Acute mechanoadaptation of vascular smooth muscle cells in response to continuous arteriolar vasoconstriction: implications for functional remodeling

Dose-response relationship of locally applied nimodipine in an ex vivo model of cerebral vasospasm

INTRODUCTION

Cerebral vasospasm is a severe complication of subarachnoid hemorrhage (SAH). The calcium channel inhibitor nimodipine has been used for treatment of cerebral vasospasm. No evidence-based recommendations for local nimodipine administration at the site of vasospasm exist. The purpose of this study was to quantify nimodipine's local vasodilatory effect in an ex vivo model of SAH-induced vasospasm.

METHODS

SAH-induced vasospasm was modeled by contracting isolated segments of rat superior cerebellar arteries with a combination of serotonin and a synthetic analog of prostaglandin A(2). A pressure myograph system was used to determine vessel reactivity of spastic as well as non-spastic arteries.

RESULTS

Compared to the initial vessel diameter, a combination of serotonin and prostaglandin induced considerable vasospasm (55 ± 2.5 % contraction; n = 12; p < 0.001). Locally applied nimodipine dilated the arteries in a concentration-dependent manner starting at concentrations as low as 1 nM (n = 12; p < 0.05). Concentrations higher than 100 nM did not relevantly increase the vasodilatory effect. Nimodipine's vasodilatory effect was smaller in spastic than in non-spastic vessels (n = 12; p < 0.05), which we assume to be due to structural changes in the vessel wall.

CONCLUSION

The described ex vivo model allows to investigate the dose-dependent efficacy of spasmolytic drugs prior to in vivo experiments. Low concentrations of locally applied nimodipine have a strong vasodilatory effect, which is of relevance when considering the local application of nimodipine in cerebral vasospasm.

Vascular smooth muscle cells remodel collagen matrices by long-distance action and anisotropic interaction

While matrix remodeling plays a key role in vascular physiology and pathology, the underlying mechanisms have remained incompletely understood. We studied the remodeling of collagen matrices by individual vascular smooth muscle cells (SMCs), clusters and monolayers. In addition, we focused on the contribution of transglutaminase 2 (TG2), which plays an important role in the remodeling of small arteries. Single SMCs displaced fibers in collagen matrices at distances up to at least 300 μm in the course of 8–12 h. This process involved both ‘hauling up’ of matrix by the cells and local matrix compaction at a distance from the cells, up to 200 μm. This exceeded the distance over which cellular protrusions were active, implicating the involvement of secreted enzymes such as TG2. SMC isolated from TG2 KO mice still showed compaction, with changed dynamics and relaxation. The TG active site inhibitor L682777 blocked local compaction by wild type cells, strongly reducing the displacement of matrix towards the cells. At increasing cell density, cells cooperated to establish compaction. In a ring-shaped collagen matrix, this resulted in preferential displacement in the radial direction, perpendicular to the cellular long axis. This process was unaffected by inhibition of TG2 cross-linking. These results show that SMCs are capable of matrix remodeling by prolonged, gradual compaction along their short axis. This process could add to the 3D organization and remodeling of blood vessels based on the orientation and contraction of SMCs.

Arteriolar vascular smooth muscle cells: mechanotransducers in a complex environment

Contraction of small artery (diameters typically less than 250 μm) vascular smooth muscle cells (VSMCs) plays a critical role in local control of blood flow and arterial pressure through its affect on vascular caliber. Specifically, contraction of small arteries in response to increased intraluminal pressure is referred to as the myogenic response and represents an important role for mechanotransduction. Critical questions remain as to how changes in pressure are sensed by VSMCs and transduced across the cell membrane to tune the contractile state of the cell. Recent studies suggest a pivotal role for interactions between VSMCs and extracellular matrix (ECM) proteins. Thus, pressure-induced deformation of ECM proteins and their cell surface receptors (for example, integrins) may initiate contraction and cytoskeletal remodeling through modulation of ion channels, membrane depolarization, increased intracellular Ca(2+) and actomyosin crossbridge cycling. Importantly, it is argued that the contractile properties of small artery VSMCs reflect an intimate and integrated interaction with their extracellular environment and the three-dimensional structure of the vessel wall.

Tensile properties of vascular smooth muscle cells: bridging vascular and cellular biomechanics

Vascular walls change their dimensions and mechanical properties adaptively in response to blood pressure. Because these responses are driven by the smooth muscle cells (SMCs) in the media, a detailed understanding of the mechanical environment of the SMCs should reveal the mechanism of the adaptation. As the mechanical properties of the media are highly heterogeneous at the microscopic level, the mechanical properties of the cells should be measured directly. The tensile properties of SMCs are, thus, important to reveal the microscopic mechanical environment in vascular tissues; their tensile properties have a close correlation with the distribution and arrangement of elements of the cytoskeletal networks, such as stress fibers and microtubules. In this review, we first introduce the experimental techniques used for tensile testing and discuss the various factors affecting the tensile properties of vascular SMCs. Cytoskeletal networks are particularly important for the mechanical properties of a cell and its mechanism of mechanotransduction; thus, the mechanical properties of cytoskeletal filaments and their effects on whole-cell mechanical properties are discussed with special attention to the balance of intracellular forces among the intracellular components that determines the force applied to each element of the cytoskeletal filaments, which is the key to revealing the mechanotransduction events regulating mechanical adaptation. Lastly, we suggest future directions to connect tissue and cell mechanics and to elucidate the mechanism of mechanical adaptation, one of the key issues of cardiovascular solid biomechanics.

