Amperometric and capacitance measurement in RBL cells. (A) Schematic illustration of the intra-patch pipette carbon electrode. Carbon electrodes were prepared to allow facile insertion and manipulation in the patch pipette holder employed. (B) In the cell-attached configuration, 2 μM of calcium ionophore, A23187, triggers profuse exocytosis. Two distinct vesicle pools are observed. One is the secretory granule (SG) pool with large-amplitude capacitance steps and amperometric spikes upon stimulation. The other pool (at the end of the trace) contains vesicles of much smaller size that do not release serotonin (non-SG pool). (C) Amperometric recording of SG fusion in an ∼20 μm (diameter) excised patch. (D) Expansion of C. In most cases, SGs are lost from membrane patches during the excision procedure. Occasionally, when SGs are preserved, fusion gives rise to capacitance steps of tens of fF, indicating that the diameter of the granules is close to micrometer range. A typical fusion event with fusion pore dilation is marked between two broken lines. A gradual increase of capacitance, transient increase of conductance, and the amperometric foot-signal is observed. Here and in all subsequent figures, numbers given between axis ticks indicate the tick interval.