Stimulation of d-aspartate efflux by mercuric chloride from rat primary astrocyte cultures

Mercuric chloride (HgCl₂; MC) was shown to increase d-aspartate release from preloaded astrocytes in a dose-dependent fashion. Two sulfhydryl (-SH) protecting agents, a cell membrane non-penetrating compound, reduced glutathione (GSH), and the membrane-permeable dithiothreitol (DTT), were found to inhibit the stimulatory action of MC on the efflux of radiolabeled d-aspartate. MC-induced d-aspartate release was completely inhibited by the addition of 1 mM DTT or GSH during the actual 5 min perfusion period with MC (5 μM). However, when added after MC treatment, this inhibition could not be sustained by GSH, while DTT fully inhibited the MC-induced release of d-aspartate. Neither DTT nor GSH alone had any effect on the rate of astrocytic d-aspartate release. Accordingly, it is postulated that the stimulatory effect exerted by MC on astrocytic d-aspartate release is associated with vulnerable -SH groups located within, but not on the surface of the cell membrane. Omission of Na⁺ from the perfusion solution did not accelerate MC-induced d-aspartate release, suggesting that reversal of the d-aspartate carrier can not be invoked to explain MC-induced d-aspartate release. Furthermore, MC did not appear to be associated with astrocytic swelling.