Nitric oxide generation by endothelial cells exposed to shear stress in glass tubes perfused with red blood cell suspensions: role of aggregation

The Impact of COVID-19 on Cellular Factors Influencing Red Blood Cell Aggregation Examined in Dextran: Possible Causes and Consequences

Several studies have indicated that COVID-19 can lead to alterations in blood rheology, including an increase in red blood cell aggregation. The precise mechanisms behind this phenomenon are not yet fully comprehended. The latest findings suggest that erythrocyte aggregation significantly influences microcirculation, causes the formation of blood clots in blood vessels, and even damages the endothelial glycocalyx, leading to endothelial dysfunction. The focus of this research lies in investigating the cellular factors influencing these changes in aggregation and discussing potential causes and implications in the context of COVID-19 pathophysiology. For this purpose, the aggregation of erythrocytes in a group of 52 patients with COVID-19 pneumonia was examined in a 70 kDa Dextran solution, which eliminates the influence of plasma factors. Using image analysis, the velocities and sizes of the formed aggregates were investigated, determining their porosity. This study showed that the process of erythrocyte aggregation in COVID-19 patients, independent of plasma factors, leads to the formation of more compact, denser, three-dimensional aggregates. These aggregates may be less likely to disperse under circulatory shear stress, increasing the risk of thrombotic events. This study also suggests that cellular aggregation factors can be responsible for the thrombotic disorders observed long after infection, even when plasma factors have normalized. The results and subsequent broad discussion presented in this study can contribute to a better understanding of the potential complications associated with increased erythrocyte aggregation.

In vitro effects of temperature on red blood cell deformability and membrane stability in human and various vertebrate species

BACKGROUND

The effects of temperature on micro-rheological variables have not been completely revealed yet.

OBJECTIVE

To investigate micro-rheological effects of heat treatment in human, rat, dog, and porcine blood samples.

METHODS

Red blood cell (RBC) - buffer suspensions were prepared and immersed in a 37, 40, and 43°C heat-controlled water bath for 10 minutes. Deformability, as well as mechanical stability of RBCs were measured in ektacytometer. These tests were also examined in whole blood samples at various temperatures, gradually between 37 and 45°C in the ektacytometer.

RESULTS

RBC deformability significantly worsened in the samples treated at 40 and 43°C, more expressed in human, porcine, rat, and in smaller degree in canine samples. The way of heating (incubation vs. ektacytometer temperation) and the composition of the sample (RBC-PBS suspension or whole blood) resulted in the different magnitude of RBC deformability deterioration. Heating affected RBC membrane (mechanical) stability, showing controversial alterations.

CONCLUSION

Significant changes occur in RBC deformability by increasing temperature, showing inter-species differences. The magnitude of alterations is depending on the way of heating and the composition of the sample. The results may contribute to better understanding the micro-rheological deterioration in hyperthermia or fever.

Examination of the relation between red blood cell aggregation and hematocrit in human and various experimental animals

BACKGROUND

Red blood cell (RBC) aggregation plays an important role in the physiological processes of the microcirculation. The complete mechanism of aggregation is still unclear, and it is influenced by several cellular and plasmatic factors. One of these factors is the hematocrit (Hct).

OBJECTIVE

We hypothesized that the relation of RBC aggregation and Hct differs between species.

METHODS

From anticoagulated blood samples of healthy volunteers, rats, dogs, and pigs, 20, 40, and 60 %Hct RBC, autologous plasma suspensions were prepared. Hematological parameters and RBC aggregation was determined by light-transmission and light-reflection method.

RESULTS

Suspensions at 20%and 60%Hct expressed lower RBC aggregation than of 40%Hct suspensions, showing inter-species differences. By curve fitting the Hct at the highest aggregation value differed in species (human: 45.25%- M 5 s, 40.86%- amp; rat: 44.44 %- M1 10 s, 39.37%- amp; dog: 42.48%- M 5 s, 44.29%- amp; pig: 47.63%- M 5 s, 52.8%- amp).

CONCLUSION

RBC aggregation - hematocrit relation shows inter-species differences. Human blood was found to be the most sensitive for hematocrit changes. The more obvious differences could be detected by M 5 s by light-transmission method and amplitude parameter using light-reflection method.

Impact of obstructive sleep apnea and intermittent hypoxia on blood rheology - a translational study

RATIONALE

Hemorheological alterations are reported in obstructive sleep apnea (OSA) and reversed with continuous positive airway pressure (CPAP), observations potentially explained by intermittent hypoxia (IH)-induced oxidative stress.

OBJECTIVE

To investigate whether IH causes hemorheological alterations viaoxidative stress.

METHODS

Wistar rats were exposed to normoxia (n=7) or IH (n=8) for 14 days. Twenty-three moderate-to-severe OSA patients were assessed at three time points: baseline, after randomisation to either 2 weeks of nocturnal oxygen (n=13) or no treatment (n=10), and after 1-month of CPAP treatment (n=17). Further, an OSA-free control group (n=13) was assessed at baseline and after time-matched follow-up.

