Potassium Attenuates Zinc-Induced Death of Cultured Cortical Astrocytes

Transient global ischemia induces CA1 hippocampal neuronal death without astrocyte death, perhaps mediated in part by the toxic translocation of zinc from presynaptic terminals to postsynaptic neurons. We tested the hypothesis that cellular depolarization, which occurs in the ischemic brain due to increased extracellular potassium and energy failure, might contribute to astrocyte resistance to zinc-induced death. We previously reported that neurons in mixed cortical neuronal-astrocyte cultures were more vulnerable to a 5-15-min exposure to Zn²⁺ than astrocytes in the same cultures. In the present report, we show that (1) neurons in isolation or in conjunction with astrocytes were 2-3-fold more sensitive to a 15-min nondepolarizing Zn ²⁺ exposure than are glia; (2) KCl-induced depolarization attenuated glial vulnerability to zinc toxicity but potentiated neuronal vulnerability to zinc toxicity; (3) Zn²⁺-induced glial death was attenuated by T-type Ca²⁺ channel blockade, as well as compounds that increase NAD ⁺ levels; and (4) both astrocytic ⁶⁵Zn²⁺ accumulation and the increase in astrocytic [Zn²⁺]_i induced by Zn²⁺ exposure were also attenuated by depolarization or T-type Ca²⁺ channel blockers. Zn²⁺-induced cell death in astrocytes was at least in part apoptotic, as caspase-3 was activated, and the caspase inhibitor Z-Val-Ala-Asp-fluoromethylketone partially attenuated Zn ²⁺-induced death. The levels of peak [Zn²⁺]_i achieved in astrocytes during this toxic nondepolarizing Zn²⁺ exposure (250 nM) were substantially greater than those achieved in neurons (40 nM). In glia, exposure to 400 μM Zn²⁺ induced a 13-mV depolarization, which can activate T-type Ca²⁺ channels. This Zn ²⁺-induced astrocyte death, like neuronal death, was attenuated by the addition of pyruvate or niacinamide to the exposure medium.