Elastin and collagen accumulation in rabbit ascending aorta and pulmonary trunk during postnatal growth. Correlation of cellular synthetic response with medial tension

Multiscale insights into postnatal aortic development

Despite its vital importance for establishing proper cardiovascular function, the process through which the vasculature develops and matures postnatally remains poorly understood. From a clinical perspective, an ability to mechanistically model the developmental time course in arteries and veins, as well as to predict how various pathologies and therapeutic interventions alter the affected vessels, promises to improve treatment strategies and long-term clinical outcomes, particularly in pediatric patients suffering from congenital heart defects. In the present study, we conducted a multiscale investigation into the postnatal development of the murine thoracic aorta, examining key allometric relations as well as relationships between in vivo mechanical stresses, collagen and elastin expression, and the gradual accumulation of load-bearing constituents within the aortic wall. Our findings suggest that the production of fibrillar collagens in the developing aorta associates strongly with the ratio of circumferential stresses between systole and diastole, hence emphasizing the importance of a pulsatile mechanobiological stimulus. Moreover, rates of collagen turnover and elastic fiber compaction can be inferred directly by synthesizing transcriptional data and quantitative histological measurements of evolving collagen and elastin content. Consistent with previous studies, we also observed that wall shear stresses acting on the aorta are similar at birth and in maturity, supporting the hypothesis that at least some stress targets are established early in development and maintained thereafter, thus providing a possible homeostatic basis to guide future experiments and inform future predictive modeling.

Ophthalmic characteristics and retinal vasculature changes in Williams syndrome, and its association with systemic diseases

BACKGROUND

We aim to characterise the ophthalmic findings and retinal vasculature changes in patients with WS, and to analyse the correlation between ophthalmic manifestations and the associated systemic diseases.

METHODS

This retrospective case-control study included 27 WS patients and 28 age-matched healthy participants. Stellate pattern of iris, central macular thickness (CMT), foveal width, retinal vessel diameter, superficial vascular density (SVD) of macula and foveal avascular zone (FAZ) were compared between WS patients and healthy participants.

RESULTS

Twenty-five patients (93%) had the classic stellate iris presentation. Compared with healthy controls, WS patients had decreased CMT, increased foveal width and a lower SVD of macula (all P < 0.001). Significantly decreased mean retinal arterial (117.9 ± 9.9 µm vs. 133.0 ± 6.7 µm in WS and controls, respectively; p < 0.001) and venous (158.9 ± 11.2 µm vs. 174.0 ± 8.0 µm in WS and controls, respectively; p < 0.001) outer diameters, as well as mean arterial wall thickness (11.2 ± 1.3 µm vs. 12.2 ± 0.8 µm in WS and controls, respectively; p < 0.01) were found in WS. Stellate iris grading was significantly associated with CMT, foveal width, retinal vessel diameter (all p < 0.05), and a significant increase in the odds of having hypertension (Odds ratio (OR), 5.63; P < 0.05). The severity of stellate iris in WS seemed to have the trend of increasing risk of having pulmonary stenosis, tricuspid regurgitation and mitral regurgitation.

CONCLUSIONS

This study provides the first in vivo evidence reflecting current knowledge on vessel morphology in WS patients that deficient circumferential growth is the predominant pathophysiologic changes resulting from elastin deficiency. The ophthalmic characteristics may serve as a complementary tool to diagnose and follow-up patients suffering from WS.

Physiological Impact of a Synthetic Elastic Protein in Arterial Diseases Related to Alterations of Elastic Fibers: Effect on the Aorta of Elastin-Haploinsufficient Male and Female Mice

Elastic fibers, made of elastin (90%) and fibrillin-rich microfibrils (10%), are the key extracellular components, which endow the arteries with elasticity. The alteration of elastic fibers leads to cardiovascular dysfunctions, as observed in elastin haploinsufficiency in mice (Eln+/-) or humans (supravalvular aortic stenosis or Williams-Beuren syndrome). In Eln+/+ and Eln+/- mice, we evaluated (arteriography, histology, qPCR, Western blots and cell cultures) the beneficial impact of treatment with a synthetic elastic protein (SEP), mimicking several domains of tropoelastin, the precursor of elastin, including hydrophobic elasticity-related domains and binding sites for elastin receptors. In the aorta or cultured aortic smooth muscle cells from these animals, SEP treatment induced a synthesis of elastin and fibrillin-1, a thickening of the aortic elastic lamellae, a decrease in wall stiffness and/or a strong trend toward a reduction in the elastic lamella disruptions in Eln+/- mice. SEP also modified collagen conformation and transcript expressions, enhanced the aorta constrictive response to phenylephrine in several animal groups, and, in female Eln+/- mice, it restored the normal vasodilatory response to acetylcholine. SEP should now be considered as a biomimetic molecule with an interesting potential for future treatments of elastin-deficient patients with altered arterial structure/function.

