Gastrointestinal Tract 4 - Pancreas Flashcards
The pancreases contains two types of glands
Exocrine and endocrine
Exocrine pancreas
- produce digestive enzymes (in excess) and secretions
- secrete HCO3 into duodenum for the neutralization of stomach acid- critical for enzyme function
Endocrine pancreas (non-digestive)
Produces hormones that regulate entire body (e.g., insulin)
Exocrine-secretion of substances into
ducts that drain onto an epithelial surface (apical surface).
Endocrine-
ductless gland, secretion occurs across epithelial basolateral surface for diffusion into blood
Main pancreatic duct joins common bile duct just before entering the
duodenum
Pancreatic ducts
- Acinar cells produce and secrete digestive enzymes- exocytosis
- Duct cells secrete H2O and HCO3-
Pancreatic juices
- isotonic, alkaline
Contains electrolytes:
- High HCO3-, low Cl-
- Na+, K+ same as in plasma
- HCO3- and H2O secreted mainly by duct cells
- HCO3- neutralizes gastric acid in duodenum
Contains digestive enzymes
The digestive enzymes in pancreatic juice secreted by
Acinar cells
Proteolytic enzymes stored and secreted in inactive forms; activation occurs in the
Duodenum
Ductular Cell Secretion of HCO3-
- Chloride channel (CFTR) opens
- Allows diffusion of Cl- into duct lumen - Cl- in lumen is exchanged for HCO3- in cell
- H2O and Na+ follow paracellularly in response to electrochemical gradient across epithelium
- Neutral pH of cytosol is maintained by exchange of H+ (exported from cell) for Na+ (imported).(in combination with carbonic anhydrase)
- Secondary active transport dependent upon the electrochemical gradient generated by the Na+/K+-ATPase
Ductular Cell Secretion of HCO3- results in watery alkaline secretion that neutralizes
gastric acid and washes digestive enzymes through
Parietal cells in stomach are producing lots of
Acid
Duct cells in pancreas are producing and secreting
HCO3- (base)
Alkaline tide.
Large amount of HCO3- pumped across basolateral surface into blood stream.
- by parietal cells in stomach
Acid tide.
Large amounts of H+ being pumped across the basolateral surface into
blood stream
- by duct cells in pancreas
HCO3- from stomach and H+ from pancreas eventually
meet up in the portal vein. Maintain acid base balance
Pancreas the source of majority of
Digestive enzymes
- proteases
- amylolytic enzymes
- lipases
- nucleases
Without the pancreas, ______ would occur
Starvation
Proteases
digest proteins into peptides and amino acids
Amylolytic enzymes
digest starch into sugars
Lipases
digest triglycerides into free fatty acids and monoglycerides
Nucleases
digest nucleic acids into free nucleotides
Acinar cells synthesize and package pro-enzymes into zymogen granules that are stored at the
apical pole of the cell
Are enxymes secreated activated or inactivated
Inactivated, activate them in the duodenum
Enterokinase-
enzyme embedded in the luminal membrane of the duodenum.
- Cleaves trypsinogen to trypsin (protease)
Trypsin is a
protease and also activates other proteases
Pancreas also secretes a variety of trypsin inhibitors to
antagonize any prematurely activated trypsin
Digestive enxymes are stored in
Inactive form, not activated until they reach the intestine
Trypsin can degrade itself if activated
prior to reaching the intestine
In cystic fibrosis, which channel is mutated ?
The Cl- channel involved in HCO3- secretion in the pancreas
CF patients can suffer from “pancreatic insufficiency”
- Still produce all of the digestive enzymes
- HCO3- and H2O secretion is minimal and enzymes do not get flushed from ducts and therefore do not reach the intestine
- Retained proteolytic enzymes = pancreatic autodigestion
How are patients with cystic fibrosis treated?
Must receive supplements of digestive enzymes and antacids to allow for adequate nutrition
The major proteases secreted by pancreas
Inactive enzyme:
- trypsinogen (actvdd by enterokinase)
- Chymotrypsinogen (actvtd by trypsin)
- pro-elastase (actvtd by trypsin)
- pro-carboxy peptidase (actvtd by trypsin)
Activate enzyme:
- trypsin
- Chymotrypsin
- elastase
- carboxypeptidase A and B
The action of trypsin, Chymotrypsin, elastase ?
Endopeptidases (hydrolyze interior peptide bonds of proteins and polypeptides)
Action of carboxypeptidase A and B ?
Exopeptidases (hydrolyze bonds at C- terminal end)
Amylolytic enzyme secreted by pancreas action is
Cleaves starches to sugars (same as salivary amylase)
End products: Maltose, maltotriose & α- limit dextrins
Lipolytic enzyme
Inactive enzyme:
- Prephospholipase A2 (trypsin)
Activate enzyme:
- Lipase (hydrolysis triglycerides)
- Phospholipase A2 (hydrolyzes phospholipids)
- Cholesterol esterase (hydrolyzes cholosterol-esters)
End product of Lipase
Free fatty acids & 2-monoglycerides
End product of Phospholipase A2
Free fatty acids and lysophospholipids
End product of Cholesterol esterase
Free fatty acids & cholesterol
Circulating CCK stimulates:
Pancreas to increase digestive enzyme secretion
Gall bladder contraction:
- Release of bile acids for fat breakdown
- Relaxation of the sphincter of Oddi
Fatty acids and amino acids in the small intestine triggers
CCK secretion from cells in small intestine into blood.
How is release of CCK stopped?
Fats and amino acids are absorbed and stimulation of CCK release is stopped-due to their removal
Secretin regulation of pancreatic HCO3-secretion
- Acid enters the duodenum from the stomach
- Reduced pH triggers secretin secretion from cells in small intestine into blood.
Circulating secretin stimulates:
- Pancreas (duct cells) to increase HCO3- secretion.
- Liver (duct cells) to increase HCO3- secretion.
When is secretion of secretin stopped
When Stomach acid is neutralized
Secretin and CCK both:
Inhibit gastrin secretion
The inhibition of gastrin secretion results in:
- Reduced stomach motility-slows stomach emptying
- Reduced acid secretion
Phases of pancreatic secretion
1) Cephalic phase:
Minor. Sight, smell, taste = stimulate pancreatic secretion via the parasympathetic nerves.
2) Gastric phase:
Minor. Distension of stomach = stimulate pancreatic secretion via the parasympathetic nerves.
3) Intestinal phase
Major.
- Acid from stomach in duodenum = secretin release
- Digested fat and protein in duodenum = CCK release
