Mechanical strength of the isolated carotid artery in SHR

Vascular elastic fiber heterogeneity in health and disease

PURPOSE OF REVIEW

Elastin has historically been described as an amorphous protein that functions to provide recoil to tissues that stretch. However, evidence is growing that elastin's role may not be limited to biomechanics. In this minireview, we will summarize current knowledge regarding vascular elastic fibers, focusing on structural differences along the arterial tree and how those differences may influence the behavior of affiliated cells.

RECENT FINDINGS

Regional heterogeneity, including differences in elastic lamellar number, density and cell developmental origin, plays an important role in vessel health and function. These differences impact cell-cell communication, proliferation and movement. Perturbations of normal cell-matrix interactions are correlated with human diseases including aneurysm, atherosclerosis and hypertension.

SUMMARY

Although classically described as a structural protein, recent data suggest that differences in elastin deposition along the arterial tree have important effects on heterotypic cell interactions and human disease.

Increased Vascular Contractility in Hypertension Results From Impaired Endothelial Calcium Signaling

Supplemental Digital Content is available in the text.

Endothelial cells line all blood vessels and are critical regulators of vascular tone. In hypertension, disruption of endothelial function alters the release of endothelial-derived vasoactive factors and results in increased vascular tone. Although the release of endothelial-derived vasodilators occurs in a Ca²⁺-dependent manner, little is known on how Ca²⁺ signaling is altered in hypertension. A key element to endothelial control of vascular tone is Ca²⁺ signals at specialized regions (myoendothelial projections) that connect endothelial cells and smooth muscle cells. This work describes disruption in the operation of this key Ca²⁺ signaling pathway in hypertension. We show that vascular reactivity to phenylephrine is increased in hypertensive (spontaneously hypertensive rat) when compared with normotensive (Wistar Kyoto) rats. Basal endothelial Ca²⁺ activity limits vascular contraction, but that Ca²⁺-dependent control is impaired in hypertension. When changes in endothelial Ca²⁺ levels are buffered, vascular contraction to phenylephrine increased, resulting in similar responses in normotension and hypertension. Local endothelial IP₃(inositol trisphosphate)-mediated Ca²⁺ signals are smaller in amplitude, shorter in duration, occur less frequently, and arise from fewer sites in hypertension. Spatial control of endothelial Ca²⁺ signaling is also disrupted in hypertension: local Ca²⁺ signals occur further from myoendothelial projections in hypertension. The results demonstrate that the organization of local Ca²⁺ signaling circuits occurring at myoendothelial projections is disrupted in hypertension, giving rise to increased contractile responses.

Fracture mechanics of shear crack propagation and dissection in the healthy bovine descending aortic media

This experimental study adopts a fracture mechanics strategy to investigate the mechanical cause of aortic dissection. Inflation of excised healthy bovine aortic rings with a cut longitudinal notch that extends into the media from the intima suggests that an intimal tear may propagate a nearly circumferential-longitudinal rupture surface that is similar to the delamination that occurs in aortic dissection. Radial and 45°-from-radial cut notch orientations, as seen in the thickness surface, produce similar circumferential crack propagation morphologies. Partial cut notches, whose longitudinal length is half the width of the ring, measure the influence of longitudinal material on crack propagation. Such specimens also produce circumferential cracks from the notch root that are visible in the thickness circumferential-radial plane, and often propagate a secondary crack from the base of the notch, visible in the intimal circumferential-longitudinal plane. Inflation of rings with pairs of cut notches demonstrates that a second notch modifies the propagation created in a specimen with a single notch. The circumferential crack propagation is likely a consequence of the laminar medial structure. These fracture surfaces are probably due to non-uniform circumferential shear deformation in the heterogeneous media as the aortic wall expands. The qualitative deformation morphology around the root of the cut notch during inflation is evidence for such shear deformation. The shear apparently results from relative slip in the circumferential direction of collagen fibers. The slip may produce shear in the longitudinal-circumferential plane between medial layers or in the radial-circumferential plane within a medial lamina in an idealized model. Circumferential crack propagation in the media is then a shear mechanical process that might be facilitated by disease of the tissue.

