The mouth of a vesicle opened and closed rapidly at low calcium concentrations. (A) Difference images from the footprint of a chromaffin cell visualized by IRM. The cytoplasmic [Ca²⁺] was gradually raised by slowly uncaging NP-EGTA. Images show the average of three frames at ∼300 nM [Ca²⁺] (left, no activity) and ∼800 nM [Ca²⁺] (right, fusion of a granule arrowed). (B) Time course of the intensity change in an ROI centered on the arrowed region in A. (C) The capacitance increase as a function of time evoked by slowly uncaging NP-EGTA (top, black). A cumulative plot of fusion events visualized by IRM is superimposed in blue. Same cell as in A. The time course of the change in [Ca²⁺] is in the bottom panel. (D) Values of D_1/2 (open circles) and C_1/2 (closed circles) as a function of the [Ca²⁺] at which fusion occurred (n = 33; 14 cells). Black symbols show mean ± SEM. (E) Time course of the change in the intensity of reflected light averaged from fusion events during slow uncaging of NP-EGTA (blue; n = 42). For comparison, the red trace shows the average of asynchronous events occurring after a depolarizing stimulus (n = 16; Fig. 4). Error bars indicate SEM. (F) Detail of the dilation phases shown in E. For comparison, the black trace shows the average of synchronous events occurring after a depolarizing stimulus (n = 73; Fig. 4). Error bars indicate SEM.