Structural adaptation of normal and tumour vascular networks

Vascular networks are dynamic structures, adapting to changing conditions by structural remodelling of vessel diameters and by growth of new vessels and regression of existing vessels. The vast number of blood vessels in the circulatory system, more than 10⁹, implies that vessels' arrangement and structure are not under individual genetic control but emerge as a result of generic responses of each segment to the various stimuli that it experiences. To obtain insight into the types of response that are needed, a network-oriented approach has been used, in which theoretical models are used to simulate structural adaptation in vascular networks, and the results are compared with experimental observations. With regard to the structural control of vessel diameters, this approach shows that responses to both haemodynamic and metabolic stimuli are needed for the formation of functionally adequate and efficient network structures. Furthermore, information transfer in both upstream and downstream directions is essential for balancing flows between long and short flow pathways. Otherwise, functional shunting occurs, that is, short pathways become enlarged and flow bypasses longer pathways. Information transfer in the upstream direction is achieved by conducted responses communicated along vessel walls. Simulations of structural adaptation in tumour microvascular networks indicate that impaired vascular communication, resulting in functional shunting, may be an important factor causing the dysfunctional microcirculation and local hypoxia typically observed in tumours. Anti-angiogenic treatment of tumours may restore vascular communication and thereby improve or normalize flow distribution in tumour vasculature.

The dynamic structure of arterioles

Arterioles are the blood vessels in the arterial side of the vascular tree that are located proximal to the capillaries and, in conjunction with the terminal arteries, provide the majority of resistance to blood flow. Consequently, arterioles are important contributors to the regulation of mean arterial pressure and tissue perfusion. Their wall consists of cellular and extracellular components that have been traditionally classified as conforming three layers: an intima containing endothelial cells sited on a basement membrane; a media made of an internal elastic lamina apposed by one or two layers of smooth muscle; and an adventitia composed mostly of collagen bundles, nerve endings and some fibroblasts. These components of the arteriolar wall are dynamically interconnected, providing a level of plasticity to the arteriolar wall that blurs the traditional boundaries of a rigid layered classification. This MiniReview focuses on the structural conformation of the arteriolar wall and shows how wall components interact spatially, functionally and temporally to control vascular diameter, regulate blood flow and maintain vascular permeability.

Smooth muscle biomechanics and plasticity: relevance for vascular calibre and remodelling

Blood vessel structure and calibre are not static. Rather, vessels remodel continuously in response to their biomechanical environment. Vascular calibre is dictated by the amount, composition and organization of the elastic extracellular matrix. In addition, the amount and organization of contractile smooth muscle cell (SMC) also need to be regulated. The SMCs are organized such that maximum contractile force generally occurs at diameters slightly below the diameter at full dilation and physiological pressure. Thus, in a remodelling vessel, not only the matrix but also the SMCs need to undergo structural adaptation. Surprisingly little is known on the adaptation of SMC contractile properties in the vasculature. The purpose of this review is to explore this SMC plasticity in the context of vascular remodelling. While not much work on this has been carried out on blood vessels, SMC plasticity is more extensively studied on other hollow structures such as airway and bladder. We therefore include studies on bladder and airway SMCs because of their possible relevance for vascular SMC behaviour. Here, plasticity is thought to form an adaptation allowing maintained function despite large volume changes. In blood vessels, the general match of active and passive diameter-tension relations suggests that SMC plasticity is part of normal vascular physiological adaptation. Vascular SMCs display similar processes and forms of adaptation as seen in nonvascular SMCs. This may become particularly relevant under strong vasoconstriction, when inward cytoskeletal adaptation possibly prevents immediate full dilation. This may contribute to structural inward remodelling as seen in hypertension and flow reduction.

Spatial distribution and mechanical function of elastin in resistance arteries: a role in bearing longitudinal stress

OBJECTIVE

Despite the role that extracellular matrix (ECM) plays in vascular signaling, little is known of the complex structural arrangement between specific ECM proteins and vascular smooth muscle cells. Our objective was to examine the hypothesis that adventitial elastin fibers are dominant in vessels subject to longitudinal stretch.

METHODS AND RESULTS

Cremaster muscle arterioles were isolated, allowed to develop spontaneous tone, and compared with small cerebral arteries. 3D confocal microscopy was used to visualize ECM within the vessel wall. Pressurized arterioles were fixed and stained with Alexa 633 hydrazide (as a nonselective ECM marker), anti-elastin, or anti-type 1 collagen antibody and a fluorescent nuclear stain. Exposure of cremaster muscle arterioles to elastase for 5 minutes caused an irreversible lengthening of the vessel segment that was not observed in cerebral arteries. Longitudinal elastin fibers were demonstrated on cremaster muscle arterioles using 3D imaging but were confirmed to be absent in cerebral vessels. The fibers were also distinct from type I collagen fibers and were degraded by elastase treatment.

CONCLUSIONS

These results indicate the importance of elastin in bearing longitudinal stress in the arteriolar wall and that these fibers constrain vascular smooth muscle cells. Differences between skeletal muscle and cerebral small arteries may reflect differences in the local mechanical environment, such as exposure to longitudinal stretch.

Small artery remodelling in hypertension

Increased blood pressure (essential hypertension) is associated with increased cardiovascular risk, and the condition is treated primarily with a view to reducing this parameter. However, in the early stages, the main pathological changes are increased peripheral resistance and altered cardiovascular structure. The aim of this MiniReview was to trace the endeavours over the past several decades to translate these findings into answering the question whether normalization of resistance vessel structure should be a target for therapy. This MiniReview describes first the altered structure of the resistance vasculature in essential hypertension, where the vessels show increased media/lumen ratio because of inward eutrophic remodelling. Secondly, evidence is presented that altered small artery structure appears to have prognostic consequences. Then, the cellular mechanisms that may be involved are discussed, where there is evidence that vasoconstriction in itself can cause inward remodelling and that this can be prevented by vasodilators. This leads to a discussion of the degree to which it may be possible to rectify the abnormal structure, where it appears that this may be achieved using a therapy that causes vasodilatation in the patient concerned. Finally, the consequences of these findings are considered as regards clues for strategies that may be able to improve the outcome of antihypertensive therapy. The MiniReview concludes that there is reasonably strong evidence that improvement in abnormal resistance vessel structure requires a treatment that reduces peripheral resistance in the individual patient.

