Gastric acid secretions and regulation

Question 1

What secretions are in the thin walled upper portion of the stomach?

Answer 1

Thin-walled upper portion of the stomach (fundus and body): mucus, HCl
and pepsinogen

Question 2

What are walls of the glands in stomachs lined with?

Answer 2

Wall of the glands is lined with parietal
cells→ HCl and intrinsic factor

Question 3

What are exocrine secretions of the stomach?

Answer 3

mucus, acid, pepsinogen

Question 4

What do enterochromaffin-like cells secrete?

Answer 4

paracrine agents, e.g. histamine

Question 5

How is gastric acid in the lumen of the stomach made?

Answer 5

1. HCO3- is exchanged for Cl- in the blood. This decreased acidity of venous blood from stomach compared to blood serving it.
2. Excess Cl- diffuses out into the stomach lumen through chloride channels as the H+ is pumped into the stomach lumen-the K+/H+-ATPase pumps H+ out into stomach lumen
3. The Cl- goes into gastric glands, which then binds H+ to form HCl

Question 6

What does the mucus lining in the stomach do?

Answer 6

↑HCO3-; forms water-insoluble gel on
epithelial surface; protects against H+ secretion

Question 7

What does Rennin(chymosin) do?

Answer 7

Curdles milk into casein clot

Question 8

What does lipase do in the stomach?

Answer 8

triglycerides → fatty acids and glycerol

Question 9

What do intrinsic factors do in the stomach?

Answer 9

Involved with absorption of vitamin B12

Question 10

What does HCl in stomach do?

Answer 10

kills bacteria; acid denaturation of digested food; activates pepsinogen
(protein digestion

Question 11

What does the cephalic phase promote the secretion of?

Answer 11

Cephalic phase (meal times- smell, sight, taste, chewing) promotes the
secretion of acetylcholine (ACh) and gastrin

Question 12

What does Ach stimulate the release of and by what cells in the cephalic phase?

Answer 12

ACh stimulates histamine release from ECL cells

Question 13

What cells do ACh directly act on in the cephalic phase?

Answer 13

ACh acts directly on parietal cells → HCl secretion

Question 14

What does gastrin stimulate release of and from what cells in the cephalic phase?

Answer 14

Gastrin stimulates histamine release from ECL cells

Question 15

What cells does gastrin act directly on and what does this secrete in the cephalic phase?

Answer 15

Gastrin acts directly on parietal cells → HCl secretion

Question 16

What does somatostatin inhibit?

Answer 16

Somatostatin inhibits acid secretion

Question 17

What does the gastric phase promote secretion of and why?

Answer 17

Promotes the secretion of acetylcholine (ACh) and gastrin due to distension of stomach and increase in peptide concentration

Question 18

What does an increase in peptide concentration and distension of stomach?

Answer 18

Distension of stomach:
1. Distension activates neural reflexes and gives a positive impulse to enteric neurons
2. This results in the release of acetylcholine which activates parietal cells releasing HCl
Peptides:
1. Stimulates G cells which secrete gastrin.
2. Gastrin stimulates ECL cells which secrete histamine that trigger parietal cells.
3. This results in a release of HCl

Question 19

What do proteins do to luminal acidity and there how does this impact HCl secretion?

Answer 19

-Proteins act as buffers in the gastric lumen
- ↓ [H +]; proteins remove the inhibitory powers of HCl on gastrin secretion and hence acid secretion

Question 20

What is the inhibitory signals when there is food in the stomach during the gastric phase?

Answer 20

1. HCl stimulates somatostatin releasing cells
2. This releases somatostatin which inhibits parietal cells, G cells and ECL cells

Question 21

What happens during the intestinal phase?

Answer 21

balances the secretory activity of the stomach and the
digestive and absorptive capacities of small intestine

Question 22

What happens if there is high acidity of duodenal contents and what does this prevent?

Answer 22

High acidity of duodenal contents reflexly inhibits acid secretion –
prevents chyme from becoming too acidic

Question 23

What does inhibition of acid secretion in the small intestine depend on?

Answer 23

1. Composition of chyme
2. Volume of chyme

Question 24

How is acid secretion inhibited during the intestinal phase?

Answer 24

1. Short and long neuronal reflexes and hormones (enterogastrones, e.g.
   secretin and CCK) inhibit acid secretion by the parietal cells or gastrin
   secretion by the G cells.
2. Somatostatin

Question 25

What happens to the sympathetic and parasympathetic discharge in order to inhibit acid secretion?

Answer 25

↑ sympathetic discharge – inhibitory;
↓ parasympathetic discharge - stimulatory

Question 26

What happens in the direct pathway of the secretagogues?

Answer 26

In the direct pathway, Ach, gastrin and histamine stimulate the parietal cell, triggering the secretion of H+ into the lumen

Question 27

What happens in the indirect pathway of the secretagogues?

Answer 27

In the indirect pathway, Ach and gastrin also stimulate the ECL cell, resulting in secretion of histamine. This histamine then acts on the parietal cell

Question 28

What receptors does Ach act on in parietal cells?

Answer 28

M3 receptors

Question 29

What receptors does histamine act on in parietal cells?

Answer 29

H2(histamine receptors)

Question 30

What receptors does gastrin act on in parietal cells?

Answer 30

CCK2(gastrin receptors)

Question 31

What is the synergism between the stimulators of acid secretion?

Answer 31

When M3, H2 and CCK-B receptors are stimulated, Ca2+ and cAMP is generated allowing the tubulovesicles with H+, K+ ATPase channels to fuse with canaliculus

Question 32

What does HCl stimulate the flow of?

Answer 32

Stimulates flow of bile and pancreatic juice:

Question 33

What does HCl do in protein digestion?

Answer 33

activates pepsinogen to pepsin

Question 34

What is pepsin secreted by?

Answer 34

Secreted by chief cells in the form of pepsinogen

Question 35

When are chief cells activated and what happens to them? What type of feedback process is it?

Answer 35

Activated if [H+] is high; shape altered by high acidity which exposes its
active site
-Autolytic feedback process

Question 36

What is secretion of pepsin inactivated upon?

Answer 36

Inactivated upon entry of food in the small intestine (HCO3- and peptides
neutralise the H+)

Question 37

What is the input to chief cells?

Answer 37

Inputs to chief cells from nerve plexus

Question 38

What does it mean by there being parallels between acid secretion and pepsinogen secretion?

Answer 38

In essence, stimulators/inhibitors of acid secretion during the cephalic
and intestinal phases exert same effect on pepsinogen secretion