Gastric Secretion: Mechanism and Regulation

1. Anatomic Basis

• Cardia and pyloric (antral) glands - secrete mucus and gastrin (G cells)
• Body/fundus glands (oxyntic glands) - contain the key secretory cells:
  • Parietal cells - secrete HCl and intrinsic factor
  • Chief cells - secrete pepsinogens and gastric lipase
  • ECL cells - secrete histamine
  • Surface mucous cells - secrete mucus, HCO3-, and trefoil peptides

Fig. 25-5: Anatomy of the stomach. Body secretes mucus, pepsinogen, and HCl; antrum secretes mucus, pepsinogen, and gastrin.

Fig. 25-6: Gastric gland structure showing parietal cells, chief cells, ECL cells, mucous neck cells (stem cell compartment), and surface mucous cells.

2. Composition of Gastric Juice (Fasting State)

3. Three Primary Stimuli of Gastric Secretion

a) Gastrin

• Released by G cells in the antrum in response to:
  • Gastrin-releasing peptide (GRP) from enteric nerve endings
  • Oligopeptides/amino acids in the gastric lumen
• Travels via the bloodstream (endocrine pathway) to the fundic glands
• Acts on CCK-B receptors on parietal cells and chief cells
• Also acts on ECL cells to stimulate histamine release

b) Histamine

• Released by ECL cells in the fundic glands
• Acts via H2 receptors on parietal cells
• Second messenger: cAMP (cyclic AMP pathway)
• The most pharmacologically important stimulus - blocking H2 alone markedly suppresses secretion

c) Acetylcholine (ACh)

• Released from enteric nerve endings in the fundus
• Acts via M3 (muscarinic) receptors on parietal cells, chief cells, and ECL cells
• Second messenger: elevated cytosolic Ca2+

Fig. 25-7: Antrum - G cells release gastrin (stimulated by GRP and peptides, inhibited by luminal H+ via D-cell somatostatin). Fundus - gastrin, ACh, and histamine converge on parietal cells. ECL cells are the key intermediate, stimulated by gastrin and ACh to release histamine.

4. Mechanism of HCl Secretion by Parietal Cells

Morphologic Changes on Activation

1. Tubulovesicles fuse with invaginations of the apical membrane (canaliculi)
2. The apical surface area is greatly amplified
3. H+K+-ATPase molecules are repositioned onto the apical membrane to begin pumping acid

Fig. 25-9: Resting parietal cell (left) - proton pumps stored in tubulovesicles. Secreting parietal cell (right) - fusion with canalicular membrane massively expands apical surface, deploying H+K+-ATPase. ACh and gastrin signal via Ca2+; histamine via cAMP.

Ion Transport Mechanism

Fig. 25-10: Ion transport proteins of parietal cells.

1. H+ generation: CO2 + H2O → H+ + HCO3- (catalyzed by carbonic anhydrase II)
2. H+ secretion: The H+,K+-ATPase (proton pump) on the apical membrane actively pumps H+ out into the lumen in exchange for K+ - against a concentration gradient of over 1,000,000-fold
3. K+ recycling: Apical K+ channels supply K+ for the proton pump exchange
4. Cl- secretion: Apical ClC chloride channels secrete Cl- into the lumen - combines with H+ to form HCl
5. HCO3- handling: HCO3- is exported across the basolateral membrane via a Cl-/HCO3- exchanger into the bloodstream (the "alkaline tide") - this HCO3- later neutralizes gastric acid after it enters the duodenum
6. Na+ homeostasis: Basolateral Na+,K+-ATPase and NHE-1 maintain intracellular ion balance

Intracellular Signal Transduction

5. Phases of Gastric Secretion

Cephalic Phase

• Initiated before food enters the stomach - anticipation, smell, sight, taste of food
• Mediated via the vagus nerve (dorsal vagal complex coordinates input from higher brain centers)
• Vagal outflow releases GRP and ACh → stimulates G cells and parietal cells
• Quantitatively limited - few local triggers yet

Gastric Phase (most significant)

• Begins when food enters the stomach
• Oligopeptides/amino acids in the lumen directly stimulate G cells to release gastrin
• Gastric distension activates stretch receptors → vago-vagal and local reflexes amplify secretion
• The meal itself buffers gastric acidity, reducing the negative feedback that would otherwise inhibit secretion
• Quantitatively the largest phase

Intestinal Phase

• Once the meal passes into the duodenum
• Duodenal factors (secretin, CCK, acidity) begin to inhibit gastric secretion

6. Inhibition of Gastric Secretion

• When luminal pH falls (excess acid), D cells in the antrum release somatostatin
• Somatostatin inhibits:
  • G cells (stops gastrin release)
  • ECL cells (stops histamine release)
  • Parietal cells directly
• This is the primary mechanism terminating secretion after the meal leaves the stomach

7. Pepsinogen / Pepsin Secretion

• Chief cells secrete pepsinogen (inactive precursor)
• Pepsinogen is activated to pepsin by HCl in the gastric lumen (autocatalytic below pH 5)
• Pepsin initiates protein digestion in the stomach
• Chief cells are stimulated by ACh and gastrin (direct pathways not fully elucidated per Ganong)

8. Gastric Mucosal Defense

1. Mucus-bicarbonate barrier secreted by surface mucous cells - creates an alkaline microenvironment adjacent to the epithelium
2. Trefoil peptides - stabilize the mucus-bicarbonate layer
3. Prostaglandins - stimulate mucus and HCO3- secretion, maintain mucosal blood flow
4. Tight junctions between epithelial cells - gastric mucosal barrier preventing back-diffusion of H+
5. Rapid cell turnover - damaged cells are quickly replaced

9. Peptic Ulcer Disease (PUD)

Causes

• The most common cause
• H. pylori disrupts the mucus-bicarbonate barrier
• Creates a local environment favoring acid injury

• Inhibit cyclooxygenase (COX) enzymes
• Reduce prostaglandin synthesis
• Consequently decrease mucus and HCO3- secretion
• Impair mucosal defense without necessarily increasing acid output

• Example: Zollinger-Ellison syndrome
  • Caused by gastrinomas (gastrin-secreting tumors)
  • Most gastrinomas are found in the pancreas, though some occur in the stomach and duodenum
  • Gastrin causes prolonged, massive acid hypersecretion
  • Results in severe, treatment-resistant peptic ulcers

10. Treatment of Peptic Ulcer (Therapeutic Highlights from Ganong)

Summary Diagram of Regulation

• Three phases: cephalic (vagal/neural) → gastric (hormonal + neural) → intestinal (inhibitory)
• Three agonists converge on parietal cells: gastrin (via CCK-B, Ca2+), histamine (via H2, cAMP), ACh (via M3, Ca2+)
• Synergism between pathways enables high secretion at low individual stimulus concentrations
• Somatostatin from D cells provides the critical acid-feedback brake
• The mucosal barrier (mucus, HCO3-, prostaglandins) protects the stomach from its own acid