Session 13

Question 1

What is the role of the parietal cells?

Answer 1

Secrete hydrochloric acid, a component of gastric juice, into large canaliculi.

Question 2

Describe the mechanism of acid secretion.

Answer 2

H+ ions generated in parietal cell from dissociation of water.
OH- ions combine with CO2 to form HCO3-.
Bicarbonate transported out in exchange for chloride.
Cl- and K+ transported into the canaliculi lumen.
H+ pumped into the canaliculi lumen, in exchange for K+, via H+K+ATPase.
Accumulation of H+ generates an osmotic gradient across the membrane, resulting in outward diffusion of water.

Question 3

Give three stimulators of acid secretion.

Answer 3

ACh, gastrin, histamine.

Question 4

Describe the role of H2 receptor antagonists.

Answer 4

E.g. ranitidine, cimetidine. Blocks the parietal cell receptor for histamine.

Question 5

Describe the role of PPIs

Answer 5

Non-competitively bind and inactivate H+K+ATPase, located in the cannalicular membrane of the parietal cell. Strong inhibition of acid secretion. E.g. Omeprazole, Lansoprazole.

Question 6

Describe the step-up process to manage GORD/oesophagitis.

Answer 6

Lifestyle modification, antacids/alginates, H2 receptor blockers, then PPIs.

Question 7

Give three ADRs of H2 receptor blockers.

Answer 7

Hypotension, headache, tiredness, dizziness, confusion, diarrhoea, constipation.

Question 8

How does Cimetidine affect CYP metabolism?

Answer 8

It is a CYP inhibitor.

Question 9

Give three ADRs of PPIs.

Answer 9

Headache, nausea, diarrhoea, fatigue, dizziness, rash, itch, flatulence, constipation.

Question 10

Describe the myogenic control of gastric motility.

Answer 10

Rhythmic contraction, passive current spread through gap junctions and interstitial cells of Cajal act as a pacemaker.

Question 11

Describe the neuronal control of gastric motility.

Answer 11

Stimulation of post-ganglionic cholinergic enteric nerves increases force of contraction of the gut. Stimulation of non-adrenergic inhibitory nerves inhibits contraction.

Question 12

Where is Auerbach’s plexus located?

Answer 12

Between circular and longitudinal muscle layers.

Question 13

What is the intestino-intestinal inhibitory reflex?

Answer 13

Distension of one intestinal segment causes complete intestinal inhibition.

Question 14

What is the anointestinal inhibitory reflex?

Answer 14

Distension of the anus causes intestinal inhibition.

Question 15

What are the gastocolic and duodenocolic reflexes?

Answer 15

Stimulate motility after material has entered the stomach or duodenum.

Question 16

Give three neurotransmitters involved in controlling gastric motility.

Answer 16

Gastrin, secretin, CCK, motilin and paracrine transmitters (histamine, somatostatin and prostaglandins).

Question 17

How does gut motility differ in the fed and fasting state?

Answer 17

Fasted: motility is highly organised into a distinct and cyclic recurring or peristalsis.
Fed: pattern is temporarily abolished and replaced by a band of random contractions.

Question 18

How can gastrointestinal motility be measured?

Answer 18

Manometry. Uses various pressure sensors to measure contractility.

Question 19

What is emesis?

Answer 19

Phyloric sphincter closes while the cardia and oesophagus relaxes. Gastric contents propelled by contraction of abdominal wall and diaphragm. Glottis closes with elevation of the soft palate preventing entry of vomitus into the trachea and nasopharynx.

Question 20

Describe the role of dopamine D2 receptor antagonists in anti-emesis medication.

Answer 20

E.g. Domperidone. Increases rate of gastric emptying. Does not cross the BBB. Also stimulates prolactin release (galactorrhoea).

Question 21

Describe the role of 5-HT3 antagonists in anti-emesis medication.

Answer 21

E.g. Ondansteron. Works against vagal afferent nerves in the GI, hence reduced vagal stimulation.

Question 22

Describe the role of Metoclopramide in anti-emesis medication.

Answer 22

D2 antagonist, anti-cholinergic effects and blocks vagal afferent 5-HT3.

Question 23

When should Metoclopramide be avoided?

Answer 23

In Parkinson’s due to extra-pyramidal reactions.

Question 24

Describe the role of anti-muscarinics in anti-emesis medication.

Answer 24

E.g. Hyoscine. Antagonist of muscarinic cholinergic receptors. Used to treat motion sickness.

Question 25

Describe the role of Histamine H1 receptor antagonists in anti-emesis medication.

Answer 25

E.g. Cyclizine. Also has anti-muscarinic effects. Can cause QT prolongation. Crosses the BBB.

Question 26

Describe the role of bulk laxatives.

Answer 26

E.g. Ispaghula. Vegetable fibre in attempt to re-establish normal bowel habits. Relieves constipation.

Question 27

Describe the role of faecal softeners.

Answer 27

Arachis oil and glycerol lubricate and soften stool.

Question 28

Describe the role of magnesium and sodium salts.

Answer 28

Osmotically active laxatives. Cause water retention in bowel to increase peristalsis. Act quickly and are severe.

Question 29

Describe the role of irritant/stimulant laxatives.

Answer 29

Excitation of sensory nerve endings leads to water and electrolyte retention and thus peristalsis. Used for rapid treatment. E.g. castor oil, bisacodyl, anthraquinones.

Question 30

Which type of laxative would you give if the history revealed hard faeces?

Answer 30

Osmotic laxatives or bulk-forming laxatives.

Question 31

Which type of laxative would you give if the history revealed soft faeces?

Answer 31

Stimulant laxatives (senna, bisacodyl, glycerol).

Question 32

Describe the role of anti-motility drugs for managing diarrhoea?

Answer 32

Act via opioid receptors in the bowel. Reduce bowel motility, increase anal tone and reduce sensory defecation reflex.

Question 33

Describe the role of bulk-forming drugs for managing diarrhoea?

Answer 33

Drugs such as ispaghula act via water absorption. Useful for patients with IBS.

Question 34

Describe the role of fluid absorbents for managing diarrhoea?

Answer 34

Kaolin acts as a fluid absorbent, therefore producing a more formed stool.

Question 35

Describe the role of Mebeverine for the treatment of IBS.

Answer 35

Direct effects on hypermotility and relieves spasms of intestinal muscle. Useful when combined with a bulk forming agent.

Question 36

Give three examples of drugs that cause diarrhoea?

Answer 36

Laxatives, antibiotics, chemo, lansoprazole, opiates, diuretics, iron preparations, anticholinergic drugs, calcium/aluminium antacids, verapamil etc.