Capacitance flickers and pseudoflickers of small granules, measured in the cell-attached configuration

We have studied exocytosis of single small granules from human neutrophils by capacitance recordings in the cell-attached configuration. We found that 2.2% of the exocytotic events were flickers. The flickers always ended with a downward step. This indicates closing of the fusion pore. During flickering, the fusion pore conductance remained below 1 nS, and no net membrane transfer was detectable. After fusion pore expansion beyond 1 nS the pore expanded irreversibly, leading to rapid full incorporation of the granule/vesicle into the plasma membrane. Following exocytosis of single granules, a capacitance decrease directly related to the preceding increase was observed in 7% of the exocytotic events. This decrease followed immediately after irreversible pore expansion, and is presumably triggered by full incorporation of the vesicle into the patch membrane. The capacitance decrease could be interpreted as endocytosis triggered by exocytosis. However, the gradual decrease could also reflect a decrease in the "free" patch area following incorporation of an exocytosed vesicle. We conclude that non-stepwise capacitance changes must be interpreted with caution, since a number of factors go into determining cell or patch admittance.