Nitric oxide reduces Ca2+ and Zn2+ influx through voltage-gated Ca2+ channels and reduces Zn2+ neurotoxicity

The translocation of synaptic Zn2+ from nerve terminals into selectively vu lnerable neurons may contribute to the death of these neurons after global ischemia. We hypothesized that cellular Zn2+ overload might be lethal for r easons similar to cellular Ca2+ overload and tested the hypothesis that Zn2 + neurotoxicity might be mediated by the activation of nitric oxide synthas e. Although Zn2+ (30-300 muM) altered nitric oxide synthase activity in cer ebellar extracts in solution, it did not affect nitric oxide synthase activ ity in cultured murine neocortical neurons. Cultured neurons exposed to 300 -500 muM Zn2+ for 5 min under depolarizing conditions developed widespread degeneration over the next 24 h that was unaffected by the concurrent addit ion of the nitric oxide synthase inhibitor N-G-nitro-L-arginine. Furthermor e, Zn2+ neurotoxicity was attenuated when nitric oxide synthase activity in the cultures was induced by exposure to cytokines, exogenous nitric oxide was added or nitric oxide production was pharmacologically enhanced. The un expected protective effect of nitric oxide against Zn2+ toxicity may be exp lained, at least in part, by reduction of toxic Zn2+ entry. Exposure to nit ric oxide donors reduced Ba2+ current through high-voltage activated calciu m channels, as well as Kr-stimulated neuronal uptake of Ca-45(2+) or Zn-65( 2+). The oxidizing agents thimerosal and 2,2'-dithiodipyridine also reduced K+-stimulated cellular Ca-45(2+) uptake, while akylation of thiols by pret reatment with N-ethylmaleimide blocked the reduction of Ca-45(2+) uptake by a nitric oxide donor. The results suggest that Zn2+-induced neuronal death is not mediated by the activation of nitric oxide synthase; rather, available nitric oxide may at tenuate Zn2+ neurotoxicity by reducing Zn2+ entry through voltage-gated Ca2 + channels, perhaps by oxidizing key thiol groups. (C) 2000 IBRO. Published by Elsevier Science Ltd. All rights reserved.