Isoflurane-induced depolarization of neural mitochondria increases with age

The mitochondrial membrane potential (DeltaPsi(m)) drives the three fundamental functions of mitochondria, namely adenosine triphosphate (ATP) generation, Ca(2+) uptake/storage, and generation/detoxification of ROS. Isoflurane depolarizes neural mitochondria. The sensitivity for general anesthetics increases with age, but the mechanism for this age-related sensitivity is still unknown. We compared the effect of isoflurane on [Ca(2+)](i) and DeltaPsi(m) in isolated pre-synaptic terminals (synaptosomes) from neonatal, adolescent, and adult rats and the influence of interventions in the respiratory chain was assessed.

Synaptosomes were loaded with the fluorescent probes fura-2 ([Ca(2+)](i)) and JC-1 (DeltaPsi(m)) and exposed to isoflurane 1 and 2 minimum alveolar concentration (MAC). The effect on the electron transport chain was investigated by blocking complexes I and V.

In neonatal rats isoflurane had no significant effect on DeltaPsi(m). In adolescent and adult synaptosomes, however, isoflurane 1 and 2 MAC decreased DeltaPsi(m). Isoflurane 2 MAC increased [Ca(2+)](i) in neonatal and adolescent rats, but not in adult synaptosomes. In Ca(2+)-depleted medium, isoflurane still decreased DeltaPsi(m), while [Ca(2+)](i) remained unaltered. By blocking complex V of the respiratory chain, the isoflurane-induced mitochondrial depolarization was enhanced in all age groups. Blocking complex I depolarized the mitochondria to the same extent as isoflurane 2 MAC, but without any additive effect.

The depolarizing effect of isoflurane on neural mitochondria is more pronounced in the adolescent and adult than in neonatal synaptosomes. The increased mitochondrial sensitivity with age seems to be related to the reversed function of the ATP synthase of the electron transport chain.