Methods
To help explain this apparent conundrum and to determine the role of intrinsic cellular defense against perfused acid, we measured duodenal epithelial cell intracellular pH (pH_i) with in vivo microscopy, and bicarbonate secretion, using duodenal loop perfusion, in anaesthetized rats. We also examined the effects of the Na⁺:HCO₃^- cotransport (NBC) inhibitor, the anion transport inhibitor DIDS, and the CFTR Cl^- channel inhibitor, NPPB on pH_i and bicarbonate secretion. Acid-induced injury was assessed with a propidium iodide (PI)-based technique.

Results
Bicarbonate secretion increased after a 15 min acid perfusion which was abolished by DIDS and NPPB. During superfusion with neutral solutions, DIDS lowered pH_i , indicating that base loading was an important determinant of basal pH_i, whereas NPPB alkalinized cells, confirming the role of the CFTR in bicarbonate secretion. A 15 min exposure to a physiologic acid solution (pH 2.2) acidified cells, followed by an increase of pH_i to over baseline (overshoot) following acid challenge. DIDS enhanced acidification during acid challenge and abolished overshoot, whereas NPPB attenuated acidification and enhanced overshoot. Supraphysiological (pH 2.0 or 1.8) acid concentrations induced epithelial cell injury, identified by the appearance of PI-positive cells. Acid-induced epithelial injury was enhanced by DIDS, but attenuated by NPPB.

Conclusions
Although DIDS and NPPB both abolished bicarbonate secretion, DIDS enhanced acid-induced injury and cellular acidification whereas NPPB had opposite effects. These studies stress the importance of cellular base loading rather than bicarbonate secretion in duodenal mucosal protection. Cellular base loading via basolateral NBC, preceding bicarbonate secretion, is a primary and acute defense mechanism to luminal acid in duodenal epithelium, and may be the means by which ulcers are prevented in CF patients.