Differential effects of pressure and flow on DNA and protein synthesis and on fibronectin expression by arteries in a novel organ culture system

High carbohydrate high fat diet causes arterial hypertension and histological changes in the aortic wall in aged rats: The involvement of connective tissue growth factors and fibronectin

BACKGROUND

Age and diabetes are risk factors for arterial hypertension. However, the relationship between age, connective tissue growth factors, vascular aging and arterial hypertension while on a the high-carbohydrate high-fat diet (HCHFD) remains poorly understood.

PURPOSE

To estimate the relationship between humoral factors, the morphological changes of aorta and impaired blood pressure regulation under the influence of age and a HCHFD.

METHODS

A study was carried out in male Wistar rats, which were divided into the following groups: 1st (n = 15) - naive young rats; 2nd (n = 15) - young rats, exposed to HCHFD; 3rd (n = 14) - naive old rats; 4th (n = 12) - old rats exposed to HCHFD. The age of old rats was 540 days, and young rats 150 days at the end of the diet. HCHFD contained proteins 16%, fats 21%, carbohydrates 46%, including 17% fructose, 0.125% cholesterol, 90 days. Blood pressure and body weight were measured weekly, carbohydrate metabolism, histological signs of changes in the aorta, serum transforming growth factor-β (TGF-β), connective tissue growth factor (CTGF), fibronectin, and endothelin-1 levels were determined one week after the onset of diet.

RESULTS

The severity of arterial hypertension and its histological signs in the aortic wall was found to be most pronounced in elderly rats kept on a HCHFD. In young rats kept on a HCHFD, arterial hypertension was transient. An increase in systolic blood pressure has a positive correlation with the degree of obesity, serum fibronectin, and endothelin-1 content, and impaired carbohydrate metabolism. The rise in diastolic blood pressure has a positive correlation with the serum CTGF, endothelin-1, fibronectin levels and aortic wall thickness, and impaired carbohydrate metabolism. A rise in the serum concentration of fibronectin was also associated with increased endothelin-1, TGFβ and CTGF serum levels.

CONCLUSION

This study indicated that an increase in blood pressure in old rats with a high-carbohydrate high-fat diet is due to a disturbance of a structure of the vascular wall, the release of fibronectin, which can occur under the influence of carbohydrate metabolism disorders, endothelin-1, TGFβ and CTGF.

An Ex Vivo Vessel Injury Model to Study Remodeling

Objective:

Invasive coronary interventions can fail due to intimal hyperplasia and restenosis. Endothelial cell (EC) seeding to the vessel lumen, accelerating re-endothelialization, or local release of mTOR pathway inhibitors have helped reduce intimal hyperplasia after vessel injury. While animal models are powerful tools, they are complex and expensive, and not always reflective of human physiology. Therefore, we developed an in vitro 3D vascular model validating previous in vivo animal models and utilizing isolated human arteries to study vascular remodeling after injury. Approach: We utilized a bioreactor that enables the control of intramural pressure and shear stress in vessel conduits to investigate the vascular response in both rat and human arteries to intraluminal injury.

Results:

Culturing rat aorta segments in vitro, we show that vigorous removal of luminal ECs results in vessel injury, causing medial proliferation by Day-4 and neointima formation, with the observation of SCA1⁺ cells (stem cell antigen-1) in the intima by Day-7, in the absence of flow. Conversely, when endothelial-denuded rat aortae and human umbilical arteries were subjected to arterial shear stress, pre-seeding with human umbilical ECs decreased the number and proliferation of smooth muscle cell (SMC) significantly in the media of both rat and human vessels.

Conclusion:

Our bioreactor system provides a novel platform for correlating ex vivo findings with vascular outcomes in vivo. The present in vitro human arterial injury model can be helpful in the study of EC-SMC interactions and vascular remodeling, by allowing for the separation of mechanical, cellular, and soluble factors.

Omics-based approaches to understand mechanosensitive endothelial biology and atherosclerosis

Atherosclerosis is a multifactorial disease that preferentially occurs in arterial regions exposed to d-flow can be used to indicate disturbed flow or disturbed blood flow. The mechanisms by which d-flow induces atherosclerosis involve changes in the transcriptome, methylome, proteome, and metabolome of multiple vascular cells, especially endothelial cells. Initially, we begin with the pathogenesis of atherosclerosis and the changes that occur at multiple levels owing to d-flow, especially in the endothelium. Also, there are a variety of strategies used for the global profiling of the genome, transcriptome, miRNA-ome, DNA methylome, and metabolome that are important to define the biological and pathophysiological mechanisms of endothelial dysfunction and atherosclerosis. Finally, systems biology can be used to integrate these 'omics' datasets, especially those that derive data based on a single animal model, in order to better understand the pathophysiology of atherosclerosis development in a holistic manner and how this integrative approach could be used to identify novel molecular diagnostics and therapeutic targets to prevent or treat atherosclerosis. WIREs Syst Biol Med 2016, 8:378-401. doi: 10.1002/wsbm.1344 For further resources related to this article, please visit the WIREs website.

Studies on the Tumor Vasculature and Coagulant Microenvironment

Angiogenesis represents one aspect in the complex process that leads to the generation of the vascular tumor stroma. The related functional constituents include responses of endothelial, mural, bone marrow-derived, and resident inflammatory cells as well as activation of coagulation and fibrinolytic systems in blood. Multiple molecular and cellular effectors participate in these events, often in a tumor-specific manner and with changes enforced through the microenvironment, genetic evolution, and responses to anticancer therapies. To capture various elements of these interactions several surrogate assays have been devised, which can be mechanistically useful and are amenable to quantification, but are individually insufficient to describe the underlying complexity and are best used in a targeted and combinatorial manner. Below, we present a survey of angiogenesis assays and experimental approaches to analyze vascular events in cancer. We also provided specific examples of validated protocols, which are less described, but enable the straightforward analysis of vascular structures and coagulant properties of cancer cells in vivo and in vitro.

The role of endothelial mechanosensitive genes in atherosclerosis and omics approaches

Atherosclerosis is the leading cause of morbidity and mortality in the U.S., and is a multifactorial disease that preferentially occurs in regions of the arterial tree exposed to disturbed blood flow. The detailed mechanisms by which d-flow induces atherosclerosis involve changes in the expression of genes, epigenetic patterns, and metabolites of multiple vascular cells, especially endothelial cells. This review presents an overview of endothelial mechanobiology and its relation to the pathogenesis of atherosclerosis with special reference to the anatomy of the artery and the underlying fluid mechanics, followed by a discussion of a variety of experimental models to study the role of fluid mechanics and atherosclerosis. Various in vitro and in vivo models to study the role of flow in endothelial biology and pathobiology are discussed in this review. Furthermore, strategies used for the global profiling of the genome, transcriptome, miR-nome, DNA methylome, and metabolome, as they are important to define the biological and pathophysiological mechanisms of atherosclerosis. These "omics" approaches, especially those which derive data based on a single animal model, provide unprecedented opportunities to not only better understand the pathophysiology of atherosclerosis development in a holistic and integrative manner, but also to identify novel molecular and diagnostic targets.

Thromboxane promotes smooth muscle phenotype commitment but not remodeling of hypoxic neonatal pulmonary artery

Background

Persistent pulmonary hypertension of the newborn (PPHN) is characterized by vasoconstriction and pulmonary vascular remodeling. Remodeling is believed to be a response to physical or chemical stimuli including pro-mitotic inflammatory mediators such as thromboxane. Our objective was to examine the effects of hypoxia and thromboxane signaling ex vivo and in vitro on phenotype commitment, cell cycle entry, and proliferation of PPHN and control neonatal pulmonary artery (PA) myocytes in tissue culture.