Inward remodeling of resistance arteries requires reactive oxygen species-dependent activation of matrix metalloproteinases

Inward eutrophic remodeling is the most prevalent structural change of resistance arteries in hypertension. Sympathetic and angiotensin (ANG)-induced vasoconstriction has been associated with hypertension and with the production of matrix metalloproteinases (MMPs) and ROS. Therefore, we hypothesize that prolonged exposure to norepinephrine (NE) and ANG II induces arteriolar inward remodeling dependent on the activation of MMPs and the production of ROS. This hypothesis was tested on rat cremaster arterioles that were isolated, cannulated, pressurized, and exposed to either NE (10(-5.5) mol/l) + ANG II (10(-7) mol/l) or vehicle (control) for 4 h. The prolonged exposure to NE + ANG II induced inward remodeling, as evidenced by the reduced maximal arteriolar passive diameter observed after versus before exposure to the vasoconstrictor agonists. NE + ANG II also increased the arteriolar expression and activity of MMP-2 and the production of ROS as determined, respectively, by real-time RT-PCR, gel and in situ zymography, and the use of ROS-sensitive dyes with multiphoton microscopy. Inhibition of MMP activation (with GM-6001) or ROS production (with apocynin or tempol) prevented the NE + ANG II-induced inward remodeling. Inhibition of ROS production prevented the activation of MMPs and the remodeling process, whereas inhibition of MMP activation did not affect ROS production. These results indicate that prolonged stimulation of resistance arterioles with NE + ANG II induces a ROS-dependent activation of MMPs necessary for the development of arteriolar inward remodeling. These mechanisms may contribute to the structural narrowing of resistance vessels in hypertension.

Matrix metalloproteinases and small artery remodeling

Inward eutrophic remodeling is a common structural change found in small resistance arteries that has been associated with an increased risk for life threatening cardiovascular events, the number one cause of death in industrialized societies. Because inward eutrophic remodeling is the most prevalent small artery structural change found in hypertension, hypertensive animals are the most common in vivo models used to study this particular remodeling process. In vitro, the isolated artery, pressure myograph has also been used as a model to study the mechanisms responsible for the development of small artery remodeling. Compelling recent evidence indicates that the matrix metalloproteinases (MMPs), a family of endopeptidases whose primary function is the cleavage and degradation of extracellular matrix components, are involved in vasoconstriction and the pathogenesis of hypertension. In this review we provide an overview of the known and potential roles that MMPs have on vascular remodeling, paying particular attention to their role on the inward eutrophic remodeling process of small resistance arteries that occurs in hypertension.

A microfluidic platform for probing small artery structure and function

Although pathologic changes to the structure and function of small blood vessels are hallmarks of various cardiovascular diseases, limitations of conventional investigation methods (i.e. pressure myography) have prohibited a comprehensive understanding of the underlying mechanisms. We developed a microfluidic device to facilitate assessment of resistance artery structure and function under physiological conditions (37 degrees C, 45 mmHg transmural pressure). The platform allows for on-chip fixation, long-term culture and fully automated acquisition of up to ten dose-response sequences of intact mouse mesenteric artery segments (diameter approximately 250 micrometres and length approximately 1.5 mm) in a well-defined microenvironment. Even abluminal application of phenylephrine or acetylcholine (homogeneous condition) yielded dose-response relationships virtually identical to conventional myography. Unilateral application of phenylephrine (heterogeneous condition) limited constriction to the drug-exposed side, suggesting a lack of circumferential communication. The microfluidic platform allows us to address new fundamental biological questions, replaces a manually demanding procedure with a scalable approach and may enable organ-based screens to be routinely performed during drug development.

Biomechanical and microstructural properties of common carotid arteries from fibulin-5 null mice

Alteration in the mechanical properties of arteries occurs with aging and disease, and arterial stiffening is a key risk factor for subsequent cardiovascular events. Arterial stiffening is associated with the loss of functional elastic fibers and increased collagen content in the wall of large arteries. Arterial mechanical properties are controlled largely by the turnover and reorganization of key structural proteins and cells, a process termed growth and remodeling. Fibulin-5 (fbln5) is a microfibrillar protein that binds tropoelastin, interacts with integrins, and localizes to elastin fibers; tropoelastin and microfibrillar proteins constitute functional elastic fibers. We performed biaxial mechanical testing and confocal imaging of common carotid arteries (CCAs) from fibulin-5 null mice (fbln5 ⁻(/)⁻) and littermate controls (fbln5 (+/+)) to characterize the mechanical behavior and microstructural content of these arteries; mechanical testing data were fit to a four-fiber family constitutive model. We found that CCAs from fbln5 ⁻(/)⁻ mice exhibited lower in vivo axial stretch and lower in vivo stresses while maintaining a similar compliance over physiological pressures compared to littermate controls. Specifically, for fbln5 ⁻(/)⁻ the axial stretch λ = 1.41 ± 0.07, the circumferential stress σ(θ) = 101 ± 32 kPa, and the axial stress σ ( z ) = 74 ± 28 kPa; for fbln5 (+/+) λ = 1.64 ± 0.03, σ(θ) = 194 ± 38 kPa, and σ(z) = 159 ± 29 kPa. Structurally, CCAs from fbln5 ⁻(/)⁻ mice lack distinct functional elastic fibers defined by the lamellar structure of alternating layers of smooth muscle cells and elastin sheets. These data suggest that structural differences in fbln5 ⁻(/)⁻ arteries correlate with significant differences in mechanical properties. Despite these significant differences fbln5 ⁻(/)⁻ CCAs exhibited nearly normal levels of cyclic strain over the cardiac cycle.

