PANCREATIC SECRETION DR. AMEL EASSAWI DR. SHAIKH MUJEEB AHMED 1

OBJECTIVES The student should be able to: Discuss pancreatic exocrine secretion and composition. Enlist pancreatic enzymes and their importance Describe Steatorrhea. Explain the mechanism of pancreatic juice secretion. Describe the regulation of pancreatic exocrine secretion hormonal and neural. 2

PANCREATIC SECRETION When food enter the small intestine [duodenum], it is mixed with pancreatic secretion and bile [pancreas and liver are accessory digestive organs]. Pancreas is elongated gland lies behind and below the stomach. Has exocrine and endocrine secretion. Exocrine Pancreas secretes pancreatic juice. Pancreas has Acini and ducts. - Pancreatic Acinar cells secrete – digestive enzyme. - Pancreatic duct – Watery [aqueous] alkaline fluid rich in NaHCO 3 3

PANCREATIC SECRETION Composition of Pancreatic Juice Pancreatic secretion per day is 1 – 2 liters. pH – approximately 8.0 It has - Water - Cations – Na, K, Ca, Mg - Anions – HCO 3, Cl, SO 4, HPO 4 - Digestive enzymes 4

5

6

PANCREATIC SECRETION 1. Pancreatic Enzymes The pancreatic enzymes are stored in zymogen granules after they are produced. They are released by exocytosis as needed. Pancreatic enzymes are important because they can completely digest food, in absence of all other digestive secretions. Pancreas Acinar Cell Secrete Three Types of Pancreatic Enzymes: i. Proteolytic enzymes for protein digestion. ii. Pancreatic amylase for carbohydrate digestion. iii. Pancreatic Lipase for fat digestion. 7

PANCREATIC SECRETION (i). Pancreatic Proteolytic Enzymes Major Proteolytic Enzymes are: - Trypsinogen - Chymotrypsinogen - Procarboxypeptidase They are secreted in inactive form. Proteolytic Enzymes [Trypsin, Chymotrypsinogen, Carboxypeptidase] attack different peptide linkages. End result is formation of small peptide chains and amino acids. 8

PANCREATIC SECRETION Activation of Pancreatic Proteolytic Enzymes When Trypsinogen is secreted into the duodenal lumen, it is activated to its active form Trypsin by Enterokinase [enteropeptidase], an enzyme which is embedded in the mucus membrane of duodenal mucosa. Trypsin once formed activates more Trypsinogen [autocatalytic]. Chymotrypsinogen and Procarboxypeptidase are converted to active form by Trypsin in duodenal lumen. IMPORTANT – We need initially Enterokinase to form trypsin. Once trypsin is formed it carries out other processes. 9

10

PANCREATIC PROTEOLYTIC ENZYMES 11

PANCREATIC SECRETION (ii). Pancreatic Amylase for Carbohydrate Digestion Pancreatic amylase causes conversion of polysaccharides into disaccharide maltose. Pancreatic amylase is secreted in the Active form as it is not the risk for auto digestion of pancreas. (iii). Pancreatic Lipase for Fat Digestion Pancreatic lipase is very important. Why? Because it is the only enzyme secreted throughout the digestive system that can digest fat. NOTE : In human, insignificant amount of lipase are secreted in the saliva and Gastric Juice called Lingual Lipase and Gastric Lipase. 12

PANCREATIC SECRETION (iii). Pancreatic Lipase for Fat Digestion Pancreatic lipase hydrolysis dietary triglycerides into Monoglyceride and Free fatty acids, which are absorbed in small intestine. Pancreatic lipase is secreted in Active form as it is not the risk for pancreatic auto-digestion. 13

PANCREATIC SECRETION Pancreatic Aqueous Alkaline Fluid As highly acidic Gastric contents are emptied into the duodenal lumen, this acidic Chyme must be neutralized quickly in the duodenal lumen. Why? - To allow the functioning of pancreatic enzymes [they work in neutral or slightly alkaline medium]. NaHCO 3 in pancreatic fluid neutralizes the acidic Chyme. - To prevent damage to duodenal mucosa. 14

REGULATION OR CONTROL OF PANCREATIC EXOCRINE SECRETION Hormonal and Neural Mechanism: 1.Mainly by hormonal mechanism: Secretin and Cholecystokinin [CCK]. 2. Parasympathetic stimulation: little contribution during cephalic phase. When Chyme enters the duodenum, Two major hormones secretin and CCK [Entrogastrones] are released in response to Chyme. 15

16

REGULATION OR CONTROL OF PANCREATIC EXOCRINE SECRETION Role of secretin and CCK. SECRETIN Primary stimulus for secretion of secretin is acid in the duodenum. Secretin is carried by blood to the pancreas where it stimulates the duct cells to increase NaHCO 3 rich watery secretion in the duodenum. NaHCO 3 neutralizes the acid content of Chyme. 17

REGULATION OR CONTROL OF PANCREATIC EXOCRINE SECRETION CHOLECYSTOKININ [CCK] CCK is released from duodenal mucosa. Main stimulus for release of CCK is Fat, to a lesser extent protein. CCK is transported by blood to pancreas Acinar cell to increase digestive enzyme secretion. These pancreatic enzymes help to digest fat, protein and carbohydrate. All three types of enzymes [for protein, fat, and carbohydrate] are packed in Zymogen granules together, therefore, all are released together by exocytosis. 18

CLINICAL APPLICATION 1. Pancreatic Insufficiency When pancreatic enzymes are deficient, digestion of food is incomplete. As pancreatic lipase is the only significant source for fat digestion, its deficiency results in Maldigestion of fats, it is called STEATORRHEA [increased undigested fat in feces. Up to 60 – 70% of fat taken maybe excreted in feces. Digestion of protein and carbohydrate is impaired to a lesser degree because salivary, gastric and small intestine enzymes contribute to their digestion. 19

REFERENCES Human Physiology by Lauralee Sherwood, seventh edition. Text book Physiology by Guyton &Hall,11 th edition. Text book of Physiology by Linda S. Contanzo, third edition. 20