Methods

To examine concurrent effects of hypoxia and thromboxane on myocyte growth, serum-fed first-passage newborn porcine PA myocytes were randomized into normoxic (21 % O₂) or hypoxic (10 % O₂) culture for 3 days, with daily addition of thromboxane mimetic U46619 (10⁻⁹ to 10⁻⁵ M) or diluent. Cell survival was detected by MTT assay. To determine the effect of chronic thromboxane exposure (versus whole serum) on activation of arterial remodeling, PPHN was induced in newborn piglets by a 3-day hypoxic exposure (FiO₂ 0.10); controls were 3 day-old normoxic and day 0 piglets. Third-generation PA were segmented and cultured for 3 days in physiologic buffer, Ham’s F-12 media (in the presence or absence of 10 % fetal calf serum), or media with 10⁻⁶ M U46619. DNA synthesis was measured by ³H-thymidine uptake, protein synthesis by ³H-leucine uptake, and proliferation by immunostaining for Ki67. Cell cycle entry was studied by laser scanning cytometry of nuclei in arterial tunica media after propidium iodide staining. Phenotype commitment was determined by immunostaining tunica media for myosin heavy chain and desmin, quantified by laser scanning cytometry.

Results

Contractile and synthetic myocyte subpopulations had differing responses to thromboxane challenge. U46619 decreased proliferation of synthetic and contractile myocytes. PPHN arteries exhibited decreased protein synthesis under all culture conditions. Serum-supplemented PA treated with U46619 had decreased G1/G0 phase myocytes and an increase in S and G2/M. When serum-deprived, PPHN PA incubated with U46619 showed arrested cell cycle entry (increased G0/G1, decreased S and G2/M) and increased abundance of contractile phenotype markers.

Conclusions

We conclude that thromboxane does not initiate phenotypic dedifferentiation and proliferative activation in PPHN PA. Exposure to thromboxane triggers cell cycle exit and myocyte commitment to contractile phenotype.

Arsenic Exposure Increases Monocyte Adhesion to the Vascular Endothelium, a Pro-Atherogenic Mechanism

Epidemiological studies have shown that arsenic exposure increases atherosclerosis, but the mechanisms underlying this relationship are unknown. Monocytes, macrophages and platelets play an important role in the initiation of atherosclerosis. Circulating monocytes and macrophages bind to the activated vascular endothelium and migrate into the sub-endothelium, where they become lipid-laden foam cells. This process can be facilitated by platelets, which favour monocyte recruitment to the lesion. Thus, we assessed the effects of low-to-moderate arsenic exposure on monocyte adhesion to endothelial cells, platelet activation and platelet-monocyte interactions. We observed that arsenic induces human monocyte adhesion to endothelial cells in vitro. These findings were confirmed ex vivo using a murine organ culture system at concentrations as low as 10 ppb. We found that both cell types need to be exposed to arsenic to maximize monocyte adhesion to the endothelium. This adhesion process is specific to monocyte/endothelium interactions. Hence, no effect of arsenic on platelet activation or platelet/leukocyte interaction was observed. We found that arsenic increases adhesion of mononuclear cells via increased CD29 binding to VCAM-1, an adhesion molecule found on activated endothelial cells. Similar results were observed in vivo, where arsenic-exposed mice exhibit increased VCAM-1 expression on endothelial cells and increased CD29 on circulating monocytes. Interestingly, expression of adhesion molecules and increased binding can be inhibited by antioxidants in vitro and in vivo. Together, these data suggest that arsenic might enhance atherosclerosis by increasing monocyte adhesion to endothelial cells, a process that is inhibited by antioxidants.

Pulmonary hypertension caused by pulmonary venous hypertension

The effect of pulmonary venous hypertension (PVH) on the pulmonary circulation is extraordinarily variable, ranging from no impact on pulmonary vascular resistance (PVR) to a marked increase. The reasons for this are unknown. Both acutely reversible pulmonary vasoconstriction and pathological remodeling (especially medial hypertrophy and intimal hyperplasia) account for increased PVR when present. The mechanisms involved in vasoconstriction and remodeling are not clearly defined, but increased wall stress, especially in small pulmonary arteries, presumably plays an important role. Myogenic contraction may account for increased vascular tone and also indirectly stimulate remodeling of the vessel wall. Increased wall stress may also directly cause smooth muscle growth, migration, and intimal hyperplasia. Even long-standing and severe pulmonary hypertension (PH) usually abates with elimination of PVH, but PVH-PH is an important clinical problem, especially because PVH due to left ventricular noncompliance lacks definitive therapy. The role of targeted PH therapy in patients with PVH-PH is unclear at this time. Most prospective studies indicate that these medications are not helpful or worse, but there is ample reason to think that a subset of patients with PVH-PH may benefit from phosphodiesterase inhibitors or other agents. A different approach to evaluating possible pharmacologic therapy for PVH-PH may be required to better define its possible utility.

Techniques and assays for the study of angiogenesis

The importance of studying angiogenesis, the formation of new blood vessels from pre-existing vessels, is underscored by its involvement in both normal physiology, such as embryonic growth and wound healing, and pathologies, such as diabetes and cancer. Treatments targeting the molecular drive of angiogenesis have been developed, but many of the molecular mechanisms that mediate vascularization, as well as how these mechanisms can be targeted in therapy, remain poorly understood. The limited capacity to quantify angiogenesis properly curtails our molecular understanding and development of new drugs and therapies. Although there are a number of assays for angiogenesis, many of them strip away its important components and/or limit control of the variables that direct this highly cooperative and complex process. Here we review assays commonly used in endothelial cell biology and describe the progress toward development of a physiologically realistic platform that will enable a better understanding of the molecular and physical mechanisms that govern angiogenesis.

Pulse pressure, arterial stiffness, and end-organ damage

Whereas larger arteries participate in the dampening of blood pressure (BP) oscillations resulting from intermittent ventricular ejection, smaller arteries steadily deliver an adequate supply of blood from the heart to the peripheral organs. Numerous active mechanisms are involved in this process. Cyclic stress acts differently from steady stress, inducing stronger and stiffer material of the vessel wall than under static conditions. Cyclic strain participates in the phenotypic plasticity of smooth muscle cells, initiates transduction mechanisms and induces the transcriptional profile of mechanically induced genes. Finally, the autoregulatory mechanism protecting the brain, heart and kidney from cardiovascular (CV) damage differ markedly according to their localization. Whereas the heart is dependent on pulsatile forces, owing to the diastolic perfusion of coronary arteries, the brain and the kidney are rather influenced by steady mechanical forces. For the kidney, the transmission of pulsatile pressure may greatly contribute to glomerular sclerosis in the elderly.

SERCA2a gene transfer prevents intimal proliferation in an organ culture of human internal mammary artery

Coronary restenosis, a major complication of percutaneous balloon angioplasty, results from neointimal proliferation of vascular smooth muscle cells (VSMCs). The sarco/endoplasmic reticulum calcium ATPase isoform 2a (SERCA2a), specific to contractile VSMCs, has been reported previously to be involved in the control of the Ca²⁺-signaling pathways governing proliferation and migration. Moreover, SERCA2a gene transfer was reported to inhibit in vitro VSMC proliferation and to prevent neointimal thickening in a rat carotid injury model.

The aim of this study was to evaluate the potential therapeutic interest of SERCA2a gene transfer for prevention of in-stent restenosis using a human ex vivo model of left internal mammary artery (hIMA) intimal thickening.

Left hIMAs, obtained at the time of aorto-coronary bypass surgeries, were subjected to balloon dilatation followed by infection for 30 min with adenoviruses encoding either human SERCA2 and GFP or control gene (beta-galactosidase) and GFP. Proliferation of subendothelial VSMCs and neointimal thickening were observed in balloon-injured hIMA maintained 14 days in organ culture under constant pressure and perfusion. SERCA2a gene transfer prevented vascular remodeling and significantly (p<0.01, n=5) reduced neointimal thickening in injured arteries (intima/media ratio was 0.07 ± 0.01 vs 0.40 ± 0.03 in βGal-infected arteries).