The mechanical and morphological properties of systemic and pulmonary arteries differ in the earth boa, a snake without ventricular pressure separation

The walls of the mammalian aorta and pulmonary artery are characterized by diverging morphologies and mechanical properties, which has been correlated with high systemic and low pulmonary blood pressures, as a result of intraventricular pressure separation. However, the relation between intraventricular pressure separation and diverging aortic and pulmonary artery wall morphologies and mechanical characteristics is not understood. The snake cardiovascular system poses a unique model for the study of this question, since representatives both with and without intraventricular pressure separation exist. In this study we perform uniaxial tensile testing on vessel samples taken from the aortas and pulmonary arteries of the earth boa, Acrantophis madagascariensis, a species without intraventricular pressure separation. We then compare these morphological and mechanical characteristics with samples from the ball python, Python regius, and the yellow anaconda, Eunectes notaeus, species with and without intraventricular pressure separation, respectively. Our data suggest that although the aortas and pulmonary arteries of A. madagascariensis respond similarly to the same intramural blood pressures, they diverge in morphology, and that this attribute extends to E. notaeus. In contrast, P. regius aortas and pulmonary arteries diverge both morphologically and in terms of their mechanical properties. Our data indicate that intraventricular pressure separation cannot fully explain diverging aortic and pulmonary artery morphologies. Following the Law of Laplace, we propose that pulmonary arteries of small luminal diameter represent a mechanism to protect the fragile pulmonary vasculature by reducing the blood volume that passes through, to which genetic factors may contribute more strongly than physiological parameters.

Cardiovascular Prevention Among Young Adults with Congenital Heart Disease

PURPOSE OF REVIEW

There are over a million adults living with congenital heart disease (CHD) in the USA. There have been improvements in CHD management which have led to an expansion of the adult congenital heart disease (ACHD) population. There is a high prevalence of atherosclerotic cardiovascular disease (ASCVD) encountered in the aging ACHD population. This review focuses on the most recent literature regarding the primary prevention of ASCVD in young ACHD patients.

RECENT FINDINGS

There are unique considerations for ASCVD risk reduction in ACHD patients. ASCVD may be as prevalent in ACHD compared in the general population. However, there may be a perceived shorter life expectancy in ACHD patients; therefore, primary prevention of ASCVD may not be considered important. Preventative strategies for ASCVD are underutilized in ACHD patients. As these patients are followed for a lifetime by cardiologists, we can truly pursue primary prevention in this aging population.

Understanding Pulmonary Autograft Remodeling After the Ross Procedure: Stick to the Facts

The Ross, or pulmonary autograft, procedure presents a fascinating mechanobiological scenario. Due to the common embryological origin of the aortic and pulmonary root, the conotruncus, several authors have hypothesized that a pulmonary autograft has the innate potential to remodel into an aortic phenotype once exposed to systemic conditions. Most of our understanding of pulmonary autograft mechanobiology stems from the remodeling observed in the arterial wall, rather than the valve, simply because there have been many opportunities to study the walls of dilated autografts explanted at reoperation. While previous histological studies provided important clues on autograft adaptation, a comprehensive understanding of its determinants and underlying mechanisms is needed so that the Ross procedure can become a widely accepted aortic valve substitute in select patients. It is clear that protecting the autograft during the early adaptation phase is crucial to avoid initiating a sequence of pathological remodeling. External support in the freestanding Ross procedure should aim to prevent dilatation while simultaneously promoting remodeling, rather than preventing dilatation at the cost of vascular atrophy. To define the optimal mechanical properties and geometry for external support, the ideal conditions for autograft remodeling and the timeline of mechanical adaptation must be determined. We aimed to rigorously review pulmonary autograft remodeling after the Ross procedure. Starting from the developmental, microstructural and biomechanical differences between the pulmonary artery and aorta, we review autograft mechanobiology in relation to distinct clinical failure mechanisms while aiming to identify unmet clinical needs, gaps in current knowledge and areas for further research. By correlating clinical and experimental observations of autograft remodeling with established principles in cardiovascular mechanobiology, we aim to present an up-to-date overview of all factors involved in extracellular matrix remodeling, their interactions and potential underlying molecular mechanisms.

