GI Secretions — Gastric Acid

Volume and composition of gastric juice:

• 2-2.5 L/day of gastric juice is produced
• It contains:
  • H2O and electrolytes (> 99.5%)
    • Gastric juice is slightly hyperosmotic (325 mOsm/L) with ↑ H+ (150-170 mmol/L), ↑ Cl- (190 mmol/L), and ↑ K+ (10 mmol/L), but ↓ Na+ (2-4 mmol/L) → cf. plasma
    • It is also more acidic (pH 1-1.5) → due to ↑ H+ content
  • Solid material (< 0.5%)
    • Digestive enzymes – Pepsin, Gastric lipase, Gastric amylase
    • Mucous in alkaline fluid (HCO3 – -rich)
    • Intrinsic factor

Gastric Acid Secretion:

Parietal cells contain an H⁺-K⁺ ATPase (exchange) pump – GPCR – cAMP dependent

• This pump is activated in response to increased levels of intracellular Ca²⁺ from stimulation by:
  • ACh
  • Histamine (H₂)
  • Gastrin
• Inhibited by:
  • Low gastric pH
  • Somatostatin

Process:

• H⁺ is produced by action of carbonic anhydrase on CO₂ and water, with ‘waste’ HCO₃^– removed from the cell in exchange for Cl^–.
  • Respiratory quotient of the stomach may become negative due to consumption of CO₂
• HCO3^– :
  • transported out of the basolateral membrane in exchange for chloride.
  • Outflow into blood results in a slight elevation of blood pH known as the “alkaline tide”. This process serves to maintain intracellular pH in the parietal cell.
• Chloride and potassium ions are transported into the lumen of the cannaliculus by conductance channels, and such is necessary for secretion of acid.
• Hydrogen ion is pumped out of the cell, into the lumen, in exchange for potassium through the action of the proton pump; potassium is thus effectively recycled.

Regulation of gastric juice secretion:

• Cephalic phase → 50% of gastric juice secretions
  • Initiated by thought, sight, taste and smell of food
  • Mediated via vagal (ACh) outflow
• Gastric phase → ~ 50% of gastric juice secretions (prolonged secretion at a slower rate)
  • Initiated by entry of food into stomach
  • Mediated by – (i) Local and vago-vagal reflexes → due to distension of body (of stomach), and (ii) Gastrin release from G-cells → due to antral distension
• Intestinal phase → < 1% gastric juice secretion
  • Initiated by chyme entering duodenum
  • Mediated by enterogastric neural (ANS/ENS) and hormonal (CCK, VIP, GIP, Etc.) reflexes

JC 2019

Answer by JC