Nitric oxide production by cultured human aortic smooth muscle cells: stimulation by fluid flow

Angiotensin II-induced arterial thickening, fibrosis and stiffening involves elevated arginase function

BACKGROUND

Arterial stiffness (AS) is an independent risk factor for cardiovascular morbidity/mortality. Smooth muscle cell (SMC) proliferation and increased collagen synthesis are key features in development of AS. Arginase (ARG), an enzyme implicated in many cardiovascular diseases, can compete with nitric oxide (NO) synthase for their common substrate, L-arginine. Increased arginase can also provide ornithine for synthesis of polyamines via ornithine decarboxylase (ODC) and proline/collagen via ornithine aminotransferase (OAT), leading to vascular cell proliferation and collagen formation, respectively. We hypothesized that elevated arginase activity is involved in Ang II-induced arterial thickening, fibrosis, and stiffness and that limiting its activity can prevent these changes.

METHODS AND RESULTS

We tested this by studies in mice lacking one copy of the ARG1 gene that were treated with angiotensin II (Ang II, 4 weeks). Studies were also performed in rat aortic Ang II-treated SMC. In WT mice treated with Ang II, we observed aortic stiffening (pulse wave velocity) and aortic and coronary fibrosis and thickening that were associated with increases in ARG1 and ODC expression/activity, proliferating cell nuclear antigen, hydroxyproline levels, and collagen 1 protein expression. ARG1 deletion prevented each of these alterations. Furthermore, exposure of SMC to Ang II (1 μM, 48 hrs) increased ARG1 expression, ARG activity, ODC mRNA and activity, cell proliferation, collagen 1 protein expression and hydroxyproline content. Treatment with ABH prevented these changes.

CONCLUSION

Arginase 1 is crucially involved in Ang II-induced SMC proliferation and arterial fibrosis and stiffness and represents a promising therapeutic target.

Simulated microgravity exposure modulates the phenotype of cultured vascular smooth muscle cells

Evidence from ground-based animal studies using tail-suspended hindlimb unloaded rats model has clearly demonstrated that simulated microgravity-induced smooth muscle cell phenotype conversion, a characteristic vascular structural and functional remodeling, may be one of the key contributors to postspaceflight orthostatic intolerance. However, the rats model involves multiple collective effects of microgravity including cephalic fluid shift and postural muscle unloading on smooth muscle cells (SMCs). It cannot isolate a single factor from the collective ones and therefore is not ideal to study the effects of gravitational vector alteration alone on SMCs. To test the hypothesis that gravitational vector alteration per se might affect smooth muscle cell phenotype, a roller culture apparatus was employed to expose cultured rat aortic smooth muscle cells (RASMCs) to simulated microgravity. Cell proliferation, cell cycle distribution, apoptosis, migration, and nitric oxide production rates were measured and compared between the control and the simulated microgravity groups. Cell cytoskeleton reorganization induced by simulated microgravity was observed by confocal microscopy. Specific contractile and synthetic Gene expression at the mRNA level was quantified by reverse transcriptional polymerase chain reaction. It was observed that simulated microgravity suppressed RASMC proliferation and migration, enhanced cell apoptosis, stimulated NO release, and destroyed the original well-organized cytoskeleton. Moreover, at the mRNA level, long-time exposure (≥ 72 h) to simulated microgravity induced a contractile phenotype tendency by up-regulating smMHC expression. All these findings suggest that the phenotype modulation of vascular smooth muscle cells may be gravity dependent.

Inhibition of nitric oxide synthesis enhances leukocyte rolling and adhesion in human microvasculature

Background

Nitric oxide (NO) is a multifunctional signaling molecule that regulates important cellular events in inflammation including leukocyte recruitment. Previous studies have shown that pharmacological inhibition of NO synthesis induces leukocyte recruitment in various in vitro and animal models. However, it is not known whether NO modulation has similar effects on leukocyte-endothelial cell interactions within the human microvasculature. The present study explored the effect of systemic L-NAME treatment on leukocyte recruitment in the SCID-hu mouse model.

Methods

Human skin xenografts were transplanted in SCID mice to study human leukocyte dynamics in human vasculature. Early events of human leukocyte recruitment in human vasculature were studied using intravital microscopy. NO synthesis was pharmacologically inhibited using N^G-nitro-L-arginine methyl ester (L-NAME). Immunohistochemical analysis was performed to elucidate E-selectin expression in human xenograft skin. Human neutrophil-endothelial cell interactions were also studied in an in vitro flow chamber assay system. P- and E-selectin expression on cultured human umbilical vein endothelial cells (HUVECs) was measured using ELISA. Platelet-activating factor (PAF) synthesis was detected using a TLC-based assay.

Results

L-NAME treatment significantly enhanced the rolling and adhesion of human leukocytes to the human vasculature. Functional blocking of P- and E-selectins significantly inhibited rolling but not adhesion induced by inhibition of NO synthesis. Systemic L-NAME treatment enhanced E-selectin expression in human xenograft skin. L-NAME treatment significantly enhanced P- and E-selectin expression on HUVECs. L-NAME treatment did not significantly modify neutrophil rolling or adhesion to HUVECs indicating that L-NAME−induced subtle P- and E-selectin expression was insufficient to elicit dynamic neutrophil-HUVEC interactions in vitro. Moreover, synthesis of endothelial-derived PAF was not significantly modified by L-NAME treatment. These results point to the accelerated leukocyte recruitment in human vasculature following suppression of NO synthesis, effects that are mediated by P- and E-selectins. The findings are, however, not supported by the in vitro data.

Conclusion

Inhibition of endogenous NO triggers early events of human leukocyte recruitment in human vasculature, involving complex cellular or molecular mechanisms in addition to P- and E-selectin-mediated leukocyte rolling.

Regulation of Inducible Nitric Oxide Synthase (iNOS) and its Potential Role in Insulin Resistance, Diabetes and Heart Failure

Nitric oxide synthases (NOS) are the enzymes responsible for nitric oxide (NO) generation. NO is a reactive oxygen species as well as a reactive nitrogen species. It is a free radical which mediates several biological effects. It is clear that the generation and actions of NO under physiological and pathophysiological conditions are regulated and extend to almost every cell type and function within the circulation. In mammals 3 distinct isoforms of NOS have been identified: neuronal NOS (nNOS), inducible NOS (iNOS) and endothelial NOS (eNOS). The important isoform in the regulation of insulin resistance (IR) is iNOS. Understanding the molecular mechanisms regulating the iNOS pathway in normal and hyperglycemic conditions would help to explain some of vascular abnormalities observed in type 2 diabetes mellitus (T2DM). Previous studies have reported increased myocardial iNOS activity and expression in heart failure (HF). This review considers the recent animal studies which focus on the understanding of regulation of iNOS activity/expression and the role of iNOS agonists as potential therapeutic agents in treatment of IR, T2DM and HF.