MEASUREMENTS

We measured hemorheological parameters [hematocrit, blood viscosity, plasma viscosity (rats only), erythrocyte aggregation and deformability (humans only)] and redox balance (SOD, GPX, protein oxidation [AOPP] and lipid peroxidation [MDA]). We also tested erythrocytes hemorheological sensitivity to reactive oxygen species (ROS) in our human participants using the oxidant t-butyl hydroperoxide (TBHP).

RESULTS

In rats, IH increased blood viscosity by increasing hematocrit without altering erythrocytes hemorheological properties. IH also reduced SOD activity and increased AOPP. In humans, baseline hemorheological properties were similar between patients and controls, and properties were unaltered following oxygen and CPAP, except erythrocyte deformability was reduced following oxygen therapy. Redox balance was comparable between patients and controls. At baseline, TBHP induced a greater reduction of erythrocyte deformability in patients while CPAP reduced TBHP-induced increase in aggregation strength.

CONCLUSION

IH and OSA per se do not cause hemorheological alterations despite the presence of oxidative stress or higher sensitivity to ROS, respectively.

Plasma viscosity pattern and erythrocyte aggregation in two patients with congenital afibrinogenemia

: In this case report, we examine the behavior of plasma viscosity, explored at high and low shear rates, and erythrocyte aggregation in two patients with congenital afibrinogenemia, a clinical disorder firstly described in 1920 and that has an estimated incidence of 1 : 1-200 0000. The two hemorheological parameters examined by us showed a marked decrease in both patients, in one of whom erythrocyte aggregation was even undetectable. Keeping in mind that spontaneous thrombosis (venous and arterial) has been often described in congenital afibrinogenemia, it can be hypothesized that the decrease in plasma viscosity and erythrocyte aggregation might cause a reduction of the endothelial synthesis and release of nitric oxide through the fall of the wall shear stress.

Beyond oxygen transport: active role of erythrocytes in the regulation of blood flow

It was classically thought that the function of mammalian red blood cells (RBCs) was limited to serving as a vehicle for oxygen, given the cells' abundance of cytosolic hemoglobin. Over the past decades, however, accumulating evidence indicates that RBCs have the capacity to sense low-oxygen tensions in hypoxic tissues, and, subsequently, release signaling molecules that influence the distribution of blood flow. The precise mechanisms that facilitate RBC modulation of blood flow are still being elucidated, although recent evidence indicates involvement of 1) adenosine triphosphate, capable of binding to purinergic receptors located on the vascular wall before initiating nitric oxide (NO; a powerful vasodilator) production in endothelial cells, and/or 2) nonvascular NO, which is now known to have several modes of production within RBCs, including an enzymatic process via a unique isoform of NO synthase (i.e., RBC-NOS), which has potential effects on the vascular smooth muscle. The physical properties of RBCs, including their tendency to form three-dimensional structures in low shear flow (i.e., aggregation) and their capacity to elongate in high shear flow (i.e., deformability), are only recently being viewed as mechanotransductive processes, with profound effects on vascular reactivity and tissue perfusion. Recent developments in intracellular signaling in RBCs, and the subsequent effects on the mechanical properties of blood, and blood flow, thus present a vivid expansion on the classic perspective of these abundant cells.

Influence of erythrocyte aggregation at pathological levels on cell-free marginal layer in a narrow circular tube

Human red blood cells (RBCs) were perfused in a circular micro-tube (inner diameter of 25 μm) to examine the dynamic changes of cell-free marginal region at both physiological (normal) and pathophysiological (hyper) levels of RBC aggregation. The cell-free area (CFA) was measured to provide additional information on the cell-free layer (CFL) width changes in space and time domains. A prominent enhancement in the mean CFL width was found in hyper-aggregating conditions as compared to that in non-aggregating conditions (P <  0.001). The frequent contacts between RBC and the tube wall were observed and the contact frequency was greatly decreased when the aggregation level was increased from none to normal (P <  0.05) and to hyper (P <  0.001) levels. In addition, the enhanced aggregation from none to hyper levels significantly enlarged the CFA (P <  0.01). We concluded that the RBC aggregation at pathophysiological levels could promote not only the CFL width (one-dimensional parameter) but also the spatiotemporal variation of CFA (two-dimensional parameter).

ENDOXY - Development of a Biomimetic Oxygenator-Test-Device

Objective

This study focusses on the development of a biomimetic oxygenator test device. Due to limited biocompatibility, current oxygenators do not allow mid- to long-term therapy. Tissue engineering uses autologous cell sources to overcome the immunogenic barriers of biomaterials. Surface coating with endothelial cells might improve hemocompatibility and thus prevent immunogenic reactions of the body. In this study this concept is applied to endothelialise a gas-permeable membrane to develop a biomimetic oxygenator test-device (ENDOXY).

Methods

ENDOXY—a multifunctional test-system was developed to endothelialise a gas-permeable membrane suitable for cell culture and to test the cell retention under shear stress and to measure gas transfer through it.

Results

Successful endothelialisation of the membrane was achieved and cells showed characteristic endothelial morphologies. They stained positive for endothelial markers. The number of cells aligned with shear stress and cell retention after blood perfusing experiments was high. Gas transfer is observed via uncoated and endothelialised membranes.

