We report the characterization of an Na⁺/H⁺ exchanger (NHE) in embryonic fibroblasts (SL-29 cells) of the chicken, a terrestrial vertebrate, where Na⁺ conservation is important. This exchanger is electroneutral, has a single Na⁺ binding site, and is highly sensitive to amiloride (IC₅₀ 2 µM), dimethyl amiloride (350 nM), and ethyl-isopropyl amiloride (25 nM). It is stimulated by serum, transforming growth factor-, hypertonicity, and okadaic acid. Although these features make it resemble mammalian NHE1, other characteristics suggest distinct differences. First, in contrast to mammalian NHE1 it is inhibited by cAMP and shows a biphasic response to phorbol esters and a highly variable response to increased intracellular Ca²⁺ concentration. Second, whereas full-length human and rat NHE1 cDNA probes recognize a 4.8-kb transcript in rat tissues, they recognize only a 3.9-kb transcript in chicken tissues. An antibody against amino acids 631-746 of human NHE1 sequence fails to recognize a protein in SL-29 cells. Rat NHE2 and NHE3 probes do not recognize any transcript in chicken fibroblasts. The SL-29 exchanger differs markedly from the previously characterized chicken intestinal apical exchanger in its amiloride sensitivity and regulation by phorbol esters. These results suggest that a modified version of mammalian NHE1 is present in chicken tissues and imply that another functionally distinct Na⁺/H⁺ exchanger is expressed in aves.

SL-29 fibroblasts; sodium/hydrogen exchanger isoform; aves; nonepithelial Na⁺/H⁺ antiporter