Nonuniform brain blood flow response to hypoxia in unanesthetized cats

In seven unanesthetized cats, radiolabeled microspheres were used to determine regional brain blood flow (rBBF) to the medulla-pons (M-P), midbrain-thalamus (M-T), cerebellum (Cb), and cortex (Cx) during three conditions: 1) control [arterial O₂ tension (Pa(O₂)) = 81 Torr, arterial CO₂ tension (Pa(CO₂)) = 26 Torr]; 2) hypocapnic hypoxia (Pa(O₂) = 39 Torr, Pa(CO₂) = 22 Torr); and 3) isocapnic hypoxia (Pa(O₂) = 47 Torr, Pa(CO₂) = 26 Torr). Hypoxia increased blood flow significantly more in the caudal brain stem (M-P) than in the Cx (P < 0.05) during both hypocapnic hypoxia (M-P/Cx: +33/+17 ml·min^-1·100 g^-1) and isocapnic hypoxia (M-P/Cx: +13/-2 ml·min^-1·100 g^-1). Since sympathetic innervation is greater anatomically to rostral than to caudal vessels, we examined the rBBF response to hypocapnia hypoxia in seven additional cats after unilateral superior cervical gangliectomy. All seven cats had a reduction in the cortical-to-caudal brain stem trend on the denervated side of the brain (M-P/Cx: +27/+28 ml·min^-1·100 g^-1) compared with the intact side of the brain (M-P/Cx: +34/+24 ml·min^-1·100 g^-1) owing to both increases in Cx and decreases in M-P flows. We conclude that 1) in unanesthetized cats hypoxia causes a greater increase in the caudal brain stem compared with cortical blood flow, and 2) this differential response is related to modulation by the sympathetic nervous system. The potential role of this phenomenon in modulating the stimuli at the acid-sensitive central respiratory chemoreceptor is indicated.