Conclusion

The study showed promising results with regard to system design, endothelialisation, and cell retention under shear stress conditions. It strongly encourages further research into the system by testing different membrane materials to design a biomimetic membrane surface and pave way for a fully hemocompatible oxygenator.

Impact of stochastic fluctuations in the cell free layer on nitric oxide bioavailability

A plasma stratum (cell free layer or CFL) generated by flowing blood interposed between the red blood cell (RBC) core and the endothelium affects generation, consumption, and transport of nitric oxide (NO) in the microcirculation. CFL width is a principal factor modulating NO diffusion and vessel wall shears stress development, thus significantly affecting NO bioavailability. Since the CFL is bounded by the surface formed by the chaotically moving RBCs and the stationary but spatially non-uniform endothelial surface, its width fluctuates randomly in time and space. We analyze how these stochastic fluctuations affect NO transport in the CFL and NO bioavailability. We show that effects due to random boundaries do not average to zero and lead to an increase of NO bioavailability. Since endothelial production of NO is significantly enhanced by temporal variability of wall shear stress, we posit that stochastic shear stress stimulation of the endothelium yields the baseline continual production of NO by the endothelium. The proposed stochastic formulation captures the natural continuous and microscopic variability, whose amplitude is measurable and is of the scale of cellular dimensions. It provides a realistic model of NO generation and regulation.

Piezo1 links mechanical forces to red blood cell volume

Red blood cells (RBCs) experience significant mechanical forces while recirculating, but the consequences of these forces are not fully understood. Recent work has shown that gain-of-function mutations in mechanically activated Piezo1 cation channels are associated with the dehydrating RBC disease xerocytosis, implicating a role of mechanotransduction in RBC volume regulation. However, the mechanisms by which these mutations result in RBC dehydration are unknown. In this study, we show that RBCs exhibit robust calcium entry in response to mechanical stretch and that this entry is dependent on Piezo1 expression. Furthermore, RBCs from blood-cell-specific Piezo1 conditional knockout mice are overhydrated and exhibit increased fragility both in vitro and in vivo. Finally, we show that Yoda1, a chemical activator of Piezo1, causes calcium influx and subsequent dehydration of RBCs via downstream activation of the KCa3.1 Gardos channel, directly implicating Piezo1 signaling in RBC volume control. Therefore, mechanically activated Piezo1 plays an essential role in RBC volume homeostasis.

DOI:http://dx.doi.org/10.7554/eLife.07370.001

Within our bodies, cells and tissues are constantly being pushed and pulled by their surrounding environment. These mechanical forces are then transformed into electrical or chemical signals by cells. This process is crucial for many biological structures, such as blood vessels, to develop correctly, and is also a key part of our senses of touch and hearing.

In 2010, researchers discovered a group of ion channels—proteins embedded in the membrane that surrounds a cell—that open up when a force is applied and allow calcium and other ions to enter the cell. This movement of ions generates the electrical response of the cell to the applied force. However, not much is known about the roles of these ‘Piezo’ ion channels.

Red blood cells experience significant forces when they pass through narrow blood vessels. In a disease called xerocytosis, the red blood cells become severely dehydrated and shrink. In 2013, researchers discovered that patients with this disease have mutations in the gene that codes for the Piezo1 protein: a Piezo protein that has also been linked to a role in blood vessel development in embryos. This suggested that Piezo1 may regulate the volume of red blood cells.

Cahalan, Lukacs et al.—including some of the researchers who worked on the 2010 and 2013 studies—have now investigated the role of Piezo1 in red blood cells in more detail. Applying strong forces to red blood cells from mice caused calcium to rapidly enter cells through Piezo1 channels.

Cahalan, Lukacs et al. then deleted the Piezo1 gene from red blood cells. This made the cells larger and more fragile than normal cells because they contained too much water. To investigate how Piezo1 regulates water content, the cells were treated with a chemical compound called Yoda1. This compound was shown in a separate study by Syeda et al. to activate Piezo1 channels. Activating Piezo1 caused a second type of ion channel to open up as well, which allowed potassium ions and water molecules to leave the cell. This resulted in the cell becoming dehydrated.

This work raises the possibility that Piezo proteins are involved in other diseases where red blood cell volume is altered. In particular, many believe that Piezo1 may be involved in sickle cell disease, a possibility that can now be tested using the tools described in this study.

DOI:http://dx.doi.org/10.7554/eLife.07370.002

Nitric oxide, vasodilation and the red blood cell

Since the identification of the elusive endothelium-derived relaxing factor as nitric oxide (NO), much attention has been devoted to understanding its physiological effects. NO is a free radical with many roles, and owing to its neutral charge and high diffusion capacity, it appears NO is involved in every mammalian biological system. Most attention has been focused on the NO generating pathways within the endothelium; however, the recent discovery of a NO synthase (NOS)-like enzyme residing in red blood cells (RBC) has increased our understanding of the blood flow and oxygen delivery modulation by RBC. In the present review, pathways of NO generation are summarized, with attention to those residing within RBC. While the bioactivity of RBC-derived NO is still debated due to its generation within proximity of NO scavengers, current theories for NO export from RBC are explored, which are supported by recent findings demonstrating an extracellular response to RBC-derived NO. The importance of NO in the active regulation of RBC deformability is discussed in the context of the subsequent effects on blood fluidity, and the complex interplay between blood rheology and NO are summarized. This review provides a summary of recent advances in understanding the role played by RBC in NO equilibrium and vascular regulation.

