Parietal Cells Acidify the Stomach Contents While Maintaining a Neutral Cytosolic pH

The mammalian stomach contains a 0.1 M solution of hydrochloric acid (HCl). This strongly acidic medium kills many ingested pathogens and denatures many ingested proteins before they are degraded by proteolytic enzymes (e.g., pepsin) that function at acidic pH. Hydrochloric acid is secreted into the stomach by specialized epithelial cells called parietal cells (also known as oxyntic cells) in the gastric lining. These cells contain a H+/K+ ATPase in their apical membrane, which faces the stomach lumen and generates a millionfold H+ concentration gradient: pH = 1.0 in the stomach lumen versus pH = 7.0 in the cell cytosol. This transport protein is a P-class ATP-powered ion pump similar in structure and function to the plasma-membrane Na+/K+ ATPase discussed earlier. The numerous mitochondria in parietal cells produce abundant ATP for use by the H+/K+ ATPase.

If parietal cells simply exported H+ ions in exchange for K+ ions, the loss of protons would lead to a rise in the concentration of OH- ions in the cytosol and thus a marked increase in cytosolic pH. (Recall that [H+] X [OH-] always is a constant, 10-14 M2.) Parietal cells avoid this rise in cy-tosolic pH in conjunction with acidification of the stomach lumen by using Cl-/HCO3- antiporters in the basolateral membrane to export the "excess" OH- ions in the cytosol into the blood. As noted earlier, this anion antiporter is activated at high cytosolic pH (see Figure 7-22).

The overall process by which parietal cells acidify the stomach lumen is illustrated in Figure 7-28. In a reaction catalyzed by carbonic anhydrase the "excess" cytosolic OH-combines with CO2 that diffuses in from the blood, forming HCO3-. Catalyzed by the basolateral anion antiporter, this bicarbonate ion is exported across the basolateral membrane (and ultimately into the blood) in exchange for a Cl- ion. The Cl- ions then exit through Cl- channels in the apical membrane, entering the stomach lumen. To preserve elec-troneutrality, each Cl- ion that moves into the stomach lumen across the apical membrane is accompanied by a K+ ion that moves outward through a separate K+ channel. In this way, the excess K+ ions pumped inward by the H+/K+

Cl-/HCO3 antiporter

hco3-

Basolateral membrane -

Cl-/HCO3 antiporter

hco3-

HCO3-

Carbonic anhydrase

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