Artery-to-vein differences in nitric oxide metabolites are diminished in sepsis

Plasma Nitric Oxide Consumption Is Elevated and Associated With Adverse Outcomes in Critically Ill Patients

OBJECTIVES

Impaired nitric oxide (NO) bioavailability may contribute to microvascular dysfunction in sepsis. Excessive plasma NO consumption has been attributed to scavenging by circulating cell-free hemoglobin. This may be a mechanism for NO deficiency in sepsis and critical illness. We hypothesized that plasma NO consumption is high in critically ill patients, particularly those with sepsis, acute respiratory distress syndrome (ARDS), shock, and in hospital nonsurvivors. We further hypothesized that plasma NO consumption is correlated with plasma cell-free hemoglobin concentration.

DESIGN

Retrospective cohort study.

SETTING

Adult ICUs of an academic medical center.

PATIENTS AND SUBJECTS

Three hundred sixty-two critically ill patients and 46 healthy control subjects.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Plasma NO consumption was measured using reductive chemiluminescence and cell-free hemoglobin was measured with a colorimetric assay. Mean (95% CI) plasma NO consumption (µM) was higher in critically ill patients versus healthy control subjects (3.9 [3.7-4.1] vs 2.1 [1.8-2.5]), septic versus nonseptic patients (4.1 [3.8-4.3] vs 3.6 [3.3-3.8]), ARDS versus non-ARDS patients (4.4 [4.0-4.9] vs 3.7 [3.6-3.9]), shock vs nonshock patients (4.4 [4.0-4.8] vs 3.6 [3.4-3.8]), and hospital nonsurvivors versus survivors (5.3 [4.4-6.4] vs 3.7 [3.6-3.9]). These relationships remained significant in multivariable analyses. Plasma cell-free hemoglobin was weakly correlated with plasma NO consumption.

CONCLUSIONS

Plasma NO consumption is elevated in critically ill patients and independently associated with sepsis, ARDS, shock, and hospital death. These data suggest that excessive intravascular NO scavenging characterizes sepsis and adverse outcomes of critical illness.

Chronic High-Altitude Hypoxia Alters Iron and Nitric Oxide Homeostasis in Fetal and Maternal Sheep Blood and Aorta

The mammalian fetus thrives at oxygen tensions much lower than those of adults. Gestation at high altitude superimposes hypoxic stresses on the fetus resulting in increased erythropoiesis. We hypothesized that chronic hypoxia at high altitude alters the homeostasis of iron and bioactive nitric oxide metabolites (NOx) in gestation. To test for this, electron paramagnetic resonance was used to provide unique measurements of iron, metalloproteins, and free radicals in the blood and aorta of fetal and maternal sheep from either high or low altitudes (3801 or 300 m). Using ozone-based chemiluminescence with selectivity for various NOx species, we determined the NOx levels in these samples immediately after collection. These experiments demonstrated a systemic redistribution of iron in high altitude fetuses as manifested by a decrease in both chelatable and total iron in the aorta and an increase in non-transferrin bound iron and total iron in plasma. Likewise, high altitude altered the redox status diversely in fetal blood and aorta. This study also found significant increases in blood and aortic tissue NOx in fetuses and mothers at high altitude. In addition, gradients in NOx concentrations observed between fetus and mother, umbilical artery and vein, and plasma and RBCs demonstrated complex dynamic homeostasis of NOx among these circulatory compartments, such as placental generation and efflux as well as fetal consumption of iron-nitrosyls in RBCs, probably HbNO. In conclusion, these results may suggest the utilization of iron from non-hematopoietic tissues iron for erythropoiesis in the fetus and increased NO bioavailability in response to chronic hypoxic stress at high altitude during gestation.

Metabolic flux between organs measured by arteriovenous metabolite gradients

Mammalian organs convert dietary nutrients into circulating metabolites and share them to maintain whole-body metabolic homeostasis. While the concentrations of circulating metabolites have been frequently measured in a variety of pathophysiological conditions, the exchange flux of circulating metabolites between organs is not easily measurable due to technical difficulties. Isotope tracing is useful for measuring such fluxes for a metabolite of interest, but the shuffling of isotopic atoms between metabolites requires mathematical modeling. Arteriovenous metabolite gradient measurements can complement isotope tracing to infer organ-specific net fluxes of many metabolites simultaneously. Here, we review the historical development of arteriovenous measurements and discuss their advantages and limitations with key example studies that have revealed metabolite exchange flux between organs in diverse pathophysiological contexts.

