Cell types
- Parietal cells
- Secrete hydrochloric acid and intrinsic factor
- Chief cells
- Pepsinogen
- This is the inactive form (zymogen) of pepsin
- Activated to pepsin initially by hydrogen ions and by pepsin
itself
- Pepsin needs acidic environment to act
- Mucous cells
- Mucous and bicarbonate
- Prostaglandins stimulate their secretion
Motility
- Increased by
- Parasympathetic activity – via acetylcholine and gastrin
- Distension
- Decreased by
- Low pH stomach contents - inhibit gastrin release
- Food in duodenum
- Fats stimulate release of cholycystokinin
- Acid stimulates release of secretin
- Hyperosmolality of duodenal contents reduces gastric emptying
- Rate of emptying also depends on the type of food
- Carbohydrate-rich is fastest, then protein-rich and fatty food last
- Receptive relaxation- relaxation of the fundus of the stomach when
food enters it
Secretions
- About 2500mls secreted per day
- pH is acidic and can be close to one
- Composition
- Ions: rich in potassium, hydrogen ions, chloride and bicarbonate
- Hydrochloric acid
- Intrinsic factor
- Pepsin
Mucous Secretion of hydrochloric acid by parietal cells
- H / K ATPase is present in the apical membrane of the parietal cells
- It pumps H+ ions into the gastric gland lumen against their
concentration gradient
- Potassium is pumped into the parietal cell actively in exchange
- Potassium diffuses back into the lumen of the gastric glands
- Chloride diffuses from the parietal cell into the lumen passively
down its electrochemical gradient through cAMP driven channels
- High concentrations of potassium and chloride are maintained within
the parietal cell through the H / K ATPase and chloride and bicarbonate
exchange on the basolateral membrane
Stimulation of Gastric acid secretion
Gastrin
- Acts in two ways
- Stimulates gastrin receptors on the parietal cells
- Stimulates histamine release from enterochromaffin-like cells
- Gastrin lead to an increase in intracellular Ca2+
- Histamine them acts on H2 on the parietal cells
- These are G protein mediated receptors
- They lead to an increase in intracellular cAMP and hence in
protein kinases which leads to activation of H / K ATPase
- This is the main mechanism of action of gastrin
Acetylcholine
- Activates M3 Muscarinic receptors
- This increases intracellular Ca2+
- Acetylcholine release is stimulated by vagal action
Histamine
- Secretion is stimulated by gastrin as described above
- Acts via H2 receptors as described above
Inhibition of Gastric acid secretion
- Prostaglandin E activates inhibitory G protein
- It thus blocks action of histamine and gastrin by inhibiting
protein kinase synthesis
- Gastric inhibitory peptide, vasoactive intestinal peptide and
secretin
- Act by inhibiting gastrin release
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