Microstructurally motivated constitutive modeling of mouse arteries cultured under altered axial stretch

Good predictions of the local mechanical environment of the tissue with known geometry and applied loads are fundamental to quantifying the biological response of tissues to mechanical stimuli. Whereas mean stresses in cylindrical sections of blood vessels may be calculated directly from measured loads and vessel geometry (e.g., Laplace's law), predicting how these stresses are distributed across the wall requires knowledge of the constitutive behavior of the tissue. Previously, we reported biaxial biomechanical data for mouse carotid arteries before and after exposure to altered axial extension in organ culture. Here we considered phenomenological and microstructurally motivated constitutive models and identified material parameters for each via nonlinear regression. Specifically, we considered the model of Chuong and Fung, a four fiber-family model, and several new variants of a rule-of-mixtures model; in the latter, we modeled the artery as a mixture of collagen, elastin, muscle, and water. We found that the four fiber-family model fitted data significantly better than the model of Chuong and Fung. When identifying parameters for the rule-of-mixtures models, we imposed penalties that required each constituent to be structurally significant; e.g., elastin contributing significantly to the overall response over low loads and collagen dominating the response over high loads. Such constraints ascribe additional microstructural "meaning" to the constitutive model. Although imposing such penalties necessarily reduces the goodness of fit of model predictions to experimental data compared to regression without such penalties, the modest reduction in the goodness of fit observed in our results was off-set by the improved structural interpretation such models provide. Such microstructurally motivated models will be useful in characterizing vascular growth and remodeling in terms of the evolution of microstructural metrics that may be quantified experimentally.

A 3-D constrained mixture model for mechanically mediated vascular growth and remodeling

In contrast to the widely applied approach to model soft tissue remodeling employing the concept of volumetric growth, microstructurally motivated models are capable of capturing many of the underlying mechanisms of growth and remodeling; i.e., the production, removal, and remodeling of individual constituents at different rates and to different extents. A 3-dimensional constrained mixture computational framework has been developed for vascular growth and remodeling, considering new, microstructurally motivated kinematics and constitutive equations and new stress and muscle activation mediated evolution equations. Our computational results for alterations in flow and pressure, using reasonable physiological values for rates of constituent growth and turnover, concur with findings in the literature. For example, for flow-induced remodeling, our simulations predict that, although the wall shear stress is restored completely, the circumferential stress is not restored employing realistic physiological rate parameters. Also, our simulations predict different levels of thickening on inner versus outer wall locations, as shown in numerous reports of pressure-induced remodeling. Whereas the simulations are meant to be illustrative, they serve to highlight the experimental data currently lacking to fully quantify mechanically mediated adaptations in the vasculature.

Structural adaptation of microvessel diameters in response to metabolic stimuli: where are the oxygen sensors?

Maintenance of functional vascular networks requires structural adaptation of vessel diameters in response to hemodynamic and metabolic conditions. The mechanisms by which diameters respond to the metabolic state are not known, but may involve the release of vasoactive substances in response to low oxygen by tissue ("tissue signaling", e.g., CO2, adenosine), by vessel walls ("wall signaling", e.g., prostaglandins, adenosine), and/or by red blood cells (RBCs) ("RBC signaling", e.g., ATP and nitric oxide). Here, the goal was to test the potential of each of these locations of oxygen-dependent signaling to control steady-state vascular diameters and tissue oxygenation. A previously developed theoretical model of structural diameter adaptation based on experimental data on microvascular network morphology and hemodynamics was used. Resulting network characteristics were analyzed with regard to tissue oxygenation (Oxdef; percentage of tissue volume with PO2<1 Torr) and the difference between estimated blood flow velocities and corresponding experimental data [velocity error (Verr); root mean square deviation of estimated vs. measured velocity]. Wall signaling led to Oxdef<1% and to the closest hemodynamic similarity (Verr: 0.60). Tissue signaling also resulted in a low oxygen deficit, but a higher Verr (0.73) and systematic diameter deviations. RBC signaling led to widespread hypoxia (Oxdef: 4.7%), unrealistic velocity distributions (Verr: 0.81), and shrinkage of small vessels. The results suggest that wall signaling plays a central role in structural control of vessel diameters in microvascular networks of given angioarchitecture. Tissue-derived and RBC-derived signaling of oxygen levels may be more relevant for the regulation of angiogenesis and/or smooth muscle tone.

The macrocirculation and microcirculation of hypertension

Changes in vascular structure that accompany hypertension may contribute to hypertensive end-organ damage. Both the macrovascular and microvascular levels should be considered, as interactions between them are believed to be critically important. Regarding the macrocirculation, the article first reviews basic concepts of vascular biomechanics, such as arterial compliance, arterial distensibility, and stress-strain relationships of arterial wall material, and then reviews how hypertension affects the properties of conduit arteries, particularly examining evidence that it accelerates the progressive stiffening that normally occurs with advancing age. High arterial stiffness may increase central systolic and pulse pressure by two different mechanisms: 1) Abnormally high pulse wave velocity may cause pressure waves reflected in the periphery to reach the central aorta in systole, thus augmenting systolic pressure; 2) In the elderly, the interaction of the forward pressure wave with high arterial stiffness is mostly responsible for abnormally high pulse pressure. At the microvascular level, hypertensive disease is characterized by inward eutrophic or hypertrophic arteriolar remodeling and capillary rarefaction. These abnormalities may depend in part on the abnormal transmission of highly pulsatile blood pressure into microvascular networks, especially in highly perfused organs with low vascular resistance, such as the kidney, heart, and brain, where it contributes to hypertensive end-organ damage.

