Kinetics of exocytosis in adrenal chromaffin cells.

Upon repetitive or maintained stimulation, chromaffin cells secrete catecholamines initially at a very high rate which then relaxes with multiple kinetic components. The complex kinetics are often modeled as resulting from the successive depletion of several functional pools of secretory granules which may reflect specific protein-mediated steps in granule maturation. The fastest component represents granules fully primed for exocytosis. This 'readily releasable pool' may, under some circumstances, consist of only about a dozen granules which can be released within tens of milliseconds. Modulating the size of this pool may be an important way for cells to regulate secretion.