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Department of Cellular and Integrative Physiology, Indiana University School of Medicine, Indianapolis, Indiana 46202-5120
Colonic
crypts can absorb fluid, but the identity of the absorptive
transporters remains speculative. Near the crypt base, the epithelial
cells responsible for vectorial transport are relatively undifferentiated and often presumed to mediate only Cl
secretion. We have applied confocal microscopy in combination with an
extracellular fluid marker [Lucifer yellow (LY)] or a pH-sensitive
dye (2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein) to
study mouse colonic crypt epithelial cells directly adjacent to the
crypt base within an intact mucosal sheet. Measurements of
intracellular pH report activation of colonocyte
Na+/H+ exchange in response to luminal or
serosal Na+. Studies with LY demonstrate the presence of a
paracellular fluid flux, but luminal Na+ does not activate
Na+/H+ exchange in the nonepithelial cells of
the lamina propria, and studies with LY suggest that the fluid bathing
colonocyte basolateral membranes is rapidly refreshed by serosal
perfusates. The apical Na+/H+ exchange in crypt
colonocytes is inhibited equivalently by luminal 20 µM
ethylisopropylamiloride and 20 µM HOE-694 but is not inhibited by
luminal 20 µM S-1611. Immunostaining reveals the presence of epitopes
from NHE1 and NHE2, but not NHE3, in epithelial cells near the base of
colonic crypts. Comparison of apical Na+/H+
exchange activity in the presence of Cl
with that in the
absence of Cl
(substitution by gluconate or nitrate)
revealed no evidence of the Cl
-dependent
Na+/H+ exchange that had been previously
reported as the sole apical Na+/H+ exchange
activity in the colonic crypt. Results suggest the presence of an
apical Na+/H+ exchanger near the base of crypts
with functional attributes similar to those of the cloned NHE2 isoform.
intracellular pH; NHE2; NHE3; NHE1; sodium absorption; epithelial polarity; laser scanning confocal microscopy; 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein; Lucifer yellow; immunofluorescence
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