Clinical Hemorheology and Microcirculation

Vascular endothelial cells form a monocellular layer on blood vessel walls with an estimated mass of 1.5 kg. One of the roles of endothelial cells is to control the hemodynamics through various metabolic activities affecting homeostasis, vascular tonus, blood fluidity, coagulating properties and blood cell adhesion. In other respects thousands of studies have underlined the crucial role of local blood flow conditions on their properties. However, the hemodynamic forces are different according to the anatomical site and to the type of blood vessels (arteries, veins, venules, ...). In microcirculation, the endothelial cells in the venules are particularly active and constitute the physiological site of liquid exchange (permeability) and above all cellular transit. During critical ischemia, the post-capillary venules are deeply involved. In other respects the properties of endothelial cells may be impaired in many diseases as atherosclerosis, hypertension, inflammation and metabolic diseases.

Although the existence of hsp90–NOS and hsp90–sGC complexes is now firmly established, their role in many pathophysiological processes remain unclear. These complexes may represent physiological mechanisms aimed at maximizing intracellular cGMP production in response to endogenous or drug-derived NO in endothelial cells and thus affecting permeability, proliferation, migration and apoptosis. Along with minimizing NO scavenging by superoxide and reducing the formation of peroxynitrite, these complexes may also prolong sGC stability by retarding its degradation. Our work and that of others have demonstrated that, depending on the environment, sGC interaction with hsp90 can optimize sGC enzyme activity or modulate sGC survival. This review addresses the functional significance of hsp90 complexes with NOS (eNOS, iNOS) and sGC in endothelial cells relevant for maintaining endothelial barrier integrity and angiogenesis. Structural and functional characteristics of sGC, its expression, transcriptional and post-translational regulation, as they relate to sGC–hsp90 interactions, will also be examined.

We have developed a novel uniaxial cyclic stretching technique to apply a ventral nonuniform strain to cells. In this system cells are grown on a glass-embedded silicone substrate instead of the commonly used uniform substrates. This unique substrate has been developed to give a strain gradient of 0.2%/μm across each individual cell. Bovine aortic endothelial cells (BAECs) were cyclically stretched up to a maximum strain of 50% at 0.5 Hz for 30 min or 3 hours, focusing on the effect of the ventral strain gradient on local cell remodeling. After the experiments, BAECs were fixed and stained with rhodamine-phalloidin to observe actin filament structure. BAECs showed local development of stress fibers and localization of cell nuclei at regions exposed to higher strain. This result suggests that BAECs may sense ventral nonuniform strain and remodel cytoskeletal structure accordingly followed by the movement of cell nuclei.

The evaluation of signaling pathways leading to gene induction by VEGF-A and IL-1 in endothelial cells supports the importance of the NF-κB pathway for the IL-1-induced gene repertoire, whereas VEGF-A is a strong and preferential trigger of signals via PLC-γ. This leads (i) via Ca⁺⁺ to the activation of calcineurin and NFAT and (ii) via PKC and the MEK/ERK MAPK pathway to the upregulation of EGR-1. Part of the VEGF-triggered gene induction depends on a cooperation of the transcription factors NFAT and EGR-1. Gene activation via PLC-γ provides VEGF with the potency to induce a wide spectrum of genes including many also upregulated by IL-1. A gene upregulated by VEGF and IL-1 is the DSCR-1 gene, which encodes an inhibitor of calcineurin. DSCR1 is induced by NFAT or NF-κB and limits Ca⁺⁺ signaling in a negative feed-back loop. Similarly, NAB2, a corepressor of EGR-1, is induced by EGR-1 and limits EGR-1 effects. Adenoviral overexpression of DSCR1 or NAB2 inhibited part of VEGF-induced gene expression and reduced sprouting in angiogenesis models.

Ca²⁺ mobilizing agonists and hemodynamic shear stress both elicit a rise in endothelial cytosolic Ca²⁺ [Ca²⁺]_i, which then acts to stimulate nitric oxide synthase and phospholipase A₂, leading to the production and release of nitric oxide (NO) and other vascular substances such as prostacyclin and endothelium-derived hyperpolarizing factors (EDHF). In this article, regulatory mechanisms of agonist-induced and mechanosensitive Ca²⁺ influx pathways in vascular endothelial cells will be discussed. Special emphasis will be placed on the regulation of agonist-induced Ca²⁺ influx by protein kinase G (PKG). Flow-induced Ca²⁺ influx in relation to vascular dilation and the vasodilator produced will also be discussed.

