Venular endothelium-derived NO can affect paired arteriole: a computational model

Multiscale homogenized constrained mixture model of the bio-chemo-mechanics of soft tissue growth and remodeling

Constrained mixture models have successfully simulated many cases of growth and remodeling in soft biological tissues. So far, extensions of these models have been proposed to include either intracellular signaling or chemo-mechanical coupling on the organ-scale. However, no version of constrained mixture models currently exists that includes both aspects. Here, we propose such a version that resolves cellular signal processing by a set of logic-gated ordinary differential equations and captures chemo-mechanical interactions between cells by coupling a reaction-diffusion equation with the equations of nonlinear continuum mechanics. To demonstrate the potential of the model, we present 2 case studies within vascular solid mechanics: (i) the influence of angiotensin II on aortic growth and remodeling and (ii) the effect of communication between endothelial and intramural arterial cells via nitric oxide and endothelin-1.

Lingering Dynamics of Type 2 Diabetes Mellitus Red Blood Cells in Retinal Arteriolar Bifurcations

It has been proven that the deformability of red blood cells (RBC) is reduced owing to changes in mechanical properties, such as diabetes mellitus and hypertension. To probe the effects of RBC morphological and physical parameters on the flow field in bifurcated arterioles, three types of RBC models with various degrees of biconcave shapes were built based on the in vitro experimental data. The dynamic behaviors of the RBCs in shear flow were simulated to validate the feasibility of the finite element-Arbitrary Lagrangian-Eulerian method with a moving mesh. The influences of the shear rate and viscosity ratios on RBC motions were investigated. The motion of RBCs in arteriolar bifurcations was further simulated. Abnormal variations in the morphological and physical parameters of RBCs may lead to diminished tank-tread motion and enhanced tumbling motion in shear flow. Moreover, abnormal RBC variations can result in slower RBC motion at the bifurcation with a longer transmit time and greater flow resistance, which may further cause inadequate local oxygen supply. These findings would provide useful insights into the microvascular complications in diabetes mellitus.

Dynamic changes in retinal vessel diameter during acute hyperglycemia in type 1 diabetes

AIMS

To investigate changes in retinal vessel diameter during acute hyperglycemia in patients with type 1 diabetes.

METHODS

We conducted a study on 11 subjects with type 1 diabetes. Euglycemia was maintained for 3h followed by induction of hyperglycemia and simultaneous bolus of rapid acting insulin. Two fundus photos were captured during euglycemia and five fundus photos, blood glucose and blood pressure were taken every 30min for 2.5h post-prandial. Central retinal artery equivalent (CRAE) and central retinal vein equivalent (CRVE) were measured over the study visit and examined using generalized linear mixed models.

RESULTS

In a multivariate mixed model, mean CRAE and CRVE were reduced at 90min post-prandial in both zones B and C. In repeated measures analysis, arterioles exhibited a significant association with change in vessel caliber per change in blood glucose. Inconsistent effects of blood pressure on vessel diameter were also measured.

CONCLUSIONS

We document a change in retinal vessel diameter during acute hyperglycemia in persons with type 1 diabetes. Larger controlled studies are required to further investigate this phenomenon and to more accurately assess if hyperglycemia has direct effects on retinal vessel diameter.

Modeling of pulsatile flow-dependent nitric oxide regulation in a realistic microvascular network

Hemodynamic pulsatility has been reported to regulate microcirculatory function. To quantitatively assess the impact of flow pulsatility on the microvasculature, a mathematical model was first developed to simulate the regulation of NO production by pulsatile flow in the microcirculation. Shear stress and pressure pulsatility were selected as regulators of endothelial NO production and NO-dependent vessel dilation as feedback to control microvascular hemodynamics. The model was then applied to a real microvascular network of the rat mesentery consisting of 546 microvessels. As compared to steady flow conditions, pulsatile flow increased the average NO concentration in arterioles from 256.8±93.1nM to 274.8±101.1nM (P<0.001), with a corresponding increase in vessel dilation by approximately 7% from 27.5±10.6% to 29.4±11.4% (P<0.001). In contrast, NO concentration and vessel size showed a far lesser increase (about 1.7%) in venules under pulsatile flow as compared to steady flow conditions. Network perfusion and flow heterogeneity were improved under pulsatile flow conditions, and vasodilation within the network was more sensitive to heart rate changes than pulse pressure amplitude. The proposed model simulates the role of flow pulsatility in the regulation of a complex microvascular network in terms of NO concentration and hemodynamics under varied physiological conditions.

