Secretin is released from S cells of the duodenum in response to acid. Its primary action on the pancreas is to:

• A Stimulate ductal cells to secrete large volumes of bicarbonate-rich, enzyme-poor pancreatic juice via cAMP-mediated activation of CFTR chloride channels and Cl-/HCO3- exchangers ✓
• B Stimulate acinar cells to secrete digestive enzymes via PLC-IP3-DAG pathway and Ca2+ release
• C Inhibit pancreatic secretion to protect the duodenal mucosa from further acid exposure
• D Directly stimulate pancreatic beta cells to release insulin in anticipation of nutrient absorption

Explanation

Secretin is the primary stimulant for pancreatic ductal bicarbonate secretion. Binding to secretin receptors on ductal cells activates Gs-protein → adenylyl cyclase → elevated cAMP → PKA phosphorylation → activation of CFTR Cl- channels. Chloride-bicarbonate (Cl-/HCO3-) exchangers on the luminal membrane then exchange intracellular HCO3- for luminal Cl-, producing HCO3--rich fluid (up to 120 mEq/L of HCO3-) that neutralises duodenal acid and creates the optimal pH for pancreatic enzyme activity. CCK (not secretin) stimulates acinar cells (option B) via Ca2+ signalling. Secretin potentiates CCK's effect on enzyme secretion. Options C and D are incorrect.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.