The pH gradient across mucus adherent to rat fundic mucosa in vivo and the effect of potential damaging agents.

We investigated the possibility that a "mucus-bicarbonate" barrier might exist in the stomach in vivo. Using an antimony microelectrode a pH gradient was demonstrated across the mucus layer on the fundic mucosa of the rat stomach in vivo. When the luminal pH was 2.0 the maximum pH reached on traversing the mucus layer was 6.68 +/- 0.71 (n = 30) and a stable gradient could be maintained across the mucus for over 100 min. Introduction of luminal solutions with a pH of less than 1.5 caused a fall in the pH gradient and its abolition at pH 1.2. N-Acetyl-L-cysteine (5%) in pH 2.0 luminal HCl solution caused a fall in pH in 12 of 18 rats within 5 min. After 30 to 60 min exposure the maximum mean pH in 12 rats was reduced to 4.18 +/- 0.92 (p less than 0.001). Ten millimolar aspirin in pH 2.0 HCl also caused a fall in pH in 12 of 24 rats within 10 min and the maximum pH in 12 rats after 30-60 min exposure was 4.13 +/- 0.84 (p less than 0.001). These effects on the mucus pH gradient were local actions because in other, untreated areas of the same stomachs there was a normal maximum pH. Ten millimolar sodium taurocholate in pH 2.0 HCl produced a visible clouding of the mucus layer, but no fall in pH. However, gentle manipulation caused the mucus to "flake" off with a subsequent fall in underlying pH. These observations indicate the presence of a pH gradient in mucus in vivo which can be compromised by agents which interfere with mucus structure and/or bicarbonate secretion and by high intraluminal hydrogen ion concentrations. The results support a role for this barrier in gastric mucosal protection.