The effect of radiographic contrast media on reperfusion injury in the isolated rat heart

Background and Objectives

We investigated the effects of commonly used contrast media (CM) on myocardial ischemia-reperfusion injury in isolated rat hearts.

Subjects and Methods

Isolated rat hearts were subjected to 30 minutes of regional ischemia and 2 hours of reperfusion. The following CM (1 mL/1 L Krebs-Henseleit buffer) were randomly perfused for 15 minutes beginning 5 minutes before reperfusion and ending 10 minutes after reperfusion: iohexol (n=8), iopromide (n=8), ioversol (n=8), iomeprol (n=8), iopamidol (n=7), ioxaglate (n=8), and iodixanol (n=7). The effects of a direct bolus injection of undiluted iohexol, iopromide, or ioxaglate (each n=6) via the aortic root immediately prior to reperfusion were also evaluated. The area of necrosis, expressed as the percentage of the area at risk (AN/AR), and cardiodynamic variables were measured.

Results

The AN/AR of the control and experimental groups in the order described in methods was 33.7±6.4%, 30.3±7.4%, 34.7±12.6%, 29.2±10.2%, 20.9±7.6%, 22.6±8.7%, 18.8±7.9%, and 19.9±11.4%, respectively. Groups that received iomeprol and ioxaglate exhibited significantly decreased AN/AR values compared to those of control hearts (p=0.042 and p=0.013). No significant differences in the AN/AR were observed between control hearts and the groups injected with a single bolus of CM. No significant hemodynamic changes were noted after reperfusion among the groups.

Conclusion

The overall effects of the CM on coronary reperfusion were not deleterious, and better effects were noted in two CM groups. However, it is unclear whether this result was attributed to a specific physiochemical property of the CM.

Hemorheology in experimental research: is it necessary to consider blood fluidity differences in the laboratory rat?

This study was designed to identify whether blood fluidity differs between commercially available laboratory rat strains. The hemorheological profiles of seven clinically healthy wild-type rat strains were analyzed to determine whether any diversity in blood fluidity might affect the outcome of cardiovascular studies. Study 1: 65 healthy adult rats (Lewis, Long-Evans, Hairless, Wistar and Fisher; mixed gender and comparable ages) were compared. In order to determine the greatest possible difference, the two strains with the greatest hematocrit (HCT) differences were selected for more detailed evaluation. Red blood cell (RBC) deformability (maximum elongation index, shear stress for half-maximal deformation of RBC; both P < 0.0001), and the effect of plasma protein concentration upon plasma viscosity (P < 0.0001) were different between Lewis and Long-Evans strains. Whole blood viscosity - although different at native HCT (P < 0.004) - was unaltered following HCT standardization of samples. Differences in RBC aggregation were statistically significant but these were small and may not be clinically relevant. Study 2: these 65 animals were compared with 21 animals (10-16 weeks old; both sexes) from mutant strains (Dahl SS/JrHsdMcwiCrl, n = 10; ZDF-Lepr(fa)/Crl, n = 11). In both mutant strains, plasma and whole blood viscosity were increased compared with commonly used strains at native and standardized HCT (P < 0.001). Unusually high RBC aggregation values were seen in the ZDF rat strain (P < 0.001). It was concluded that the variability in blood fluidity among clinically healthy adult laboratory rat strains was both statistically and clinically significant. A hemorheological profile should be added to a routine phenotyping process, since both variables can significantly influence study outcomes.

Two-dimensional transient model for prediction of arteriolar NO/O2 modulation by spatiotemporal variations in cell-free layer width

Despite the significant roles of the cell-free layer (CFL) in balancing nitric oxide (NO) and oxygen (O2) bioavailability in arteriolar tissue, many previous numerical approaches have relied on a one-dimensional (1-D) steady-state model for simplicity. However, these models are unable to demonstrate the influence of spatiotemporal variations in the CFL on the NO/O2 transport under dynamic flow conditions. Therefore, the present study proposes a new two-dimensional (2-D) transient model capable of predicting NO/O2 transport modulated by the spatiotemporal variations in the CFL width. Our model predicted that NO bioavailability was inversely related to the CFL width as expected. The enhancement of NO production by greater wall shear stress with a thinner CFL could dominate the diffusion barrier role of the CFL. In addition, NO/O2 availability along the vascular wall was inhomogeneous and highly regulated by dynamic changes of local CFL width variation. The spatial variations of CFL widths on opposite sides of the arteriole exhibited a significant inverse relation. This asymmetric formation of CFL resulted in a significantly imbalanced NO/O2 bioavailability on opposite sides of the arteriole. The novel integrative methodology presented here substantially highlighted the significance of spatiotemporal variations of the CFL in regulating the bioavailability of NO/O2, and provided further insight about the opposing effects of the CFL on arteriolar NO production.