Fluid flow mechanotransduction in vascular smooth muscle cells and fibroblasts

Understanding how vascular wall endothelial cells (ECs), smooth muscle cells (SMCs), and fibroblasts (FBs) sense and transduce the stimuli of hemodynamic forces (shear stress, cyclic strain, and hydrostatic pressure) into intracellular biochemical signals is critical to prevent vascular disease development and progression. ECs lining the vessel lumen directly sense alterations in blood flow shear stress and then communicate with medial SMCs and adventitial FBs to regulate vessel function and disease. Shear stress mechanotransduction in ECs has been extensively studied and reviewed. In the case of endothelial damage, blood flow shear stress may directly act on the superficial layer of SMCs and transmural interstitial flow may be elevated on medial SMCs and adventitial FBs. Therefore, it is also important to investigate direct shear effects on vascular SMCs as well as FBs. The work published in the last two decades has shown that shear stress and interstitial flow have significant influences on vascular SMCs and FBs. This review summarizes work that considered direct shear effects on SMCs and FBs and provides the first comprehensive overview of the underlying mechanisms that modulate SMC secretion, alignment, contraction, proliferation, apoptosis, differentiation, and migration in response to 2-dimensional (2D) laminar, pulsatile, and oscillating flow shear stresses and 3D interstitial flow. A mechanistic model of flow sensing by SMCs is also provided to elucidate possible mechanotransduction pathways through surface glycocalyx, integrins, membrane receptors, ion channels, and primary cilia. Understanding flow-mediated mechanotransduction in SMCs and FBs and the interplay with ECs should be helpful in exploring strategies to prevent flow-initiated atherosclerosis and neointima formation and has implications in vascular tissue engineering.

Vascular smooth muscle cell glycocalyx modulates shear-induced proliferation, migration, and NO production responses

The endothelial cell glycocalyx, a structure coating the luminal surface of the vascular endothelium, and its related mechanotransduction have been studied by many over the last decade. However, the role of vascular smooth muscle cells (SMCs) glycocalyx in cell mechanotransduction has triggered little attention. This study addressed the role of heparan sulfate proteoglycans (HSPGs), a major component of the glycocalyx, in the shear-induced proliferation, migration, and nitric oxide (NO) production of the rat aortic smooth muscle cells (RASMCs). A parallel plate flow chamber and a peristaltic pump were employed to expose RASMC monolayers to a physiological level of shear stress (12 dyn/cm(2)). Heparinase III (Hep.III) was applied to selectively degrade heparan sulfate on the SMC surface. Cell proliferation, migration, and NO production rates were determined and compared among the following four groups of cells: 1) untreated with no flow, 2) Hep.III treatment with no flow, 3) untreated with flow of 12 dyn/cm(2) exposure, and 4) Hep.III treatment with flow of 12 dyn/cm(2) exposure. It was observed that flow-induced shear stress significantly suppressed SMC proliferation and migration, whereas cells preferred to aligning along the direction of flow and NO production were enhanced substantially. However, those responses were not found in the cells with Hep.III treatment. Under flow condition, the heparinase III-treated cells remained randomly oriented and proliferated as if there were no flow presence. Disruption of HSPG also enhanced wound closure and inhibited shear-induced NO production significantly. This study suggests that HSPG may play a pivotal role in mechanotransduction of SMCs.

Shear stress increases nitric oxide production in thick ascending limbs

We showed that luminal flow stimulates nitric oxide (NO) production in thick ascending limbs. Ion delivery, stretch, pressure, and shear stress all increase when flow is enhanced. We hypothesized that shear stress stimulates NO in thick ascending limbs, whereas stretch, pressure, and ion delivery do not. We measured NO in isolated, perfused rat thick ascending limbs using the NO-sensitive dye DAF FM-DA. NO production rose from 21 ± 7 to 58 ± 12 AU/min (P < 0.02; n = 7) when we increased luminal flow from 0 to 20 nl/min, but dropped to 16 ± 8 AU/min (P < 0.02; n = 7) 10 min after flow was stopped. Flow did not increase NO in tubules from mice lacking NO synthase 3 (NOS 3). Flow stimulated NO production by the same extent in tubules perfused with ion-free solution and physiological saline (20 ± 7 vs. 24 ± 6 AU/min; n = 7). Increasing stretch while reducing shear stress and pressure lowered NO generation from 42 ± 9 to 17 ± 6 AU/min (P < 0.03; n = 6). In the absence of shear stress, increasing pressure and stretch had no effect on NO production (2 ± 8 vs. 8 ± 8 AU/min; n = 6). Similar results were obtained in the presence of tempol (100 μmol/l), a O(2)(-) scavenger. Primary cultures of thick ascending limb cells subjected to shear stresses of 0.02 and 0.55 dyne/cm(2) produced NO at rates of 55 ± 10 and 315 ± 93 AU/s, respectively (P < 0.002; n = 7). Pretreatment with the NOS inhibitor l-NAME (5 mmol/l) blocked the shear stress-induced increase in NO production. We concluded that shear stress rather than pressure, stretch, or ion delivery mediates flow-induced stimulation of NO by NOS 3 in thick ascending limbs.

Interplay between calcium and reactive oxygen/nitrogen species: an essential paradigm for vascular smooth muscle signaling

Signaling cascades initiated or regulated by calcium (Ca(2+)), reactive oxygen (ROS), and nitrogen (RNS) species are essential to diverse physiological and pathological processes in vascular smooth muscle. Stimuli-induced changes in intracellular Ca(2+) regulate the activity of primary ROS and RNS, producing enzymes including NADPH oxidases (Nox) and nitric oxide synthases (NOS). At the same time, alteration in intracellular ROS and RNS production reciprocates through redox-based post-translational modifications altering Ca(2+) signaling networks. These may include Ca(2+) pumps such as sarcoplasmic endoplasmic reticulum Ca(2+)-ATPase (SERCA), voltage-gated channels, transient receptor potential canonical (TRPC), melastatin2 (TRPM2), and ankyrin1 (TRPA1) channels, store operated Ca(2+) channels such as Orai1/stromal interaction molecule 1 (STIM1), and Ca(2+) effectors such as Ca(2+)/calmodulin-dependent protein kinase II (CaMKII). In this review, we summarize and highlight current experimental evidence supporting the idea that cross-talk between Ca(2+) and ROS/RNS may represent a well-integrated signaling network in vascular smooth muscle.

