Characterization of l-α-aminoadipic acid transport in cultured rat astrocytes

The mechanism of the selective gliotoxicity of l-α-aminoadipate ( l-αAA) is thought to involve its entry into glia as a substrate for glutamate transporters or, alternatively, its ability to inhibit glial glutamate transport. To clarify the properties of l-αAA as a transport substrate, we explored the ionic dependence, kinetics and pharmacology of l-[ 3H]αAA uptake in rat cortical astrocytes. We observed two components of saturable l-αAA uptake, one Na +-dependent and the other Na +-independent. These components exhibited the characteristics of system XA AG −, the widespread family of Na +-cotransporters of aspartate and glutamate, and system x c − , a Cl −-dependent glutamate/cystine exchanger, respectively. The K m value of Na +-dependent l-αAA uptake was 629 ± 42 μM, and V max was 62 ± 4nmol· min −1 · mg −1 protein, which was more than twice the capacity of Na +-dependent glutamate uptake. The kinetic parameters of Na +-dependent l-αAA uptake ( K m of 20 ± 2 μM, V maxof1.7 ± 0.4nmol· min −1 · mg −1 protein did not differ from the values for Na +-independent glutamate uptake, indicating that l-αAA and glutamate are equally good substrates for system x c −.