Estimation of sodium chloride diffusion coefficient in gastric mucin.

Estimates of the diffusion coefficient of hydrogen and sodium ions were made in solutions of artificial porcine mucin. When stirred compartmental systems were used, the calculated diffusion coefficients were one order of magnitude higher than free solution values, if the contribution from the support membranes was evaluated. It is likely that the stirred systems induce forced convection in a central compartment, leading to overestimation of path length and consequently erroneous values for the diffusion coefficients. In these systems, mucus probably only reduces the forced convection and increases the apparent path length. In a static diffusion cell, mucus reduced the diffusion coefficient for sodium ion significantly. No attempt was made to estimate DH+ because of the compromising effect of buffering by the mucus. The reduction in DNa+ depended on the concentration of mucin used but was not pH dependent. The calculated activation energy indicated factors other than steric hindrance to be involved in the restriction of free diffusion in mucus solutions.