GI secretory products Flashcards
intrinsic factor: source, action, notes
secreted by parietal cells of the stomach
will bind vitamin B12. this is required for B12 absorption in the terminal ileum.
autoimmune destruction of parietal cells causes chronich gastritis and pernicious anemia
gastric acid: source, action, regulation
parietal cells of the stomach
action: decrease stomach pH
regulation: increased by histamine, ACh, and gastrin. decreased by secretin, somatostatin, GIP, prostaglandin.
what is a gastrinoma?
gastrin-secreting tumor that cuases high levels of acid secretion and ulcers refractory to medical therapy
Pepsin: source, action, regulation
secreted by chief cells in the stomach
digests proteins
increased by vagal stimulation and local acid.
note that it is secreted in its inactive form and converted to its active form by protons in the stomach
HCO3- in the GI tract: source, action, regulation
secreted by mucosal cells in the stomach, duodenum, salivary glands and pancreas, and by Brunner glands in the duodenum.
neutralizes acid.
increased by pancreatic and biliary secretion with secretin
it is trapped in the mucus that covers the gastric epithelium
What is the mechanism by which gastrin influences parietal cells?
it has an effect on eneterochromaffin-like cells, which leads to histamine release (which, in turn, stimulates acid release).
gastrin seems to directly stimulate release of intrinsic factor from parietal cells.
How does the vagus nerve stimulate secretion in the stomach?
two synapses:
in one, preganglionics symapse in the cardia of the stomach. post-ganglionics go directly to parietal cells and release ACh.
in the other, preganglionics synapse in the body of the stomach, and post-ganglionics synapse on G cells (which release gastrin).
What is the effect of atropine on the stomach?
atropine blocks vagal stimulation of parietal cells only. vagal stimulation of G cells in uneffected because a different transmitter (not ACh) is used.
describe the gastric parietal cell (drawing is ok too- answer on pg 347)
gastric acid secretion established by an ATP-dependent H-K exchanger.
Many factors influence action of the proton pump through many mechanisms:
1. ACh and Gastrin are both controlled by the vagus nerve. ACh binds M3 receptors, while gastrin binds CCKa receptors. these are both Gq receptors, which stimulate IP3/Ca and promote proton pump action. Remember that gastrin’s biggest mechanism of action, though, is through promoting histamine release from eneterochromaffin cells.
2. histamine binds H2 receptors. H2 receptors are Gs receptors. they promote cAMP activity and, therefore, pump activity.
3. prostaglandins (misoprostol) and somatostatin bind Gi receptors. they oppose cAMP and decrease proton pump activity.
what are the enzymes secreted by the pancreas? What should I know about the tonicity of pancreatic secretions?
alpha amylase, lipase/phospholipase A/colipase, proteases, trypsinogen.
pancreatic fluids are isotonic. in low-flow situations, there is lots of Cl-, in high flow situations, there is lots of HCO3-.
what are the functions and forms of alpha amylase, lipase, phospholipase A, colipase, and proteases?
alpha amylase: starch digestion; secreted in active form.
lipase, phospholipase A, and colipase are all for fat digestion.
proteases include trypsin, chymotrypsin, elastase carboxypeptidases and digest proteins. secreted as proenzymes known as zymogens.
trypsinogen: function, activation
converted to active enzyme trypsin. trypsin activates other proenzymes and cleaves additional trypsinogen into active trypsin.
trypsinogen is converted to trypsin by enterokinase/enteropeptidase, a brush-border enzyme on the duodenal and jejunal mucosa
describe carbohydrate absorption
only monosaccharides are absorbed by enterocytes. glucose and galactose are taken up by SGLT1 (Na dependent). fructose is taken up by facilitated diffusion by GLUT-5. all are transported to the blood by GLUT-2.
What is the D-xylose absorption test?
used to distinguish GI mucosal damage from other causes of malabsorption.
iron absorption
absorbed as Fe2+ in the duodenum.
