Does exocrine portion of pancreas secrete inactive pancreatic enzymes?
There are two types of exocrine secretions produced by the pancreas. Acinar cells produce digestive enzymes: amylase, lipase, and peptidases. Pancreatic peptidases are produced as inactive zymogens that are only activated after they reach the duodenum (see webpage on Zymogens).
The other major secretion is bicarbonate (HCO3-), which is produced by the duct cells. The bicarbonate acts to neutralize acidic chyme coming from the stomach. As well, fluid that is produced by duct cells flushes enzymes and zymogens out into the large pancreatic duct.
At right is a picture from one of our histology slides. The majority of exocrine tissue in the pancreas consists of acinar cells, which are the dark clusters visible throughout. The pointer points to a small intercalated duct. These ducts receive the acinar cell secretions, and secrete bicarbonate and fluid. They connect to larger ducts that eventually lead to the large pancreatic duct that has its outlet at the duodenal papilla.
The figure at left summarizes the regulation of pancreatic secretion. A small amount of pancreatic secretion occurs due to neural inputs that are triggered by cephalic phase and gastric phase stimuli. The majority of pancreatic secretion arises from intestinal phase stimuli (when chyme reaches the duodenum). The hormones secretin and cholecystokinin (CCK) are released by endocrine cells that are located in the duodenal epithelium.
As shown in the figure, secretin release is triggered by H+ ions (low pH). Secretin then travels via the circulation to stimulate bicarbonate secretion by duct cells. CCK release is triggered by digestive products (fats and peptides). CCK then travels via the circulation to stimulate secretion by acinar cells.