Cloning and expression of a rabbit cDNA encoding a serum-activated ethylisopropylamiloride-resistant epithelial Na⁺/H⁺ exchanger isoform (NHE- 2)

A unique Na⁺/H⁺ exchanger isoform, NHE-2, was cloned and characterized. NHE-2 is a protein of 809 amino acids with a calculated size of 90,787. It exhibits overall amino acid identity of 50, 44, and 60% with other cloned mammalian Na⁺/H⁺ exchangers NHE-1, NHE-3, and NHE-4, respectively. Northern blot analysis of poly(A⁺) RNA isolated from rabbit ileum, kidney cortex, and kidney medulla using NHE-2 cDNA as a probe revealed messages of 5.2, 4.2, and 3.2 kilobases with relative abundance (in descending order) kidney medulla > kidney cortex > ileum. More detailed tissue distribution of message was performed by ribonuclease protection assay. NHE-2 was predominantly expressed in kidney, intestine, and adrenal gland with a small amount in skeletal muscle and trachea. Stable expression of NHE-2 in PS120 fibroblasts confirmed that NHE-2 is a functional Na⁺/H⁺ exchanger which is defined by amiloride- sensitive Na⁺-dependent alkalinization of acid-loaded cells. NHE-2 has the same K(i) for amiloride inhibition as NHE-1 (1 μM) but is 25-fold more resistant to ethylisopropylamiloride inhibition than is NHE-1 (500 versus 20 nM). Like NHE-1, NHE-2 can be activated by serum. Expression of NHE-2 in a polarized human intestinal epithelial cell line, Caco-2 cells, results in functional expression of NHE-2 in the apical membrane. Thus, we conclude that NHE-2 is a candidate to be an apical membrane Na⁺/H⁺ exchanger in intestinal and renal epithelial cells.