STATEMENT OF SIGNIFICANCE

An intimal tear of an apparently healthy aortic wall near the aortic arch is life-threatening because it may lead to full rupture or to wall dissection in which delamination of the medial layer extends around most of the aortic circumference. The mechanical events underlying dissection are not definitively established. This experimental fracture mechanics study provides evidence that shear rupture is the main mechanical process underlying aortic dissection. The commonly performed tensile strength tests of aortic tissue are not clinically useful to predict or describe aortic dissection. One implication of the study is that shear tests might produce more fruitful simple assessments of the aortic wall strength. A clinical implication is that when presented with an intimal tear, those who guide care might recommend steps to reduce the shear load on the aorta.

Primary Blast Brain Injury Mechanisms: Current Knowledge, Limitations, and Future Directions

Mild blast traumatic brain injury (bTBI) accounts for the majority of brain injury in United States service members and other military personnel worldwide. The mechanisms of primary blast brain injury continue to be disputed with little evidence to support one or a combination of theories. The main hypotheses addressed in this review are blast wave transmission through the skull orifices, direct cranial transmission, skull flexure dynamics, thoracic surge, acceleration, and cavitation. Each possible mechanism is discussed using available literature with the goal of focusing research efforts to address the limitations and challenges that exist in blast injury research. Multiple mechanisms may contribute to the pathology of bTBI and could be dependent on magnitudes and orientation to blast exposure. Further focused biomechanical investigation with cadaver, in vivo, and finite element models would advance our knowledge of bTBI mechanisms. In addition, this understanding could guide future research and contribute to the greater goal of developing relevant injury criteria and mandates to protect our soldiers on the battlefield.

Crack Propagation and Its Shear Mechanisms in the Bovine Descending Aorta

Aortic dissection and rupture may involve circumferential shear stress in the circumferential-longitudinal plane. Inflation of bovine descending aortic ring specimens provides evidence of such shear from the non-uniform circumferential distortion of radial lines drawn on the circumferential-radial ring face. Delamination without tensile peeling induces cracks that propagate nearly circumferentially in the circumferential-longitudinal plane from the root of a radial cut representing rupture initiation in a ring. Translational shear deformation tests of small rectangular aortic wall blocks in the circumferential and longitudinal direction measure the consequences of such shear on substructures in the aortic wall, in particular the collagen fibers. The two directions of shear deformation produce no statistical difference in the shear stress response of the wall. Possibly, the interfiber connections between collagen fibers are put into tension by either translational shear deformation so that the stress measured reflects the tensile response of these connections. Wall rupture may involve failure of these connections; such failure is supported by the voids parallel to the collagen fibers observed in a histological study after translational shear. Further, interstitial fluid is redistributed by shear as evidenced by the measured weight loss of a set of specimens during the translational shear of blocks. Because the mass changes, mathematical modeling of aortic tissue in vitro as incompressible is an approximation. These observations suggest that no simple modification of classical rupture theories, whether based on energy functions, stress or strain, suffices to predict the rupture of hydrated soft biological tissue that has complex substructures.

Multilayer material properties of aorta determined from nanoindentation tests

In a wide range of biomechanical modeling of aorta from traumatic injury to stent grafts, the arterial wall has been considered as a single homogeneous layer vessel, ignoring the fact that arteries are composed of distinct anatomical layers with different mechanical characteristics. In this study, using a custom-made nanoindentation technique, changes in the mechanical properties of porcine thoracic aorta wall in the radial direction were characterized using a quasi-linear viscoelastic model. Two layers of equal thickness were mechanically distinguishable in descending aorta based on the radial variations in the instantaneous Young's modulus E and reduced relaxation function G(t). Overall, comparison of E and G(∞) of the outer half (70.27±2.47 kPa and 0.35±0.01) versus the inner half (60.32±1.65 kPa and 0.33±0.01) revealed that the outer half was stiffer and showed less relaxation. The results were used to explain local mechanisms of deformation, force transmission, tear propagation and failure in arteries.

Different biomechanical properties of medial and adventitial layers of thoracic aorta in Wistar-Kyoto and spontaneously hypertensive rats

AIM

To evaluate the biomechanical properties of thoracic aorta with or without adventitia, and to determine whether there are corresponding changes with hypertension.

