Apoptosis has been implicated in ischemic renal injury. Thus one strategy of renal protection is to antagonize apoptosis. However, apoptosis inhibitory approaches remain to be fully explored. Zn(2+) has long been implicated in apoptosis inhibition; but systematic analysis of the inhibitory effects of Zn(2+) is lacking. Moreover, whether Zn(2+) blocks renal cell apoptosis following ischemia is unknown. Here, we demonstrate that Zn(2+) is a potent apoptosis inhibitor in an in vitro model of renal cell ischemia. ATP depletion induced apoptosis in cultured renal tubular cells, which was accompanied by caspase activation. Zn(2+) at 10 microM inhibited both apoptosis and caspase activation, whereas Co(2+) was without effect. In ATP-depleted cells, Zn(2+) partially prevented Bax activation and cytochrome c release from mitochondria. In isolated cell cytosol, Zn(2+) blocked cytochrome c-stimulated caspase activation at low-micromolar concentrations. In addition, Zn(2+) could directly antagonize the enzymatic activity of purified recombinant caspases. We conclude that Zn(2+) is a potent inhibitor of apoptosis in renal tubular cells following ATP depletion. Zn(2+) blocks apoptosis at multiple steps including Bax activation, cytochrome c release, apoptosome function, and caspase activation.