Abstract
We report here the functional characteristics of Na +-independent choline transport system in primary cultures of neurons from mouse cerebral cortex. Na +-independent choline transport was saturable with a Michaelis constant ( K t) of 26.7 ± 1.2 μM and a maximal velocity ( V max) of 1.04 ± 0.02 nmol/mg protein/10 min. Choline uptake was significantly influenced by extracellular pH and by membrane depolarization. This uptake system was inhibited by various organic cations including unlabeled choline, guanidine, diphenhydramine and the choline analog hemicholinium-3. However, the prototypical organic cation tetraethylammonium and cimetidine showed very little affinity for the Na +-independent choline uptake system in neurons. These results indicate that mouse cerebrocortical neurons express a Na +-independent, high-affinity choline transport system. RT-PCR revealed that choline transporter-like protein 1 (CTL1) and its spliced variant CTL1a, which have been reported to be novel Na +-independent choline transporter, are expressed in mouse cerebrocortical neurons. The Na +-independent transport properties of choline in mouse neurons is similar or identical to that of CTL1 and/or CTL1a. This choline transport system seems to have relevance not only for neuronal physiology but also for the uptake of pharmacologically important organic cation drugs.
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