Diffusive shunting of gases and other molecules in the renal vasculature: physiological and evolutionary significance

Countercurrent systems have evolved in a variety of biological systems that allow transfer of heat, gases, and solutes. For example, in the renal medulla, the countercurrent arrangement of vascular and tubular elements facilitates the trapping of urea and other solutes in the inner medulla, which in turn enables the formation of concentrated urine. Arteries and veins in the cortex are also arranged in a countercurrent fashion, as are descending and ascending vasa recta in the medulla. For countercurrent diffusion to occur, barriers to diffusion must be small. This appears to be characteristic of larger vessels in the renal cortex. There must also be gradients in the concentration of molecules between afferent and efferent vessels, with the transport of molecules possible in either direction. Such gradients exist for oxygen in both the cortex and medulla, but there is little evidence that large gradients exist for other molecules such as carbon dioxide, nitric oxide, superoxide, hydrogen sulfide, and ammonia. There is some experimental evidence for arterial-to-venous (AV) oxygen shunting. Mathematical models also provide evidence for oxygen shunting in both the cortex and medulla. However, the quantitative significance of AV oxygen shunting remains a matter of controversy. Thus, whereas the countercurrent arrangement of vasa recta in the medulla appears to have evolved as a consequence of the evolution of Henle’s loop, the evolutionary significance of the intimate countercurrent arrangement of blood vessels in the renal cortex remains an enigma.

Endothelial NO and O₂·⁻ production rates differentially regulate oxidative, nitroxidative, and nitrosative stress in the microcirculation

Endothelial dysfunction causes an imbalance in endothelial NO and O₂·⁻ production rates and increased peroxynitrite formation. Peroxynitrite and its decomposition products cause multiple deleterious effects including tyrosine nitration of proteins, superoxide dismutase (SOD) inactivation, and tissue damage. Studies have shown that peroxynitrite formation during endothelial dysfunction is strongly dependent on the NO and O₂·⁻ production rates. Previous experimental and modeling studies examining the role of NO and O₂·⁻ production imbalance on peroxynitrite formation showed different results in biological and synthetic systems. However, there is a lack of quantitative information about the formation and biological relevance of peroxynitrite under oxidative, nitroxidative, and nitrosative stress conditions in the microcirculation. We developed a computational biotransport model to examine the role of endothelial NO and O₂·⁻ production on the complex biochemical NO and O₂·⁻ interactions in the microcirculation. We also modeled the effect of variability in SOD expression and activity during oxidative stress. The results showed that peroxynitrite concentration increased with increase in either O₂·⁻ to NO or NO to O₂·⁻ production rate ratio (QO₂·⁻/QNO or QNO/QO₂·⁻, respectively). The peroxynitrite concentrations were similar for both production rate ratios, indicating that peroxynitrite-related nitroxidative and nitrosative stresses may be similar in endothelial dysfunction or inducible NO synthase (iNOS)-induced NO production. The endothelial peroxynitrite concentration increased with increase in both QO₂·⁻/QNO and QNO/QO₂·⁻ ratios at SOD concentrations of 0.1-100 μM. The absence of SOD may not mitigate the extent of peroxynitrite-mediated toxicity, as we predicted an insignificant increase in peroxynitrite levels beyond QO₂·⁻/QNO and QNO/QO₂·⁻ ratios of 1. The results support the experimental observations of biological systems and show that peroxynitrite formation increases with increase in either NO or O₂·⁻ production, and excess NO production from iNOS or from NO donors during oxidative stress conditions does not reduce the extent of peroxynitrite mediated toxicity.

Retinal microvascular responses to short-term changes in particulate air pollution in healthy adults

BACKGROUND

Microcirculation plays an important role in the physiology of cardiovascular health. Air pollution is an independent risk factor for the development and progression of cardiovascular diseases, but the number of studies on the relation between air pollution and the microcirculation is limited.

OBJECTIVES

We examined the relationship between short-term changes in air pollution and microvascular changes.

METHODS

We measured retinal microvasculature using fundus image analysis in a panel of 84 healthy adults (52% female), 22-63 years of age, during January-May 2012. Blood vessels were measured as central retinal arteriolar/venular equivalent (CRAE/CRVE), with a median of 2 measurements (range, 1-3). We used monitoring data on particulate air pollution (PM10) and black carbon (BC). Mixed-effect models were used to estimate associations between CRAE/CRVE and exposure to PM10 and BC using various exposure windows.