These findings could have potential implications for treatment of pathological in stent-restenosis.

Cyclic strain upregulates VEGF and attenuates proliferation of vascular smooth muscle cells

Objective

Vascular smooth muscle cell (VSMC) hypertrophy and proliferation occur in response to strain-induced local and systemic inflammatory cytokines and growth factors which may contribute to hypertension, atherosclerosis, and restenosis. We hypothesize VSMC strain, modeling normotensive arterial pressure waveforms in vitro, results in attenuated proliferative and increased hypertrophic responses 48 hrs post-strain.

Methods

Using Flexcell Bioflex Systems we determined the morphological, hyperplastic and hypertrophic responses of non-strained and biomechanically strained cultured rat A7R5 VSMC. We measured secretion of nitric oxide, key cytokine/growth factors and intracellular mediators involved in VSMC proliferation via fluorescence spectroscopy and protein microarrays. We also investigated the potential roles of VEGF on VSMC strain-induced proliferation.

Results

Protein microarrays revealed significant increases in VEGF secretion in response to 18 hours mechanical strain, a result that ELISA data corroborated. Apoptosis-inducing nitric oxide (NO) levels also increased 43% 48 hrs post-strain. Non-strained cells incubated with exogenous VEGF did not reproduce the antimitogenic effect. However, anti-VEGF reversed the antimitogenic effect of mechanical strain. Antibody microarrays of strained VSMC lysates revealed MEK1, MEK2, phospo-MEK1^{T385, T291, T298}, phospho-Erk1/2^{T202+Y204/T185+T187}, and PKC isoforms expression were universally increased, suggesting a proliferative/inflammatory signaling state. Conversely, VSMC strain decreased expression levels of Cdk1, Cdk2, Cdk4, and Cdk6 by 25-50% suggesting a partially inhibited proliferative signaling cascade.

Conclusions

Subjecting VSMC to cyclic biomechanical strain in vitro promotes cell hypertrophy while attenuating cellular proliferation. We also report an upregulation of MEK and ERK activation suggestive of a proliferative phenotype. Hhowever, the proliferative response appears to be aborogated by enhanced antimitogenic cytokine VEGF, NO secretion and downregulation of Cdk expression. Although exogenous VEGF alone is not sufficient to promote the quiescent VSMC phenotype, we provide evidence suggesting that strain is a necessary component to induce VSMC response to the antimitogenic effects of VEGF. Taken together these data indicate that VEGF plays a critical role in mechanical strain-induced VSMC proliferation and vessel wall remodeling. Whether VEGF and/or NO inhibit signaling distal to Erk 1/2 is currently under investigation.

Animal, in vitro, and ex vivo models of flow-dependent atherosclerosis: role of oxidative stress

Atherosclerosis is an inflammatory disease preferentially occurring in curved or branched arterial regions, whereas straight parts of the arteries are protected, suggesting a close relationship between flow and atherosclerosis. However, evidence directly linking disturbed flow to atherogenesis is just emerging, thanks to the recent development of suitable animal models. In this article, we review the status of various animal, in vitro, and ex vivo models that have been used to study flow-dependent vascular biology and atherosclerosis. For animal models, naturally flow-disturbed regions such as branched or curved arterial regions as well as surgically created models, including arterio-venous fistulas, vascular grafts, perivascular cuffs, and complete, incomplete, or partial ligation of arteries, are used. Although in vivo models provide the environment needed to mimic the complex pathophysiological processes, in vitro models provide simple conditions that allow the study of isolated factors. Typical in vitro models use cultured endothelial cells exposed to various flow conditions, using devices such as cone-and-plate and parallel-plate chambers. Ex vivo models using isolated vessels have been used to bridge the gap between complex in vivo models and simple in vitro systems. Here, we review these flow models in the context of the role of oxidative stress in flow-dependent inflammation, a critical proatherogenic step, and atherosclerosis.

Alterations of pulse pressure stimulate arterial wall matrix remodeling

The effect of pulse pressure on arterial wall remodeling has not been clearly defined. The objective of this study was to evaluate matrix remodeling in arteries under nonpulsatile and hyperpulsatile pressure as compared with arteries under normal pulsatile pressure. Porcine carotid arteries were cultured for 3 and 7 days under normal, nonpulsatile, and hyperpulsatile pressures with the same mean pressure and flow rate using an ex vivo organ culture model. Fenestrae in the internal elastic lamina, collagen, fibronectin, and gap junction protein connexin 43 were examined in these arteries using confocal microscopy, immunoblotting, and immunohistochemistry. Our results showed that after 7 days, the mean fenestrae size and the area fraction of fenestrae decreased significantly in nonpulsatile arteries (51% and 45%, respectively) and hyperpulsatile arteries (45% and 54%, respectively) when compared with normal pulsatile arteries. Fibronectin decreased (29.9%) in nonpulsatile arteries after 3 days but showed no change after 7 days, while collagen I levels increased significantly (106%) in hyperpulsatile arteries after 7 days. The expression of connexin 43 increased by 35.3% in hyperpulsatile arteries after 7 days but showed no difference in nonpulsatile arteries. In conclusion, our results demonstrated, for the first time, that an increase or a decrease in pulse pressure from its normal physiologic level stimulates structural changes in the arterial wall matrix. However, hyperpulsatile pressure has a more pronounced effect than the diminished pulse pressure. This effect helps to explain the correlation between increasing wall stiffness and increasing pulse pressure in vivo.

Comparison of artery organ culture and co-culture models for studying endothelial cell migration and its effect on smooth muscle cell proliferation and migration

Arterial restenosis associated with intimal hyperplasia is the major cause of long-term failure of vascular interventions. Endothelium injury and the proliferation and migration of smooth muscle cells (SMC) are key events in the development of intimal hyperplasia. The objectives of this study were to develop an ex vivo artery injury model for studying endothelial cell (EC) migration and to compare it with an in vitro co-culture arterial wall injury model in terms of the effect of flow on EC migration and its effect on SMC migration and proliferation. Our results demonstrated that shear flow improves reendothelialization in the injured area by promoting EC migration. The migration distance of ECs is much smaller in the arteries than in an in vitro cell culture model (3.57+/-1.29 mm vs. 5.2+/-1.4 cm, p<0.001). SMC proliferation was significantly less in the EC intact and reendothelialization areas than in the EC denuded areas indicating that reendothelialization suppresses SMC proliferation. Our models provide a new approach to study techniques to enhance endothelium healing.

Transmural pressure and axial loading interactively regulate arterial remodeling ex vivo

Physiological axial strains range between 40 and 60% in arteries, resulting in stresses comparable to those due to normal blood pressure or flow. To investigate the contribution of axial strain to arterial remodeling and function, porcine carotid arteries were cultured for 9 days at physiological and reduced axial stretch ratios in the presence of normotensive and hypertensive transmural pressures by ex vivo perfusion techniques. Consistent with previous in vivo studies, vessels cultured with physiological levels of axial strain and exposed to hypertensive pressure had greater mass, wall area, and outer diameter relative to those cultured at the same axial stretch ratio and normotensive pressure. Reducing the amount of axial strain resulted in mass loss and decreased cell proliferation. Culture in a hypertensive pressure environment at reduced axial strain produced arteries with greater contractility in response to norepinephrine. Arteries cultured at reduced axial strain with the matrix metalloproteinase inhibitor GM6001 maintained their masses over culture, indicating a possible mechanism for this model of axial stretch-dependent remodeling. Although not historically considered one of the primary stimuli for remodeling, multiple linear regression analysis revealed that axial strain had an impact similar to or greater than transmural pressure on various remodeling indexes (i.e., outer diameter, wall area, and wet mass), suggesting that axial strain is a primary mediator of vascular remodeling.

