Contributions of endothelial and neuronal nitric oxide synthases to cerebrovascular responses to hyperoxia

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 (NOx) decreased in WT and eNOS(-/-) mice within 30 minutes of HBO2, but after 45 minutes, NOx 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.