Sex differences in the mitochondrial bioenergetics of astrocytes but not microglia at a physiologically relevant brain oxygen tension.

Biological sex is thought to influence mitochondrial bioenergetic function. Previous respiration measurements examining brain mitochondrial sex differences were made at atmospheric oxygen using isolated brain mitochondria. Oxygen is 160 mm Hg (21%) in the atmosphere, while the oxygen tension in the brain generally ranges from ∼5 to 45 mm Hg (∼1-6% O2). This study tested the hypothesis that sex and/or brain physiological oxygen tension influence the mitochondrial bioenergetic properties of primary rat cortical astrocytes and microglia. Oxygen consumption was measured with a Seahorse XF24 cell respirometer in an oxygen-controlled environmental chamber. Strikingly, male astrocytes had a higher maximal respiration than female astrocytes when cultured and assayed at 3% O2. Three percent O2 yielded a low physiological dissolved O2 level of ∼1.2% (9.1 mm Hg) at the cell monolayer during culture and 1.2-3.0% O2 during assays. No differences in bioenergetic parameters were observed between male and female astrocytes at 21% O2 (dissolved O2 of ∼19.7%, 150 mm Hg during culture) or between either of these cell populations and female astrocytes at 3% O2. In contrast to astrocytes, microglia showed no sex differences in mitochondrial bioenergetic parameters at either oxygen level, regardless of whether they were non-stimulated or activated to a proinflammatory state. There were also no O2- or sex-dependent differences in proinflammatory TNF-α or IL-1β cytokine secretion measured at 18 h activation. Overall, results reveal an intriguing sex variance in astrocytic maximal respiration that requires additional investigation. Findings also demonstrate that sex differences can be masked by conducting experiments at non-physiological O2.