Therapeutic potential of pharmacologically targeting arteriolar myogenic tone

The arteriolar myogenic response, which is defined as vasoconstriction to increases in intraluminal pressure and, conversely, dilation to a reduction in pressure, is key in the setting of vascular resistance, local control of microvascular blood flow through autoregulation, and in the control of capillary hydrostatic pressure. Although considerable progress has been made in the quest for understanding the underlying sensory apparatus and cellular mechanisms, fundamental questions remain - particularly if this pathway is to be considered as a target for novel strategies of pharmacological intervention. We propose that an ability to 're-set' myogenic tone would enable modification of systemic vascular resistance and pressure while at the same time preserving existing interactions with neurohumoral regulatory mechanisms. The challenge, therefore, is to identify steps unique to the myogenic signaling pathway to enable specific pharmacological targeting.

The plastic nature of the vascular wall: a continuum of remodeling events contributing to control of arteriolar diameter and structure

The diameter of resistance arteries has a profound effect on the distribution of microvascular blood flow and the control of systemic blood pressure. Here, we review mechanisms that contribute to the regulation of resistance artery diameter, both acutely and chronically, their temporal characteristics, and their interdependence. Furthermore, we hypothesize the existence of a remodeling continuum that allows for the vascular wall to rapidly modify its structural characteristics, specifically through the re-positioning of vascular smooth muscle cells. Importantly, the concepts presented more closely link acute vasoregulatory responses with adaptive changes in vessel wall structure. These rapid structural adaptations provide resistance vessels the ability to maintain a desired diameter under presumed optimal energetic and mechanical conditions.

Small artery remodelling in hypertension: causes, consequences and therapeutic implications

Essential hypertension is treated primarily with a view to reducing blood pressure, and not with regard to normalizing the main pathological changes: the peripheral resistance and the cardiovascular structure. The aim of this review is to discuss whether normalization of the latter parameters, in particular resistance vessel structure, may also be a target for therapy. The review presents first the evidence for altered structure of the resistance vasculature, an increase in the media:lumen ratio of the vessels due to inward eutrophic remodelling. Secondly the degree to which it may be possible to rectify the abnormal structure is discussed, where it is shown that there is strong evidence that this requires a therapy which causes vasodilatation in the patient concerned. Thirdly evidence is presented that altered small artery structure appears to have prognostic consequences. Fourthly, the cellular mechanisms which may be involved are discussed, where there is evidence that vasoconstriction in itself can cause inward remodelling, and that this can be prevented by vasodilators. Finally, the consequences of these findings are considered as regards clues for strategies that may be able to improve the outcome of antihypertensive therapy. The review concludes that there is reasonably strong evidence that a treatment which reduces peripheral resistance in the individual patient will, apart from reducing blood pressure, also improve the abnormal structure.

A mechanism for arteriolar remodeling based on maintenance of smooth muscle cell activation

Structural adaptation in arterioles is part of normal vascular physiology but is also seen in disease states such as hypertension. Smooth muscle cell (SMC) activation has been shown to be central to microvascular remodeling. We hypothesize that, in a remodeling process driven by SMC activation, stress sensitivity of the vascular wall is a key element in the process of achieving a stable vascular structure. We address whether the adaptive changes in arterioles under different conditions can arise through a common mechanism: remodeling in a stress-sensitive wall driven by a shift in SMC activation. We present a simple dynamic model and show that structural remodeling of the vessel radius by rearrangement of the wall material around a lumen of a different diameter and driven by differences in SMC activation can lead to vascular structures similar to those observed experimentally under various conditions. The change in structure simultaneously leads to uniform levels of circumferential wall stress and wall strain, despite differences in transmural pressure. A simulated vasoconstriction caused by increased SMC activation leads to inward remodeling, whereas outward remodeling follows relaxation of the vascular wall. The results are independent of the specific myogenic properties of the vessel. The simulated results are robust in the face of parameter changes and, hence, may be generalized to vessels from different vascular beds.

Hypertension and microvascular remodelling

In the present review, microvascular remodelling refers to alterations in the structure of resistance vessels contributing to elevated systemic vascular resistance in hypertension. We start with some historical aspects, underscoring the importance of Folkow's contribution made half a century ago. We then move to some basic concepts on the biomechanics of blood vessels, and explicit the definitions proposed by Mulvany for specific forms of remodelling, especially inward eutrophic and inward hypertrophic. The available evidence for the existence of remodelled resistance vessels in hypertension comes next, with relatively more weight given to human, in comparison with animal data. Mechanisms are discussed. The impact of antihypertensive drug treatment on remodelling is described, again with emphasis on human data. Some details are given on the three studies to date which point to remodelling of subcutaneous resistance arteries as an independent predictor of cardiovascular risk in hypertensive patients. We terminate by considering the potential role of remodelling in the pathogenesis of end-organ damage and in the perpetuation of hypertension.

