Question

Explain how the stomach secretes acid at a cellular level. How could you prevent the excessive acid production that triggers heartburn? What is protecting the walls of the stomach against acid?

Answer 1

Mechanism of Acid Secretion:

- The hydrogen ion concentration in parietal cell secretions is roughly 3 million fold higher than in blood, and chloride is secreted against both a concentration and electric gradient.

- Partietal cell secretes acid using active transport.

- The most important transport proteins in acid secretion is a H+/K+ ATPase or "proton pump" located in the cannalicular membrane.

- Hydrogen ions are generated within the parietal cell from dissociation of water. The hydroxyl ions formed in this process rapidly combine with carbon dioxide to form bicarbonate ion, a reaction cataylzed by carbonic anhydrase.

- Bicarbonate is transported out of the basolateral membrane in exchange for chloride..

- The outflow of bicarbonate 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.

- Accumulation of osmotically-active hydrogen ion in the cannaliculus generates an osmotic gradient across the membrane that results in outward diffusion of water - the resulting gastric juice is 155 mM HCl and 15 mM KCl with a small amount of NaCl.

Control of acid secretion :

Parietal cells bear receptors for three stimulators of acid secretion:

1- Acetylcholine (muscarinic type receptor)

2- Gastrin

3- Histamine (H2 type receptor)

Stimulation of these receptors results in acid secretion. Inhibitors of these receptors such as anticholinergic drugs and antihistamine are used that prevent excessive acid secretion by the stomach thus preventing the heart burn.

Stomach is prevented from acid action by mucin lining the mucosa.