GI Secretion II
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- Is fluid hyperosmotic or hyposmotic when it leaves the stomach?
- hyperosmotic
- What area is considered to be the endocrine portion of the pancreas and what do they secrete?
- islets of langerhans - secrete insulin, somatostatin, glucagon, and pancreatic polypeptide; these are all secreted into blood
- What is secreted from the exocrine pancreas?
- enzyme, water, HCO3- secretion into pancreatic and common bile duct and thru sphincter of oddi
- What is the purpose of having both acinar and duct cells providing secretion in the pancreas?
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acinar cells - secrete enzymes
ductile cells - secrete water and electrolytes
** having 2 different secretion systems allows better adjustment of enzyme/liquid components - What is the significance of osmolarity dropping rapidly in the duodenum?
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1) maintains fluid balance
2) provide digestive enzymes with ideal conditions
3) protection of mucosa - What is the difference between proximal and distal duct cells in the pancreas?
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proximal duct cells - secrete water and bicarbinate
distal duct cells - reabsorbs water and bicarbonate - What is the effect of increased flow rate on pancreatic secretions?
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1) high rates - net secretion is high in bicarbonate and low in chloride - reflective of primary secretion
2) vice versa at low rates - secondary secretion
*virually no change in cations wih change in flow rate - What is stored in a zymogen granule?
- inactive proteolytic enzymes (proenzyme), and a trypsin inhibior (to avoid autodigestion of the pancreas)
- What are the different types of enzymes found in the exocrine pancreas?
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1) lipolytic (dig lipids)
2) amylases (dig CHO)
3) proteolytic (dig proteins) - What does pancreatic lipase do?
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1) splits TGs into glycerol and FFAs (2FAs and a 2-monoglyceride)
2) requires oil/water interface, bile salts, and colipase - What linkage does pancreatic amylase cleave?
- alpha 1,4 linkage of starch (creates maltose, maltotriose etc.)
- Trypsin, chymotrypsin, and carboxypeptidase A and B are considered to be what general type of pancreatic enzyme?
- proteolytic (digests proteins)
- Describe the action of trypsin.
- Endopeptidase active at basic amino acids.
- Describe the action of chymotrypsin.
- Endopeptidase active at aromatic amino acids.
- Describe the action of carboxypeptidase A & B.
- Exopeptidases active at C-term end; A active at aromatic and neutral amino acids and B active at basic amino acids
- When pancreatic cells release granules, do all have the same % of enzymes?
- YES!
- What type of pancreatic enzymes make up 75% of the pancreatic enzymes?
- Proteolytic (proteases including endopeptidases, and exopeptidases)
- Which is a more important method of pancreatic secretion control, neural or hormonal?
- Hormonal (secretin, CCK, and PP)
- What pH triggers the release of secretin from the mucosal cells?
- pH<4 in the duodenal lumen
- Which hormones/neural peptides bind to Acinar cells? via their receptors that are located there?
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1) VIP
2) GRP
3) CCK
4) ACh - Which hormones/neural peptides bind to duct cell in the pancreas via their receptors that are located there?
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1) secretin
2) ACh - What cells does secretin stimulate to secrete bicarbonate and water?
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1) pancreatic duct cells
2) duodenal glands
3) hepatocytes - What receptors does CCK act on in the pancreas?
- CCK-A receptors
- What type of response does most of CCKs activity come from?
- long arc vago-vagal reflex
- When is PP secreted and what does it do?
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pancreatic polypeptide (PP) is an inhibitory regulator (inhibits vagal output from CNS)that is secreted during the meal and turns off the excitatory inputs on acinar cells
*proximal feedback loop - What is PYY and when is it secreted?
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PYY is secreted from the ileum after a meal and inhibis gastric acid secretion, pancreatic enzyme secretion, and decreases transport
*distal feedback loop - What are the 3 phases of postprandial pancreatic secretion?
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1) cephalic (25% of response)
2) gastric (10%)
3) intestinal (50-75%) - Which pancreatic secretion phases are neurally regulated and which are hormonally regulated?
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1) neural - cephalic and gastric
2) hormonal - intestinal - Explain stimulus secretion coupling of pancreatic acinar cell secretion.
- Receptors for two different things (ex. VIP and secretin) couple throuh the same pathway (ex. G protien) and potentiate each other.
- How much fluid is secreted by enterocytes/day?
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1000ml/day
(about 3000 ml total is secreted into the intestine, but much of that is from non-enterycte origin) - Where is the oldest enterocyte located on a villus structure?
- at the tip of the villus where it will be sloughed off
- Explain how the enterocyte changes its function as it moves up the villus.
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crypt - net secretory cell
tip - net absorptive cell - What ion is coupled with the secretion of water in an enterocyte?
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Cl-
(via the cystic fibrosis transmembrane regulator (CFTR) on the apical membrane) - How does cholera toxin work?
- irreversibly activates G protein coupled to adenylate cyclase, increasing cAMP and keeping CFTR channel open (lose Cl-, Na+, and water)
- What makes up most of the colonic secretion?
- mucus
- What does interdigestive secretion do?
- gastric, bilary, and pancreatic fluid and enzymes is released periodically between meals to clear and clean the gut