Characterization of umbilical cord blood-derived late outgrowth endothelial progenitor cells exposed to laminar shear stress

Endothelial progenitor cells isolated from umbilical cord blood (CB-EPCs) represent a promising source of endothelial cells for synthetic vascular grafts and tissue-engineered blood vessels since they are readily attainable, can be easily isolated, and possess a high proliferation potential. The objective of this study was to compare the functional behavior of late outgrowth CB-EPCs with human aortic endothelial cells (HAECs). CB-EPCs and HAECs were cultured on either smooth muscle cells in a coculture model of a tissue-engineered blood vessels or fibronectin adsorbed to Teflon-AF-coated glass slides. Late outgrowth CB-EPCs expressed endothelial cell-specific markers and were negative for the monocytic marker CD14. CB-EPCs have higher proliferation rates than HAECs, but are slightly smaller in size. CB-EPCs remained adherent under supraphysiological shear stresses, oriented and elongated in the direction of flow, and expressed similar numbers of alpha(5)beta(1) and alpha(v)beta(3) integrins and antithrombotic genes compared to HAECs. There were some differences in mRNA levels of E-selectin and vascular cell adhesion molecule 1 between CB-EPCs and HAECs; however, protein levels were similar on the two cell types, and CB-EPCs did not support adhesion of monocytes in the absence of tumor necrosis factor-alpha stimulation. Although CB-EPCs expressed significantly less endothelial nitric oxide synthase protein after exposure to flow than HAECs, nitric oxide levels induced by flow were not significantly different. These results suggest that late outgrowth CB-EPCs are functionally similar to HAECs under flow conditions and are a promising cell source for cardiovascular therapies.

Role of shear stress on nitrite and NOS protein content in different size conduit arteries of swine

AIM

Inherent fundamental difference exists among arteries of different sizes. The purpose of this study was to evaluate the relation between regional difference of wall shear stress (WSS) in various sizes arteries and contents of nitrite and NO synthase (NOS) isoforms.

METHODS

Five different conduit arteries in a wide range of diameter (1-8 mm) were examined in the hind limbs of 13 pigs. Blood flow rate and outer diameter were measured in vivo to determine WSS. Arterial tissues were harvested for the measurement of nitrite and NOS protein contents. The concentration of nitrite, a product of NO synthesis, was determined by high-performance liquid chromatography method. Western blot analysis was used to assess the protein contents of endothelial NOS (eNOS), inducible NOS (iNOS) and neuronal NOS (nNOS).

RESULTS

Our data show that WSS increases with a decrease in artery diameter. Nitrite level increases with increasing WSS and hence decreases with artery diameter. The eNOS protein contents decrease with an increase in diameter. No significant difference for iNOS and nNOS protein contents was found with different artery diameter. A significant positive correlation between tissue nitrite and eNOS protein contents was also observed. Finally, the WSS-normalized eNOS is not significantly different in various size vessels.

CONCLUSION

Regional difference in blood flow has no effect on iNOS and nNOS protein contents in these conduit arteries. Regional difference in eNOS expression and nitrite contents may be related to the WSS-induced NO by the endothelium under normal physiological conditions.

The study of wall deformation and flow distribution with transmural pressure by three-dimensional model of thoracic aorta wall

The sensitivity of shear stress over smooth muscle cells (SMCs) to the deformability of media layer due to pressure is investigated in thoracic aorta wall using three-dimensional simulations. A biphasic, anisotropic model assuming the radius, thickness, and hydraulic conductivity of vessel wall as functions of transmural pressure is employed in numerical simulations. The leakage of interstitial fluid from intima to media layer is only possible through fenestral pores on the internal elastic lamina (IEL). The media layer is assumed a heterogeneous medium containing SMCs embedded in a porous extracellular matrix of elastin, proteoglycan, and collagen fibers. The applicable pressures for the deformation of media layer are varied from 0 to 180 mmHg. The SMCs are cylindrical objects of circular cross section at zero pressure. The cross sectional shape of SMCs changes from circle to ellipse as the media is compressed. The local shear stress over the nearest SMC to the IEL profoundly depends on pressure, SMCs configurations, and the corresponding distance to the IEL. The consideration of various SMC configurations, namely the staggered and square arrays, mimics various physiological conditions that can happen in positioning of an SMC. The results of our simulations show that even the second nearest SMCs to the IEL can significantly change their functions due to high shear stress levels. This is in contrast to earlier studies suggesting the highest vulnerability to shear stress for the innermost layer of SMCs at the intimal-medial interface.

Regulation of smooth muscle by inducible nitric oxide synthase and NADPH oxidase in vascular proliferative diseases

Inflammation plays a critical role in promoting smooth muscle migration and proliferation during vascular diseases such as postangioplasty restenosis and atherosclerosis. Another common feature of many vascular diseases is the contribution of reactive oxygen (ROS) and reactive nitrogen (RNS) species to vascular injury. Primary sources of ROS and RNS in smooth muscle are several isoforms of NADPH oxidase (Nox) and the cytokine-regulated inducible nitric oxide (NO) synthase (iNOS). One important example of the interaction between NO and ROS is the reaction of NO with superoxide to yield peroxynitrite, which may contribute to the pathogenesis of hypertension. In this review, we discuss the literature that supports an alternate possibility: Nox-derived ROS modulate NO bioavailability by altering the expression of iNOS. We highlight data showing coexpression of iNOS and Nox in vascular smooth muscle demonstrating the functional consequences of iNOS and Nox during vascular injury. We describe the relevant literature demonstrating that the mitogen-activated protein kinases are important modulators of proinflammatory cytokine-dependent expression of iNOS. A central hypothesis discussed is that ROS-dependent regulation of the serine/threonine kinase protein kinase Cdelta is essential to understanding how Nox may regulate signaling pathways leading to iNOS expression. Overall, the integration of nonphagocytic NADPH oxidase with cytokine signaling in general and in vascular smooth muscle in particular is poorly understood and merits further investigation.

