Hormones and neurotransmitters release: four mechanisms of secretion.

Distinct mechanisms operate in different secreting systems, they are 1) free diffusion through the plasma membrane; 2) exocytosis resulting from fusion of a secretory granule with the plasma membrane; 3) fleeting release from a granule through a transient pore without full fusion; 4) release through a specialised plasmalemmal molecule such as the mediatophore. The latter mechanism is proposed to operate in rapid synapses in which the neurotransmitter is emitted as an abrupt chemical impulse of quantal composition. There, release is momentarily signalled in the plasma membrane by large intramembrane particles. Synaptic vesicles are also essential for regulation of this type of release. They fuse with the plasma membrane only late after activity and seem to be involved in calcium sequestration and extrusion.