Fig. 3. A model depicting the mechanism of ductal fluid and HCO₃⁻ secretion.

Shown are the major transporters in the basolateral and luminal membranes of duct cells. In the pancreas ductal secretion is driven by HCO₃⁻ secretion. The major HCO₃⁻ loading mechanism is the basolateral Na⁺-HCO₃⁻ cotransporter NBCe1-B. Luminal HCO₃⁻ secretion is mediated by the CFTR-Slc26a6 complex. The duct also expresses the HCO₃⁻ salvage mechanisms NHE3 and NBCn1-A. The salivary glands, but not the pancreatic, duct also expresses ENaC at the luminal membrane. The functioning of the transporters in ductal fluid and HCO₃⁻ secretion is illustrated in Fig. 6.