Aqueous Component of Pancreatic Juice
The columnar epithelial cells of the pancreatic ducts secrete most of the aqueous component of the pancreatic juice. Na+ and K+ concentrations are similar to those in the plasma. HCO3- increases threefold above basal levels during high secretory rates. HCO3- and Cl- concentrations usually vary reciprocally. After initial secretion the aqueous component contains a high concentration of HCO3- and is slightly hypertonic. Water equilibrates with the aqueous component rich in HCO3-, causing it to be isotonic, and some Cl- is exchanged for HCO3- at rest. Most of the aqueous component is synthesized by cells of the intercalated ducts. When secretin strongly stimulates the aqueous component, it is secreted mostly by extralobular ducts. CCK stimulates secretion of small quantities of isotonic aqueous component, but the secretin-stimulated component is of greater quantity and has a greater concentration of bicarbonate.
Note that the data presented in the tables above are for the rat; similar measurements are not available for humans.