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Recessed oxygen microelectrodes (tip diameter < 2 (j.m) were positioned stereotactically into either the cerebral cortex or the hippocampus of sodium pentobarbital anesthetized gerbils (n = 21). The mean tissue P02 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 P02 distributions were seen. The disappearance rate for oxygen ( -dPo^dt) was measured after brief (< 15 seconds) bilateral carotid artery (total brain blood flow) occlusion. The mean (±SEM) disappearance rate was significantly higher (p<0.05) in the cortex (Z3.8 ± 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 (Vc^max) for Michaelis-Menten kinetics were calculated from the disappearance rates, correcting for gerbil oxyhemoglobin. The mean Vo^max was 8.28 ± 0.51 and 6.13 ± 0.25 ml O 2 /100 g/rain for the cortex and hippocampus, respectively. The apparent Michaelis-Menten kinetic constants (K_) for the 2 regions were not significantly different (overall mean 3.3 ± 0.4 torr The present study was undertaken to quantify oxygen metabolism at highly localized sites by the oxygen disappearance technique, correcting for the influence of oxyhemoglobin. Measurements were made in 2 regions of the gerbil brain, the cortex and the hippocampus, which are of interest due to marked differences in susceptibility to ischemic damage (selective vulnerability), even within a region. The gerbil model of ischemia was chosen because blood flow to the brain can be stopped completely with bilateral carotid artery occlusion since the circle of Willis has no connections to the basilar artery for a source of collateral blood flow. Materials and MethodsMongolian gerbils, aged 3-4 months, weighing 50-90 g, were maintained on standard laboratory food and water ad libitum. They were anesthetized with 30-40 mg/kg i.p. sodium pentobarbital, tracheotomized, and allowed to breathe spontaneously. Both common carotid arteries were exposed through ventral midline cervical incisions and isolated from the accompanying nerves and vein. Surgical sutures were placed around the arteries for causing occlusions. In by guest on May 9, 2018 http://stroke.ahajournals.org/ Downloaded from

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