THE MITOCHONDRIAL PERMEABILITY TRANSITION - INTERACTIONS OF SPERMINE,ADP, AND INORGANIC-PHOSPHATE

Mitochondria that have accumulated Ca2+ can be induced to undergo a pe rmeability transition: the inner membrane becomes nonselectively perme able to small (<1500 daltons) solutes. Our laboratory has recently ide ntified the polyamine spermine as an inhibitor of the permeability tra nsition of isolated rat heart and liver mitochondria. Here, we have us ed swelling of liver mitochondria as an indicator of transition occurr ence to investigate the connection between spermine, another transitio n antagonist, ADP, and several key triggering agents: P-i, Ca2+, and t -butyl hydroperoxide (t-BH). Our results demonstrate that: 1) ADP stro ngly inhibits only the swelling induced by P-i; transitions induced by t-BH and Ca2+ are minimally affected. 2) The sensitivity of the perme ability transition to P-i is enhanced in mitochondria depleted of aden ine nucleotides. 3) Incubation with P-i decreases mitochondrial ADP an d ATP content. 4) Spermine inhibits less well in adenine nucleotide-de pleted than control mitochondria, regardless of triggering agent. 5) S permine and ADP act synergistically to inhibit the transition. 6) ADP replenishment makes P-i a worse triggering agent. Triggering by Ca2+ a nd 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+ an d t-BH.