Regulation of DRA and AE1 in rat colon by dietary Na depletion

Two distinct Cl/anion exchange activities (Cl/HCO3 and Cl/OH) identified in apical membranes of rat distal colon are distributed in cell type-specific patterns. Cl/HCO3 exchange is expressed only in surface cells, whereas Cl/OH exchange is localized in surface and crypt cells. Dietary Na depletion substantially inhibits Cl/HCO3 but not Cl/OH exchange. We determined whether anion exchange isoforms (AE) and/or downregulated in adenoma (DRA) are expressed in and related to apical membrane anion exchanges by examining localization of AE isoform-specific and DRA mRNA expression in normal and Na-depleted rats. Amplification of AE cDNA fragments by RT-PCR with colonic mRNA as template indicates that AE1 and AE2 but not AE3 mRNAs are expressed. In situ hybridization study revealed that AE1 mRNA is expressed predominantly in surface but not crypt cells. In contrast, AE2 polypeptide is expressed in basolateral membranes and DRA protein is expressed in apical membranes of both surface and crypt cells. AE1 mRNA is only minimally present in proximal colon, and DRA mRNA abundance is similar in distal and proximal colon. Dietary Na depletion reduces AE1 mRNA abundance but did not alter DRA mRNA abundance. This indicates that AE1 encodes surface cell-specific aldosterone-regulated Cl/HCO3 exchange, whereas DRA encodes aldosterone-insensitive Cl/OH exchange.