A randomized pilot study of nitrate supplementation with beetroot juice in acute respiratory failure

Nitrate rich beetroot juice (BRJ) can enhance nitric oxide signaling, leading to improved physical function in healthy and diseased populations, but its safety and biologic efficacy have not been evaluated in a critically ill population. We randomized 22 previously functional acute respiratory failure patients to either BRJ or placebo daily until day 14 or discharge. We measured blood nitrate and nitrite levels and quantified strength and physical function at intensive care unit (ICU) and hospital discharge. Participants were predominantly male (54%), aged 68.5 years with an APACHE III score of 62. BRJ increased plasma nitrate (mean 219.2 μM increase, p = 0.002) and nitrite levels (mean 0.144 μM increase, p = 0.02). We identified no adverse events. The unadjusted and adjusted effect sizes of the intervention on the short physical performance battery were small (d = 0.12 and d = 0.17, respectively). In this pilot trial, administration of BRJ was feasible and safe, increased blood nitrate and nitrate levels, but had a small effect on physical function. Future studies could evaluate the clinical efficacy of BRJ as a therapy to improve physical function in survivors of critical illness.

Microcirculation dysfunction in endotoxic shock rabbits is associated with impaired S-nitrosohemoglobin-mediated nitric oxide release from red blood cells: a preliminary study

Background

Microcirculation dysfunction with blood flow heterogeneity is an important characteristic in sepsis shock. We hypothesized that impaired ability of red blood cells to release nitric oxide resulted in microcirculation dysfunction in sepsis shock.

Methods

4,4′-Diisothiocyanatostilbene-2,2′-disulfonic acid disodium salt hydrate (DIDS), an inhibitor of band3 protein, was used to inhibit S-nitrosohemoglobin-mediated nitric oxide release. Rabbits were randomly divided into four groups: control (n = 6), lipopolysaccharide (LPS) (n = 6), LPS + DIDS (n = 6), and control + DIDS group (n = 6). Macrocirculation (cardiac output and mean arterial pressure) and microcirculation (microvascular flow index and flow heterogeneity index) parameters were recorded. At 2-h time point, arterial and venous S-nitrosohemoglobin concentrations were measured.

Results

The arterial–venous difference for S-nitrosohemoglobin in the LPS group was lower than the control group (27.3 ± 5.0 nmmol/L vs. 40.9 ± 6.2 nmmol/L, P < 0.05) but was higher than the LPS + DIDS group, with a statistically significant difference (27.3 ± 5.0 nmmol/L vs. 16.0 ± 4.2 nmmol/L, P < 0.05). Microvascular flow index for the LPS group at 2 h was lower than the control group (1.13 ± 0.16 vs. 2.82 ± 0.08, P < 0.001) and higher than the LPS + DIDS group (1.13 ± 0.16 vs. 0.84 ± 0.14, P < 0.05). Flow heterogeneity index for the LPS group at 2 h was higher than the control group (1.03 ± 0.27 vs. 0.16 ± 0.06, P < 0.001) and lower than the LPS + DIDS group (1.03 ± 0.27 vs. 1.78 ± 0.46, P < 0.001).

Conclusions

In endotoxic shock rabbits, the ability of S-nitrosohemoglobin-mediated nitric oxide release from RBC was impaired, and there was an association between the ability and microcirculation dysfunction especially increased blood flow heterogeneity.

Electronic supplementary material

The online version of this article (10.1186/s40635-018-0215-0) contains supplementary material, which is available to authorized users.

Nitrite and S -Nitrosohemoglobin Exchange Across the Human Cerebral and Femoral Circulation

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 O 2 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% O 2 ) and hypoxic (10% O 2 ) 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.

