LECTURE 5 (Pancreatic secretion) Flashcards
What are the properties of the pancreas?
- Elongated gland
- Lies behind & below stomach and above the first loop of the duodenum
- Contains both exocrine and endocrine tissue
The exocrine pancreas secretes a pancreatic juice consisting of which components?
- Pancreatic enzymes actively secreted by ACINAR CELLS
- Aqueous alkaline solution actively secreted by DUCT CELLS
What are the pancreatic enzymes for digesting proteins?
- Trypsin
- Chymotrypsin
- Carboxypolypeptidase
What is the difference between Trypsin, Chymotrypsin and Carboxypolypeptidase?
Trypsin & Chymotrypsin = split whole and partially digested proteins into peptides of various sizes
[do not cause release of amino acids]
Carboxypolypeptidase = splits some peptides into individual amino acids
When first synthesised in the pancreatic cells, the __________ ___________ enzymes are in their enzymatically inactive forms trypsinogen, chymotrypsinogen and procarboxypolypeptidase
Proteolytic digestive
Which enzyme activates Trypsinogen?
Enteropeptidase/Enterokinase
Explanation: Secreted by intestinal mucosa when chyme comes in contact with the mucosa
Secretion of what prevents digestion of the pancreas?
Trypsin Inhibitor
What is the pancreatic enzyme for digesting carbohydrates?
Pancreatic amylase
Explanation: Hydrolyses starches, glycogen and other carbohydrates to form mostly disaccharides and a few trisaccharides
What are the main enzymes for fat digestion?
- Pancreatic lipase
[hydrolyse neutral fat into fatty acids and monoglycerides] - Cholesterol esterase
[hydrolysis of cholesterol esters] - Phospholipase
[splits fatty acids from phospholipids]
Explain why alkaline fluid secreted by pancreatic duct cells into the duodenum is necessary
Pancreatic enzymes function best in a neutral/slightly alkaline environment -> Acidic chyme must be neutralised in duodenal lumen -> Allows optimal functioning of pancreatic enzymes but also to prevent acid damage to DUODENAL MUCOSA
What are the two sources that HCO3- secreted into the duct lumen comes from?
- Within PANCREATIC DUCT CELL from CO2 that has either diffused in from PLASMA or been produced by CELLULAR METABOLISM
[CO2 combine with H2O to form HCO3 -> H+ enters plasma across basolateral border by Na-H anti porter -> acidifies venous blood that leaves pancreas] - Enters cell from plasma via Na-HCO3 symporter at the BASOLATERAL MEMBRANE
[the basolateral Na-K pump provides the driving energy for the secondary active transport mechanisms]
HCO3 enters the duct lumen by which mechanisms?
- Cl-HCO3 antiporters in the luminal membrane move HCO3 into the lumen in exchange for Cl2
- HCO3 enters lumen by diffusing through a cystic fibrosis transmembrane conductance regulator (CFTR) channel
How does Na get transported into the pancreatic lumen?
Na diffuses down an electrochemical gradient via PARACELLULAR TRANSPORT through the “leaky” tight junctions between the duct cells into the lumen
What happens when pancreatic enzymes are deficient?
- Digestion of food is incomplete
- Serious maldigestion and malabsorption of dietary fat
[since pancreas is only significant source of lipase]
What is the main clinical manifestation of pancreatic exocrine insufficiency?
Steatorrhea/Excessive undigested fat in faeces
Why is digestion of protein and carbohydrates impaired to a lesser degree?
Salivary, gastric and small-intestinal enzymes contribute to the digestion
What are the stimuli that cause pancreatic secretion?
- Acetylcholine
[released from parasympathetic vagus nerve endings and other cholinergic nerves in enteric nervous system] - Cholecystokinin
[secreted by duodenal and upper jejunal mucosa when food enters small intestine] - Secretin
[secreted by duodenal and jejunal mucosa when highly acidic food enters small intestine]
Describe the control of pancreatic aqueous NaHCO3 solution into duodenal lumen
1) Acid in duodenal lumen
2) Increase Secretin release from duodenal mucosa to PANCREATIC DUCT CELLS
[secretin carried by blood]
3) Increase secretion of aqueous NaHCO3 solution into duodenal lumen which neutralises acid
Describe the control of pancreatic digestive enzyme secretion
1) Fat and protein products in duodenal lumen
2) Increase CCK release from duodenal mucosa to PANCREATIC ACINAR CELLS
[CCK carried by blood]
3) Increase in secretion of pancreatic digestive enzymes into duodenal lumen
What are the different phases of Pancreatic secretion?
- Cephalic phase
[20% of total secretion of pancreatic enzymes] - Gastric phase
[5-10% of pancreatic enzyme secretion] - Intestinal phase
[pancreatic secretion becomes abundant]
What do the predominant and smaller endocrine parts consist of?
- Predominant = ACINI
[secretory cells that connect to ducts that empty into duodenum] - Smaller - ISLETS OF LANGERHANS
[isolated islands of endocrine tissues dispersed throughout pancreas and secrete INSULIN and GLUCAGON]
Where are pancreatic enzymes stored?
Within ZYMOGEN GRANULES (secretory vesicles) then are released by exocytosis
What are the three types of pancreatic enzymes that acinar cells release?
- proteolytic enzymes
- pancreatic amylase
- pancreatic lipase
What is important to remember about the proportion of enzymes released per meal?
Although the total amount of enzymes released varies depending on the type of meal consumed, the proportion does not vary on a meal-to-meal basis -> A high protein meal does not cause the release of a greater proportion of proteolytic enzymes
