Molecular biology of renal Na(+)-H+ exchangers

Na(+)-H+ exchangers are plasma membrane proteins that are responsible for Cl- and HCO3- reabsorption and regulation of intracellular pH in several nephron segments. Recently, a cDNA encoding a human Na(+)-H+ exchanger of unknown tissue origin was cloned, and Northern blot analysis revealed that structurally similar transcripts were expressed in rabbit kidney and porcine renal epithelial cells (LLC-PK1). To clone these renal transcripts we employed cDNA library screening and the polymerase chain reaction (PCR). We obtained 2.5 kb and 1.4 kb cDNAs corresponding to the 5' untranslated region and the membrane-associated domain of a rabbit renal Na(+)-H+ exchanger. From LLC-PK1 cells we obtained 1.5 kb and 1.3 kb cDNAs encoding the membrane-associated and cytoplasmic domains. The sequences of cDNAs from these three species were very similar and suggested a high degree of evolutionary conservation. Immunolocalization of synthetic oligopeptides derived from the deduced amino acid sequences indicated that the cloned cDNAs encoded the amiloride-sensitive form of the Na(+)-H+ exchanger present in basolateral membranes of epithelia. cDNAs were also used to study regulation of Na(+)-H+ exchanger gene expression in the kidney, and we found that metabolic acidosis stimulated both the transport rate and steady-state transcript levels of the basolateral Na(+)-H+ exchanger in LLC-PK1 cells.