Effects of serosal-side acidosis on cell pH (pH_i) and membrane electrical properties in gastric mucosa

Acute gastric mucosal injury and bleeding occur in the settings of both respiratory acidosis or metabolic acidosis secondary to systemic sepsis or shock. Respiratory acidosis, however, is more predictably associated with acute injury than metabolic acidosis. We hypothesized that the gastric surface epithelial cells are more susceptible to acute increases in PCO₂ than to acute decreases in HCO^-₃, even for the same level of extracellular acidification. To evaluate this hypothesis, we used intracellular microelectrodes to measure pH_i, cell membrane potential (V_c), as well as ion conductances of the apical (G_a) and basolateral (G_b) membranes and the paracellular pathway (G_s) in gastric mucosal cells during acute changes in serosal PCO₂ or HCO^-₃. Necturus antral mucosae were mounted in Ussing chambers, perfused on both sides by Ringer solution (40 mmHg PCO₂, 18 mM HCO^-₃, pH 7.3). Measurements were performed before and during increases in PCO₂ (80 mmHg, pH 7.0) or decreases in HCO^-₃ (7-2 mM, pH 6.8 or 2.4 mM, pH 6.4). Both forms of acidosis acidified pH_i, depolarized membrane potentials, and decreased ion conductances across apical and basolateral membranes, but not the paracellular pathways. For the same level of extracellular acidification, increases in PCO₂ were more effective than acute decreases in HCO^-₃ in acidifying pH_i and eliciting disturbances in voltage-generating and ion permeability properties of the cell membranes. These findings suggest that pH-buffering mechanisms in gastric surface cells respond less effectively to high PCO₂ than low HCO₃.