Spermine inhibition of the permeability transition of isolated rat liver mitochondria: an investigation of mechanism.

Mitochondria that have accumulated Ca2+ can be induced to undergo a permeability transition: the inner membrane becomes nonselectively permeable to small (< 1500 Da) solutes. The molecular mechanism(s) underlying this transition, which is Ca(2+)-dependent and cyclosporin A-sensitive, has yet to be clearly elucidated. Our laboratory has recently identified the polyamine spermine as an inhibitor of the permeability transition of isolated rat heart mitochondria. In this study, we have used spermine, in combination with a series of triggering agents, to clarify several mechanistic details of the transition process in isolated rat liver mitochondria. Mitochondrial swelling was monitored as an indicator of transition occurrence. Our results indicate that: (1) spermine inhibits the permeability transition of isolated rat liver mitochondria; (2) the sensitivity of the permeability transition of liver mitochondria to spermine is highly dependent on the ionic composition of the assay medium; (3) K+ interacts with a site outside the mitochondria to decrease spermine effectiveness; (4) spermine likewise acts at an external site; and (5) the Ca2+ uniporter in its inactive form is not the protein responsible for mediating the permeability transition.