Secretory functions of GI tract peptides ( choudhury) Flashcards
are all GI hormones peptides?
yes
how are GI peptides classified ? and what are the three classifications
based on method by which peptide is delivered to target site
endocrine
paracrine
neurocrine
what are the 5 GI endocrine hormones
Secretin Gastrin CCK (Cholecystokinin) Gastric inhibitory peptide (GIP) Motilin
where are endocrine peptides released and what do they reach
released into circulation and reach all tissues
specific receptors are found on target tissues
what are paracrine peptides released from and where do they diffuse and have effects
released from endocrine cells and diffuse through extracellular space to target
have effects in areas where cells are contained which release them
can release or inhibit release of endocrine substances
what is an example of a paracrine substance
histamine
what effect does ACH have on the GI tract
its not a peptide BUT is a neuroregulator of the GI tract which stimulates acid secretion from gastric parietal cells
what are the three candidate endocrine peptides
PP
peptide YY
enteroglucagon
where are endocrine factors released in the GI tract and what are the three stimuli for release
released from stomach and small intestine mucosa into portal circulation
after stimulation from nerves, distention, chemical stimulation of food intake
pass through liver and heart back to digestive system to regulate movement, secretions, growth, release of other hormones and absorption
where is the site of release of gastrin
G cells in gastric mucosa
duodenum
pancreas
what molecules stimulate the release of gastrin
protein
distention due to food
nerves
also:
Ca
Decaffeinated coffee
wine
what inhibits the release gastrin release
acid (feedback)
secretin
glucagon
what are the sites of release of CCK
duodenum
jejunum
ileum (minor)
what stimulates the release of CCK
protein
fat
Acid (of secondary importance)
what part of the GI tract release secretin
duodenum
what stimulates the release of secretin
acid
fatty acids(of secondary importance)
what part of GI tract releases GIP
duodenum
jejunum
what part of GI tract releases Motilin
Duodenum
jejunum
what stimulates the release of GIP
glucose (primarily)
protein
fat
what stimulates the release of motilin
nerves (Main)
of secondary importance:
fat
acid
what is the function of microvilli on the surface of endocrine cells
contain receptors that sample luminal contents
what is the major regulatory action of secretin
Natures antacid
released when pH falls below 4.5
stimulates pancreatic and liver bicarb and fluid secretion
inhibits gastric emptying (acid secretion)
stimulates pancreatic growth
inhibits gastrin release
what is the major regulatory action of gastrin
stimulates release of HCl by parietal cells of the stomach via release of histamine from ECL cells and direct action
aids in gastric motility
stimulates GI mucosal growth (can lead to carcinoid tumors)
conditions leading to decreased acid release lead to what….
high serum gastrin levels
what are the major regulatory actions of CCK on the GI tract
stimulates gallbladder contractions to release bile for fat digestion and pancreatic bicarb release
stimulates pancreatic growth
inhibits gastric emptying
what is the function of GIP
inhibits gastric secretions and stimulates insulin release
what does motilin do
stimulates gastric motility and upper GI motility via the interdigestive migrating myoelectric complex (MMC)
what is similar about gastrin and CCK
they have 5 identical amino acids on their C terminal
what are the two forms of gastrin
G17 (little gastrin) G 34 (big gastrin)
gastrin is synthesized as a larger precursor molecule called progastrin
what form does most gastrin exist in
G 34 in the basal or interdigestive state
after a meal what is the predominant form of gastrin and what does this cause
G17
stimulates gastric secretion
because of the similar structure of CCK and gastrin, what can happen.
desulfated CCK can activate gastrin receptors responsible for acid secretion (CCK-B receptors)
gastrin can activate CCK receptors responsible for gall bladder contraction (CCK-A receptors)
what molecules are similar in structure to secretin
Glucagon has 14 AA’s identical to secretin
GIP and VIP have 9 amino acids identical to secretin
what is the function of enteroglucagon
this is a lesser known candidate endocrine hormone similar to secretin
formed by same gene in pancreatic alpha cells which forms glucagon
releases insulin
inhibits gastric secretion
delays gastric emptying
what stimulates the release of enteroglucagon
fat
what is the function of pancreatic polypeptide
“PP”
inhibits pancreatic bicarb and enzyme secretion
what stimulates PP release
protein, fat, glucose
what is the function of peptide YY and what stimulates its release
inhibits gastric secretion and emptying as well as intestinal motility
what is the function of substance P
stimulates intestinal motility
stimulates gallbladder contraction
(neurocrine peptide)
what is the function of neurotensin
increases blood glucose by stimulation of glycogenolysis and release of glucagon
inhibits release of insulin
what is the function of VIP
relaxation of smooth muscle (intestine)
vasodilator via promotion of production of NO
stimulates pancreatic secretion
inhibits gastric secretion
stimulates intestinal secretion
(this is a neurocrine peptide)
what is the function of Bombesin (aka GRP)
released by vagal stimulation with resulting release of gastrin
this is a neurocrine peptide
what is the function of enkephalins
what are these used to treat
activation of opiate receptors on circular smooth muscle cells to mediate their contraction and also contraction of lower esophageal, pyloric, and ileocelcal sphincters
opiates slow intestinal motility
used to treat diarrhea
what is the function of somatostatin (paracrine peptide) and where is it found
found in gastric and duodenal mucosa AND pancreas
inhibits gastrin release and gastric acid secretion
what is the function of histamine
what cell is histamine produced in
what causes the release of histamine
produced in ECL cells
released by gastrin and stimulates acid secretion from parietal cells
potentiates action of gastrin and ACh on acid secretion
how do we treat hypersecretion of acid
histamine (H2) blockers
what is Gastrinoma or Zollinger-Ellison syndrome
non-beta cell tumor of the pancreas or duodenal tumors
continually produce and release gastrin into the blood
what does gastrinoma result in
hypersecretion of gastric acid by parietal cells and increased acid secretory capacity by hyperplastic mucosa
peptic ulcers
diarrhea
steatorrhea (excess fat in feces)
hypokalemia
what contributes to the diarrhea that is seen in gastrinoma
high gastrin levels inhibit absorption of fluid and electrolytes by the intestine
what contributes to the steatorrhea in gastrinoma (fat in the stool)
inactivation of pancreatic lipase by gastrin along with precipitation of bile salts at a low luminal pH
how is gatrinoma (zollinger-ellison) treated
treated with drugs which inhibit acid secretion along with gastrectomy if needed
what are the blood tests that indicate gastrinoma
elevated serum gastrin
what are three tests that can definitively diagnose gastrinoma (considering gastrin is also elevated in duodenal ulcer disease)
stimulation by protein meal (no measurable change in serum gastrin levels)
IV Calcium (increased acid and serum gastrin level) (ca causes increases gatrin release)
secretin infusion (increased serum gastrin level) (secretin typically decreases gastrin levels... but in gastrinoma for unknown reason secretin causes gatrin release)
what is the cause of pancreatic cholera (watery diarrhea syndrome)
what does it result in
overproduction of VIP due to pancreatic islet cell tumor
results in significant intestinal secretion of fluid and electrolytes with production of lots of diarrhea
frequently lethal due to large volume of fluid and electrolyte loss
