Hypoxia compounds exercise-induced free radical formation in humans; partitioning contributions from the cerebral and femoral circulation.

This study examined to what extent the human cerebral and femoral circulation contribute to free radical formation during basal and exercise-induced responses to hypoxia. Healthy participants (5♂, 5♀) were randomly assigned single-blinded to normoxic (21% O2) and hypoxic (10% O2) trials with measurements taken at rest and 30 min after cycling at 70% of maximal power output in hypoxia and equivalent relative and absolute intensities in normoxia. Blood was sampled from the brachial artery (a), internal jugular and femoral veins (v) for non-enzymatic antioxidants (HPLC), ascorbate radical (A•-, electron paramagnetic resonance spectroscopy), lipid hydroperoxides (LOOH) and low density lipoprotein (LDL) oxidation (spectrophotometry). Cerebral and femoral venous blood flow was evaluated by transcranial Doppler ultrasound (CBF) and constant infusion thermodilution (FBF). With 3 participants lost to follow up (final n = 4♂, 3♀), hypoxia increased CBF and FBF (P = 0.041 vs. normoxia) with further elevations in FBF during exercise (P = 0.002 vs. rest). Cerebral and femoral ascorbate and α-tocopherol consumption (v < a) was accompanied by A•-/LOOH formation (v > a) and increased LDL oxidation during hypoxia (P < 0.043-0.049 vs. normoxia) implying free radical-mediated lipid peroxidation subsequent to inadequate antioxidant defense. This was pronounced during exercise across the femoral circulation in proportion to the increase in local O2 uptake (r = -0.397 to -0.459, P = 0.037-0.045) but unrelated to any reduction in PO2. These findings highlight considerable regional heterogeneity in the oxidative stress response to hypoxia that may be more attributable to local differences in O2 flux than to O2 tension.

Nitrite and S-Nitrosohemoglobin Exchange Across the Human Cerebral and Femoral Circulation: Relationship to Basal and Exercise Blood Flow Responses to Hypoxia.

BACKGROUND: The mechanisms underlying red blood cell (RBC)-mediated hypoxic vasodilation remain controversial, with separate roles for nitrite () and S-nitrosohemoglobin (SNO-Hb) widely contested given their ability to transduce nitric oxide bioactivity within the microcirculation. To establish their relative contribution in vivo, we quantified arterial-venous concentration gradients across the human cerebral and femoral circulation at rest and during exercise, an ideal model system characterized by physiological extremes of O2 tension and blood flow. METHODS: Ten healthy participants (5 men, 5 women) aged 24±4 (mean±SD) years old were randomly assigned to a normoxic (21% O2) and hypoxic (10% O2) trial with measurements performed at rest and after 30 minutes of cycling at 70% of maximal power output in hypoxia and equivalent relative and absolute intensities in normoxia. Blood was sampled simultaneously from the brachial artery and internal jugular and femoral veins with plasma and RBC nitric oxide metabolites measured by tri-iodide reductive chemiluminescence. Blood flow was determined by transcranial Doppler ultrasound (cerebral blood flow) and constant infusion thermodilution (femoral blood flow) with net exchange calculated via the Fick principle. RESULTS: Hypoxia was associated with a mild increase in both cerebral blood flow and femoral blood flow (P<0.05 versus normoxia) with further, more pronounced increases observed in femoral blood flow during exercise (P<0.05 versus rest) in proportion to the reduction in RBC oxygenation (r=0.680-0.769, P<0.001). Plasma gradients reflecting consumption (arterial>venous; P<0.05) were accompanied by RBC iron nitrosylhemoglobin formation (venous>arterial; P<0.05) at rest in normoxia, during hypoxia (P<0.05 versus normoxia), and especially during exercise (P<0.05 versus rest), with the most pronounced gradients observed across the bioenergetically more active, hypoxemic, and acidotic femoral circulation (P<0.05 versus cerebral). In contrast, we failed to observe any gradients consistent with RBC SNO-Hb consumption and corresponding delivery of plasma S-nitrosothiols (P>0.05). CONCLUSIONS: These findings suggest that hypoxia and, to a far greater extent, exercise independently promote arterial-venous delivery gradients of intravascular nitric oxide, with deoxyhemoglobin-mediated reduction identified as the dominant mechanism underlying hypoxic vasodilation.