RESULTS

CRAE and CRVE were associated with PM10 and BC concentrations, averaged over the 24 hr before the retinal examinations. Each 10-µg/m3 increase in PM10 was associated with a 0.93-µm decrease (95% CI: -1.42, -0.45; p = 0.0003) in CRAE and a 0.86-µm decrease (95% CI: -1.42, -0.30; p = 0.004) in CRVE after adjusting for individual characteristics and time varying conditions such as ambient temperature. Each 1-µg/m3 increase in BC was associated with a 1.84-µm decrease (95% CI: -3.18, -0.51; p < 0.001) in CRAE.

CONCLUSIONS

Our findings suggest that the retinal microvasculature responds to short-term changes in air pollution levels. These results support a mechanistic pathway through which air pollution can act as a trigger of cardiovascular events at least in part through effects on the microvasculature.

Local oxidative and nitrosative stress increases in the microcirculation during leukocytes-endothelial cell interactions

Leukocyte-endothelial cell interactions and leukocyte activation are important factors for vascular diseases including nephropathy, retinopathy and angiopathy. In addition, endothelial cell dysfunction is reported in vascular disease condition. Endothelial dysfunction is characterized by increased superoxide (O₂^•−) production from endothelium and reduction in NO bioavailability. Experimental studies have suggested a possible role for leukocyte-endothelial cell interaction in the vessel NO and peroxynitrite levels and their role in vascular disorders in the arterial side of microcirculation. However, anti-adhesion therapies for preventing leukocyte-endothelial cell interaction related vascular disorders showed limited success. The endothelial dysfunction related changes in vessel NO and peroxynitrite levels, leukocyte-endothelial cell interaction and leukocyte activation are not completely understood in vascular disorders. The objective of this study was to investigate the role of endothelial dysfunction extent, leukocyte-endothelial interaction, leukocyte activation and superoxide dismutase therapy on the transport and interactions of NO, O₂^•− and peroxynitrite in the microcirculation. We developed a biotransport model of NO, O₂^•− and peroxynitrite in the arteriolar microcirculation and incorporated leukocytes-endothelial cell interactions. The concentration profiles of NO, O₂^•− and peroxynitrite within blood vessel and leukocytes are presented at multiple levels of endothelial oxidative stress with leukocyte activation and increased superoxide dismutase accounted for in certain cases. The results showed that the maximum concentrations of NO decreased ∼0.6 fold, O₂^•− increased ∼27 fold and peroxynitrite increased ∼30 fold in the endothelial and smooth muscle region in severe oxidative stress condition as compared to that of normal physiologic conditions. The results show that the onset of endothelial oxidative stress can cause an increase in O₂^•− and peroxynitrite concentration in the lumen. The increased O₂^•− and peroxynitrite can cause leukocytes priming through peroxynitrite and leukocytes activation through secondary stimuli of O₂^•− in bloodstream without endothelial interaction. This finding supports that leukocyte rolling/adhesion and activation are independent events.

Nitric oxide signaling in the microcirculation

Several apparent paradoxes are evident when one compares mathematical predictions from models of nitric oxide (NO) diffusion and convection in vasculature structures with experimental measurements of NO (or related metabolites) in animal and human studies. Values for NO predicted from mathematical models are generally much lower than in vivo NO values reported in the literature for experiments, specifically with NO microelectrodes positioned at perivascular locations next to different sizes of blood vessels in the microcirculation and NO electrodes inserted into a wide range of tissues supplied by the microcirculation of each specific organ system under investigation. There continues to be uncertainty about the roles of NO scavenging by hemoglobin versus a storage function that may conserve NO, and other signaling targets for NO need to be considered. This review describes model predictions and relevant experimental data with respect to several signaling pathways in the microcirculation that involve NO.

3D network model of NO transport in tissue

We developed a mathematical model to simulate shear stress-dependent nitric oxide (NO) production and transport in a 3D microcirculatory network based on published data. The model consists of a 100 μm × 500 μm × 75 μm rectangular volume of tissue containing two arteriole-branching trees, and nine capillaries surrounding the vessels. Computed distributions for NO in blood, vascular walls, and surrounding tissue were affected by hematocrit (Hct) and wall shear stress (WSS) in the network. The model demonstrates that variations in the red blood cell (RBC) distribution and WSS in a branching network can have differential effects on computed NO concentrations due to NO consumption by RBCs and WSS-dependent changes in NO production. The model predicts heterogeneous distributions of WSS in the network. Vessel branches with unequal blood flow rates gave rise to a range of WSS values and therefore NO production rates. Despite increased NO production in a branch with higher blood flow and WSS, vascular wall NO was predicted to be lower due to greater NO consumption in blood, since the microvascular Hct increased with redistribution of RBCs at the vessel bifurcation. Within other regions, low WSS was combined with decreased NO consumption to enhance the NO concentration.

