Although the enzymes of the pancreatic juice are secreted entirely by the acini of the pancreatic glands, the other two important components of pancreatic juice, bicarbonate ions and water, are secreted mainly by the epithelial cells of the ductules and ducts that lead from the acini. When the pancreas is stimulated to secrete copious quantities of pancreatic juice, the bicarbonate ion concentration can rise to as high as 145 mEq/L, a value about five times that of bicarbonate ions in the plasma. This high concentration provides a large quantity of alkali in the pancreatic juice that serves to neutralize the hydrochloric acid emptied into the duodenum from the stomach.
The basic steps in the cellular mechanism for secreting sodium bicarbonate solution into the pancreatic ductules and ducts, shown in Figure 65-8, are as follows:
1. Carbon dioxide diffuses to the interior of the cell from the blood and, under the influence of carbonic anhydrase, combines with water to form carbonic acid (H2CO3). The carbonic acid dissociates into bicarbonate ions and hydrogen ions (HCO3 − and H+). Additional bicarbonate ions enter the cell through the basolateral membrane by co-transport with sodium ions (Na+). The bicarbonate ions are then exchanged for chloride ions (Cl−) by secondary active transport through the luminal border of the cell into the lumen of the duct. The chloride that enters the cell is recycled back into the lumen by special chloride channels.
2. The hydrogen ions formed by dissociation of carbonic acid inside the cell are exchanged for sodium ions through the basolateral membrane of the cell by secondary active transport. Sodium ions also enter the cell by co-transport with bicarbonate across the basolateral membrane. Sodium ions are then trans ported across the luminal border into the pancreatic duct lumen. The negative voltage of the lumen also pulls the positively charged sodium ions across the tight junctions between the cells.
3. The overall movement of sodium and bicarbonate ions from the blood into the duct lumen creates an osmotic pressure gradient that causes osmosis of water also into the pancreatic duct, thus forming an almost completely isosmotic bicarbonate solution.