Influence of serological factors and BMI on the blood pressure/hematocrit association in healthy young men and women

The association between mean arterial blood pressure (MAP) and hematocrit (Hct) as a surrogate for blood viscosity was investigated in a young (average 20.0±2.3 years), healthy population of 174 men and 442 women. Health status was assessed by clinical examination and serological evaluation. Individuals with severe anemia or hemoconcentration, prior traumas or major surgical intervention, smokers, and pregnant or lactating women were excluded from the study. The MAP/Hct association was positive and significant (P=0.04) for women and negative, albeit not significantly so, for men. The MAP/Hct association was also evaluated in subgroups of the same population with a progressive step-by-step exclusion of: individuals with cholesterol >200 mg/dL; triglycerides >200 mg/dL; body mass index >25 kg/m(2); and glucose >100 mg/dL. This consecutively reduced the strength of the positive MAP/Hct association in women, which became negative - although not significantly so - when all anomalously high factors were excluded. The same trend was found in men. Our study indicates that previously reported positive trends in the relationship between the MAP and Hct in the population are not present in a young, healthy population of men or women that excludes individuals with the confounding factors of above normal serological values and BMI.

Tissue oxygen demand in regulation of the behavior of the cells in the vasculature

The control of arteriolar diameters in microvasculature has been in the focus of studies on mechanisms matching oxygen demand and supply at the tissue level. Functionally, important vascular elements include EC, VSMC, and RBC. Integration of these different cell types into functional units aimed at matching tissue oxygen supply with tissue oxygen demand is only achieved when all these cells can respond to the signals of tissue oxygen demand. Many vasoactive agents that serve as signals of tissue oxygen demand have their receptors on all these types of cells (VSMC, EC, and RBC) implying that there can be a coordinated regulation of their behavior by the tissue oxygen demand. Such functions of RBC as oxygen carrying by Hb, rheology, and release of vasoactive agents are considered. Several common extra- and intracellular signaling pathways that link tissue oxygen demand with control of VSMC contractility, EC permeability, and RBC functioning are discussed.

Effect of erythrocyte aggregation on spatiotemporal variations in cell-free layer formation near on arteriolar bifurcation

OBJECTIVE

To investigate how red blood cell aggregation could modulate the spatial variations in cell-free layer formation in the vicinity of an arteriolar bifurcation.

METHODS

Visualization of blood flow was performed in upstream and downstream vessels of arteriolar bifurcations in the rat cremaster muscles under reduced flow conditions before and after induction of red blood cell aggregation to both physiological normal- and pathological hyperlevels seen in humans.

RESULTS

Large asymmetries of layer widths on opposite sides of the downstream vessel were attenuated along the vessel and this effect could be prominently enhanced by the hyperaggregation due to a higher formation rate of the layer which was greater on one side than the other of the vessel. The proportion of downstream layer formation constituted by the smaller downstream vessel generally increased with a thicker layer width at the wall of the upstream vessel adjacent it. A greater tendency of the layer formation in the smaller downstream vessel was found under the hyperaggregating condition than normal-aggregating and nonaggregating conditions.

CONCLUSION

Red blood cell aggregation could attenuate the asymmetry in cell-free layer formation on opposite sides of the downstream vessel, but enhances the heterogeneity of the layer formation between downstream vessels.

Nitric oxide synthase dysfunction contributes to impaired cerebroarteriolar reactivity in experimental cerebral malaria

Cerebrovascular dysfunction plays a key role in the pathogenesis of cerebral malaria. In experimental cerebral malaria (ECM) induced by Plasmodium berghei ANKA, cerebrovascular dysfunction characterized by vascular constriction, occlusion and damage results in impaired perfusion and reduced cerebral blood flow and oxygenation, and has been linked to low nitric oxide (NO) bioavailability. Here, we directly assessed cerebrovascular function in ECM using a novel cranial window method for intravital microscopy of the pial microcirculation and probed the role of NOS isoforms and phosphorylation patterns in the impaired vascular responses. We show that pial arteriolar responses to endothelial NOS (eNOS) and neuronal NOS (nNOS) agonists (Acetylcholine (ACh) and N-Methyl-D-Aspartate (NMDA)) were blunted in mice with ECM, and could be partially recovered by exogenous supplementation of tetrahydrobiopterin (BH4). Pial arterioles in non-ECM mice infected by Plasmodium berghei NK65 remained relatively responsive to the agonists and were not significantly affected by BH4 treatment. These findings, together with the observed blunting of NO production upon stimulation by the agonists, decrease in total NOS activity, augmentation of lipid peroxidation levels, upregulation of eNOS protein expression, and increase in eNOS and nNOS monomerization in the brain during ECM development strongly indicate a state of eNOS/nNOS uncoupling likely mediated by oxidative stress. Furthermore, the downregulation of Serine 1176 (S1176) phosphorylation of eNOS, which correlated with a decrease in cerebrovascular wall shear stress, implicates hemorheological disturbances in eNOS dysfunction in ECM. Finally, pial arterioles responded to superfusion with the NO donor, S-Nitroso-L-glutathione (GSNO), but with decreased intensity, indicating that not only NO production but also signaling is perturbed during ECM. Therefore, the pathological impairment of eNOS and nNOS functions contribute importantly to cerebrovascular dysfunction in ECM and the recovery of intrinsic functionality of NOS to increase NO bioavailability and restore vascular health represents a target for ECM treatment.