Arterial hypoxaemia and its impact on coagulation: significance of altered redox homeostasis.

AIMS: Arterial hypoxaemia stimulates free radical formation. Cellular studies suggest this may be implicated in coagulation activation though human evidence is lacking. To examine this, an observational study was designed to explore relationships between systemic oxidative stress and haemostatic responses in healthy participants exposed to inspiratory hypoxia. RESULTS: Activated partial thromboplastin time and international normalised ratio were measured as routine clinical biomarkers of coagulation and ascorbate free radical (A(•-)) as a direct global biomarker of free radical flux. Six hours of hypoxia activated coagulation, and increased formation of A(•-), with inverse correlations observed against oxyhaemoglobin saturation. CONCLUSIONS: This is the first study to address the link between free radical formation and coagulation in vivo. This 'proof-of-concept' study demonstrated functional associations between hypoxaemia and coagulation that may be subject to redox activation of the intrinsic pathway. Further studies are required to identify precisely which intrinsic factors are subject to redox activation.

Transcompartmental inflammatory responses in humans: IV versus endobronchial administration of endotoxin*.

OBJECTIVES: Transcompartmental signaling during early inflammation may lead to propagation of disease to other organs. The time course and the mechanisms involved are still poorly understood. We aimed at comparing acute transcompartmental inflammatory responses in humans following lipopolysaccharide-induced pulmonary and systemic inflammation. DESIGN: Randomized, double-blind, placebo-controlled, crossover study. SETTING ICU SUBJECTS: Healthy male volunteers. INTERVENTIONS: Fifteen volunteers (mean age, 23; SD, 2 yr) received Escherichia coli endotoxin (lipopolysaccharide, 4 ng/kg) IV or endobronchially on two different study days. Groups were evaluated by bronchoalveolar lavage at baseline (0 hr) and 2, 4, 6, 8, or 24 hours postchallenge. Cardiorespiratory variables were continuously recorded throughout the study day, and plasma and bronchoalveolar lavage fluid markers of inflammation were measured. MEASUREMENTS AND MAIN RESULTS: IV endotoxin elicited a systemic inflammatory response with a time-dependent increase and peak in tumor necrosis factor-α, interleukin-6, and leukocyte counts (all p < 0.001). Furthermore, a delayed (6-8 hr) increase in bronchoalveolar lavage fluid interleukin-6 concentration (p < 0.001) and alveolar leukocyte count (p = 0.03) and a minor increase in bronchoalveolar lavage fluid tumor necrosis factor-α were observed (p = 0.06). Endobronchial endotoxin was followed by progressive alveolar neutrocytosis and increased bronchoalveolar lavage fluid tumor necrosis factor-α, interleukin-6, and albumin (all p < 0.001); a systemic inflammatory response was observed after 2-4 hours, with no change in plasma tumor necrosis factor-α. CONCLUSIONS: Acute lung or systemic inflammation in humans is followed by a transcompartmental proinflammatory response, the degree and differential kinetics of which suggests that the propagation of inflammation may depend on the primary site of injury.

Lipopolysaccharide infusion enhances dynamic cerebral autoregulation without affecting cerebral oxygen vasoreactivity in healthy volunteers.

