Lectire 36: GIT 4

Question 0

What are the enzymes that are secreted by the pancreas?

Answer 0

1. Proteolytic
   - trypsin
   - chymotrypsin
   - carboxypeptidase

Lipase
Amylase
DNAase
RNAase

Question 1

Tell me about the exocrine pancreas

Answer 1

Functional unit is acinus + drainage ductules

• acinar cells synthesise, store and secreted digestive enzymes
• bicarbonate is secreted by epithelial cells
• fluid flows along duct that empties into duodenum

Question 2

How are all the inactive pancreatic enzymes converted to active?

Answer 2

Firstly enterokinase converts trypsiongen➡ trypsin

Which then converts all the others to their active form

Question 3

What is the function of proteolytic enzymes?
Amylase?
Lipase?

Answer 3

Proteolytic enzymes:

• main ones are trypsin, chymotrypsin and carboxytrypsin
• secrete as inactive forms
• trypsinogen is converted to trypsin by enterokinase which activates the other 2 enzymes
• trypsin and chymotrypsin splits proteins into peptides
• carboxypeptidase splits them into individual aa
• trypsin inhibitor produced by same acinar cells that secrete proteolytic enzymes (prevents enzyme damaging pancreas)

Amylase:

• secreted by pancreas and salivary glands
• digests starch to maltose
• complete hydrolysis to monosaccharides performed by brush boarder enzymes
• amylase secretion stimulated by diets high in starch

Lipase:

• important for fat digestion but bile salts also required
• in presence of bile salts, hydrolysis of fats by lipase allows formation of water soluble micelles
• lipid secretion stimulated by diets high in fat or protein

Question 4

Secretion of Bicarbonate, Water and Electrolytes

Answer 4

• Secreted mainly by epithelial cells of duct system
• When secretion stimulated, HCO3- can reach 150 mmol/L
• HCO3- in pancreatic fluid neutralises acid that enters duodenum
• As HCO3- concentration increases, Cl- concentration decreases (sum remains constant)
• Pancreatic fluid also contains water, sodium and potassium

Question 5

What are the phases of pancreatic secretion

Answer 5

• Cephalic, gastric and intestinal phases
• In cephalic and gastric phases, vagal cholinergic fibres stimulate acinar muscarinic M3 receptors, stimulating enzyme secretion
• During these phases, only small amounts of water and electrolytes secreted with enzymes
• Minimal flow through ducts

Intestinal Phase: Regulation of Pancreatic Secretion by Secretin:
•Secretin released from S cells in response to H+ entering duodenum in gastric chyme
•Stimulates copious HCO3- and water secretion from ducts
•Secretin-induced secretion also requires cholinergic input

Intestinal Phase: Regulation of Pancreatic Secretion by CCK
•CCK released from
I cells, mainly in response to fat entering in gastric chyme
•Mediates acinar enzyme secretion
•Also requires cholinergic input

Question 6

Stimulation of Pancreatic Bicarbonate Secretion by Acid in Duodenum

Answer 6

Yep

Question 7

Bile production: bile acids

Answer 7

• Bile acids make up 90% of organic portion of bile
• Hepatocytes synthesise bile acids from cholesterol
• Cholesterol insoluble in water
• 2 additional hydroxyl groups give bile acids a hydrophilic side, whilst still maintaining a hydrophobic side

Bile acids:
•Main bile acid formed in liver is cholic acid, with some chenodeoxycholic acid (primary bile acids)
•In liver, bile acids conjugated with either taurine or glycine and excreted in bile
•This reduces the pK, so in bile exist as bile salts
•As bile salts are not fat soluble they tend to remain within lumen of small intestine
•This enables them to carry out role in formation of micelles (important for fat absorption)
•Reabsorbed by active process in distal ileum or degraded to secondary bile acids by intestinal bacteria
•95% recycled to liver via portal circulation (enterohepatic circulation)
Recirculation of Bile Salts (Enterohepatic Circulation)

Question 8

Anther component of bile: bilirubin

Answer 8

• Breakdown product of haemoglobin from red blood cells that have passed their “use by” date
• Iron recycled whilst pigment processed by liver
• Following excretion in bile, conjugated bilirubin reduced by intestinal bacteria to form urobilinogen
• Urobilinogen either absorbed by small intestine or oxidised stercobilin, which is responsible for brown colour of stoo

Question 9

Modification of Bile Composition by Ducts and Gall Bladder

Answer 9

• Bile produced by hepatocytes has similar ionic composition to blood
• Volume and ionic composition altered during flow through ducts and storage in gall bladder
• In ducts, secretin stimulates secretion of bicarbonate
• In gall bladder, bile concentrated by active reabsorption of sodium, chloride and bicarbonate – passively followed by water

Question 10

Check out the diagrams on last 2 pages and know the shit out of them

Answer 10

Yeee