Spatial and temporal coordination of bone marrow-derived cell activity during arteriogenesis: regulation of the endogenous response and therapeutic implications

Arterial occlusive disease is the leading cause of morbidity and mortality throughout the developed world, which creates a significant need for effective therapies to halt disease progression. Despite success of animal and small-scale human therapeutic arteriogenesis studies, this promising concept for treating arterial occlusive disease has yielded largely disappointing results in large-scale clinical trials. One reason for this lack of successful translation is that endogenous arteriogenesis is highly dependent on a poorly understood sequence of events and interactions between bone marrow derived cells (BMCs) and vascular cells, which makes designing effective therapies difficult. We contend that the process follows a complex, ordered sequence of events with multiple, specific BMC populations recruited at specific times and locations. Here, we present the evidence suggesting roles for multiple BMC populations-from neutrophils and mast cells to progenitor cells-and propose how and where these cell populations fit within the sequence of events during arteriogenesis. Disruptions in these various BMC populations can impair the arteriogenesis process in patterns that characterize specific patient populations. We propose that an improved understanding of how arteriogenesis functions as a system can reveal individual BMC populations and functions that can be targeted for overcoming particular impairments in collateral vessel development.

A computational model for nitric oxide, nitrite and nitrate biotransport in the microcirculation: effect of reduced nitric oxide consumption by red blood cells and blood velocity

Bioavailability of vasoactive endothelium-derived nitric oxide (NO) in vasculature is a critical factor in regulation of many physiological processes. Consumption of NO by RBC plays a crucial role in maintaining NO bioavailability. Recently, Deonikar and Kavdia (2009b) reported an effective NO-RBC reaction rate constant of 0.2×10(5)M(-1)s(-1) that is ~7 times lower than the commonly used NO-RBC reaction rate constant of 1.4×10(5)M(-1)s(-1). To study the effect of lower NO-RBC reaction rate constant and nitrite and nitrate formation (products of NO metabolism in blood), we developed a 2D mathematical model of NO biotransport in 50 and 200μm ID arterioles to calculate NO concentration in radial and axial directions in the vascular lumen and vascular wall of the arterioles. We also simulated the effect of blood velocity on NO distribution in the arterioles to determine whether NO can be transported to downstream locations in the arteriolar lumen. The results indicate that lowering the NO-RBC reaction rate constant increased the NO concentration in the vascular lumen as well as the vascular wall. Increasing the velocity also led to increase in NO concentration. We predict increased NO concentration gradient along the axial direction with an increase in the velocity. The predicted NO concentration was 281-1163nM in the smooth muscle cell layer for 50μm arteriole over the blood velocity range of 0.5-4cms(-1) for k(NO-RBC) of 0.2×10(5)M(-1)s(-1), which is much higher than the reported values from earlier mathematical modeling studies. The NO concentrations are similar to the experimentally measured vascular wall NO concentration range of 300-1000nM in several different vascular beds. The results are significant from the perspective that the downstream transport of NO is possible under the right circumstances.

Distributed self-regulation of living tissue: beyond the ideal limit

The present paper is devoted to mathematical description of the vascular network response to local perturbations in the cellular tissue state, being one of the basic mechanisms controlling the inner environment of human body. Keeping in mind individual organs we propose a model for distributed self-regulation of living tissue, which is regarded as an active hierarchical system without any controlling center. This model is based on the self-processing of information about the cellular tissue state and cooperative interaction of blood vessels governing redistribution of blood flow over the vascular network. The information self-processing is implemented via mass conservation, i.e., conservation of blood flow as well as special biochemical compounds called activators transported by blood. The cooperative interaction of blood vessels stems from the response of individual vessels to activators in blood flowing through them. The general regularities are used to specify the vessel behavior. The arterial and venous beds are considered to be individually of the tree form. The constructed governing equations are analyzed numerically. In particular, first, we show that the blood perfusion rate approximately (in the analyzed case within 10% accuracy) depends only on the local concentration of activators in the cellular tissue. It is due to the hierarchical structure of the vascular network rather than the ideal behavior of individual vessels accepted previously. Second, we demonstrate the distinction between the reaction thresholds of individual vessels and that of the vascular network as a whole. The latter effect is the cause for introducing the notion of activators instead of using such quantities as temperature in describing the living tissue self-regulation.