METHODS

Normotensive Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR) at the age of 16 and 32 weeks were used. Thoracic aortic adventitial layer was mechanically separated from thoracic aorta and the adventitia-denuded artery ring was viewed as thoracic media. A load-strain curve was obtained by stretching the ring-shaped intact thoracic aorta or thoracic media with a tensile testing machine. Then, the slope of the load-stain curve at 30%-40% strains was viewed as the elastic stiffness at physiological load, whereas the slope near the breaking point was calculated as maximum stiffness. The maximum load is the load at the breaking point.

RESULTS

There was no significant difference in elastic stiffness and maximum stiffness of intact thoracic aorta between SHR and age-matched WKY. The elastic stiffness of intact thoracic aorta showed no significant difference from that of thoracic media in WKY and SHR at both ages. In contrast, both maximum stiffness and maximum load were reduced in thoracic media compared with intact thoracic aorta in SHR and WKY at both ages.

CONCLUSION

These results indicated that vascular adventitia contributes to maximum stiffness, but not elastic stiffness in both SHR and WKY.

Alterations in mechanical properties of mesenteric resistance arteries in experimental portal hypertension

Splanchnic vasodilation is the pathophysiological hallmark in the development of the hyperdynamic circulatory syndrome in liver cirrhosis and portal hypertension. This has been attributed so far mainly to a marked vascular hyporeactivity to endogenous vasoconstrictors. However, myogenic tone and vessel stiffness have not been addressed in mesenteric arteries in liver cirrhosis. CCl(4)(-)-induced ascitic cirrhotic (LC) and age-matched control rats, portal vein-ligated (PVL) rats, and sham-operated rats were investigated. Third-order mesenteric resistance arteries were studied under no-flow conditions using a pressure myograph measuring media thickness and lumen diameter in response to incremental increases in intramural pressure, from which wall mechanics were calculated. Electron microscopy was used for investigation of wall ultrastructure, especially the fenestrae in internal elastic lamina (IEL). In PVL animals, no significant change in passive vessel strain, stress, media-to-lumen ratio, or cross-sectional area was noted. In contrast, in LC rats, vessel strain was markedly elevated compared with healthy control rats, indicating a marked reduction in vessel stiffness. In addition, the strain-stress curve was shifted to the right, and the elastic modulus in dependency on vessel stress decreased, demonstrating predominantly structure-dependent factors to be involved. The media-to-lumen quotient was not significantly altered, but cross-sectional area was highly increased in LC rats, indicating hypertrophic outward remodeling. These findings were paralleled by enlarged fenestrae in the IEL but no change in thickness of IEL or proportion of extracellular matrix or vascular smooth muscle in LC rats. We concluded that, in long-standing severe portal hypertension such as ascitic LC but not in short-term conditions such as PVL, mesenteric resistance arteries exhibit vascular remodeling and markedly less resistant mechanical properties, leading to decreased vessel stiffness accompanied by structural changes in the IEL. This may well contribute to the maintenance and severity of splanchnic arterial vasodilation in LC.

Development of a reinforced porcine elastin composite vascular scaffold

Elastin, a principal structural component of native arteries, has distinct biological and mechanical advantages when used as a biomaterial; however, its low ultimate tensile strength has limited its use as an arterial conduit. We have developed a scaffold, consisting of a purified elastin tubular conduit strengthened with fibrin bonded layers of acellular small intestinal submucosa (aSIS) for potential use as a small diameter vascular graft. The addition of aSIS increased the ultimate tensile strength of the elastin conduits nine-fold. Burst pressures for the elastin composite vascular scaffold (1,396 +/- 309 mmHg) were significantly higher than pure elastin conduits (162 +/- 36 mmHg) and comparable to native saphenous veins. The average suture pullout strength of the elastin composite vascular scaffolds was 14.612 +/- 3.677 N, significantly higher than the pure elastin conduit (0.402 +/- 0.098 N), but comparable to native porcine carotid arteries (13.994 +/- 4.344 N). Cyclic circumferential strain testing indicated that the composite scaffolds were capable of withstanding physiological loading conditions for at least 83 h. Implantation of the elastin composites as carotid interposition grafts in swine demonstrated its superiority to clinically acceptable ePTFE with significantly longer average patency times of 5.23 h compared to 4.15 h. We have developed a biologically based elastin scaffold with suitable mechanical properties and low thrombogenicity for in vivo implantation, and with the potential for cellular repopulation and host integration reestablishing an appropriate elastic artery.