Human saphenous vein organ culture under controlled hemodynamic conditions

INTRODUCTION

Saphenous vein grafting is still widely used to revascularize ischemic myocardium. The effectiveness of this procedure is limited by neointima formation and accelerated atherosclerosis, which frequently leads to graft occlusion. A better understanding of this process is important to clarify the mechanisms of vein graft disease and to aid in the formulation of strategies for prevention and/or therapeutics.

OBJECTIVE

To develop an ex vivo flow system that allows for controlled hemodynamics in order to mimic arterial and venous conditions.

METHODS

Human saphenous veins were cultured either under venous (flow: 5 ml/min) or arterial hemodynamic conditions (flow: 50 ml/min, pressure: 80 mmHg) for 1-, 2- and 4-day periods. Cell viability, cell density and apoptosis were compared before and after these intervals using MTT, Hoeschst 33258 stain, and TUNEL assays, respectively.

RESULTS

Fresh excised tissue segments were well preserved prior to the study. Hoechst 33258 and MTT stains showed progressive losses in cell density and cell viability in veins cultured under arterial hemodynamic conditions from 1 to 4 days, while no alterations were observed in veins cultured under venous conditions. Although the cell density from 1-day cultured veins under arterial conditions was similar to that of freshly excised veins, the TUNEL assay indicated that most of these cells were undergoing apoptosis.

CONCLUSION

The results observed resemble the events taking place during early in vivo arterial-vein grafting and provide evidence that an ex vivo perfusion system may be useful for the identification of new therapeutic targets that ameliorate vein graft remodeling and increase graft patency over time.

The early- and late stages in phenotypic modulation of vascular smooth muscle cells: differential roles for lysophosphatidic acid

Lysophosphatidic acid (LPA) has been implicated as causative in phenotypic modulation (PM) of cultured vascular smooth muscle cells (VSMC) in their transition to the dedifferentiated phenotype. We evaluated the contribution of the three major LPA receptors, LPA1 and LPA2 GPCR and PPARgamma, on PM of VSMC. Expression of differentiated VSMC-specific marker genes, including smooth muscle alpha-actin, smooth muscle myosin heavy chain, calponin, SM-22alpha, and h-caldesmon, was measured by quantitative real-time PCR in VSMC cultures and aortic rings kept in serum-free chemically defined medium or serum- or LPA-containing medium using wild-type C57BL/6, LPA1, LPA2, and LPA1&2 receptor knockout mice. Within hours after cells were deprived of physiological cues, the expression of VSMC marker genes, regardless of genotype, rapidly decreased. This early PM was neither prevented by IGF-I, inhibitors of p38, ERK1/2, or PPARgamma nor significantly accelerated by LPA or serum. To elucidate the mechanism of PM in vivo, carotid artery ligation with/without replacement of blood with Krebs solution was used to evaluate contributions of blood flow and pressure. Early PM in the common carotid was induced by depressurization regardless of the presence/absence of blood, but eliminating blood flow while maintaining blood pressure or after sham surgery elicited no early PM. The present results indicate that LPA, serum, dissociation of VSMC, IGF-I, p38, ERK1/2, LPA1, and LPA2 are not causative factors of early PM of VSMC. Tensile stress generated by blood pressure may be the fundamental signal maintaining the fully differentiated phenotype of VSMC.

Plaque-prone hemodynamics impair endothelial function in pig carotid arteries

Hemodynamic forces play an active role in vascular pathologies, particularly in relation to the localization of atherosclerotic lesions. It has been established that low shear stress combined with cyclic reversal of flow direction (oscillatory shear stress) affects the endothelial cells and may lead to an initiation of plaque development. The aim of the study was to analyze the effect of hemodynamic conditions in arterial segments perfused in vitro in the absence of other stimuli. Left common porcine carotid segments were mounted into an ex vivo arterial support system and perfused for 3 days under unidirectional high and low shear stress (6 ± 3 and 0.3 ± 0.1 dyn/cm2) and oscillatory shear stress (0.3 ± 3 dyn/cm2). Bradykinin-induced vasorelaxation was drastically decreased in arteries exposed to oscillatory shear stress compared with unidirectional shear stress. Impaired nitric oxide-mediated vasodilation was correlated to changes in both endothelial nitric oxide synthase (eNOS) gene expression and activation in response to bradykinin treatment. This study determined the flow-mediated effects on native tissue perfused with physiologically relevant flows and supports the hypothesis that oscillatory shear stress is a determinant factor in early stages of atherosclerosis. Indeed, oscillatory shear stress induces an endothelial dysfunction, whereas unidirectional shear stress preserves the function of endothelial cells. Endothelial dysfunction is directly mediated by a downregulation of eNOS gene expression and activation; consequently, a decrease of nitric oxide production and/or bioavailability occurs.

A New Biomechanical Perfusion System for ex vivo Study of Small Biological Intact Vessels

The vascular endothelium transduces physical stimuli within the circulation into physiological responses, which influence vascular remodelling and tissue homeostasis. Therefore, a new computerized biomechanical ex vivo perfusion system was developed, in which small intact vessels can be perfused under well-defined biomechanical forces. The system enables monitoring and regulation of vessel lumen diameter, shear stress, mean pressure, variable pulsatile pressure and flow profile, and diastolic reversal flow. Vessel lumen measuring technique is based on detection of the amount of flourescein over a vessel segment. A combination of flow resistances, on/off switches, and capacitances creates a wide range of pulsatile pressures and flow profiles. Accuracy of the diameter measurement was evaluated. The diameters of umbilical arteries were measured and compared with direct ultrasonographic measurement of the vessel diameter. As part of the validation the pulsatile pressure waveform was altered, e.g., in terms of pulse pressure, frequency, diastolic shape, and diastolic reversal flow. In a series of simulation experiments, the hemodynamic homeostasis functions of the system were successfully challenged by generating a wide range of vascular diameters in artificial and intact human vessels. We conclude that the system presented may serve as a methodological and technical platform when performing advanced hemodynamic stimulation protocols.

Arterial Wall Response to ex vivo Exposure to Oscillatory Shear Stress

BACKGROUND

The aim of this study was to analyze the arterial wall response to plaque-prone hemodynamic environments, known to occur mainly in areas of arterial trees such as bifurcations and branching points. In these areas, the vasculature is exposed to cyclically reversing flow that induces an endothelial dysfunction predisposing thus arteries to local development of atherosclerotic plaques.

METHODS

We used an ex vivo perfusion system that allows culturing arterial segments under different hemodynamic conditions. Porcine carotid arteries were exposed for 3 days to unidirectional high and low shear stress (6 +/- 3 and 0.3 +/- 0.1 dyn/cm(2)) as well as to oscillatory shear stress (0.3 +/- 3 dyn/cm(2)). This latter condition mimics the hemodynamics present at plaque-prone areas. At the end of the perfusion, the influence of different flow patterns on arterial metabolism was assessed in terms of matrix turnover as well as of smooth muscle cell function, differentiation and migration.

RESULTS

Our results show that after 3 days of perfusion none of the applied conditions influence smooth muscle cell phenotype retaining their full contraction capacity. However, an increase in the expression level of matrix metalloproteinase-2 and -9, as well as a decrease in plasminogen activator inhibitor-1 expression were observed in arteries exposed to oscillatory shear stress when compared to arteries exposed to unidirectional shear stress.

CONCLUSION

These observations suggest that plaque-prone hemodynamic environment triggers a vascular wall remodelling process and promotes changes in arterial wall metabolism, with important implication in atherogenesis.

