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

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