Methylmercury-induced alterations in excitatory amino acid transport in rat primary astrocyte cultures

To determine whether methylmercury (MeHg) has the potential to interfere with homeostatic functions in neonatal rat cortical primary astrocyte cultures, the effects of MeHg on the uptake and efflux of both l-glutamate and d-aspartate were examined. Uptake of both of these excitatory amino acids (EAAs) was significantly (P < 0.05) reduced in the presence of MeHg concentrations as low as 10^-5 M. Efflux of both glutamate and aspartate from preloaded astrocytes was also increased by MeHg in a dose- and time-dependent fashion. Since in our earlier studies we had found that MeHg causes dose-dependent astrocytic swelling, which could have been the mechanism of the increased efflux, we examined whether blockage of conductive ion fluxes, which have been implicated in astrocytic swelling, could reverse the MeHg-induced increase in l-glutamate and d-aspartate efflux. Three compounds which inhibit the hypotonic-media-induced efflux of EAA, 4-acetamido-4′-isothiocyanatostilbene-2,2′-disulfonic acid (SITS), L-644,711 (a derivative of ethacrynic acid) and furosemide were tested at their maximal concentrations for their ability to reverse MeHg-induced EAA efflux. Only furosemide (5 mM) could sustain the reversal for the entire 120 min duration of the efflux measurement. Since hypotonic-media swelling-induced release of EAAs is inhibited by these anion inhibitors (in the following rank order: L-644,711 > SITS > furosemide), we conclude that different mechanisms account for EAA release from primary astrocyte cultures during MeHg exposure as compared to hypotonic media-induced efflux.