EPITHELIAL LOCALIZATION OF A REPTILIAN NA+ H+ EXCHANGER HOMOLOGOUS TONHE-1/

Basolateral membranes of turtle (Pseudemys scripta) colon epithelial c ells exhibit robust Na+/H+ exchange activity that can be activated by cell shrinkage and is blocked by amiloride [M. A. Post and D. C. Dawso n. Am. J. Physiol. 262 (Cell Physiol. 31): C1089-C1094, 1992]. The col onic epithelium actively absorbs Na+ and secretes K+ and HCO3-, but th e role of basolateral Na+/H+ exchange, if any, in transepithelial tran sport is unknown. The current studies were undertaken to identify the gene product(s) responsible for the observed basolateral Na+/H+ exchan ge activity and to determine the cellular localization of the reptilia n Na+/H+ exchange protein. We cloned and sequenced partial-length cDNA s that are likely to encode a reptilian homologue of the mammalian NHE -1 Na+/H+ exchanger isoform. The partial-length cDNAs were >80% identi cal to mammalian NHE-1 homologues at the nucleotide level and recogniz ed a transcript (similar to 5.8-6.0 kb) in RNA isolated from turtle co lon, small intestine, stomach, kidney, urinary bladder, heart, and liv er. In situ hybridization showed that mRNA encoding the reptile homolo gue of NHE-1 was expressed predominantly in the epithelial cells of th ese tissues. Immunofluorescent localization of the reptilian Na+/H+ ex changer protein using an antibody raised against a human NHE-1 fusion protein confirmed that protein expression paralleled abundant mRNA exp ression in epithelial cells of turtle stomach and colon, as well as in some nephron segments, and showed that the reptile NHE-1 homologue wa s localized exclusively to the basolateral membranes of these cells. T he relatively high level of NHE-1 expression in epithelial cells, part icularly those of the colon and stomach, suggests that NHE-1 function is important for the maintenance or regulation of ion transport proces ses that occur in these cell types.