Increased spontaneous tone in renal arteries of spontaneously hypertensive rats

The spontaneous tone of vascular smooth muscle is augmented in hypertension. The present study examined the role of nitric oxide (NO), cyclooxygenase (COX), thromboxane A2/prostanoid (TP) and PGE2/prostanoid (EP-1) receptors, reactive oxygen species, and large-conductance Ca2+-activated K+ (BKCa) channels in the regulation of spontaneous tone in renal arteries of young and mature Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). Rings of arteries, with and without endothelium, were suspended in a myograph for isometric force recording. Spontaneous tone (increase above initial tension) was observed only in arteries of mature SHR and was greater in arteries without endothelium. Nω-nitro-l-arginine methyl ester (l-NAME, an inhibitor of NO synthases) induced larger contractions in arteries of SHR than WKY. Indomethacin (a COX inhibitor), SC-19220 (an EP-1 receptor antagonist), and terutroban (a TP receptor antagonist) reduced the l-NAME-evoked contractions. Tiron (a superoxide anion scavenger), catalase (an enzyme that degrades H2O2), and deferoxamine (a hydroxyl radical scavenger) augmented the l-NAME-induced contractions in arteries of mature SHR. Charybdotoxin (a BKCa channel blocker) caused contractions in arteries of mature SHR without endothelium and in arteries with endothelium incubated with l-NAME. A decreased protein level of endothelial NO synthase, an increased release of prostacyclin, and an increased expression of EP-1 receptors were observed in arteries of mature SHR. The present study suggests that spontaneous tone is precipitated by age and hypertension. The reduced production of NO, leading to decreased activation of BKCa channels, may leave the actions of endogenous vasoconstrictors unopposed. COX products that activate EP-1 and TP receptors are involved in the development of spontaneous tone.

Nongenomic, endothelium-independent effects of estrogen on human coronary smooth muscle are mediated by type I (neuronal) NOS and PI3-kinase-Akt signaling

Sex steroids exert profound and controversial effects on cardiovascular function. For example, estrogens have been reported to either ameliorate or exacerbate coronary heart disease. Although estrogen dilates coronary arteries from a variety of species, the molecular basis for this acute, nongenomic effect is unclear. Moreover, we know very little of how estrogen affects human coronary artery smooth muscle cells (HCASMC). The purpose of this study was to elucidate nongenomic estrogen signal transduction in HCASMC. We have used tissue (arterial tension studies), cellular (single-channel patch clamp, fluorescence), and molecular (protein expression) techniques to now identify novel targets of estrogen action in HCASMC: type I (neuronal) nitric oxide synthase (nNOS) and phosphatidylinositol 3-kinase (PI3-kinase)Akt. 17beta-Estradiol (E(2)) increased NO-stimulated fluorescence in HCASMC, and cell-attached patch-clamp experiments revealed that stimulation of nNOS leads to increased activity of calcium-activated potassium (BK(Ca)) channels in these cells. Furthermore, overexpression of nNOS protein in HCASMC greatly enhanced BK(Ca) channel activity. Immunoblot studies demonstrated that E(2) enhances Akt phosphorylation in HCASMC and that wortmannin, an inhibitor of PI3-kinase, attenuated E(2)-stimulated channel activity, NO production, Akt phosphorylation, and estrogen-stimulated coronary relaxation. These studies implicate the PI3-kinase/Akt signaling axis as an estrogen transduction component in vascular smooth muscle cells. We conclude, therefore, that estrogen opens BK(Ca) channels in HCASMC by stimulating nNOS via a transduction sequence involving PI3-kinase and Akt. These findings now provide a molecular mechanism that can explain the clinical observation that estrogen enhances coronary blood flow in patients with diseased or damaged coronary arteries.

Role of Oxidative Stress in the Pathogenesis of Abdominal Aortic Aneurysms

The role of inflammation in the pathogenesis of abdominal aortic aneurysms (AAA) is well established. The inflammatory process leads to protease-mediated degradation of the extracellular matrix and apoptosis of smooth muscle cells (SMC), which are the predominant matrix synthesizing cells of the vascular wall. These processes act in concert to progressively weaken the aortic wall, resulting in dilatation and aneurysm formation. Oxidative stress is invariably increased in, and contributes importantly to, the pathophysiology of inflammation. Moreover, reactive oxygen species (ROS) play a key role in regulation of matrix metalloproteinases and induction of SMC apoptosis. ROS may also contribute to the pathogenesis of hypertension, a risk factor for AAA. Emerging evidence suggests that ROS and reactive nitrogen species (RNS) are associated with AAA formation in animal models and in humans. Although experimental data are limited, several studies suggest that modulation of ROS production or activity may suppress AAA formation and improve experimental outcome in rodent models. Although a number of enzymes can produce injurious ROS in the vasculature, increasing evidence points toward a role for NADPH oxidase as a source of oxidative stress in the pathogenesis of AAA.

Microvascular engineering in perfusion culture: immunohistochemistry and CLSM findings

BACKGROUND

One of the most challenging problems in tissue engineering is the establishment of vascular supply. A possible approach might be the engineering of microvasculature in vitro and the supply by engineered feeder vessels.

METHODS

An in vitro model for a small-diameter vessel was developed and made from adipose tissue stromal cells and human umbilical vein endothelial cells in a tube-like gelatine scaffold. The number of "branches" emerging from the central lumen and the morphology of the central lumen of the vessel equivalent were assessed after 16 days of either pulsatile perfusion culture or culture in rotating containers by evaluation of immunohistochemically stained sections (n = 6 pairs of cultures). Intramural capillary network formation was demonstrated in five experiments with confocal laser scanning microscopy.

RESULTS

Perfused specimens showed a round or oval lumen lined by a single layer of endothelial cells, whereas following rotation culture the lumen tended to collapse. Confocal laser scanning microscopy showed more extended network formation in perfused specimens as compared to specimens after rotation culture. Partially highly interconnected capillary-like networks were imaged which showed orientation around the central lumen. Perfused specimens exhibited significantly more branches emerging from the central lumen. There were, however, hardly any capillary branches crossing the whole vessel wall.

CONCLUSION

Pulsatile perfusion supports the development of vascular networks with physiological appearance. Advances in reactor development, acquisition of functional data and imaging procedures are however necessary in order to attain the ultimate goal of a fully functional engineered supplying vessel.