Extracellular diffusion and permeability effects on NO-RBCs interactions using an experimental and theoretical model

Nitric oxide (NO) is a potent vasodilator and its homeostasis depends on interaction with RBCs. A key factor in understanding NO-RBC interactions in vascular lumen is a comprehensive analysis of product identification and quantification. In this context, administration of NO during in vitro NO-RBC interactions becomes a crucial variable. In this study, we designed a bioreactor that maintains a precise NO concentration in the headspace that diffuses to RBCs suspension to study the quantitative effect of NO concentration and hematocrit (Hct) on NO-RBC interactions. The products of NO-RBC reaction (nitrite and total nitrogen species (total NOx)) were measured by chemiluminescence assay. A mathematical model simulating NO biotransport to a single RBC was developed to (1) estimate NO-RBC reaction rate constant; (2) predict the NO concentrations in the bulk RBC suspension and at the RBC membrane for RBC membrane NO permeability (P(m)) values of 0.0415-40 cm/s. Measured nitrite and total NOx concentrations increased with increase in headspace NO concentration whereas nitrite concentrations decreased with hematocrit and total NOx concentrations increased with hematocrit. This indicates that the extracellular resistance is a controlling factor for RBC uptake of NO. Modeling results showed that the effective reaction rate constant (k(eff)) for NO-RBC interactions was 2.32 x 10(4)-1.08 x 10(6) M(-1) s(-1). Results also predict that the membrane permeability in the range of 0.0415-0.4 cm/s is required to maintain physiologically relevant levels of NO at the smooth muscle cell layer. The effective reaction rate constant increased with increase in P(m) and magnitude of increase was higher at 45% Hct. For all P(m) values, the k(hb)/k(eff) ratios were lower for 45% Hct as compared to 5% Hct indicating extracellular resistance is important for RBC NO uptake. Our experimental and mathematical analyses of NO-RBC interactions indicate that both unstirred layer and RBC membrane have a significant effect on NO transport to RBCs. In addition, the membrane permeability in the range of 0.0415-0.4 cm/s is required to maintain sufficient NO concentrations at the smooth muscle cell layer.

NO/peroxynitrite dynamics of high glucose-exposed HUVECs: chemiluminescent measurement and computational model

Pathogenesis of many of diabetes-related vascular complications is associated with endothelial cell (EC) dysfunction, which is reduced bioavailability of EC-released nitric oxide (NO). Interaction dynamics of NO, superoxide (O(2)(-)) and peroxynitrite (ONOO(-)) are dependent on both their productions and consumptions through various pathways. Quantitative knowledge of these interaction dynamics in high glucose-induced EC dysfunction remains poorly understood. We developed an integrated experimental and computational approach to gain a quantitative understanding of the interactions of NO, O(2)(-) and ONOO(-) in high glucose-exposed ECs. End-products, nitrite and nitrate, were measured using a chemiluminescence analyzer. A computational biochemical reaction network model was developed to predict the effect of high glucose on ECs NO, O(2)(-) and ONOO(-). ECs NO and O(2)(-) productions increased in high glucose as evidenced by increased total NOx concentration, primarily increasing nitrate concentration. The model predicted an increase in O(2)(-) and ONOO(-) concentrations and a decrease in NO concentration in high glucose conditions. Administration of superoxide dismutase (SOD) decreased O(2)(-) concentration and increased NO concentration, thus SOD improved high glucose-induced changes in these interactions. An important finding of this study was that the NO bioavailability decreased in high glucose conditions even though NO production of EC increased. The integrated approach provides a framework to predict NO, O(2)(-) and ONOO(-) concentrations and productions that are difficult to measure in one experiment and will be useful in further EC dysfunction studies.

