Acid transport across the continuous gastric mucus layer
S. Schreiber and P. Scheid
The gastric mucus protects the mucosa from the acidic gastric juice; at the same time, it must allow protons secreted by the mucosa to reach the lumen. We have substantiated the first feature - mucosa protection against acid gastric juice - by measurement of the pH profile across the mucus layer in the guinea pig, using fine-tipped pH microelectrodes. At a luminal pH of 1.8 the pH became incresingly alkaline across the mucus layer, reaching a value of about 5.5 at the epithelial surface. We found that the mucus layer increased continuously by 170 µm/hr in the unstimulated and by 450 µm/hr in the histamine-stimulated preparation. We have further analyzed the buffer properties of the gastric mucus in vitro.
We have titrated fresh guinea pig mucus at 10°C from pH 6.5 to 0.7, incubating it at pH 0.7 for 30 min at 37°C. We observed spontaneous proton release from the mucus, about 100 mmol per liter of mucus. This proton release was inhibited by the pepsin inhibitor pepstatin. We have interpreted these data to propose a model for proton transport across the gastric mucus. We suggest that protons released by the mucosa are buffered by the mucus proteins and thus transported with the continously secreted mucus towards the gastric lumen. Pepsinogen, convereted within the mucus to pepsin, modifies the buffer properties of the mucus, thereby irreversibly releasing protons, which then end up in the gastric lumen.
Schreiber, S. and Scheid, P. (1997). Gastric mucus of the guinea pig: Proton carrier and diffusion barrier. Am. J. Physiol. 272: G63 - G70. |
