A comparison of Zn2+- and Ca2+-triggered depolarization of liver mitochondria reveals no evidence of Zn2+-induced permeability transition

Abstract Intracellular Zn 2+ toxicity is associated with mitochondrial dysfunction. Zn 2+ depolarizes mitochondria in assays using isolated organelles as well as cultured cells. Some reports suggest that Zn 2+ -induced depolarization results from the opening of the mitochondrial permeability transition pore (mPTP). For a more detailed analysis of this relationship, we compared Zn 2+ -induced depolarization with the effects of Ca 2+ in single isolated rat liver mitochondria monitored with the potentiometric probe rhodamine 123. Consistent with previous work, we found that relatively low levels of Ca 2+ caused rapid, complete and irreversible loss of mitochondrial membrane potential, an effect that was diminished by classic inhibitors of mPT, including high Mg 2+ , ADP and cyclosporine A. Zn 2+ also depolarized mitochondria, but only at relatively high concentrations. Furthermore Zn 2+ -induced depolarization was slower, partial and sometimes reversible, and was not affected by inhibitors of mPT. We also compared the effects of Ca 2+ and Zn 2+ in a calcein-retention assay. Consistent with the well-documented ability of Ca 2+ to induce mPT, we found that it caused rapid and substantial loss of matrix calcein. In contrast, calcein remained in Zn 2+ -treated mitochondria. Considered together, our results suggest that Ca 2+ and Zn 2+ depolarize mitochondria by considerably different mechanisms, that opening of the mPTP is not a direct consequence of Zn 2+ -induced depolarization, and that Zn 2+ is not a particularly potent mitochondrial inhibitor.