L9 Digestive Secretions

Question 1

How can the liver be divided?

Answer 1

The liver is divided into a right and left lobe and the falciform ligament. The liver can be also be divided into 8 segments (Coninaud segments) determined by vasculature.

Question 2

Give the source, stimulus, pathway and 1st target of. the following hormone:

Gastrin

Answer 2

Source: G cells
Stimulus: Neural, amino acids and peptides
Pathway: Endocrine
1st target: ECL and parietal cells

Question 3

Give the source, stimulus, pathway and 1st target of. the following hormone:

Cholecystokinin

Answer 3

Source: I Cells (mainly from the duodenum and jejunum)
Stimulus: Fatty acids and some amino acids
Pathway: Endocrine and paracrine
1st target: Vagal and afferent terminals of the gall bladder

Question 4

Give the source, stimulus, pathway and 1st target of. the following hormone:

Secretin

Answer 4

Source: S cells (Duodenum)
Stimulus: Acid in the SI
Pathway: Endocrine and paracrine
1st Target: Vagal afferent terminals, pancreatic duct cells and Cholangiocytes (epithelial cells of the bile duct).

Secretin helps regulate the pH of the duodenum by (1) inhibiting the secretion of gastric acid from the parietal cells of the stomach and (2) stimulating the production of bicarbonate from the ductal cells of the pancreas.

Question 5

Give the source, stimulus, pathway and 1st target of. the following hormone:

Motilin

Answer 5

Source: Mo cells (Intestine)
Stimulus: Neural, fasting
Pathway: Paracrine
1st Target: Upper GI motility

Question 6

Give the source, pathway and 1st target of. the following hormone:

Vasoactive intestinal peptide

Answer 6

Source: Nerve terminals
Pathway: Neurocrine
1st Target: smooth muscle and secretory cells

Question 7

What are the sites of secretion of Gastrin, CCK, Secretin and motilin?

Answer 7

Gastrin is secreted mainly in the antrum. Secretoon decreases at the duodenum.

CCK is secreted at the duodenum and jejunum; secretion decreases at the ileum.

Secretin is secreted at the duodenum.

Motilin is secreted at the duodenum and jejunum,

Question 8

What are the phases of pancreatic juice secretion?

Answer 8

Cephalic phase when you see or taste food stimulates through the vagus nerve, the pancreas to produce digestive juices (20%). Enzyme rich and low level of bicarbonate.

Gastric phase when food is in the stomach (10% of secretion). This is protein and enzyme rich. Sensors in the wall of the stomach or SI get stretched causes more secretion due to the enteric nervous system. This also occurs through the ANS and the vagal system at the level of the brainstem.

Intestinal phase when food is in the intestines (70%). Contents in gastric chyme. Food has lots of acid. Less protein but more bicarbonate rich to neutralise the acid to help increase the pH to allow onward digestion.

Question 9

What is the effect of CCK?

Answer 9

Main effect is on the pancreatic acini to increase secretion and the gall bladder to increase emptying.

Question 10

What is the difference between the CCK receptors?

Answer 10

CCK-A receptors (main receptor for the intetsine but also found in the pituitary and the brain)are better for CCK, CCK-B receptors are better for gastrin (mainly found in the brain but also in pancreatic islet cells and stomach).

Question 11

How is CCK modified?

Answer 11

CCK is an peptide hormone secreted by I cells in the small intestine. It is a 33 aa. Synthesised as a zymogen called procholecyctokinin. The last 5 amino acids are identical to the last 5 of gastrin. Amidation of the C-terminal increases function and sulphation of a C-terminal Tyr residue is essential for binding to CCK-A receptor. Tyrosine residue also goes sulphation.

Question 12

What is the length of CCK?

Answer 12

33aa

Question 13

What is the length of secretin?

Answer 13

121 aa

Question 14

Give examples of pancreatic lipase.

Answer 14

• Pancreatic lipase
• Nonspecific esterase
• Prophospholipase A2
(Procolipase)

Question 15

Give examples of nucleases.

Answer 15

• Deoxribonuclease

* Ribonuclease

Question 16

What is monitor peptide?

Answer 16

Monitor peptide stimulates cholecystokinin (CCK) release from the intestine.

Question 17

How is water potential maintained when proteins are secreted?

Answer 17

CCK-A receptors are found on the membrane of acinar cells. When stimulated with CCK, protein synthesis is stimulated and there is fusion of vesicles containing the pro-enzymes to the membrane. CCK works by increasing intracellular calcium.

Acinar cells also secrete an isotonic plasma rich fluid to dilute the protein being secreted out. This happens by an ATP driven potassium pump. This builds up chloride. Increasing intraluminal chlorine. This means that sodium and water can pass through tight junctions by passive diffusion to diffuse the pancreatic juice.

Question 18

What is bile?

Answer 18

Consists of a complex mixture of components.
Bile acids/salts - steroid, xenobiotics, vitamins, cholesterol, amino acids, enzymes, heavy metals, exogenous drugs and environmentall toxins. Bile pigments from the breakdown of haemoglobin. Synthesised by the liver
Recycled (up to 8 times a day!)
Amphipathic – polar groups are all on one side
Form micelles at high concentration

Question 19

What are the functions of bile?

Answer 19

(i) Bile is the major excretory route for potentially harmful exogenous lipophilic substances, noted above, as well as other endogenous substrates such as bilirubin and bile salts.
(ii) Emulsify fats
(iii) Bile is the major route for elimination of cholesterol.
(iv) Bile protects the organism from enteric infections by excreting immune globulin A (IgA),inflammatory cytokines, and stimulating the innate immune system in the intestine.
(v) many hormones and pheromones are excreted in bile.

Question 20

How is bile recycled?

Answer 20

95% of the bile acids which are delivered to the duodenum will be recycled by the enterohepatic circulation. Along the proximal and distal ileum, these conjugated primary bile salts are reabsorbed actively into hepatic portal circulation.

Question 21

What is the role of the gall bladder?

Answer 21

The gallbladder stores bile, and concentrates it.

Question 22

What is the role of trypsinogen?

Answer 22

Trypsin hydrolyse peptides in which the basic amino acid supplies the carboxyl group

Question 23

What is the role fo chymotrypsinogen?

Answer 23

Hydrolyse peptide bonds in which aromatic amino acids supply carboxyl group

Question 24

What is the role of proelastase?

Answer 24

Hydrolyse peptide bonds in which neutral aliphatic amino acids supply carboxyl group