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. P-31-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.