Flow-Induced Cerebral Vasodilatation in Vivo Involves Activation of Phosphatidylinositol-3 Kinase, NADPH-Oxidase, and Nitric Oxide Synthase

Abstract

Reactive oxygen species (ROS) such as superoxide (O^•−₂) and hydrogen peroxide (H₂O₂) are known cerebral vasodilators. A major source of vascular ROS is the flavin-containing enzyme nicotinamide adenine dinucleotide phosphate (NADPH)-oxidase. Activation of NADPH-oxidase leads to dilatation of the basilar artery in vivo via production of H₂O₂, but the endogenous stimuli for this unique vasodilator mechanism are unknown. Shear stress is known to activate both NADPH-oxidase and phosphatidylinositol-3 kinase (PI3-K) in cultured cells. Hence, this study used a cranial window preparation in anesthetized rats to investigate whether increased intraluminal blood flow could induce cerebral vasodilatation via the activation of NADPH-oxidase and/or PI3-K. Bilateral occlusion of the common carotid arteries to increase basilar artery blood flow caused reproducible, reversible vasodilatation. Topical treatment of the basilar artery with the NADPH-oxidase inhibitor diphenyleneiodonium (DPI) (0.5 and 5 μmol/L) inhibited flow-induced dilatation by up to 50% without affecting dilator responses to acetylcholine. Treatment with the H₂O₂ scavenger, catalase similarly attenuated flow-induced dilatation, suggesting a role for NADPH-oxidase-derived H₂O₂ in this response. The nitric oxide synthase inhibitor N^G-nitro-l-arginine methyl ester (l-NAME) partially reduced flow-induced dilatation, and combined treatment with a ROS inhibitor (DPI or catalase) and l-NAME caused a greater reduction in flow-induced dilatation than that seen with any of these inhibitors alone. Flow-induced dilatation was also markedly inhibited by the PI3-K inhibitor, wortmannin. Increased O^•−₂ production in the endothelium of the basilar artery during acute increases in blood flow was confirmed using dihydroethidium. Thus, flow-induced cerebral vasodilatation in vivo involves production of ROS and nitric oxide, and is dependent on PI3-K activation.