Bioavailable estradiol concentrations are elevated and predict mortality in septic patients: a prospective cohort study

BACKGROUND

Experimental studies demonstrate beneficial immunological and hemodynamic effects of estradiol in animal models of sepsis. This raises the question whether estradiol contributes to sex differences in the incidence and outcomes of sepsis in humans. Yet, total estradiol levels are elevated in sepsis patients, particularly nonsurvivors. Bioavailable estradiol concentrations have not previously been reported in septic patients. The bioavailable estradiol concentration accounts for aberrations in estradiol carrier protein concentrations that could produce discrepancies between total and bioavailable estradiol levels. We hypothesized that bioavailable estradiol levels are low in septic patients and sepsis nonsurvivors.

METHODS

We conducted a combined case-control and prospective cohort study. Venous blood samples were obtained from 131 critically ill septic patients in the medical and surgical intensive care units at the University of Rochester Medical Center and 51 control subjects without acute illness. Serum bioavailable estradiol concentrations were calculated using measurements of total estradiol, sex hormone-binding globulin, and albumin. Comparisons were made between patients with severe sepsis and control subjects and between hospital survivors and nonsurvivors. Multivariable logistic regression analysis was also performed.

RESULTS

Bioavailable estradiol concentrations were significantly higher in sepsis patients than in control subjects (211 [78-675] pM vs. 100 [78-142] pM, p < 0.01) and in sepsis nonsurvivors than in survivors (312 [164-918] pM vs. 167 [70-566] pM, p = 0.04). After adjustment for age and comorbidities, patients with bioavailable estradiol levels above the median value had significantly higher risk of hospital mortality (OR 4.27, 95 % CI 1.65-11.06, p = 0.003). Bioavailable estradiol levels were directly correlated with severity of illness and did not differ between men and women.

CONCLUSIONS

Contrary to our hypothesis, bioavailable estradiol levels were elevated in sepsis patients, particularly nonsurvivors, and were independently associated with mortality. Whether estradiol's effects are harmful, beneficial, or neutral in septic patients remains unknown, but our findings raise caution about estradiol's therapeutic potential in this setting. Our findings do not provide an explanation for sex-based differences in sepsis incidence and outcomes.

Methionine Metabolites in Patients With Sepsis

OBJECTIVE

Sepsis is characterized by microvascular dysfunction and thrombophilia. Several methionine metabolites may be relevant to this sepsis pathophysiology. S-adenosylmethionine (SAM) serves as the methyl donor for trans-methylation reactions. S-adenosylhomocysteine (SAH) is the by-product of these reactions and serves as the precursor to homocysteine. Relationships between plasma total homocysteine concentrations (tHcy) and vascular disease and thrombosis are firmly established. We hypothesized that SAM, SAH, and tHcy levels are elevated in patients with sepsis and associated with mortality.

METHODS

This was a combined case-control and prospective cohort study consisting of 109 patients with sepsis and 50 control participants without acute illness. The study was conducted in the medical and surgical intensive care units of the University of Rochester Medical Center. Methionine, SAM, SAH, and tHcy concentrations were compared in patients with sepsis versus control participants and in sepsis survivors versus nonsurvivors.

RESULTS

Patients with sepsis had significantly higher plasma SAM and SAH concentrations than control participants (SAM: 164 [107-227] vs73 [59-87 nM], P < .001; SAH: 99 [60-165] vs 35 [28-45] nM, P < .001). In contrast, plasma tHcy concentrations were lower in sepsis patients compared to healthy control participants (4 [2-6]) vs 7 [5-9] μM; P = .04). In multivariable analysis, quartiles of SAM, SAH, and tHcy were independently associated with sepsis ( P = .006, P = .05, and P < .001, respectively). Sepsis nonsurvivors had significantly higher plasma SAM and SAH concentrations than survivors (SAM: 223 [125-260] vs 136 [96-187] nM; P = .01; SAH: 139 [81-197] vs 86 [55-130] nM, P = .006). Plasma tHcy levels were similar in survivors vs nonsurvivors. The associations between SAM or SAH and hospital mortality were no longer significant after adjusting for renal dysfunction.

CONCLUSIONS

Methionine metabolite concentrations are abnormal in sepsis and linked with clinical outcomes. Further study is required to determine whether these abnormalities have pathophysiologic significance.

Cardiovascular effects of ozone in healthy subjects with and without deletion of glutathione-S-transferase M1

CONTEXT

Exposure to ozone has acute respiratory effects, but few human clinical studies have evaluated cardiovascular effects.

