INNER MEDULLARY COLLECTING DUCT NA+-H+ EXCHANGER

Cells from the inner medullary collecting duct (IMCD) exhibit Na+-H+ exchange. The present studies were performed to address certain important characteristics of this process in cultured IMCD cells. First, Na+-H+ exchange was found to be present both at 37-degrees-C and at 25-degrees-C, in contrast to Na+-independent H+ extrusion, which was only observed in some cultures and only at 37-degrees-C. Second, with the use of image analysis techniques, virtually all cells in IMCD cultures were demonstrated to possess Na+-H+ exchange, whether or not the cells exhibited Na+-independent intracellular pH recovery from acid loads. Also, Na+-H+ exchange was found to be expressed on the basolateral aspect of these cells, but not on the apical membrane. These properties of IMCD Na+-H+ exchange are consistent with a function to regulate intracellular pH rather than mediate transepithelial acid-base transport. Na+-H+ exchange in IMCD cells was also compared with that in cultured renal proximal tubule cells. Despite physiologically distinct roles in vivo, Na+-H+ exchange in these two cell types in culture was found to be similar with respect to the K(m) for Na+ and the K(i) for 5-(N-ethyl-N-isopropyl)amiloride. These data are consistent with functionally similar (if not identical) processes mediating Na+-H+ exchange in these two cell types, but with opposite polarity.