Gastric acid secretions and regulation Flashcards
What secretions are in the thin walled upper portion of the stomach?
Thin-walled upper portion of the stomach (fundus and body): mucus, HCl
and pepsinogen
What are walls of the glands in stomachs lined with?
Wall of the glands is lined with parietal
cells→ HCl and intrinsic factor
What are exocrine secretions of the stomach?
mucus, acid, pepsinogen
What do enterochromaffin-like cells secrete?
paracrine agents, e.g. histamine
How is gastric acid in the lumen of the stomach made?
- HCO3- is exchanged for Cl- in the blood. This decreased acidity of venous blood from stomach compared to blood serving it.
- Excess Cl- diffuses out into the stomach lumen through chloride channels as the H+ is pumped into the stomach lumen-the K+/H+-ATPase pumps H+ out into stomach lumen
- The Cl- goes into gastric glands, which then binds H+ to form HCl
What does the mucus lining in the stomach do?
↑HCO3-; forms water-insoluble gel on
epithelial surface; protects against H+ secretion
What does Rennin(chymosin) do?
Curdles milk into casein clot
What does lipase do in the stomach?
triglycerides → fatty acids and glycerol
What do intrinsic factors do in the stomach?
Involved with absorption of vitamin B12
What does HCl in stomach do?
kills bacteria; acid denaturation of digested food; activates pepsinogen
(protein digestion
What does the cephalic phase promote the secretion of?
Cephalic phase (meal times- smell, sight, taste, chewing) promotes the
secretion of acetylcholine (ACh) and gastrin
What does Ach stimulate the release of and by what cells in the cephalic phase?
ACh stimulates histamine release from ECL cells
What cells do ACh directly act on in the cephalic phase?
ACh acts directly on parietal cells → HCl secretion
What does gastrin stimulate release of and from what cells in the cephalic phase?
Gastrin stimulates histamine release from ECL cells
What cells does gastrin act directly on and what does this secrete in the cephalic phase?
Gastrin acts directly on parietal cells → HCl secretion
What does somatostatin inhibit?
Somatostatin inhibits acid secretion
What does the gastric phase promote secretion of and why?
Promotes the secretion of acetylcholine (ACh) and gastrin due to distension of stomach and increase in peptide concentration
What does an increase in peptide concentration and distension of stomach?
Distension of stomach:
1. Distension activates neural reflexes and gives a positive impulse to enteric neurons
2. This results in the release of acetylcholine which activates parietal cells releasing HCl
Peptides:
1. Stimulates G cells which secrete gastrin.
2. Gastrin stimulates ECL cells which secrete histamine that trigger parietal cells.
3. This results in a release of HCl
What do proteins do to luminal acidity and there how does this impact HCl secretion?
-Proteins act as buffers in the gastric lumen
- ↓ [H +]; proteins remove the inhibitory powers of HCl on gastrin secretion and hence acid secretion
What is the inhibitory signals when there is food in the stomach during the gastric phase?
- HCl stimulates somatostatin releasing cells
- This releases somatostatin which inhibits parietal cells, G cells and ECL cells
What happens during the intestinal phase?
balances the secretory activity of the stomach and the
digestive and absorptive capacities of small intestine
What happens if there is high acidity of duodenal contents and what does this prevent?
High acidity of duodenal contents reflexly inhibits acid secretion –
prevents chyme from becoming too acidic
What does inhibition of acid secretion in the small intestine depend on?
- Composition of chyme
- Volume of chyme
How is acid secretion inhibited during the intestinal phase?
- Short and long neuronal reflexes and hormones (enterogastrones, e.g.
secretin and CCK) inhibit acid secretion by the parietal cells or gastrin
secretion by the G cells. - Somatostatin
What happens to the sympathetic and parasympathetic discharge in order to inhibit acid secretion?
↑ sympathetic discharge – inhibitory;
↓ parasympathetic discharge - stimulatory
What happens in the direct pathway of the secretagogues?
In the direct pathway, Ach, gastrin and histamine stimulate the parietal cell, triggering the secretion of H+ into the lumen
What happens in the indirect pathway of the secretagogues?
In the indirect pathway, Ach and gastrin also stimulate the ECL cell, resulting in secretion of histamine. This histamine then acts on the parietal cell
What receptors does Ach act on in parietal cells?
M3 receptors
What receptors does histamine act on in parietal cells?
H2(histamine receptors)
What receptors does gastrin act on in parietal cells?
CCK2(gastrin receptors)
What is the synergism between the stimulators of acid secretion?
When M3, H2 and CCK-B receptors are stimulated, Ca2+ and cAMP is generated allowing the tubulovesicles with H+, K+ ATPase channels to fuse with canaliculus
What does HCl stimulate the flow of?
Stimulates flow of bile and pancreatic juice:
What does HCl do in protein digestion?
activates pepsinogen to pepsin
What is pepsin secreted by?
Secreted by chief cells in the form of pepsinogen
When are chief cells activated and what happens to them? What type of feedback process is it?
Activated if [H+] is high; shape altered by high acidity which exposes its
active site
-Autolytic feedback process
What is secretion of pepsin inactivated upon?
Inactivated upon entry of food in the small intestine (HCO3- and peptides
neutralise the H+)
What is the input to chief cells?
Inputs to chief cells from nerve plexus
What does it mean by there being parallels between acid secretion and pepsinogen secretion?
In essence, stimulators/inhibitors of acid secretion during the cephalic
and intestinal phases exert same effect on pepsinogen secretion
