Comparisons of oxygen metabolism and tissue PO2 in cortex and hippocampus of gerbil brain

Recessed oxygen microelectrodes (tip diameter less than 2 microns) were positioned stereotactically into either the cerebral cortex or the hippocampus of sodium pentobarbital anesthetized gerbils (n = 21). The mean tissue PO2 levels (+/- SEM) were not significantly different between cortex (35.4 +/- 1.7 torr) and hippocampus (33.6 +/- 1.4 torr) although differences in the tissue PO2 distributions were seen. The disappearance rate for oxygen (-dPO2/dt) was measured after brief (less than 15 seconds) bilateral carotid artery (total brain blood flow) occlusion. The mean (+/- SEM) disappearance rate was significantly higher (p less than 0.05) in the cortex (23.8 +/- 1.5 torr/sec, 160 locations in 21 gerbils) than in the hippocampus (17.0 +/- 0.7 torr/sec, 119 locations in 16 gerbils). The maximum oxygen consumption rates (VO2max) for Michaelis-Menten kinetics were calculated from the disappearance rates, correcting for gerbil oxyhemoglobin. The mean VO2max was 8.28 +/- 0.51 and 6.13 +/- 0.25 ml O2/100 g/min for the cortex and hippocampus, respectively. The apparent Michaelis-Menten kinetic constants (Km) for the 2 regions were not significantly different (overall mean 3.3 +/- 0.4 torr). Differences in the recovery of tissue PO2 after releasing the occlusion were seen, with more hyperemic responses in the hippocampus.