Cholecystokinin

Overview

Cholecystokinin (CCK) is a gastrointestinal peptide hormone first isolated from porcine intestinal tissue in 1928. It is produced by enteroendocrine I-cells distributed throughout the duodenal and jejunal mucosa. CCK exists in multiple molecular forms, with the 33-amino acid variant (CCK-33) being one of the most characterized circulating forms. The biologically active core resides in the C-terminal octapeptide (CCK-8), which shares structural homology with gastrin.

Structure and Receptors

The mature CCK-33 peptide has a molecular weight of approximately 2196 Da. Post-translational processing of prepro-CCK yields several bioactive fragments, including CCK-58, CCK-33, CCK-22, and CCK-8. A conserved tyrosine residue undergoes sulfation in approximately 70% of circulating CCK molecules, a modification essential for full biological potency at the CCK-A (CCK1) receptor.

Two G-protein-coupled receptor subtypes mediate CCK signaling:

• CCK-A (CCK1) receptors: Found predominantly on gallbladder smooth muscle, pancreatic acinar cells, pyloric sphincter, and vagal afferent neurons. They mediate the classic digestive actions of CCK.
• CCK-B (CCK2) receptors: Predominantly expressed in the central nervous system, gastric ECL cells, and certain peripheral tissues. They bind CCK and gastrin with comparable affinity.

Physiological Functions

CCK performs several critical roles in gastrointestinal physiology. Upon nutrient entry into the duodenum, CCK stimulates pancreatic enzyme secretion and gallbladder contraction while relaxing the sphincter of Oddi, facilitating bile flow into the intestinal lumen. It simultaneously inhibits gastric emptying, slowing the transit of chyme to optimize enzymatic digestion.

Beyond digestion, CCK functions as a potent satiety signal. Vagal afferent fibers expressing CCK-A receptors transmit satiety information to the nucleus tractus solitarius in the brainstem, reducing meal size and duration. Centrally expressed CCK-B receptors also modulate anxiety, memory, and pain perception.

Clinical Significance

Dysregulation of CCK signaling has been implicated in obesity, panic disorder, and irritable bowel syndrome. CCK-A receptor antagonists have been investigated as potential anti-obesity agents, though clinical outcomes have been mixed. Mutations in the CCK gene or its receptor are rare but associated with gallbladder hypomotility and impaired satiety responses.