This review presents some of the recent technological developments in biomaterials used for the construction of synthetic cardiovascular vessels that are capable of simulating specific biological responses. However, with respect to the problems of stiffness, a major hypertensive risk factor, it is necessary to underline the important role of mechanical properties, such as vessel strength and composition, in vascular reconstructive surgery. Biomaterials occupy a central place in many cardiovascular disease treatments and they depend on the chemical nature of the polymers, on the biotechnology used, and also on cellular and gene therapy. Several methodologies using animal or human cells have emerged for constructing blood vessel replacements. Tissue-engineered blood vessel (TEBV) substitutes begin to motivate much work and have contributed to the restoration, maintenance, and/or improvement in tissue and organ function. Each methodology has it benefits, its promises, and holds many challenges in future biological, biomaterial and clinical research.

Imaging thick and opaque tissue, like blood vessel, in a noninvasive mode with high resolution, is nowadays possible with multiphoton technology. A near-infrared excitation presents the advantage to be compatible with living specimens and allows a deep penetration into tissues. The nonlinear excitation process is followed by several deactivation ways, among which fluorescence emission can be represented with Spectral or Lifetime imaging. Applied to ex vivo blood vessel imaging, these techniques enabled us to discriminate cell structures (nucleus, cytoskeleton) by fluorescent labelling (Hoechst, QDots). Another method, based on 2-photon excitation and which doesn't need any exogenous dye has also been experimented on arteries: SHG (Second Harmonic Generation) is a diffusion process generated from organized structures. Collagen molecules give rise to a strong SHG signal, enabling us to image the arterial wall (3-dimensional extracellular matrix).

Although autogenous vessels are useful in surgery, often patients cannot furnish suitable vessels. If there are not available, two possible alternatives for vessel replacements are to use vascular synthetic prostheses such as Dacron^® and polytetrafluoroethylene (PTFE) or cryopreserved allografts. However, their success has been limited to replace small-diameter (<6 mm) arterial vessel because of their high thrombogenicity and compliance mismatch. On account of a clear clinical need for a functional arterial substitute, tissue engineering techniques have been developed. This review encompasses the use of mature endothelial, endothelial progenitor and bone marrow cells combined with natural or synthetic scaffolds whose surface has been modified with multiple origin matrices.

In the vascular system, the shear applied to the vascular wall activates mechano-sensors located on endothelial cells (ECs) leading to a modification in the gene expression profile. We applied laminar shear stress at 1 Pa on ECs for 6 h and measured by quantitative real time PCR the expression modulation of genes implied in inflammation (ICAM-1 and E-selectin), oxidative stress sensing (HO-1) and vascular tone modulation (eNOS). We showed that all these genes are shear stress inducible. ICAM-1 is more up-regulated than E-selectin suggesting different levels of implication in inflammatory responses and different modes of induction (SSRE, cytokine). Laminar shear stress induces an oxidative stress translated into HO-1 up-regulation, and a possible vasodilatation through the induction of eNOS. Our laminar shear stress system opens a novel and interesting frame in the evaluation of the impact on ECs and blood cells of new pharmacological substances injected in the bloodstream.

An increasing number of studies implicate oxidative stress in the development of endothelial dysfunction and the pathogenesis of cardiovascular disease. Further, this oxidative stress has been shown to be associated with alterations in both the endothelin-1 (ET-1) and nitric oxide (NO) signaling pathways such that bioavailable NO is decreased and ET-1 signaling is potentiated. However, recent data, from our groups and others, have shown that oxidative stress, ET-1, and NO are co-regulated in a complex fashion that appears to be dependent on the cellular levels of each species. Thus, when ROS levels are transiently elevated, NO signaling is potentiated through transcriptional, post-transcriptional, and post-translational mechanisms. However, in pediatric pulmonary hypertensive disorders, when reactive oxygen species (ROS) increases are sustained by ET-1 mediated activation of smooth muscle cell ET_A subtype receptors, NOS gene expression and NO signaling are reduced. Further, increases in oxidative stress can stimulate both the expression of the ET-1 gene and the secretion of the ET-1 peptide. Thus, this manuscript will review the available data regarding the interaction of NO, ET-1, and ROS in the endothelial dysfunction of pediatric pulmonary hypertension. In addition, we will suggest avenues of both basic and clinical research that will be important to develop novel pulmonary hypertension treatment and prevention strategies.

