Functional and molecular characterization of luminal and basolateral Cl-/HCO-3, exchangers of rat thick limbs

Cl-/HCO3- exchange was measured in luminal (LMV) and basolateral (BLMV) membrane vesicles purified from rat medullary thick ascending limb (MTAL). Cl-/HCO3- exchange in BLMV and LMV was inhibited by DIDS, with respective IC50 values of 3.2 +/- 0.9 and 15.2 +/- 5.2 mu M, whereas Cl- conductances were DIDS insensitive. At constant external pH, BLMV Cl-36(-)/HCO3- and Cl-36(-)/Cl- exchanges exhibited a sigmoidal pattern of activation as internal pH (pH(i)) increased from 6.1 to 8.0, whereas LMV Cl-36(-)/Cl- exchange was unchanged between pH(i) 6.7 and 7.8. The 165-kDa AE2 polypeptide and similar to 115-kDa AE1-related polypeptide were present only in BLMV. In contrast, AE1-related polypeptides of similar to 90 and 95 kDa were present not only in BLMV but also (in variable abundance) in LMV. mie conclude that rat MTAL BLMV and LMV express distinct anion exchange activities and distinct sets of AE polypeptides. AE2 (and perhaps AE1) in BLMV likely contribute to HCO3- absorption. In contrast, LMV exchangers may contribute to NaCl absorption via parallel coupling with the luminal Na+/H+ antiporters and/or may provide negative feedback regulation of HCO3- absorption.