Mathematical model of nitric oxide convection and diffusion in a renal medullary vas rectum

In this study, the generation, convection, diffusion, and consumption of nitric oxide (NO) in and around a single renal medullary descending or ascending vas rectum in rat were modeled using CFD. The vascular lumen (with a core RBC-rich layer and a parietal layer), the endothelium, the pericytes and the interstitium were represented as concentric cylinders. We accounted for the generation of NO by vascular endothelial cells, and that by the epithelial cells of medullary thick ascending limbs (mTALs) and inner medullary collecting ducts (IMCDs), the latter via interstitial boundary conditions. Luminal velocity profiles were obtained by modeling blood flow dynamics. Our results suggest that convection (i.e., blood flow per se) does not significantly affect NO concentrations along the cortico-medullary axis, because the latter are mostly determined by the rate of NO production and that of NO consumption by hemoglobin. However, the shear stress-mediated effects of blood flow on NO generation rates, and therefore NO concentrations, were predicted to be important. Finally, we found that unless epithelial NO generation rates (per unit tubular surface area) are at least 10 times lower than endothelium NO generation rates, NO production by mTALs and IMCDs affects vascular NO concentrations, with possible consequences for medullary blood flow distribution.

Changes in vascular reactivity following administration of isoproterenol for 1 week: a role for endothelial modulation

1. The aim of this study was to assess the effects of treatment with isoproterenol (ISO, 0.3 mg kg-1 day-1, s.c.) for 7 days on the vascular reactivity of rat-isolated aortic rings. Additionally, potential mechanisms underlying the changes that involved the endothelial modulation of contractility were investigated. 2. Treatment with ISO induced cardiac hypertrophy without changes in haemodynamic parameters. Aortic rings from ISO-treated rats showed an increase in the contraction response to phenylephrine (PHE) and serotonin, but did not change relaxations produced by acetylcholine or isoproterenol. Removal of the endothelium increased the responses to PHE in both groups. However, this procedure was less effective in ISO-treated as compared with control rats. Endothelial cell removal abolished the increase in the response to PHE in ISO-treated rats. The presence of Nomega-nitro-L-arginine methyl ester shifted the concentration-response curve to PHE to the left in both groups of rats. However, this effect was more pronounced in the ISO group. In addition, aminoguanidine (50 microM) potentiated the actions of PHE only in the ISO group. ISO treatment increased nitric oxide synthase (NOS) activity and neuronal NOS and endothelial NOS protein expression in the aorta. 3. Neither losartan (10 microM) nor indomethacin (10 microM) abolished the effects of ISO on the actions of PHE. Superoxide dismutase (SOD, 150 U ml-1) and L-arginine (5 mM), but neither catalase (300 U ml-1) nor apocynin (100 microM), blocked the effect of ISO treatment. In addition, we observed an increase in superoxide anion levels as measured by ethidium bromide fluorescence and of copper and zinc superoxide dismutase protein expression in ISO-treated rats. 4. In conclusion, our data suggest that ISO treatment alters the endothelial cell-mediated modulation of the contraction to PHE in rat aorta. The increased maximal response of PHE seems to be due to an increase in superoxide anion generation, which inactivates some of the basal NO produced and counteracts NO-mediated negative modulation even in the presence of high NO production and antioxidant defence.

Hypoxia induces a functionally significant and translationally efficient neuronal NO synthase mRNA variant

We tested the hypothesis that induction of neuronal NO synthase (nNOS) impairs vascular smooth muscle contractility after hypoxia. nNOS protein was increased in aorta, mesenteric arterioles, pulmonary arteries, brain, and diaphragm from rats exposed to 8% O2 for 48 hours and in human aortic SMCs after hypoxic incubation (1% O2). Ca-dependent NO synthase activity was increased in endothelium-denuded aortic segments from hypoxia-exposed rats. N-nitro-L-arginine methyl ester enhanced the contractile responses of endothelium-denuded aortic rings and mesenteric arterioles from hypoxia-exposed but not normoxic rats (P < 0.05). The hypoxia-inducible mRNA transcript expressed by human cells was found to contain a novel 5'-untranslated region, consistent with activation of transcription in the genomic region contiguous with exon 2. Translational efficiency of this transcript is markedly increased compared with previously described human nNOS mRNAs. Transgenic mice possessing a lacZ reporter construct under control of these genomic sequences demonstrated expression of the construct after exposure to hypoxia (8% O2, 48 hours) in the aorta, mesenteric arterioles, renal papilla, and brain. These results reveal a novel human nNOS promoter that confers the ability to rapidly upregulate nNOS expression in response to hypoxia with a functionally significant effect on vascular smooth muscle contraction.

Shear stress inhibits smooth muscle cell migration via nitric oxide-mediated downregulation of matrix metalloproteinase-2 activity

Vascular smooth muscle cell (SMC) migration is a hallmark of intimal hyperplasia (IH), the progression of which is affected by hemodynamic conditions at the diseased site. The realization that SMCs are exposed to blood flow in both denuded vessels (direct blood flow) and intact vessels (interstitial blood flow) motivated this study of the effects of fluid flow shear stress (SS) on SMC migration. Rat aortic SMCs were seeded onto Matrigel-coated cell culture inserts, and their migratory activity toward PDGF-BB when exposed to SS in a rotating disk apparatus was quantified. Four hours of either 10 or 20 dyn/cm2 SS significantly inhibited SMC migration to the bottom side of the insert. This inhibition was associated with downregulation of SMC matrix metalloproteinase (MMP)-2 activation. Four hours of 10 dyn/cm2 SS also drastically increased SMC production of NO. A NO synthase inhibitor (N(G)-nitro-L-arginine methyl ester; 100 microM) abolished the shear-induced increase in SMC NO production as well as the inhibition of migration and MMP-2 activity. A NO donor (S-nitroso-N-acetyl-penicillamine; 500 microM) suppressed SMC migration via the reduction of both total and active MMP-2 levels. Addition of 10 microM MMP-2 inhibitor I to inserts significantly reduced SMC migration. Western blots showed no effect of 4 h of 20 dyn/cm2 SS on SMC production of PDGF-AA, another chemical known to suppress SMC migration. Thus it appears that SS acts to suppress SMC migration by upregulating the cellular production of NO, which in turn inhibits MMP-2 activity.