Shear stress has been shown to influence endothelial cell gene expression and morphology. In particular, low and bi-directional shear stress, mimicking conditions at plaque-prone areas, down-regulates the expression of several atheroprotective genes, and up-regulates that of other genes considered as pro-inflammatory. Another mechanical situation thought to have a negative influence on vascular functions is arterial stiffness. Loss of arterial compliance occurs during ageing, in diabetic as well as in hypertensive patients. In this work we investigated the effects of these two particular hemodynamic environments (bi-directional shear stress and reduced compliance), using a recently developed perfusion system allowing to expose native arteries in vitro to complex hemodynamic environments. We were able to show that both plaque-prone shear stress and reduced compliance trigger endothelial dysfunction, but via different mechanisms. Only reduced compliance affected vascular contractility, inducing a dedifferentiation of smooth muscle cells and a consequent loss of norepinephrine sensitivity.

Endothelial cells (ECs) which participate the interface between the blood and the vessel wall undergo morphologic changes in response to shear stress induced by blood flow, liable for the important regulation on physiologic and pathophysiologic function of blood vessels. Shear stress induced changes in cell morphology, begin with elongation in the direction of shearing and end by a reorientation and assembly of F-actin stress fibers. Shear stress is also implicated in many important ECs functions such as: decrease of platelet aggregation, anti-thrombogenic and anti-adhesive effects, inhibition of vascular smooth muscle cell (SMC) proliferation and regulation of their contraction and arterial tonicity, via a regulation of vasodilator and vasoconstrictor secretion molecules such as nitric oxide (NO), endothelin I, prostacyclin and angiotensin II. Besides, many of human diseases such as hypercholesterolemia, diabetes and hypertension, are strongly linked to a disturbance of the production of several vasodilator or vasoconstrictor molecules.

The aim of this in-vitro study was to evaluate the potential balance between time and rate effects of shearing in cell shape changes and e-NOS activity. Two unidirectional steady laminar flow rates (1.2 Pa and 2.0 Pa) were applied on EC monolayers, each one for a short and a long period, (6 h and 24 h). Cytoskeleton reorganization was evaluated by actin filaments labelling and observed by confocal microscopy. NO production was evaluated by a colorimetric method using the Griess reagent kit for nitrite determination.

Results showed that laminar flow affected cell rearrangement by inducing cytoskeleton reorientation and increased production of NO. Laminar shear rate at 2.0 Pa for 24 h did not upregulate NO release. Whereas at 1.2 Pa for 24 h, NO release increased by 33% compared with the static conditions. Both 1.2 Pa and 2.0 Pa for 6 h increased NO release by 17% and 24% respectively as compared with the static conditions. These observations suggested that stress fiber assembly, which controls EC reorientation and NO production, are dependent on rate and time of shearing. In addition, there appear to be a relation between the cytoskeleton reorganization stage and NO production. These results could promote the parameters to evaluate the more appropriate pattern of shearing, to evaluate a potential pharmacological effect on hypertension disorder decrease.

Aging is the major risk factor for the development of cardiovascular diseases, the leading cause of morbidity, mortality and disability in western countries. Mounting data suggest that cardiovascular structure and function change with time as result of an “aging process”, regarded as an independent process which accompanies aging, interwines and modulates superimposed traditional cardiovascular risk factors to determine the peculiar occurrence, presentations and prognosis of heart disease in the elderly. A whole body of data underlies the impairment of endothelial function due to oxidative stress as a crucial feature of the aging process acting on the cardiovascular system. Insights into molecular and cellular mechanisms of age-associated endothelial dysfunction may provide new strategies to treat age-related cardiovascular diseases.

We have assessed the NO system in the cardiovascular and renal systems of young, adult and old normotensive (WKY) and hypertensive rats (SHR). The NO pathway was assessed analytically, by measuring the concentration of nitrate in plasma as well as the activity of NO synthases in the left ventricle and kidney; and functionally, by measuring the isometric forces generated upon addition of the NO blocker, L-NAME, to aortic segments. All these procedures consistently revealed that the NO pathway is upregulated in hypertension or senescence. In addition, we have performed immunohistochemical studies of NO synthases in the kidney of adult animals (WKY and SHR). NO synthases are expressed throughout the kidney in both rat strains. Immunoreactivity of neuronal NOS was higher in the tubular cells of the renal medulla of the SHR. Staining with the inducible and endothelial NOS antibodies was similar in normo- and hypertension. In summary, hypertension and ageing upregulate the NO pathway in structures involved in the regulation of blood pressure (heart, vessels and kidney).

