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Digestive and Renal Sciences > Secretions of the stomach > Flashcards

Secretions of the stomach Flashcards

1
Q

What are the functions of the stomach?

A

Store food
Mixes food with secretions
Regulates release of food into duodenum
secretes gastric juices

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2
Q

What is the anatomy of the stomach?

A 
Oesophagus
Caria
Fundus
Corpus
Antrum
Pylorus
Duodenum
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3
Q

What are the different cell types within the stomach?

A 
Mucous Neck cells
Parietal Cell 
ECL cells 
Chief cells
D cell 
G cells
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4
Q

How do the parietal cells produce acid?

A

Mediated ultimately by the parietal H-K pump embedded in apical membrane
The H-K pump requires both and alpha and beta subunit for full activity

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5
Q

What is the process of the parietal cell making stomach acid?

A
  1. CO2 from the blood enters into the parietal cell
  2. The CO2 combines with H2O to form H2CO3 (carbonic acid).
  3. The carbonic acid splits into H+ ion and HCO3- ions via the Carbonic anhydrase enzyme
  4. The H+ ion travels in the lumen of the gastric gland in exchange for a K+ ion via a H-K pump.
  5. The HCO3- ion exits the parietal cell across the basolateral membrane via a Cl-HCO3- exchanger.
  6. The Cl brought into the parietal cell diffuses into the lumen through a Cl ion channel where it combines with the H+ ion to form HCl
  7. The K+ ion in the parietal cell returns to the lumen through its own ion channel.
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6
Q

What regulates acid secretion?

A

Paritetal cell has 3 Direct triggers:
Ach (from vagus nerve) via PKC pathway
Histamine - onto H2 histamine receptor via PKA pathway (greatest contribution)
Gastrin - locks onto the CCKb receptor via PKC pathway

Indirect Triggers:
Ach and gastrin mediated by histamine release by ECL cells (Ach triggers ECL cells to release histamine)

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7
Q

How is acid secretion stimulated?

A

mediated by a intracellular signal-transduction processes involving G-protein-coupled receptors on the parietal-cell membrane . Notably PKA and PKC

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8
Q

Explain how the 3 secretagogues (ACh, gastrin and histamine) stimulate secretion

A
  • Acetylcholine binds to an M3 muscarinic receptor. This culminates in activation of protein kinase Ca2+ channel (PKC) and intracellular calcium
  • Gastrin binds to the cholecystokinin B (CCKB) receptor. Similarly activates PKC.
  • Histamine activation of the H2 receptor stimulates the enzyme adenyl cyclase, which results in production of cAMP and activation of PKA.
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9
Q

How are G cells involved with cell acid secretion in the corpus?

A

G cells are stimulated by GRP which indirectly stimulates the parietal cells.

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10
Q

What affect does ACh have on D cells?

A

ACh inhibits D cells preventing the release of somatostatin which antagonises histamine production

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11
Q

What affect do amino acids have on G cells?

A

peptides and amino acids are able to stimulate G cells to produce gastrin which can directly interact with the parietal cells and trigger acid secretion.

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12
Q

How does the D cell regulate the amount of acid in the corpus?

A

D cells in the antrum detect high acid levels and therefore secrete somatostatin inhibiting the function of the G cells

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13
Q

What are the inhibitors of acid secretion?

A

Somatostatin - main inhibitor
Secretin - stimulated release by fat and low pH in duodenum. Secretin inhibits antral gastrin release and stimulates somatostatin release
CCK - produced by I cells and release in response to fat. CCK directly reduces parietal cell acid secretion

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14
Q

How can we pharmacologically inhibit acid secretion?

A

By inhibiting the proton pump we can prevent acid secretion instead of reducing it by only targeting one of the 3 triggers

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15
Q

Give an example of a drug which blocks the proton pump

A

Omeprazole has direct interaction on the proton pump itself

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16
Q

Give an example of a drug which inhibits the H2 receptors

A

Cimetidine and ranitidine are H2 antagonists which prevent histamine trigger on acid secretion. (histamine targeted as this is the most effecting trigger)

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17
Q

What are the 4 phases of gastric acid secretion?

A
  1. Basal

2. Cephalic 3.Gastric 4.Intestinal

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18
Q

What happens in the basal phase of acid secretion?

