Lecture 46: GI Regulatory Substances Flashcards
List some functions of the GIT.
Contraction/relaxation of smooth muscle and sphincters
Secretion of enzymes, fluid, and electrolytes
Trophic (growth) effects on tissues
Secretion of other GI peptides
What substances regulate the functions of the GIT?
GI peptides, including hormones, neurocrines, and paracrines
What are the four GI hormones?
Gastrin
Cholecystokinin (CCK)
Secretin
Glucose-dependent insulinotropic peptide, AKA gastric inhibitory peptide (GIP)
Read the following criteria and determine if it is describing a hormone, paracrine, or neurocrine.
-GI peptides released from endocrine cells of GIT, not glands
-Enter portal circulation, liver, systemic circulation, target cells
-Target could be in GIT or somewhere else
Hormones
Read the following criteria and determine if it is describing a hormone, paracrine, or neurocrine.
-Secreted by endocrine cells of the GIT
-Act locally within the same tissue that secretes them
GI Paracrine
What is the major GI paracrine hormone?
Somatostatin- major inhibitory actions in the GIT
Read the following criteria and determine if it is describing a hormone, paracrine, or neurocrine.
synthesized in neurons of GIT and released after an action potential
GI neurocrines
What are some of the major GI neurocrines?
Acetylcholine
NE
vasoactive intestinal polypeptide (VIP)
gastrin-releasing pepide (GRP)
Enkephalins
Neuropeptide Y
substance P
What causes gastrin to be released?
Secreted in response to physiologic stimulus, carried in blood to a distant site
**Functions independent of neural activity
Gastrin is secreted by what type of cells?
Secreted by G cells in the stomach
What are the functions of gastrin?
Promote hydrogen ion secretion by gastric parietal cells
Stimulate growth of gastric mucosa (trophic effect)
Increase gastric motility
There are two forms of gastrin. What are they?
Big gastrin: secreted between meals and secreted at low basal levels
Little gastrin: secreted in response to a meal aka distension of stomach and presence of things like amino acids and peptides
What inhibits gastrin secretion?
Low gastric pH AND somatostatin
What is Zollinger-Ellison syndrome?
Gastrinoma (gastrin secreting tumor) in non-B cell pancreas in humans, dogs, and cats
this tumor causes an increase in H+ ion secretion and hypertrophy of gastric mucosa, this can lead to duodenal ulcers
Cholecystokinin (CCK) is secreted by what type of cells?
Secreted by I cells of duodenal and jejunal mucosa
CCK is secreted in response to?
Presence of fatty acids, amino acids, and peptides
What are the five actions of CCK?
1) Contraction of gallbladder: eject bile into the small intestine, emulsify lipids
2) Secretion of pancreatic enzymes: lipases to release FA, amylase for starch, need proteases for proteins
3) Secretion of bicarbonate from pancreas: buffering, increases pH of digesta entering the small intestine
4) Growth of exocrine pancreas and gallbladder
5) Inhibition of gastric emptying: increases gastric emptying time, allow adequate time for digestion and absorption
Secretion is secreted by what type of cells?
S cells of the duodenum
Secretin is secreted in response to what?
H ions and fatty acids present in the small intestine
What actions does secretin have in the small intestine?
stimulates the secretion of pancreatic and biliary HCO3-
Inhibits gastrin release from G cells in stomach (same as somatostatin)
Gastric-inhibitory peptide (GIP) is secreted by what type of cells?
K cells of duodenum and jejunum
GIP is secreted in response to what?
response to glucose, amino acids, and fatty acids
***Oral glucose load stimulates GIP, intravenous glucose does not
What effect does GIP have on the duodenum and jejunum?
Stimulates insulin secretion (causes tissues to take up blood glucose)
Inhibits gastric H ion secretion and gastric emptying
What are the 4 candidate hormones of the GIT?
1) Motilin
2) Pancreatic polypeptide
3) Enteroglucagon
4) Glucagon-like peptide-1 (GLP-1)
Not considered true GI hormones because they fail to meet one or more criteria
Motilin is secreted where? What does it do?
Secreted by upper duodenum during fasting, it increases GI motility, initiates inter-digestive myoelectric complexes @ 90 minute intervals, which triggers peristaltic waves
Pancreatic polypeptide is secreted in response to what? What is its effect?
Secreted by pancreas in response to ingestion of carbs, proteins, and lipids
Self-regulates pancreatic secretion- inhibits pancreatic secretion of HCO3- and enzymes
Enteroglucagon is secreted in response to what? What is its effect?
Secreted by intestinal cells in response to decreased blood glucose
Stimulates the lover to increase glycogenolysis and gluconeogenesis
Glucagon-like peptide-1 (GLP-1) is secreted by what kind of cells? What is its role?
Secreted by L cells of the SI in response to nutrient ingestion
Secreted in response to nutrient ingestion, it inhibits glucagon secretion, decrease gastric emptying, inhibit appetite
Where are GI paracrines released from?
Released from endocrine cells, they act locally and don’t enter systemic circulation
What are the two GI paracrines?
Somatostatin
Histamine
Somatostatin is released from where? What is its effect?
Secreted by D cells of GI mucosa in response to decreased luminal pH (if luminal pH is decreases, we know food is present)
Inhibits secretion of other GI hormones and gastric H ion secretion
Histamine is released from where in the GIT? What is its effect?
Major secretion from H ion-secreting area of the stomach
Stimulates H ion secretion by gastric parietal cells- released locally and acts locally
Where is the satiety center located in the body?
Located in ventromedial nucleus (VPN) of the hypothalamus
Anorexigenic neurons release pro-opiomelanocortin (POMC)- decreases appetite
The feeding center is located where?
Located in the lateral hypothalamic area (LHA)
Orexigenic neurons release neuropeptide Y (which increases appetite)
What substances influence anorexigenic neurons (decreases appetite)?
Leptin- secreted by fat cells
Insulin
GLP-1- synthesized and secreted by intestinal cells
Peptide YY- secreted by intestinal L cells following a meal
What substances influence orexigenic neurons (increases appetite)?
Ghrelin- secreted by gastric cells