Hemodynamic conditions alter axial and circumferential remodeling of arteries engineered ex vivo

We previously demonstrated that growth and remodeling was stimulated in arteries elongated ex vivo using step increases in axial strain. Viability and vasoactivity were similar to fresh arteries, however there was a substantial decrease in the ultimate circumferential stress. To test the hypothesis that the subphysiological perfusion conditions (i.e., low pressure and flow) previously used caused the reduction, arteries were subjected to the identical elongation protocol (50% increase over 9 days) while being perfused with physiological levels of flow, viscosity and pulsatile pressure. A significant increase in unloaded length was achieved by elongation under both perfusion conditions, although the increase was less under physiological (7 +/- 1%) than under subphysiological conditions (19 +/- 2%, p < 0.005). When length at physiological stress was estimated using mechanical testing data the values were similar. The ultimate circumferential stress of arteries elongated under physiological conditions was increased (33%), whereas the ultimate axial stress was decreased (50%) as compared with arteries elongated under subphysiological conditions. Elongated arteries under both perfusion conditions showed significant increases in proliferation and collagen mass, and similar viability and appearance to fresh arteries. These data suggest that there is substantial cross-talk between perfusion conditions and axial strain that modulates arterial remodeling and length.

Cell proliferation in mesenteric microvascular network remodeling in response to elevated hemodynamic stress

The objective of this study was to quantify the proliferation of existing vascular and perivascular cells during a specific form of microvascular remodeling characterized by increased coverage by smooth muscle cells (SMCs), in response to increased mechanical stress. Coordinated ligations of artery/vein pairs in the rat mesentery resulted in hemodynamic stress elevations within the targeted microvascular network. BRDU incorporation per unit length of smooth muscle (SM) alpha-actin positive vessel was evaluated following ligation at 2, 5, and 10 days. At 2 days, BRDU incorporation was significantly increased for both sham and ligated treatments, but the ligated response was not elevated over the sham response. After 5 days, proliferation for both groups returned to unstimulated levels. The results indicate that moderate elevations in hemodynamic stress do not cause perivascular cell proliferation along rat mesenteric microvessels, therefore, the increased coverage of differentiated SMCs along the same microvessels does not involve proliferation of vascular or perivascular cells.

Sustained axial loading lengthens arteries in organ culture

Although it has been recognized for many years that arteries in vivo exist under significant axial strain, studies of the adaptation of arteries to elevated axial strain have only recently been conducted. To determine the effects of sustained elevation of axial loading on arterial structure and function, axial stresses of 250 kPa or greater were applied to porcine common carotid arteries maintained in a perfusion organ culture system for 7 days at physiologic pressure and flow conditions. Our results demonstrated that axial stretch could lead to an increase in unloaded length that was proportional to the axial stretch ratio (stretched length divided by unloaded length) when the axial stretch ratio was above a threshold value of 2.14. Below this threshold, no significant length change occurred. Above this threshold, a significant increase in unloaded length (13 +/- 7%,) and the number of smooth muscle cell nuclei (20 +/- 7%) was observed. Permanent length change was associated with a significant decrease in axial stiffness, and the maximum elongation achieved was limited by rupture of the arterial wall. All tested arteries demonstrated good viability and strong vasomotor responses. These results show that arteries in organ culture can elongate under sustained axial loading.

Changes of opening angle in hypertensive and hypotensive arteries in 3-day organ culture

To study the effect of pressure changes on the opening angle of arteries in organ culture, tubular segments of porcine common carotid arteries were cultured with pulsatile flow perfusion under hypertensive (150+/-20 mmHg), normotensive (100+/-20 mmHg), or hypotensive (30+/-10 mmHg) pressure while maintaining the arteris at a physiological wall shear stress of approximately 15 dyn/cm(2) for up to 3 days. Arteries were then cut into short ring segments by sections perpendicular to the axis and then cut open radially to observe the opening angle in aerated phosphate buffered saline solution (37 degrees C). Norepinephrine (NE, 10 microM), carbacol (CCh, 100 microM), and sodium nitroprusside (SNP, 10 microM) were added after the radial cut at 30, 20, and 30 min intervals, the opening angles were measured, respectively. Results show that hypertensive arteries developed a significantly larger opening angle than normotensive and hypotensive arteries, associated with a significant increase in cell proliferation. In addition, with smooth muscle contraction activated by NE, the opening angle decreases significantly in hypertensive arteries but has little change in hypotensive and normotensive arteries, indicating an enhancement of smooth muscle contraction on the lumen side of the hypertensive arterial wall. In comparison, hypotensive pressure has little effect on arterial opening angle and cell proliferation.

Systolic hypertension in the elderly: arterial wall mechanical properties and the renin-angiotensin-aldosterone system

BACKGROUND

Systolic hypertension in the elderly involves an increase of arterial stiffness and early wave reflections, both of them causing a predominant or selective increase of systolic blood pressure. The mechanisms for such alterations remain largely unknown.

DESCRIPTION AND RESULTS

The development of systolic hypertension includes constantly an age-related increase of sodium sensitivity and of endothelial dysfunction, both responsible for phenotypic changes of aortic smooth muscle cells with collagen accumulation and increased stiffness. In the presence of a high sodium diet and under the influence of angiotensin II and aldosterone, a higher number of attachments between vascular smooth muscle cells and collagen fibers develop, causing a supplementary increase in stiffness independent of the mean blood pressure together with the occurrence of early wave reflections. Gene polymorphisms related to the renin-angiotensin system may participate in this evolution.

CONCLUSION

This process contributes to accelerating the increase in pulse pressure and arterial stiffness with age, and therefore to the development of cardiovascular risk.

A multiaxial computer-controlled organ culture and biomechanical device for mouse carotid arteries

Much of our understanding of vascular mechanotransduction has come from studies using either cell culture or in vivo animal models, but the recent success of organ culture systems offers an exciting alternative. In studying cell-mediated vascular adaptations to altered loading, organ culture allows one to impose well-controlled mechanical loads and to perform multiaxial mechanical tests on the same vessel throughout the culture period, and thereby to observe cell-mediated vascular adaptations independent of neural and hormonal effects. Here, we present a computer-controlled perfused organ culture and biomechanical testing device designed for small caliber (50-5000 micron) blood vessels. This device can control precisely the pulsatile pressure, luminal flow, and axial load (or stretch) and perform intermittent biaxial (pressure-diameter and axial load-length) and functional tests to quantify adaptations in mechanical behavior and cellular function, respectively. Device capabilities are demonstrated by culturing mouse carotid arteries for 4 days.

Fluid shear stress inhibits vascular inflammation by decreasing thioredoxin-interacting protein in endothelial cells

Regions in the vasculature that are exposed to steady laminar blood flow are protected from atherosclerosis as compared with regions where flow is disturbed. We found that flow decreased TNF-mediated VCAM1 expression by inhibiting JNK and p38. JNK inhibition correlated with inhibition of apoptosis signal-regulating kinase 1 (ASK1), a JNK and p38 activator. Thioredoxin-interacting protein (TXNIP) is a stress-responsive protein that inhibits thioredoxin (TRX) activity. Since thioredoxin inhibits ASK1, we hypothesized that changes in TXNIP-TRX-ASK1 interactions mediate the antiinflammatory effects of flow. To explore this, we used perfused vessels and cultured ECs. Exposure of rabbit aortae or ECs to normal flow (12 dyn/cm2, 24 hours) was associated with decreased TXNIP expression and increased TRX activity compared with exposure to low flow (0.4 dyn/cm2). Normal flow inhibited TNF activation of JNK/p38 and VCAM1 expression. In cultured ECs, reduction of TXNIP expression by small interfering RNA increased TRX binding to ASK1 and inhibited TNF activation of JNK/p38 and VCAM1 expression. Conversely, overexpression of TXNIP stimulated JNK and p38. In aortae from TXNIP-deficient mice, TNF-induced VCAM1 expression was inhibited. The data suggest that TXNIP and TRX are key components of biomechanical signal transduction and establish them as potentially novel regulators of TNF signaling and inflammation in ECs.