Williams syndrome

Williams syndrome (WS) is a relatively rare microdeletion disorder that occurs in as many as 1:7,500 individuals. WS arises due to the mispairing of low-copy DNA repetitive elements at meiosis. The deletion size is similar across most individuals with WS and leads to the loss of one copy of 25-27 genes on chromosome 7q11.23. The resulting unique disorder affects multiple systems, with cardinal features including but not limited to cardiovascular disease (characteristically stenosis of the great arteries and most notably supravalvar aortic stenosis), a distinctive craniofacial appearance, and a specific cognitive and behavioural profile that includes intellectual disability and hypersociability. Genotype-phenotype evidence is strongest for ELN, the gene encoding elastin, which is responsible for the vascular and connective tissue features of WS, and for the transcription factor genes GTF2I and GTF2IRD1, which are known to affect intellectual ability, social functioning and anxiety. Mounting evidence also ascribes phenotypic consequences to the deletion of BAZ1B, LIMK1, STX1A and MLXIPL, but more work is needed to understand the mechanism by which these deletions contribute to clinical outcomes. The age of diagnosis has fallen in regions of the world where technological advances, such as chromosomal microarray, enable clinicians to make the diagnosis of WS without formally suspecting it, allowing earlier intervention by medical and developmental specialists. Phenotypic variability is considerable for all cardinal features of WS but the specific sources of this variability remain unknown. Further investigation to identify the factors responsible for these differences may lead to mechanism-based rather than symptom-based therapies and should therefore be a high research priority.

Impact of Modified Anesthesia Management for Pediatric Patients With Williams Syndrome

OBJECTIVE

This study compared the percent change in systolic blood pressure and the incidence of adverse cardiac events (ACEs; defined as cardiac arrest, cardiopulmonary resuscitation, arrhythmias, or ST-segment changes) during anesthesia induction in patients with Williams syndrome (WS) before and after implementation of a perioperative management strategy.

DESIGN

Retrospective observational cohort study.

SETTING

Single quaternary academic referral center.

PARTICIPANTS

The authors reviewed the records of all children with WS at the authors' institution who underwent general anesthesia for cardiac catheterization, diagnostic imaging, or any type of surgery between November 2008 and August 2019. The authors identified 142 patients with WS, 48 of whom underwent 118 general anesthesia administrations. A historic group (HG) was compared with the intervention group (IG).

INTERVENTIONS

Change in perioperative management (three-stage risk stratification: preoperative intravenous hydration, intravenous anesthesia induction, and early use of vasoactives).

MEASUREMENTS AND MAIN RESULTS

The authors determined event rates within 60 minutes of anesthesia induction. Standardized mean difference (SMD) was calculated (SMD >0.2 suggests clinically meaningful difference). Sixty-seven general anesthesia encounters were recorded in the HG (mean age, 4.8 years; mean weight, 16.3 kg) and 51 in the IG (mean age, 6.0 years; mean weight, 18.2 kg). The change in systolic blood pressure was -17.5% (-30.0, -5.0) in the HG versus -9% (-18.0, 5.0) in the IG (p = 0.015; SMD = 0.419), and the incidence of ACEs was 6% in the HG and 2% in the IG (p = 0.542; SMD = 0.207).

CONCLUSIONS

Preoperative risk stratification, preoperative intravenous hydration, intravenous induction, and early use of continuous vasoactives resulted in greater hemodynamic stability, with a 2% incidence of ACEs.

Factors Predisposing to Hypertension in Subjects Formerly Born Preterm: Renal Impairment, Arterial Stiffness, Endothelial Dysfunction or Something Else?

Subjects formerly born preterm subsequently develop arterial - particularly isolated systolic- hypertension more frequently than their peers born at term.

Numerous factors may influence this predisposition, including an incomplete nephrogenesis, implying the presence of kidneys with a reduced number of nephrons and consequent reduction in haematic filtration, increased sodium absorption and activation of renin-angiotensin-aldosterone system, increased arterial rigidity produced by an elastin deficiency previously observed in anatomic specimens of human immature aorta, and reduced endothelial nitric oxide excretion, due to high blood levels of ADMA, a strong direct inhibitor of nitric oxide that exerts a vasoconstrictor effect.

