Pancreatic Exocrine Secretions and Control Flashcards
State the secretions produced by the pancreas
- Duct cells secrete NaHCO3
- Pancreatic islet cells secrete hormones - insulin from beta cells, glucagon from alpha cells and somatostatin from delta cells
- Acinar cells secrete digestive enzymes
What cells form the exocrine portion of the pancreas?
Acinar cells
Where do the secretions travels through once secreted?
Secretions go from intercalated ducts -> intralobular ducts -> extralobular ducts -> interlobular ducts -> main duct
State briefly the effect of secretin and CCK on the pancreas
CCK effects acinar cells and causes them to secrete digestive enzymes - done when there is lipid heavy chyme in the duodenum
Secretin acts on duct cells and causes them to secrete aqueous NaHCO3 solution - done when there is acidic chyme in the duodenum
Describe the structure of the pancreas including the cells of the ducts
- Acinar - berry like secretory portion similar to a grape- duct resembles the stalk - is primarily pyramidal epithelial cells
- Microvilli extend from the apical surface of the epithelium into the lumen of the acini
- Acini form sacs - connect to the ductal tree - empties into the duodenum
- Intercalated ducts - squamous/cuboidal epithelial cells
- Intralobular ducts - cuboidal or low columnar epithelial cells
- Extralobular ducts - columnar epithelial cells
- Interlobular ducts - columnar epithelial/goblet cells
- Pancreatic duct which enters the duodenum at the common bile duct
How much solution does the pancreas secrete a day?
1.5L of fluid a day
In what form are enzymes secreted in and why?
Inactive form (zymogens) to prevent auto digestion
Where are pancreatic enzymes activated?
Duodenum
State the anions and cations present in the pancreatic juice
Anions that are secreted in the pancreatic juice - HCO3-, Cl-, SO4 2-, HPO4 2-
Cations that are secreted in the pancreatic juice - Na+, K+, Ca2+, Mg2+
State the alkaline secretions of the pancreas
HCO3- rich juice from the pancreas and gallbladder help to neutralise gastric acid so pH increases from 6 to 7
Secretion from pancreas is similar to that in salivary glands but it is a HCO3- rich juice
Describe how composition of pancreatic juice changes through the duct
- Epithelial cells actively exchange Cl-/HCO3- - Cl- comes into the duct cell and -HCO3- goes out of the duct cell
- H+ is actively eliminated by Na+/H+ exchanger - H+ exits the duct cell and Na+ enters the duct cell via the exchanger
- H+ is exchanged for K+ driven by Na+/K+ ATPase- Na+ exists the duct cell and K+ enters the duct cell
- H+ neutralises HCO3- (H2CO3 forms)
- CO2 and HCO3- are produced in the blood
- CO2 diffuses in and forms H2CO3 with H2O
What is concentration of HCO3- in the pancreatic juice proportional to?
The concentration of HCO3- in the blood
Describe a study in which secretin was given to an experimental animal and state what effects it had on HCO3-, Cl-, amylase and K+
- The concentration of HCO3- increases significantly
- The concentration of Cl- decreases
- The concentration of amylase decreases slightly
- The concentration of K+ decreases slightly
- Hence there is a reciprocal change in concentrations of Cl- and HCO3- upon secretin infusion
- Reduced amylase and Cl- concentrations upon secretin infusion may be a dilution effect due to the increase in volume of the secreted pancreatic juice
- This is because the stimulated pancreas causes an increase of [HCO3-] rich secretions
State the enzymes that are secreted from the pancreas
- Proteolytic enzymes (trypsin, chymotrypsin and carboxypeptidases)
- Amylase
- Lipase
- Ribonuclease
- Deoxyribonuclease
What is the activator and substrate for trypsin?