Isolation of functional human endothelial cells from small volumes of umbilical cord blood

Endothelial cells (ECs) isolated from endothelial progenitor cells in blood have great potential as a therapeutic tool to promote vasculogenesis and angiogenesis and treat cardiovascular diseases. However, current methods to isolate ECs are limited by a low yield with few colonies appearing during isolation. In order to utilize blood-derived ECs for therapeutic applications, a simple method is needed that can produce a high yield of ECs from small volumes of blood without the addition of animal-derived products. For the first time, we show that human ECs can be isolated without the prior separation of blood components through the technique of diluted whole blood incubation (DWBI) utilizing commercially available human serum. We isolated ECs from small volumes of blood (~10 mL) via DWBI and characterized them with flow cytometry, immunohistochemistry, and uptake of DiI-labeled acetylated low density lipoprotein (DiI-Ac-LDL). These ECs are functional as demonstrated by their ability to form tubular networks in Matrigel, adhere and align with flow under physiological fluid shear stress, and produce increased nitric oxide under fluid flow. An average of 7.0 ± 2.5 EC colonies that passed all functional tests described above were obtained per 10 mL of blood as compared to only 0.3 ± 0.1 colonies with the traditional method based on density centrifugation. The time until first colony appearance was 8.3 ± 1.2 days for ECs isolated with the DWBI method and 12 ± 1.4 days for ECs isolated with the traditional isolation method. A simplified method, such as DWBI, in combination with advances in isolation yield could enable the use of blood-derived ECs in clinical practice.

Two-phase model for prediction of cell-free layer width in blood flow

This study aimed to develop a numerical model capable of predicting changes in the cell-free layer (CFL) width in narrow tubes with consideration of red blood cell aggregation effects. The model development integrates to empirical relations for relative viscosity (ratio of apparent viscosity to medium viscosity) and core viscosity measured on independent blood samples to create a continuum model that includes these two regions. The constitutive relations were derived from in vitro experiments performed with three different glass-capillary tubes (inner diameter=30, 50 and 100 μm) over a wide range of pseudoshear rates (5-300 s(-1)). The aggregation tendency of the blood samples was also varied by adding Dextran 500 kDa. Our model predicted that the CFL width was strongly modulated by the relative viscosity function. Aggregation increased the width of CFL, and this effect became more pronounced at low shear rates. The CFL widths predicted in the present study at high shear conditions were in agreement with those reported in previous studies. However, unlike previous multi-particle models, our model did not require a high computing cost, and it was capable of reproducing results for a thicker CFL width at low shear conditions, depending on aggregating tendency of the blood.

Effects of cell-free layer formation on NO/O2 bioavailability in small arterioles

We developed a new time-dependent computational model for coupled NO/O(2) transport in small arterioles that incorporates potential physiological responses (temporal changes in NO scavenging rate and O(2) partial pressure in blood lumen and NO production rate in endothelium) to the temporal cell-free layer width variations. Two relations between wall shear stress (WSS) and NO production rate based on the linear and sigmoidal functions were considered in this simulation study. The cell-free layer data used for the simulation were acquired from arteriolar flows (D=48.3 ± 1.9 μm) in the rat cremaster muscles under normal flow conditions (WSS=3.4-5.6 Pa). For both cases of linear and sigmoidal relations, temporal layer width variations were found to be capable of significantly enhancing NO bioavailability and this effect was more pronounced in the latter (P<0.0005) than the former (P<0.005). In contrast, O(2) bioavailability in the arteriolar wall was not considerably altered by the temporal layer width variations, irrespective of the relation. Prominent enhancement (P<0.005) of soluble guanylyl cyclase (sGC) activation in the smooth muscle by the temporal layer width variations were predicted for both relations. The extent of sGC activation was generally lower (P<0.01) in the case of the sigmoidal relation than that of the linear relation, suggesting a lesser tendency for arterioles to dilate with the former.