Other possible factors (i.e. excretion of neuroendocrine compounds) may also be implicated. The aim of this paper was to review all possible mechanisms involved in the observed increase in blood pressure in individuals who had been born preterm and/or with intrauterine growth restriction. The outlook for new and promising laboratory techniques capable of identifying alterations in the metabolic pathways regulating blood pressure levels, such as metabolomics, is also provided.

Cardiovascular response and sequelae after minimally invasive surfactant therapy in growth-restricted preterm infants

OBJECTIVE

To study cardiovascular response to minimally invasive surfactant therapy in preterm infants with and without foetal growth restriction (FGR).

DESIGN

Poractant alfa was administered and echocardiograms were performed before and 30 min after. FGR infants were compared with those appropriate for gestational age (AGA).

RESULTS

Ten FGR infants were compared with 20 AGA infants (gestation [weeks], 28.9 ± 2 vs. 28.6 ± 1, p = 0.55 and birthweight [g], 813 ± 157 vs. 1141 ± 257, p = 0.01, respectively). The change in echocardiographic parameters was more prominent in AGA infants ([global contractility] fractional area change [FAC, %], FGR, 24.7 ± 2.2 to 27.9 ± 0.4, p = 0.08 vs. AGA, 26.6 ± 3 to 30.5 ± 1, p < 0.01, and [longitudinal contractility] tricuspid annular plane systolic excursion [mm], FGR, 3.9 ± 0.3 to 4.6 ± 0.5, p = 0.003 vs. AGA, 4.6 ± 0.3 to 5.5 ± 0.4, p = 0.0001). Significant difference was noted for change in FAC (%), FGR 2.1 ± 1.7 vs. AGA 4.1 ± 1.2, p = 0.02.

CONCLUSIONS

Differential cardiovascular response to minimally invasive surfactant therapy amongst FGR infants may reflect an in-utero maladaptive state.

Captopril treatment during development alleviates mechanically induced aortic remodeling in newborn elastin knockout mice

Deposition of elastin and collagen in the aorta correlates with increases in blood pressure and flow during development, suggesting that the aorta adjusts its mechanical properties in response to hemodynamic stresses. Elastin knockout (Eln^-/-) mice have high blood pressure and pathological remodeling of the aorta and die soon after birth. We hypothesized that decreasing blood pressure in Eln^-/- mice during development may reduce hemodynamic stresses and alleviate pathological remodeling of the aorta. We treated Eln^+/+ and Eln^-/- mice with the anti-hypertensive medication captopril throughout embryonic development and then evaluated left ventricular (LV) pressure and aortic remodeling at birth. We found that captopril treatment decreased Eln^-/- LV pressure to values near Eln^+/+ mice and alleviated the wall thickening and changes in mechanical behavior observed in untreated Eln^-/- aorta. The changes in thickness and mechanical behavior in captopril-treated Eln^-/- aorta were not due to alterations in measured elastin or collagen amounts, but may have been caused by alterations in smooth muscle cell (SMC) properties. We used a constitutive model to understand how changes in stress contributions of each wall component could explain the observed changes in composite mechanical behavior. Our modeling results show that alterations in the collagen natural configuration and SMC properties in the absence of elastin may explain untreated Eln^-/- aortic behavior and that partial rescue of the SMC properties may account for captopril-treated Eln^-/- aortic behavior.

Williams syndrome

Williams syndrome affects approximately one in 10 000 people and is caused by the deletion of genes on chromosome 7q11.23 which code for elastin. The phenotypic appearance of people with Williams syndrome is well characterized, but there continues to be new genetic and therapeutic discoveries. Patients with Williams syndrome have increased morbidity and mortality under sedation and anesthesia, largely as a result of cardiovascular abnormalities. This review article focuses on new information about Williams syndrome and outlines a structured approach to patients with Williams syndrome in the perioperative period.

Cardiovascular disease in Williams syndrome

PURPOSE OF REVIEW

Williams syndrome is a multisystem disorder seen with some regularity at most pediatric centers and usually fairly often at larger centers. Cardiovascular abnormalities, because of elastin deficiency, are the leading cause of morbidity and mortality in patients with Williams syndrome. The present article presents a review of the most recent developments regarding the cardiovascular issues in Williams syndrome.