INTRODUCTION: Sepsis may be associated with disturbances in cerebral oxygen transport and cerebral haemodynamic function, thus rendering the brain particularly susceptible to hypoxia. The purpose of this study was to assess the impact of isocapnic hypoxia and hyperoxia on dynamic cerebral autoregulation in a human-experimental model of the systemic inflammatory response during the early stages of sepsis. METHODS: A total of ten healthy volunteers were exposed to acute isocapnic inspiratory hyperoxia (FIO2 = 40%) and hypoxia (FIO2 = 12%) before and after a 4-hour lipopolysaccharide (LPS) infusion (2 ng kg-1). Middle cerebral artery blood follow velocity was assessed using transcranial Doppler ultrasound, and dynamic autoregulation was evaluated by transfer function analysis. RESULTS: Transfer function analysis revealed an increase in the phase difference between mean arterial blood pressure and middle cerebral artery blood flow velocity in the low frequency range (0.07-0.20 Hz) after LPS (P<0.01). In contrast, there were no effects of either isocapnic hyperoxia or hypoxia on dynamic autoregulation, and the cerebral oxygen vasoreactivity to both hyperoxia and hypoxia was unaffected by LPS. CONCLUSIONS: The observed increase in phase suggests that dynamic cerebral autoregulation is enhanced after LPS infusion and resistant to any effects of acute hypoxia; this may protect the brain from ischaemia and/or blood-brain barrier damage during the early stages of sepsis.

Hypoxia and exercise provoke both lactate release and lactate oxidation by the human brain.

Lactate is shuttled between organs, as demonstrated in the Cori cycle. Although the brain releases lactate at rest, during physical exercise there is a cerebral uptake of lactate. Here, we evaluated the cerebral lactate uptake and release in hypoxia, during exercise and when the two interventions were combined. We measured cerebral lactate turnover via a tracer dilution method ([1-(13)C]lactate), using arterial to right internal jugular venous differences in 9 healthy individuals (5 males and 4 females), at rest and during 30 min of submaximal exercise in normoxia and hypoxia (F(i)o(2) 10%, arterial oxygen saturation 72 ± 10%, mean ± sd). Whole-body lactate turnover increased 3.5-fold and 9-fold at two workloads in normoxia and 18-fold during exercise in hypoxia. Although middle cerebral artery mean flow velocity increased during exercise in hypoxia, calculated cerebral mitochondrial oxygen tension decreased by 13 mmHg (P<0.001). At the same time, cerebral lactate release increased from 0.15 ± 0.1 to 0.8 ± 0.6 mmol min(-1) (P<0.05), corresponding to ∼10% of cerebral energy consumption. Concurrently, cerebral lactate uptake was 1.0 ± 0.9 mmol min(-1) (P<0.05), of which 57 ± 9% was oxidized, demonstrating that lactate oxidation may account for up to ∼33% of the energy substrate used by the brain. These results support the existence of a cell-cell lactate shuttle that may involve neurons and astrocytes.

Sea-level assessment of dynamic cerebral autoregulation predicts susceptibility to acute mountain sickness at high altitude.

BACKGROUND AND PURPOSE: Dynamic cerebral autoregulation is impaired in subjects who develop acute mountain sickness (AMS), a neurological disorder characterized by headache. The present study examined if the normoxic sea-level measurement of dynamic cerebral autoregulation would predict subsequent susceptibility to AMS during rapid ascent to terrestrial high altitude. METHODS: A dynamic cerebral autoregulation index was determined in 18 subjects at sea level from continuous recordings of middle cerebral artery blood flow velocity (Doppler ultrasonography) and arterial blood pressure (finger photoplethysmography) after recovery from transiently induced hypotension. Six hours after passive ascent to 3800 m (Mt Elbrus, Russia), the Lake Louise and Environmental Symptoms Cerebral Symptoms questionnaires were used to assess AMS. RESULTS: AMS scores increased markedly at high-altitude (Lake Louise: +3±2 points, P=0.001 and Environmental Symptoms Cerebral Symptoms: +0.6±0.9 points, P=0.0003 versus sea level). Inverse relationships were observed between the sea-level autoregulation index score and the high-altitude-induced increases in the Lake Louise (r=-0.62, P=0.007) and Environmental Symptoms Cerebral Symptoms (r=-0.78, P=0.01) scores. One subject with a history of high-altitude pulmonary and cerebral edema presented with the lowest sea-level autoregulation index score (3.7 versus group: 6.2±1.0 points) and later developed high-altitude cerebral edema at 4800 m during the summit bid. CONCLUSIONS: These findings suggest that a lower baseline autoregulation index may be considered a potential risk factor for AMS. This laboratory measurement may prove a useful screening tool for the expedition doctor when considering targeted pharmacological prophylaxis in individuals deemed "AMS-susceptible."

