Plasma levels of nitrite ions have been used as an index of nitric oxide synthase (NOS) activity in vivo. Recent data suggest that nitrite is a potential intravascular repository for nitric oxide (NO), bioactivated by a nitrite reductase activity of deoxyhemoglobin. The precise levels and compartmentalization of nitrite within blood and erythrocytes have not been determined. Nitrite levels in whole blood and erythrocytes were determined using reductive chemiluminescence in conjunction with a ferricyanide-based hemoglobin oxidation assay to prevent nitrite destruction. This method yields sensitive and linear measurements of whole blood nitrite over 24 hours at room temperature. Nitrite levels measured in plasma, erythrocytes, and whole blood from 15 healthy volunteers were 121 plus or minus 9, 288 plus or minus 47, and 176 plus or minus 17 nM, indicating a surprisingly high concentration of nitrite within erythrocytes. The majority of nitrite in erythrocytes is located in the cytosol unbound to proteins. In humans, we found a significant artery-to-vein gradient of nitrite in whole blood and erythrocytes. Shear stress and acetylcholine-mediated stimulation of endothelial NOS significantly increased venous nitrite levels. These studies suggest a dynamic intravascular NO metabolism in which endothelial NOS-derived NO is stabilized as nitrite, transported by erythrocytes, and consumed during arterial-tovenous transit. IntroductionNitric oxide (NO) is a gas that is continuously synthesized in endothelial cells and executes multiple functions that maintain vascular homeostasis. In the vascular system NO is synthesized by the type III isoform of NO synthase (endothelial NOS [eNOS]). 1,2 When NO is released from the endothelium, it may diffuse abluminally into smooth muscle cells causing vasodilation; when released luminally into the bloodstream NO reacts with intraerythrocytic oxyhemoglobin to form nitrate, and a portion of the remaining NO is oxidized to nitrite. 3,4 We have recently shown that nitrite has the potential to be a major intravascular NO storage molecule in humans that is capable of transducing NO bioactivity distal to its site of formation. 5 Plasma nitrite has been described as an index of eNOS activity in the regional 6 and systemic circulation in humans and various mammals. 7 Despite the growing appreciation of an important potential role for nitrite in physiology and as a disease marker, the actual circulating levels of nitrite in humans have been difficult to measure, owing to the relative instability of nitrite in blood, as well as contaminating nitrite in clinical blood collection tubes and laboratory ware. This has resulted in reported levels ranging from undetectable 8 to 20 M. 9 A recent report identified some of the analytical problems of measuring nitrite in plasma, potentially explaining the wide range of reported levels. 7 In that study plasma nitrite was determined with 3 independent analytical methods and rapid sample preparations in 7 mammalian species, strongly suggesting that in vivo p...

Rationale: Although pulmonary hypertension (PH) is a common complication of sickle cell disease (SCD) associated with high mortality, there exist few data characterizing hemodynamics and cardiopulmonary function in this population. Objectives: To characterize hemodynamics and cardiopulmonary function in patients with SCD with and without PH. Methods: Patients with SCD with PH (n ϭ 26) were compared with control subjects with SCD but without PH (n ϭ 17), matched for age, hemoglobin levels, and fetal hemoglobin levels. Measurements and Main Results: Upon catheterization, 54% of the patients with PH had pulmonary arterial hypertension, and 46% had pulmonary venous hypertension. When compared with control subjects, patients with PH exhibited lower six-minute-walk distance (435 Ϯ 31 vs. 320 Ϯ 20 m, p ϭ 0.002) and oxygen consumption (50 Ϯ 3% vs. 41 Ϯ 2% of predicted, p ϭ 0.02), and also had mild restrictive lung disease and more perfusion abnormalities on radionuclide lung scans. The six-minute-walk distance in this population inversely correlated with tricuspid regurgitant jet velocity (r ϭ Ϫ0.55, p Ͻ 0.001), and mean pulmonary artery pressure (r ϭ Ϫ0.57, p Ͻ 0.001), and directly correlated with maximal oxygen consumption (r ϭ 0.49, p ϭ 0.004), even after adjustment for hemoglobin, supporting an independent contribution of increasing pulmonary artery pressures to loss of exercise capacity. Conclusions: Patients with SCD-associated PH have both pulmonary arterial and venous PH associated with severe limitations in exercise capacity, likely compounded by interstitial lung fibrosis and severe anemia. These data support the use of the six-minute-walk distance as an index of PH and cardiopulmonary function in patients with SCD.Keywords: sickle cell disease; pulmonary hypertension; six-minute walk; hemodynamics; echocardiogram Pulmonary arterial hypertension is an increasingly recognized complication of chronic hereditary and acquired hemolytic anemias, including sickle cell disease (SCD), thalassemia intermedia and major, paroxysmal nocturnal hemoglobinuria, hereditary spherocytosis and stomatocytosis, microangiopathic hemolytic anemias, pyruvate kinase deficiency, alloimmune hemolytic anemia, and possibly malaria (1, 2). In addition, certain conditions AT A GLANCE COMMENTARY Scientific Knowledge on the SubjectPulmonary hypertension is an emerging complication of sickle cell disease with high mortality. There are few data characterizing hemodynamics and cardiopulmonary function in this population. What This Study Adds to the FieldPatients with sickle cell disease-associated pulmonary hypertension have both pulmonary arterial and venous pulmonary hypertension associated with severe limitations in exercise capacity, likely compounded by interstitial lung fibrosis and severe anemia.are associated with intravascular hemolysis, and consequently there is the potential risk for the development of pulmonary hypertension, such as schistosomiasis (3, 4), and iatrogenic hemolysis from mechanical heart valves (5, 6), left ventricula...

The blood anion nitrite contributes to hypoxic vasodilation through a heme-based, nitric oxide (NO)-generating reaction with deoxyhemoglobin and potentially other heme proteins. We hypothesized that this biochemical reaction could be harnessed for the treatment of neonatal pulmonary hypertension, an NO-deficient state characterized by pulmonary vasoconstriction, right-to-left shunt pathophysiology and systemic hypoxemia. To test this, we delivered inhaled sodium nitrite by aerosol to newborn lambs with hypoxic and normoxic pulmonary hypertension. Inhaled nitrite elicited a rapid and sustained reduction ( approximately 65%) in hypoxia-induced pulmonary hypertension, with a magnitude approaching that of the effects of 20 p.p.m. NO gas inhalation. This reduction was associated with the immediate appearance of NO in expiratory gas. Pulmonary vasodilation elicited by aerosolized nitrite was deoxyhemoglobin- and pH-dependent and was associated with increased blood levels of iron-nitrosyl-hemoglobin. Notably, from a therapeutic standpoint, short-term delivery of nitrite dissolved in saline through nebulization produced selective, sustained pulmonary vasodilation with no clinically significant increase in blood methemoglobin levels. These data support the concept that nitrite is a vasodilator acting through conversion to NO, a process coupled to hemoglobin deoxygenation and protonation, and evince a new, simple and inexpensive potential therapy for neonatal pulmonary hypertension.