Na(+)-Ca2+ exchanger of rat proximal tubule: gene expression and subcellular localization

The activity of the Na(+)-Ca2+ exchanger, a membrane transporter that mediates Ca2+ efflux, has been described in amphibian and mammalian renal proximal tubules. However, demonstration of cell-specific expression of the Na(+)-Ca2+ exchanger in proximal renal tubules has been restricted to functional assays. In this work, Na(+)-Ca2+ exchanger gene expression in rat proximal tubules was characterized by three additional criteria: functional assay of transport activity in membrane vesicles derived from proximal tubules, expression of specific Na(+)-Ca2+ exchanger protein detected on Western blots, and determination of specific mRNA encoding Na(+)-Ca2+ exchanger protein on Northern blots. A new transport activity assay showed that proximal tubule membranes contained the highest Na(+)-Ca2+ exchanger transport activity reported in renal tissues. In dog renal proximal tubules and sarcolemma, a specific protein of approximately 70 kDa was detected, whereas in rat proximal tubules and sarcolemma, the specific protein approximated 65 kDa and was localized to the basolateral membrane. On Northern blots, a single 7-kb transcript isolated from rat proximal tubules, whole kidney, and heart hybridized under high-stringency conditions with rat heart cDNA. These data indicate that Na(+)-Ca2+ exchanger protein expressed in rat proximal tubule is similar, if not identical, to the cardiac protein. We suggest that the tubular Na(+)-Ca2+ exchanger characterized herein represents the Na(+)-Ca2+ exchanger described in functional assays of renal proximal tubules.