The exocytotic event in chromaffin cells revealed by patch amperometry

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Abstract
In mast cells and granulocytes, exocytosis starts with the formation of a fusion pore1,2,3. It has been suggested that neurotransmitters may be released through such a narrow pore without full fusion4,5. However, owing to the small size of the secretory vesicles containing neurotransmitter, the properties of the fusion pore formed during Ca2+-dependent exocytosis and its role in transmitter release are still unknown. Here we investigate exocytosis of individual chromaffin granules by using cell-attached capacitance measurements3,6 combined with electrochemical detection of catecholamines7,8, achieved by inserting a carbon-fibre electrode into the patch pipette. This allows the simultaneous determination of the opening of individual fusion pores and of the kinetics of catecholamine release from the same vesicle. We found that the fusion-pore diameter stays at 8 which precedes the amperometric spike. Occasionally, the narrow pore forms only transiently and does not expand, allowing complete transmitter release without full fusion of the vesicle with the plasma membrane.