OBJECTIVE

We hypothesized that ozone exposure alters pulmonary and systemic vascular function, and cardiac function, with more pronounced effects in subjects with impaired antioxidant defense from deletion of the glutathione-S-transferase M1 gene (GSTM1 null).

METHODS

Twenty-four young, healthy never-smoker subjects (12 GSTM1 null) inhaled filtered air, 100 ppb ozone and 200 ppb ozone for 3 h, with intermittent exercise, in a double-blind, randomized, crossover fashion. Exposures were separated by at least 2 weeks. Vital signs, spirometry, arterial and venous blood nitrite levels, impedance cardiography, peripheral arterial tonometry, estimation of pulmonary capillary blood volume (Vc), and blood microparticles and platelet activation were measured at baseline and during 4 h after each exposure.

RESULTS

Ozone inhalation decreased lung function immediately after exposure (mean ± standard error change in FEV1, air: -0.03 ± 0.04 L; 200 ppb ozone: -0.30 ± 0.07 L; p < 0.001). The immediate post-exposure increase in blood pressure, caused by the final 15-min exercise period, was blunted by 200 ppb ozone exposure (mean ± standard error change for air: 16.7 ± 2.6 mmHg; 100 ppb ozone: 14.5 ± 2.4 mmHg; 200 ppb ozone: 8.5 ± 2.5 mmHg; p = 0.02). We found no consistent effects of ozone on any other measure of cardiac or vascular function. All results were independent of the GSTM1 genotype.

CONCLUSIONS

We did not find convincing evidence for early acute adverse cardiovascular consequences of ozone exposure in young healthy adults. The ozone-associated blunting of the blood pressure response to exercise is of unclear clinical significance.

Asphyxia and therapeutic hypothermia modulate plasma nitrite concentrations and carotid vascular resistance in preterm fetal sheep

In this study, we tested the hypothesis that cerebral hypoperfusion after asphyxia and induced hypothermia is associated with reduced circulating nitrite levels as an index of nitric oxide synthase (NOS) activity. The preterm fetal sheep at 0.7 gestation (103-104 days, term = 147 days) received 25-minute umbilical cord occlusion, followed by mild whole-body cooling from 30 minutes to 72 hours after occlusion. Occlusion and induced hypothermia were independently associated with reduced carotid vascular conductance (CaVC) from 2 to 72 hours, and with transiently suppressed plasma nitrite levels at 6 hours. There was a significant within-subjects correlation (r(2) = 0.33, P = .002) between CaVC and plasma nitrite values in the first 24 hours after occlusion but not after sham occlusion. These findings suggest that in preterm fetal sheep, changes in NOS activity are an important mediator of changes in carotid vascular tone in the early recovery phase after asphyxia and may help mediate some of the vascular effects of induced hypothermia.

Brachial artery reactivity in patients with severe sepsis: an observational study

Introduction

Ultrasound measurements of brachial artery reactivity in response to stagnant ischemia provide estimates of microvascular function and conduit artery endothelial function. We hypothesized that brachial artery reactivity would independently predict severe sepsis and severe sepsis mortality.

Methods

This was a combined case-control and prospective cohort study. We measured brachial artery reactivity in 95 severe sepsis patients admitted to the medical and surgical intensive care units of an academic medical center and in 52 control subjects without acute illness. Measurements were compared in severe sepsis patients versus control subjects and in severe sepsis survivors versus nonsurvivors. Multivariable analyses were also conducted.

Results

Hyperemic velocity (centimeters per cardiac cycle) and flow-mediated dilation (percentage) were significantly lower in severe sepsis patients versus control subjects (hyperemic velocity: severe sepsis = 34 (25 to 48) versus controls = 63 (52 to 81), P < 0.001; flow-mediated dilation: severe sepsis = 2.65 (0.81 to 4.79) versus controls = 4.11 (3.06 to 6.78), P < 0.001; values expressed as median (interquartile range)). Hyperemic velocity, but not flow-mediated dilation, was significantly lower in hospital nonsurvivors versus survivors (hyperemic velocity: nonsurvivors = 25 (16 to 28) versus survivors = 39 (30 to 50), P < 0.001; flow-mediated dilation: nonsurvivors = 1.90 (0.68 to 3.41) versus survivors = 2.96 (0.91 to 4.86), P = 0.12). Lower hyperemic velocity was independently associated with hospital mortality in multivariable analysis (odds ratio = 1.11 (95% confidence interval = 1.04 to 1.19) per 1 cm/cardiac cycle decrease in hyperemic velocity; P = 0.003).

