Figure 1. Synaptic and endocrine Ca²⁺-triggered exocytosis.

At a synapse (left), neurotransmitters are packaged into small synaptic vesicles, which are docked at the active zone adjacent to voltage-dependent Ca²⁺-channels. A presynatpic action potential (insert) gates Ca²⁺-influx into the terminal, thereby triggering vesicle exocytosis. The released transmitters produce a postsynaptic current (insert) which can be recorded by whole-cell patch clamping. In endocrine cells (right), hormones are packaged into LDCVs, which are generally not docked. Upon sustained increases in cytosolic Ca²⁺, as obtained during stimulation or Ca²⁺-uncaging (insert), exocytosis is triggered with a significantly slower time course than at a synapse, as measured by amperometry or capacintance (Cm; insert). Note that Ca²⁺-channel and release sites are not tightly coupled in endocrine cells. ER, endoplasmic reticulum; M, mitochondrion. Traces are shown purely for demonstration purposes.