A unique Na+/H+ exchanger, analogous to NHE1, in the chicken embryonic fibroblast

We report the characterization of an Na+/H+ exchanger (NHE) in embryonic fi broblasts (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 mu M), dimet hyl amiloride (350 nM), and ethyl-isopropyl amiloride (25 nM). It is stimul ated by serum, transforming growth factor-alpha, hypertonicity, and okadaic acid. Although these features make it resemble mammalian NHE1, other chara cteristics suggest distinct differences. First, in contrast to mammalian NH E1 it is inhibited by cAMP and shows a biphasic response to phorbol esters and a highly variable response to increased intracellular Ca2+ concentratio n. 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 seq uence 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 exchange r differs markedly from the previously characterized chicken intestinal api cal exchanger in its amiloride sensitivity and regulation by phorbol esters . These results suggest that a modified version of mammalian NHE1 is presen t in chicken tissues and imply that another functionally distinct Na+/H+ ex changer is expressed in aves.