Characteristics of block by Pb2+ of function of human neuronal L-, N-, and R-type Ca2+ channels transiently expressed in human embryonic kidney 293 cells.
Lead (Pb(2+)) is a well-known inhibitor of voltage-dependent Ca(2+) channels in their native environments in several types of cells. However, its effects on discrete Ca(2+) channel phenotypes in isolation have not been well studied. We compared how specific subtypes of human neuronal high-voltage-activated Ca(2+) channels were affected by acute exposure to Pb(2+). Expression cDNA clones of human alpha(1C), alpha(1B), or alpha(1E) subunit genes encoding neuronal L-, N-, and R-subtypes of Ca(2+) channels, respectively, along with a constant alpha(2)delta and beta(3) subunits were transfected into human embryonic kidney 293 cells. Currents through the respective transiently expressed channels were measured using whole-cell recording techniques with Ba(2+) (20 mM) as charge carrier. Extracellular bath applications of Pb(2+) significantly reduced current amplitude through all three types of Ca(2+) channels in a concentration-dependent manner. The order of potency was: alpha(1E) (IC(50) = 0.10 microM), followed by alpha(1C) (IC(50) = 0.38 microM) and alpha(1B) (IC(50) = 1.31 microM). Pb(2+)-induced perturbation of function of alpha(1C) and alpha(1B) containing Ca(2+) channels was more easily reversed than for alpha(1E)-containing Ca(2+) channels after washing with Pb(2+) free solution. The current-voltage relationships were not altered after 3-min exposure to Pb(2+) for any of the three types. However, the steady-state inactivation relationships were shifted to more negative potentials for channels containing alpha(1B) and alpha(1E) subunits, but not for those containing alpha(1C) subunits. Pb(2+) accelerated the inactivation time of current in all three subtypes of Ca(2+) channels in a concentration- and voltage-dependent manner. Therefore, different subtypes of Ca(2+) channels exhibit differential susceptibility to Pb(2+) even when expressed in the same cell type. Current expressed by alpha(1E)-containing channels is more sensitive to Pb(2+) than that expressed by alpha(1C)- or alpha(1B)-containing channels. Several Ca(2+) channel phenotypes are quite sensitive to the inhibitory action of Pb(2+). Furthermore, it seems that Pb(2+) is more likely to combine with Ca(2+) channels in the closed state.
Peng
Shuangqing
S
Department of Pharmacology and Toxicology, Institute of Environmental Toxicology and Neuroscience Program, Michigan State University, East Lansing, Michigan, USA.
Hajela
Ravindra K
RK
Atchison
William D
WD
D002121:Calcium Channel Blockers / Q000494:pharmacology*; D020746:Calcium Channels, L-Type / Q000096:biosynthesis* / Q000378:metabolism; D020864:Calcium Channels, N-Type / Q000096:biosynthesis* / Q000378:metabolism; D020908:Calcium Channels, R-Type / Q000096:biosynthesis* / Q000378:metabolism; D002478:Cells, Cultured; D004305:Dose-Response Relationship, Drug; D006801:Humans; D007668:Kidney / Q000166:cytology / Q000196:embryology; D007700:Kinetics; D007854:Lead / Q000494:pharmacology*; D009474:Neurons / Q000187:drug effects* / Q000378:metabolism; D010084:Oxidation-Reduction / Q000187:drug effects; D014162:Transfection
D002121:Calcium Channel Blockers; D020746:Calcium Channels, L-Type; D020864:Calcium Channels, N-Type; D020908:Calcium Channels, R-Type; D007854:Lead