Role of alpha1beta1-integrin in arterial stiffness and angiotensin-induced arterial wall hypertrophy in mice

We examined the arterial phenotype of mice lacking alpha(1)-integrin (alpha(1)(-/-)) at baseline and after 4 wk of ANG II or norepinephrine (NE) administration. Arterial mechanical properties were determined in the carotid artery (CA). Integrin expression, MAPK kinases, and focal adhesion kinase (FAK) were assessed in the aorta. No change in arterial pressure was observed in alpha(1)(-/-) mice. Elastic modulus-wall stress curves were similar in alpha(1)(-/-) and alpha(1)(+/+) animals, indicating no change in arterial stiffness. The rupture pressure was lower in alpha(1)(-/-) mice, demonstrating decreased mechanical strength. Lack of alpha(1)-integrin was accompanied by an increase in beta(1)-, alpha(v)-, and alpha(5)-integrins but no change in alpha(2)-integrin. ANG II increased medial cross-sectional area of the CA in alpha(1)(+/+), but not alpha(1)(-/-), mice, whereas equivalent pressor doses of NE did not produce a significant increase in either group. In alpha(1)(+/+) mice, ANG II induced alpha(1)-integrin expression and smooth muscle cell (SMC) hypertrophy in the CA in association with increased aortic expression of alpha-smooth muscle actin and smooth muscle myosin heavy chain and phosphorylation of ERK1/2, p38 MAPK, and FAK. ANG II did not induce SMC hypertrophy or phosphorylation of p38 MAPK and FAK in alpha(1)(-/-) mice. A functional anti-alpha(1)-integrin antibody inhibited in vitro the ANG II-induced phosphorylation of FAK and p38 MAPK. In conclusion, alpha(1)(-/-) mice exhibit a reduced mechanical strength at baseline and a lack of ANG II-induced SMC hypertrophy. These results emphasize the importance of alpha(1)beta(1)-integrin in p38 MAPK and FAK phosphorylation during vascular hypertrophy in response to ANG II.

Mechanics of Fresh, Refrigerated, and Frozen Arterial Tissue

Arterial grafts and experimental soft tissues are commonly preserved using refrigeration and freezing. The present study was designed to investigate effects of common storage protocols on arterial mechanics. Porcine aortas were axially distracted to failure implementing fresh, refrigerated, and frozen storage conditions. Fresh tissues were tested within 24 h of sacrifice; refrigerated tissues were stored at +4 degrees C for 24 or 48 h prior to testing, and frozen tissues were stored at -20 or -80 degrees C for 3 months prior to testing. Blunt arterial injury experimentally occurred in distraction with intimal subfailure before ultimate failure in 82% of specimens. Subfailure stress decreased in refrigerated (0.59 +/- 0.19 MPa) compared to fresh (0.83 +/- 0.39 MPa) and frozen (0.99 +/- 0.41 MPa) specimens. Ultimate stress was also significantly decreased in refrigerated (0.83 +/- 0.19 MPa) compared to fresh (1.15 +/- 0.39 MPa) and frozen (1.32 +/- 0.31 MPa) specimens. Subfailure and ultimate strain were not significantly dependent upon storage technique. Young's modulus significantly decreased in refrigerated (1.89 +/- 0.63 MPa) compared to fresh (2.98 +/- 1.45 MPa) and frozen (3.49 +/- 1.32 MPa) specimens. Physiological, subfailure, and ultimate failure mechanics between fresh and frozen specimens were not significantly different. Clinically relevant intimal failures can be reproduced and injury mechanics determined while adhering to experimental protocols of freezing specimens before testing. However, short-term tissue refrigeration may affect mechanics.

