Analysis of hydrogen ion concentration in the gastric gel mucus layer.

Secretion of HCO-3 by the gastric epithelium has been thought to lower the concentration of H+ in the gastric mucus layer. This has been analyzed mathematically to include the HCO-3-H+ reaction, bulk water flow, diffusion, ion-ion electrical interaction, and ion-fixed charge interaction. The reaction-electrodiffusion problem is solved by use of singular perturbation theory. We show that there is a very thin layer for the reaction, equivalent to a sink of H+. In this layer there is negligible HCO-3 accumulation. A steady-state model is satisfactory if gastric mixing motions are more frequent than every 3 min. H+ concentration at the epithelium decreases with increased bicarbonate secretion, increased volume flow associated with bicarbonate secretion, increased thickness of the mucus layer, increased fixed negative charge of the mucus, and decreased cation flux into the lumen. The resultant lowering of H+ concentration may be as small as 5 mM but is probably considerably larger. Determining the actual drop will depend on more precise experimental measurements of the parameters of the problem.