Potassium-induced increases in oxygen consumption are diminished by age in rat hippocampal slices

Previous reports showed that cytochrome α,α₃ responded to heightened brain activity with shifts toward oxidation in adult rats but toward reduction in aged animals. To determine whether this change indicates an age-associated limitation in mitochondrial respiratory capacity, the present study compared oxygen consumption in hippocampal slices of young adult (6 month, control) and aged (26 month) rats. Slices were used to insure that results were independent of cerebrovascular factors. Age was without effect on oxygen consumption under 'resting' conditions (i.e. with slices bathed at 3.5 mM K⁺), but oxygen consumption was not increased as much in hippocampal slices from aged rats under heightened energy demands (produced by raising the extracellular potassium ion activity ([K⁺]_o to 50 mM). This lesser oxygen consumption response to enhanced metabolic demand suggests that there are age-associated limits to the brain's ability to increase its metabolic rate.