Enhanced Depolarization-Evoked Calcium Signal and Reduced [ATP]/[ADP] Ratio Are Unrelated Events Induced by Oxidative Stress in Synaptosomes

Abstract: Oxidative insult elicited by hydrogen peroxide (H₂O₂) was previously shown to increase the basal intracellular Ca²⁺ concentration in synaptosomes. In the present study, the effect of H₂O₂ on the depolarization-evoked [Ca²⁺] signal was investigated. Pretreatment of synaptosomes with H₂O₂ (0.1–1 mM) augmented the [Ca²⁺] rise elicited by high K⁺ depolarization with essentially two alterations, the sudden sharp rise of [Ca²⁺]_i due to K⁺ depolarization is enhanced and, instead of a decrease to a stable plateau, a slow, steady rise of [Ca²⁺]_i follows the peak [Ca²⁺]_i. H₂O₂ in the same concentration range lowered the ATP level and the [ATP]/[ADP] ratio. When carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (FCCP) (1 µM) or rotenone (2 µM)/oligomycin (10 µM) was applied initially to block mitochondrial ATP production, the lowered [ATP]/[ADP] ratio was further reduced by subsequent addition of 0.5 mM H₂O₂. The decline of the [ATP]/[ADP] ratio was parallel with but could not explain the enhanced K⁺-evoked [Ca²⁺]_i signal, indicated by experiments in which the [ATP]/[ADP] ratio was decreased by FCCP (0.1 µM) or rotenone (2 µM) to a similar value as by H₂O₂ without causing any alteration in the [Ca²⁺]_i signal. These results indicate that H₂O₂-evoked oxidative stress, in its early phase, gives rise to a complex dysfunction in the Ca²⁺ homeostasis and, parallel with it, to an impaired energy status.