Cl(-)-HCO3- Exchanger Isoform AE2 is Restricted to the Basolateral Surface of Alveolar Epithelial Cell Monolayers

We investigated the polarized distribution and isoform specificity of anion exchange (Cl(-)-HCO3- exchange) in alveolar epithelial cell monolayers. Rat alveolar type II epithelial cell monolayers were grown in primary culture on detachable tissue culture-treated nuclepore filters. Each filter was mounted in a cuvette containing two fluid compartments (apical and basolateral) separated by the monolayer, the cells loaded with pH-sensitive dye, and intracellular pH (pHi) measured spectrofluorometrically. To assay for Cl(-)-HCO3- exchange, monolayers were incubated in medium containing 24 mM HCO3-/5% CO2 and 140 mM NaCl at pH 7.4 and acutely alkalinized by replacement of the fluid by HCO3(-)-free buffer containing Hepes (6 mM) at pH 7.4. Monolayers exhibited basolateral (but not apical) Cl(-)-dependent, Na(+)-independent recovery from an alkaline load that was abolished when Cl- was substituted by equimolar gluconate in the basolateral fluid, or if DIDS (500 microM) was present basolaterally. Substitution of gluconate for Cl- in the basolateral fluid, but not the apical fluid, resulted in a rise in steady-state pHi that was reversible on replacement of the basolateral fluid with Cl(-)-containing buffer, which occurred in HCO3(-)- but not Hepes-buffered medium. These data indicate that alveolar epithelial cells express basolateral membrane domain of these cells. Northern analysis of alveolar epithelial cell mRNA using anion exchanger (AE) isoform-specific cDNA probes indicates that alveolar epithelial cells express the AE2 isoform predominantly, if not exclusively, and do not express detectable AE1 (i.e., band-3 protein) or AE3.(ABSTRACT TRUNCATED AT 250 WORDS)