Cholecystokinin (CCK)

Endogenous Hormone

Cholecystokinin (CCK) is a classical gut hormone secreted by I-cells located in the mucosa of the duodenum and jejunum. It is released in response to a meal, particularly the arrival of fats and proteins in the small intestine. CCK's name reflects its most historically recognized action: "chole-cysto-kinin" literally means "bile-sac mover," describing its ability to make the gallbladder contract. In addition to stimulating gallbladder emptying, CCK triggers the pancreas to secrete digestive enzymes and relaxes the sphincter of Oddi, coordinating the delivery of bile and pancreatic juice into the intestine to digest the incoming meal. CCK exists in several molecular forms of different lengths (such as CCK-8, CCK-33, and CCK-58) that share a common biologically active C-terminal region. Beyond digestion, CCK is one of the body's key satiety signals. Acting on the CCK1 receptor (formerly called CCK-A), CCK released during a meal communicates fullness to the brain, in part by signaling through vagal afferent nerves, and helps bring eating to an end. It also slows gastric emptying, which prolongs the sense of fullness. A second receptor, CCK2 (formerly CCK-B), is found in the brain and stomach and is closely related to the receptor for the hormone gastrin; through it, CCK participates in central nervous system functions and, together with gastrin, in the regulation of gastric acid. This dual role — as both a digestive coordinator and a satiety hormone — makes CCK a central node in the gut–brain axis. CCK itself is an endogenous hormone and is not sold as a drug, but its pharmacology is directly applied in clinical medicine through a synthetic analog. Sincalide, a synthetic form of the biologically active CCK-8 fragment, is FDA-approved and used to stimulate gallbladder contraction during imaging studies. In a hepatobiliary (HIDA) scan, sincalide is administered to provoke the gallbladder to empty so that its ejection fraction can be measured — a test used to evaluate suspected gallbladder dysfunction such as biliary dyskinesia. Sincalide can also be used to stimulate pancreatic secretion for diagnostic collection. In this way, the natural biology of CCK is harnessed diagnostically even though native CCK is not itself a marketed medicine. CCK is best understood within the network of digestive and satiety peptides. It works in concert with secretin, another duodenal hormone that stimulates bicarbonate-rich pancreatic and biliary secretion, and it is functionally and structurally intertwined with gastrin through their shared receptor family. For readers exploring how the gut senses food and signals fullness, CCK is one of the most important and well-characterized examples, and its clinical footprint via sincalide illustrates how understanding an endogenous hormone can yield a practical diagnostic tool.