alimentary mechanisms Flashcards
gut hormones: explain the role of key gut hormones (gastrin, somatostatin, cholecystokinin, glucose-dependent insulinotropic peptide & secretin) in the regulation of gut function
what cells secrete gut hormones (paracrine of neurocrine factors)
enteroendocrine cells of alimentary canal
specialisations of enteroendocrine cells for production and secretion of hormones
small apical membrane with lots of sensory apparatus (fingers of cytoplasm) and mitochondria (changes in gut contents or activation by neurotransmitters), broad basolateral surface which is close to blood vessels for rapid distribution of many secretory vesicles into blood by exocytosis
where are most gut hormones secreted from
endocrine cells in mucosa or submucosa of stomach, small intestine and pancreas
distance of effects of gut hormones
most regulate nearby organs as primary effect, but to get there must travel long way via liver, heart and lungs
2 examples of paracrine cells in GIT
D-cells (in stomach), enterochromaffin-like cells (in gastric mucosa)
what do D-cells secrete and what does it do
somatostatin, which inhibits secretion of acid from parietal cells in gastric pits
what do enterchromaffin-like cells secrete and what does it do
histamine, which binds to H2 receptors on parietal cells and stimulates acid secretion
hormones which affect gut hormone function
somatostatin, secretin, cholecystokinin, gastrin, glucose-dependent insulinotropic peptide
what cells produce gastrin and where are they
G-cells in distal end of stomach (gastric antrum), proximal duodenum, pancreas
lengths of gastrin produced (3)
34 amino-acid mostly, 17 and 14 also
3 stimuli of gastrin secretion
presence of single amino acids and small peptides in stomach; mechanoreceptors in stomach wall detect presence of meal (stretch); parasympathetic nervous system (vagus nerve causing release of gastrin-releasing peptide from vagal efferents)
3 effects of gastrin secretion
increased gastric acid secretion, gastric emptying, pepsinogen secretion
negative feedback loop of gastrin secretion
if pH of chyme <3, secretion reduced to protect duodenal mucosa
what cells produce secretin and where are they
S-cells in duodenum and jejunum
stimuli of secretin secretion
drop in pH (<4.5) in duodenal kumen following acid chyme leaving stomach through pyloric sphincter
3 effects of secretin secretion
stimulation of secretion of pancreatic HCO3- to neutralise acid; at high concentrations: reduced acid secretion by stomach, reduced gastric emptying to prevent further exacerbation of acidic environment in small intestine
what cells secrete somatostatin and where are they
D-cells in stomach, gut mucosa, pancreas
stimuli of somatostatin secretion
mixed meal and reduced pH
inhibitory effects of somatostatin (hence universal inhibitor)
acid secretion, mobility, exocrine secretion, endocrine secretion, absorption
how does somatostatin reduce acid secretion
via inhibiting G-cells from secreting gastrin and inhibiting enterochromaffin-like cells from secreting histamine
what cells secrete cholecystokinin and where are they
I-cells in small intestine
stimuli of CCK secretion
small peptides and fats after mixed meal
effects of CCK secretion without input from ANS
increased secretion of pancreatic enzymes, reduced gastric emptying, increased gall bladder contraction, reduced appetite
what cells secrete glucose-dependent insuliotropic peptide and where are they
K-cells in duodenum and jejunum
stimuli of glucose-dependent insuliotropic peptide secretion
mixed meal nutrients (especially glucose)
effects of glucose-dependent insuliotropic peptide secretion
increased insulin secretion; in high concentrations causes reduced acid secretion and reduced gastric emptying
functions of GI endocrine system
regulate mechanical processes of digestion, chemical and enzymatic processes of digestion, control of post-absorptive processes, effect on growth and development
somatostatin analogue use (e.g. octreotide)
treat neuroendocrine tumours
why use somatostatin analogue not somatostatin (3)
somatostatin has short half life (be able to be turned off and broken down quickly), whereas analogue lasts longer in circulation; selectivity; peptide hormones must inject as otherwise broken down and not absorbed
where is peptide YY (PYY) located
L cells throughout mucosa of terminal ileum, colon and rectum
stimulus of peptide Y
post prandially (particularly protein)
peptide YY functions
reduces intestinal motility, gallbladder contraction and pancreatic exocrine secretion; inhibitor of intestinal fluid and electrolyte secretion; inhibits food intake
