MECHANISMS DETERMINING THE TIME-COURSE OF SECRETION IN NEUROENDOCRINECELLS

Transmitter release from chromaffin cells differs from that in synapse s in that it persists for a longer time after Ca2+ entry has stopped. This prolonged secretion is not due to a delay between vesicle fusion and transmitter release, nor to slow detection of released substance: step increases in capacitance due to single vesicle fusion precede the release detected by amperometry by only a few milliseconds. The persi stence of secretion after a depolarization is reduced by addition of m obile calcium buffer. This suggests that most of the delay is due to d iffusion of Ca2+ between channels and release sites, implying that Ca2 + channels and secretory vesicles are not colocalized in chromaffin ce lls, in contrast to presynaptic active zones.