Lecture 31 - Pancreatic and Biliary Secretion Flashcards
Where is CCK released from?
I-cells in the duodenum due to fat and protein digestive products
What does CCK do?
Inhibits gastrin secretion in the antrum (via D-cell)
Increases protein secretion in the pancreatic acini
Where is secretin released from?
S-cells in the small intestine due to acidification of the duodenum
What does secretin do?
Inhibits gastrin secretion in the antrum (via D-cell)
Increases fluid secretion in the pancreatic duct
What is the structure of the exocrine pancreas?
Lobules
- acini
Ducts
- intercalated
- common bile
Secretory units
- acini
- intercalated ducts
What are the secretory cells of the pancreas?
Acinus
Intercalated ducts
What is the site of majority of fluid secretion in the pancreas?
Duct cells
What do the acinar cells in pancreas secrete?
Isotonic NaCl solution - 25% of secretion
What do the duct cells in pancreas secrete?
HCO3- rich fluid - 75% of secretion
What is the difference in osmolarity between the saliva and pancreas?
Saliva: 100 mOsm
Pancreas: 300 mOsm
What is the volume and composition of secretions from pancreas?
1-1.5 L of alkaline fluid and 5 -15 g protein secreted per day
Where are the proteins in pancreatic secretions made?
Acinar cells
What proteins do the pancreatic acinar cells secrete?
20 different proteins secreted - mainly digestive enzymes
- zymogens - inactive precursors of digestive enzymes
- (trypsinogen, procarboxypeptidase)
- amylase, lipase
Enzymes for all major nutrient groups
What cell produces the alkaline fluid in pancreatic secretion?
Intercalated duct cells
What does the alkaline fluid in pancreatic secretion do?
Neutralises acidic chyme from the stomach
What is the alkaline fluid in pancreatic secretion?
Isosmotic NaHCO3 solution
When do we have the largest volume of pancreatic secretion?
Intestinal phase due to CCK and secretin
What stimulates pancreatic secretion?
CCK
Secretin
- minor regulation via vagus (ACh)
What stimulates pancreatic acinar cells?
CCK and vagus (ACh)
What stimulates pancreatic duct cells?
Secretin
Pancreatic secretion
A. is hypotonic due to high Na+ re-absorption in pancreatic ducts.
B. is stimulated by somatostatin.
C. contains trypsinogen released from zymogen granules.
D. is elevated during the gastric phase.
C. contains trypsinogen released from zymogen granules.
A - Salivary gland not pancreatic
B - Somatostatin always switch off - not involved with pancreatic secretion
D - Elevated in intestinal phase
What are pancreatic acinar cells specialised for?
Protein secretion
- zymogen granules
How does CCK and ACh stimulate secretion of pancreatic acinar cells?
Increase in intracellular Ca2+
How do acinar cells secrete isotonic NaCl solution/What is fluid secretion in pancreatic acinar cells driven by?
By electrogenic Cl- secretion
Why does increase in Ca+ in pancreatic acinar cells increase secretion?
By activating a Ca2+ dependent apical Cl- channel (CFTR)
- also stimulates secretion of enzymes
What is secretion by pancreatic duct cell dependent on?
Dependent on apical Cl-/HCO3- exchanger
Why is Cl-/HCO3- exchanger inactive at rest?
Due to little Cl- in duct lumen
What is Cl- in pancreatic duct lumen controlled by?
Controlled indirectly via a cAMP-dependent Cl- channel (CFTR) activated by secretin
How does secretin increase pancreatic duct cell secretion?
Increasing cAMP
Activation of apical Cl- channel (cAMP-activated)
Increased Cl- in duct lumen
luminal Cl- exchanged for HCO3-
Result: HCO3- secretion
Why does Na+ diffuse paracellular in pancreatic duct cells?
Due to electric gradient set up by Cl- and HCO3- movement
Why do we also have the secretion of NaHCO3 in pancreatic duct cells?
Na+ diffuses across the paracellular pathway due to potential set up by Cl- and HCO3- movement
Water follows because of the osmotic gradient
The secretion of a bicarbonate-rich solution by pancreatic duct cells is driven by CCK, BECAUSE
bicarbonate secretion in stimulated pancreatic duct cells is facilitated by CFTR.
D - First false, second true
Driven by secretin not CCK, CCK for acinar cells
What epithelia are the pancreatic duct cells?
Leaky
What are the differences in salivary and pancreatic duct secretion?
Tight vs leaky epithelium
Chloride secreted to drive bicarbonate secretion (boosted by secretin in pancreas)
NO change in sodium content (pancreas)
Major site of fluid secretion in pancreas
How does the liver process absorbed nutrients and control metabolism?
Gluconeogenesis, glucose buffering, fatty acid oxidation, synthesis of plasma proteins (e.g., albumin)
What is the exocrine function of the liver?
Secretion and excretion
What is secretion and excretion of liver?
Provision of bile acids and alkaline fluid to:
- aid digestion and absorption of fats
- neutralise gastric acid
Degradation and conjugation of waste products of metabolism
Detoxification of poisonous substances
Excretion of waste metabolites and detoxified substance in bile
What is function of liver assisted by?
Blood supply
Where is bile secreted in liver?
Bile secreted by hepatocytes lining canaliculi
What do ducts in liver secrete?
HCO3- rich fluid
Where is liver secretion stored and concentrated?
Gall bladder between meals
What is the volume and composition of liver secretion?
0.5 L day-1 - secreted continuously
Excretory products
- bile pigments - waste products
- cholesterol - excreted by liver
- steroids, heavy metal, drugs
Products associated with digestion
- HCO3- rich fluid - secreted by duct cells
- bile salts
What makes up bile salts?
Amphipathic compounds
- hydrophillic and hydrophobic surface
Primary bile acid
- Cholic acid
What are the two sources of bile salts in liver secretion?
Newly synthesised
Enterohepatic circulation
What percentage of bile salts are absorbed in the enterohepatic circulation?
95%
What is the mechanism of bile salt absorption in the enterohepatic circulation?
Small intestine
- limited passive absorption
- active absorption in ileum
Colon
- limited role
What are the three mechanisms of biliary secretion?
Nervous
Hormonal
Bile salts
What is the control of biliary secretion in the cephalic and gastric phases?
Minor, PNS
What is the control of biliary secretion in the intestinal phase?
Hormonal
- secretin stimulates production of HCO3- rich solution by ducts
- CCK stimulates contraction of gallbladder
Bile salts
- arrival of bile salts in portal venous blood stimulates absorption by liver and subsequent secretion
The liver produces a bicarbonate-rich bile, BECAUSE bile acids are required to reabsorb protein in the intestine.
C - First true, second false
Bile acids to reabsorb fat
What stimulates biliary secretion?
CCK and secretin
What is the purpose of bile acids in biliary secretion?
To reabsorb fat