Fluid shear stress inhibits vascular inflammation by decreasing thioredoxin-interacting protein in endothelial cells

Regions in the vasculature that are exposed to steady laminar blood flow are protected from atherosclerosis as compared with regions where flow is disturbed. We found that flow decreased TNF-mediated VCAM1 expression by inhibiting JNK and p38. JNK inhibition correlated with inhibition of apoptosis signal-regulating kinase 1 (ASK1), a JNK and p38 activator. Thioredoxin-interacting protein (TXNIP) is a stress-responsive protein that inhibits thioredoxin (TRX) activity. Since thioredoxin inhibits ASK1, we hypothesized that changes in TXNIP-TRX-ASK1 interactions mediate the antiinflammatory effects of flow. To explore this, we used perfused vessels and cultured ECs. Exposure of rabbit aortae or ECs to normal flow (12 dyn/cm2, 24 hours) was associated with decreased TXNIP expression and increased TRX activity compared with exposure to low flow (0.4 dyn/cm2). Normal flow inhibited TNF activation of JNK/p38 and VCAM1 expression. In cultured ECs, reduction of TXNIP expression by small interfering RNA increased TRX binding to ASK1 and inhibited TNF activation of JNK/p38 and VCAM1 expression. Conversely, overexpression of TXNIP stimulated JNK and p38. In aortae from TXNIP-deficient mice, TNF-induced VCAM1 expression was inhibited. The data suggest that TXNIP and TRX are key components of biomechanical signal transduction and establish them as potentially novel regulators of TNF signaling and inflammation in ECs.

Comparative assessment of intimal hyperplasia development after 14 days in two different experimental settings: tissue culture versus ex vivo continuous perfusion of human saphenous vein

BACKGROUND

Intimal hyperplasia (IH) is a vascular remodeling process which often leads to failure of arterial bypass or hemodialysis access. Experimental and clinical work have provided insight in IH development; however, further studies under precise controlled conditions are required to improve therapeutic strategies to inhibit IH development. Ex vivo perfusion of human vessel segments under standardized hemodynamic conditions may provide an adequate experimental approach for this purpose. Therefore, chronically perfused venous segments were studied and compared to traditional static culture procedures with regard to functional and histomorphologic characteristics as well as gene expression.

MATERIALS AND METHODS

Static vein culture allowing high tissue viability was performed as previously described. Ex vivo vein support system (EVVSS) was performed using a vein support system consisting of an incubator with a perfusion chamber and a pump. EVVSS allows vessel perfusion under continuous flow while maintaining controlled hemodynamic conditions. Each human saphenous vein was divided in two parts, one cultured in a Pyrex dish and the other part perfused in EVVSS for 14days. Testing of vasomotion, histomorphometry, expression of CD 31, Factor VIII, MIB 1, alpha-actin, and PAI-l were determined before and after 14days of either experimental conditions.

RESULTS

Human venous segments cultured under traditional or perfused conditions exhibited similar IH after 14 days as shown by histomorphometry. Smooth-muscle cell (SMC) was preserved after chronic perfusion. Although integrity of both endothelial and smooth-muscle cells appears to be maintained in both culture conditions as confirmed by CD31, factor VIII, and alpha-actin expression, a few smooth-muscle cells in the media stained positive for factor VIII. Cell-proliferation marker MIB-1 was also detected in the two settings and PAI-1 mRNA expression and activity increased significantly after 14 days of culture and perfusion.

CONCLUSION

This study demonstrates the feasibility to chronically perfuse human vessels under sterile conditions with preservation of cellular integrity and vascular contractility. To gain insights into the mechanisms leading to IH, it will now be possible to study vascular remodeling not only under static conditions but also in hemodynamic environment mimicking as closely as possible the flow conditions encountered in reconstructive vascular surgery.

Differential activation of potassium channels in cerebral and hindquarter arteries of rats during simulated microgravity

The purpose of this study was to test the hypothesis that differential autoregulation of cerebral and hindquarter arteries during simulated microgravity is mediated or modulated by differential activation of K(+) channels in vascular smooth muscle cells (VSMCs) of arteries in different anatomic regions. Sprague-Dawley rats were subjected to 1- and 4-wk tail suspension to simulate the cardiovascular deconditioning effect due to short- and medium-term microgravity. K(+) channel function of VSMCs was studied by pharmacological methods and patch-clamp techniques. Large-conductance Ca(2+)-activated K(+) (BK(Ca)) and voltage-gated K(+) (K(v)) currents were determined by subtracting the current recorded after applications of 1 mM tetraethylammonium (TEA) and 1 mM TEA + 3 mM 4-aminopyridine (4-AP), respectively, from that of before. For cerebral vessels, the normalized contractility of basilar arterial rings to TEA, a BK(Ca) blocker, and 4-AP, a K(v) blocker, was significantly decreased after 1- and 4-wk simulated microgravity, respectively. VSMCs isolated from the middle cerebral artery branches of suspended rats had a more depolarized membrane potential (E(m)) and a smaller K(+) current density compared with those of control rats. Furthermore, the reduced total current density was due to smaller BK(Ca) and smaller K(v) current density in cerebral VSMCs after 1- and 4-wk tail suspension, respectively. For hindquarter vessels, VSMCs isolated from second- to sixth-order small mesenteric arteries of both 1- and 4-wk suspended rats had a more negative E(m) and larger K(+) current densities for total, BK(Ca), and K(v) currents. These results indicate that differential activation of K(+) channels occur in cerebral and hindquarter VSMCs during short- and medium-term simulated microgravity. It is further suggested that different profiles of channel remodeling might occur in VSMCs as one of the important underlying cellular mechanisms to mediate and modulate differential vascular adaptation during microgravity.

Functional, mechanical and geometrical adaptation of the arterial wall of a non-axisymmetric artery in vitro

OBJECTIVE

Vascular remodeling is an adaptive response to variations in the hemodynamic environment acting on the arterial wall. Remodeling translates into changes of structure, geometry and mechanical properties of the artery. Our aim was to study the remodeling response of pig right common carotid arteries in vitro.

METHODS

In vivo right carotid arteries are exposed to a non-uniform hemodynamic environment and exhibit a strong wall asymmetry in the circumferential direction that allows the study of two regions separately, as the artery remodels under in vitro perfusion. Porcine right common carotid arteries were cultured during 1 day (n = 6), 3 days (n = 6) or 8 days (n = 6) in an in vitro organ culture system, at a constant perfusion pressure of 100 mmHg. Geometrical, histological, biomechanical and biological analysis of the perfused segments was performed at the end of each study.

RESULTS

Smooth muscle cell nuclei density and wall thickness remain constant along the culture periods. Elastin and collagen are significantly redistributed to equilibrate their relative content along the vessel circumference. The distensibility profile is significantly different at day 8. Matrix metalloproteinase-2 expression and activity increase significantly at days 3 and 8.

CONCLUSION

The non-axisymmetric arterial wall adapts to a uniform hemodynamic environment by redistributing the structural components of the extracellular matrix. The changes of collagen and elastin density may result from a vascular remodeling process involving matrix metalloproteinase-2 up-regulation and enzymatic activity. The remodeling response results in a new vascular wall configuration that is more distensible at physiological pressures (30-120 mmHg) and stiffer at higher pressures.

