Exocytotic

Exocytotic refers to the processes and machinery involved in exocytosis, the cellular mechanism by which substances are expelled from the cell. During exocytotic secretion, vesicles derived from the Golgi apparatus or endosomes fuse with the plasma membrane to release their lumenal contents into the extracellular space. Exocytotic activity contrasts with endocytosis, which internalizes material from the outside.

Mechanism: Vesicles approach the plasma membrane, dock, and become primed for fusion. Fusion is triggered by a rise in intracellular calcium and is mediated by SNARE proteins: v-SNAREs on the vesicle (such as synaptobrevin/VAMP) pairing with t-SNAREs on the target membrane (such as syntaxin and SNAP-25). Accessory proteins, including synaptotagmin as a calcium sensor and Rab family GTPases, regulate trafficking and docking. The fusion merges the vesicle and plasma membranes, releasing contents and adding membrane to the surface.

Exocytosis operates in two main modes. Constitutive exocytosis occurs continuously to maintain membrane and extracellular matrix turnover, while regulated exocytosis occurs in response to specific stimuli, enabling rapid release of neurotransmitters, hormones, and enzymes in neurons and endocrine cells.

Physiological roles include neurotransmission, hormone secretion (e.g., insulin), delivery of enzymes to the extracellular space, and the release of immune mediators. Materials can be stored in secretory granules before release; membrane components are replenished after fusion.

Clinical relevance: Defects in exocytotic pathways, SNARE function, or calcium signaling can contribute to disease, including impaired insulin secretion in diabetes and disrupted neurotransmitter release in certain neurological disorders. Toxins such as botulinum and tetanus target SNAREs, inhibiting exocytosis.