Elevations of intracellular Ca2+ as a probable contributor to decreased viability in cerebellar granule cells following acute exposure to methylmercury

In these experiments we examined whether the elevations in intracellular Ca2+ concentration ([Ca2+](i)) induced by methylmercury (MeHg) (described in our previous study) might contribute to cerebellar granule cell mortality following exposure to MeHg in vitro. Cells were exposed to 0.5 mu M MeHg for 45 min or 1 mu M MeHg for 38 min, conditions previously shown to induce elevations in [Ca2+](i) in these cells. Control cells were exposed to buffer alone for 60 min. Viability was assessed using the Live/Dead viability/cytotoxicity kit. At 30 min post-MeHg exposure, there was no immediate increase in cell mortality; however, by 3.5 h after the onset of MeHg exposure, cell viability decreased to 74 and 54% of control values for 0.5 and 1.0 mu M MeHg, respectively. At 24.5 h after MeHg exposure, cell viability declined to approximately 27%. Losses in cell viability at 3.5 h were prevented by pretreating the granule cells for 65 min with the Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl)ester (BAPTA; 10 mu M), then exposing the cells to MeHg in the continued presence of BAPTA; however, at 24.5 h, BAPTA no longer prevented MeHg-induced cell death. Exposure to the Ca2+ channel blockers omega-conotoxin MVIIC (1 mu M) or nifedipine (1 mu M), previously shown to delay elevations in [Ca2+](i) with MeHg exposure in vitro, protected granule cells from MeHg-induced mortality at 3.5 h postexposure. These data suggest that at early time points, MeHg-induced increases in [Ca2+](i) may contribute to granule cell mortality; however, the role of Ca2+ at later time points is unclear. (C) 1998 Academic Press.