Uptake and efflux of methylmercury in vitro: Comparison of transport mechanisms in C6, B35 and RBE4 cells

Abstract

Methylmercury (MeHg) is a neurotoxicant which enters the brain and may cause permanent change. Thus, the properties of MeHg transport across cell membranes are a key factor in designing an appropriate model for MeHg neurotoxicity. This study uses cell cultures to examine the uptake and efflux mechanisms of methylmercury in C6 glioma, B35 neuroblastoma and rat brain endothelial (RBE4) cells. The cellular uptake and efflux of MeHg was investigated using 14C-labeled MeHg. The uptake of MeHg-chloride was temperature-independent while the uptake of MeHg- l-cysteine was temperature-dependent in all the three cell types. This indicates that uptake of MeHg-chloride is due to passive diffusion and uptake of MeHg- l-cysteine is due to a protein carrier. Substrates of the amino acid transport system L inhibited uptake of MeHg- l-cysteine in C6 and RBE4 cells, but not B35 cells, indicating a role for system L in MeHg-uptake in the former two. Probenecid, Na +-free medium, MeHg and several l-amino acids did not alter the efflux of MeHg from C6 and RBE4 cells. The amino acids l-cysteine and cystine however, increased the efflux. Both cysteine and cystine are important in the generation of glutathione (GSH), suggesting the involvement of GSH in MeHg efflux. HgCl 2 at low concentrations (0.5 and 1.0 μM) decreased the MeHg efflux and at high concentrations (5.0 and 10.0 μM) increased the efflux. This inhibiting effect of HgCl 2 at low concentrations is possibly due to binding to GSH while the effect of high HgCl 2 concentrations is attributed to disrupted membrane integrity, as measured by Trypan blue. This study demonstrates differing transport mechanisms of MeHg in the cell lines C6, B35 and RBE4.