Changes of synaptosomal energy metabolism induced by hypoxia during aging

Abstract

Synaptosomes were isolated from the motor area of the cerebral cortex of normoxic or hypoxic (PaO₂=17–19 mmHg, for 15 min) beagle dogs of different ages. Synaptosomes were incubated in Krebs-Henseleit-Hepes buffer (for 10 min at 24°C) and the energetic state was defined by: the balance of the labile phosphates (ATP, ADP, AMP, and creatine phosphate); the respiratory rate; the redox state of the intramitochondrial NAD-couple. By the present experimental model, it is possible to evaluate the potential damage (induced by the “in vivo” hypoxic insult) that synaptosomes cannot reverse under optimal incubation. Aging affected the phosphorylation state of the post-hypoxic incubated synaptosomes. The oxygen consumption rate was quite similar in the synaptosomal fractions from the motor area of hypoxic beagle dogs of different ages, but the cytochromec anda contents were lower in the preparations from hypoxic older brains. In dogs of different ages, hypoxia always lowered the respiration of the synaptosomes, but aging affected the oxygen consumption rates only in post-hypoxic synaptosomes incubated with succinate. The synaptosomal energetic state was defined also by the redox state of the intramitochondrial NAD-couple (ΔG_ox-red) and the phosphorylation state of adenine nucleotide system (ΔG_ATP). The free-energy change (ΔΔG) for the coupled reactions was calculated, too. In synaptosomes isolated from the cerebral cortex of dogs submitted to hypoxia, the equilibrium (calculated for the mitochondrial electron transfer chain and the phosphorylation of adenine nucleotides) was markedly altered as function of aging. The extensive age-related ΔΔG changes were largely supported by alteration of the phosphorylation state of adenine nucleotides, rather than by modification of the redox state of the electron transfer chain. All present data suggest that the bioenergetic derangement caused by hypoxia and aging may be interpreted also in terms of modification of the biophysical and biochemical mechanisms involving the mitochondrial membranes and particularly the inner mitochondrial membrane.