Critical oxygen tension in rat brain: a combined P-NMR and EPR oximetry study

Rolett, Ellis L., Ali Azzawi, Ke Jian Liu, Martin N. Yongbi, Harold M. Swartz, and Jeff F. Dunn. Critical oxygen tension in rat brain: a combined P-NMR and EPR oximetry study. Am J Physiol Regulatory Integrative Comp Physiol 279: R9–R16, 2000.—The relationship between cerebral interstitial oxygen tension (PtO2) and cellular energetics was investigated in mechanically ventilated, anesthetized rats during progressive acute hypoxia to determine whether there is a “critical” brain PtO2 for maintaining steadystate aerobic metabolism. Cerebral PtO2, measured by electron paramagnetic resonance oximetry, decreased proportionately to inspired oxygen fraction. P-nuclear magnetic resonance measurements revealed no changes in Pi, phosphocreatine (PCr)/Pi ratio, or intracellular pH when arterial blood oxygen tension (PaO2) was reduced from 145.1 6 11.7 to 56.5 6 4.4 mmHg (means 6 SE). Intracellular acidosis, a sharp rise in Pi, and a decline in the PCr/Pi ratio developed when PaO2 was reduced further to 40.7 6 2.3 mmHg. The corresponding PtO2 values were 15.1 6 1.8, 8.8 6 0.4, and 6.8 6 0.3 mmHg. We conclude that over a range of decreasing oxygen tensions, cerebral oxidative metabolism is not sensitive to oxygen concentration. Oxygen becomes a regulatory substrate, however, when PtO2 is decreased to a critical level. hypoxia; electron paramagnetic resonance; nuclear magnetic resonance spectroscopy; energy metabolism; pH