RELATION BETWEEN ACCUMULATION OF PHOSPHOLIPASE A(2) REACTION-PRODUCTSAND CA2-MITOCHONDRIA( RELEASE IN ISOLATED LIVER)

A Ca2+-dependent stimulation of mitochondrial phospholipase A(2) is of ten assumed to play a role in mitochondrial Ca2+ release. We sought to clarify this relation by measuring Ca2+ transport and determining pho spholipase A(2) reaction products from the same sample of isolated, in cubated rat liver mitochondria. When mitochondria had accumulated and spontaneously released again Ca2+, most probably by membrane permeabil ity transition, there was no increase of phospholipase A(2) reaction p roducts. However, when the incubation was continued after Ca2+ release , significant increases of the content of lysophosphatidylcholine and unesterified fatty acids could be seen. Quinacrine, an inhibitor of ph ospholipase A(2) activity, prevented Ca2+ release and p-hydroxymercuri benzoic acid, an inhibitor of lysophospholipid reesterification, induc ed a fast release of Ca2+ from isolated mitochondria. Such effects are usually taken as indirect evidence for a participation of phospholipa se A(2) in mitochondrial Ca2+ release, but analysis of the mitochondri al lipids revealed that no significant changes of the mass of phosphol ipase A(2) reaction products had occurred. These experiments suggest t hat the accumulation of phospholipase A(2) reaction products in mitoch ondria is the consequence rather than the cause of the membrane permea bility transition. Exogenous phospholipase A(2) products, lysophosphat idylcholine and arachidonic acid, induced mitochondrial Ca2+ release a fter a time lag, which decreased with aging of the mitochondrial prepa ration. The amount of lysophosphatidylcholine taken up by the mitochon dria from the incubation medium during these experiments was measured and compared to the amount of lysophosphatidylcholine produced endogen ously by mitochondrial phospholipase A(2). From these data it appears likely that the amount of lysophosphatidylcholine generated in the mit ochondria after the permeability transition is sufficient to sustain t he permeable state. An accumulation of mitochondrially generated phosp holipase A(2) reaction products after the permeability transition coul d thus be a decisive factor for the limited reversibility of the membr ane permeability transition.