Characterization of L-alpha-aminoadipic acid transport in cultured rat astrocytes

The mechanism of the selective gliotoxicity of L-alpha-aminoadipate (L-alpha AA) 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-alpha AA as a transport substrate, we explored the ionic dependence, kinetics and pharmacology of L-[H-3]alpha AA uptake in rat cortical astrocytes. We observed two components of saturable L-alpha AA uptake, one Na+-dependent and the other Na+-independent. These components exhibited the characteristics of system X(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-alpha AA uptake was 629 +/- 42 mu M, and V-max was 62 +/- 4 nmol . min(-1). mg(-1) protein, which was more than twice the capacity of Na+-dependent glutamate uptake. The kinetic parameters of Na+-dependent L-alpha AA uptake (K-m of 20 +/- 2 mu M, V-max of 1.7 +/- 0.4 nmol . min(-1). mg(-1) protein did not differ from the values for Na+-independent glutamate uptake, indicating that L-alpha AA and glutamate are equally good substrates for system x(c)(-).