DIFFERENTIAL EXPRESSION OF AE1 IN RENAL HCO3-SECRETING AND HCO3-REABSORBING INTERCALATED CELLS

The cortical collecting duct of the kidney contains two types of inter calated cells that transport HCO3 in opposite directions. HCO3 reabsor ption takes place in the alpha-type intercalated cells, which express a Cl/HCO3 exchanger on the basolateral membrane. This exchanger is the product of the anion exchanger 1 (AE1) or band 3 gene. HCO3 secretion occurs in the beta-intercalated cells, which have a Cl/HCO3 exchanger on the apical membrane. Based on studies in an immortalized cell line , recently it was proposed that the apical anion exchanger of beta-int ercalated cells is also AE1 (van Adelsberg, J. S., Edwards, J. C., and Al-Awqati, Q. (1993) J. Biol. Chem. 268, 11283-11289). In the present study we reinvestigated this issue by determining the distribution of AE1 mRNA and protein in the two intercalated cell types using cells f reshly isolated from the native epithelium. Using quantitative reverse transcriptase polymerase chain reaction, we found that alpha-intercal ated cells, isolated from rabbit kidney by fluorescence-activated cell sorting, have high levels of AE1 mRNA, whereas beta-intercalated cell s express very low levels. The ratio of AE1 mRNA levels in alpha- vers us beta-intercalated cells averaged 10.1 +/- 2.6. In addition, metabol ic acidosis increased the levels of AE1 mRNA by 3-5-fold in cortical c ollecting duct cells. This difference was confirmed by Northern blotti ng. Western blotting using an antibody against rabbit AE1 revealed a m ajor immunoreactive product with a molecular weight of similar to 110 kDa in cortical collecting duct cells. Deglycosylation reduced the siz e of the immunoreactive product to similar to 90 kDa, which is compati ble with the presence of a truncated form of AE1. Metabolic acidosis i ncreased the intensity of the AE1 immunoreactive band. The level of AE 1-immunoreactive protein was significantly higher in alpha-intercalate d cells than in beta-intercalated cells. In aggregate, these data prov ide evidence for the differential expression of AE1 in HCO3-reabsorbin g versus HCO, secreting renal intercalated cells both at the mRNA and at the protein level. These results give no support to the concept tha t AE1 functions both as a basolateral and an apical anion exchanger in cortical collecting duct cells.