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Lecture 61

Physiology 3120 Lecture Notes - Lecture 61: Pancreatic Lipase Family, Pancreatic Duct, Common Bile Duct

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Tom Stavraky

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Lecture 61 Accessory Organs
- Pancreas is the most important accessory organ when it comes to digestion
- Both exocrine and endocrine function,
o Endocrine secretions: insulin, glucagon and somatostatin
- Exocrine secretions from the pancreas are secreted into the duodenum
o Acinar cells are ductal clusters that look similar to salivary glands
- Acinar cells make enzymatic secretions which are modified as they flow through the larger
ducts of the pancreas
o Larger ducts are surrounded by ductal cells
o Ductal cells secrete water and bicarbonate the enzymatic solution made my acinar
cells to make the volume of the secretions larger and provide alkaline environment
for which the enzymes will enter into the acidic duodenum
o Chyme is acidic and comes into contact with enzyme and bicarbonate rich solution
from the exocrine pancreas
Secretions from the exocrine pancreas
- 1. Bicarbonate - from ductal cells
o As secretions travel through ducts of the pancreas, water and bicarbonate is added
in from ductal cells
- 2. Pancreatic Amylase
o Similar to salivary amylase
o Digests starch (complex carbohydrates)
o Has more time to interact with starches in our diet because it interacts with starches
it acts in our duodenum and small intestine can control how long the enzyme is
o Most important starch digesting enzyme
- Zymogens: secreted as inactive and must be cleaved to become activated (3-6)
o They are protein digesting enzymes once they are activated
o Proteins become enzymes when cleaved
- Pancreas makes inactive proteins so it does not digest itself
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o Pancreas is made of cells made of proteins
o If there are protein digesting enzymes in the body of the pancreas, it will digest itself
o = it secretes zymogens that are activated when in the small intestine
o Small intestine is a safe place for proteases to become active
- 3. Trypsinogen > Trypsin
- 4. Chymotrypsinogen > Chymotrypsin
- 5. Procarboxypeptidase > Carboxypeptidase
- 6. Proelastase > Elastase
- 7. Pancreatic Lipase
o Lipid digesting enzyme
o Vs. lingual and gastric lipase: pancreatic lipase is the most important and digests the
most amount of fat
o This is not because it is a better enzyme but because it has help in the small intestine
- 8. Ribonuclease
o Breaks down RNA, DNA
- 9. Deoxyribonuclease
o Breaks down RNA, DNA
- Pancreatic secretions enter ducts from acinar cells and then into the small intestine
- Pancreatic duct joins and becomes part of the common bile duct
- Common bile duct is closed off by the spinchter of Odi and when it is open, secretions from
the pancreas go into the duodenum
- Trypsinogen (zymogen) comes into contact with enterocytes and enterokinase is a brush
border enzyme on absorptive cells
- Enterokinase is embedded in the microvilli membrane of absorptive cells
o Takes trypsinogen and cleaves it into trypsin (smaller peptide)
o Activation of trypsinogen occurs because of the action of enterokinase
- Trypsin goes in a cascade fashion to digest other zymogens produced by the pancreas
- Chymotrypsinogen is cleaved to chymotrypsin because of the action of trypsin
- Procarboxypeptidase is converted into carboxypeptidase because of a cleavage event
- Inactive enzymes get activated in the small intestine
- Small intestine is safe for this to occur because it occurs when food is present
- Food contains protein = protein digesting enzymes are activated in the presence of dietary
- Trypsin, chymotrypsin, carboxypeptidase act on the dietary protein ingested and do not
autodigest the wall of the intestine or pancreas
- This is a specific event to prevent the pancreas from digesting itself
- PANCREATITIS: occurs because of inappropriate activation of trypsin within the ducts of
the pancreas
o Activation occurs within acinar cells where proteases within acinar cell may
accidentally or pathologically cleave trypsinogen into trypsin
o This can occur in acinar cell or ducts and lead to the activation of all other zymogens
early on in the pancreas = pancreas digests itself
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- All zymogens are activated by trypsin
o Coenzyme procolipase is made by acinar cells and activated in the cascade
o Colipase is not a protein digesting enzyme (it is a coenzyme for pancreatic lipase)
Regulation of pancreatic secretions
- Secretions of the pancreas must occur when there is food in the intestine
- Do not want acinar secretions when we have an empty intestine
- Acinar cell secretions from the pancreas are homogeneous
o All acinar cells make the same ratio of all of the enzymes
o Every acinar cell secretes vesicles containing all enzymes at the same time
o This is not like the salivary glands with mixed populations
o Acinar cells are all the same and all make the same thing
- Can change diet if you ingest less animals protein, you make less proelastase
o Elastase is good at digesting animal proteins
o As you shift what you eat, the pancreas will shift what zymogens and enzymes it
o E.g. vegetarian eating a lot of animal protein at once not a good idea because
pancreas has shifted enzymes to accommodate a diet that is high in plant protein,
plant fibers and starches (more pancreatic amylase instead of animal protein
digesting enzymes)
o Making large dietary changes recommended that it be done slowly
o Hard to digest food that you are not used to digesting when foods aren’t in the same
- Ratio of zymogens and digestive enzymes will change with changing diets but each acini will
still secrete the same composition
- Ductal cells modify secretions by adding bicarbonate and water
- Body has the ability to trigger ductal cells to trigger more water or bicarbonate if necessary
to the total secretion product from the pancreas
- Hormone action triggers this
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