Gastric Acid Secretion – Regulation & Output

GASTRIC ACID SECRETION – REGULATION & OUTPUT

• Gastric acid – Secreted by gastric glands in gastric mucosa.

GASTRIC GLANDS IN STOMACH:

1. In pyloric & cardiac regions:

• Gastric glands are “Mucous” type.
• Secrete only mucous & HCO3–.
• In pylorus referred “Mucus-secreting pylorus gland”.
• Contains less parietal & chief cells.

2. In body & fundus regions:

• Glands contain secretory cells.

Cell types:

2a. Parietal/Oxyntic cells –

• Secrete hydrochloric acid (HCl) & Intrinsic factor of castle.

2b. Chief/Zymogen/Peptic cells –

• Secrete pepsinogen.

2c. G cells –

• Secretes Gastrin.

2d. Delta (D) cells –

• Secretes Somatostatin.

2e. Entero-chromaffin like (ECL) cells –

• Secretes Histamine.

COMPOSITION OF GASTRIC ACID:

• In human – 2.5 L (105ml/hr) gastric juice secreted daily.
• Gastric acid pH is less than 3 (usually 1-2).

Important constituents:

• HCl (H+ & Cl–).
• Anions (HPO4-2 & SO4+2).
• Cations (Na2+, K+, & Mg2+).
• Pepsinogen & Pepsin.
• Mucus.
• Lipase.
• Intrinsic factor of castle.

REGULATION OF ACID SECRETION:

• Gastric acid secreted by parietal cell activation in gastric mucosa.

1. ACTIVATORS FOR ACID SECRETION: 

• Mainly Histamine – H2 receptors.
• Acetylcholine (M3 receptor).
• Gastrin.

Mechanism:

Ach & gastrin acts indirectly.

• By releasing histamine.
• Also by stimulating H+-K+-ATPase (proton pump).

Vagal (Parasympathetic) stimulation –

• Increases acid secretion.
• Through acetylcholine.

Clinical application:

Proton Pump/H+ inhibitors:

• By blocking H+ receptors/inhibiting proton pump –> Blocking acid secretion.

Anti-muscarinic drugs:

• Blocks muscarinic receptors.
• Ex: Pirenzepine.

2. INHIBITORS OF ACID SECRETION:

• Somatostatin –.
• By directly inhibiting parietal cells & gastrin secretion
• Secretin
• Cholecystokinin (CCK).
• GIP & VIP –  
• Inhibit gastric secretion by stimulating somatostatin release.

PGE2 –

• By inhibiting parietal cells & gastrin release.
• Best established inhibitor – Low pH (Acid itself).
• Stomach & duodenal acidification inhibits acid secretion.
• By inhibiting gastrin release from G cells.

CONDITIONS:

1. Increased gastric acid secretion:

• In duodenal ulcer.
• Gastric ulcer.
• ZES.

2. Reduced gastric acid secretion:

• After vagotomy.
• In Pernicious anemia.

GASTRIC ACID OUTPUT:

• Measurement of gastric acid output – Clinical importance.
• Two important parameters – 

1. Basal Acid Output (BAO) –

• Rate of acid secretion in absence of any stimulations.
• Normal BAO – < 10 (2 – 5) mmol/hr.

2. Maximal Acid Output (MAO)/Total Acid Output –

• Rate of acid secretion produced by stimulants like histamine/ pentagastrin.
• Normal MAO – < 50 mmol/hr.
• Values reflects total parietal cell population in stomach.
• Hence, referred “Parietal cell mass”.