Interactions of methylmercury with rat primary astrocyte cultures: methylmercury efflux

Methylmercury (MeHg) efflux from rat astrocyte cultures was studied to complement our previous studies on uptake of MeHg in these cells. Exchange with extracellular MeHg was not obligatory for the efflux of [²⁰³Hg]MeHg into the extracellular media, because efflux occurred into MeHg-free extracellular media, but stimulation of [²⁰³Hg]MeHg net efflux was shown when astrocytes were equilibrated in the presence of 'cold' MeHg and graded concentrations of l-cysteine. Net efflux of MeHg was most rapid for the first 5 min, and approximately 20% of preloaded [²⁰³Hg]MeHg was lost from the astrocytes by 60 min. Uptake of [²⁰³Hg]MeHgCl was maximal by 30 min and did not increase when the loading period was extended up to 4 h. However, the total amount of intracellular ²⁰³Hg that was available for net efflux gradually decreased as the duration of the preloading period increased. MeHg net efflux from astrocytes was unchanged when [²⁰³Hg]MeHgCl preloaded astrocytes were equilibrated in hypotonic buffer, suggesting that unlike ions and amino acids swollen astrocytes remain impervious to MeHg efflux. Thus, the main MeHg efflux transport system is apparently specific for the MeHg-l-cysteine conjugate and represents transport by the same neutral amino acid System L that facilitates its uptake.