Conclusions

Brachial artery hyperemic blood velocity is a noninvasive index of microvascular function that independently predicts mortality in severe sepsis. In contrast, brachial artery flow-mediated dilation, reflecting conduit artery endothelial function, was not associated with mortality in our severe sepsis cohort. Brachial artery hyperemic velocity may be a useful measurement to identify patients who could benefit from novel therapies designed to reverse microvascular dysfunction in severe sepsis and to assess the physiologic efficacy of these treatments.

Nitric oxide formation versus scavenging: the red blood cell balancing act

Nitric oxide (NO) is a key modulator of vascular homeostasis controlling critical functions related to blood flow, respiration, cell death and proliferation, and protecting the vasculature from pro-inflammatory and coagulative stresses. Inhibition of NO formation, and/or diversion of NO away from its physiological signalling targets lead to dysregulated NO bioavailability, a hallmark of numerous vascular and pulmonary diseases. Current concepts suggest that the balance between NO formation and NO scavenging is critical in disease development, with the corollary being that redressing the balance offers a target for therapeutic intervention. Evidence presented over the last two decades has seen red blood cells (RBCs) and haemoglobin specifically emerge as prominent effectors in this paradigm. In this symposium review article, we discuss recent insights into the mechanisms by which RBCs may modulate the balance between NO-formation and inhibition. We discuss how these mechanisms may become dysfunctional to cause disease, highlight key questions that remain, and discuss the potential impact of these insights on therapeutic opportunities.

The ratio of arginine to dimethylarginines is reduced and predicts outcomes in patients with severe sepsis

OBJECTIVES

Arginine deficiency may contribute to microvascular dysfunction, but previous studies suggest that arginine supplementation may be harmful in sepsis. Systemic arginine availability can be estimated by measuring the ratio of arginine to its endogenous inhibitors, asymmetric and symmetric dimethylarginine. We hypothesized that the arginine-to-dimethylarginine ratio is reduced in patients with severe sepsis and associated with severity of illness and outcomes.

DESIGN

Case-control and prospective cohort study.

SETTING

Medical and surgical intensive care units of an academic medical center.

PATIENTS AND SUBJECTS

One hundred nine severe sepsis and 50 control subjects.

MEASUREMENTS AND MAIN RESULTS

Plasma and urine were obtained in control subjects and within 48 hrs of diagnosis in severe sepsis patients. The arginine-to-dimethylarginine ratio was higher in control subjects vs. sepsis patients (median, 95; interquartile range, 85-114; vs. median, 34; interquartile range, 24-48; p < .001) and in hospital survivors vs. nonsurvivors (median, 39; interquartile range, 26-52; vs. median, 27; interquartile range, 19-32; p = .004). The arginine-to-dimethylarginine ratio was correlated with Acute Physiology and Chronic Health Evaluation II score (Spearman's correlation coefficient [ρ] = - 0.40; p < .001) and organ-failure free days (ρ = 0.30; p = .001). A declining arginine-to-dimethylarginine ratio was independently associated with hospital mortality (odds ratio, 1.63 per quartile; 95% confidence interval, 1.00-2.65; p = .048) and risk of death over the course of 6 months (hazard ratio, 1.41 per quartile; 95% confidence interval, 1.01-1.98; p = .043). The arginine-to-dimethylarginine ratio was correlated with the urinary nitrate-to-creatinine ratio (ρ = 0.46; p < .001).

CONCLUSIONS

The arginine-to-dimethylarginine ratio is associated with severe sepsis, severity of illness, and clinical outcomes. The arginine-to-dimethylarginine ratio may be a useful biomarker, and interventions designed to augment systemic arginine availability in severe sepsis may still be worthy of investigation.

