MECHANISM OF METHYLMERCURY EFFLUX FROM CULTURED ASTROCYTES

To study the mechanism of methylmercury (MeHg) efflux from the central nervous system cells, cultured astroglia obtained from neonatal rats were incubated with 10 mu M MeHg-cysteine (CySH) for 30 min. After bei ng washed four times, cells were incubated in Hg-free medium, and the release of MeHg from the cells was monitored. The amount of MeHg relea sed in the medium approached a plateau level (ca. 31% of the loaded am ount) at 4 hr. Treatment of the cells with a CySH precursor, 2-oxothia zolidine-4-carboxylic acid (OTC), resulted in a significant increase o f cellular levels of CySH and glutathione (GSH). OTC also increased 1. 5-fold the MeHg efflux from the loaded cells. Another GSH enhancer, GS H isopropyl ester, also stimulated MeHg export from the cells. Ion-exc hange column chromatography using DEAE-Sephadex revealed that the MeHg metabolite thus released was exclusively MeHg-GSH conjugate, both wit h and without OTC. Since the MeHg efflux was suppressed significantly by the presence of probenecid, the efflux occurred via the probenecid- sensitive organic acid transport system. Even though the cellular GSH levels were depleted drastically by treatment with L-buthionine(S,R)-s ulfoximine (BSO), a considerable level (90% of the control) of Hg effl ux was detected. Since neither GSH- nor CySH-MeHg was detected in the culture medium of the BSO-treated cells, GSH depletion may trigger som e other secretion system(s) in the cells. These results suggest that c onjugation with GSH is the major pathway for MeHg efflux in rat astrog lia, and that elevation in the cellular GSH level would possibly be a logical therapy for MeHg poisoning, promoting the accelerated eliminat ion of MeHg from the critical tissues.