Expression of multiple Na+/H+ exchanger isoforms in rat parotid acinar andductal cells

Several members of the Na+/H+ exchanger gene family (NHE1, NHE2, NHE3, and NHE4) with unique functional properties have been cloned from rat epithelia l tissues. The present study examined the molecular and pharmacological pro perties of Na+/H+ exchange in rat parotid salivary gland cells. In acinar c ells superfused with a physiological salt solution (145 mM Na+), Na+/H+ exc hanger activity was inhibited by low concentrations of the amiloride deriva tive ethylisopropyl amiloride (EIPA; IC50 = 0.014 +/- 0.005 mu M), suggesti ng the expression of amiloride-sensitive isoforms NHE1 and/or NHE2. Semiqua ntitative RT-PCR confirmed that NHE1 transcripts are most abundant in this cell type. In contrast, the intermediate sensitivity of ductal cells to EIP A indicated that inhibitor-sensitive and -resistant Na+/H+ exchanger isofor ms are coexpressed. Ductal cells were about one order of magnitude more res istant to EIPA (IC50 = 0.754 +/- 0.104 mu M) than cell lines expressing NHE 1 or NHE2 (IC50 = 0.076 +/- 0.013 or 0.055 +/- 0.015 mu M, respectively). C onversely, ductal cells were nearly one order of magnitude more sensitive t o EIPA than a cell line expressing the NHE3 isoform (IC50 = 6.25 +/- 1.89 m u M) Semiquantitative RT-PCR demonstrated that both NHE1 and NHE3 transcrip ts are expressed in ducts. NHE1 was immunolocalized to the basolateral memb ranes of acinar and ductal cells, whereas NHE3 was exclusively seen in the apical membrane of ductal cells. Immunoblotting, immunolocalization, and se miquantitative RT-PCR experiments failed to detect NHE2 expression in eithe r cell type. Taken together, our results demonstrate that NHE1 is the domin ant functional Na+/H+ exchanger in the plasma membrane of rat parotid acina r cells, whereas NHE1 and NHE3 act in concert to regulate the intracellular pH of ductal cells.