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Gastrointestinal physiology > Gastric secretion > Flashcards

Gastric secretion Flashcards

1
Q

What does the stomach secrete (5) ?

A 
H+
Pepsinogen
Mucus
HCO3-
Intrinsic factor (IF)
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2
Q

What part of the stomach secretes the most acid, pepsinogen and IF?

A

Body

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3
Q

What does antrum do?

A

Secrete somatostain, gastrin and holds food

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4
Q

What is the structure of the gastric gland?

A

Gastric pits which open into necks and leads to base, deep invaginations in the epithelial layer

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5
Q

What 6 cells make up gastric pits?

A

Parietal (oxyntic) cells in the base and neck: secrete HCl and intrinsic factor (for vitamin B12 absorption in the ileum)

Chief (peptic) cells in the base and neck: secrete pepsinogen

Endocrine cells in the base: secrete regulators such as gastrin, somatostatin via the bloodstream

Mucous neck cells: secrete mucus

Superficial epithelial cells in the pit and on the surface lining: secrete mucus along with HCO3- ions

Basal regenerative cells

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6
Q

What cells secrete IF?

A

Parietal

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7
Q

What secretions are there in an unstimulated stomach, what cells from?

A

Non-parietal surface epithelial cells

Juice is rich in Na+, Cl-, and is isotonic.

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8
Q

What secretions are there in a stimulated stomach, what cells from?

A

Secreted by parietal cells

Concentrated HCl.

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9
Q

What factor determines which juice is secreted by the stomach, how does it vary in a stimulated vs unstimulated stomach?

A

Secretory rate

When stimulates the secretory rate is much faster

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10
Q

What do parietal tubovesicles contain?

A

H+/K+ATPase.

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11
Q

What does stimulation of parietal cell trigger tubovesicles to do?

A

Merge into the apical membrane

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12
Q

How does the shape of the parietal cell membrane change when stimulated?

A

Stimulated parietal cells posses deep invaginations of apical membrane called secretory canaliculus.

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13
Q

How does the surface area of the parietal cell change?

A

When stimulated H+/K+ATPase containing tubovesicles merge with the membrane and the number of canaliculi ride

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14
Q

Why does the H+/K+ATPase have no effect on cytoplasm pH when in vesicles?

A

Vesicle is arranged so that ATPase is ‘inside out’ (H+ pumped in K+ pumped out)

K+ cannot recycle by entering the vesicle (vesicle impermeable to K+)

[K+] in the vesicle falls, low K+ availability within the vesicle ‘brakes’ the activity of the ATPase.

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15
Q

Describe the mechanism and transporters behind secretion of H+ and Cl-

A

Intracellular CA catalyses hydration of CO2 to yield H+ and HCO3-

H+/K+ATPase pumps H+ ions into the lumen in exchange for K+

K+ recycles out of the cell through apical K+ channels

HCO3- exits across basolateral on Cl-/HCO3- exchanger

Cl- ions diffuse through apical channels to join H+ ions in the lumen.

Water follows by osmosis

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16
Q

What are 3 ways of regulating gastric acid secretion?

A

Neurocrine: ACh from vagus binds M3 and stimulate GRP (gastrin releasing peptide) and histamine release

Endocrine: gastrin (in response to GRP from vagus nerve or AA in lumen) binds CCKB receptors, triggers IP3 cascade

Paracrine: histamine from ECL cells, binds H2 receptors (adenylyl cyclase cascade)

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17
Q

What underpins all the mechanisms behind activating parietal cells?

A

Kinase activation leads to cytoskeletal rearrangement via phosphorylation which leads to tubulovesicle insertion

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18
Q

What is the common mediator hypothesis?

A

ACh and gastrin bind M3 and CCKB receptors on ECL (enterochromaffin-like cells) cells to induce histamine release, as well are their role on parietal cells.

19
Q

Why is histamine action arguably the most important factor for affecting parietal cell secretions?

A

It is unregulated by ACh and gastrin

20
Q

What are 3 phases of gastric acid secretion?

A

Cephalic: ACh and GRP release after vagal stimulation

Gastric: distension initiates vagovagal reflex (triggers acid & histamine secretion) and protein digestion products trigger gastrin release

Intestinal: protein digestion products stimulate duodenal G cells

21
Q

What do endocrine D cells do?

A

Release somatostatin which reduces gastric acid secretion

22
Q

What do endocrine D cells do?

A

Release somatostatin which reduces gastric acid secretion

23
Q

What hormones does secretin stimulate and inhibit?

A

Stimulates somatostatin

Inhibits gastrin

24
Q

What do chief cells secrete?

A

Pepsinogens

25
Q

How are pepsinogens activated?

A

Activated by N-terminal truncation, spontaneous activation occurs in acidic lumen.

Low pH is also required for optimal activity of pepsin.

26
Q

What % of ingested protein do pepsins digest?

A

20%

27
Q

What is pepsin release stimulated by (&pathway)?

A

ACh, via M3 receptors, Ca2+ signalling

Gastrin and CCK, via CCKB receptor, Ca2+signalling

Secretin, via adenylyl cyclase-coupled receptors, cAMP signalling

28
Q

What is mucin?

A

Large glycoprotein with high viscosity

29
Q

Why is it essential to have a protective mucous layer?

A

Gel is a barrier to H+ ion diffusion from the acidic bulk solution in the lumen

Protects the gastric mucosa

30
Q

What cells secrete HCO3-?

A

Surface cells

31
Q

How does mucous move out of the pits?

A

Secretion of gel raises the hydrostatic pressure inside the gastric pits, this forces the secretions upwards in the pit.

32
Q

How does pressure in pit act as a protective mechanism?

A

Encourages the acid to bore through the mucous without lateral spread

33
Q

What is the stream of H+ exiting the pit termed as?

A

Viscous finger

34
Q

What induces mucous secretions?

A

ACh induced Ca2+ signalling

35
Q

What prevents autodigestion of stomach mucosa from acidic contents?

A

Mucous cells produce mucus which is barrier to H+ diffusion

Surface epithelial cells secrete HCO3- to neutralise H+ which penetrates mucous lining

36
Q

What are the causes of peptic ulcers?

A

Diet e.g. spicy food, alcohol

H pylori

Excess acid from hypo/hyperglycaemia or Zollinger Ellison

NSAID drugs directly damage mucosa

37
Q

What is the secondary messenger for histamine induced acid release?

A

cAMP

38
Q

What action does CCK have on gastric acid secretion?

A

Inhibits it

39
Q

What does ranitidine inhibit?

A

H2 receptors - acid secretion by histamine action

40
Q

What does omeprazole inhibit?

A

H+/K+ATPase - acid secretion

41
Q

What is the common mediator for stimulation of gastric acid secretion?

A

Histamine

42
Q

What is the main inhibitor of gastric acid secretion?

A

Somatostatin (secretin also does by upregulating it)

CCK? (if only option)

43
Q

What drug most completely suppresses gastric acid secretion?

A

Omeprazole

Gastrointestinal physiology (5 decks)

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