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gihep 10: GIT hormones (i) (Factors inhibiting acid secretion (1)…
gihep 10: GIT hormones (i)
Factors inhibiting acid secretion
1) distension in SI
enterogastric reflex
when intestine is full
slows food entry
2)acid/fat/protein in duodenum
causes secretion of 2 peptide hormones
cholecystokinin (CCK)
released from I cells in upper SI when they're stimulated with CCK releasing peptide
I cells directly sense fat + protein
CCK binds to CCKA Rs in D cells, which then secrete somatostatin (which inhibits cAMP)
has same 5 terminal AAs as gastrin
both act @ CCKB to increase acid, but only CCK @ CCKA
secretin
released from S cells in upper SI when they're stimulated with secretin releasing peptide
S cells directly sense acid
stimulates bicarb, somatostatin + inhibits motility (vagal reflex)
3) excess acid in stomach
4) Drugs
H2 antagonists (e.g. cimetidine) inhibit by 60-80%
proton pump inhibitors (e.g. omeprazole) by 100%
Exocrine pancreatic secretions
acinar cells make enzymes
ductal epithelial cells make H2O + HCO3 (washes enzymes into intestine)
control...
ACh (cephalic + gastric phase)
gastrin (gastric phase)
secretin (more H2O + HCO3- via cAMP in intestinal phase)
neutralises acide to provide suitable pH for enzymes
CCK (more enzymes in intestinal phase)
Secretion of bile/mucus
CCK contracts gallbladder + relaxes sphincter of Oddi
also controlled by vagus + enteric Ns
allows secretions to flow into 2nd part of duodenum
Brunner's glands
mucus secreting
in first few cm of duodenum (vulnerable to acid)
activated by food, vagus, hormone (esp secretin + CCK)
inhibited by sympathetics
Motility hormones
Motilin
peptide from mucosa of upper GIT
causes smooth muscle contraction
migrating motor complex 1.5-2 hrs after food
helps intestine clear foreign bodies
Serotonin (5 HT)
from enterochromaffin cells
causes vomiting via activation of sensory afferents via 5-HT-3 Rs
anti-emetics (eg ondansetron) block these Rs
Gastrin inhibiting polypeptide
from upper SI
inhibits secretion + motility
stimulates insulin in response to food in SI
somatostatin
from D cells in stomach + pancreas
stimulated by CCK+ACh+increased blood glucose
slows digestion: inhibits BF,secretins,motility