3D network model of NO transport in tissue

Influence of aortic aneurysm on the local distribution of NO and O2 using image-based computational fluid dynamics

There is a pressing need to establish novel biomarkers to predict the progression of thoracic aortic aneurysm (TAA) dilatation. Aside from hemodynamics, the roles of oxygen (O₂) and nitric oxide (NO) in TAA pathogenesis are potentially significant. As such, it is imperative to comprehend the relationship between aneurysm presence and species distribution in both the lumen and aortic wall. Given the limitations of existing imaging methods, we propose the use of patient-specific computational fluid dynamics (CFD) to explore this relationship. We have performed CFD simulations of O₂ and NO mass transfer in the lumen and aortic wall for two cases: a healthy control (HC) and a patient with TAA, both acquired using 4D-flow magnetic resonance imaging (MRI). The mass transfer of O₂ was based on active transport by hemoglobin, while the local variations of the wall shear stress (WSS) drove NO production. Comparing hemodynamic properties, the time-averaged WSS was considerably lower for TAA, while the oscillatory shear index and endothelial cell activation potential were notably elevated. O₂ and NO showed a non-uniform distribution within the lumen and an inverse correlation between the two species. We identified several locations of hypoxic regions for both cases due to lumen-side mass transfer limitations. In the wall, NO varied spatially, with a clear distinction between TAA and HC. In conclusion, the hemodynamics and mass transfer of NO in the aorta exhibit the potential to serve as a diagnostic biomarker for TAA. Furthermore, hypoxia may provide additional insights into the onset of other aortic pathologies.

Delivery of Nitric Oxide in the Cardiovascular System: Implications for Clinical Diagnosis and Therapy

Nitric oxide (NO) is a key molecule in cardiovascular homeostasis and its abnormal delivery is highly associated with the occurrence and development of cardiovascular disease (CVD). The assessment and manipulation of NO delivery is crucial to the diagnosis and therapy of CVD, such as endothelial dysfunction, atherosclerotic progression, pulmonary hypertension, and cardiovascular manifestations of coronavirus (COVID-19). However, due to the low concentration and fast reaction characteristics of NO in the cardiovascular system, clinical applications centered on NO delivery are challenging. In this tutorial review, we first summarized the methods to estimate the in vivo NO delivery process, based on computational modeling and flow-mediated dilation, to assess endothelial function and vulnerability of atherosclerotic plaque. Then, emerging bioimaging technologies that have the potential to experimentally measure arterial NO concentration were discussed, including Raman spectroscopy and electrochemical sensors. In addition to diagnostic methods, therapies aimed at controlling NO delivery to regulate CVD were reviewed, including the NO release platform to treat endothelial dysfunction and atherosclerosis and inhaled NO therapy to treat pulmonary hypertension and COVID-19. Two potential methods to improve the effectiveness of existing NO therapy were also discussed, including the combination of NO release platform and computational modeling, and stem cell therapy, which currently remains at the laboratory stage but has clinical potential for the treatment of CVD.

A dynamic computational network model for the role of nitric oxide and the myogenic response in microvascular flow regulation

OBJECTIVES

The effect of NO on smooth muscle cell contractility is crucial in regulating vascular tone, blood flow, and O₂ delivery. Quantitative predictions for interactions between the NO production rate and the myogenic response for microcirculatory blood vessels are lacking.

METHODS

We developed a computational model of a branching microcirculatory network with four representative classes of resistance vessels to predict the effect of endothelium-derived NO on the microvascular pressure-flow response. Our model links vessel scale biotransport simulations of NO and O₂ delivery to a mechanistic model of autoregulation and myogenic tone in a simplified microcirculatory network.

RESULTS

The model predicts that smooth muscle cell NO bioavailability significantly contributes to resting vascular tone of resistance vessels. Deficiencies in NO seen during hypoxia or ischemia lead to a decreased vessel diameter for all classes at a given intravascular pressure. At the network level, NO deficiencies lead to an increase in pressure drop across the vessels studied, a downward shift in the pressure-flow curve, and a decrease in the effective range of the autoregulatory response.

CONCLUSIONS

Our model predicts the steady state and transient behavior of resistance vessels to perturbations in blood pressure, including effects of NO bioavailability on vascular regulation.

Patient-Specific, Multi-Scale Modeling of Neointimal Hyperplasia in Vein Grafts

Neointimal hyperplasia is amongst the major causes of failure of bypass grafts. The disease progression varies from patient to patient due to a range of different factors. In this paper, a mathematical model will be used to understand neointimal hyperplasia in individual patients, combining information from biological experiments and patient-specific data to analyze some aspects of the disease, particularly with regard to mechanical stimuli due to shear stresses on the vessel wall. By combining a biochemical model of cell growth and a patient-specific computational fluid dynamics analysis of blood flow in the lumen, remodeling of the blood vessel is studied by means of a novel computational framework. The framework was used to analyze two vein graft bypasses from one patient: a femoro-popliteal and a femoro-distal bypass. The remodeling of the vessel wall and analysis of the flow for each case was then compared to clinical data and discussed as a potential tool for a better understanding of the disease. Simulation results from this first computational approach showed an overall agreement on the locations of hyperplasia in these patients and demonstrated the potential of using new integrative modeling tools to understand disease progression.