Function and structure of pressurized and perfused porcine carotid arteries: effects of in vitro balloon angioplasty

In this report we describe the application of an in vitro pressure-perfusion system for study of functional/structural changes after in vitro balloon dilation injury. Pig carotid arteries were perfused at P = 100 mm Hg and Q = 100 ml/min, balloon angioplastied (BA), and cultured under these hemodynamic conditions for 4 or 8 days (n = 5 BA and 6 controls for each time point). To assess endothelial function, outer diameter changes in response to bradykinin (BK) were measured daily. Remodeling was determined from the shift in pressure-passive diameter relation, as obtained after papaverine addition. Arterial samples were processed for histology. Control arteries showed spontaneous tone, BK-induced relaxation, and inward remodeling that was more pronounced at day 8 (ratio end-to-start passive diameter at P = 100 mm Hg, 0.69 +/- 0.04; P < 0.001) than at day 4 (0.85 +/- 0.03, P = 0.03). Intimal hyperplasia was detectable in these control vessels at day 8 with accumulation of alpha-smooth muscle actin-positive cells around the lumen. Angioplasty caused ruptures and dissections and abolished tone that returned after 5 days of perfusion along with BK-dependent relaxation. No significant inward remodeling or intimal hyperplasia was observed at day 8 after angioplasty. Thus, BA inhibits remodeling, which occurs after in vitro perfusion of conductance arteries.

Intraluminal pressure increases vascular neuronal nitric oxide synthase expression

BACKGROUND

Development of high blood pressure (BP) is associated with an increased expression of neuronal nitric oxide synthase (nNOS) in vascular smooth muscle cells.

METHODS

We investigated whether or not changes in intraluminal pressure affect nNOS expression in carotid arteries of normotensive rats. Expression of nNOS and other NOS isoforms was determined by Western blot analysis in rat carotid arteries maintained up to 24 h at different levels of intraluminal pressure in an organ culture system.

RESULTS

Expression of nNOS in arteries exposed to 80 mmHg was stable for the duration of the experiment. Increasing intraluminal pressure to 200 mmHg transiently augmented nNOS expression at 9 h, both in intact arteries and in arteries where the endothelium and the adventitia were removed. The expression of endothelial NOS (eNOS) was also augmented under similar experimental conditions, but only after 24 h exposure. The ERK1/2 kinase cascade inhibitor PD 98059 significantly impaired the expression of nNOS in arteries exposed to 200 mmHg for 9 h. However, the angiotensin AT(1) antagonist candesartan and the angiotensin converting enzyme inhibitor perindoprilat did not have any effect under the same experimental conditions. Finally, the preferential nNOS inhibitor S-methyl-L-thiocitrulline significantly augmented the contraction evoked by angiotensin II in arteries exposed to 200 mmHg, but not in those maintained at 80 mmHg intraluminal pressure for 9 h.

CONCLUSION

These results show that transmural pressure increases nNOS expression and NO release in rat smooth muscle cells by a mechanism involving the mitogen-activated protein kinase pathway, but independent from the local formation of angiotensin II.

Induction of SM-alpha-actin expression by mechanical strain in adult vascular smooth muscle cells is mediated through activation of JNK and p38 MAP kinase

Mechanical forces have direct effects on the growth and differentiation of vascular smooth muscle. The goal of this study was to examine the effects of cyclic mechanical strain on expression of smooth muscle-alpha-actin (SM-alpha-actin), a marker for the differentiated state of vascular smooth muscle, in cultured rat aortic smooth muscle cells (VSMC). Cells grown on dishes coated with either laminin or pronectin were subjected to mechanical strain and effects on expression of SM-alpha-actin were evaluated using the Flexercell Strain Unit. Application of mechanical strain to cells in full media increased SM-alpha-actin protein expression and promoter activity. This was not associated with any effect on growth. Mechanical strain increased activity of all three members of the MAP kinase family (ERKs, JNKs, and p38 MAP kinase), with similar kinetics. Inhibition of either JNKs or p38 MAP kinase blocked the strain-induced increase in SM-alpha-actin promoter activity, and expression of constitutively active forms of JNK or MKK6, a p38 kinase, increased promoter activity. These studies indicate that in adult VSMC, mechanical strain leads to increased expression of smooth muscle markers, resulting in a more contractile phenotype.

Design of a new pulsatile bioreactor for tissue engineered aortic heart valve formation

Evidence has been gathered that biomechanical factors have a significant impact on cell differentiation and behavior in in vitro cell cultures. The aim of this bioreactor is to create a physiological environment in which tissue engineered (TE) aortic valves seeded with human cells can be cultivated during a period of several days. The bioreactor consists of 2 major parts: the left ventricle (LV) and the afterload consisting of a compliance, representing the elastic function of the large arteries, and in series a resistance, mimicking the arterioles and capillaries. The TE aortic valve is placed between the LV and the compliance. With controllable resistance, compliance, stroke volume and frequency, and hydrodynamic conditions can be changed over a wide physiological range. This study resulted in a prototype of a compact pulsatile flow system for the creation of TE aortic valves. In addition a biocompatibility study of the used materials is performed.

Effects of oxygen tension on energetics of cultured vascular smooth muscle

Chronic hypoxia is a clinically important condition known to cause vascular abnormalities. To investigate the cellular mechanisms involved, we kept rings of a rat tail artery for 4 days in hypoxic culture (HC) or normoxic culture (NC) (PO(2) = 14 vs. 110 mmHg) and then measured contractility, oxygen consumption (JO(2)), and lactate production (J(lac)) in oxygenated medium. Compared with fresh rings, basal ATP turnover (J(ATP)) was decreased in HC, but not in NC, with a shift from oxidative toward glycolytic metabolism. JO(2) during mitochondrial uncoupling was reduced by HC but not by NC. Glycogen stores were increased 40-fold by HC and fourfold by NC. Maximum tension in response to norepinephrine and the JO(2) versus tension relationship (JO(2) vs. high K(+) elicited force) were unaffected by either HC or NC. Force transients in response to caffeine were increased in HC, whereas intracellular Ca(2+) wave activity during adrenergic stimulation was decreased. Protein synthesis rate was reduced by HC. The results show that long-term hypoxia depresses basal energy turnover, impairs mitochondrial capacity, and alters Ca(2+) homeostasis, but does not affect contractile energetics. These alterations may form a basis for vascular damage by chronic hypoxia.

Biomechanical adaptation of porcine carotid vascular smooth muscle to hypo and hypertension in vitro

Previous research in arterial remodeling in response to changes in blood pressure seldom included both hyper- and hypotension. To compare the effects of low and high pressure on arterial remodeling and vascular smooth muscle tone and performance, we have utilized an in vitro model. Porcine carotid arteries were cultured for 3 days at 30 and 170mmHg and compared to controls cultured at 100mmHg for 1 and 3 days. On the first and last day of culture, pressure-diameter and pressure-wall thickness curves were measured under normal smooth muscle tone using a high-resolution ultrasonic device. Last-day experiments included measurements where vascular smooth muscle was contracted or totally relaxed. From the data wall cross-sectional area, Hudetz elastic modulus and a contraction index related to the diameter reduction under normal smooth muscle tone were calculated. We found that although wall cross-sectional area (indicating wall mass) did not change much, Hudetz elastic modulus was significantly reduced in the 3-day hypotension group. Inspection of the wall contraction index suggests that this is due to a reduction in the vascular smooth muscle tone. Further, the peak of contraction index was found to be shifted to higher pressures in the 3-day 170mmHg group. We conclude that vascular smooth muscle performance adapts to both hypo- and hypertension at short time scales and can alter the biomechanics of the vascular wall in vitro.

Small artery remodeling in hypertension

Hypertension is associated with altered structure of the resistance vessels, a process known as remodeling. This review summarizes current concepts concerning the structure of a subgroup of the resistance vessels, the small arteries, and the modes of remodeling, some of the determinants of remodeling, and some signaling pathways for remodeling. It is shown that the available evidence points to important roles for blood flow and growth factors, in addition to blood pressure, as causes of resistance artery remodeling. Finally, the relationship between vascular structure and blood pressure is discussed, in particular with regard to the effects of antihypertensive therapy. Here again, it appears that blood flow plays an important role in allowing the correction of the abnormal resistance vessel structure seen in hypertension.

