Na+/H+ exchanger isoforms NHE-2 and NHE-1 in inner medullary collecting duct cells. Expression, functional localization, and differential regulation

Recent cloning experiments have identified the existance of four distinct Na+/H+ exchanger isoforms designated as NHE-1, NHE-2, NHE-3, and NHE-4. The cellular distribution, subcellular localization, and regulation of one of these isoforms, NHE-2, in the kidney remains unknown. Northern hybridization showed that NHE-2, along with NHE-1, is expressed in cultured renal medullary collecting duct (mIMCD-3) cells. Acid-stimulated, dimethyl amiloride-sensitive 22Na+ uptake and sodium-dependent pHi recovery occurred only from the basolateral surface of the cells, indicating localization of Na+/H+ exchanger to the basolateral membrane domain. Incubation of IMCD cells in high osmolality media (510 mosm/liter) for 72 h stimulated the Na+/H+ exchanger activity by 59% (p < 0.001). NHE-1 mRNA abundance decreased, whereas NHE-2 mRNA increased in high osmolality media. Incubation of IMCD cells in acid media (pH 7.1) for 48 h did not affect the Na+/H+ exchanger activity compared with control (pH 7.4) (p > 0.05). Northern hybridization, however, indicated that NHE-1 mRNA increased, whereas NHE-2 mRNA decreased in acid media. In conclusion, mIMCD-3 cells express NHE-1 and NHE-2 mRNAs. The cell functional studies in mIMCD-3 cells strongly suggest that NHE-2, along with NHE-1, is expressed in the basolateral membrane domain. They further demonstrate differential regulation of NHE-1 and NHE-2 mRNAs in response to acidosis and high osmolality and suggest that NHE-2 may be involved in volume regulation of IMCD cells.