PREEPITHELIAL MUCUS-HCO3-LAYER PROTECTS AGAINST INTRACELLULAR ACIDOSIS IN ACID-EXPOSED GASTRIC-MUCOSA

The role of the preepithelied mucus-HCO3- layer in protection against intracellular acidosis was investigated in isolated Necturus gastric antral mucosa exposed to luminal acid by simultaneous measurement of intracellular pH (pH(i)) and extracellular surface pH (pH(s)) in surface epithelium with microelectrode technique. Acidification of the luminal perfusate to pH 2.5 acidified pH(i) in surface epithelial cells from 7.33 +/- 0.02 to 7.20 +/- 0.04, whereas pH(s) fell from 6.75 +/- 0.21 to 5.20 +/- 0.25 (P < 0.01; n = 9), followed by a steady state for at least 2 h. Inhibition of epithelial HCO3- secretion and transport by removal of serosal HCO3- and CO2 (HEPES and O2 substitution) during acid exposure provoked a progressive acidification of pH(s) from 5.60 0.41 to 2.74 +/- 0.14 in 30 min (P < 0.01; n = 9), which was accompanied, after a 5- to 10-min delay, by acidification of pH(i) from 7.21 +/- 0.03 to 5.68 +/- 0.26 (P < 0.01). Digestion of the surface mucus gel by pepsin (5% wt/vol) at pH 2.5 caused a slow acidification of pH(s) from 5.22 +/- 0.59 to 3.60 +/- 0.46 within 2 h. This was followed by a more rapid acidification to 2.53 +/- 0.38 (P < 0.01; n = 7), with concomitant acidification of pH(i) from 7.19 +/- 0.05 to 6.03 +/- 0.33 (P < 0.01). Decomposing the structural integrity of the mucus gel by luminal exposure to 5% N-acetyl-L-cysteine at pH 2.5 caused, after a 30- to 45-min relatively stable period a fall of pH(s) from 5.35 +/- 0.66 to 2.70 +/- 0.31 in 15 min (P < 0.01; n = 7), with concomitant acidification of pH(i) from 7.24 +/- 0.04 to 6.35 +/- 0.36 (P < 0.01). Data indicate that the Necturus antral mucosa has a preepithelial buffer layer with pH gradient, which has an important defense function against luminal acid.