Secretions of the stomach
Terms in this set (35)
What are the functions of the stomach?
-storage of food -release of food into the duodenum via pylorus -antrum is thick muscle for propulsion and grinding -secretes gastric juices
what are the names of the different parts of the stomach? draw it out maybe. (6 portions including eosophagus)
What types of gastric gland cells are there? what do they secrete? (6 types)
mucous neck cell=mucous and bicarb G cell=gastrin D cell= somatostatin Parietal Cell= acid and IF EC like cell=histamine chief cell=pepsinogen and lipase
where are the main locations of these cells?
Corpus=Parietal, Chief, D cells, ECL cells Antrum=G and D cells
What are the layers of the stomach lining? (6 layers)
Describe the mechanism of secretion of H+ into the lumen of the stomach...
What are the subunits of the H-K pump?
-Parietal H-K pump in apical membrane
-Alpha and beta subunits
How is H+ formed?
-CO2+H20 goes to HCO3- and H+
How is an alkaline tide into the blood created?
-for every H+ into lumen There is a HCO3- that crosses basolateral membrane into blood via Cl- HCO3- exchanger
what are the phases of acid secretion? How are the phases triggered? BCGI
1.Basal-follows circadian rhythms (low AM)
2.Cephalic-Smell, sight, taste, thought mediated via vagus nerve thus triggering 30% of acid secretion
3.Gastric-50-60% of acid secretion triggered by distension (triggers release of ACH)
4.Intestinal-Partially digested peptides/amino acids in proximal section of small bowel stimulates duodenal G cells to release gastrin.
what is the direct and indirect stimuli for acid secretion?
Direct-ACh, Gastrin, Histamine
Indirect-Ach and Gastrin mediated Histamine Release.
What receptors and pathways do ACh, Gastrin and Histamine use?
ACH-Muscarinic M3 receptor
How is indirect stimulation of parietal cells mediated?
Involves Enterochromaffin like cells
-ACh and Gastrin trigger it to make histamine
How is acid secretion stimulated in the corpus? What does it do in responses? (3 things) DISTENSION
-Distension of the corpus causes ACh release by vagus nerve ending
-directly stimulates parietal cells to make acid
-increases production of histamine from ECL cells
-ACh inhibits release of somatostatin via D cells in antrum (see below)
How is acid secretion stimulated in the antrum? (2 things)
G and D cells subject to vagal stimulation due to remnant peptides
1. Vagal stimulation of G cells mediated by Gastrin releasing peptide
2. Vagal stimulation of D cells via ACh inhibits release of somatostatin.
so evidently the antrum is very receptive to changes in environment...what changes mediate what?
-Increase in protein/remnant peptides increases Gastrin release from G cells
-Increase in luminal H+ stimulates somatostatin release from D cells.
How do D Cells inhibit acid secretion?
-Secrete peptide hormone somatostatin
-direct and indirect inhibitor of parietal cell secretion
-ANTAGONISES HISTAMINE AT PARIETAL CELL LEVEL
How is somatostatin release triggered? Think CA
-D cells in corpus stimulated by neuronal/hormonal mechanism
-D cells in antrum triggered by low intraluminal pH
How is indirect regulation of histamine regulated by somatostatin? CA
-Somatostatin can inhibit release of histamine from ECL cells in the corpus.
-Somatostatin can inhibit release of gastrin from G cells in the antrum.
What are names of other naturally produced Acid secretion inhibitors?
How do these work?
Secretin-stimulated by fat in duodenum
released by S cells in duodenum
stimulates Somatostatin release and reduces antral gastrin release
CCK-produced by I cells of duodenum in response to fat
directly reduce parietal acid secretion
What are the names of some pharmalogical inhibitors how do they work?
PPI's e.g. Omeprazole (directly act on H-K pumps not through mediators)
H2 receptor antagonists e.g. cimetidine, ranitidine
competitive antagonists of histmamine at parietal cell level thus only block effects of Histamine but not gastrin or ACH.
Which cells produce pepsinogens? what is the trigger?
Secreted by chief cells of the corpus
Ach Is major trigger for pepsinogen secretion
How is secretion of pepsinogens interlinked with acid secretion?
-Resulting peptides trigger antral G cell gastrin secretion
How is pepsinogen converted to pepsin-trigger?
Occurs at low pH (pH3)
Pepsin autoactivates pepsinogen
how does the mucous layer of the stomach protect itself from acid?
Traps local HCO3- to neutralise epithelium
describe the role of prostaglandins in gastric mucosal production?
-COX-1 constitutive enzyme converts arachidonic acid to prostaglandins which are used in physiology-gastric mucosa production, platelet aggregation etc.
-Prostaglandins have important role in maintaining diffusion barrier they do this by inhibiting acid secretion and stimulating HCO3- and mucous secretion.
How does aspirin cause GI side effects?
-NSAID's in general affect prostaglandins thus reduce effectiveness of Gastric mucosa.
-Affect COX enzymes!
-they block the inflammatory pathway involving COX 2 swell as the physiological pathway.
-causes gastric ulceration and dyspepsia.
What is helicobacter pylori?
-Colonises stomach of 40% of humans
Leading cause of peptic ulceration
What effects does it have?
Increases acid production
increases barrier damage
increases cancer rate
Conditions caused by HP ?
Gastric and duodenal ulcers
How do HP affect the antrum?
G cells hyper secrete Gastrin
decreases Somatostatin release from D cells
causes hypergastrinaemia and thus duodenal ulcers
How does HP affect the corpus?
Reduces acid secretion as it damages parietal cells causes hyperchlorrhydria (due to damaged parietal cells)
causes gastric ulcers
how is HP detected?
-Urea breath test
-Urease production by HP converts Urea into HCO3-
-Used by HP to neutralise the stomach acid around itself.
Where is intrinsic factor produced what is its role?
Produced by parietal cells of the corpus
-necessary for absorption of B12 in terminal ileum
-important in red cell synthesis and proliferation
What could cause loss of IF?
-Autoimmune atrophic gastritis
-Results in megaloblastic/pernicious anaemia (red blood cells get bigger but dont divide)