Regulated alkali secretion acts in tandem with unstirred layers to regulate mouse gastric surface pH

The physical basis for the protective pH gradient at the gastric surface is unconfirmed. This study examined the role of mucus, the unstirred layer, and acid/alkali secretion in controlling gastric surface pH in vivo.

Stomachs of anesthetized mice were exteriorized, and exposed gastric mucosa was imaged by confocal microscopy.

Accessibility of molecules at the gastric surface was determined by monitoring the decrease in probe fluorescence over time after dyes were removed from perfusate. On dye removal, Cl-NERF (400 molecular weight) fluorescence decreased more slowly at the gastric surface in the presence of mucus (rate constant [k] = 0.08 +/- 0.02 per second) than in the absence of mucus (k = 0.15 +/- 0.02 per second) or 90 microm distant from the surface (k = 0.22 +/- 0.03 per second). In contrast, 70-kilodalton Cl-NERF/dextran washed from the gastric surface more slowly in the presence (k = 0.05 +/- 0.01 per second) or absence (k = 0.09 +/- 0.01 per second) of mucus compared with 90 microm from the tissue surface (k = 0.36 +/- 0.08 per second). Two-photon uncaging of fluorescein near nonsuperfused gastric surface showed that diffusion was not slowed at the gastric surface compared with diffusion in free solution. Surface pH was measured by Cl-NERF ratio imaging. Increasing the superfusion rate decreased the thickness of the surface pH gradient without significantly changing surface pH values, suggesting a pH set point of approximately 4 for control of surface pH. Increasing perfusate pH buffers decreased surface pH toward perfusate values.

Proton concentration at the gastric surface is the result of regulating acid/alkali secretion to a set point in combination with an unstirred layer and not by trapping of proton or small-molecular-weight buffers in the unstirred layer.