Nickel block of a family of neuronal calcium channels: subtype- and subunit-dependent action at multiple sites

Nickel ions have been reported to exhibit differential effects on distinct subtypes of voltage-activated calcium channels. To more precisely determine the effects of nickel, we have investigated the action of nickel on four classes of cloned neuronal calcium channels (alpha1A, alpha1B, alpha1C, and alpha1E) transiently expressed in Xenopus oocytes. Nickel caused two major effects: (i) block detected as a reduction of the maximum slope conductance and (ii) a shift in the current-voltage relation towards more depolarized potentials which was paralleled by a decrease in the slope of the activation-curve. Block followed 1:1 kinetics and was most pronounced for alpha1C, followed by alpha1E > alpha1A > alpha1B channels. In contrast, the change in activation-gating was most dramatic with alpha1E, with the remaining channel subtypes significantly less affected. The current-voltage shift was well described by a simple model in which nickel binding to a saturable site resulted in altered gating behavior. The affinity for both the blocking site and the putative gating site were reduced with increasing concentration of external permeant ion. Replacement of barium with calcium reduced both the degree of nickel block and the maximal effect on gating for alpha1A channels, but increased the nickel blocking affinity for alpha1E channels. The coexpression of Ca channel beta subunits was found to differentially influence nickel effects on alpha1A, as coexpression with beta2a or with beta4 resulted in larger current-voltage shifts than those observed in the presence of beta1b, while elimination of the beta subunit almost completely abolished the gating shifts. In contrast, block was similar for the three beta subunits tested, while complete removal of the beta subunit resulted in an increase in blocking affinity. Our data suggest that the effect of nickel on calcium channels is complex, cannot be described by a single site of action, and differs qualitatively and quantitatively among individual subtypes and subunit combinations.