Uterine artery myosin phosphatase isoform switching and increased sensitivity to SNP in a rat L-NAME model of hypertension of pregnancy

Dramatic and vascular bed-specific hemodynamic changes occur in pregnancy and hypertension of pregnancy (HtP). Because myosin phosphatase (MP) is the primary effector of smooth muscle relaxation and a key target of signaling pathways that regulate vascular tone, we hypothesized that MP expression would be altered in these conditions. The abundance of the targeting/regulatory subunit of MP (MYPT1) mRNA and protein was increased 1.7- to 2.0-fold specifically in the uterine arteries (UAs) of late-pregnant rats without isoform switching. In a model of HtP in which nitric oxide (NO) synthesis is blocked by the chronic administration of N(omega)-nitro-L-arginine methyl ester, MYPT1 was downregulated and switched to the splice variant isoform that codes for the COOH-terminal leucine zipper motif. This was associated with increased sensitivity of the main UA and its subbranches to the vasorelaxant effects of the NO donor drug sodium nitroprusside. This difference was abolished by pretreatment with the phosphatase inhibitor tautomycetin. The sensitivity of relaxation to the NO second messenger cGMP was also increased under calcium-clamp conditions in permeabilized UAs, indicating heightened activation of MP. The changes in MP expression in HtP were largely prevented by treatment with the antihypertensive medicine hydralazine. We propose that MYPT1 isoform switching is an adaptive response to reduce vascular resistance and maintain uterine blood flow in the setting of hypertension-triggered inward remodeling of the UAs in hypertension of pregnancy.

Vascular adaptation and mechanical homeostasis at tissue, cellular, and sub-cellular levels

Blood vessels exhibit a remarkable ability to adapt throughout life that depends upon genetic programming and well-orchestrated biochemical processes. Findings over the past four decades demonstrate, however, that the mechanical environment experienced by these vessels similarly plays a critical role in governing their adaptive responses. This article briefly reviews, as illustrative examples, six cases of tissue level growth and remodeling, and then reviews general observations at cell-matrix, cellular, and sub-cellular levels, which collectively point to the existence of a "mechanical homeostasis" across multiple length and time scales that is mediated primarily by endothelial cells, vascular smooth muscle cells, and fibroblasts. In particular, responses to altered blood flow, blood pressure, and axial extension, disease processes such as cerebral aneurysms and vasospasm, and diverse experimental manipulations and clinical treatments suggest that arteries seek to maintain constant a preferred (homeostatic) mechanical state. Experiments on isolated microvessels, cell-seeded collagen gels, and adherent cells isolated in culture suggest that vascular cells and sub-cellular structures such as stress fibers and focal adhesions likewise seek to maintain constant a preferred mechanical state. Although much is known about mechanical homeostasis in the vasculature, there remains a pressing need for more quantitative data that will enable the formulation of an integrative mathematical theory that describes and eventually predicts vascular adaptations in response to diverse stimuli. Such a theory promises to deepen our understanding of vascular biology as well as to enable the design of improved clinical interventions and implantable medical devices.

Modulation of folate uptake in cultured human colon adenocarcinoma Caco-2 cells by dietary compounds

Folate is a water-soluble B vitamin with a crucial role in the synthesis and methylation of DNA and in the metabolism of several amino acids. In the present study we investigated whether beverages like wine, beer and tea, or some of their specific constituents, affect the intestinal uptake of (3)H-folic acid or (3)H-methotrexate (an antifolate). All tested beverages significantly inhibited the uptake of (3)H-folic acid by Caco-2 cells. Most of these beverages, with the exception of wines (not tested), also inhibited (3)H-methotrexate uptake in these cells. Additionally, ethanol, when tested separately, inhibited the uptake of both compounds. Some of the tested phenolic compounds, namely myricetin, epigallocatechin gallate (EGCG) and isoxanthohumol, markedly inhibited (3)H-folic acid uptake. Myricetin and EGCG also had a concentration-dependent inhibitory effect upon the uptake of (3)H-methotrexate by Caco-2 cells. Resveratrol, quercetin and kaempferol were able to inhibit the transport of both compounds, but only in the concentration of 100 microM. In conclusion, dietary constituents may impact on intestinal folate uptake, as here shown for phenolic compounds.

The influence of serum aldosterone and the aldosterone-renin ratio on pulse wave velocity in hypertensive patients

OBJECTIVE

The aldosterone-renin ratio (ARR) is widely used to screen for primary aldosteronism, and may reflect a relative excess of aldosterone secretion compared with renin secretion. Excess aldosterone may have a detrimental effect on vascular stiffness. We therefore hypothesized that ARR and aldosterone are independently correlated with pulse wave velocity (PWV) in hypertensive patients.

METHODS

The study consisted of 438 hypertensive patients. Serum aldosterone and plasma renin activity were measured in a sitting position using standard techniques. The PWV was determined by measuring the heart to femoral PWV (hfPWV) and brachial to ankle PWV with a VP-2000 pulse wave unit.

RESULTS

Group 1 was defined as patients with ARR of at least 20 (n = 53) with serum aldosterone >or= 12 ng/dl, while the remainder comprised group 2. Comparisons between the two groups reveal group 1 tended to have higher age, significantly higher proportion of women and higher systolic/diastolic blood pressure. Patients in group 1 also had a significantly higher index of central arterial stiffness (hfPWV 1048 +/- 202 vs 978 +/- 182, P = 0.010) compared with group 2. Multiple linear regression revealed that aldosterone, but not the ARR, is significantly associated with hfPWV but not brachial to ankle PWV, after controlling for age, systolic blood pressure and heart rate at the time of PWV measurement, body mass index, gender, low-density lipoprotein-cholesterol, triglyceride, high-density lipoprotein-cholesterol, blood pressure medication and statins.

CONCLUSIONS

Serum aldosterone is significantly associated with central aortic PWV in hypertensive patients. The results demonstrate a possible role for aldosterone in developing central aortic stiffness and increased PWV in hypertensive patients.