RECENT FINDINGS

Cardiovascular abnormalities occur in 80% of patients with Williams syndrome, the majority of which are arterial stenoses. The stenoses seen in Williams syndrome now appear to arise from deficient circumferential arterial growth. Pharmacological therapies aimed at improving the vascular stenoses have shown some promise in animal models. Surgical outcomes for supravalvar aortic stenosis are good at most centers. Transcatheter interventions are largely ineffective in Williams syndrome. Multilevel surgical pulmonary artery reconstruction has excellent results for peripheral pulmonary artery stenosis. Periprocedural risk stratification and management algorithms may decrease the risk of cardiovascular complications.

SUMMARY

Cardiovascular abnormalities are a major determining factor in the clinical picture and trajectory of patients with Williams syndrome. Advances in surgical techniques, medical therapeutic options, and periprocedural management hold promise for significant improvements in the cardiovascular outcomes of these patients.

Williams Syndrome and Anesthesia for Non-cardiac Surgery: High Risk Can Be Mitigated with Appropriate Planning

Patients with Williams syndrome are considered at high risk for anesthesia-related adverse events. At our institution, all William syndrome patients undergoing cardiac surgical, cardiac catheterization/interventional procedures, and cardiac imaging studies are cared for by cardiac anesthesiologists. All William syndrome patients undergoing non-cardiac surgical, interventional, or imaging studies are cared for by main operating room pediatric anesthesiologists with consultative input from a cardiac anesthesiologist. We reviewed our experience with 75 patients undergoing 202 separate anesthetics for 95 non-cardiac procedures and 107 cardiac procedures from 2012 to 2016. The mean age was 7.5 ± 7.0 years and the mean weight was 22.3 ± 17.0 kg. One hundred and eighty-seven patients had a general anesthetic (92.6%). Medications used included etomidate in 26.2%, propofol in 37.6%, isoflurane in 47.5%, and sevoflurane in 68.3%. Vasopressors and inotropes were required including calcium (22.8%), dopamine (10.4%), norepinephrine (17.3%), phenylephrine (35.1%), vasopressin (0.5%), and ephedrine (5.4%). The median length of stay after anesthesia was 2.8 days (range 0-32). No adverse events occurred in 89.6% of anesthetics. There were two cases of cardiac arrest, one of which required extracorporeal life support for resuscitation. Of the non-cardiac surgical procedures, 95.7% did not have a cardiovascular adverse event. Patients with Williams syndrome are at high risk for anesthesia, especially when undergoing cardiac procedures. The risk can be mitigated with appropriate planning and adherence to the hemodynamic goals for non-cardiac surgical procedures.

Preterm growth restriction and bronchopulmonary dysplasia: the vascular hypothesis and related physiology

KEY POINTS

Approximately 5-10% pregnancies are affected by fetal growth restriction. Preterm infants affected by fetal growth restriction have a higher incidence of bronchopulmonary dysplasia. The present study is the first to measure pulmonary artery thickness and stiffness. The findings show that impaired vasculogenesis may be a contributory factor in the higher incidence of bronchopulmonary dysplasia in preterm growth restricted infants. The study addresses the mechanistic link between fetal programming and vascular architecture and mechanics.

ABSTRACT

Bronchopulmonary dysplasia is the most common respiratory sequelae of prematurity and histopathologically features fewer, dysmorphic pulmonary arteries. The present study aimed to characterize pulmonary artery mechanics and cardiac function in preterm infants with fetal growth restriction (FGR) compared to those appropriate for gestational age (AGA) in the early neonatal period. This prospective study reviewed 40 preterm infants between 28 to 32 weeks gestational age (GA). Twenty infants had a birthweight <10th centile and were compared with 20 preterm AGA infants. A single high resolution echocardiogram was performed to measure right pulmonary arterial and right ventricular (RV) indices. The GA and birthweight of FGR and AGA infants were 29.8 ± 1.3 vs. 30 ± 0.9 weeks (P = 0.78) and 923.4 g ± 168 vs. 1403 g ± 237 (P < 0.001), respectively. Assessments were made at 10.5 ± 1.3 days after birth. The FGR infants had significantly thicker right pulmonary artery inferior wall (843.5 ± 68 vs. 761 ± 40 μm, P < 0.001) with reduced pulsatility (51.6 ± 7.6 μm vs. 59.7 ± 7.5 μm, P = 0.001). The RV contractility [fractional area change (28.7 ± 3.8% vs 32.5 ± 3.1%, P = 0.001), tricuspid annular peak systolic excursion (TAPSE) (5.2 ± 0.3% vs. 5.9 ± 0.7%, P = 0.0002) and myocardial performance index (0.35 ± 0.03 vs. 0.28 ± 0.02, P < 0.001)] was significantly impaired in FGR infants. Significant correlation between RV longitudinal contractility (TAPSE) and time to peak velocity/RV ejection time (measure of RV afterload) was noted (r²  =  0.5, P < 0.001). Altered pulmonary vascular mechanics and cardiac performance reflect maladaptive changes in response to utero-placental insufficiency. Whether managing pulmonary vascular disease will alter clinical outcomes remains to be studied prospectively.

