A secretagogue is a substance that causes another substance to be secreted. One example is gastrin, which stimulates the H/K ATPase in the parietal cells (increased gastric acid production by the stomach). Pentagastrin, a synthetic gastrin, histamine, and acetylcholine are also gastric secretagogues.
Sulfonylureas are insulin secretagogues, triggering insulin release by direct action on the KATP channel of the pancreatic beta cells. Blockage of this channel leads to depolarization and secretion of vesicles.
Angiotensin II is a secretagogue for aldosterone from the adrenal gland.
References
Secretagogue Wikipedia(Text) CC BY-SA