l-Glutamate in the extracellular space regulates endogenous d-aspartate homeostasis in rat pheochromocytoma MPT1 cells

In previous studies [FEBS Lett. 434 (1998) 231, Arch. Biochem. Biophys. 404 (2002) 92], we demonstrated for the first time that d-aspartate (d-Asp) is synthesized in cultured mammalian cell lines, such as pheochromocytoma 12 (PC12) and its subclone, MPT1. Our current focus is analysis of the dynamics of d-Asp homeostasis in these cells. In this communication, we show that l-glutamate (Glu) and l-Glu transporter substrates in the extracellular space regulate the homeostasis of endogenous d-Asp in MPT1 cells. d-Asp is apparently in dynamic homeostasis, whereby endogenous d-Asp is constantly released into the extracellular space by an undefined mechanism, and continuously and intensively taken up into cells by an l-Glu transporter. Under these conditions, l-Glu and its transporter substrates in the medium may competitively inhibit the uptake of d-Asp via the transporter, resulting in accumulation of the amino acid in the extracellular space. We additionally demonstrate that dl-TBOA, a well-established l-Glu transporter inhibitor, is taken up by the transporter during long time intervals, but not on a short time-scale.