Effects of prolonged exposure to nanomolar concentrations of methylmercury on voltage-sensitive sodium and calcium currents in PC12 cells.

The neurotoxicant methylmercury (CH(3)Hg(+)) inhibits voltage-sensitive Na(+) and Ca(2+) currents in neuronal preparations following acute, in vitro, exposure. In the present study, effects on voltage-sensitive Na(+) (I(Na)) and Ca(2+) (I(Ca)) currents in pheochromocytoma (PC12) cells were examined following prolonged exposure to CH(3)Hg(+). When PC12 cells cultured in the presence of nerve growth factor (NGF) for 7 days ('primed') were replated in the presence of NGF and 30 nM CH(3)Hg(+), I(Ca), but not I(Na), amplitude was reduced (29%) significantly approximately 24 h later. Quantitative assessment of morphology indicated that this approximately 24 h exposure to CH(3)Hg(+) significantly reduced neurite length. The N-type voltage-sensitive Ca(2+) channel (VSCC) antagonist omega-conotoxin GVIA (500 pM) was without significant effect on current amplitude or morphology in this exposure protocol. When undifferentiated cells were cultured in the presence of NGF and 10 nM CH(3)Hg(+) for 6 days, I(Ca) and I(Na) amplitude were reduced by 36 and 52%, respectively. I(Ca) at the end of a 150 ms test pulse was also reduced by 40% in CH(3)Hg(+)-treated cells. Thus, both inactivating and non-inactivating I(Ca) were reduced equally. There was no change in [(3)H]saxitoxin or omega-[(125)I]conotoxin GVIA binding, nor were there any morphological alterations in cells treated with CH(3)Hg(+) for 6 days. Omega-conotoxin GVIA (500 pM, 6 days), reduced significantly I(Ca), but not I(Na), but was without effect on morphology. These results demonstrate that prolonged exposure to low concentrations of CH(3)Hg(+) reduces cationic currents in differentiating PC12 cells, but that current reduction is not always associated with morphological alteration.