Exocrine functions of the pancreas

Each day the pancreas secretes approximately 1 L of alkaline isosmotic pancreatic juice that originates from the pancreatic acinar cells and pancreatic ducts. The colorless, bicarbonate-rich, and protein-rich pancreatic juice plays key roles in duodenal alkalinization and food digestion. The acinar cells secrete the enzymes required for the digestion of the three main food types: amylase for carbohydrate (starch) digestion, proteases (e.g., trypsin) for protein digestion, and lipases for fat digestion. The acinar cells are pyramidal in shape with the apices facing the lumen of the acinus, where the enzyme-containing zymogen granules fuse with the apical cell membrane for release. Acinar cells, unlike the endocrine cells of the pancreas, are not specialized and produce all three types of pancreatic enzymes from the same cell type.

Exocrine functions of the pancreas

Amylase is secreted in its active form and hydrolyzes starch and glycogen to the simple sugars of dextrins and maltose; maltose is then metabolized to glucose by intestinal maltase. 

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The proteolytic enzymes are secreted as proenzymes and must be activated in the duodenum. For example, trypsinogen is converted in the duodenum to trypsin by enterokinase. Intrapancreatic conversion of trypsinogen is prevented by a pancreatic secretory trypsin inhibitor, a step that prevents pancreatic autodigestion. Another example of a proteolytic enzyme that is secreted as a proenzyme is chymotrypsinogen, which is activated in the duodenum to chymotrypsin. The actions of trypsin, chymotrypsin, and other proteolytic enzymes (e.g., elastase, carboxypeptidase A and B, intestinal peptidases) cleave bonds between amino acids in peptide chains, yielding smaller peptides that stimulate the intestinal endocrine cells to release cholecystokinin and secretin, which further stimulate the pancreas to release more digestive enzymes and bicarbonate. The amino acids and dipeptides are actively transported into enterocytes.

Pancreatic lipase is secreted in its active form, and it hydrolyzes triglycerides to fatty acids and glycerol. Phospholipase A cleaves the fatty acid off lecithin to form lysolecithin. Phospholipase B cleaves the fatty acid off lysolecithin to form glycerol phosphatidylcholine. Phospholipase A2 is activated by trypsin in the duodenum, where it serves to hydrolyze phospholipids. Hydrolyzed fat is organized in micelles and is transported into the enterocytes.

There are approximately 40 acinar cells per acinus. The acinar cells near the center of the acinus are termed centroacinar cells. Centroacinar cells and pancreatic duct cells secrete electrolytes, bicarbonate, and water into the pancreatic juice. At rest, secretion occurs at a low basal rate (-2% of maximal). The pancreas’ response to a meal occurs in three phases. The cephalic phase—in response to the smell, sight, and taste of food—accounts for 10% of meal-stimulated pancreatic secretion and is mediated by peripherally released acetylcholine. The gastric phase—in response to gastric distension from food—accounts for 10% of meal-stimulated pancreatic secretion. With gastric distension, gastrin is released, and vagal afferents are stimulated to directly mediate pancreatic enzyme secretion and enhance gastric acid secretion and duodenal acidification. The intestinal phase accounts for 80% of meal-stimulated pancreatic secretion. The duodenal hormone secretin is released in response to acid chyme (pH <3.0) and bile passing into the duodenum. Secretin then stimulates increased production of centroacinar cell bicarbonate to buffer the acidic chyme. Cholecystokinin is also released in response to protein and fat in the proximal small intestine, and it enhances the centroacinar cell response to secretin.

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