Two mechanisms by which ATP depletion potentiates induction of the mitochondrial permeability transition.

The present and a previous study [J. W. Snyder, J. G. Pastorino, A. M. Attie, and J. L. Farber, Am. J. Physiol. 264 (Cell Physiol. 33): C709-C714, 1993] define two mechanisms whereby ATP depletion promotes liver cell death. ATP depletion and cell death are linked by the mitochondrial permeability transition (MPT). Mitochondrial deenergization promotes the MPT, and ATP maintains a membrane potential by reversal of ATP synthase. With an increased influx of Ca2+ induced by the ionophore A-23187, oligomycin depleted the cells of ATP without loss of the mitochondrial membrane potential and further elevated the intracellular Ca2+ concentration. Cyclosporin A (CyA) prevented the accompanying cell killing. Fructose also preserved the viability of the cells. With the increased cytosolic Ca2+ imposed by A-23187, viability is maintained by ATP-dependent processes. Upon depletion of ATP, Ca2+ homeostasis cannot be maintained, and the MPT is induced. Rotenone also depleted the cells of ATP, and A-23187 accelerated the loss of the mitochondrial membrane potential occurring with rotenone alone. CyA and fructose prevented the cell killing with rotenone and A-23187. Oligomycin did not prevent this action of fructose. We conclude that ATP is needed to maintain Ca2+ homeostasis to prevent the MPT and the resultant liver cell death. ATP is also needed to maintain mitochondrial energization when electron transport is inhibited.