NICKEL BLOCK OF A FAMILY OF NEURONAL CALCIUM CHANNELS - SUBTYPE-DEPENDENT AND SUBUNIT-DEPENDENT ACTION AT MULTIPLE SITES

Nickel ions have been reported to exhibit differential effects on dist inct subtypes of voltage-activated calcium channels. To more precisely determine the effects of nickel, we have investigated the action of n ickel on four classes of cloned neuronal calcium channels (alpha(1A), alpha(1B), alpha 1C, and alpha(1E)) transiently expressed in Xenopus o ocytes. Nickel caused two major effects: (i) block detected as a reduc tion of the maximum slope conductance and (ii) a shift in the current- voltage relation towards more depolarized potentials which was paralle led by a decrease in the slope of the activation-curve. Block followed 1:1 kinetics and was most pronounced for alpha(1C) followed by alpha( 1E) > alpha(1A) > alpha(1B) channels. In contrast, the change in activ ation-gating was most dramatic with alpha(1E) with the remaining chann el subtypes significantly less affected. The current-voltage shift was well described by a simple model in which nickel binding to a saturab le site resulted in altered gating behavior. The affinity for both the blocking site and the putative gating site were reduced with increasi ng concentration of external permeant ion. Replacement of barium with calcium reduced both the degree of nickel block and the maximal effect on gating for alpha(1A) channels, but increased the nickel blocking a ffinity for alpha(1E) channels. The coexpression of Ca channel beta su bunits was found to differentially influence nickel effects on alpha(1 A), as coexpression with beta(2a) or with beta(4) resulted in larger c urrent-voltage shifts than those observed in the presence of beta(1b), while elimination of the beta subunit almost completely abolished the gating shifts. In contrast, block was similar for the three beta subu nits tested, while complete removal of the beta subunit resulted in an increase in blocking affinity. Our data suggest that the effect of ni ckel on calcium channels is complex, cannot be described by a single s ite of action, and differs qualitatively and quantitatively among indi vidual subtypes and subunit combinations.