Nitric oxide in the vasculature: where does it come from and where does it go? A quantitative perspective

Nitric oxide (NO) affects two key aspects of O2 supply and demand: It regulates vascular tone and blood flow by activating soluble guanylate cyclase (sGC) in the vascular smooth muscle, and it controls mitochondrial O2 consumption by inhibiting cytochrome c oxidase. However, significant gaps exist in our quantitative understanding of the regulation of NO production in the vascular region. Large apparent discrepancies exist among the published reports that have analyzed the various pathways in terms of the perivascular NO concentration, the efficacy of NO in causing vasodilation (EC50), its efficacy in tissue respiration (IC50), and the paracrine and endocrine NO release. In this study, we review the NO literature, analyzing NO levels on various scales, identifying and analyzing the discrepancies in the reported data, and proposing hypotheses that can potentially reconcile these discrepancies. Resolving these issues is highly relevant to improving our understanding of vascular biology and to developing pharmaceutical agents that target NO pathways, such as vasodilating drugs.

Impact of superoxide dismutase on nitric oxide and peroxynitrite levels in the microcirculation--a computational model

Interactions of free radicals such as superoxide (O2-), nitric oxide (NO), and peroxynitrite (ONOO-) are important in pathophysiological conditions such as hypertension, atherosclerosis, diabetes and the resulting cardiovascular diseases. Excessive levels of superoxide during oxidative stress cause a reduction in NO bioavailability by forming peroxynitrite and resulting in endothelial dysfunction. Superoxide dismutase (SOD) competes with NO for superoxide, and reduces the formation of peroxynitrite. In this study, we developed a mathematical model for free radical transport within and around an arteriolar vessel based on the fundamental principles of mass balance, reaction kinetics, and vascular geometry. We used the model to study the effect of the three types of SOD, viz. CuZn-SOD, Mn-SOD and extra cellular-SOD, on the bioavailability of NO. Results indicate that SOD location and concentration in the arteriole significantly affect superoxide concentration. The model predicts that a reduction in SOD levels results in increased superoxide and peroxynitrite concentrations and decreased NO concentration in the vessel. The results also suggest a role of SOD in the amelioration of oxidative stress and NO bioavailability in microcirculation. This model will help in furthering our knowledge of endothelial dysfunction in pathological conditions and the impact of specific SODs on free radical interactions.

Nitric oxide from nitrite reduction by hemoglobin in the plasma and erythrocytes

Experimental evidence has shown that nitrite anion plays a key role in one of the proposed mechanisms for hypoxic vasodilation, in which the erythrocyte acts as a NO generator and deoxygenated hemoglobin in pre-capillary arterioles reduces nitrite to NO, which contributes to vascular smooth muscle relaxation. However, because of the complex reactions among nitrite, hemoglobin, and the NO that is formed, the amount of NO delivered by this mechanism under various conditions has not been quantified experimentally. Furthermore, paracrine NO is scavenged by cell-free hemoglobin, as shown by studies of diseases characterized by extensive hemolysis (e.g., sickle cell disease) and the administration of hemoglobin-based oxygen carriers. Taking into consideration the free access of cell-free hemoglobin to the vascular wall and its ability to act as a nitrite reductase, we have now examined the hypothesis that in hypoxia this cell-free hemoglobin could serve as an additional endocrine source of NO. In this study, we constructed a multicellular model to characterize the amount of NO delivered by the reaction of nitrite with both intraerythrocytic and cell-free hemoglobin, while intentionally neglecting all other possible sources of NO in the vasculature. We also examined the roles of hemoglobin molecules in each compartment as nitrite reductases and NO scavengers using the model. Our calculations show that: (1) approximately 0.04pM NO from erythrocytes could reach the smooth muscle if free diffusion were the sole export mechanism; however, this value could rise to approximately 43pM with a membrane-associated mechanism that facilitated NO release from erythrocytes; the results also strongly depend on the erythrocyte membrane permeability to NO; (2) despite the closer proximity of cell-free hemoglobin to the smooth muscle, cell-free hemoglobin reaction with nitrite generates approximately 0.02pM of free NO that can reach the vascular wall, because of a strong self-capture effect. However, it is worth noting that this value is in the same range as erythrocytic hemoglobin-generated NO that is able to diffuse freely out of the cell, despite the tremendous difference in hemoglobin concentration in both cases (microM hemoglobin in plasma vs. mM in erythrocyte); (3) intraerythrocytic hemoglobin encapsulated by a NO-resistant membrane is the major source of NO from nitrite reduction, and cell-free hemoglobin is a significant scavenger of both paracrine and endocrine NO.

