Pancreatic exocrine secretions and its control Flashcards
What do the secretions of the pancreas join with and where?
→The secretions from the pancreas join with the secretions from the gallbladder
→via the common bile duct
Where are the pancreatic secretions emptied?
→emptied into the duodenum via the Sphincter of Oddi
What are the functional units of the pancreas?
→ Acinar cells
What do acinar cells do?
→ Release enzymes
What are acinar cells connected to?
→Duct that secretes bicarbonate
What do the acini form?
→Acini form sacs that connect to the ductal tree and empty into the duodenum
What are intercalated ducts made from?
→ squamous epithelium
What are interlobular ducts made from?
→ cuboidal or low columnar epithelium
What are interlobular ducts made from?
→columnar epithelium/goblet cells
What do the ductal cells release?
→bicarbonate rich secretions that are identical to salivary secretions
→except that there is a lot more bicarbonate in it.
→This neutralizes acid.
Where do the bicarbonate secretions come from?
→ Ductal and acinar cells
What control are bicarbonate secretions under?
→ Secretin
What are the enzyme secretions of the acinar cells under?
→CCK
What is the pathway that enzymes take?
Acini → microvilli→ intercalated duct → intralobular duct → interlobular duct → pancreatic duct
How much fluid does the pancreas secrete per day?
→1.5L
What are the pancreatic secretions made from?
→Sodium and HCO3- rich juice, albumin, globulin and digestive enzymes
What do acinar cells secrete?
→digestive enzymes to break down carbohydrates, fats, proteins and nucleic acids
Why are enzymes secreted as zymogens?
→prevent autodigestion
Where does enzyme activation occur?
→in the duodenum
What are the anions that are secreted in pancreatic juice?
→HCO3-,
→Cl-
→SO42-
→HPO42-
What are the cations that are secreted in pancreatic juice?
→Na+
→K+,
→Ca2+
→Mg2+
What do the HCO3- secretions occur?
→ Secretions from the gallbladder help to neutralize gastric acid
How is the composition of the juice varied as it moves through the ducts?
→The composition of the juice is modified as it travels through the duct : epithelial cells actively exchange Cl-/HCO3-
How does H2CO3 form?
→H+ is actively eliminated by Na+/H+ exchanger
→H+ is exchanged for K+ driven by Na+/K+ ATPase
→H+ neutralizes HCO3- (H2CO3 is formed)
→More CO2 and HCO3- is produced in the blood
→CO2 diffuses in and forms H2CO3 with H2O
What is the rate of pancreatic juice production proportional to?
→ [HCO3-]pancreatic juice and its rate of production∝ [HCO3-]blood
What happens to Cl- and amylase after you infuse secretin?
→There is reciprocal change in the concentrations of Cl- and HCO3- after the infusion of secretin
Why does Cl- and amylase decrease after infusing secretin?
→The amount of chloride and amylase decreases
→ there are more increased watery secretions so the other components decrease.
What are the 3 major types of enzymes?
→Proteolytic enzymes (trypsin, chymotrypsin, carboxypeptidase)
→Amylase
→Lipase
What is the activator and substrate of trypsin?
→A : Secreted as trypsinogen - activated by enterokinase/enteropeptidase and by trypsin
→S: Proteins and polypeptides
What is the substrate of pancreatic lipase?
→triglycerides
What is the activator and substrate of Pancreatic α amylase?
→ A: Cl-
→S: Starch
What is the substrate of ribonuclease?
→ RNA
What is the substrate of deoxyribonuclease?
→ DNA
What is the activator and substrate of elastase?
→ A: Trypsin
→ S: Elastin
What is the activator and substrate of Phospholipase A2?
→ A: Trypsin
→ S: Phospholipids (lecithin)
What is the activator and substrate of carboxypeptidase A+B?
→ A: Trypsin
→ S: Proteins and polypeptides
What is the activator and substrate of colipase?
→ A: trypsin
→ B: fat droplets
How are proteolytic enzymes secreted and why?
→Enzymes secreted as proenzymes
→ Prevent autodigestion
Where is enterokinase and when is it secreted?
→ Brush border of duodenum
→ Secreted in response to CCK
What does enterokinase do?
→ trypsinogen to trypsin
What is meant by an autocatalytic reaction?
→ Once one trypsin is formed it can go on and convert more trypsinogens to trypsin
What are inhibitors of autodigestion?
→Kazal inhibitor - enzyme Y (chymotrypsin C, caldecrin)
→Intracellular pH of zymogen granule = acidic (keeps zymogen inactive)
What happens in acute pancreatitis?
→Trypsin activates phospholipase A2 in the pancreatic duct
→Increase in pancreatic alpha amylase - diagnostic test
What does activation of PLA2 in the pancreatic duct do?
→converts Lecithin → Isolecithin (PLA2)
→Isolecithin → disruption of pancreatic tissue, membrane damage and necrosis
What reaction does amylase catalyze?
→polysaccharides → disaccharides
What reaction does lipase catalyze?
→triglycerides → fatty acids and glycerol
What happens in pancreatic insufficiency?
→poor digestion of lipids and their malabsorption which causes steatorrhoea
What is the percentage of the enzymes that reach the small intestine?
→Amylase - 75%
→Trypsin - 20%
→Lipase - 1%
What membranes do chymotrypsinogen and amylase cross in the pancreas and what does this suggest?
→Chymotrypsinogen and amylase cross the basolateral membrane of the pancreas
→Suggests bidirectional permeability of the basolateral membrane to digestive enzymes
How are pancreatic secretions controlled?
→ Neuroendocrine secretions
What happens to pancreatic secretions with parasympathetic stimulation?
→enhances the rate of secretion of enzymes and aqueous components of pancreatic juice
What happens to pancreatic secretions with sympathetic stimulation?
→inhibits secretion
What do secretin and CCK do?
→ stimulate secretion of pancreatic fluid ( enzyme and bicarbonate rich secretions)
What happens during the cephalic phase?
→Vagal (ACh and VIP) stimulation of gastrin release from the antrum
→ some protein-rich pancreatic juice
What happens during the gastric phase?
→Distension (vagal reflex on fundus or antrum) amino acid and peptide ( in antrum) stimulated gastrin secretion → release of enzyme rich pancreatic juice
What happens as the result of a vagotomy?
Vagotomy → 50% decrease in response to acidic chyme
What happens in the intestinal phase?
→Secretin and CCK
→Acidic chyme in the duodenum and jejunum induces secretion of pancreatic juice with bicarbonate in it to neutralize the acid.
Where is secretin produced?
→from mucosa of duodenal and jejunum
What does secretin induce?
→Induces pancreatic duct cells to secrete HCO3- rich pancreatic juice but decreased enzyme content
What does secretin stimulate in the liver?
→ the production of bile by the liver
Where is CCK secreted from and in response to what?
→ from duodenal and jejunum in response to fatty acids
What does CCK stimulate?
→stimulates pancreatic acinar cells to synthesize and release enzyme rich pancreatic juice
→stimulates the secretion of concentrated bile from gallbladder for fat absorption
What effect does CCK have on the gallbladder?
→Contracts the gallbladder and relaxes the sphincter of Oddi → release of bile into the duodenum
What does CCK potentiate?
→Potentiates the effect of secretin which is a weak agonist of acinar cells
What is a consequence of cystic fibrosis?
→The pancreatic ducts are blocked and the intestines cannot absorb nutrients fully
→Steatorrhea
