Mechanisms of Zn2+ efflux in cultured cortical neurons.

Zinc dyshomeostasis in brain might be involved in the pathogenesis of brain diseases such as Alzheimer's disease and stroke. Resting neurons tightly regulate and maintain low to subnanomolar levels of intracellular free Zn2+, but mechanisms of normal Zn2+ homeostasis are poorly understood. In this study, the mechanisms of transporter-mediated Zn2+extrusion across the plasma membrane of cultured cortical neurons were studied. Changes in intracellular free Zn2+ levels were tracked in individual neurons by microfluorometry using a Zn2+ selective fluorophore, FluoZin3. Unopposed Zn2+efflux was measured by first loading cultured cortical neurons with Zn2+ then reducing extracellular Zn2+ to near zero by addition of EDTA. Studies revealed that the primary means of Zn2+ efflux in cortical neurons required both extracellular Na+ and Ca2+. The actions of either Na+ or Ca2+ on Zn2+ efflux were blunted in the absence of the other cation. Reversed Na+ gradients could induce Zn2+ uptake. The Na+ dependence of Zn2+ efflux was not affected by a small pHo shift (7.6-8);whereas an effect of Ca2+ was not observed at pHo 8. In summary, a Na+, Ca2+/Zn2+ exchanger mechanism is proposed to be the primary transport mechanism that extrudes Zn2+ when neuronal intracellular free Zn2+ levels rise.