Proton/hydroxyl transport in gastric and intestinal epithelia.

Proton transport across the plasma membrane of the gastrointestinal epithelium occurs by various pathways. There is the permeability of H+ across the lipid components of the membranes, probably of minor significance at physiological pH, but at the pH of the secretory surface of the parietal cell a factor that cannot be neglected. Transport of H+ dependent on the protein components of the plasma membrane involves various mechanisms. For example Na+ :H+ or Cl- :HCO-3 antiport (exchange) are generally electroneutral mechanisms (i.e., neither affected by potential gradients nor affecting membrane conductance) that are widely distributed throughout the body. Plasma membranes may contain proton or bicarbonate conductances (i.e., gradients of either ion may be determined by the potential across the membrane). This type of pathway is often of minor significance, hence the electrical component of hydrogen ion gradients across the plasma membrane can often be neglected. In the case of the gastric parietal cell, proton transport depends on the activity of a specific ATPase. This ATPase may be present elsewhere in the intestinal tract. This review will consider many of these proton pathways. In the case of brush border pathways, some of the data presented on Na+ :H+ antiport wil be derived from studies done on renal brush border rather than those of the small intestine, on the assumption that the properties of the antiporter are similar in the two tissues.