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SECRETIONS OF THE STOMACH Flashcards

1
Q

What are the 4 functions of the stomach?

A
  • Stores food
  • Mixes food with secretions
  • Regulates release of food into the duodenum
  • Secretes gastric juices
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2
Q

What cell types does the antrum, pylorus and duodenum contain?

A
  • G cells
  • D cells
  • No parietal cells
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3
Q

What cell types does the fungus and corpus contain?

A
  • Parietal cells
  • Chief cells
  • ECL and D cells
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4
Q

What mechanism does the parietal cells use to make acid and where is it found?

A
  • ATP driven H-K pump
  • Found in an apical membrane
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5
Q

What is the alkaline tide, explain how it works.

A

Every time a proton is made, the equivalent amount of bicarbonate is made

  • The H-K pump pumps a H+ into the lumen of the stomach
  • The H+ combines with a Cl- ion to form HCl
  • K+ recycled back to the lumen via K+ channels
  • The original H+ is generated from carbonic acid (H2CO3)
  • The remaining HCO3- is transported across the basolateral membrane via the HCO3-/ Cl- exchanger
  • HCO3- ends up in the blood
  • This means every time the stomach pumps a proton into the lumen of the stomach a bicarbonate ion ends up in the blood
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6
Q

What 2 things does the H-K pump require for full activity?

A
  • An alpha subunit for catalytic function
  • A beta subunit which is apical membrane targeting
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7
Q

Where are protons from the H-K pump, pumped into?

A

The lumen of the stomach

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8
Q

How does the pH of the blood change after a meal?

A

pH temporarily increases and becomes more alkaline because parietal cells are stimulated which results in HCO3- being transported into the blood

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9
Q

Write out the equilibrium that exists in relation to carbonic acid

A

H+ + HCO3- <—> H2CO3 <—-> CO2 + H2O

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10
Q

What are the 3 direct triggers of acid release?

A
  • ACh
  • Gastrin
  • Histamine
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11
Q

What stimulates ACh release?

A

Stimulation of the vagus nerve

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12
Q

What drives acid production in the cephalic phase?

A

ACh

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13
Q

Which cells produce histamine and what stimulates them to produce histamine?

A
  • ECL cells produce histamine
  • ACh stimulates ECL cells to produce histamine
  • Gastrin also stimulates ECL cells to produce histamine
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14
Q

What stimulates parietal cells to produce acid?

A

Histamine

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15
Q

What stimulates G cells to produce gastrin?

A
  • GRPs - gastrin releasing peptides
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16
Q

What two things does gastrin stimulate?

A
  • Gastrin stimulates ECL cells to produce histamine
  • Gastrin stimulates parietal cells to produce acid
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17
Q

What inhibits acid release and when is it produced?

A
  • Somatostatin produced by D cells inhibits acid release
  • Somatostatin is produced when the pH of the stomach is too low
  • Somatostatin also inhibits G cells which inhibits gastrin release and therefore parietal cell stimulation
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18
Q

What mechanism does somatostatin use to inhibit acid release?

A

Somatostatin antagonises histamine mediated acid production

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19
Q

Which pathways mediate production of ACh, gastrin and histamine?

A
  • ACh - (PLC/PKC)/ Ca2+ mediated intracellular signalling cascade
  • Histamine - PKA pathway mediated by adenylyl cyclase
  • Gastrin - (PLC/PKC)/ Ca2+ mediated intracellular signalling cascade
20
Q

Which receptors do ACh, histamine and gastrin bind to?

A
  • ACh - M3 receptors which stimulate parietal cells to produce acid
  • Histamine - H2 receptors on parietal cells which triggers acid production
  • Gastrin - binds to gastrin receptors on parietal cells which stimulates acid production
21
Q

What effect does ACh have on D cells?

A

Inhibits somatostatin from inhibiting acid release

22
Q

Describe the negative loop and the positive loop in the antrum of the stomach

A
  • Negative loop - High luminal H+ stimulates D cells to produce somatostatin which inhibits acid release
  • Positive loop - Products of protein digestion stimulates the G cell to produce gastrin, stimulating acid secretion
23
Q

Give 2 examples of other things that inhibit acid production and how

A
  • Secretin - released from duodenal S cells inhibits acid production by inhibiting gastrin release
  • CCK - produced by duodenal I cells. Is part of the same family as gastrin and acts as a competitive antagonist. Directly reduces acid secretion from parietal cells
24
Q

Give 2 pharmacological inhibitors of acid secretion

A
  • Proton pump inhibitors - acts directly on the H-K pumps
  • H2 receptor antagonists - competitive antagonists to histamine which binds to H2 receptors
25
What are the 4 phases of gastrin acid secretion?
- Basal - Cephalic - Gastrin - Intestinal
26
What is the basal phase of gastrin acid secretion?
- Follows a circadian rhythm where acid secretion is low a.m and high p.m
27
What is the cephalic phase and what triggers it?
- This phase is triggered by sights, smell, taste and thought of swallowing of food - It is mediated by the vagus nerve which triggers acid secretion - It accounts for 30% of acid production - This occurs before food is even swallowed
28
How much of gastric secretion is the gastric phase responsible for?
50-60% of gastrin secretion
29
What happens in the intestinal phase?
- Partially digested peptides/ AA in the proximal section of the small intestine stimulates acid secretion via G cells which secrete gastrin - This accounts for 5% to 10% of total gastrin secretion
30
What are pepsinogens, what are they secreted by and what are the active and inactive forms ?
- Pepsinogens are proteolytic proenzymes - They are secreted by chief cells - The active form is pepsin (an endopeptidase) - The inactive form is pepsinogen
31
What triggers pepsinogen secretion?
Acetylecholine
32
What is an endopeptidase?
A protease that cleaves proteins in the middle of the molecule. Mostly end up with oligopeptides
33
At what pH are pepsinogens irreversibly inactivated?
7.2
34
Where does the mucous barrier sit and how does it protect the stomach?
- Sits on mucosal surface - In the mucus there is bicarbonate trapped in there meaning if any acid gets into the mucus layer it can be neutralised
35
What are prostaglandins made from and what two things are they involved in?
- Made from precursors known as arachidonic acid - Involved in inflammation as well as maintaining normal physiology
36
Which enzymes produce prostaglandins that are involved in inflammation?
- COX - 2 - These enzymes are induceable meaning they aren't produced all of the time
37
Which enzymes produce prostaglandins that are involved in normal physiology?
- COX -1
38
What blocks COX-1 and COX-2 enzymes?
- NSAIDS e.g ibuprofen or aspirin - They can inhibit prostaglandins that are involved in gastric mucosal protection
39
What 3 things can H pylori bacteria cause?
- Gastritis - Gastric and duodenal ulcers - Gastric cancer
40
Give 2 adaptations of H pylori
- Flagella --> Mobility - Has urease which converts urea to ammonia -
41
What 2 things can H pylori infection in the antrum cause?
- Hyper secretion of gastrin from G cells - Decreased somatostatin release - This leads to hypergastrinemia
42
How is hypochlorhydia caused?
H. Pylori infection of the corpus leads to reduced acid secretion and this is linked with gastric ulcers
43
Location of hypochlorhydia and hypergastrinaemia?
- Hypochlorhydia - Antrum - Hypergastrinaemia - Corpus
44
Where is IF produced and what does it do?
- Absorption of B12 in the terminal ileum - Loss of this leads to Addisons anaemia
45
What is Addisons anaemia?
Autoimmune atrophic gastritis where antibodies attack parietal cells

DIG (6 decks)

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