Whole-cell capacitance/admittance recordings resolving single granule fusion in RBL subclone −1 and −2H3 cells were analyzed to test for a relation between granule size and the kinetics of the exocytotic event. Compared to RBL-2H3 cells, capacitance steps in RBL-1 cells had larger median amplitude (68.78 vs. 9.46 fF) corresponding to larger median diameter (1.48 vs. 0.55 μm) and displayed a significantly higher percentage (35 vs. 1.6%) of capacitance fluctuations before full pore expansion (flicker). In both cell types, loading of granules with serotonin (5-HT) by overnight incubation (verified by amperometry) resulted in a significant increase in median granule size (RBL-1: 111.41 fF or 2.58 μm; RBL-2H3: 61.39 fF or 1.4 μm). Interestingly, in RBL-2H3 cells, the growth in granule diameter (to 250% of control) was accompanied by a strong increase in the prevalence of capacitance flicker (to 41.5%), while in RBL-1 cells with granule size of 170% of control, capacitance flicker occured in 41.5% of cases after 5HT loading. These results suggest that granule size may influence fusion pore kinetics e.g. as a result of membrane curvature favoring rapid pore expansion.View Large Image | View Hi-Res Image | Download PowerPoint Slide