Biomechanical Characterization of Internal Layer Subfailure in Blunt Arterial Injury

Blunt carotid artery injuries occur in 0.3% of blunt injured patients and may lead to devastating neurological consequences. However, arterial mechanics leading to internal layer subfailure have not been quantified. Twenty-two human carotid artery segments and 18 porcine thoracic aorta segments were opened to expose the intimal side and longitudinally distracted to failure. Porcine aortas were a geometrically accurate model of human carotid arteries. Internal layer subfailures were identified using videography and correlated with mechanical data. Ninety-three percent (93%) of vessels demonstrated subfailure prior to catastrophic failure. All subfailures occurred on the intimal surface. Initial subfailure occurred at 79% of the stress and 85% of the strain to catastrophic failure in younger porcine specimens, compared to 44% and 60%, respectively, in older human specimens. In most cases, multiple subfailures occurred prior to catastrophic failure. Due to limitations in human specimen quality (age, prior storage), young and fresh porcine aorta specimens are likely a more accurate model of clinical blunt carotid artery injuries. Present results indicate that vessels are acutely capable of maintaining physiologic function following initial subfailure. Delayed symptomatology commonly associated with blunt arterial injuries is explained by this mechanics-based and experimentally quantified onset of subcatastrophic failure.

Mechanics of arterial subfailure with increasing loading rate

Arterial subfailure leads to delayed symptomatology and high morbidity and mortality rates, particularly for the thoracic aorta and carotid arteries. Although arterial injuries occur during high-velocity automotive collisions, previous studies of arterial subfailure focused on quasi-static loading. This investigation subjected aortic segments to increasing loading rates to quantify effects on elastic, subfailure, and ultimate vessel mechanics. Sixty-two specimens were axially distracted, and 92% demonstrated subfailure before ultimate failure. With increasing loading rate, stress at initial subfailure and ultimate failure significantly increased, and strain at initial subfailure and ultimate failure significantly decreased. Present results indicate increased susceptibility for arterial subfailure and/or dissection under higher-rate extension. According to the present results, automotive occupants are at greater risk of arterial injury under higher velocity impacts due to greater body segment motions in addition to decreased strain tolerance to subfailure and catastrophic failure.

Postnatal alterations in elastic fiber organization precede resistance artery narrowing in SHR

Resistance artery narrowing and stiffening are key elements in the pathogenesis of essential hypertension, but their origin is not completely understood. In mesenteric resistance arteries (MRA) from spontaneously hypertensive rats (SHR), we have shown that inward remodeling is associated with abnormal elastic fiber organization, leading to smaller fenestrae in the internal elastic lamina. Our current aim is to determine whether this alteration is an early event that precedes vessel narrowing, or if elastic fiber reorganization in SHR arteries occurs because of the remodeling process itself. Using MRA from 10-day-old, 30-day-old, and 6-mo-old SHR and normotensive Wistar Kyoto rats, we investigated the time course of the development of structural and mechanical alterations (pressure myography), elastic fiber organization (confocal microscopy), and amount of elastin (radioimmunoassay for desmosine) and collagen (picrosirius red). SHR MRA had an impairment of fenestrae enlargement during the first month of life. In 30-day-old SHR, smaller fenestrae and more packed elastic fibers in the internal elastic lamina were paralleled by increased wall stiffness. Collagen and elastin levels were unaltered at this age. MRA from 6-mo-old SHR also had smaller fenestrae and a denser network of adventitial elastic fibers, accompanied by increased collagen content and vessel narrowing. At this age, elastase digestion was less effective in SHR MRA, suggesting a lower susceptibility of elastic fibers to enzymatic degradation. These data suggest that abnormal elastic fiber deposition in SHR increases resistance artery stiffness at an early age, which might participate in vessel narrowing later in life.

