Secretion of proteins may be constitutive or regulated. Constitutive secretion replenishes membrane lipids and proteins as part of normal cellular physiology. Cells may also release molecules to remodel their immediate environment, such as the extracellular matrix. Regulated secretion occurs in response to an external stimulus.
Secretory vesicles are held near the membrane and released in response to a specific stimulus.
In nerve terminals, action potentials cause a rise in intracellular [Ca2+ ], which activates CaM kinase II to promote release of synaptic vesicles from the cytoskeleton and their fusion with the membrane. Secretion in many other cells is also controlled by intracellular [Ca2+ ].
In many cases the basal expression of surface molecules is controlled by constitutive production and secretion, but release of molecules from intracellular stores can produce a rapid increase in surface expression if required.