Effects of T- and R-state stabilization on deoxyhemoglobin-nitrite reactions and stimulation of nitric oxide signaling

Recent data suggest that transitions between the relaxed (R) and tense (T) state of hemoglobin control the reduction of nitrite to nitric oxide (NO) by deoxyhemoglobin. This reaction may play a role in physiologic NO homeostasis and be a novel consideration for the development of the next generation of hemoglobin-based blood oxygen carriers (HBOCs, i.e. artificial blood substitutes). Herein we tested the effects of chemical stabilization of bovine hemoglobin in either the T- (THb) or R-state (RHb) on nitrite-reduction kinetics, NO-gas formation and ability to stimulate NO-dependent signaling. These studies were performed over a range of fractional saturations that is expected to mimic biological conditions. The initial rate for nitrite-reduction decreased in the following order RHb>bHb>THb, consistent with the hypothesis that the rate constant for nitrite reduction is faster with R-state Hb and slower with T-state Hb. Moreover, RHb produced more NO-gas and inhibited mitochondrial respiration more potently than both bHb and THb. Interestingly, at low oxygen fractional saturations, THb produced more NO and stimulated nitrite-dependent vasodilation more potently than bHb despite both derivatives having similar initial rates for nitrite reduction and a more negative reduction potential in THb versus bHb. These data suggest that cross-linking of bovine hemoglobin in the T-state conformation leads to a more effective coupling of nitrite reduction to NO-formation. Our results support the model of allosteric regulation of nitrite reduction by deoxyhemoglobin and show that cross-linking hemoglobins in distinct quaternary states can generate products with increased NO yields from nitrite reduction that could be harnessed to promote NO-signaling in vivo.

The potential role of the red blood cell in nitrite-dependent regulation of blood flow

Nitrite was once thought to have little physiological relevance. However, nitrite is now being increasingly recognized as a therapeutic or possibly even physiological precursor of nitric oxide (NO) that is utilized when needed to increase blood flow. It is likely that different mechanisms for nitrite bioconversion occur in different tissues, but in the vascular system, there is evidence that erythrocyte haemoglobin (Hb) is responsible for the oxygen-dependent reduction of nitrite to modulate blood flow. Here, we review the complex chemical interactions of Hb and nitrite and discuss evidence supporting its role in vasodilation. We also discuss ongoing work focused on defining the precise mechanisms for export of NO activity from red blood cells and of other pathways that may mediate nitrite-dependent vasodilation.

Erythrocyte-dependent regulation of human skeletal muscle blood flow: role of varied oxyhemoglobin and exercise on nitrite, S-nitrosohemoglobin, and ATP

The erythrocyte is proposed to play a key role in the control of local tissue perfusion via three O(2)-dependent signaling mechanisms: 1) reduction of circulating nitrite to vasoactive NO, 2) S-nitrosohemoglobin (SNO-Hb)-dependent vasodilatation, and 3) release of the vasodilator and sympatholytic ATP; however, their relative roles in vivo remain unclear. Here we evaluated each mechanism to gain insight into their roles in the regulation of human skeletal muscle blood flow during hypoxia and hyperoxia at rest and during exercise. Arterial and femoral venous hemoglobin O(2) saturation (O(2)Hb), plasma and erythrocyte NO and ATP metabolites, and leg and systemic hemodynamics were measured in 10 healthy males exposed to graded hypoxia, normoxia, and graded hyperoxia both at rest and during submaximal one-legged knee-extensor exercise. At rest, leg blood flow and NO and ATP metabolites in plasma and erythrocytes remained unchanged despite large alterations in O(2)Hb. During exercise, however, leg and systemic perfusion and vascular conductance increased in direct proportion to decreases in arterial and venous O(2)Hb (r(2) = 0.86-0.98; P = 0.01), decreases in venous plasma nitrite (r(2) = 0.93; P < 0.01), increases in venous erythrocyte nitroso species (r(2) = 0.74; P < 0.05), and to a lesser extent increases in erythrocyte SNO (r(2) = 0.59; P = 0.07). No relationship was observed with plasma ATP (r(2) = 0.01; P = 0.99) or its degradation compounds. These in vivo data indicate that, during low-intensity exercise and hypoxic stress, but not hypoxic stress alone, plasma nitrite consumption and formation of erythrocyte nitroso species are associated with limb vasodilatation and increased blood flow in the human skeletal muscle vasculature.