Gastroenterology

Gastroenterology

Volume 126, Issue 3, March 2004, Pages 774-783
Gastroenterology

Basic-alimentary tract
Regulated alkali secretion acts in tandem with unstirred layers to regulate mouse gastric surface pH

https://doi.org/10.1053/j.gastro.2003.11.059Get rights and content

Abstract

Background & Aims: 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. Methods: Stomachs of anesthetized mice were exteriorized, and exposed gastric mucosa was imaged by confocal microscopy. Results: 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 μm 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 μm 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. Conclusions: 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.

Section snippets

Animal preparation

The surgical preparation for mice has been described previously.9 Briefly, the stomachs of anesthetized mice were exteriorized and everted to expose the gastric mucosa. The mouse was placed supine on the stage of an inverted confocal microscope (LSM 510; Zeiss, Jena, Germany) such that a portion of the exposed mucosa protruded into a perfusion chamber. The perfusion chamber and stage were warmed to keep the animal’s body temperature at 37°C.

Cl-NERF pH imaging

The protocol for imaging the mouse gastric surface pH

Results

In previous confocal microscopy measurements of gastric surface pH in rats and mice, results showed that surface pH remained at a near-constant value (pH 3.9–4.2) despite a dramatic conversion of the tissue secretions from alkali to acid when luminal pH was increased.5, 9 Our goal was to learn why surface pH was nearly constant around pH 4.

Discussion

This study investigated the physical basis for the pH gradient maintained at the gastric mucosal surface. Using in vivo confocal microscopy, we examined the influence of restricted diffusion, the unstirred layer, and juxtamucosal pH buffering on surface pH control. This technique also allowed us to compare results in the absence vs. presence of surface mucus.

This study is in agreement with previous work suggesting the presence of an unstirred layer at the gastric surface that limits entry or

Acknowledgements

We are grateful to Oleg Gerasimenko (Department of Medicine, University of Liverpool, UK) for his generosity in helping us with the uncaging experiments and George Sachs (Department of Physiology, University of California, Los Angeles) for his helpful urging to do the pH 2 experiments.

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  • Cited by (0)

    Supported by the National Institutes of Health (Grant RO1DK 54940) and the American Heart Association (predoctoral fellowship; H.K.B.).

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