Effects of Cd2+, Pb2+ and CH3Hg+ on high voltage-activated calcium currents in pheochromocytoma (PC12) cells: potency, reversibility, interactions with extracellular Ca2+ and mechanisms of block.

Effects of the neurotoxic heavy metals Cd2+, Pb2+ and CH3Hg+ on current carried by Ca2+ ions (I(Ca)) through high-voltage activated Ca2+ channels in nerve growth factor (NGF)-differentiated pheochromocytoma (PC12) cells were examined to characterize possible differences in the mechanism of action of these metals on Ca2+ channel function. Specifically, the potency and reversibility of effect on I(Ca) by each metal was examined, as well as the relationship between extracellular [Ca2+] and potency of block of I(Ca) by Cd2+ and Pb2+. In addition, the effect of each of these metals on Ca2+ channels when applied to the intracellular side of the membrane was also examined. When extracellular solution contained 20, 10 or 5 mM Ca2+, the estimated IC50 values (total metal concentration) for block of I(Ca) were 15, 10, and 6.5 microM for Cd2+ and 7.5, 2.0 and 1.1 microM for Pb2+, respectively. CH3Hg+ (1-10 microM) blocked I(Ca) (20 mM Ca2+) in a time- and concentration-dependent manner. When cells were washed with metal-free solutions, block of I(Ca) by Cd2+ was reversed rapidly, whereas block by Pb2+ was reversed only partially, and block of I(Ca) by CH3Hg+ was not reversed. When Pb2+ and CH3Hg+ treated cells were washed in metal-free solutions containing 50 microM D-penicillamine (DPEN), block of I(Ca) by 10 microM Pb2+ was rapidly and completely reversed, whereas, block of I(Ca) by 5 microM CH3Hg+ was not reversed. Higher concentrations (500 microM) of 2,3-dimercapto-1-propane sulfonic acid (DMPS) did reverse partially the block of I(Ca) by 5 and 10 microM CH3Hg+. When Cd2+, Pb2+ or CH3Hg+ was present in the intracellular solution, Ca2+ channel currents were significantly reduced. These results characterize effects of Cd2+ on Ca2+ channels and demonstrate that Cd2+, Pb2+ and CH3Hg+ differ in their actions on Ca2+ channels.