ABSTRACT
Background: Brain metabolism depends largely on oxygen and therefore constant delivery of oxygen to the brain is more important than to any other organ. Previously we reported a newly developed method to automatically calculate red blood cell (RBC) flow and its temporal modulations at the layer 1 of the rat cerebral cortex. Objective: To examine a general tendency of frame-rate dependency of RBC velocities and heterogeneity of RBC movements in single capillaries identified by fluorescein isothiocyanate (FITC)-dextran staining. Methods: In urethane-anesthetized rats with a closed cranial window, intravenously administered FITC-labeled RBCs were traced at 125, 250 or 500 frames/s (fps) with a laser-scanning confocal fluorescence microscope and their velocities were automatically calculated with home-made software KEIO-IS2. Results: RBC velocities in single capillaries were not constant but variable, and were dependent on frame rate, with average values of 0.85 ± 0.43 mm/s at 125 fps, 1.34 ± 0.73 mm/s at 250 fps, and 2.05¬1.59 mm/s at 500 fps. When all capillary RBCs were plotted against their velocities (frequency distribution function of RBC velocities), RBC velocities were clustered at around 1.0 mm/s, smearing at higher velocities up to 9.4 mm/s. High RBC velocity was only detected from frame analysis with high frame rates since such high-flow RBCs were uncounted at low frame rates. RBC velocities higher than 6.5 mm/s were statistically significantly outlined from the main population (p \< 0.01). Such a group of capillaries was considered to belong to thoroughfare channels, although their diameter was almost the same as that of ordinary ones. Conclusion: Extra-high flow capillaries are confirmed in the cerebral cortex and these may be thoroughfare channels or non-nutritional capillaries carrying 42% of the blood in reserve.