A

Follows a circadian rhythm; acid secretion is low a.m. and high p.m.
• Acid secretion is a direct function of the number of parietal cells, so size does matter

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19
Q

What happens in the Cephalic phase of acid secretion

A

The smell, sight, taste, thought, and swallowing of food initiate the cephalic phase, which is mediated by the vagus nerve, thus triggering acid secretion

• Accounts for approximately 30% of total acid secretion and occurs before the entry of any food into the stomach

20
Q

What happens in the Gastric phase of acid secretion

A

Digestion of food in the stomach releases ACh which then triggers both directly and indirectly acid secretion in the parietal cell
(accounts for 50-60% of total gastric secretion)

21
Q

What happens in the Intestinal phase of acid secretion

A

Partially digested peptides/amino acids in the proximal portion of the small intestine stimulates acid secretion predominantly by stimulation of duodenal G cells to secrete gastrin

22
Q

What are pepsinogens?

A

Proteolytic proenzymes secreted by chief cells

23
Q

What is the trigger for pepsinogen secretion?

A

Acetylcholine

24
Q

How does pepsinogen work?

A

Conversion of pepsinogen to pepsin occurs spontaneously at pH<5. Most active at <3
• Pepsin auto-activates pepsinogen
• Irreversibly inactivated at pH 7.2
• Interlinked with acid secretion because of similar trigger (Ach) and resulting peptides trigger antral Gcell gastrin secretion
• Pepsin is an endopeptidase which initiates protein digestion

25
Q

What is gastric mucosal protection?

A

Protection from the harsh pH environment by a mucous layer which traps HCO3- ions and maintains the mucosal pH at 7

26
Q

Why are Prostaglandins important?

A

Important in maintaining the mucosal diffusion barrier by inhibiting acid secretion.

27
Q

What are the functions of prostaglandins?

A

Functions in both physiology and pathology. Maintains pH levels and has a role in inflammation

28
Q

What do COX-2 enzymes do?

A

Inhibit prostaglandins and therefore can be uses to purposefully cause an inflammatory response

29
Q

What are some common side effects of COX enzyme inhibitors?

A

Common GI side effects experienced in (non selective) NSAID users including gastric ulceration and dyspepsia

30
Q

What are Helicobacter pylori?

A

Gram-negative microaerophilic bacteria

They are acid loving microbes colonising in the stomach of 40% of humans.

31
Q

What do Helicobacter Pylori do to infected individuals?

A

Cause peptic ulceration

32
Q

How does Helicobacter Pylori cause Peptic Ulcer disease?

A

By urease which causes urea to be converted into ammonium and bicarbonate ions which will neutralise the stomach acid.
Urea—->NH3 + CO2+ H20 —-> NH4+ + HCO3-

33
Q

What does Helicobacter Pylori infection cause in the antrum?

A

G-cells to hypersecrete gastrin 2. Decrease antral D-cell somotastatin release

This leads to hypergastinaemia and duodenal ulcers

34
Q

What does Helicobacter Pylori infection cause in the corpus?

A

Hyperchlorhydria largely associated with gastric ulcers

35
Q

What is intrinsic factor?

A

produced by parietal cells

Necessary for the absorption of vitamin B12

36
Q

What is the result of a loss of intrinsic factor?

A

causes megaloblastic anaemia or pernicious anaemia which is an autoimmune atrophic gastritis.

37
Q

What do the mucous neck cells secrete?

A

Mucous and bicarbonate

38
Q

What do the parietal cells secrete?

A

Acid and intrinsic factors

39
Q

What do the ECL cells secrete?

A

Histamine

40
Q

What do the Chief cells secrete?

A

Pepsinogen and lipase

41
Q

What do the D cells secrete?

A

Somatostatin

42
Q

What do the G cells secrete?

A

Gastrin

43
Q

What is Alkaline tide?

A

The process of bicarbonate ions entering the blood as a consequence of acid secretion.

44
Q

What is vitamin B12 important for?

A

normal function of brain and nervous system and for the production of red blood cells.

45
Q

What is hypergastrinaemia?

A

presence of excess gastrin in the blood

46
Q

What is hyperchlorhydria?

A

excess stomach acid levels

47
Q

What is megaloblastic / pernicious anaemia?

A

deficiency in red blood cell production.

Digestive and Renal Sciences (26 decks)

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