Functional characterisation of the active ascorbic acid transport into cerebrospinal fluid using primary cultured choroid plexus cells

Abstract

Crossing the blood–CSF barrier is an important pathway for certain nutrients to enter the CNS. Cultured choroid plexus epithelial cells are a potent model system to study active transport properties of this tissue in vitro. In the present study this in vitro model was used to analyse ascorbic acid transport across the blood–CSF barrier that is supposedly mediated by the Na +-dependent transporter SVCT2. The expression of SVCT2 in the cultured cells was proven by RT-PCR. Active transport across the cell monolayer resulted in ascorbic acid enrichment at the CSF mimicking side. Ascorbic acid transport and uptake were decreased to 13 and 27%, respectively, in the presence of 200 μM phloretin. Inhibition of both transepithelial substrate transport (to 7.5%) and cytoplasmatic uptake (to 20%) was observed in Na +-free medium indicating that a basolaterally located and Na +-dependent transporter mediates ascorbic acid uptake. Substituting Cl − by either iodide or d-gluconate increased ascorbic acid uptake by factors of 3.7 or 2.5, respectively. Similar observations were made when Na +-dependent myo-inositol transport was analysed. Additionally, in presence of 100 μM bumetanide, an inhibitor of Na +-Cl −-cotransport, indirectly increased ascorbic acid and myo-inositol transport rates were observed showing that ascorbic acid-Na +-cotransport might balance low intracellular Na + concentration.