Imaging Collagen in Intact Viable Healthy and Atherosclerotic Arteries Using Fluorescently Labeled CNA35 and Two-Photon Laser Scanning Microscopy

We evaluated CNA35 as a collagen marker in healthy and atherosclerotic arteries of mice after both ex vivo and in vivo administration and as a molecular imaging agent for the detection of atherosclerosis. CNA35 conjugated with fluorescent Oregon Green 488 (CNA35/OG488) was administered ex vivo to mounted viable muscular (uterine), elastic (carotid), and atherosclerotic (carotid) arteries and fresh arterial rings. Two-photon microscopy was used for imaging. CNA35/OG488 labeling in healthy elastic arteries was compared with collagen type I, III, and IV antibody labeling in histologic sections. For in vivo labeling experiments, CNA35/OG488 was injected intravenously in C57BL6/J and apolipoprotein E−/− mice. Ex vivo CNA35/OG488 strongly labeled collagen in the tunica adventitia, media, and intima of muscular arteries. In healthy elastic arteries, tunica adventitia was strongly labeled, but labeling in tunica media and intima was prevented by endothelium and elastic laminae. Histology confirmed the affinity of CNA35 for type I, III, and IV collagen in arteries. Strong CNA35/OG488 labeling was found in atherosclerotic plaques. In vivo applied CNA35/OG488 minimally labeled the tunica intima of healthy carotid arteries. Atherosclerotic plaques in apolipoprotein E−/− mice exhibited large uptake. CNA35/OG488 imaging in organs revealed endothelium as a limiting barrier for in vivo uptake. CNA35/OG488 is a good molecular imaging agent for atherosclerosis.

Arterial adaptations to chronic changes in haemodynamic function: coupling vasomotor tone to structural remodelling

Healthy mature arteries are usually extremely quiescent tissues with cell proliferation rates much below 1%/day and with extracellular matrix constituents exhibiting half-lives of years to decades. However, chronic physiological or pathological changes in haemodynamic function elicit arterial remodelling processes that may involve substantial tissue synthesis, degradation or turnover. Although these remodelling processes accommodate changing demands placed upon the cardiovascular system by physiological adaptations, they can compromise further perfusion in the context of arterial occlusive disease and they entrench hypertension and may exacerbate its progression. Recent findings indicate that some of the most important such remodelling responses involve the integrated effects of persistently altered vascular tone that feed into restructuring responses, with common signalling pathways frequently interacting in the control of both phases of the response. Current efforts to define these signals and their targets may provide new directions for therapeutic interventions to treat important vascular disorders.

Biochemomechanics of cerebral vasospasm and its resolution: I. A new hypothesis and theoretical framework

The etiology, and hence most effective treatment, of cerebral vasospasm remains unknown, thus this devastating sequela to subarachnoid hemorrhage continues to be responsible for significant morbidity and mortality. Based on abundant and diverse clinical and laboratory observations, we hypothesize that vasospasm and its subsequent resolution result from a short-term chemo-dominated turnover of cells and matrix in evolving vasoconstricted states that produces a narrowed lumen and thicker wall, which is stiffer and largely unresponsive to exogenous vasodilators, and a subsequent mechano-dominated turnover of cells and matrix in evolving vasodilated states that restores the vessel toward normal. There is, however, a pressing need for a mathematical model of arterial growth and remodeling that can guide the design and interpretation of experiments to test this and competing hypotheses. Toward this end, we present a new biochemomechanical framework that couples a 2-D model of the evolving geometry, structure, and properties of the affected arterial wall, a 1-D model of the blood flow within the affected segment, and a 0-D model of the biochemical insult to the segment. We submit that such a framework can capture salient features of the time-course of vasospasm and its potential resolution, as illustrated numerically in part II of this paper.

Two-photon microscopy of vital murine elastic and muscular arteries. Combined structural and functional imaging with subcellular resolution

Understanding vascular pathologies requires insight in the structure and function, and, hence, an imaging technique combining subcellular resolution, large penetration depth, and optical sectioning. We evaluated the applicability of two-photon laser-scanning microscopy (TPLSM) in large elastic and small muscular arteries under physiological conditions. Elastic (carotid) and muscular (uterine, mesenteric) arteries of C57BL/6 mice were mounted in a perfusion chamber. TPLSM was used to assess the viability of arteries and to visualize the structural components elastin, collagen, nuclei, and endothelial glycocalyx (EG). Functionality was determined using diameter changes in response to noradrenaline and acetylcholine. Viability and functionality were maintained up to 4 h, enabling the assessment of structure-function relationships. Structural vessel wall components differed between elastic and muscular arteries: size (1.3 vs. 2.1 microm) and density (0.045 vs. 0.57 microm(-2)) of internal elastic lamina fenestrae, smooth muscle cell density (3.50 vs. 1.53 microm(-3)), number of elastic laminae (3 vs. 2), and adventitial collagen structure (tortuous vs. straight). EG in elastic arteries was 4.5 microm thick, covering 66% of the endothelial surface. TPLSM enables visualization and quantification of subcellular structures in vital and functional elastic and muscular murine arteries, allowing unraveling of structure-function relationships in healthy and diseased arteries.

Biaxial biomechanical adaptations of mouse carotid arteries cultured at altered axial extension

Many have studied the roles of altered blood flow and pressure on adaptive responses of blood vessels, but few have studied the role of altered axial loads. We exposed common carotid arteries from wild-type mice to low, medium, or high axial extensions while maintaining the same pressure and luminal flow rate for two days in culture, and studied adaptations in vessel geometry, in vitro loads, and stresses while collecting biaxial biomechanical (pressure-diameter and axial force-length) data on Day 0 (initial control conditions), Day 1, and Day 2. In addition, we compared vasoreactive responses to phenylephrine, carbamylcholine chloride, and sodium nitroprusside at the end of the 2-day culture period. We found significant differences in the structural (e.g., pressure-axial force and axial force-length) responses between groups as well as within each group over time. These adaptations seem to be aimed at restoring the mechanical state from a perturbed condition (e.g., low or high axial extension) toward a normal 'homeostatic' condition. Although structural responses (e.g., pressure-axial force and axial force-length) differed between groups on Day 2, the material behavior (e.g., circumferential and axial stress-stretch responses) did not differ significantly between groups.

