L18 - Biliary & pancreas secretion control

Question 1

Endocrine Secretion of pancreas

Answer 1

Secreted by cells in Islets of Langerhans (1-2% of the pancreas):

1) Glucagon (by A cells)

2) Insulin (by B cells)

3) Somatostatin (by D cells)

• Minor secretion:*
• 4) pancreatic polypeptide (by PP cells)*
• 5) VIP Vasoactive intestinal peptide (by D-1 cells)*
• 5) Serotinin, secretin, motilin, substance P (by EC cells)*

Question 2

Exocrine secretion of pancreas

Answer 2

• Enzymatic component (Stored as zymogen granules om acinar cells; release by exocytosis):*
  1) α-amylase
  2) Lipase
  3) Protease proenzymes (i.e. inactive forms)
  i) Trysinogen
  ii) Chymotrypsinogen
  iii) Proelastase
  iv) Procarboxypeptidase A
  v) Procarboxypeptidase B
  4) Ribonuclease
  5) Deoxyribonuclease
• Aqueous component (Secreted by epithelial cells of pancreatic ductules & ducts; centroacinar cells):*
  6) HCO₃^-
  7) Water

Question 3

Function of enzymatic exocrine pancreatic secretion

Answer 3

1) Carbohydrate digestion (α-amylase) - splits polysacchrides to glucose & maltose

2) Fat digestion (lipase) - splits triglycerides into free fatty acids & monoglycerides

3) Protein digestion (Pancreatic protease proenzymes are activated only in small intestine by enterokinases at the microvilli (typsinogen to trypsin); trypsin then converts other inactive enzymes into active enzymes as well as digestion of protein)

i) break peptide bonds in protein to form peptide fragments (elastase, trypson, chymotrypsin)
ii) splits off terminal amino acid on carboxyl end of protein (carboxypeptidase A & B)

4) Nucleic acid digestion (ribonuclease, deoxyribonuclease) - splits nucleic acids to mononucleotides

Question 4

Function of aqueous exocrine pancreatic secretion

Answer 4

Bicarbonate ions and water can:

1) Neutralizes acid chyme from stomach, stops action of pepsin
2) Provides optimal pH (7-8) for the action of pancreatic enzymes

Question 5

Activation of Pancreatic protease Proenzymes

Answer 5

Pancreatic proteases are activated only in small intestine by enterokinases at the microvilli (typsinogen to trypsin); trypsin then converts other inactive enzymes into active enzymes

Question 6

Prevention of premature activation of pancreatic proteases

Answer 6

Normally, prematurely activated trypsin within the pancreas is:

(1) inhibited by the human pancreatic secretory trypsin inhibitor (SPINK1, serine protease inhibitor Kazel type 1) and;
(2) chymotrypsin C (CTRC) and trypsin will degrade trypsinogen and trypsin

Question 7

Mechanism of Bicarbonate Secretion

Answer 7

1. H⁺ released to blood from Na⁺-H⁺ exchanger & H⁺-K⁺ ATPase in basolateral membrane, with entry of Na⁺ and K⁺
2. H⁺ released in blood and HCO₃^- in blood forms CO₂ and H₂O
3. CO₂ enters cell, and combines with H₂O in cell to form intracellular CO₃^-
4. Active transport of HCO₃^- into lumen via Cl^--HCO₃^- exchanger, with Cl^- entering the cell
5. Cl^- recycled back to lumen through Cl^- channel (CFTR cystic fibrosis transmembrane conductance regulator)
6. K⁺ efflux back to blood through K⁺ channel in basolateral membrane
7. Osmosis of water down osmotic gradient