Cholecystokinin (CCK) is released from I cells in the duodenum in response to luminal fat and protein. CCK stimulates gallbladder contraction and relaxes the sphincter of Oddi primarily through which mechanism?

• A CCK acts on CCK-B receptors on parietal cells to inhibit gastric acid, indirectly relaxing the sphincter
• B CCK directly activates secretin release, which inhibits sphincter of Oddi via cAMP
• C CCK activates delta cells in the pancreatic islets to release somatostatin, which relaxes the sphincter
• D CCK acts on CCK-A receptors on gallbladder smooth muscle and via vagal afferent-mediated reflex relaxation of the sphincter of Oddi ✓

Explanation

CCK stimulates gallbladder contraction directly via CCK-A receptors on gallbladder smooth muscle and indirectly through activation of vagal afferents (capsaicin-sensitive C fibers), which relay to the brainstem and mediate a vagovagal reflex causing sphincter of Oddi relaxation. CCK-B receptors are primarily gastric (same as gastrin receptors), mediating acid secretion. The coordinated contraction of the gallbladder with relaxation of the sphincter of Oddi is essential for efficient delivery of bile into the duodenum; failure of this coordination (e.g., achalasia of the sphincter) can cause biliary colic.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.