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1 Center for Oral Biology,
Several members of the
Na+/H+
exchanger gene family (NHE1, NHE2, NHE3, and NHE4) with unique
functional properties have been cloned from rat epithelial tissues. The
present study examined the molecular and pharmacological properties of
Na+/H+
exchange in rat parotid salivary gland cells. In acinar cells superfused with a physiological salt solution (145 mM
Na+),
Na+/H+
exchanger activity was inhibited by low concentrations of the amiloride
derivative ethylisopropyl amiloride (EIPA;
IC50 = 0.014 ± 0.005 µM),
suggesting the expression of amiloride-sensitive isoforms NHE1
and/or NHE2. Semiquantitative RT-PCR confirmed that NHE1
transcripts are most abundant in this cell type. In contrast, the
intermediate sensitivity of ductal cells to EIPA indicated that
inhibitor-sensitive and -resistant
Na+/H+
exchanger isoforms are coexpressed. Ductal cells were about one order
of magnitude more resistant to EIPA
(IC50 = 0.754 ± 0.104 µM) than cell lines expressing NHE1 or NHE2
(IC50 = 0.076 ± 0.013 or 0.055 ± 0.015 µM, respectively). Conversely, ductal cells were nearly
one order of magnitude more sensitive to EIPA than a cell line
expressing the NHE3 isoform (IC50 = 6.25 ± 1.89 µM). Semiquantitative RT-PCR demonstrated that both
NHE1 and NHE3 transcripts are expressed in ducts. NHE1 was
immunolocalized to the basolateral membranes of acinar and ductal
cells, whereas NHE3 was exclusively seen in the apical membrane of
ductal cells. Immunoblotting, immunolocalization, and semiquantitative
RT-PCR experiments failed to detect NHE2 expression in either cell
type. Taken together, our results demonstrate that NHE1 is the dominant
functional
Na+/H+
exchanger in the plasma membrane of rat parotid acinar cells, whereas
NHE1 and NHE3 act in concert to regulate the intracellular pH of ductal cells.
salivary gland; fluid secretion; pH regulation; sodium chloride absorption
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