Elastic fibres and vascular structure in hypertension

Blood vessels are dynamic structures composed of cells and extracellular matrix (ECM), which are in continuous cross-talk with each other. Thus, cellular changes in phenotype or in proliferation/death rate affect ECM synthesis. In turn, ECM elements not only provide the structural framework for vascular cells, but they also modulate cellular function through specific receptors. These ECM-cell interactions, together with neurotransmitters, hormones and the mechanical forces imposed by the heart, modulate the structural organization of the vascular wall. It is not surprising that pathological states related to alterations in the nervous, humoral or haemodynamic environment-such as hypertension-are associated with vascular wall remodeling, which, in the end, is deleterious for cardiovascular function. However, the question remains whether these structural alterations are simply a consequence of the disease or if there are early cellular or ECM alterations-determined either genetically or by environmental factors-that can predispose to vascular remodeling independent of hypertension. Elastic fibres might be key elements in the pathophysiology of hypertensive vascular remodeling. In addition to the well known effects of hypertension on elastic fibre fatigue and accelerated degradation, leading to loss of arterial wall resilience, recent investigations have highlighted new roles for individual components of elastic fibres and their degradation products. These elements can act as signal transducers and regulate cellular proliferation, migration, phenotype, and ECM degradation. In this paper, we review current knowledge regarding components of elastic fibres and discuss their possible pathomechanistic associations with vascular structural abnormalities and with hypertension development or progression.

Methodology to study intimal failure mechanics in human internal carotid arteries

While the incidence of blunt carotid artery injuries is low, the mortality rate is extremely high (40%). Clinical evidence indicates that the intimal region of the artery often sustains failure, while maintaining the integrity of the outer layers. This condition may lead to delayed ischemic symptoms, commonly reported in clinical literature. To date, the mechanical properties of the intima relative to the outer vessel layers have not been quantified in the human carotid artery. The purpose of the present study was to develop a methodology to determine the longitudinal mechanical properties of the human internal carotid artery in tension, with an emphasis on intimal failure. This was accomplished by opening the vessel at the mid-diameter level, creating an 'I'-shaped testing specimen, subjecting the specimen to failure loading, documenting the stretch characteristics of the intimal and adventitial sides in the temporal domain, and correlating the synchronized videography with mechanical loading. Intimal failure data were quantified using stress and strain parameters in conjunction with digital videography of the intimal and adventitial sides. The present methodology can be used to determine the mechanical properties of the intima relative to ultimate carotid artery failure. These data will assist in the understanding of blunt carotid artery injuries, its diagnosis and treatment.

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

BACKGROUND

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

DESCRIPTION AND RESULTS

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

CONCLUSION

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

Incorporation of intact elastin scaffolds in tissue-engineered collagen-based vascular grafts

Although collagen-based tissue-engineered blood vessels (TEBVs) have many interesting properties and have been utilized to study aspects of vascular biology, these constructs are too weak to be implanted as bypass grafts for in vivo investigations. This study presents a method to incorporate organized, intact elastin into collagen-based TEBVs to form hybrid constructs that better mimic arterial physiology and exhibit improved mechanical properties. Porcine carotids were digested with a series of autoclave and chemical treatments to elicit isolated elastin scaffolds. Elastin purity was verified via immunohistochemistry and amino acid analysis. Isolated scaffolds were combined with type I collagen and either human dermal fibroblasts (HDFs) or rat smooth muscle cells (RASMs) to form an elastin hybrid TEBV. Hybrid constructs exhibited increased tensile strengths (11-fold in HDFs; 7.5-fold in RASMs) and linear stiffness moduli (4-fold in HDFs; 1.8-fold in RASMs) compared with collagen control constructs with no exogenous elastin scaffold. Viscoelastic properties of the TEBVs also improved with the addition of an ancillary elastin scaffold as determined through stepwise stress relaxation analysis. Whereas the majority of resistance to deformation in collagen control constructs stemmed from viscous fluidlike effects, elastin hybrid constructs exhibited more ideal elastic solid mechanical behavior. Thus, elastin scaffolds can help recreate the elastic properties of native arteries. Future challenges include stimulating appropriate reorganization or synthesis of the collagen matrix to provide the necessary strength and viscoelastic properties for implantation.

