Pancreatic Exocrine Secretions and Control *** Flashcards
What are the three types of cells found in the pancreas?
ACINAR CELLS (exocrine)
ISLETS OF LANGERHAAN (endocrine)
DUCT CELLS
What are acinar cells stimulated by, and what do they secrete?
- stimulated by CCK
- secretes digestive enzymes into the pancreatic ducts
What do the Islets of Langerhan secrete?
Hormones into bloodstream
What do duct cells secrete and what are they stimulated by?
- Secrete NaHCO3
- Stimulated by secretin
List the structures a substance has to go through to get from acinar cells to the pancreatic duct.
- acini (a cluster of acinar cells)
- intercalated ducts
- intralobular ducts
- extralobular ducts
- interlobular ducts
- pancreatic duct
The pancreas secretes around 1.5 litres of fluid every day.
What are some components of this fluid?
- Sodium and HCO3-
- Albumin, globulin and digestive enzymes.
Name some anions that are secreted in the pancreatic juice.
- HCO3-
- Cl-
- SO4 2-
- HPO4 2-
Name some cations that are secreted in the pancreatic juice.
- Na+
- K+
- Ca2+
- Mg2+
The composition of the juice is modified as it travels through the duct.
Describe how this happens. PART 1
- Cl- is actively exchanged for HCO3- by the epithelial cells
- H+ is actively eliminated and exchanged for K+ and Na+
- H+ neutralises the HCO3- (and H2CO3 is formed)
The composition of the juice is modified as it travels through the duct.
Describe how this happens. PART 2
- CO2 and HCO3- are produced in the blood
- CO2 diffuses in and forms H2CO3 with H2O
What is HCO3- concentration in juice, and rate of production proportional to?
Concentration of HCO3- in the blood
Describe the effect of stimulating the pancreas (via secretin) on the components of pancreatic juice. PART 1
Change in the concentration of Cl- and HCO3- after secretin infusion.
- [HCO3-]-rich secretions
Describe the effect of stimulating the pancreas (via secretin) on the components of pancreatic juice. PART 1
- Na+ levels don’t change much, but the Cl- goes down while the HCO3- goes up
Normally it would be expected that [HCO3-] secretions are high following secretin-induced stimulation but this is not the case. Suggest a reason why.
- Less time for HCO3- and Cl- exchange
- Flow rate is greater following stimulation
What are the three major types of enzymes secreted by the pancreas?
- proteolytic enzymes
- amylase
- lipase
Give examples of proteolytic enzymes secreted by the pancreas.
- Trypsin
- Carboxypeptidase
- Chymotrypsin
What is the fourth additional enzyme type sometimes secreted by the pancreas?
(Deoxy)ribonucleases
Trypsin activates many other enzymes.
List some enzymes that it activates.
- Phospholipase A2
- Chymotrypsins
- Carboxypeptidase
- Colipase
What does phospholipase A2 do?
- Breakdown of membrane phospholipids.
- Converts lecithin to isolecithin.
What does colipase do?
Enhance pancreatic lipase activity
How does colipase enhance pancreatic lipase activity? PART 1
- Acts on some of the fat droplets
- Enhances binding capacity of lipase
- Allows lipase to access the lipid core.
How does colipase enhance pancreatic lipase activity? PART 2
- Allows lipase to break down the fat droplet further to make micelles
- Small enough to travel to the surface of the epithelial cells to get absorbed.
What is the percentage of secreted enzymes that reach the small intestine?
amylase: 75%
trypsin: 20%
lipase: 1%
How is pancreatic secretion controlled?
Neuroendocrine signalling
What is the effect of parasympathetic stimulation of the pancreas?
- Enhances rate of secretion of enzymes and aqueous components of pancreatic juice
What is the effect of sympathetic stimulation of the pancreas?
- Inhibits secretion
- (Possibly) decreases blood flow - (diverting blood flow from GI tract)
Which two chemicals help stimulate secretion of pancreatic fluid?
- Secretin
- CCK
What are the 3 stages of digestion?
