Role of basolateral Na⁺-K⁺-Cl^- cotransport in HCl secretion by amphibian gastric mucosa

In amphibians and mammals, luminal H⁺ secretion by the stomach requires Cl^-. It is widely accepted that a basolateral Cl^-/HCO₃/^- exchanger in the acid-secreting oxyntic cell restores the Cl^- deficit resulting from apical HCl secretion. In this study, we used reverse transcriptase-polymerase chain reaction (RT-PCR) to generate a 1,200-bp fragment specific to a basolateral isoform of the Na⁺-K⁺-Cl^- cotransporter in the gastric fundus of Necturus maculosus. By Northern analysis, we observed that gastric mucosa expresses greater than fivefold higher levels of mRNA encoding this cotransporter than any other tissue in the gastrointestinal tract. Feeding results in > 100% increases in mRNA levels in acid-secreting fundic mucosa but does not alter mRNA levels in the neighboring and non-acid-secreting antral mucosa or duodenum. Flux measurements using in vitro fundic mucosae indicate that acid secretion requires Na⁺ in the nutrient (i.e., serosal side) perfusate, is modulated by changes in nutrient K⁺ levels, and is inhibited by nutrient solutions containing 50 μM bumetanide, a recognized blocker of Na⁺-K⁺- Cl^- cotransport. These findings suggest that this basolateral transporter plays a dominant and previously unsuspected role in secretion of HCl across the apical membrane.