Effects of methylmercury on human neuronal L-type calcium channels transiently expressed in human embryonic kidney cells (HEK-293)

Methylmercury (MeHg) disrupts the function of native, high voltage-activated neuronal Ca2+ channels in several types of cells. However, the effects of MeHg on isolated Ca2+ channel phenotypes have not been examined. The aim of the present study was to examine the action of MeHg on recombinant, neuronal L-type voltage-sensitive Ca2+ channels. Human embryonic kidney cells (HEK-293) were transfected with human neuronal cDNA clones of the alpha(1C-1) subunit in combination with alpha(2b) and beta(3a) Ca2+ channel subunits and the reporter jellyfish green fluorescent protein for transient expression. Current from expressed channels (I-Ba) and their response to MeHg applied acutely were measured using whole-cell voltage-clamp recording techniques and Ba2+ (5 mM) as charge carrier. Amplitude of IBa in these cells was reduced by the dihydropyridine (DHP), nimodipine, and enhanced by Bay K8644 [S-(-)-1,4-dihydro-2,6- dimethyl-5-nitro-4-(2-[ trifluoromethyl] phenyl)- 3 pyridine carboxylic acid methyl ester]. MeHg (0.125-5.0 muM) caused a time- and concentration-dependent reduction in amplitude of the peak and sustained current through these channels. However, even at the highest concentration of MeHg tested, reduction of current amplitude by MeHg was incomplete. Washing with MeHg-free solution could not reverse its effects. The steady-state inactivation curve was unaltered by MeHg. Increasing the stimulation frequency or the extracellular Ba2+ concentration each attenuated slightly the reduction in amplitude of I-Ba by MeHg. In the presence of MeHg (5.0 muM), Bay K8644 still increased the remaining current, and nimodipine (10 muM) reduced residual current that was resistant to MeHg. Thus, although MeHg reduces the amplitude of recombinant, heterologously expressed L-type channel current, a portion of current is resistant to reduction by MeHg. Furthermore, DHP agonists and antagonists retain their ability to affect L-type Ca2+ channel current even in the presence of MeHg.