Whole-cell capacitance/admittance recordings were used to resolve single granule exocytosis as step-wise increases in membrane capacitance in RBL-1 and RBL-2H3 cells. The size of the granules differ in these two cell lines. The median capacitance step due to granule fusion in RBL-1 cells was 68.78 fF, which corresponds to a granule diameter of 1.48 µm. RBL-2H3 cells have smaller granules with a median diameter of 0.55 µm (9.46 fF). Exocytosis of the larger granules in RBL-1 cells in 35 % of the observed events was associated with capacitance fluctuation before fusion pore expansion (flickering fusion pore), whereas flickering in RBL-2H3 cells could only be observed in 1,6 % of events. Loading both granules with serotonin (5HT) by overnight incubation (verified by amperometric recording of 5HT-release upon exocytosis) led to an increase in granule size (RBL-1: 1.88 µm (111,41 fF), RBL-2H3: 1.4 µm (61.39 fF)). Interestingly, this increase in granule size in RBL-2H3 cells was accompanied by a strong increase in the incidence fusion pore flickering (43.7 %). In RBL-1 cells loaded with 5HT, flickering could be observed in 41.5 % of events. The flicker dwell time in unloaded cells was significant different (Median dwell time: RBL-1: 151,15 ms ms, RBL-2H3: 158,2 ms; KS-Test p<0.01) whereas loaded with 5HT no significant difference in both cell lines could be observed (Median dwell time: RBL-1+5HT:367,1 ms, RBL-2H3: 182,3 ms; KS-Test p>0,1). These observations suggest that granule size may influence fusion pore formation as a result of membrane curvature favoring rapid fusion pore expansion.

Figure 1