Increased Oxidative-nitrative Stress In The Hypoxic Human Brain

Abstract

Oxidative stress may be responsible for the reduction in systemic nitric oxide (NO) bioavailability and impaired neurovascular reactivity recently observed in hypoxia (DM Bailey et al., J. Physiol. in-the-press). However, the brain's contribution to these systemic changes in redox homeostasis remains unknown. PURPOSE: The present study determined the trans-cerebral exchange kinetics of oxidative-nitrative stress biomarkers in response to hypoxia. Specifically, we hypothesized that hypoxia would increase the net cerebral output of free radicals that would inactivate NO as indicated by a decrease in the net uptake or consumption of nitrite (NO2) and increased output of 3-nitrotyrosine (3-NT). METHODS: Ten healthy males aged 27 (mean) ± 4 (SD) years were examined in normoxia (N: 20.9%O2) and following 9h passive exposure to hypoxia (H: 12.9%O2). Global cerebral blood flow (CBF) was measured using the Kety-Schmidt technique with paired samples obtained from the radial artery and jugular venous bulb. Global cerebral plasma flow (CPF) was determined as CBF × (1-hematocrit). The serum concentration of spin-trapped a-phenyl-tert-butylnitrone (PBN)-adducts was assessed via X-band electron paramagnetic resonance spectroscopy. Plasma NO2 was determined by ozone-based chemiluminescence using modified tri-iodide reagent and 3-NT via ELISA. Trans-cerebral net exchange was calculated as the arterio-jugular venous concentration difference x CPF. Data were analyzed with a two-way repeated measures ANOVA and post-hoc Bonferroni-corrected paired samples t-tests. RESULTS: Despite a marked reduction in PaO2 (N: 107 ± 6 to H: 46 ± 3 mmHg, P < 0.05), the cerebral metabolic rate for O2 was preserved (N: 2.4 ± 0.5 vs.H: 2.5 ± 0.3 μmol/g/min) due to a trend towards increased CBF (N: 85 ± 15 to H: 94 ± 17 mL/100g/min). Hypoxia increased the net cerebral output of PBN-adducts from -73 ± 192 to -360 ± 253 AU/min/g (P < 0.05). This was associated with an attenuation in the net uptake of NO2 (N: 126.4 ± 93.9 to H: 16.0 ± 46.7 nmol/min/g, P < 0.05) and increased output of 3-NT (N: -3.1 ± 9.0 to H: -10.7 ± 18.2 nmol/min/g, P < 0.05). CONCLUSION: These findings provide direct evidence for increased oxidative-nitrative stress across the hypoxic human brain and suggest that neurovascular NO bioavailability may be subject to redox-regulation.