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DEMS: Unit 1 > GI Secretion & Digestion > Flashcards

Flashcards in GI Secretion & Digestion Deck (6):
1

Mechanism of gastric secretion

  • Parietal cells secrete hydrochloric acid & intrinstic factor into stomach
  1. H+ actively transported across apical membrane in exchange for K+ by H+/K+ ATP-ase [primary active transport]
    1. drives uphill movement of Cl- and H2O
  2. HCO3- transported across basolateral membrane in exchange for Cl- via Cl-/HC03- anion exchanger [secondary active transport]
  3. Cl- transported across apical membrane via facilitated diffusion [passive transport]
  4. H2O follows net transport of HCl for osmotic reasons (==> alkaline tide = high pH of venous blood leaving stomach)

2

Characteristics of acid secretion stimulation

  • acid secretion stimulated by ACh, gastrin, paracrine histamine (via direct and indirect pathways)
  • ACh ==[muscarinic receptors @ basolateral] ==> activate G-protein ==> increase intracell Ca2+
  • Gastrin ==> increased intracell Ca2+
  • Histamine =={histaminergic H2 receptors] ==> activate G-protein ==> activate adenylate cyclase ==> increased cAMP
  • Ca2+ and cAMP ==> protein kinase activation ==> phosphorylation of H+/K+ ATPase ==> increase in H+/K+ ATPase activity

3

Indirect vs. Direct pathway of acid secretion stimulation

  • direct pathway: ACh, gastrin and histamine directly stimulate the parietal cell, triggering the secretion of H+ into the lumen
  • indirect pathway: ACh and gastrin stimulate the ECL (enterochromaffin-like) cells, resulting in secretion of histamine ==> histamine then acts on the parietal cell 

4

Phases of gastric acid secretion (general)

  • basal phase = interdigestive phase
    • rate of acid secretion follows circadian rhythm:
    • morning = lowest
    • evening = highest
    • resting pH = 3-7
  • 3 phases assoc. w/eating

5

Gastric secretion during eating

  • cephalic phase = @ sight, smell, taste and swallowing of food; mediated by vagus nerve
    • ACh release
    • histamine release from ECL cells
    • gastrin-releasing peptide (vagal/enteric neurons)
    • inhibition of somatostatin release from delta cells
  • gastric phase = @ entry of food in stomach
    • distension ==> vasovagal reflex + ENS reflexes
    • proteins ==> gastrin release from G cells
  • intestinal phase = @ amino acids/peptides @ proximal small intestine
    • duodenal G cells secrete gastrin

6

Characteristics of protective barrier of gastric surface

  • barrier = surface epithelial cells + mucous + bicarbonate secretion
    • protects from harmful effect of acid
    • aids in maintenance of pH gradient
  • mucous secreted by goblet cells (= specialized epithelial cells)