Nitric oxide release by deoxymyoglobin nitrite reduction during cardiac ischemia: A mathematical model

Interactions between cardiac myoglobin (Mb), nitrite, and nitric oxide (NO) are vital in regulating O₂ storage, transport, and NO homeostasis. Production of NO through the reduction of endogenous myocardial nitrite by deoxygenated myoglobin has been shown to significantly reduce myocardial infarction damage and ischemic injury. We developed a mathematical model for a cardiac arteriole and surrounding myocardium to examine the hypothesis that myoglobin switches functions from being a strong NO scavenger to an NO producer via the deoxymyoglobin nitrite reductase pathway. Our results predict that under ischemic conditions of flow, blood oxygen level, and tissue pH, deoxyMb nitrite reduction significantly elevates tissue and smooth muscle cell NO. The size of the effect is consistent at different flow rates, increases with decreasing blood oxygen and tissue pH and, in extreme pathophysiological conditions, NO can even be elevated above the normoxic levels. Our simulations suggest that cardiac deoxyMb nitrite reduction is a plausible mechanism for preserving or enhancing NO levels using endogenous nitrite despite the rate-limiting O₂ levels for endothelial NO production. This NO could then be responsible for mitigating deleterious effects under ischemic conditions.

A mathematical model for the role of N2O3 in enhancing nitric oxide bioavailability following nitrite infusion

Nitrite infusion into the bloodstream has been shown to elicit vasodilation and protect against ischemia-reperfusion injury through nitric oxide (NO) release in hypoxic conditions. However, the mechanism by which nitrite-derived NO escapes scavenging by hemoglobin in the erythrocyte has not been fully elucidated, owing in part to the difficulty in measuring the reactions and transport on NO in vivo. We developed a mathematical model for an arteriole and surrounding tissue to examine the hypothesis that dinitrogen trioxide (N2O3) acts as a stable intermediate for preserving NO. Our simulations predict that with hypoxia and moderate nitrite concentrations, the N2O3 pathway can significantly preserve the NO produced by hemoglobin nitrite reductase in the erythrocyte and elevate NO reaching the smooth muscle cells. Nitrite retains its ability to increase NO bioavailability even at varying flow conditions, but there is minimal effect under normoxia or very low nitrite concentrations. Our model demonstrates a viable pathway for reconciling experimental findings of potentially beneficial effects of nitrite infusions despite previous models showing negligible NO elevation associated with hemoglobin nitrite reductase. Our results suggest that additional mechanisms may be needed to explain the efficacy of nitrite-induced vasodilation at low infusion concentrations.

Nitric oxide transport in normal human thoracic aorta: effects of hemodynamics and nitric oxide scavengers

Despite the crucial role of nitric oxide (NO) in the homeostasis of the vasculature, little quantitative information exists concerning NO transport and distribution in medium and large-sized arteries where atherosclerosis and aneurysm occur and hemodynamics is complex. We hypothesized that local hemodynamics in arteries may govern NO transport and affect the distribution of NO in the arteries, hence playing an important role in the localization of vascular diseases. To substantiate this hypothesis, we presented a lumen/wall model of the human aorta based on its MRI images to simulate the production, transport and consumption of NO in the arterial lumen and within the aortic wall. The results demonstrated that the distribution of NO in the aorta was quite uneven with remarkably reduced NO bioavailability in regions of disturbed flow, and local hemodynamics could affect NO distribution mainly via flow dependent NO production rate of endothelium. In addition, erythrocytes in the blood could moderately modulate NO concentration in the aorta, especially at the endothelial surface. However, the reaction of NO within the wall could only slightly affect NO concentration on the luminal surface, but strongly reduce NO concentration within the aortic wall. A strong positive correlation was revealed between wall shear stress and NO concentration, which was affected by local hemodynamics and NO reaction rate. In conclusion, the distribution of NO in the aorta may be determined by local hemodynamics and modulated differently by NO scavengers in the lumen and within the wall.

Nitric oxide transport in an axisymmetric stenosis

To test the hypothesis that disturbed flow can impede the transport of nitric oxide (NO) in the artery and hence induce atherogenesis, we used a lumen-wall model of an idealized arterial stenosis with NO produced at the blood vessel-wall interface to study the transport of NO in the stenosis. Blood flows in the lumen and through the arterial wall were simulated by Navier-Stokes equations and Darcy's Law, respectively. Meanwhile, the transport of NO in the lumen and the transport of NO within the arterial wall were modelled by advection-diffusion reaction equations. Coupling of fluid dynamics at the endothelium was achieved by the Kedem-Katchalsky equations. The results showed that both the hydraulic conductivity of the endothelium and the non-Newtonian viscous behaviour of blood had little effect on the distribution of NO. However, the blood flow rate, stenosis severity, red blood cells (RBCs), RBC-free layer and NO production rate at the blood vessel-wall interface could significantly affect the transport of NO. The theoretical study revealed that the transport of NO was significantly hindered in the disturbed flow region distal to the stenosis. The reduced NO concentration in the disturbed flow region might play an important role in the localized genesis and development of atherosclerosis.

Another role for nitric oxide in blood flow control?

In the current issue, Chen and co-authors present a mathematical model to simulate shear stress-dependent nitric oxide (NO) transport in a small reconstructed microvascular network. Here their results are discussed in the context of NO-dependent blood flow control. Furthermore, other NO-dependent blood flow control mechanisms are briefly reviewed.