Abstract
Expression cDNA clones of α1B-1 or α1E-3 subunits coding for human neuronal N-(Cav2.2) or R-subtype (Cav2.3) Ca2+ channels, respectively, was combined with α2-bδ and β3-a Ca2+ channel subunits, and transfected into human embryonic kidney cells for transient expression to determine whether specific types of neuronal voltage-sensitive Ca2+ channels are affected differentially by methylmercury (MeHg) and Hg2+. For both Ca2+ channel subtypes, MeHg (0.125-5.0 μM) or Hg2+ (0.1-5 μM) caused a time- and concentration-dependent reduction of current. MeHg caused an initial, rapid component and a subsequent more gradual component of inhibition. The rapid component of block was completed between 100 and 150 s after beginning treatment. At 0.125 to 1.25 μM, MeHg caused a more gradual decline in current. Apparent IC50 values were 1.3 and 1.1 μM for MeHg, and 2.2 and 0.7 μM for Hg2+ on N- and R-types, respectively. For N-type current, effects of Hg2+ were initially greater on the peak current than on the sustained current remaining at the end of a test pulse; subsequently, Hg2+ blocked both components of current. For R-type current, Hg2+ affected peak and sustained current approximately equally. Kinetics of inactivation also seemed to be affected by Hg2+ in cells expressing N-type but not R-type current. Washing with MeHg-free solution could not reverse effects of MeHg on either type of current. The effect of Hg2+ on N- but not R-type current was partially reversed by Hg2+-free wash solution. Therefore, different types of Ca2+ channels have differential susceptibility to neurotoxic mercurials even when expressed in the same cell type.
Footnotes
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↵1 R.K.H. and S.-Q.P. contributed equally to this study.
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This study was supported by National Institutes of Health Grants R01ES03299 and R01ES05822 (to W.D.A.). A preliminary report of these findings was presented at the 2001 Annual Meeting of the Society of Toxicology in San Francisco, CA and published in Toxicologist60:185.
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Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
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DOI: 10.1124/jpet.103.049429.
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ABBREVIATIONS: MeHg, methylmercury; IBa, Ba2+ current; HEK, human embryonic kidney; GFP, green fluorescent protein; V1/2, voltage for half-maximal activation.
- Received January 22, 2003.
- Accepted May 23, 2003.
- The American Society for Pharmacology and Experimental Therapeutics
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