Inorganic mercury and methylmercury inhibit the Cav3.1 channel expressed in human embryonic kidney 293 cells by different mechanisms

Part of the neurotoxic effects of inorganic mercury (Hg2+) and methylmercury (MeHg) was attributed to their interaction with voltage-activated calcium channels. Effects of mercury on T-type calcium channels are controversial. Therefore, we investigated effects of Hg2+ and MeHg on neuronal Ca(v)3.1 (T-type) calcium channel stably expressed in the human embryonic kidney (HEK) 293 cell line. Hg2+ acutely inhibited current through the Ca(v)3.1 calcium channel in concentrations 10 nM and higher with an IC50 of 0.63 +/- 0.11 mu M and a Hill coefficient of 0.73 +/- 0.08. Inhibition was accompanied by strong deceleration of current activation, inactivation, and deactivation. The current-voltage relation was broadened, and its peak was shifted to a more depolarized membrane potentials by 1 mu M Hg2+. MeHg in concentrations between 10 nM and 100 mu M inhibited the current through the Ca(v)3.1 calcium channel with an IC50 of 13.0 +/- 5.0 mu M and a Hill coefficient of 0.47 +/- 0.09. Low concentration of MeHg (10 pM to 1 nM) had both positive and negative effects on the current amplitude. Micromolar concentrations of MeHg reduced the speed of current activation and accelerated current inactivation and deactivation. The current-voltage relation was not affected. Up to 72 h of exposure to 10 nM MeHg had no significant effect on current amplitude, whereas 72-h-long exposure to 1 nM MeHg increased significantly current density. Acute treatment with Hg2+ or MeHg did not affect HEK 293 cell viability. In conclusion, interaction with the Ca(v)3.1 calcium channel may significantly contribute to neuronal symptoms of mercury poisoning during both acute poisoning and long-term environmental exposure.