Arterial stiffness and stroke in hypertension: therapeutic implications for stroke prevention

Stroke is the second leading cause of mortality worldwide, and the leading cause of death in China and Japan. Its prevention represents a major goal. Identification of primary stroke risk, particularly through newly individualised risk factors including biomarkers of large artery damage such as arterial stiffening, is necessary for determining the appropriate level of intervention. The purpose of this review is to focus on the pathophysiology of arterial stiffness, its predictive value for stroke and the therapeutic implications of this risk factor for stroke prevention. The predictive value of arterial stiffness for stroke was demonstrated in a longitudinal study that included 1715 patients with essential hypertension and measurements of carotid-femoral pulse wave velocity (PWV) [an indicator of arterial stiffness] at entry. Over a mean follow-up period of 7.9 years, during which 25 fatal strokes occurred, PWV significantly predicted stroke (relative risk = 1.39 [(95% CI 1.08, 1.72]; p = 0.02 for each 4 m/sec increase) independently of classical cardiovascular risk factors, including age, cholesterol level, diabetes mellitus, smoking and mean blood pressure. Additional longitudinal studies are needed to confirm the predictive value of aortic stiffness on primary and secondary events, in low- and high-risk populations, in various countries, and using different methodologies of arterial stiffness measurement. Drug treatment could prevent stroke through a reduction in arterial stiffness in parallel with correction of cardiovascular risk factors such as hypertension, dyslipidaemia, diabetes mellitus and smoking, all of which are associated with arterial stiffening. In view of the important local actions of angiotensin II on arterial stiffening, drugs interfering with the renin-angiotensin-aldosterone system should be particularly effective. Promising therapeutic strategies to reduce arterial stiffness include taking advantage of the non-lipid-lowering effects of statins and directly targeting the molecular events leading to arterial stiffening, such as formation of advanced glycation end products.

Role of elastin in spontaneously hypertensive rat small mesenteric artery remodelling

Chronic hypertension is associated with resistance artery remodelling and mechanical alterations. However, the contribution of elastin has not been thoroughly studied. Our objective was to evaluate the role of elastin in vascular remodelling of mesenteric resistance arteries (MRA) from spontaneously hypertensive rats (SHR). MRA segments from Wistar Kyoto rats (WKY) and SHR were pressurised under passive conditions at a range of physiological pressures with pressure myography. Confocal microscopy was used to determine differences in the quantity and organisation of elastin in intact pressure-fixed arteries. To assess the contribution of elastin to MRA structure and mechanics, myograph-mounted vessels were studied before and after elastase incubation. When compared with WKY, MRA from SHR showed: (1) a smaller lumen, (2) decreased distensibility at low pressures, (3) a leftward shift of the stress-strain relationship, (4) redistribution of elastin within the internal elastic lamina (IEL) leading to smaller fenestrae but no change in fenestrae number or elastin amount. Elastase incubation (1) fragmented the structure of IEL in a concentration-dependent fashion, (2) abolished all the structural and mechanical differences between strains, and (3) decreased distensibility at low pressures. The study shows the overriding role of elastin in determining vascular dimensions and mechanical properties in a resistance artery. In addition, it informs hypertensive remodelling. MRA remodelling and increased stiffness are accompanied by elastin restructuring within the IEL and elastin degradation reverses structural and mechanical alterations of SHR MRA. Differences in elastin organisation are, therefore, a central element in small artery remodelling in hypertension.

A biological hybrid model for collagen-based tissue engineered vascular constructs

Various approaches to tissue engineering a small diameter blood vessel have historically relied upon extended culturing periods and/or synthetic materials to create mechanical properties suitable to withstand the hemodynamic stresses of the vasculature. In this work, we present the concept of a construct-sleeve hybrid (CSH) graft, which uses a biological support to provide temporary reinforcement while cell-mediated remodeling of the construct occurs. Support sleeves were fabricated from Type I collagen gels and crosslinked with glutaraldehyde, ultraviolet, or dehydrothermal treatments. Uniaxial tensile testing of acellular sleeves revealed increased stiffness moduli and tensile stresses with crosslinking treatments. A second collagen layer containing cells was molded about the sleeve to create a CSH. After in vitro culture, CHSs with uncrosslinked (UnXL) and glutaraldehyde treated (Glut) sleeves exhibited significant increases in mechanical strength (20.4-fold and 121-fold increases in ultimate stress, respectively) compared to unreinforced control constructs. Burst testing produced similar findings with peak pressures of 100 and 650mmHg in the UnXL and Glut CSHs, respectively. Construct compaction, cell viability, and histological examination demonstrated that the function of most cells remained unimpaired with the incorporation of the biological support sleeve.