788 Effect of Genetic Ablation of the Acid/Base Transporters PNBC1, NHE1, NHE2, and Slc26a7, On Gastric Epithelial Functions

Abstract

Background: Recent investigations have revealed the expression of a multitude of basolateral acid/base transporters in gastric parietal cells, but the physiological significance of many of them is not established. Aim and methods: The study was designed to elucidate the physiological role of the Na/HCO3 cotransporter pNBC1, the Na+/H+ exchangers NHE1 and NHE2, all abundantly expressed in parietal cells. Isolated gastric mucosae from ten days old mice (as both NBC1and NHE1-deficient mice have a short life expectancy) were placed in classical Ussing chambers and basal and forskolin-stimulated acid secretory rate determined by back titration. Identical experiments were performed with selective pharmacological inhibitors of NBC and NHEs. Parietal cell ultrastructure was examined by electron microscopy, expression of H+/K+ATPase was assessed by quantitative RT-PCR and immunohistochemistry, epitheial cell apoptosis assessed by the TUNEL assay, and proliferation by immunohistochemistry with anti-Ki67, anti-histone. Results: Forskolin-stimulated acid secretion was significantly reduced (by >50%) in NBC1-deficient gastric mucosa, or after pharmacological inhibition of NBC1, but remained unaltered in NHE1or Slc26a7 deficient gastric mucosa in ten days old mice, or after pharmacological inhibition of NHE1 and NHE2. Parietal cell number, ultrastructure, H+/K+-ATPase expression and epithelial cell proliferation were normal in NBC1as well as NHE1 deficient mice. This indicates that pNBC1 likely serves as an additional base extrusion pathway in addition to the Cl/HCO3 exchanger AE2. In contrast, NHE2-deficient mucosae secreted acid only as a short burst, and displayed a reduction of H+/K+-ATPase but not AE2 expression. At postnatal day 10, no difference in apoptotic rate, proliferative indices, histology, and parietal cell numbers were detected in NHE2 +/+ and -/mice, whereas a complete change of the gastric proliferative zone and marked loss of parietal cell number was seen in adult stomach. Summary and conclusions: The electrogenic Na/HCO3 cotransporter NBC1 is an important base extruder during stimulation of acid secretion, and its absence or inhibition leads to a significant reduction in maximal acid secretory rates. In contrast, NHE1 and SLC26a7 anion transporter are not essential components of the ion transport machinery for gastric acid secretion in young mice. NHE2 ablation leaves parietal cell structure intact in young mice, but renders them non-functional.