Interaction of Lipid Peroxidation and Calcium in the Pathogenesis of Neuronal Injury

Abstract

The interactions between lipid peroxidation and calcium in mediating damage to central nervous system membranes have been examined in several in vitro systems. Using isolated rat brain synaptosomes, brain mitochondria, or cultured fetal mouse spinal cord neurons, Ca²⁺ was found to markedly enhance lipid peroxidation-induced disruption of membrane function. Gamma-aminobutyric acid (GABA) uptake by synaptosomes was inhibited 25% by either lipid peroxidation (induced with xanthine and xanthine oxidase) or Ca²⁺ alone, whereas inhibition was 46% with their combination. Ca²⁺ enhancement of lipid peroxidation-induced damage to synaptosomes was intensified by the Ca²⁺ ionophore, A23187, and was partially blocked by the Ca²⁺ channel blocker, verapamil. Similarly, inhibition of state 3 respiration in isolated rat brain mitochondria was observed with Ca²⁺ and a free radical generating system (xanthine and xanthine oxidase) under conditions where either insult alone failed to cause detectable damage. Na⁺,K⁺-ATPase activity of cultured fetal mouse spinal cord neurons was inhibited 32% when cells were incubated for 30 minutes in the presence of both A23187 and a free radical generating system. However, Na⁺,K⁺-ATPase was not affected during a 30 minute incubation with either A23187 or radical generating system alone. In further studies, peroxidation of rat brain synaptosomes by ferrous iron (Fe²⁺) and H₂O₂ was coupled with a rapid and large (2−7-fold) uptake of Ca²⁺ by synaptosomes. Fe²⁺ also enhanced Ca²⁺ uptake by spinal cord neurons in culture, an effect that was coincident with peroxidation of neuronal membranes and the release of arachidonic acid from cells. Iron-induced Ca²⁺ uptake was blocked by high concentrations of either desferrioxamine or methylprednisolone, whereas Ca²⁺ channel blockers did not affect Ca²⁺ uptake induced by Fe²⁺. Finally, peroxidation of membrane lipids by Fe²⁺ was stimulated by Ca²⁺. Concentrations of Ca²⁺ as low as 10⁻⁹ M increased peroxidation reactions within brain synaptosomal membranes. The results of these studies indicate that lipid peroxidation and Ca²⁺ can synergistically act to damage biologic membranes. The findings suggest that Ca²⁺ and lipid peroxidation cannot be considered as separate entities in the pathophysiology of CNS trauma. A hypothesis proposing an inseparable interplay between lipid peroxidation and Ca²⁺ in the pathogenesis of traumatic and ischemic cell injury is presented. Key Words: Calcium–Neuronal injury–Lipid peroxidation–Synaptosomes–Free radicals–Calcium ionophore–Xanthine oxidase–Na⁺,K⁺-ATPase–Spinal cord–Tissue culture–GABA–Iron.