Stimuli that trigger vesicle fusion act by increasing intracellular Ca.
* Synaptic vesicles commit vesicle fusion by a nerve impulse reaching the synapse, activating voltage-dependent calcium channels that cause influx of Ca into the cell.
* In the endocrine system, many hormones are released by their releasing hormones binding to G protein coupled receptors coupled to the G alpha subunit, activating the IP3/DAG pathway to increase Ca. Examples of this mechanism include:
**Gonadotropin releasing hormone
**Thyrotropin releasing hormone
**Growth hormone releasing hormone (minor pathway - main one is cAMP dependent pathway)