Studies of the pH gradient and thickness of frog gastric mucus gel

The role of mucus in protecting the underlying gastric epithelium is unknown. We investigated the pH gradient and thickness of the mucus gel layer in in vitro gastric fundic mucosa mounted in a special chamber using an antimony microelectrode, a slit lamp, and a dissecting microscope (x40). This system allowed visualization of the tip of the electrode as it entered the mucus gel and as it reached the mucosa. With luminal pH maintained at 3.0-3.5, the maximum pH at interface between mucus and cells was 6.50 ± 0.10 (n = 9). At luminal pH 3.0-3.5, an increase in nutrient [HCO ₃ ^-] from 17.8 to 47.8 μM resulted in an increase in maximum pH within the mucus from 6.80 ± 0.20 to 7.60 ± 0.20 (p < 0.01), whereas when nutrient HCO ₃ ^- was replaced with 5 mM N-2-hydroxyethylpiperazine-N ¹-2-ethane sulfonic acid, the maximum pH decreased from 6.30 ± 0.10 to 4.80 ± 0.20 within 60 min. Decreasing luminal pH from 3.5 to 1.8 produced a fall in the pH gradient without alteration in potential difference, and even after 3 h at a luminal pH of 1.8 the tissues, when remounted in a standard Ussing chamber, showed healthy potential difference, short-circuit current, and resistance and secreted acid at a rate of 1.13 ± 0.08 μEq · cm ^-2 · h ^-1 2,4 Dinitrophenol, 4-acetoamino-4'-isothiocyanostilbene-2,2-disulfonic acid, or acetazolamide had no significant effect on the pH gradient or mucus gel thickness at luminal pH 3.0, whereas KCN caused a small but significant reduction in maximum pH without a change in thickness. A significant increase in mucus gel thickness was observed after treatment with 16,16-dimethyl prostaglandin E ₂ (441 ± 33 to 636 ± 47 μm). A luminal pH of 1.7-1.8 produced a time-dependent dissipation in pH gradient, a phenomenon partially prevented by pretreatment with 16,16-dimethyl prostaglandin E ₂ or incubation in a high [HCO ₃ ^-] nutrient solution. These observations indicate that (a) a pH gradient exists and is maintained across gastric mucus, probably by a continuous alkaline secretion, (b) the gradient is steepened by increasing nutrient [HCO ₃ ^- but is reduced by increasing luminal [H ⁺], (c) 16,16-dimethyl prostaglandin E ₂ increases the thickness of the mucus gel but has no effect on the gradient, and (d) the 'mucus HCO ₃ ^- barrier' offers a first line of defense, but other mechanisms are probably required to ensure mucosal protection.