Distinct NF-κB Regulation by Shear Stress Through Ras-Dependent IκBα Oscillations

NF-kappaB, a transcription factor central to inflammatory regulation during development of atherosclerosis, is activated by soluble mediators and through biomechanical inputs such as flow-mediated shear- stress. To investigate the molecular mechanisms underlying shear stress mediated signal transduction in vascular cells we have developed a system that applies flow-mediated shear stress in a controlled manner, while inserted in a confocal microscope. In combination with GFP-based methods, this allows continuous monitoring of flow induced signal transduction in live cells and in real time. Flow-mediated shear stress, induced using the system, caused a successive increase in NF-kappaB-regulated gene activation. Experiments assessing the mechanisms underlying the NF-kappaB induced activity showed time and flow rate dependent effects on the inhibitor, IkappaBalpha, involving nuclear translocation characterized by a biphasic or cyclic pattern. The effect was observed in both endothelial- and smooth muscle cells, demonstrated to impact noncomplexed IkappaBalpha, and to involve mechanisms distinct from those mediating cytokine signals. In contrast, effects on the NF-kappaB subunit relA were similar to those observed during cytokine stimulation. Further experiments showed the flow induced inter-compartmental transport of IkappaBalpha to be regulated through the Ras GTP-ase, demonstrating a pronounced reduction in the effects following blocking of Ras activity. These studies show that flow-mediated shear stress, regulated by the Ras GTP-ase, uses distinct mechanisms of NF-kappaB control at the molecular level. The oscillatory pattern, reflecting inter-compartmental translocation of IkappaBetaalpha, is likely to have fundamental impact on pathway regulation and on development of shear stress-induced distinct vascular cell phenotypes.

Vascular smooth muscle cell glycocalyx influences shear stress-mediated contractile response

This study addressed the influence of the rate of shear stress application on aortic smooth muscle cell (SMC) contraction and the role of specific glycosaminoglycans in this mechanotransduction. Rat aortic SMCs were exposed to either a step increase in shear stress (0 to 25 dyn/cm(2)) or a ramp increase in shear stress (0 to 25 dyn/cm(2) over 5 min) in a parallel plate flow chamber, and cell contraction was characterized by cell area reduction. SMCs contracted at levels similar to those reported previously and equally in response to both a step and ramp increase in shear stress. When the cells were pretreated with heparinase III or chondroitinase ABC to remove the glycosaminoglycans heparan sulfate and chondroitin sulfate, respectively, from the glycocalyx, the contraction response to increases in shear stress was significantly inhibited. These studies indicate that specific components of the SMC glycocalyx play an important role in the mechanotransduction of shear stress into a contractile response and that the rate of application of shear stress does not affect the SMC contraction.

Neuronal Nitric Oxide Synthase as a Novel Anti-Atherogenic Factor

Nitric oxide (NO) has multiple important actions that contribute to the maintenance of vascular homeostasis. NO is synthesized by three different isoforms of NO synthase (NOS), all of which have been reported to be expressed in human atherosclerotic vascular lesions. Although the regulatory roles of endothelial NOS (eNOS) and inducible NOS (iNOS) on the development of atherosclerosis have been described, little is known about the role of neuronal NOS (nNOS). Recent studies have demonstrated that nNOS also exerts important vasculoprotective effects in vivo. In a carotid artery ligation model, nNOS-knockout mice exhibited accelerated neointimal formation and constrictive vascular remodeling caused by blood flow disruption. In a rat balloon injury model, the selective inhibition of nNOS activity potently enhanced vasoconstrictor responses to a variety of calcium-mobilizing stimuli, and exacerbated neointimal formation. Moreover, in apolipoprotein E-knockout mice, deficiency of nNOS induced progression of aortic vascular lesion formation. In these models, nNOS was up-regulated in vascular lesions, and was predominantly expressed in the neointima and medial smooth muscle cells. These results provide the first direct evidence that nNOS plays important roles in suppressing arteriosclerotic vascular lesion formation. Thus, nNOS could be regarded as a novel anti-atherogenic factor.

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.

Fluid shear stress-induced alignment of cultured vascular smooth muscle cells

The study objectives were to quantify the time- and magnitude-dependence of flow-induced alignment in vascular smooth muscle cells (SMC) and to identify pathways related to the orientation process. Using an intensity gradient method, we demonstrated that SMC aligned in the direction perpendicular to applied shear stress, which contrasts with parallel alignment of endothelial cells under flow SMC alignment varied with the magnitude of and exposure time to shear stress and is a continuous process that is dependent on calcium and cycloskeleton based mechanisms. A clear understanding and control of flow-induced SMC alignment will have implications for vascular tissue engineering.

Flow through internal elastic lamina affects shear stress on smooth muscle cells (3D simulations)

We describe a three-dimensional numerical simulation of interstitial flow through the medial layer of an artery accounting for the complex entrance condition associated with fenestral pores in the internal elastic lamina (IEL) to investigate the fluid mechanical environment around the smooth muscle cells (SMCs) right beneath the IEL. The IEL was modeled as an impermeable barrier to water flow except for the fenestral pores, which were assumed to be uniformly distributed over the IEL. The medial layer was modeled as a heterogeneous medium composed of a periodic array of cylindrical SMCs embedded in a continuous porous medium representing the interstitial proteoglycan and collagen matrix. Depending on the distance between the IEL bottom surface and the upstream end of the proximal layer of SMCs, the local shear stress on SMCs right beneath the fenestral pore could be more than 10 times higher than that on the cells far removed from the IEL under the conditions that the fenestral pore diameter and area fraction of pores were kept constant at 1.4 microm and 0.05, respectively. Thus these proximal SMCs may experience shear stress levels that are even higher than endothelial cells exposed to normal blood flow (order of 10 dyn/cm(2)). Furthermore, entrance flow through fenestral pores alters considerably the interstitial flow field in the medial layer over a spatial length scale of the order of the fenestral pore diameter. Thus the spatial gradient of shear stress on the most superficial SMC is noticeably higher than computed for endothelial cell surfaces.

