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1 Dept. for Physiology and Biophysics, University of Aarhus, Aarhus, Denmark; Water and Salt Research Center, University of Aarhus, Aarhus, Denmark
2 Dept. of Anatomy, University of Aarhus, Aarhus, Denmark; Water and Salt Research Center, University of Aarhus, Aarhus, Denmark
3 Dept. of Cellular & Molecular Physiology, University of Yale, School of Medicine, New Haven, Connecticut, USA
* To whom correspondence should be addressed. E-mail: evb{at}fi.au.dk.
Several studies suggest the involvement of Na+ and HCO3- transport in the formation of cerebrospinal fluid. Two Na+ dependent HCO3- transporters were recently localized to the epithelial cells of the rat choroid plexus (NBCn1 and NCBE), and the mRNA for a third protein was also detected (NBCe2) (Praetorius et al., 2004b). We aimed to immunolocalize the NBCe2 to the choroid plexus by immunohistochemistry and immuno-gold electronmicroscopy as well as to functionally characterize the bicarbonate transport in the isolated rat choroid plexus by measurements of intracellular pH (pHi) using a dual excitation wavelength pH sensitive dye (BCECF). Both antisera derived from c-terminal and n-terminal NBCe2 peptides localized NBCe2 to the brush border membrane domain of choroid plexus epithelial cells. Steady state pHi in choroidal cells increased from 7.03 ± 0.02 to 7.38 ± 0.02 (n = 41) after addition of CO2/HCO3- into the bath solution. This increase was Na+ dependent and inhibited by the Cl- and HCO3- transport inhibitor, DIDS (200 µM). This suggests the presence of Na+ dependent partially DIDS sensitive HCO3- uptake. The pHi recovery after acid loading revealed an initial Na+ and HCO3- dependent net base flux of 0.828 ± 0.116 mM/s (n = 8). The initial flux in presence of CO2/HCO3- was unaffected by DIDS. Our data support the existence of both DIDS-sensitive and -insensitive Na+ and HCO3- dependent base loaders uptake into the rat choroid plexus epithelial cells. This is consistent with the localization of the three base transporters NBCn1, NCBE, and NBCe2 in this tissue.
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