Endothelin-1 (ET-1) is a powerful vasoconstrictor and mitogen that contributes to blood pressure elevation and related vascular remodeling and target organ damage. ET-1 also influences salt and water homeostasis through effects on the renin-angiotensin-aldosterone system and vasopressin, thus elevating blood pressure and increasing vascular tone. Circulating ET-1 levels are elevated in a variety of animal models of hypertension, particularly those that are salt-dependent, and in a subset of human hypertensives, i.e. African-Americans and those with renal dysfunction. ET type B receptors, which normally have vasodilator functions, mediate vasoconstriction in some hypertensives, and hypertensive African-American patients may have increased numbers of vasoconstrictor ET-B receptors in their vascular smooth muscle. Whether selective ET-A or combined ET-A/ET-B receptor antagonists are more efficacious in treating hypertension and related cardiovascular disease is controversial. ET antagonists have only modest BP lowering effects in the general population of essential hypertensives, but show promise in patients with severe, treatment resistant hypertension.

Statins can protect endothelial activation independent from their lipid-lowering effects. To gain more insight in mechanisms via which HMG-CoA inhibition may attenuate endothelial activation, we assessed the effects of mevastatin on eNOS expression of non- and modified-LDL treated endothelial cells and on basal and hydrogen peroxide-induced lipid peroxidation. Oxidized-LDL (Ox-LDL), but not glycated or acylated LDL decreased eNOS expression in human endothelial cells. The extent to which Ox-LDL decreases eNOS expression depends on the extent of modification of Ox-LDL. Mevastatin increased eNOS-expression, both in the presence and absence of Ox-LDL. In addition, mevastatin decreased the H₂O₂-induced (100 μM) lipid peroxidation in endothelial cells. This study shows that mevastatin has protective effects on endothelial cells by inducing eNOS and by inhibiting lipid peroxidation.

Oxidative stress contributes to homeostasis in vascular cells. However, excessive ROS is pathophysiological and contributes to impaired endothelium-dependent dilation in hypertension by decreasing NO bio-availability. NADPH oxidase is upregulated in hypertension by humoral and mechanical signals, and quantitatively this enzyme makes the largest contribution to ROS production. Genetic and chemical manipulation of NADPH oxidase and of antioxidant enzymes cause predictable changes in oxidative stress and endothelium-dependent function in hypertension. The chemical antioxidant glutathione also impacts endothelium-mediated vascular function, possibly through maintenance of S-nitrosothiols and via other independent antioxidant effects. The effects of changes in thiols and nitrosothiols on vasomotor function in hypertension need to be examined. H₂O₂ is believed to act as an EDHF physiologically. Thus, there must be competition between the influence of ROS and oxidative stress on NO-dependent dilation versus EDHF-dependent dilation. The crossover effects of ROS on the three main endothelium-dependent dilatory pathways must be examined in hypertension models.

The sympathetic system is central in the understanding of numerous physiological and physiopathological phenomena. During the last decade, the characterization of a new β-adrenoceptor subtype, β₃, in addition to β₁ and β₂-adrenoceptor in the cardiovascular system has changed the view of the roles of the sympathetic system. In heart, β₃-adrenoceptor stimulation produce an opposite effect to that induced by β₁ and β₂-adrenoceptors suggesting that in normal heart, the negative inotropic effect induced by the β₃-adrenergic stimulation might play a role of a “safety-valve” during intense adrenergic stimulation. In vessels, all β-adrenoceptors subtypes, β₁, β₂ and β₃, mediate a vasodilation. As β₃-adrenoceptors are activated at higher concentrations of catecholamines than β₁ and β₂-adrenoceptors, they could play the roll of a receptor reserve. β₃-adrenoceptors are overexpressed in heart failure and hypertension and could constitute a new therapeutic target. In addition, the efficiency of some β-blockers such as nebivolol could result from an action on β₃-adrenoceptors.

Hypertension is associated with vascular structural alterations known as “vascular remodelling”, which initially are adaptive but in the long run, lead to vascular damage and loss of function. Despite decades of study, there is still modest information on the 3-dimensional (3D) arrangement of vascular cells and extracellular matrix (ECM) and how they change under pathological situations. To address this problem we developed a technique which combines fluorescence confocal microscopy, pressure myography and image analysis, “confocal myography”, which permits the study of intact resistance-sized vessels at cellular level and at physiological pressure. With the aid of this method, we have identified, in arteries from hypertensive rats, abnormal orientation of endothelial, smooth muscle cells (SMC) and elastic fibres; elongation and denudation of endothelial cells, and adventitial hypercellularity. Confocal myography offers a new approach to the study of vascular remodelling in intact small arteries from a 3D point of view.