Role of the renin–angiotensin–aldosterone system in vascular remodeling and inflammation: a clinical review

The concept of hypertension as primarily a consequence of altered hemodynamics has changed. Many factors are now implicated in the development of hypertensive vascular disease, and the renin-angiotensin-aldosterone system (RAAS) appears to be one of the most significant. Angiotensin II, the principal effector peptide of the RAAS, has far-reaching effects on vascular structure, growth and fibrosis, and is a key regulator of vascular remodeling and inflammation. Reactive oxygen species and a network of signaling pathways mediate angiotensin II and cellular mechanisms that promote remodeling and inflammation. The involvement of aldosterone in vessel-wall and myocardial remodeling has also come under intensive research scrutiny. Treatments that block the pathologic effects of the RAAS at several points have been shown to limit target-organ damage in hypertension and to decrease cardiovascular morbidity and mortality. Understanding the molecular and cellular mechanisms that participate in the early development of hypertensive vascular disease may lead to more targeted treatment and improved outcomes.

Middle cerebral artery structure and distensibility during developing and established phases of hypertension in the spontaneously hypertensive rat

OBJECTIVE

The aims of the current study were to examine the structural properties of middle cerebral arteries (MCA) from young (5-7 weeks) and adult (20-24 weeks) spontaneously hypertensive rats (SHR), compared with age-matched Wistar-Kyoto (WKY) control rats.

DESIGN

MCA segments (8-10 per group) were secured onto glass pipettes in a small vessel chamber and studied using a pressure arteriograph system. Vessels were perfused in Ca2+-free physiological salt solution to ensure the absence of tone. The wall thickness and lumen diameter were recorded at intraluminal pressures ranging from 3 to 180 mmHg using a video dimension analyser.

RESULTS

There was a borderline increase in systolic pressure of the young SHR, compared with WKY controls, but the systolic pressure of the older SHR was significantly raised. The MCA lumen diameter from young SHR was reduced across the entire pressure range and arterial distensibility was not reduced, compared with WKY vessels. The MCA lumen diameter from adult SHR was reduced at high pressure, but converged with the lumen diameter of the WKY vessels at 3 mmHg, and the stress-strain relation was shifted to the left, compared with the WKY vessels; nevertheless, the slope of the tangential elastic modulus-stress relation was not significantly increased. The pressure-wall cross-sectional area relationship did not differ between strains at either time point.

CONCLUSIONS

These data demonstrate eutrophic inward remodelling of the MCA from young SHR, compared with WKY controls. In the adult SHR the structural changes are probably a consequence of a reduced arterial distensibility.

αV Integrins Are Necessary for Eutrophic Inward Remodeling of Small Arteries in Hypertension

Human essential hypertension is characterized by eutrophic remodeling of small arteries, with little evidence of hypertrophy. Likewise, vessels of young hypertensive TGR(mRen2)27 animals have undergone similar structural alterations. The role of integrins in resistance arteries of TGR(mRen2)27 during the eutrophic-remodeling process was examined as blood pressure rose. Initially, 8 alpha and 3 beta integrins were identified and levels of expression investigated using RT-PCR. As pressure increased and remodeling advanced, integrin expression profiles revealed that only alphaV was significantly raised. In conjunction, we confirmed elevated integrin alphaV protein levels in TGR(mRen2)27 rat arteries and localization to the media using immunofluorescence. beta1 and beta3, but not beta5 integrin subunits were coprecipitated with integrin alphaV and are implicated in the eutrophic remodeling process. Administration of a peptide antagonist of alphaVbeta3 abolished remodeling but enhanced growth, indicating that hypertrophy supervened as a response to hypertension-induced increases in wall stress. We have established that the only upregulated integrin, the alphaV subunit of integrin alphaVbeta3, has a crucial role in the hypertensive remodeling process of TGR(mRen2)27 rat resistance arteries. During hypertensive remodeling, functions of specific alphaVbeta3-extracellular matrix interactions are likely to allow vascular smooth muscle cell-length autoregulation, which includes a migratory process, to maintain a narrowed lumen after a prolonged constricted state.

Myosin filament assembly in an ever-changing myofilament lattice of smooth muscle

A major development in smooth muscle research in recent years is the recognition that the myofilament lattice of the muscle is malleable. The malleability appears to stem from plastic rearrangement of contractile and cytoskeletal filaments in response to stress and strain exerted on the muscle cell, and it allows the muscle to adapt to a wide range of cell lengths and maintain optimal contractility. Although much is still poorly understood, we have begun to comprehend some of the basic mechanisms underlying the assembly and disassembly of contractile and cytoskeletal filaments in smooth muscle during the process of adaptation to large changes in cell geometry. One factor that likely facilitates the plastic length adaptation is the ability of myosin filaments to form and dissolve at the right place and the right time within the myofilament lattice. It is proposed herein that formation of myosin filaments in vivo is aided by the various filament-stabilizing proteins, such as caldesmon, and that the thick filament length is determined by the dimension of the actin filament lattice. It is still an open question as to how the dimension of the dynamic filament lattice is regulated. In light of the new perspective of malleable myofilament lattice in smooth muscle, the roles of many smooth muscle proteins could be assigned or reassigned in the context of plastic reorganization of the contractile apparatus and cytoskeleton.