Focal cortical distribution of blood flow and brain pHi determined by in vivo fluorescent imaging

A technique was developed for panoramic imaging of intracellular focal brain pH (pHi) and focal cortical blood flow to measure the pattern of changes in these parameters during arterial PCO2 (PaCO2) and arterial PO2 (PaO2) alterations. Eleven overnight-fasted New Zealand White rabbits were operated and studied under 2.0 and 1.0% halothane anesthesia, respectively. The exposed cortex was imaged by a 512 x 512 pixel image intensifier system with a resolution of 10 microns2/pixel. Focal brain pHi and focal cortical blood flow were measured by umbelliferone fluorescence. The mean focal cortical blood flow was 49 +/- 4.5 ml.100 g-1.min-1, and focal brain pHi was 7.05 +/- 0.02 during normocapnia. At a stable PaCO2, focal cortical blood flow and focal brain pHi within 10 microns of surface-conducting vessels were 2-50% greater and 0.02 pH units lower than that observed in the border-zone regions between vessels. Focal cortical blood flow demonstrated a significant heterogeneity of 51% among various cortical regions, despite a stable PaCO2. Alternatively, focal brain pHi was homogeneous, varying no more than 0.05 pH units. These results demonstrate the dependence of focal cortical blood flow on topical locations from surface conducting vessels and the tight regulation of brain pHi.