Cross talk between prostacyclin and nitric oxide under shear in smooth muscle cell: role in monocyte adhesion

We tested the hypothesis that at sites of vascular damage, vessel homeostasis is maintained through the cross talk of shear-induced production of prostacyclin and nitric oxide (NO) in vascular smooth muscle cells (VSMC). Confluent A7r5 cells derived from rat aortic VSMC and mesenteric VSMC were exposed to shear stress at 15 dyn/cm(2) for 90 min with the use of a cone-plate device, and productions of prostacyclin and NO were examined. Shear stress increased cumulative production of prostacyclin by 3- to 3.5-fold and that of NO by 6- to 7.5-fold. Western blot analysis showed that inducible NO synthase protein was expressed after shear stress in both types of VSMC. Inhibition of NO synthase enhanced the shear-induced production of prostacyclin from 40 to 60%. Shear-induced production of NO was suppressed by 70% after treatment with 10(-4) M of indomethacin. A7r5 cells adhesiveness for monocytes was suppressed by 50% after shear stress. This suppression was abolished by pretreatment with 10(-4) M of indomethacin, whereas inhibition of NO synthase only minimally inhibited it. We conclude that there is a cross talk of shear-induced production of prostacyclin and NO in VSMC. At sites of vascular damage, prostacyclin synthesis may prevent monocyte adhesiveness for VSMC through the concomitant enhancement of NO production.

Shear stress induces iNOS expression in cultured smooth muscle cells: role of oxidative stress

After deendothelialization, the most luminal smooth muscle cells of the neointima are in contact with blood flow and express inducible nitric oxide synthase (iNOS) in vivo. We hypothesized that shear stress may be a stimulus for this iNOS overexpression. We have thus submitted smooth muscle cells to laminar shear and measured the iNOS expression. Shear stress (20 dyn/cm(2)) induced iNOS mRNA and protein expression, whereas brain NOS mRNA expression was decreased. Conversely, nitrite production was increased. This production was blocked by a selective iNOS inhibitor. Pyrrolidine dithiocarbamate, an antioxidant molecule, and BXT-51072, a gluthation peroxidase mimic, both inhibited the shear-induced iNOS expression. Shear stress also increased the expression of both membrane subunits of NADPH oxidase p22(phox) and Mox-1. Shear stress activated the redox-sensitive nuclear translocation of the transcription nuclear factor-kappaB (NF-kappaB) and stimulated the degradation of both cytosolic inhibitors kappaB alpha and beta. These results show that shear stress can induce iNOS expression and nitrite production in smooth muscle cells and suggest that this regulation is probably mediated by oxidative stress-induced NF-kappaB activation.

Effect of fluid flow on smooth muscle cells in a 3-dimensional collagen gel model

A 3D collagen gel model was developed to simulate interstitial fluid flow and to assess the importance of this flow on the biochemical production rates of vascular smooth muscle cells (SMCs). Rat aortic SMCs were suspended in type I collagen, and the gel was supported by nylon fibers that allowed a 9-cm length of the SMC-gel model to withstand 90 cm H(2)O differential pressure over a 6-hour period without significant compaction. Up to 1 dyne/cm(2) shear stress on the suspended SMCs could be induced by the pressure-driven interstitial flow. The suspended SMCs were globular, had a diameter of approximately 10 microm, and were distributed uniformly throughout the gel. The collagen fibers formed a network that was connected randomly with the surface of SMCs and nylon fibers. The diameter of the collagen fibers was approximately 100 nm, and the concentration of collagen was 2.5 mg/mL. Using these parameters, fiber matrix theory predicted a Darcy permeability coefficient (K:(p)) of 1.22x10(-)(8) cm(2), which was close to the measured value of K:(p). The production rates of prostaglandin (PG) I(2) and PGE(2) were used as markers of biochemical responsiveness of SMCs to fluid shear stress. Both PGI(2) and PGE(2) production rates under 1 dyne/cm(2) shear stress were significantly elevated relative to static (no-flow) controls. The production rates, however, were approximately 10 times lower than observed when the same cells were plated on collagen-treated glass slides (2D model) and exposed to the same level of shear stress by use of a rotating disk apparatus. The results indicate that interstitial flow can affect SMC biology and that SMCs are more quiescent in 3D cultures than in 2D cultures. The 3D collagen gel model should be useful for future studies of interstitial flow effects on SMC function.

Shear stress-mediated NO production in inner medullary collecting duct cells

Recent evidence suggests that nitric oxide (NO) within the inner medullary collecting duct (IMCD) functions to regulate sodium and water reabsorption. Because fluid shear stress has been shown to increase NO production in endothelial and vascular smooth muscle cells, experiments were designed to determine whether a similar mechanism exists in IMCD cells. Cultured IMCD-3 cells derived from murine IMCD were subjected to 60 min of pulsatile shear stress. Nitrite production (2,3-diaminonaphthalene fluorometric assay) increased 12-, 16-, and 23-fold at 3.3, 10, and 30 dyn/cm(2), respectively, compared with static control cultures. Preincubation with the non-isoform-specific NO synthase inhibitor nitro-L-arginine methyl ester reduced nitrite production by 83% in response to 30 dyn/cm(2). Western blotting and immunofluorescence analysis of static IMCD-3 cell cultures revealed the expression of all three NO synthase isoforms (NOS-1 or neuronal NOS, NOS-2 or inducible NOS, and NOS-3 or endothelial NOS) in IMCD-3 cultures. These results indicate that NO production is modulated by shear stress in IMCD-3 cells and that fluid shear stress within the renal tubular system may play a role in the regulation of sodium and water excretion by control of NO production in the IMCD.

Interstitial flow through the internal elastic lamina affects shear stress on arterial smooth muscle cells

Interstitial flow through the tunica media of an artery wall in the presence of the internal elastic lamina (IEL), which separates it from the subendothelial intima, has been studied numerically. A two-dimensional analysis applying the Brinkman model as the governing equation for the porous media flow field was performed. In the numerical simulation, the IEL was modeled as an impermeable barrier to water flux, except for the fenestral pores, which were uniformly distributed over the IEL. The tunica media was modeled as a heterogeneous medium composed of a periodic array of cylindrical smooth muscle cells (SMCs) embedded in a fiber matrix simulating the interstitial proteoglycan and collagen fibers. A series of calculations was conducted by varying the physical parameters describing the problem: the area fraction of the fenestral pore (0. 001-0.036), the diameter of the fenestral pore (0.4-4.0 microm), and the distance between the IEL and the nearest SMC (0.2-0.8 microm). The results indicate that the value of the average shear stress around the circumference of the SMC in the immediate vicinity of the fenestral pore could be as much as 100 times greater than that around an SMC in the fully developed interstitial flow region away from the IEL. These high shear stresses can affect SMC physiological function.