Activator - Secreted as trypsinogen activated by enterokinase/enteropeptidase and by trypsin
Substrate - Proteins and polypeptides
State the substrate of lipases
Triglycerides
State the activator and substrate of pancreatic alpha amylase
Activator - Activated by Cl-
Substrate - Starch
State the substrate of ribonuclease and deoxyribonuclease
Ribonuclease - RNA
Deoxyribonuclease - DNA
State the activators and substrates of elastase and phospholipase A2
- Elastase -
Activated by trypsin
Elastin - Phospholipase A2 -
Activated by trypsin
Phospholipids
State the activator and substrates of chymotrypsins
Activator - Secreted by chymotrypsinogen, activated by trypsin
Substrate - Proteins and polypeptides
State the activators and substrates of carboxypeptidase A/B and colipase
Carboxypeptidase A/B -
Activated by trypsin
Proteins and polypeptides
Colipase -
Activated by trypsin
Fat droplets in concert with pancreatic lipase
What are proteolytic enzymes secreted as and why?
- Enzymes secreted as proenzymes
- Digestion of proteins so the pancreas is at risk of autodigestion
Describe where enterokinase is secreted, why it is secreted and what it does
- In the brush border of duodenum
- Secreted in response to CCK
- Converts trypsinogen to trypsin
- Trypsin causes an auto acatalytic chain reaction (auto activation and activation of other proenzymes)
What are inhibitors of autodigestion?
Kazal inhibitor, enzyme Y
Intracellular pH of zymogen granule - acidic to keep zymogens inactive
What happens during acute pancreatitis?
- Trypsin activates phospholipase A2 in the pancreatic duct
- This causes the conversion of lecithin to isolecithin
- Isolecithin causes disruption of pancreatic tissue, membrane damage and necrosis
Describe what amylase does
- Active
- CHO digestion
- Polysaccharides -> disaccharides
Describe what lipases do
- Fat digestion - triglycerides -> fatty acids and glycerol
- Pancreatic insufficiency - poor digestion of lipids and their malabsorption which causes steatorrhoea (fatty stool)
State the % of amylase, trypsin and lipase that reach the small intestine
- Amylase- 75%
- Trypsin 20%
- Lipase 1%
Describe how pancreatic secretions and controlled including in the 3 phases
- Neuroendocrine signals - neural and hormonal factors
- Vagal (parasympathetic) stimulation - enhances the rate of secretion of enzymes and aqueous components of pancreatic juice
- Sympathetic stimulation inhibits secretion
- Secretin and CCK stimulate secretion of pancreatic fluid (bicarbonate and enzyme rich secretions respectively)
- Cephalic phase - vagal (acetylcholine and VIP) stimulation of gastrin release from the antrum - some enzyme rich pancreatic juice
- Gastric phase - distension (vago vagal reflex on fundus or antrum) , amino acids and peptides in the antrum stimulate gastrin secretion causing the release of enzyme rich pancreatic juice (shown by the fact that a vagotomy causes a 50% reduction in response to acidic chyme)
- Intestinal phase - secretin and CCK; enterohepatic reflexes - acidic chyme in duodenum and jejunum induce secretion of pancreatic juice (HCO3- and enzymes) - H+ stimulates S cells in duodenum to secrete secretin which acts on duct cell receptors to cause HCO3- secretion - protein+lipid breakdown stimulates I cells to secrete CCK which acts on acinar cells to cause secretions
Summarise the actions of secretin and CCK
Secretin - from mucosa of duodenum and jejunum
- Induces pancreatic duct cells to secrete HCO3- rich pancreatic juice but decreases enzyme content
- Secretin also stimulates production of bile by the liver
CCK from duodenum and jejunum in response to fatty acids (amino acids and peptides)
- Stimulates pancreatic acinar cells to synthesise and release enzyme rich pancreatic juice (and stimulates the secretion of concentrated bile from gallbladder for fat absorption)
- CCK contracts the gallbladder and relaxes the sphincter of Oddi - bile is released into duodenum
- CCK potentiates the effects of secretin which is a weak agonist of acinar cells
Describe why those with cystic fibrosis have GI issues
- Causes the pancreatic ducts to become blocked due to thick mucus build up
- The bicarbonate rich pancreatic secretions therefore cannot enter the duodenum
- Hence food is not digested and broken down properly as there are less enzymes in the duodenum
- This has a knock on effect in the intestines as they are unable to fully absorb all the nutrients
