Molecular and functional characterization of an Na + -independent choline transporter in rat astrocytes

In this study, we examined the molecular and functional characterization of choline uptake into cultured rat cortical astrocytes. Choline uptake into astrocytes showed little dependence on extracellular Na + .N a + -independent choline uptake was saturable and mediated by a single transport system, with an apparent Michaelis–Menten constant (Km) of 35.7 ± 4.1 lM and a maximal velocity (Vmax) of 49.1 ± 2.0 pmol/mg protein/ min. Choline uptake was significantly decreased by acidification of the extracellular medium and by membrane depolarization. Na + -independent choline uptake was inhibited by unlabeled choline, acetylcholine and the choline analogue hemicholinium-3. The prototypical organic cation tetrahexylammonium (TEA), and other n-tetraalkylammonium compounds such as tetrabutylammonium (TBA) and tetrahexylammonium (THA), inhibited Na + -independent choline uptake, and their inhibitory potencies were in the order THA > TBA > TEA. Various organic cations, such as 1-methyl-4-tetrahydropyridinium (MPP + ), clonidine, quinine,