Temporal variations of the cell-free layer width may enhance NO bioavailability in small arterioles: Effects of erythrocyte aggregation

Recently, we have shown that temporal variations in the cell-free layer width can potentially enhance nitric oxide (NO) bioavailability in small arterioles. Since the layer width variations can be augmented by red blood cell aggregation, we tested the hypothesis that an increase in the layer width variations due to red blood cell aggregation could provide an underlying mechanism to improve NO bioavailability in the endothelium and promote vasodilatory effects. Utilizing cell-free layer width data acquired from arterioles of the rat cremaster muscle before and after dextran infusion in reduced flow conditions (wall shear stress=0.13-0.24Pa), our computational model predicted exponential enhancements of NO bioavailability in the endothelium and soluble guanylyl cyclase (sGC) activation in the smooth muscle layer with increasing temporal variability of the layer width. These effects were mediated primarily by the transient responses of wall shear stress and NO production rate to the layer width variations. The temporal variations in the layer width were significantly enhanced (P<0.05) by aggregation, leading to significant improvements (P<0.05) in NO bioavailability and sGC activation. As a result, the significant reduction (P<0.05) of sGC activation due to the increased width of the layer after aggregation induction was diminished by the opposing effect of the layer variations. These findings highlighted the possible enhancement of NO bioavailability and vascular tone in the arteriole by the augmented layer width variations due to the aggregation.

An automated method for cell-free layer width determination in small arterioles

Histogram-based thresholding techniques utilized for cell-free layer width measurement in arteriolar flow may produce an overestimation of the layer width since they do not consider faint shaded regions near the vessel wall as part of the erythrocyte column. To address this problem, we developed a new method for detecting the boundary of the erythrocyte column based on an edge detection algorithm. This automated method (grayscale method) provides local detections of the inner vessel wall as well as the boundary between the cell-free layer and the erythrocyte column without binarization of grayscale images. The cell-free layer width measurements using the grayscale method and existing techniques (minimum method and Otsu's method) were compared with those determined manually in arteriolar flows of the rat cremaster muscle. In the absence of the shaded regions, values obtained by the grayscale method and minimum method were statistically in good agreement with the manual method but not in the case of Otsu's method. When the faint shaded regions were present, the grayscale method appeared to produce more accurate results than the minimum method and Otsu's method.

Modulation of NO bioavailability by temporal variation of the cell-free layer width in small arterioles

The cell-free layer exhibits dynamic characteristics in the time domain that may be capable of altering nitric oxide (NO) bioavailability in small arterioles. However, this effect has not been fully elucidated. This study utilized a computational model on NO transport to predict how temporal variations in the layer width could modulate NO bioavailability in the arterioles. Data on the layer width was acquired from high-speed video recordings in arterioles (ID = 48.4 ± 1.8 μm) of the rat cremaster muscle. We found that when wall shear stress response was not considered, the layer variability could lead to a slight decrease (1.6-6.6%) in NO bioavailability that was independent of transient changes in NO scavenging rate. Conversely, the transient response in wall shear stress and NO production rate played a dominant role in reversing this decline such that a significant augmentation (5.3-21.0%) in NO bioavailability was found with increasing layer variability from 24.6 to 63.8%. This study highlighted the importance of the temporal changes in wall shear stress and NO production rate caused by the layer width variations in prediction of NO bioavailability in arterioles.

Effect of cell-free layer variation on arteriolar wall shear stress

Relationship between a cell-free layer and wall shear stress (WSS) in small arterioles has been of interest in microcirculatory research. However, influence of temporal variation in the cell-free layer width on the WSS in vivo has not been fully elucidated. In this study, we tested the hypothesis that the layer variation would increase the WSS, and this effect would be enhanced by red blood cell aggregation. The cell-free layer width in arterioles (29.5–67.1 μm ID) in rat cremaster muscles were obtained with a high-speed video camera, and the layer width data were introduced into WSS estimation. Dextran 500 was administrated to elevate the aggregation level of red blood cells to those seen in normal human blood. The variation of the layer was quantified by the variability (coefficient of variation), and its effect on WSS was studied under normal and reduced flow conditions. We found that the dextran-induced red blood cell aggregation significantly elevated the variability (p < 0.01) at low pseudoshear rates of 9.2 ± 0.6 s⁻¹. The WSS estimated without taking account of the variability showed underestimation of its value than that of with consideration of the variability under all flow conditions, and this effect became more pronounced with increasing the variability. The variation of the cell-free layer should, therefore, be considered in the determination of the WSS particularly in the presence of red blood cell aggregation under reduced flow condition.

Effect of erythrocyte aggregation and flow rate on cell-free layer formation in arterioles

Formation of a cell-free layer is an important dynamic feature of microcirculatory blood flow, which can be influenced by rheological parameters, such as red blood cell aggregation and flow rate. In this study, we investigate the effect of these two rheological parameters on cell-free layer characteristics in the arterioles (20-60 mum inner diameter). For the first time, we provide here the detailed temporal information of the arteriolar cell-free layer in various rheological conditions to better describe the characteristics of the layer variation. The rat cremaster muscle was used to visualize arteriolar flows, and the extent of aggregation was raised by dextran 500 infusion to levels seen in normal human blood. Our results show that cell-free layer formation in the arterioles is enhanced by a combination of flow reduction and red blood cell aggregation. A positive relation (P < 0.005) was found between mean cell-free layer widths and their corresponding SDs for all conditions. An analysis of the frequency and magnitudes of cell-free layer variation from their mean value revealed that the layer deviated with significantly larger magnitudes into the red blood cell core after flow reduction and dextran infusion (P < 0.05). In accordance, the disparity of cell-free layer width distribution found in opposite radial directions from its mean became greater with aggregation in reduced flow conditions. This study shows that the cell-free layer width in arterioles is dependent on both flow rate and red blood cell aggregability, and that the temporal variations in width are asymmetric with a greater excursion into the red blood cell core than toward the vessel wall.