Biomechanical evaluation of a personalized external aortic root support applied in the Ross procedure

A commonly heard concern in the Ross procedure, where a diseased aortic valve is replaced by the patient's own pulmonary valve, is the possibility of pulmonary autograft dilatation. We performed a biomechanical investigation of the use of a personalized external aortic root support or exostent as a possibility for supporting the autograft. In ten sheep a short length of pulmonary artery was interposed in the descending aorta, serving as a simplified version of the Ross procedure. In seven of these cases, the autograft was supported by an external mesh or so-called exostent. Three sheep served as control, of which one was excluded from the mechanical testing. The sheep were sacrificed six months after the procedure. Samples of the relevant tissues were obtained for subsequent mechanical testing: normal aorta, normal pulmonary artery, aorta with exostent, pulmonary artery with exostent, and pulmonary artery in aortic position for six months. After mechanical testing, the material parameters of the Gasser-Ogden-Holzapfel model were determined for the different tissue types. Stress-strain curves of the different tissue types show significantly different mechanical behavior. At baseline, stress-strain curves of the pulmonary artery are lower than aortic stress-strain curves, but at the strain levels at which the collagen fibers are recruited, the pulmonary artery behaves stiffer than the aorta. After being in aortic position for six months, the pulmonary artery tends towards aorta-like behavior, indicating that growth and remodeling processes have taken place. When adding an exostent around the pulmonary autograft, the mechanical behavior of the composite artery (exostent + artery) differs from the artery alone, the non-linearity being more evident in the former.

Role of mechanotransduction in vascular biology: focus on thoracic aortic aneurysms and dissections

Thoracic aortic diseases that involve progressive enlargement, acute dissection, or rupture are influenced by the hemodynamic loads and mechanical properties of the wall. We have only limited understanding, however, of the mechanobiological processes that lead to these potentially lethal conditions. Homeostasis requires that intramural cells sense their local chemomechanical environment and establish, maintain, remodel, or repair the extracellular matrix to provide suitable compliance and yet sufficient strength. Proper sensing, in turn, necessitates both receptors that connect the extracellular matrix to intracellular actomyosin filaments and signaling molecules that transmit the related information to the nucleus. Thoracic aortic aneurysms and dissections are associated with poorly controlled hypertension and mutations in genes for extracellular matrix constituents, membrane receptors, contractile proteins, and associated signaling molecules. This grouping of factors suggests that these thoracic diseases result, in part, from dysfunctional mechanosensing and mechanoregulation of the extracellular matrix by the intramural cells, which leads to a compromised structural integrity of the wall. Thus, improved understanding of the mechanobiology of aortic cells could lead to new therapeutic strategies for thoracic aortic aneurysms and dissections.

Alterations in viscoelastic properties following premature birth may lead to hypertension and cardiovascular disease development in later life

The aim of this review was to identify the underlying relationship between preterm birth and the development of cardiovascular diseases. Preterm birth significantly affects the elastin content and viscoelastic properties of the vascular extracellular matrix in human arteries. Inadequate elastin synthesis during early development may cause a permanent increase in arterial stiffness in adulthood.

CONCLUSION

Early and permanent alterations in viscoelastic properties may lead to hypertension and cardiovascular disease development in adults born prematurely.

