pH dependence of hydrochloric acid diffusion through gastric mucus: correlation with diffusion through a water layer using a membrane-mounted glass pH electrode.

Solute diffusion coefficients (D) can indicate a dependence upon actual solute concentrations. Here a single compartment has been utilized, in which effective HCl diffusion to a membrane-mounted glass pH electrode can be measured across the pH spectrum. The study has investigated HCl diffusion through both mucus and water layers as a function of HCl concentration. The observed dynamic responses of a liquid-film and mucus-coated electrodes over a range of HCl concentrations suggest that the speed at which equilibrium is attained is pH dependent; equilibrium was reached rapidly under more acidic and alkaline conditions. Estimated values of DHCl also indicate a strong pH dependence for both liquid film and mucus. In both instances, a greater than 10-fold reduction in DHCl at pH 7.5 as compared with that at pH 3.5 has been demonstrated. Furthermore, estimated values of DHCl are approximately 4-fold smaller through the mucus gel, as compared with a water layer. The findings indicate that the most powerful influence on diffusional resistance is pH itself, whereby a marked drop in H+ diffusion is likely to occur towards neutral pH irrespective of the composition of the gel barrier. Possible implications of the findings are discussed in relation to mucosal protection from acid.