SNAP23 deletion increases exocytosis of predocked granules during biphasic insulin release. (A and B) Fasting-refeeding experiments showing the enhanced glucose tolerance in BcKO mice. Blood glucose concentrations (A) and plasma insulin concentrations (B) under conditions of feeding (Free), 2-h fasting (Rest), 18-h fasting (Fast), and refeeding after 18-h fasting (Refed). Four mice per genotype were examined for blood glucose concentrations, and three mice per genotype were examined for plasma insulin concentrations. Blood glucose concentrations (C) and plasma insulin concentrations (D) during IPGTT. Three mice per genotype were examined. (E) Percentage of initial blood glucose concentration during ITT. Three mice per genotype were examined. (F) Insulin secretion from isolated islets as measured by a batch assay. Islets were isolated from control (Ctrl) and BcKO mice by collagenase digestion, and, after a 30-min incubation with 2.2 mM or 16.7 mM glucose, the secreted insulin was measured by ELISA. The total contents of insulin were obtained by solubilizing and sonicating the islets after the experiments. Islets from three mice per genotype were examined. (G and H) Perfusion analysis of isolated islets demonstrating the increased insulin secretion of BcKO islets in the first phase (4 min after 22 mM glucose induction). Islets from three mice per genotype were examined. (I and J) TIRFM analysis showing that the predocked insulin granules fusion was increased in the BcKO islets. (I) Histogram of the number of fusion events in control and BcKO β cells at 1-min intervals after 22 mM glucose stimulation observed by TIRFM. (J) The total number of fusion events of predocked or newcomer granules calculated by the values of I. Analysis was performed on 28 control β cells and 20 BcKO β cells scored in six mice per genotype. Data are mean ± SEM. Significance was calculated by the two-tailed paired Student’s t test. *, P < 0.05; **, P < 0.01; ***, P < 0.001. a.u., arbitrary units.