Acid production and regulation

Question 1

4 main types of cells in stomach

Answer 1

Mucous cells, parietal cells (HCl and intrinsic factor), chief cells and enterendocrine cells (hormones, G cells secrete gastrin)

Question 2

Approx __ litres of acid produced per day

Answer 2

2

Question 3

Hydrogen pump is…

Answer 3

energy requiring, as H+ pumped against gradient

Question 4

Gastric acid secretion mechanism

Answer 4

H2O + CO2 –> H2CO3), catalysed by carbonic anhydrase. Spontaneously dissociates into a H+ and HCO3–.

The H+ is transported into stomach lumen via the H+– K+ ATPase ion pump, uses ATP to exchange K+ with H+.

HCO3– transported out of cell into blood via a transporter protein called anion exchanger in exchange for Cl–. This chloride ion is then transported into the stomach lumen via a chloride channel.

This results in both hydrogen and chloride ions being present within the stomach lumen. Their opposing charges leads to them associating with each other to form hydrochloric acid (HCl).

Question 5

Gastric phase (2) of regulating gastric acid secretion

Answer 5

Gastric distension, presence of peptides and amino acids

Gastrin released
Acts directly on parietal cells.

Triggers release of hormone histamine.
Acts directly on parietal cells.
Increased acid production.

Question 6

Proteins as regulator of acid production

Question 7

Effect of pH

Answer 7

Low luminal pH directly inhibits gastrin secretion, indirectly inhibits histamine release. Stimulates somatastatin (turns parietal cells off, inhibits acid production)

Question 8

Duodenum in regulation

Answer 8

duodenal distension, senses chyme is hypertonic, low pH and presence of amino acids and fatty acids so triggers release of enterogastrones.

Question 9

2 Enterogastrones

Answer 9

Secretin - inhibits gastrin release, promotes somatostatin release

Cholecystokinin (CKK)

Question 10

Cephalic phase (1) of acid production regulation

Answer 10

Parasympathetic nervous system
sight, smell, taste and chewing
Acetylcholine release
ACh acts directly on parietal cells
ACh triggers release of gastrin and histamine
Increased acid production.

Question 11

What are paracrine factors

Answer 11

Cells signal to each other, acts on neighbouring cells eg. histamine and somatostatin.

Question 12

What is a peptic ulcer?

Answer 12

An ulcer is a breach in a mucosal surface. (epithelium stripped away due to gastric acid)

Caused by:
helicobacter pylori infection
Drugs (NSAIDS - ibuprofen, aspirin)
Chemical irritants (alcohol, bile salts)
Gastrinoma (tumour, too much acid made and not regulated)

Question 13

How does gastric mucosa defend against attacks leading to ulcers?

Answer 13

Alkaline mucus,
tight junctions between epithelial cells,
replacement of damaged cells,
feedback loops to prevent too much acid being produced (negative feedback loop).

Question 14

How does helicobacter pylori work

Answer 14

Lives in gastric mucus. Secretes urease, splitting urea into CO2 and ammonia.
Ammonia + H+ –> ammonium

Damages gastric epithelium along with proteases, phospholipases and toxin known as vacuolating cytotoxin A.

Question 15

How do NSAIDs attack epithelium?

Answer 15

Non-steroidal anti-inflammatory drugs
Inhibit cyclo-oxygenase 1 which is needed for prostaglandin synthesis which stimulates mucus secretion.

Question 16

How do Bile salts attack epithelium?

Answer 16

Duodeno-gastric reflux
Strips away mucus layer

Question 17

Treating helicobacter pylori

Answer 17

Proton pump inhibitor
antibiotics

Question 18

What do omeprazole, lansoprazole and esomeprazole do?

Answer 18

Turn off hydrogen pump

Question 19

What does ranitidine (Zantac) do?

Answer 19

Histamine blocker

Question 20

Where is pepsinogen made

Answer 20

By chief cells, is a zymogen (inactive form)

Question 21

Secretion of pepsinogen is parallel to

Answer 21

HCl secretion

Question 22

Pepsin production is __ feedback loop

Answer 22

A positive feedback loop.

Pepsin catalyses reaction of pepsinogen to pepsin. Rapidly cascades to make lots of pepsin.

Question 23

Why is pepsinogen made not pepsin from beginning?

Answer 23

Pepsin would break down the chief cells themselves as made from protein! (would be autodigestion)

Question 24

Conversion of pepsinogen to pepsin is dependent on what

Answer 24

pH
Most efficient when pH < 2.

Irreversible inactivation in small intestine by HCO3-.

Question 25

Can you survive without pepsin?

Answer 25

Yes - it is not essential. Only accelerates protein digestion.
You can even live without a stomach.

Question 26

Receptive relaxation

Answer 26

Mediated by vagus nerve (parasympathetic nervous system acting on enteric nerve plexuses)
Triggered by sight/smell/chewing and presence of food.

Question 27

Peristalsis of stomach

Answer 27

Waves begin in gastric body, weak contraction.
More powerful contraction in gastric antrum. Pylorus closes as peristaltic waves reaches it (mixes).

Question 28

Pacemaker cells of stomach

Answer 28

Interstitial cells of Cajal.
In muscularis propria, undergo slow depolarisation-repolarisation cycles. 3 per minute.
Depolarisation waves transmitted through gap junctions to adjacent smooth muscle cells.

Question 29

Factors effecting strength of peristaltic contractions

Answer 29

Excitatory neurotransmitters and hormones (gastrin) further depolarise membranes.

Reduced peristaltic contraction by:
increase duodenal luminal fat, osmolarity, sympathetic NS action or decreased duodenal luminal pH, parasympathetic NS action.