Flashcards in lecture 6: gastric physio Deck (39):
What does the stomach's production of IF do?
intrinsic factor helps for intestinal uptake of vitamin B12
What does gastric acid activate?
Gastric pepsinogen gets activated to pepsin and the gastric acid provides the optimal pH activity for pepsin
what cells produce gastric acid? where in the stomach are they most prevalent?
how do proton pumps change in their localization within an activated parietal cell?
parietal cell activation fuses proton pumps with the apical membrane for gastric acid secretion to the apical side/lumen
What are the net effects of gastric acid secretion by parietal cells
Lumen = H, Cl, K (minor)
Bloodstream = bicarbonate
how do PPIs work?
from the bloodstream, activated PPIs enter parietal cells from the basolateral end and then covalently bind to the proton pumps.
the binding inactivates the pumps selectively and permanently
What are stimulators of parietal cell activation (and gastric acid secretion)?
Histamine (on H2 receptor)
ACh (on M3 receptor)
Gastrin (on CCK receptor)
What are the products of stimulation at the H2, M3 and CCK receptors by histamine, acetylcholine, and gastrin, respectively?
CCK receptor = more Ca
H2 receptor = more cAMP
M3 receptor = more Ca
What are products of the EEC cells in the stomach?
gastrin (G cells)
Histamine (ECL cells)
Somatostatin (D cells)
Where do parietal cell stimulators come from?
ACh = neurons (direct stimulation of parietal cells or stimulation of G, ECL cells)
Histamine = from ECL cells
Gastrin = from G cells (via circulation) to ECL or parietal cells
GRP, which stimulates gastrin release
What is the source of gastrin?
G cells in gastric antrum and duodenum
What is gastrin release induced by?
peptides + amino acids
vagal innervation (via GRP and ACh)
What is gastrin release inhibited by?
What are the functions of gastrin?
1) gastric acid production (primarily through histamine release by ECL cells)
2) proliferation of gastric epithelial cells and ECL cells
3) mediated by CCK2 receptors via Ca
what is histamine release induced by from ECL cells?
what is the signaling associated with H2 receptors?
stimulation = increased cAMP
where are H2 receptors found?
gastric parietal cells
What is the source of somatostatin?
D cells in stomach, small intestine and pancreas
What is another name for somatostatin?
What stimulates the release of somatostatin?
in antrum: acid
in corpus: hormones (CCK and secretin, both which inhibit gastric acid secretion)
What is the function of somatostatin?
1) inhibit hormone release (gastrin, CCK, insulin, glucagon)
2) inhibit gastric acid and pepsinogen secretion
3) inhibit pancreatic exocrine functions
4) inhibit gallbladder contraction
Describe the feedback regulation of gastrin secretion.
Acid stimulates somatostatin secretion by antral D cells
Somatostatin inhibits gastrin secretion by antral G cells, resulting in reduced gastrin levels to reduce HCl secretion.
ACh inhibits somatostatin for more gastrin release
what are the four phases of gastric acid secretion in response to a meal?
describe the four phases of gastric acid secretion
basal = circadian rhythm
cephalic = response with sight, smell or taste of food
gastric = gastric distension, food compounds, increased gastric pH due to buffering of food
intestinal = mostly inhibitory
which phase of gastric acid secretion prepares the stomach for digestion?
cephalic phase = vagal nerve stimulation via acetylcholine
Describe neural reflexes in regulating gastric secretion.
gastric distension triggers vago-vagal reflexes, leading to: receptive relaxation, gastrin, acid, and pepsinogen secretion, antral contraction
what stimuli take part in the gastric and intestinal phases of gastric meal response?
chemical = peptide, amino acids, acid
mechanical = stomach distension
what pathways do each stimulus activate in the gastric meal response
chemical: hormonal (gastrin, SST, HA, CCK)
mechanical: neural (ACh, GRP, NO, VIP)
what is the secretory response?
acid, pepsinogen, IF, mucus
What is the motor response?
receptive relaxation, antral peristalsis, blood flow
what are forms of protection against gastric acid?
limit flux of gastric contents
neutralization of acid
protection of epithelial barrier
how is production of gastric acid limited?
SST from D cells
how is flux of gastric contents limited?
How is gastric acid neutralized?
local bicarbonate secretion
what protects the epithelial barrier?
mucus (controlled by PGE2)
trefoil peptides, growth factors
What is bicarbonate produced by?
small intestinal epithelium
(also in saliva)
What produces mucus in GIT?
esophagus = esophageal glands
stomach = gastric mucous cells
small intestine = goblet cells
what do surface mucus cells in the stomach do?
produce mucus and bicarbonate to protect against gastric acid
stimulated by PGE2