolarization, but the peak rise was not significantly altered (97 ± 6% of control; n = 5;p = 0.66). B, The τ_fast [Ca²⁺]_cyto decay was slowed by mitochondrial depolarization and TPP⁺but not by thapsigargin (129 ± 9% of control;n = 3; p = 0.08). C, D, Trains of presynaptic depolarization (−80 to 0 mV, 1 msec stimulation repeated 20 times at 200 Hz) produced rises in [Ca²⁺]_cyto that were also significantly slowed by mitochondrial depolarization or 1 μm Ru360. E, F, [Ca²⁺]_mito recorded with rhod-FF increased during stimulation (at arrow as inC). This increase was blocked by mitochondrial depolarization or Ru360. Asterisks indicate statistical significance (p < 0.05).