EXPRESSION OF NA+ H+ EXCHANGER ISOFORMS IN INNER SEGMENT OF INNER MEDULLARY COLLECTING DUCT (IMCD3)/

Na+/H+ exchangers (NHE) play a critical role in many cellular and tran sport processes in the inner medullary collecting duct (IMCD). Morphol ogically, the IMCD is divided into the outer (IMCD1), middle (IMCD2), and inner (IMCD3) segments. The inner, IMCD3 segment contains only one cell type, the IMCD cell, which is distinct in ultrastructure and in function from the principal acid intercalated cells that are present i n other portions of the IMCD. NHEs constitute a gene family containing several isoforms (NHE1, NHE2, NHE3, NHE4 and NHE5) which possess dist inct characteristics and serve specialized functions. To understand th e molecular basis of NHE-related processes in the IMCD, it is critical to know the molecular identity of the NHEs in this tubule segment. Th e purpose of the present study was to identify the NHE isoforms presen t and their polar distribution in IMCD3. Applying the reverse transcri ption-polymerase chain reaction (RT-PCR) technique to IMCD3 (obtained from distal 50% of inner medulla) of mouse and rat kidneys, we found t hat NHE1, NHE2 and NHE4, but not NHE3 were expressed in both species. The polar localization of NHE in IMCD3 was examined in tubules isolate d from rats and perfused in vitro with HEPES-buffered solutions under isotonic conditions, pHi was measured by BCECF fluorescence. Naf-depen dent, amiloride-inhibitable pHi recovery from cell acidification (cons istent with NHE) was detected in the basolateral, but not the apical, membrane of IMCD3. We conclude that NHE1, NHE2 and NHE4, but not NHE3, are present in both the mouse and rat IMCD3. Functionally, NHE is lim ited to the basolateral membrane. Additional studies are needed to det ermine the physiological roles and regulation of basolateral NHE isofo rms in this tubule segment.