Critical oxygen tension in rat brain: A combined ³¹P-NMR and EPR oximetry study

The relationship between cerebral interstitial oxygen tension (Pt(O₂)) and cellular energetics was investigated in mechanically ventilated, anesthetized rats during progressive acute hypoxia to determine whether there is a 'critical' brain Pt(O₂) for maintaining steady-state aerobic metabolism. Cerebral Pt(O₂), measured by electron paramagnetic resonance oximetry, decreased proportionately to inspired oxygen fraction. ³¹P-nuclear magnetic resonance measurements revealed no changes in P(i), phosphocreatine (PCr)/P(i) ratio, or intracellular pH when arterial blood oxygen tension (Pa(O₂)) was reduced from 145.1 ± 11.7 to 56.5 ± 4.4 mmHg (means ± SE). Intracellular acidosis, a sharp rise in P(i), and a decline in the PCr/P(i) ratio developed when Pa(O₂) was reduced further to 40.7 ± 2.3 mmHg. The corresponding Pt(O₂) values were 15.1 ± 1.8, 8.8 ± 0.4, and 6.8 ± 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 Pt(O₂) is decreased to a critical level.