CEPHALIC PHASE
GASTRIC PHASE
INTESTINAL PHASE
Describe the control of pancreatic secretion in the cephalic phase.
Vagal (ACh and VIP) stimulation of gastrin release from the antrum leads to some pancreatic secretion
Describe the control of pancreatic secretion in the gastric phase.
- Distention (vagal reflex on the fundus or antrum)
- Amino acid and peptide stimulated gastrin secretion
- Both lead to the release of enzyme-rich pancreatic juice
Describe the control of pancreatic secretion in the intestinal phase.
- Secretin and CCK
- Chyme in the duodenum and jejunum
- Both induce secretion of pancreatic juice
How does secretin work?
- Induces pancreatic duct cells to secrete HCO3- rich pancreatic juice, with a lowered enzyme content.
- Stimulates bile production
Where is CCK released from?
Duodenum and jejunum
What is CCK released in response to?
- Fatty acids
- Amino acids and peptides
What is the effect of CCK stimulation of acinar cells?
Synthesis and release of enzyme-rich pancreatic juice
What is the effect of CCK stimulation of bile release?
Stimulates the secretion of concentrated bile for fat absorption
How does CCK cause bile release into the duodenum?
- Contracts the gallbladder
- Relaxes the Sphincter of Oddi
How does CCK enhance acinar cell activity?
- Potentiates the effects of secretin - weak agonist for acinar cells.
How would cystic fibrosis affect the pancreas?
- Blocked pancreatic duct
- Secretion will cease to end up in the duodenum
- Nutritional deficiencies, due to improper digestion of food
- Causes pancreatic insufficiency leading to steatorrhea
What do the secretions of the pancreas join with and where?
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
Where is secretin produced?
→from mucosa of duodenal and jejunum
What happens as a result of a vagotomy during the intestinal phase?
50% decrease in response to acidic chyme
What membranes do chymotrypsinogen and amylase cross in the pancreas and what does this suggest?
→Cross the basolateral membrane of the pancreas
→Suggests bidirectional permeability of membrane to digestive enzymes
What are inhibitors of autodigestion?
→Kazal inhibitor - inhibits chymotrypsin C, caldecrin etc.
→Intracellular pH of zymogen granule = naturally acidic (keeps zymogen inactive)
Give an example of an autocatalytic reaction.
→ Once one trypsin is formed it can go on and convert more trypsinogens to trypsin
- Trypsin is a reaction product and is able to catalyse its own formation
Where is enterokinase and when is it secreted?
→ Brush border of duodenum
→ Secreted in response to CCK
What does enterokinase do?
→ trypsinogen to trypsin
How are proteolytic enzymes secreted and why?
→Enzymes secreted as proenzymes
→ Prevent autodigestion
What is the activator and substrate of colipase?
→ A: trypsin
→ B: fat droplets
What is the activator and substrate of carboxypeptidase A+B?
→ A: Trypsin
→ S: Proteins and polypeptides
What is the activator and substrate of Phospholipase A2?
→ A: Trypsin
→ S: Phospholipids (lecithin)
What is the activator and substrate of elastase?
→ A: Trypsin
→ S: Elastin
What is the activator and substrate of Pancreatic α amylase?
→ A: Cl-
→S: Starch
In general, what are the substrates for most lipases?
Triglycerides
What is the activator and substrate of trypsin?
→A : Secreted as trypsinogen - activated by enterokinase/enteropeptidase autocatalytically
→S: Proteins and polypeptides
Why do Cl- and amylase decrease after infusing secretin?
→ there are more increased watery secretions so the other components decrease.
What do secretions from the gall bladder help to do?
Help neutralise gastric acid
Where does enzyme activation occur?
Duodenum
Why are enzymes secreted as zymogens?
→prevent autodigestion
What are acinar cells connected to?
→Duct that secretes bicarbonate
What do the acini form?
Sacs that connect to the ductal tree and empty into the duodenum
What are intercalated ducts made from?
→ squamous epithelium
What are intralobular ducts made from?
→ cuboidal or low columnar epithelium
What are interlobular ducts made from?
→columnar epithelium
