REGULATION OF AE2 MESSENGER-RNA EXPRESSION IN THE CORTICAL COLLECTINGDUCT BY ACID BASE BALANCE/

AE2 mRNA and protein is expressed in several nephron segments, one of which is the cortical collecting duct (CCD). However, the distribution of AE2 among the different cell types of the CCD and the function of AE2 in the kidney are not known. The purpose of this study was to dete rmine the distribution of AE2 mRNA among the three CCD cell types and to examine the effects of changes in acid/base balance on its expressi on. Following NH4Cl (acid) or NaHCO3 (base) loading of rabbits for sim ilar to 18 h, CCD cells were isolated by immunodissection. AE2 mRNA le vels were determined by RT-PCR and were normalized for beta-actin leve ls. We found that CCD cells express high levels of AE2 mRNA (similar t o 500 copies/cell). AE2 mRNA levels were significantly higher in CCD c ells originating from base-loaded than acid-loaded rabbits, with an av erage increase of 3.7 +/- 1.07-fold. The effect of pH on AE2 mRNA leve ls was also tested directly using primary cultures of CCD cells. CCD c ells incubated in acidic media expressed significantly lower levels of AE2 mRNA than those in normal or alkaline media. Experiments with iso lated principal cells, alpha-intercalated cells, and beta-intercalated cells (separated by fluorescence-activated cell sorting) demonstrated that AE2 mRNA levels are comparable in the three collecting duct cell subtypes and are similarly regulated by changes in acid/base balance. Based on these results, we conclude that adaptation to changes in ext racellular H+ concentration is accompanied by opposite changes in AE2 mRNA expression. The observations that AE2 mRNA is not expressed in a cell-type-specific manner and that changes in acid/base balance have s imilar effects on each CCD cell subtype suggest that AE2 might serve a housekeeping function rather than being the apical anion exchanger of beta-intercalated cells.