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3
cotransporter in human pancreas
1 Veterans Affairs Medical Center, Departments of 2 Medicine and of 3 Physiology and Biophysics, University of Tennessee, Memphis, Tennessee 68163; and 4 Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, Connecticut 06520
The cellular
mechanisms of HCO
3 secretion in the
human pancreas are unclear. Expression of a
Na+-HCO3
cotransporter (NBC) mRNA has been observed recently, but the
distribution and physiological role of the NBC protein are not known.
Here we examined the expression and localization of NBC in human
pancreas by Northern blot, immunoblot, and immunofluorescence microscopy. Rat kidney NBC probes detected a single 9.5-kb band by
Northern blot. On immunoblots, two polyclonal antisera directed against
different epitopes of rat kidney NBC identified a single ~130-kDa
protein. In cryosections of normal human pancreas, both antisera
labeled basolateral membranes of large, morphologically identifiable
ducts and produced a distinct labeling pattern in the remainder of the
parenchyma. In double-labeling experiments, NBC immunoreactivity in the
parenchyma colocalized with the
Na+-K+
pump, a basolateral marker. In contrast, NBC and cystic fibrosis transmembrane conductance regulator, an apical membrane marker, were
detected within the same histological structures but at different subcellular localizations. The NBC antisera did not label acinar or
islet cells. Our observations suggest that secretion of
HCO3 by human pancreatic duct cells
involves the basolateral uptake of
Na+ and
HCO3 via NBC, an electrogenic
Na+-HCO3 cotransporter.
ion transport; immunofluorescence; membrane proteins; cell physiology; pancreatic ducts
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