Effects of phospholipase A2 inhibitors on ruthenium red-induced Ca2+ release from mitochondria

The pharmacologic agents verapamil, nifedipine, diltiazem, prenylamine, N-oleoylethanolamine, R 24571, trifluoperazine, dibucaine, and quinacrine are examined as potential inhibitors of rat liver mitochondrial phospholipase A2 acting on endogenous phospholipid. Their potency as inhibitors of the enzyme is compared to their activities as inhibitors of phospholipase A2-dependent swelling and ruthenium red-induced Ca2+ release in intact mitochondria. For verapamil, diltiazem, trifluoperazine, dibucaine, and quinacrine, there is complete agreement between the relative potencies as inhibitors of phospholipase A2 and the two other processes. Nifedipine and prenylamine, which are weak inhibitors of phospholipase A2, produce a permeable inner membrane, provided that the mitochondrial have accumulated Ca2+. R 24571, which strongly inhibits the enzyme, disrupts mitochondria by a Ca2+-independent mechanism. N-Oleoylethanolamine, which is an effective inhibitor of swelling, does not inhibit phospholipase A2 or ruthenium red-induced Ca2+ release. The results support a proposed scheme wherein ruthenium red-induced Ca2+ release is viewed as reverse activity of the Ca2+-uptake uniporter occurring subsequent to decline in the proton motive force. The latter effect is proposed to arise from a specific phospholipase A2-dependent increase in inner-membrane H+ conductance of mitochondrial subpopulations. It is further shown that mitochondrial membranes display cyclic oscillations in free fatty acid content which are not dependent on the presence of Ca2+ or on the capacity to generate acylcoenzyme A.