Closure of the vesicle mouth and membrane retrieval were blocked by Sr²⁺. (A) The change in capacitance in response to stimulation in the presence of 2.5 mM Ca²⁺ (black; 9 cells) or 10 mM Sr²⁺ (red; 11 cells). Error bars indicate SEM. Strontium triggered fusion and a large amount of asynchronous release, but there was no net retrieval of membrane, indicating that endocytosis was inhibited. Dashed line indicates the baseline capacitance. (B) Comparison of the IRM response in Ca²⁺ (black) and Sr²⁺ (red). The response in Sr²⁺ is averaged from 9 synchronous and 11 asynchronous events because the two populations could not be distinguished. Error bars indicate SEM. (C) Average images of a single granule fusion in 10 mM Sr²⁺ (n = 6). Bar, 0.5 μm