Background/aims: The gastric mucosa is covered by a continuous layer of bicarbonate-containing mucus gel; the question arises how acid, formed in the gastric glands, moves into the lumen.

Methods: The pH in the gastric mucus gel and gel thickness were measured in anesthetized rats with pH-sensitive microelectrodes (tip diameter, 1-5 microns).

Results: During pentagastrin (40 micrograms.kg-1.h-1) stimulation of acid secretion, the pH was higher in the gel than in the lumen (pH 2) up to a distance of 115 +/- 18 microns from the epithelial surface and maximal (pH 7.2 +/- 0.1) at the surface. A similar pH gradient was recorded at luminal pH 3. After omeprazole (10 mumol/kg) inhibition of endogenous acid secretion and with exogenous acid in the lumen, the pH profile was broader: 204 +/- 26 microns at luminal pH 2 and 231 +/- 63 microns at luminal pH 3. In contrast, the pH at the epithelial surface was lower (pH 6.8-6.9). The gel thickness (200-300 microns) was similar in all groups.

Conclusions: The significantly higher surface pH in acid-secreting stomachs probably reflects better availability of interstitial mucosal bicarbonate. Bulk transport of secreted acid in channels created by the gland luminal hydrostatic pressure may additionally act to limit acidification of the mucus gel.