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 microM MeHg for 45 min or 1 microM 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 microM 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 microM), 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 microM) or nifedipine (1 microM), 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.