Na⁺-dependent HCO₃⁻ uptake into the rat choroid plexus epithelium is partially DIDS sensitive

Abstract

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 J, Nejsum LN, and Nielsen S. Am J Physiol Cell Physiol 286: C601-C610, 2004). Our goal was to immunolocalize the NBCe2 to the choroid plexus by immunohistochemistry and immunogold electronmicroscopy and 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 COOH-terminal and NH2-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 muM). 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 the presence of CO2/HCO3- was unaffected by DIDS. Our data support the existence of both DIDS-sensitive and -insensitive Na+- and HCO3- -dependent base loader uptake into the rat choroid plexus epithelial cells. This is consistent with the localization of the three base transporters NBCn1, Na+-driven Cl- bicarbonate exchanger, and NBCe2 in this tissue.

Mice with targeted Slc4a10 gene disruption have small brain ventricles and show reduced neuronal excitability

“…We confirmed that Slc4a10 did not colocalize with the apical Na ϩ /K ϩ -ATPase, but was expressed at the basolateral side of choroid plexus epithelial cells as described previously ( Fig. 2A) (5,6). The signal was absent in tissue from KO mice (Fig.…”

“…Electron micrographs localized Slc4a10 to dendritic membranes within the stratum oriens and stratum radiatum. We also confirmed the previously reported basolateral localization of Slc4a10 in choroid plexus epithelial cells (5,6).…”

“…Na ϩ -dependent acid extrusion was not completely blocked in choroid plexus epithelial cells from Slc4a10 KO mice. The anion transporter inhibitor DIDS causes only a partial inhibition of solute transport or CSF production (6,21). Furthermore, the carbonic anhydrase inhibitor, acetazolamide, has been shown to inhibit CSF production rate or Na ϩ secretion (20,22,23).…”

“…Thereby, SLC4A10 may play an important role in intracellular pH (pH i ) regulation in various kinds of cells. In the choroid plexus, SLC4A10 is strongly expressed within the basolateral membrane of epithelial cells (5)(6)(7), suggesting a role in transepithelial electrolyte transport and production of cerebrospinal fluid (CSF). In previous studies, the mRNA has been primarily detected in the brain, including the hippocampus (1,7,8), but the protein distribution in the CNS has not been resolved.…”

Antibody-independent localization of the electroneutral Na+-HCO3− cotransporter NBCn1 (slc4a7) in mice

“…Subsequently, NBCn1 has been proposed to be important in a number of cell types: mouse duodenal enterocytes (26), rat outer medullary thick ascending limb epithelial cells (24), mouse retinal and inner ear sensory cells (7), rat inner medullary collecting duct epithelial cells (27), rat hippocampal neurons (12), rat choroid plexus epithelial cells (9), rat osteoclasts (8), and mouse vascular smooth muscle cells (6).…”