Secretory functions of pancreas and liver

Question 1

Exocrine secretions of pancreas

Answer 1

-acini are arranged in anastomosing tubular network
-acini surrounded by myoepithelial cells that can contract to expel contents of the acini into the duct

Question 2

What stimulates exocrine secretions?

Answer 2

**stimulated by CCK in acini
**stimulated by secretin in ductal system

Question 3

Pancreas exocrine cell functions in ducts

Answer 3

-Centroacinar and proximal duct cells produce HCO3-, Cl, Na, K ions and water
-intercalated duct cells-minimal secretory function
-interlobular duct cells secrete HCO3, Cl, Na, K, water
-main collecting duct- bicarbonate is exchanged for chloride

Question 4

pH of pancreas

Answer 4

-typically 7.8
-if the pH in duodenum is low, secretin is released from duodenum causing increased Cl removal and pH rises to ~8.2

Question 5

Regulation and function of pancreatic exocrine secretion

Answer 5

1.Secretion of HCO3 to neutralize the gastric contents entering the duodenum from stomach (stimulated by secretin in ductal system but CCK in acini
2.Secretion of enzymes (most zymogens) for the luminal digestion of carbohydrates, fat, proteins . Controlled by CCK that is released by fat and protein presence
3.Cholinergic stimulation also enhances the effect of secretin on the pancreas. During cephalic and gastric phases the pancreatic secretions are controlled by nerves
>both enzymes and electrolytes can be stimulated by cholinergic stimulation however it is species dependent

Question 6

Pancreatic flow and bicarbonate

Answer 6

-as flow increases, bicarbonate also increases
*mediated by secretin

Question 7

Pancreatic secretion during different phases

Answer 7

-intestinal phase (60-70%; high volume, low proteins)
-cephalic phase (20-25%)
-gastric phase (10%; low volume, high protein)

Question 8

CCK regulation of pancreatic secretion

Answer 8

-stimulator of acinar secretion. Acts via monitor peptide produced by the pancreatic acinar cells
-synthesized and stored by endocrine cells, predominantly in the duodenum and released in response to digesta

Question 9

CCK-RP

Answer 9

-CCK releasing peptides
-derived from cells in duodenum and released in response to fatty acids and hydrophobic amino acids

Question 10

Secretin regulation of pancreatic secretion

Answer 10

-responsible for post meal secretion of HCO3-
-S cells in duodenal mucosa release secretin when pH decreases (due to gastric acid)
-Secretin binds to receptors on pancreatic ducts
**If pH increases secretin release decreases

Question 11

Function of biliary secretion

Answer 11

-provides source of bile acids needed for fat digestion and absorption
-provide excretory route for certain metabolites and drugs
-provides buffer secreted into duodenum

Question 12

Structure of liver lobule

Answer 12

-cords of hepatocytes arranged around a central vein
-sinusoids (specialized capillaries) present between the hepatic cords
-portal triad: portal vein, hepatic artery, bile duct

Question 13

Bile secretion

Answer 13

-bile exits the hepatocytes into the ductules and eventually the bile duct of the triad

Question 14

Sinusoid regulation

Answer 14

-macrophages (Kupffer cells)- important for RBC removal
-large pores that allow albumin into the extravascular space. Hepatocytes can metabolize/filter

Question 15

Bile acid/salts

Answer 15

-bile acids are steroid acid (derived from cholesterol) comprising about 80% of the organic compounds in bile (others are phospholipids and cholesterol)

Question 16

Function of bile acids

Answer 16

-facilitate digestion of dietary fats and oils; serve as micelle-forming surfactants which encapsulate nutrients and facilitate absorption

Question 17

How are bile acids created?

Answer 17

-created from cholesterol via hydroxylations, oxidations, isomerization, saturations, and reductions
*7alpha-hydroxylase (rate limiting step)

Question 18

Primary bile acids

Answer 18

-cholic and chenodeoxycholic acid

Question 19

Bile salt synthesis

Answer 19

-cholic and chenodeoxycholic acid conjugated to taurine and glycine to form taurocholic and glycocholic acids forming bile salts
>this reduced the pKa from around 6 to 4 for glycine conjugates and 2 for taurine acids
»increases solubility and makes them impermeable to cell membranes so high concentrations persist in bile and intestinal contents

Question 20

Cats and Bile acids

Answer 20

-cats are obligate carnivores because they only use taurine (from protein) to make bile salts. Do not use glycine

Question 21

Secondary bile salts

Answer 21

-arise from the deconjugation and dihydroxylation of bile acids by gut microbes
>these are deoxycholic and lithocholic acids
>at an increase in pH, they become insoluble and are excreted in the feces

Question 22

Bile salt absorption

Answer 22

-occurs in intestines
-90% occurring in ilium

Question 23

Bile salt recycling

Answer 23

-recycled to increase the amount in the GI for proper digestion of fat
*turnover: 6-8x in 24 hrs
-if this is impaired, then fat and fat soluble vitamin malabsorption will occur

Question 24

Blood bile acids during fasting

Answer 24

0-10 umol/L

Question 25

Blood bile acids post meal

Answer 25

0-20 umol/L

Question 26

Extrahepatic biliary tract components

Answer 26

-bile duct
-common bile duct
-gallbladder & sphincter of Oddi

Question 27

Ruminant and pig bile secretion

Answer 27

-almost continuous secretion of bile into intestines
-sphincter is poorly defined

Question 28

Horse bile secretion

Answer 28

-continuous flow
-no gallbladder

Question 29

Other species (humans, moose, elephant, rat, elk, deer) bile secretion

Answer 29

-intermittent flow depending on eating patterns
-sphincter closes when animal not eating and gallbladder is relaxed
-sphincter opens (relaxes) and gallbladder contracts when CCK released

Question 30

Hepatic excretory function

Answer 30

-hepatocytes perform many detoxifying functions
-bilirubin (end product of hemoglobin) is a very water-insoluble carried in blood bound to albumin
-hepatocytes conjugate bilirubin with glucuronic acid to form bilirubin diglucuronide (more soluble and can be excreted in canaliculi)
**when not excreted=jaundice