Exercise-induced oxidative-nitrosative stress is associated with impaired dynamic cerebral autoregulation and blood-brain barrier leakage.

The present study examined whether dynamic cerebral autoregulation and blood-brain barrier function would become compromised as a result of exercise-induced oxidative-nitrosative stress. Eight healthy men were examined at rest and after an incremental bout of semi-recumbent cycling exercise to exhaustion. Changes in a dynamic cerebral autoregulation index were determined during recovery from continuous recordings of blood flow velocity in the middle cerebral artery (MCAv) and mean arterial pressure during transiently induced hypotension. Electron paramagnetic resonance spectroscopy and ozone-based chemiluminescence were employed for direct detection of spin-trapped free radicals and nitric oxide metabolites in venous blood. Neuron-specific enolase, S100ß and 3-nitrotyrosine were determined by ELISA. While exercise did not alter MCAv, it caused a mild reduction in the autoregulation index (from 6.9 ± 0.6 to 5.5 ± 0.9 a.u., P < 0.05) that correlated directly against the exercise-induced increase in the ascorbate radical, 5-(diethoxyphosphoryl)-5-methyl-1-pyrroline N-oxide and N-tert-butyl-α-phenylnitrone adducts, 3-nitrotyrosine and S100ß (r = -0.66 to -0.76, P < 0.05). In contrast, no changes in neuron-specific enolase were observed. In conclusion, our findings suggest that intense exercise has the potential to increase blood-brain barrier permeability without causing structural brain damage subsequent to a free radical-mediated impairment in dynamic cerebral autoregulation.

Haemostatic response to hypoxaemic/exercise stress: the dilemma of plasma volume correction.

OBJECTIVE: Patients with arterial occlusive disease are typically hypoxaemic, and exercise is prescribed for rehabilitation. Both stressors independently contract plasma volume (PV), which may influence clinical interpretation of a patient's thrombogenicity. The aim of the study was to emphasise the conceptual significance of PV correction. METHODS AND RESULTS: Venous plasma samples were obtained from 18 healthy men at rest in normoxia for the measurement of fibrinogen, prothrombin (PT), thrombin (TT) and activated partial thromboplastin (aPTT) times. Additional samples were obtained in hypoxia (12% oxygen) after 6 h of rest and immediately after a maximal exercise challenge. Haemostatic parameters were expressed before and after volume-shift correction. Passive hypoxia reduced PV by 3±5% (p<0.05 vs normoxia), with a further decrease observed during exercise (14±5%, p<0.05). The latter increased the absolute concentration of fibrinogen and shortened aPTT (p<0.05), but these changes were no longer apparent after PV correction (p>0.05). Likewise, the lack of change in absolute values for PT and TT (p>0.05) translated into an elongation after correction (p<0.05). CONCLUSIONS: These findings highlight the important, but previously ignored, interpretive implications of PV correction when haemostasis is assessed.

High-altitude pulmonary hypertension is associated with a free radical-mediated reduction in pulmonary nitric oxide bioavailability.

High altitude (HA)-induced pulmonary hypertension may be due to a free radical-mediated reduction in pulmonary nitric oxide (NO) bioavailability. We hypothesised that the increase in pulmonary artery systolic pressure (PASP) at HA would be associated with a net transpulmonary output of free radicals and corresponding loss of bioactive NO metabolites. Twenty-six mountaineers provided central venous and radial arterial samples at low altitude (LA) and following active ascent to 4559 m (HA). PASP was determined by Doppler echocardiography, pulmonary blood flow by inert gas re-breathing, and vasoactive exchange via the Fick principle. Acute mountain sickness (AMS) and high-altitude pulmonary oedema (HAPE) were diagnosed using clinical questionnaires and chest radiography. Electron paramagnetic resonance spectroscopy, ozone-based chemiluminescence and ELISA were employed for plasma detection of the ascorbate free radical (A(·-)), NO metabolites and 3-nitrotyrosine (3-NT). Fourteen subjects were diagnosed with AMS and three of four HAPE-susceptible subjects developed HAPE. Ascent decreased the arterio-central venous concentration difference (a-cv(D)) resulting in a net transpulmonary loss of ascorbate, α-tocopherol and bioactive NO metabolites (P < 0.05 vs. LA). This was accompanied by an increased a-cv(D) and net output of A(·-) and lipid hydroperoxides (P < 0.05 vs. sea level, SL) that correlated against the rise in PASP (r = 0.56-0.62, P < 0.05) and arterial 3-NT (r = 0.48-0.63, P < 0.05) that was more pronounced in HAPE. These findings suggest that increased PASP and vascular resistance observed at HA are associated with a free radical-mediated reduction in pulmonary NO bioavailability.

