L5 - Gastric secretions

Question 1

what are the components and functions of gastric secretions

Answer 1

gastric acid:
- protective role
- denature proteins
- optimum pH (1-2) for digestive enzymes (pepsin)
pepsin:
- protein digestion
intrinsic factor:
- vitamin B12 absorption (DNA, red blood cells, pernicious anaemia)
mucus:
- protects against acid and mechanical forces

Question 2

what are the secretions of each section of the stomach?

Answer 2

lower oesophageal sphyncter and cardia:
- mucus, HCO3-
fundus and body:
- H+, intrinsic factor, mucus, HCO3-, pepsinogens
Antrum and pylorus:
- mucus, HCO3-

Question 3

functional anatomy of the stomach (cell types)

Answer 3

gastric pits have (from superficial to deep);
- surface cells: HCO3-, mucus
- mucus secreting cells
- oxyntic (parietal) cells: acid, intrinsic factor
- enterochromaffin-like (ECL) cells: histamine
- chief cells: pepsingoen

Question 4

what is the volume and composition of secretion in the stomach?

Answer 4

2-3 litres per day:
- composition varies depending on eating or fasting
between meals:
- 15-30 mL h-1
- secreted constantly whether you are eating or not
- secreted by surface cells to protect the epithelium
while eating:
- superimposed on basal section are much larger volumes of secretion
- 5010x higher at 140mL h-1

Question 5

how does a non-secreting cell begin secreting?

Answer 5

vesicles containing the H+/K+ ATPase transporter come to the cell surface so that secretion can begin
- when the cell gets activated the vesicle fuses to the cell membrane and this cotransporter gets incorporated to the membrane

Question 6

how does omeprazole act to stop gastric secretion?

Answer 6

• blocks H+/K+ ATPase on apical side (lumen side) of gastric gland
• bicarbonate/Cl- cotransporter also gets blocked on basolateral/blood side of cell

Question 6

describe/draw what happens to a secreting parietal cell on the apical side

Answer 6

on stimulation:
- fusion of vesicles inserts H+/K+ ATPase into apical membrane
- activation of apical K+ and Cl- channel
K+ and Cl- move into the lumen
presence of K+ in the lumen stimulates the H+/K+ ATPase to exchange H+ for K+
- H+ obtained from carbonic anhydrase reaction
water is driven out of cells by the efflux of ions (osmotic gradient)
result is a secretion of Hal (gastric acid)

Question 7

describe what happens to a secreting parietal cell on the basolateral side

Answer 7

Na+/K+ ATPase:
- maintains high intracellular K+
K+ channel:
- contributes to membrane potential
Na+/H+ exchanger:
- minimal role, buffers H+ if required
Cl-/HCO3- exchanger:
- for each mole of H+ ion secreted, an equivalent amount of HCO3- is produced
- this is transported across the basolateral membrane by the Cl-/HCO3- exchanger (alkaline tide after a meal)
- this provides the Cl- ion that is secreted on the apical side together with H+

Question 8

how do surface cells protect themselves?

Answer 8

• by secreting bicarbonate to neutralise the gastric acid

H+ + HCO3 <-> H2CO3 <-> H2O + CO2

Question 9

what are the three modes of action of gastric acid secretion ?

Answer 9

• neurocrine: release of neurotransmitter
• endocrine: hormones into blood
• paracrine: cell takes effect on neighbouring cell

Question 10

what are the switches on for regulation of gastric acid secretion and what mode of action are they? + draw/explain that diagram from the lecture

Answer 10

• acetylcholine: neurocrine
• gastrin: endocrine
• histamine: paracrine

Question 11

what are the switches off for regulation of gastric acid secretion and what mode of action are they? + draw/explain that diagram from the lecture

Answer 11

• less acetylcholine: neurocrine
• inhibition of gastric release by somatostatin: paracrine
• inhibition of histamine release by somatostatin: paracrine
• somatostatin inhibits gastric acid release on parietal cells: endocrine