Secretin from S cells of the duodenum is the primary stimulant of pancreatic HCO3- secretion. The mechanism by which ductal cells of the pancreas secrete HCO3- involves which transport system?
- A H+/K+-ATPase on the apical membrane directly secreting H+ while retaining HCO3-
- B AQP8-mediated HCO3- transport from cytoplasm to lumen, driven by carbonic anhydrase activity
- C CFTR (Cl- channel) on the apical membrane working in tandem with SLC26A6 (Cl-/HCO3- exchanger) to secrete HCO3-; driven by basolateral NBC1 (Na+-HCO3- cotransporter) ✓
- D NHE3 (sodium-hydrogen exchanger 3) in reverse mode, importing Na+ and exporting HCO3- into the duct lumen
Explanation
Pancreatic ductal HCO3- secretion is a multitransporter process. Basolateral NBC1 imports HCO3- from blood. On the apical membrane, CFTR (the ABCC7 Cl- channel, mutated in cystic fibrosis) opens under cAMP stimulation (from secretin), providing luminal Cl- which is then exchanged for HCO3- via SLC26A6 (anion exchanger). The net result is HCO3- secretion into the duct lumen. Dysfunction of CFTR in cystic fibrosis impairs this mechanism, leading to viscous pancreatic secretions, ductal obstruction, and eventual exocrine pancreatic insufficiency. Carbonic anhydrase II provides intracellular CO2/HCO3- interconversion to replenish HCO3- stores.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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