title: Proton secretion by the gastric parietal cell.
abstract: The parietal cell occupies a unique niche among eukaryotic cells in that it develops a proton gradient of more than 4 million-fold across the membrane of the secretory canaliculus. At rest, the cell is still able to develop a proton gradient across intracellular membranes, such that the acid compartment has a pH of less than 4. Acidification depends on the simultaneous presence of ATP, K+ and Cl- as demonstrated in permeabilized cells. With acidification of the luminal side of the proton pump, there is a corresponding alkalinization of the cytosolic face as revealed by carboxyfluorescein fluorescence enhancement. Disposal of the resultant alkali depends on carbonic anhydrase activity and the functioning of a coupled Na+:H+ and Cl-:OH-antiport across the basal lateral membrane. Accordingly, with secretion there is an increased cellular Cl- level, which is exported across the apical membrane in association with K+. The Na+ pump dependent secretion of KCl across this membrane is one of the major sites of the gastric ATPase. Membranes isolated from secreting tissue contain a KCl permeation pathway largely absent from membranes isolated from resting tissue. The pump itself acts as an H+ for K+ exchange ATPase which is most probably composed of at least two peptides of 100 000 Mr. That catalytic cycle consists of formation and breakdown of a covalent aspartyl phosphate. Formation of the intermediate depends on loss of K+ from cytosolic binding sites, and breakdown of the intermediate depends on K+ binding to the luminal face of the enzyme. During breakdown, an acid labile E . P is formed, and, at high ATP concentrations, loss of this form of the enzyme is probably the rate limiting step.
mesh_terms: D000251:Adenosine Triphosphatases / Q000502:physiology; D000818:Animals; D002118:Calcium / Q000502:physiology; D002712:Chlorides / Q000502:physiology; D000242:Cyclic AMP / Q000502:physiology; D004553:Electric Conductivity; D005089:Exocytosis; D005750:Gastric Juice / Q000378:metabolism*; D005753:Gastric Mucosa / Q000166:cytology / Q000378:metabolism*; D017506:H(+)-K(+)-Exchanging ATPase; D006863:Hydrogen-Ion Concentration; D008564:Membrane Potentials; D011522:Protons
chemical_list: D002712:Chlorides; D011522:Protons; D000242:Cyclic AMP; D000251:Adenosine Triphosphatases; D017506:H(+)-K(+)-Exchanging ATPase; D002118:Calcium