Intact endothelial and smooth muscle function in small resistance arteries after 48 h in vessel culture

Long-term culture of resistance vessels allows introduction of molecular biology techniques for use in microvascular research. The aim of the present study was to establish a culture protocol that preserved vascular integrity and function in microvessels for 48 h in culture. Skeletal muscle resistance arteries were excised from the hamster gracilis muscle. Segments were assigned to immediate functional tests or to vessel culture, during which segments were perfused and superfused at a transmural pressure of 45 mmHg with Leibovitz (L15) medium containing 15% fetal calf serum and antibiotics for 48 h. Cultured and freshly isolated vessels showed similar levels of spontaneous tone, myogenic responses, changes in smooth muscle intracellular calcium (Ca(i)(2+)) (fura 2), and vascular diameter (video microscopy) in response to 0.3 M norepinephrine and similar concentration-response curves for acetylcholine (endothelium dependent, +/-N(omega)-nitro-L-arginine) and sodium nitroprusside (endothelium independent). Measurements of endothelial Ca(i)(2+) revealed similar acetylcholine-induced increases in endothelial Ca(i)(2+) in both groups. It is concluded that vascular function can be preserved while maintaining vessels in culture. Thus it is possible to utilize protocols that require long-term treatment.

Development of an ex vivo model to investigate the effects of altered haemodynamics on human bypass grafts

The insertion of vein grafts into the arterial circulation may contribute to vessel wall thickening and accelerated atherosclerosis, a common feature of late vein graft failure. We aimed to develop a model suitable for investigation of the effects of altered haemodynamics on human saphenous vein following its implantation into the arterial circulation. Segments of human saphenous vein obtained from patients undergoing coronary artery bypass surgery were sutured at each end to PTFE and placed into a flow system. Pressure and flow rates to stimulate the arterial and venous systems were achieved. A theoretical model of the flow chamber was created and computational fluid dynamics software (FLOTRAN, Swanson Analysis Systems) was used to determine the flow profile within the model. In summary, a flow model has been developed to investigate the effect of altered haemodynamics on the molecular and pathological changes that occur in vein grafts incorporated into the arterial circulation.

A simple physiologic pulsatile perfusion system for the study of intact vascular tissue

Perfusion vascular culture models may provide a useful link between cell culture models and animal culture models by allowing a high level of control over important parameters while maintaining physiologic structure. The purpose of this study was to develop and test a new vascular culture system for pulsatile perfusion culture of intact vascular tissue. The system generates a pulsatile component of flow by means of a cam-driven syringe and a peristaltic pump and compliance chamber. Cams were designed, constructed and tested to simulate canine femoral and common carotid artery flows. The mean pressure was adjusted between 60 and 200 mmHg without significantly affecting flow rate, flow waveform, or the pressure waveform. Porcine common carotid artery segments were cultured in this pulsatile perfusion system. The viability of vascular segments was tested after various culture times with a functional assay that demonstrated both smooth muscle cell and endothelial cell response to vasomotor challenge.

Organoid culture of cannulated rat resistance arteries: effect of serum factors on vasoactivity and remodeling

We developed an organoid culture technique to study the mechanisms involved in arterial remodeling. Resistance arteries were isolated from rat cremaster muscle and mounted in a pressure myograph at 75 mmHg. Vessels were studied during a 4-day culture period in DMEM with either 2% albumin, 10% heat-inactivated FCS (HI-FCS) or 10% dialyzed HI-FCS (12 kDa cut off) added to the perfusate. The albumin group showed a gradual loss of endothelial function and integrity, whereas smooth muscle agonist and myogenic responses were retained. No remodeling was observed. Vessels cultured in the presence of serum showed a progressive constriction. Smooth muscle responses and substance P-induced endothelium-dependent dilation were maintained. An inward remodeling of 17 +/- 4% in the HI-FCS group and 26 +/- 3% in the dialyzed HI-FCS group was found, while media cross-sectional areas were unchanged. These data show that pressurized resistance arteries can be maintained in culture for several days and undergo eutrophic remodeling in vitro in the presence of high molecular weight serum factors.

Transmural pressure induces matrix-degrading activity in porcine arteries ex vivo

Extracellular matrix components must be degraded and resynthesized for vascular remodeling to occur. We hypothesized that the hemodynamic environment regulates activity of matrix metalloproteinases (MMPs), the primary agents for in vivo matrix degradation, during vascular remodeling in response to changes in transmural pressure and shear stress. Pathological hemodynamic conditions were reproduced in an ex vivo system in which we maintained porcine carotid arteries for 24 and 48 h. Total levels of MMP-2 and MMP-9 extracted from tissue homogenates and analyzed by SDS-PAGE zymography were stimulated by transmural pressure and were unaffected by shear stress changes. Degradation of two specific gelatinase substrates, gelatin and elastin, increased with increasing pressure, but the degradation was not affected by shear stress changes in tissue specimens analyzed using in situ zymography (gelatin) and fluorescent measurement of endogenous elastin degradation (elastin). Our results suggest that transmural pressure activates at least two members of the MMP family and that activity of these enzymes is accompanied by degradation of matrix components, effects that may be implicated in hypertensive vascular remodeling.

Long-term effects of Ca(2+) on structure and contractility of vascular smooth muscle

Culture of dispersed smooth muscle cells is known to cause rapid modulation from the contractile to the synthetic cellular phenotype. However, organ culture of smooth muscle tissue, with maintained extracellular matrix and cell-cell contacts, may facilitate maintenance of the contractile phenotype. To test the influence of culture conditions, structural, functional, and biochemical properties of rat tail arterial rings were investigated after culture. Rings were cultured for 4 days in the absence and presence of 10% FCS and then mounted for physiological experiments. Intracellular Ca(2+) concentration ([Ca(2+)](i)) after stimulation with norepinephrine was similar in rings cultured with and without FCS, whereas force development after FCS was decreased by >50%. The difference persisted after permeabilization with beta-escin. These effects were associated with the presence of vasoconstrictors in FCS and were dissociated from its growth-stimulatory action. FCS treatment increased lactate production but did not affect ATP, ADP, or AMP contents. The contents of actin and myosin were decreased by culture but similar for all culture conditions. There was no effect of FCS on calponin contents or myosin SM1/SM2 isoform composition, nor was there any appearance of nonmuscle myosin. FCS-stimulated rings showed evidence of cell degeneration not found after culture without FCS or with FCS + verapamil (1 microM) to lower [Ca(2+)](i). The decreased force-generating ability after culture with FCS is thus associated with increased [Ca(2+)](i) during culture and not primarily caused by growth-associated modulation of cells from the contractile to the synthetic phenotype.

Mechanical strain-induced posttranscriptional regulation of fibronectin production in fetal lung cells

We have shown that intermittent mechanical strain, simulating fetal breathing movements, stimulated fetal rat lung cell proliferation. Because normal lung growth requires proper coordination between cell proliferation and extracellular matrix remodeling, we investigated the effect of strain on fibronectin metabolism. Organotypic cultures of fetal rat lung cells, subjected to intermittent strain, showed increased fibronectin content in the culture media. Fibronectin-degrading activity in media from strained cells was similar to that of static cultures. Northern analysis revealed that strain inhibited fibronectin mRNA accumulation seen during static culture. Synthesis of fibronectin, determined by metabolic labeling, was increased by strain despite lower mRNA levels or presence of actinomycin D. This increase was not mediated via a rapamycin-sensitive mechanism. Strain stimulated prelabeled fibronectin secretion even in the presence of cycloheximide. These results suggest that strain differentially regulates fibronectin production of fetal lung cells at the transcriptional and posttranscriptional levels. Mechanical strain increases soluble fibronectin content by stimulating its synthesis and secretion without increasing fibronectin message levels.