['The gastric mucosa must have efficient protective mechanisms to maintain physiological intracellular pH. The aim of this study was to investigate the effect of low luminal pH on apical membrane permeability.', "Chambered Necturus antral mucosa was perfused with Ringer's/95% O2-5% CO2 at pH 7.25. The mucosal side was exposed to pH 4.0-2.0 with four microelectrodes placed in surface cells. Two-dimensional cable analysis was used to measure apical, basolateral, and shunt resistances. In some experiments, liquid sensor pH or Na(+)-selective microelectrodes were used.", 'Luminal acidification hyperpolarized apical cell membrane potential and increased apical cell membrane resistance from 21.3 +/- 2.6 (pH 7.25) to 38.0 +/- 2.3 k omega.cm2 (pH 3.0; n = 8). The increase in apical cell membrane resistance was preceded by transient intracellular acidosis from 7.32 +/- 0.07 (pH 4.0) to 7.23 +/- 0.06 (pH 3.0; n = 6). Similar intracellular acidosis (provoked by NH4+ prepulse) failed to cause the effects observed with luminal acid. The increase in apical cell membrane resistance caused by luminal acid was eliminated when N-methyl-D-glucamine+, but not Na+, was substituted for all cations in the luminal solution.'] Does luminal acid increase apical cell membrane resistance in isolated Necturus antral mucosa?