Functional expression of NOS 1 in vascular smooth muscle

Substances that increase intracellular calcium concentration ([Ca(2+)](i)), such as serotonin, are known to induce vascular smooth muscle (VSM) contraction. However, increases in [Ca(2+)](i) also activate Ca(2+)/calmodulin-dependent nitric oxide synthases (NOS), which leads to increases in cGMP and activation of cGMP-dependent protein kinase (PKG). One recently identified substrate protein of PKG is the small heat shock protein, HSP20. The purpose of this study was to determine if serotonin activates a Ca(2+)-dependent NOS in VSM. Strips of bovine carotid arterial smooth muscle denuded of endothelium were stimulated with serotonin in the presence and absence of the nonspecific NOS inhibitor N-monomethyl-L-arginine (L-NMMA). Activation of NOS was determined by increases in cGMP and in the phosphorylation of HSP20. Immunohistochemical and Western blotting techniques were performed to identify specific NOS isoforms in bovine carotid arterial smooth muscle preparations. Serotonin stimulation led to significant increases in cGMP and in the phosphorylation of HSP20, which were inhibited by pretreatment with L-NMMA. Antibodies against NOS 1 stained the media of bovine carotid and human renal arteries, whereas antibodies against NOS 3 stained only the endothelium. Additionally, the conversion of radiolabeled L-arginine to L-citrulline NOS activity demonstrated a consistent amount of activity present in the endothelium-denuded smooth muscle preparations that was reduced by 99% with an NOS 1 specific inhibitor. Finally, an NOS 1 specific inhibitor, 7-nitroindazole, augmented contractions induced by high extracellular KCl. This study demonstrates that NOS 1 is present in VSM and may effect physiological contractile responses.

Fluid flow releases fibroblast growth factor-2 from human aortic smooth muscle cells

This study tested the hypothesis that fluid shear stress regulates the release of fibroblast growth factor (FGF)-2 from human aortic smooth muscle cells. FGF-2 is a potent mitogen that is involved in the response to vascular injury and is expressed in a wide variety of cell types. FGF-2 is found in the cytoplasm of cells and outside cells, where it associates with extracellular proteoglycans. To test the hypothesis that shear stress regulates FGF-2 release, cells were exposed to flow, and FGF-2 amounts were measured from the conditioned medium, pericellular fraction (extracted by heparin treatment), and cell lysate. Results from the present study show that after 15 minutes of shear stress at 25 dyne/cm(2) in a parallel-plate flow system, a small but significant fraction (17%) of the total FGF-2 was released from human aortic smooth muscle cells. FGF-2 levels in the circulating medium increased 10-fold over medium from static controls (P<0.01). A 50% increase in FGF-2 content versus control (P<0.01) was found in the pericellular fraction (extracted by heparin treatment). Furthermore, a significant decrease in FGF-2 was detected in the cell lysate, indicating that FGF-2 was released from inside the cell. Cell permeability studies with fluorescent dextran were performed to examine whether transient membrane disruption caused FGF-2 release. Flow cytometry detected a 50% increase in mean fluorescence of cells exposed to 25 dyne/cm(2) versus control cells. This indicates that the observed FGF-2 release from human aortic smooth muscle cells is likely due to transient membrane disruption on initiation of flow.

Protection by shear stress from collar-induced intimal thickening: role of nitric oxide

Nitric oxide (NO) has potent relaxant and antiproliferative effects on vascular smooth muscle cells, which may represent an important antiatherosclerotic mechanism. Since one of the major stimuli for NO release is flow-related shear stress, we have investigated (1) the effect of increased shear stress on neointimal formation induced in the rabbit carotid artery by enclosing the vessel in a nonconstrictive silicone soft collar and (2) the role of NO in the antiproliferative effect of increased shear stress. Forty-three New Zealand White rabbits were used. High shear stress in the left common carotid artery (CCA) was induced by ligature of the contralateral right internal carotid artery; intimal thickening was produced by the positioning a nonconstrictive silicone soft collar around the left CCA. To evaluate the role of NO, N(G)-nitro-L-arginine methyl ester (L-NAME) was orally administered at a subpressor dose. In all rabbits, arterial blood pressure, heart rate, arterial diameters, and blood flow velocities of both CCAs were determined at days 0, 3, 7, and 14. At the end of the study, all rabbits were euthanized, and histological analyses were performed on both CCAs of each animal. The presence of the collar was associated with a marked degree of intimal hyperplasia (intimal/medial area ratio 29+/-3.0% in collared arteries compared with 3+/-0.7% in sham control [noncollared] arteries, P<0.001). The increase in blood flow almost completely inhibited neointimal formation and induced an increase in arterial diameter of approximately 30%. The effects of increased blood flow were reversed by the administration of L-NAME. In conclusion, we demonstrate that in collar-induced intimal thickening, a chronic increase in shear stress (1) almost completely inhibits intimal thickening, and (2) this protective effect is mediated by NO production.

Potential functional significance of brain-type and muscle-type nitric oxide synthase I expressed in adventitia and media of rat aorta

Skeletal muscle and myocardium express microNOS I, an elongated splice variant of neuronal-type nitric oxide (NO) synthase (NOS I), and NOS III, endothelial-type NO synthase, respectively. This study was designed to elucidate whether vascular smooth muscle also contains a constitutively expressed NO synthase isoform. In the rat, microNOS I contains an insert of 102 nucleotides after nucleotide 2865 of the cDNA, yielding a protein of 164 kd. Reverse transcription-polymerase chain reaction with primers flanking this insert and with insert-specific primers indicated that endothelium-denuded rat aorta expresses both brain-type NOS I and microNOS I. RNase protection analyses with an antisense RNA probe overlapping the microNOS I insert detected significant amounts of NOS I mRNA and lesser amounts of microNOS I mRNA in endothelium-denuded aorta. Western blots using a specific polyclonal antibody recognizing NOS I and microNOS I showed a major band of the 160-kd NOS I and a lesser band of a slightly larger protein in endothelium-denuded aorta. Immunohistochemistry demonstrated low levels of NOS I/microNOS I immunoreactivity in the medial layer of rat aorta, whereas the endothelium expressed only NOS III immunoreactivity. When the adventitia also was removed, NOS I and microNOS I mRNA decreased markedly but remained detectable in the medial layer. In functional experiments with endothelium-denuded rat aortic rings (that contained no NOS III), contractions induced by KCl were markedly increased in the presence of the NOS inhibitor N(G)-nitro-L-arginine. These data demonstrate that 2 subforms of NOS I are expressed in nonendothelial components of rat aorta: NOS I and lesser amounts of microNOS I. Under certain conditions, this NOS I/microNOS I expression could serve as a backup system to the functionally predominant NOS III.