Microscopy of acid transport at the gastric surface in vivo

In vivo microscopy of the gastric surface, and pH-sensitive dyes, were used to study the movement of acid formed in the gastric crypts across the mucus layer adherent to the gastric surface and into the lumen. Rats were anaesthetized and the stomach gently exteriorized. When the pH-sensitive dye Congo red was applied luminally to stain the gel, predominantly red spots (pH greater than 5) and occasional blue spots (pH less than 3), located above the outlets of the crypts, were observed in spontaneously-secreting mucosae. Maximal stimulation of acid secretion (pentagastrin, 40 micrograms kg-1 h-1) resulted in the appearance only of blue spots, but the pH in the mucus gel between the crypts remained more alkaline, as indicated by pink staining. The fluorescence dye acridine orange was injected intravenously to an estimated blood concentration of 10(-5)M in another type of experiment. This dye is concentrated and secreted by the parietal cells. Pronounced fluorescence was observed within spots of the gastric surface corresponding to the outlets of the gastric crypts, but no fluorescence was detected outside these areas. The results obtained with both dyes strongly suggest that acid (and pepsin) is transported across the mucus gel only at restricted sites.