Exocytosis is a general concept describing secretion of enzymes,
hormones and transmitters that are otherwise sequestered in intracellular
granules. Chemical evidence for this concept was first gathered from studies
on chromaffin cells in perfused adrenal glands, in which it was found that
granule contents, including both large protein and small molecules such as
adrenaline and ATP, were released together while the granule membrane was
retained in the cell. A number of exhaustive reviews of this early work have
been published and are summarized in Reference 1. The critical experiments
demonstrating the importance of extracellular calcium for exocytosis per se
were also first performed in this system (2,3), further indicating the
substantial service given by chromaffin cells to those interested in
secretory phenomena over the years.
