Loss of SNAP23 reduces both the primary and sequential exocytosis fusion events in acinar cells. (A) Fluorescence images of SRB during 100 pM CCK stimulation of control (Ctrl) and AcKO acinar cells. Arrows indicate primary exocytosis, and arrowheads indicate sequential exocytosis. Bar, 10 µm. (B–E) Quantification of exocytotic events in response to CCK stimulation. The numbers of primary (B) and sequential (C) exocytotic events detected by two-photon microscopy were counted in control and AcKO acinar cells. (D and E) show the total numbers of primary and sequential exocytosis per acinar cell during a 15-min period. 10 acinar cells from 4 control mice and 18 acinar cells from 6 AcKO mice were analyzed. (F) The ratio of sequential exocytotic events to primary exocytotic events calculated by the values of D and E. (G) Time courses of intracellular Ca²⁺ concentrations in the control (top) and AcKO (bottom) acinar cells during CCK stimulation as represented by (F₀ − F)/F₀, where F₀ and F are the resting and poststimulation fluorescence, respectively. (H) The maximum increment of intracellular Ca²⁺ concentrations. Nine acinar cells from two control mice and nine acinar cells from three AcKO mice were analyzed. Data are mean ± SEM. Significance was calculated by the two-tailed paired Student’s t test. **, P < 0.01; ***, P < 0.001.