Zaczek RS 3- Secretions Functional in the GI Tract Flashcards
What are the stimuli for the release of CCK?
Partially digested fats, partially digested proteins (peptones) and CCK-releasing peptides (proteins released by epithelial cells that line the intestine. Bind to and stimulate I cells)
What are the stimuli for the release of secretin?
Acid (primarily), medium and long chain fatty acids (secondarily)
What organs does CCK bind? How does CCK exert its actions directly and indirectly?
Binds to cells in the pancreas and gallbladder
Direct actions on pancreas
-Binds to acinar cells to stimulate the fusion of vesicles containing digestive enzymes with the lumen
Indirect Actions
-Binds to pancreas and sends a signal to the CNS, which sends efferents to the pancreas which release ACh. ACh then causes vesicles containing digestive enzymes to fuse with the lumen
In the gallbladder, CCK causes the sphincter of Oddi to relax and contracts the gallbladder
What is enterokinase? How does it protect the pancreas?
Enterokinase is a brush border enzyme located in the small intestine.. It converts trypsinogen to trypsin, which can then activate the rest of the pancreatic proteases
What is the mechanism of action of secretin?
Secretin binds to duct cells in the pancreas and phosphorylates the CFTR receptor, making it more likely to be open
Duct cells have a cystic fibrosis transmembrane regulator (CFTR) channel, which keeps Cl- at a high concentration outside of the cell.
This high extracellular concentration of Cl- is used to antiport HCO3- and Cl-
The HCO3-, now in the lumen, is osmotically active and draws cations and fluids into the lumen.
What are the two most common causes of acute pancreatitis?
Alcohol abuse and gallstones
What pancreatic enzymes are indicative of acute pancreatitis?
Lipase and amylase are elevated in pancreatitis, with lipase being 4x more than amylase
What is the difference between primary and secondary bile acids?
Primary bile acids are made in the liver by the conjugation, hydroxylation, carboxylation and shortening of tails of fatty acids
Secondary bile acids are made in the colon by modifications of these primary bile acids
What function does conjugating bile salts serve?
Conjugation makes bile salts more water soluble, making them less likely to diffuse out of the small intestine and thereby giving them more time to exert their actions.
Why can’t fatty acids enter the small intestine without micelles?
The intestine has an unstirred water layer that hydrophobic lipids cannot enter.
Micelles form a slow-release lipid reservoir that allows them to enter.
Why are chylomicrons necessary inside of enterocytes?
Once fats are inside an enterocyte, they cannot leave without the help of chylomicrons.
The large size of the chylomicrons allows lipids to leave through lacteals or capillaries.
Where does bilirubin come from? How does it make it to the liver? What are it’s fates once in the liver?
Bilirubin comes from the breaking down of red blood cells.
It is carried to the liver bound to albumin.
Once in the liver, it is taken up by hepatocytes and conjugated by the enzyme UDP-glucuronyl transferase. Then, it is released into the small intestine.
Conjugated bilirubin makes it to the large intestine, where it is deconjugated and converted to urobilinogen.
Urobilinogen has 3 fates
1) Stay in the gut, become converted to stercobilin, and excreted in feces
2) Leave the gut, enter circulation and get urinated out as urobilin
3) Leave the gut, return to the liver and be excreted as bile
What is direct bilirubin? What is indirect bilirubin?
Direct bilirubin is conjugated bilirubin.
Indirect bilirubin is unconjugated bilirubin.
Which liver enzyme is naturally more abundant? Which liver enzyme is also expressed in bone and gut?
ALT is the more abundant liver enzyme and is typically elevated in liver disease, more than AST.
ALP is expressed in the bone and gut.
Which liver enzyme is decreased in alcoholic hepatitis/cirrhosis?
ALT