Increased cerebral output of free radicals during hypoxia: implications for acute mountain sickness?

This study examined whether hypoxia causes free radical-mediated disruption of the blood-brain barrier (BBB) and impaired cerebral oxidative metabolism and whether this has any bearing on neurological symptoms ascribed to acute mountain sickness (AMS). Ten men provided internal jugular vein and radial artery blood samples during normoxia and 9-h passive exposure to hypoxia (12.9% O(2)). Cerebral blood flow was determined by the Kety-Schmidt technique with net exchange calculated by the Fick principle. AMS and headache were determined with clinically validated questionnaires. Electron paramagnetic resonance spectroscopy and ozone-based chemiluminescence were employed for direct detection of spin-trapped free radicals and nitric oxide metabolites. Neuron-specific enolase (NSE), S100beta, and 3-nitrotyrosine (3-NT) were determined by ELISA. Hypoxia increased the arterio-jugular venous concentration difference (a-v(D)) and net cerebral output of lipid-derived alkoxyl-alkyl free radicals and lipid hydroperoxides (P < 0.05 vs. normoxia) that correlated with the increase in AMS/headache scores (r = -0.50 to -0.90, P < 0.05). This was associated with a reduction in a-v(D) and hence net cerebral uptake of plasma nitrite and increased cerebral output of 3-NT (P < 0.05 vs. normoxia) that also correlated against AMS/headache scores (r = 0.74-0.87, P < 0.05). In contrast, hypoxia did not alter the cerebral exchange of S100beta and both global cerebral oxidative metabolism (cerebral metabolic rate of oxygen) and neuronal integrity (NSE) were preserved (P > 0.05 vs. normoxia). These findings indicate that hypoxia stimulates cerebral oxidative-nitrative stress, which has broader implications for other clinical models of human disease characterized by hypoxemia. This may prove a risk factor for AMS by a mechanism that appears independent of impaired BBB function and cerebral oxidative metabolism.

Transcerebral exchange kinetics of nitrite and calcitonin gene-related peptide in acute mountain sickness: evidence against trigeminovascular activation?

BACKGROUND AND PURPOSE: High-altitude headache is the primary symptom associated with acute mountain sickness, which may be caused by nitric oxide-mediated activation of the trigeminovascular system. Therefore, the present study examined the effects of inspiratory hypoxia on the transcerebral exchange kinetics of the vasoactive molecules, nitrite (NO(2)(*)), and calcitonin gene-related peptide (CGRP). METHODS: Ten males were examined in normoxia and after 9-hour exposure to hypoxia (12.9% O(2)). Global cerebral blood flow was measured by the Kety-Schmidt technique with paired samples obtained from the radial artery and jugular venous bulb. Plasma CGRP and NO(2)(*) were analyzed via radioimmunoassay and ozone-based chemiluminescence. Net cerebral exchange was calculated by the Fick principle and acute mountain sickness/headache scores assessed via clinically validated questionnaires. RESULTS: Hypoxia increased cerebral blood flow with a corresponding increase in acute mountain sickness and headache scores (P<0.05 vs normoxia). Hypoxia blunted the cerebral uptake of NO(2)(*), whereas CGRP exchange remained unaltered. No relationships were observed between the change (hypoxia-normoxia) in cerebral NO(2)(*) or CGRP exchange and acute mountain sickness/headache scores (P>0.05). CONCLUSIONS: These findings argue against sustained trigeminovascular system activation as a significant event in acute mountain sickness.