Metallothionein induction protects swollen rat primary astrocyte cultures from methylmercury-induced inhibition of regulatory volume decrease

Metallothionein (MT) proteins have been postulated to play a role in the detoxification of heavy metals. Since methylmercury (MeHg) preferentially accumulates in astrocytes, and MT-1 and MT-2 are astrocyte-specific MT isoforms, we investigated the ability of MTs to attenuate MeHg-induced cytotoxicity. The toxic effects of MeHg on astrocytes were investigated in a model of regulatory volume decrease (RVD) in which the cells are swollen by exposure to a hypotonic buffer. Preexposure to CdCl₂ (1 μM) for 72, 96 or 120 h, prior to acute exposure to hypotonic buffer and MeHg (10 μM) led to a time-dependent increase in the intracellular levels of astrocyte MT proteins. The acute MeHg-induced inhibition of RVD was significantly, and almost fully reversed by preexposure to CdCl₂. This reversal was time-dependent, 120-h preexposure to CdCl₂ producing the greatest reversibility. Furthermore, the ability of astrocytes to efficiently volume regulate in the presence of MeHg-containing hypotonic buffer was highly correlated (r = 0.99) with the intracellular levels of MT proteins. The release of [³H]taurine, an osmolyte involved in the RVD process was also measured. The inhibitory effect of MeHg on [³H]taurine in swollen cells was significantly, and fully reversed by CdCl₂ preexposure. The study suggests that astrocytes induced to express high levels of MT proteins are resistant to the acute inhibitory effect of MeHg on RVD.