Tissue oxygenation after exchange transfusion with ultrahigh-molecular-weight tense- and relaxed-state polymerized bovine hemoglobins

Hemoglobin (Hb)-based O(2) carriers (HBOCs) constitute a class of therapeutic agents designed to correct the O(2) deficit under conditions of anemia and traumatic blood loss. The O(2) transport capacity of ultrahigh-molecular-weight bovine Hb polymers (PolybHb), polymerized in the tense (T) state and relaxed (R) state, were investigated in the hamster chamber window model using microvascular measurements to determine O(2) delivery during extreme anemia. The anemic state was induced by hemodilution with a plasma expander (70-kDa dextran). After an initial moderate hemodilution to 18% hematocrit, animals were randomly assigned to exchange transfusion groups based on the type of PolybHb solution used (namely, T-state PolybHb and R-state PolybHb groups). Measurements of systemic parameters, microvascular hemodynamics, capillary perfusion, and intravascular and tissue O(2) levels were performed at 11% hematocrit. Both PolybHbs were infused at 10 g/dl, and their viscosities were higher than nondiluted blood. Restitution of the O(2) carrying capacity with T-state PolybHb exhibited lower arterial pressure and higher functional capillary density compared with R-state PolybHb. Central arterial O(2) tensions increased significantly for R-state PolybHb compared with T-state PolybHb; conversely, microvascular O(2) tensions were higher for T-state PolybHb compared with R-state PolybHb. The increased tissue Po(2) attained with T-state PolybHb results from the larger amount of O(2) released from the PolybHb and maintenance of macrovascular and microvascular hemodynamics compared with R-state PolybHb. These results suggest that the extreme high O(2) affinity of R-state PolybHb prevented O(2) bound to PolybHb from been used by the tissues. The results presented here show that T-state PolybHb, a high-viscosity O(2) carrier, is a quintessential example of an appropriately engineered O(2) carrying solution, which preserves vascular mechanical stimuli (shear stress) lost during anemic conditions and reinstates oxygenation, without the hypertensive or vasoconstriction responses observed in previous generations of HBOCs.

Hemorrhagic shock and nitric oxide release from erythrocytic nitric oxide synthase: a quantitative analysis

A large loss of blood during hemorrhage can result in profound shock, a state of hypotension associated with hemodynamic abnormalities. One of the hypotheses to account for this collapse of homeostasis is that the production of nitric oxide (NO), a gas molecule that dilates blood vessels, is significantly impaired during hemorrhage, resulting in a mismatch between O(2) delivery and the metabolic activity in the tissues. NO can be released from multiple sources in the vasculature. Recent studies have shown that erythrocytes express functional endothelial nitric oxide synthase (NOS3), which potentially serves as an intraluminal NO source. NO delivery from this source is complex: erythrocytes are not only NO producers but also act as potent sinks because of the high affinity of NO for hemoglobin. To test our hypothesis that the loss of erythrocytic NOS3 during hemorrhage contributes to NO deficiency-related shock, we have constructed a multicellular computational model that simulates NO production and transport to allow us to quantify the loss of NO under different hemorrhagic conditions. Our model shows that: (1) during mild hemorrhage and subsequent hemodilution (hematocrit >30%), NO from this intraluminal source is only slightly decreased in the vascular smooth muscle, but the NO level is significantly reduced under severe hemorrhagic conditions (hematocrit <30%); (2) whether a significant amount of NO from this source can be delivered to vascular smooth muscle is strongly dependent on the existence of a protective mechanism for NO delivery; (3) if the expression level of NOS3 on erythrocytes is similar to that on endothelial cells, we estimate approximately 13 pM NO at the vascular smooth muscle from this source when such a protective mechanism is involved. This study provides a basis for detailed studies to characterize the impairment of NO release pathways during hemorrhage and yield important insights for the development of resuscitation methods.

Nitric oxide production pathways in erythrocytes and plasma

Nitric oxide (NO) is a potent regulator of vascular tone and hemorheology. The signaling function of NO was largely unappreciated until approximately 30 years ago, when the endothelium-derived relaxing factor (EDRF) was identified as NO. Since then, NO from the endothelium has been considered the major source of NO in the vasculature and a contributor to the paracrine regulation of blood hemodynamics. Because NO is highly reactive, and its half-life in vivo is only a few seconds (even less in the bloodstream), any NO bioactivity derived from the intraluminal region has traditionally been considered insignificant. However, the availability and significance of NO signaling molecules derived from intraluminal sources, particularly erythrocytes, have gained attention in recent years. Multiple potential sources of NO bioactivity have been identified in the blood, but unresolved questions remain concerning these proposed sources and how the NO released via these pathways actually interacts with intravascular and extravascular targets. Here we review the hypotheses that have been put forward concerning blood-borne NO and its contribution to hemorheological properties and the regulation of vascular tone, with an emphasis on the quantitative aspects of these processes.