Contributions of Endothelial and Neuronal Nitric Oxide Synthases to Cerebrovascular Responses to Hyperoxia

Abstract

Hyperoxia causes a transient decrease in CBF, followed by a later rise. The mediators of these effects are not known. We used mice lacking endothelial or neuronal nitric oxide synthase (NOS) isoforms (eNOS^−/− and nNOS^−/− mice) to study the roles of the NOS isoforms in mediating changes in cerebral vascular tone in response to hyperoxia. Resting regional cerebral blood flow (rCBF) did not differ between wild type (WT), eNOS^−/− mice, and nNOS^−/− mice. eNOS^−/− mice showed decreased cerebrovascular reactivities to N^G-nitro-L-arginine methyl ester (L-NAME), PAPA NONOate, acetylcholine (Ach), and SOD1. In response to hyperbaric oxygen (HBO₂) at 5 ATA, WT and nNOS^−/− mice showed decreases in rCBF over 30 minutes, but eNOS^−/− mice did not. After 60 minutes HBO₂, rCBF increased more in WT mice than in eNOS^−/− or nNOS^−/− mice. Brain NO-metabolites (NO_x) decreased in WT and eNOS^−/− mice within 30 minutes of HBO₂, but after 45 minutes, NO_x rose above control levels, whereas they did not change in nNOS^−/− mice. Brain 3NT increased during HBO₂ in WT and eNOS^−/− but did not change in nNOS^−/− mice. These results suggest that modulation of eNOS-derived NO by HBO₂ is responsible for the early vasoconstriction responses, whereas late HBO₂-induced vasodilation depends upon both eNOS and nNOS.