Vascular smooth muscle NO exposure from intraerythrocytic SNOHb: a mathematical model

We previously constructed computational models based on the biochemical pathway analysis of different nitric oxide (NO) synthase isoforms and found a large discrepancy between our predictions and perivascular NO measurements, suggesting the existence of nonenzymatic sources of NO. S-nitrosohemoglobin (SNOHb) has been suggested as a major source to release NO in the arteriolar lumen and induce hypoxic vasodilation. In the present study, we formulated a multicellular computational model to quantify NO exposure in arteriolar smooth muscle when the NO released by intraerythrocytic SNOHb is the sole NO source in the vasculature. Our calculations show an NO exposure of approximately 0.25-6 pM in the smooth muscle region. This amount does not account for the large discrepancy we encountered regarding perivascular NO levels. We also found that the amount of NO delivered by SNOHb to smooth muscle strongly depends on the SNOHb concentration and half-life, which further determine the rate of NO release, as well as on the membrane permeability of red blood cells (RBCs) to NO. In conclusion, our mathematical model predicts that picomolar amounts of NO can be delivered to the vascular smooth muscle by intraerythrocytic SNOHb; this amount of NO alone appears not sufficient to induce the hypoxic vasodilation.

A model of NO/O2 transport in capillary-perfused tissue containing an arteriole and venule pair

The goal of this study was to investigate the complex co-transport of nitric oxide (NO) and oxygen (O2) in a paired arteriole-venule, surrounded by capillary-perfused tissue using a computer model. Blood flow was assumed to be steady in the arteriolar and venular lumens and to obey Darcy's law in the tissue. NO consumption rate was assumed to be constant in the core of the arteriolar and venular lumen and to decrease linearly to the endothelium. Average NO consumption rate by capillary blood in a unit tissue volume was assumed proportional to the blood flux across the volume. Our results predict that: (1) the capillary bed, which connects the arteriole and venule, facilitates the release of O2 from the vessel pair to the surrounding tissue; (2) decreasing the distance between arteriole and venule can result in a higher NO concentration in the venular wall than in the arteriolar wall; (3) in the absence of capillaries in the surrounding tissue, diffusion of NO from venule to arteriole contributes little to NO concentration in the arteriolar wall; and (4) when capillaries are added to the simulation, a significant increase of NO in the arteriolar wall is observed.

Computation of plasma hemoglobin nitric oxide scavenging in hemolytic anemias

Intravascular hemoglobin limits the amount of endothelial-derived nitric oxide (NO) available for vasodilation. Cell-free hemoglobin scavenges NO more efficiently than red blood cell-encapsulated hemoglobin. Hemolysis has recently been suggested to contribute to endothelial dysfunction based on a mechanism of NO scavenging by cell-free hemoglobin. Although experimental evidence for this phenomenon has been presented, support from a theoretical approach has, until now, been missing. Indeed, due to the low amounts of cell-free hemoglobin present in these pathological conditions, the role of cell-free hemoglobin scavenging of NO in disease has been questioned. In this study, we model the effects of cell-free hemoglobin on NO bioavailability, focusing on conditions that closely mimic those under known pathological conditions. We find that as little as 1 microM cell-free intraluminal hemoglobin (heme concentration) can significantly reduce NO bioavailability. In addition, extravasation of hemoglobin out of the lumen has an even greater effect. We also find that low hematocrit associated with anemia increases NO bioavailability but also leads to increased susceptibility to NO scavenging by cell-free hemoglobin. These results support the paradigm that cell-free hemoglobin released into plasma during intravascular hemolysis in human disease contributes to the experimentally observed reduction in NO bioavailability and endothelial dysfunction.

Nitrite as a vascular endocrine nitric oxide reservoir that contributes to hypoxic signaling, cytoprotection, and vasodilation

Accumulating evidence suggests that the simple and ubiquitous anion salt, nitrite (NO(2)(-)), is a physiological signaling molecule with potential roles in intravascular endocrine nitric oxide (NO) transport, hypoxic vasodilation, signaling, and cytoprotection after ischemia-reperfusion. Human and animal studies of nitrite treatment and NO gas inhalation provide evidence that nitrite mediates many of the systemic therapeutic effects of NO gas inhalation, including peripheral vasodilation and prevention of ischemia-reperfusion-mediated tissue infarction. With regard to nitrite-dependent hypoxic signaling, biochemical and physiological studies suggest that hemoglobin possesses an allosterically regulated nitrite reductase activity that reduces nitrite to NO along the physiological oxygen gradient, potentially contributing to hypoxic vasodilation. An expanded consideration of nitrite as a hypoxia-dependent intrinsic signaling molecule has opened up a new field of research and therapeutic opportunities for diseases associated with regional hypoxia and vasoconstriction.

