Control 3 Flashcards
Gut hormones
Where are gut hormones secreted from?
Enteroendocrine cells of the alimentary canal
What are enteroendocrine cells?
A large family of cells that are specialised for their own stimuli and secretions
What is the structure of an enteroendocrine cell?
Small apical membrane with lots of sensory apparatus
Broad basolateral surface close to blood vessels
What happens in the apical membrane of an enteroendocrine cell
it has lots of sensory apparatus so - receptors and intracellular signalling techniques that can sense changes in the gut contents or activation by neurotransmitters
What happens in the basolateral membrane of an enteroendocrine cell
For rapid distribution, and near basolateral membrane there are vesicles with their secretatory products ready for exocytosis
What organs do most gut hormones regulate?
Nearby gut organs,
although for hormones to get there they need to travel a long way via liver, heart and lungs
What organs are the ones that mainly secrete hormones?
Stomach, small intestine and pancreas
What do D cells in the stomach do?
secrete the gut hormone somatostatin, which inhibits the secretion of acid from parietal cells in the gastric pits.
What do enterochrommaffin like cells ECl in gastric mucosa do?
Secrete histamine. Which binds to H2 receptors on the parietal cells, stimulating acid secretion.
What are the particular functions controlled by enteroendocrine cells
GLUCOSE REGULATION (e.g. insulin & glucagon) APPETITE REGULATION (e.g. ghrelin & peptide YY) GUT FUNCTION REGULATION (e.g. somatostatin, secretin, cholecystokinin, gastrin and glucose-dependent insulinotropic peptide)
Where is gastrin synthesised?
G-cells in the distal end of the stomach (gastric antrum), the proximal duodenum, and to a lesser extent the pancreas.
How is the secretion of gastrin stimulated?
presence of single amino acids and small peptides in the stomach (remember that the stomach is the first site of protein digestion). Also, mechanoreceptors in the stomach wall that detect the presence of a meal (inferred by stretching) stimulate gastrin secretion. Lastly, the parasympathetic nervous system can stimulate gastrin release by releasing gastrin-releasing peptide from vagal efferents.
What does gastrin do?
causes an increase in gastric acid secretion from parietal cells in the stomach.
What is the negative feedback loop of gastrin secretion? What’s it for?
if the pH of chyme in the duodenum drops below 3 (pH<3), secretion will be reduced. This is to protect the duodenal mucosa.
Where is Secretin synthesised?
S-cells in the proximal small intestine (duodenum and the jejunum).
What stimulates secretin synthesis?
a drop in pH in the duodenal lumen (usually following acid chyme leaving the stomach through the pyloric sphincter).
what does secretin do?
stimulates the secretion of bicarbonate rich juice from the exocrine pancreas (to neutralise the acid).
What does excessive levels of secretin do?
inhibit the secretion of acid by the stomach and inhibit gastric emptying (to prevent further exacerbation of the acidic environment in the small intestine, as this will be causing mucosal damage).
Where is somatostasin synthesised?
D (delta) cells in stomach, pancreas and small intestine in response
What stimulates somatostatin secretion?
in response to a mixed meal
Increased [H+] or decrease pH
What does somatostatin do?
a number of inhibitory effects. It decreases secretion of gastric acid via two mechanisms. It inhibits G-cells form secreting gastrin (which promotes acid secretion), and it inhibits ECL cells from secreting histamine. It also decreases motility of the gut and decreases gastric emptying.
What is somatostatin described as? Why?
Universal inhibitor because it has an enormous number of inhibitory functions by vagus nerve
Where is cholecystokinin secreted?
I-cells in the small intestine
What stimulates cholecystokinin secretion?
Fats and peptide ie a mixed meal in lumen
What effects does cholecystokinin have?
increased secretion of pancreatic enzymes
reduced gastric emptying
gall bladder contraction
reduced appetite
What is special about cholecystokinin?
able to be secreted and triggers these effects without any input from the autonomic nervous system (act independent of vagus nerve)
What is the Glucose-dependent insuliotropic peptide also called? (GIP)
Gastric inhibitory peptide
Where is GIP secreted?
K cells in the duodenum and jejunum
What does GIP do?
involved with glucose storage, with its main physiological effect being the upregulation of insulin secretion in response to glucose concentration in the small intestine
What does GIP in high conc do?
have inhibitory effects on stomach function
