The effects of vesicular volume on secretion through the fusion pore in exocytotic release from PC12 cells.

Many spikes in amperometric records of exocytosis events initially exhibit a prespike feature, or foot, which represents a steady-state flux of neurotransmitter through a stable fusion pore spanning both the vesicle and plasma membranes and connecting the vesicle lumen to the extracellular fluid. Here, we present the first evidence indicating that vesicular volume before secretion is strongly correlated with the characteristics of amperometric foot events. L-3,4-dihydroxyphenylalanine and reserpine have been used to increase and decrease, respectively, the volume of single pheochromocytoma cell vesicles. Amperometry and transmission electron microscopy have been used to determine that as vesicle size is decreased the frequency with which foot events are observed increases, the amount and duration of neurotransmitter released in the foot portion of the event decreases, and vesicles release a greater percentage of their total contents in the foot portion of the event. This previously unidentified correlation provides new insight into how vesicle volume can modulate the activity of the exocytotic fusion pore.

Measurement of the dynamics of exocytosis and vesicle retrieval at cell populations using a quartz crystal microbalance.

The quartz crystal microbalance-dissipation technique (QCM-D) is used in two different measurement strategies to monitor the mass change and rigidity of populations of excitable cells during exocytosis and subsequent retrieval of dense-core vesicles. Two cell lines, NG 108-15 and PC 12, were grown to confluence on piezoelectric quartz crystals and were examined separately to demonstrate differences in release and retrieval with cells of different morphology, size, and number of dense-core vesicles. Stimulating the cells to exocytosis with media containing an elevated potassium concentration resulted in an increase in the frequency response corresponding to loss of mass from the cells owing to release of vesicles. In Ca2+-free media, the response was completely abolished. The amplitude and peak area in the frequency response corresponding to mass change with stimulated release was larger for PC 12 cells than for NG 108-15 cells, whereas the initial rate constants for the frequency responses were similar. The data suggest (1) that a greater number and larger size of vesicles in PC 12 cells results in a greater amount of release from these cells vs NG 108-15 cells, (2) the recycling of vesicles utilizes similar fusion/retrieval mechanisms in both cell types, (3) that the control of excess retrieval might be related to the number and size of released vesicles, and (4) that measured retrieval has a rapid onset, masking exocytosis and implying a rapid retrieval mechanism in the early stages of release. These results demonstrate that measurements of complex dynamic processes relating to dense-core vesicle release and retrieval can be simultaneously accomplished using the QCM-D technique.