The mitochondrial permeability transition. Interactions of spermine, ADP, and inorganic phosphate.

Mitochondria that have accumulated Ca2+ can be induced to undergo a permeability transition: the inner membrane becomes nonselectively permeable to small (< 1500 daltons) solutes. Our laboratory has recently identified the polyamine spermine as an inhibitor of the permeability transition of isolated rat heart and liver mitochondria. Here, we have used swelling of liver mitochondria as an indicator of transition occurrence to investigate the connection between spermine, another transition antagonist, ADP, and several key triggering agents: P(i), Ca2+, and t-butyl hydroperoxide (t-BH). Our results demonstrate that: 1) ADP strongly inhibits only the swelling induced by P(i); transitions induced by t-BH and Ca2+ are minimally affected. 2) The sensitivity of the permeability transition to P(i) is enhanced in mitochondria depleted of adenine nucleotides. 3) Incubation with P(i) decreases mitochondrial ADP and ATP content. 4) Spermine inhibits less well in adenine nucleotide-depleted than control mitochondria, regardless of triggering agent. 5) Spermine and ADP act synergistically to inhibit the transition. 6) ADP replenishment makes P(i) a worse triggering agent. Triggering by Ca2+ and t-BH is enhanced. 7) P(i) overcomes spermine inhibition; Ca2+ and t-BH do not. We propose that P(i) triggers the transition by lowering the matrix concentration of the inhibitor ADP and that spermine inhibits the transition by enhancing ADP effectiveness. In addition, these data clearly distinguish the triggering action of P(i) from that of Ca2+ and t-BH.