Within the framework of studying the mechanisms of acute toxicity of arachidonic acid and the role of ambient cations, we have investigated the effects of extracellular La3+ on arachidonic acid-induced death (lactate dehydrogenase release) and mitochondrial depolarization (rhodamine 123 fluorescence) in PC12 cells. Micromolar La3+ profoundly suppressed arachidonic acid toxicity and this effect was dependent on the presence of other cations. Whereas in the cation-free solution 10-20 mu M La3+ protected most cells from death caused by a 2 hour-long exposure to 20 mu M arachidonic acid, the cytoprotective effect of 100 mu M La3+ was reduced to similar to 70% in the presence of a normal complement of monovalent cations and was hardly detectable with 5 mM Ca2+ in the bath. Increasing the concentration of arachidonic acid could defeat La3+ cytoprotection. In fluorescence experiments, arachidonic acid caused a decrease in the mitochondrial membrane potential, with the rate and extent of depolarization increasing with an increase in the concentration of arachidonic acid. La3+ countered the depolarizing effect of arachidonic acid in a manner consistent with a decrease in the effective arachidonic acid concentration. The results suggest that extracellular cations modulate cellular effects of arachidonic acid by reducing its ability to pass through the plasma membrane, possibly by binding the fatty acid. The similarities of the La3+ effects on arachidonic acid-induced cell death and arachidonic acid-induced mitochondrial depolarization strongly support the causal relations between the two events and suggest that mitochondria are the primary target of arachidonic acid at the cellular level. (c) 2007 Elsevier B.V. All rights reserved.
