Barrier function of the gastric mucus gel.

The gastric epithelium is covered by a continuous layer of secreted mucus and bicarbonate. The function of this mucobicarbonate layer in terms of protecting the epithelial cells from luminal acid is controversial. Several studies conducted in vitro have shown that gastric mucus can slow proton diffusion and can enable the formation of a pH gradient across the mucobicarbonate layer. In our laboratory, simultaneous measurements of intracellular pH and the thickness of the mucus gel overlying gastric surface cells in vivo indicated that surface cell acidification rates and mucus gel thickness were inversely related. This suggests that the gastric mucobicarbonate layer delays proton permeation into gastric surface cells, enabling secreted bicarbonate to neutralize luminal acid. Several theoretical models, including the effects of mucus and bicarbonate secretion, convection, stirring, and lipids are offered as a possible explanation for the experimental observations. Lipid content and additional unstirred layers outside of the mucus gel are offered as possible explanations for the experimental observations. On the basis of the available data and theoretical considerations, we can conclude that all of these factors probably interact in an integrated manner to protect the gastric epithelial cells from damage due to luminal acid.