Comparative effects of methylmercury and Hg(2+) on human neuronal N- and R-type high-voltage activated calcium channels transiently expressed in human embryonic kidney 293 cells.
Expression cDNA clones of alpha1B-1 or alpha1E-3 subunits coding for human neuronal N-(Cav2.2) or R-subtype (Cav2.3) Ca2+ channels, respectively, was combined with alpha2-bdelta and beta3-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 microM) or Hg2+ (0.1-5 microM) 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 microM, MeHg caused a more gradual decline in current. Apparent IC50 values were 1.3 and 1.1 microM for MeHg, and 2.2 and 0.7 microM 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.
Hajela
Ravindra K
RK
Department of Pharmacology and Toxicology, Michigan State University, East Lansing, MI 48824-1317, USA.
Peng
Shuang-Qing
SQ
Atchison
William D
WD
D002118:Calcium / Q000378:metabolism; D020864:Calcium Channels, N-Type / Q000187:drug effects / Q000235:genetics / Q000502:physiology*; D020908:Calcium Channels, R-Type / Q000187:drug effects / Q000235:genetics / Q000502:physiology*; D002478:Cells, Cultured; D004305:Dose-Response Relationship, Drug; D004594:Electrophysiology; D004622:Embryo, Mammalian; D006801:Humans; D007668:Kidney / Q000166:cytology; D008628:Mercury / Q000494:pharmacology*; D008767:Methylmercury Compounds / Q000494:pharmacology*; D009474:Neurons / Q000378:metabolism; D013997:Time Factors; D014162:Transfection
D020864:Calcium Channels, N-Type; D020908:Calcium Channels, R-Type; D008767:Methylmercury Compounds; D008628:Mercury; D002118:Calcium