Acinar Cells Secrete EnzymesfWhereas Centroacinar Cells and Duct Cells Secrete a Sodium Bicarbonate Solution
The exocrine pancreas is a typical acinar gland in which the end-pieces, or acini, are connected by an arborizing system of ducts; thus the gland conceptually resembles a bunch of grapes.
Its general structure resembles the salivary gland, as illustrated in Figure 29-1. The cells of the acini contain a generous portion of rough endoplasmic reticulum, on which large amounts of secretory proteins, the digestive enzymes, are synthesized. Each pancreatic acinar cell can produce all the more than 10 different enzymes secreted by the pancreas. Chapter 30 discusses the functions of the major digestive enzymes of the pancreas (see Table 30-1). Protein-digesting enzymes, which are potentially harmful to the pancreatic cells, are synthesized as zymogens in the same manner as pepsinogen synthesis in the gastric glands. After synthesis, the enzymes and proenzymes are stored in vesicles, or zymogen granules, near the cellular apex. When the cells are stimulated, the zymogen granules fuse with the plasma membrane and release their contents into the lumen of the gland and eventually into the duodenal lumen, where they are converted to the activated form of the enzyme.Specialized cells near the junction of the acini and ducts are called centroacinar cells. The function of these cells, and to a lesser degree the duct epithelial cells, is to modify the electrolyte composition of the fluid secreted by the acinar cells. The electrolyte makeup of the acinar secretion initially resembles extracellular fluid, having a relatively high concentration of sodium and chloride. The centroacinar cells have on their luminal surface a chloride-bicarbonate exchange protein that transports bicarbonate out of the cell in exchange for chloride, thus greatly enriching the bicarbonate concentration of pancreatic fluid. This exchange protein does not require an energy input, and its action is driven by a high intracellular concentration ofbicarbonate. 'Γhis system is facilitated by electrolyte transport proteins on the basolateral surface of the cell (see Chapter 30).
These transport proteins consist of Na+,IC-ATPase, an Na+-HCO3- co-transporter, an H’-Na1 exchanger, and an Hf-ATPase. The Na+-HCO3' co-transporter, in combination with carbonic anhydrase, generates bicarbonate within the cell, thus driving the chloride-bicarbonate exchange at the luminal membrane. The H* remaining from the dissociation of carbonic acid is removed from the cell at the basolateral membrane by Na4 - Hf exchange and by the H ’ -ATPase pump. The net result is that pancreatic fluid is a bicarbonate-rich, alkaline fluid that neutralizes the acid ingesta arriving in the duodenum from the stomach. In addition, the H+ ions transported into the basal interstitial fluid of the pancreas are absorbed into the blood, balancing the ‘"alkaline tide” that was created by gastric acid secretion.In broad generality, the ion transport activities of the pancreatic duct cells are similar, but directionally opposite, to those of the parietal cells, as illustrated in Figure 29-4. The overall effect of the two secretory cell types is to mix hydrochloric acid with ingesta in the stomach and to neutralize the acid with sodium bicarbonate in the duodenum.
The ducts of the pancreatic lobules coalesce in an arborizing pattern to form either one or two main pancreatic ducts, depending on the species. The pancreatic duct or ducts may empty directly into the duodenum or, as in sheep, into the common bile duct. In the latter case, the pancreatic secretions enter the intestinal lumen along with bile.