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This study investigated the effects 2 h after administration of acetazolamide on cerebral blood flow and the pattern of cerebral capillary perfusion. Arterial blood pressure, heart rate, arterial blood gases, and pH were recorded in two groups of rats along with either regional cerebral blood flow or the percentage of capillary volume per cubic millimeter and number per square millimeter perfused as determined in cortical, thalamic, pontine, and medullary regions of the brain. Blood pressure, heart rate, and arterial PCO2 were not significantly different between the rats receiving acetazolamide (100 mg/kg) and the controls. Arterial blood pH was significantly lower in the acetazolamide rats. Blood flow increased significantly in the cortical (+ 102%), thalamic (+ 89%), and pontine (+ 88%) regions receiving acetazolamide. In control rats, approximately 60% of the capillaries were perfused in all of the examined regions. The percentage of capillaries per square millimeter perfused was significantly greater in the cortical (+ 52%), thalamic (+ 49%), and pontine (+ 47%) regions of acetazolamide rats compared with controls. In the medulla the increases in blood flow and percentage of capillaries perfused were not significant. Thus in the regions that acetazolamide increased cerebral blood flow, it also increased the percentage of capillaries perfused.

Alterations in regional cerebral blood flow (rCBF) and percent perfused capillaries (indicative of functional intercapillary distance) were determined in conscious male Long-Evans rats after reducing their blood O2-carrying capacity by exposing them to 1% CO for 12 min. rCBF was determined by the iodoantipyrine method. rCBF increased from a mean of 106 +/- 8 (SE) ml.min-1.100 g-1 before CO exposure to 173 +/- 14 ml.min-1.100 g-1 after CO exposure. There was a greater flow increase (126%) in the cerebral cortex than in the lower brain stem [pons (45%), medulla (39%)]. Presence of fluorescein isothiocyanate-labeled dextran identified the perfused capillaries before and after CO exposure. The volume fraction (Vv) and number/mm2 (Na) of all capillaries (perfused and nonperfused) in a given area of brain were determined after staining for alkaline phosphatase. The percent Vv and percent Na of perfused capillaries increased uniformly (from approximately 50% to approximately 80%) in all parts of the brain after CO exposure. In the presence of tissue hypoxia with undiminished plasma PO2, the brain vasculature allowed greater flow of blood while the microvasculature adjusted to reduce the diffusion distance for O2.