How do red blood cells cause hypoxic vasodilation? The SNO-hemoglobin paradigm

One of the most intriguing areas of research in erythrocyte physiology is the interaction of hemoglobin with nitric oxide (NO). These two molecules independently fulfill diverse and complex physiological roles, while together they subtly modulate microvascular perfusion in response to second-by-second changes in local metabolic demand, contributing to hypoxic vasodilation. It is through an appreciation of the temporal and structural constraints of the microcirculation that the principal requirements of the physiological interplay between NO and hemoglobin are revealed, elucidating the role of the erythrocyte in hypoxic vasodilation. Among the candidate molecular mechanisms, only S-nitrosohemoglobin (SNO-hemoglobin) directly fulfills the physiological requirements. Thus, NO is transported by red blood cells to microvascular sites of action in protected form as an S-nitrosothiol on the highly conserved hemoglobin beta-93 Cys residue, invariant in birds and mammals. SNO-hemoglobin dispenses NO bioactivity to microvascular cells on the release of oxygen, physiologically coupling hemoglobin deoxygenation to vasodilation. SNO-hemoglobin is the archetype for the role of S-nitrosylation in a newly identified class of biological signals, and disturbances in SNO-hemoglobin activity are associated with the pathogenesis of several important vascular diseases.

Leukocyte adherence inhibits adenosine-dependent venular control of arteriolar diameter and nitric oxide

Venular control of arteriolar perfusion has been the focus of several investigations in recent years. This study investigated 1) whether endogenous adenosine helps control venule-dependent arteriolar dilation and 2) whether venular leukocyte adherence limits this response via an oxidant-dependent mechanism in which nitric oxide (NO) levels are decreased. Intravital microscopy was used to assess changes in arteriolar diameters and NO levels in rat mesentery. The average resting diameter of arterioles (27.5 +/- 1.0 microm) paired with venules with minimal leukocyte adherence (2.1 +/- 0.3 per 100-microm length) was significantly larger than that of unpaired arterioles (24.5 +/- 0.8 microm) and arterioles (23.3 +/- 1.3 microm) paired with venules with higher leukocyte adherence (9.0 +/- 0.5 per 100-microm length). Local superfusion of adenosine deaminase (ADA) induced significant decreases in diameter and perivascular NO concentration in arterioles closely paired to venules with minimal leukocyte adherence. However, ADA had little effect on arterioles closely paired to venules with high leukocyte adherence or on unpaired arterioles. To determine whether the attenuated response to ADA for the high-adherence group was oxidant dependent, the responses were also observed in arterioles treated with 10(-4) M Tempol. In the high-adherence group, Tempol fully restored NO levels to those of the low-adherence group; however, the ADA-induced constriction remained attenuated, suggesting a possible role for an oxidant-independent vasoconstrictor released from the inflamed venules. These findings suggest that adenosine- and venule-dependent dilation of paired arterioles may be mediated, in part, by NO and inhibited by venular leukocyte adherence.

Heterogeneous response of microvascular endothelial cells to shear stress

We investigated changes in calcium concentration in cultured bovine aortic endothelial cells (BAECs) and rat adrenomedulary endothelial cells (RAMECs, microvascular) in response to different levels of shear stress. In BAECs, the onset of shear stress elicited a transient increase in intracellular calcium concentration that was spatially uniform, synchronous, and dose dependent. In contrast, the response of RAMECs was heterogeneous in time and space. Shear stress induced calcium waves that originated from one or several cells and propagated to neighboring cells. The number and size of the responding groups of cells did not depend on the magnitude of shear stress or the magnitude of the calcium change in the responding cells. The initiation and the propagation of calcium waves in RAMECs were significantly suppressed under conditions in which either purinergic receptors were blocked by suramin or extracellular ATP was degraded by apyrase. Exogenously applied ATP produced similarly heterogeneous responses. The number of responding cells was dependent on ATP concentration, but the magnitude of the calcium change was not. Our data suggest that shear stress stimulates RAMECs to release ATP, causing the increase in intracellular calcium concentration via purinergic receptors in cells that are heterogeneously sensitive to ATP. The propagation of the calcium signal is also mediated by ATP, and the spatial pattern suggests a locally elevated ATP concentration in the vicinity of the initially responding cells.