Insights into regional adaptations in the growing pulmonary artery using a meso-scale structural model: effects of ascending aorta impingement

As the next step in our investigations into the structural adaptations of the main pulmonary artery (PA) during postnatal growth, we utilized the extensive experimental measurements of the growing ovine PA from our previous study (Fata et al., 2013, "Estimated in vivo Postnatal Surface Growth Patterns of the Ovine Main Pulmonary Artery and Ascending Aorta," J. Biomech. Eng., 135(7), pp. 71010-71012). to develop a structural constitutive model for the PA wall tissue. Novel to the present approach was the treatment of the elastin network as a distributed fiber network rather than a continuum phase. We then utilized this model to delineate structure-function differences in the PA wall at the juvenile and adult stages. Overall, the predicted elastin moduli exhibited minor differences remained largely unchanged with age and region (in the range of 150 to 200 kPa). Similarly, the predicted collagen moduli ranged from ∼1,600 to 2700 kPa in the four regions studied in the juvenile state. Interestingly, we found for the medial region that the elastin and collagen fiber splay underwent opposite changes (collagen standard deviation juvenile = 17 deg to adult = 28 deg, elastin standard deviation juvenile = 35 deg to adult = 27 deg), along with a trend towards more rapid collagen fiber strain recruitment with age, along with a drop in collagen fiber moduli, which went from 2700 kPa for the juvenile stage to 746 kPa in the adult. These changes were likely due to the previously observed impingement of the relatively stiff ascending aorta on the growing PA medial region. Intuitively, the effects of the local impingement would be to lower the local wall stress, consistent with the observed parallel decrease in collagen modulus. These results suggest that during the postnatal somatic growth period local stresses can substantially modulate regional tissue microstructure and mechanical behaviors in the PA. We further underscore that our previous studies indicated an increase in effective PA wall stress with postnatal maturation. When taken together with the fact that the observed changes in mechanical behavior and structure in the growing PA wall were modest in the other three regions studied, our collective results suggest that the majority of the growing PA wall is subjected to increasing stress levels with age without undergoing major structural adaptations. This observation is contrary to the accepted theory of maintenance of homeostatic stress levels in the regulation of vascular function, and suggests alternative mechanisms might regulate postnatal somatic growth. Understanding the underlying mechanisms will help to improve our understanding of congenital defects of the PA and lay the basis for functional duplication in their repair and replacement.

Regional structural and biomechanical alterations of the ovine main pulmonary artery during postnatal growth

The engineering foundation for novel approaches for the repair of congenital defects that involve the main pulmonary artery (PA) must rest on an understanding of changes in the structure-function relationship that occur during postnatal maturation. In the present study, we quantified the postnatal growth patterns in structural and biomechanical behavior in the ovine PA in the juvenile and adult stages. The biaxial mechanical properties and collagen and elastin fiber architecture were studied in four regions of the PA wall, with the collagen recruitment of the medial region analyzed using a custom biaxial mechanical-multiphoton microscopy system. Circumferential residual strain was also quantified at the sinotubular junction and bifurcation locations, which delimit the PA. The PA wall demonstrated significant mechanical anisotropy, except in the posterior region where it was nearly isotropic. Overall, we observed only moderate changes in regional mechanical properties with growth. We did observe that the medial and lateral locations experience a moderate increase in anisotropy. There was an average of about 24% circumferential residual stain present at the luminal surface in the juvenile stage that decreased to 16% in the adult stage with a significant decrease at the bifurcation, implying that the PA wall remodels toward the bifurcation with growth. There were no measurable changes in collagen and elastin content of the tunica media with growth. On average, the collagen fiber recruited more rapidly with strain in the adult compared to the juvenile. Interestingly, the PA thickness remained constant with growth. When this fact is combined with the observed stable overall mechanical behavior and increase in vessel diameter with growth, a simple Laplace Law wall stress estimate suggests an increase in effective PA wall stress with postnatal maturation. This observation is contrary to the accepted theory of maintenance of homeostatic stress levels in the regulation of vascular function and suggests alternative mechanisms regulate postnatal somatic growth. Understanding the underlying mechanisms, incorporating important structural features during growth, will help to improve our understanding of congenital defects of the PA and lay the basis for functional duplication in their repair and replacement.

Extracellular matrix and the mechanics of large artery development

The large, elastic arteries, as their name suggests, provide elastic distention and recoil during the cardiac cycle in vertebrate animals. The arteries are distended from the pressure of ejecting blood during the active contraction of the left ventricle (LV) during systole and recoil to their original dimensions during relaxation of the LV during diastole. The cyclic distension occurs with minimal energy loss, due to the elastic properties of one of the major structural extracellular matrix (ECM) components, elastin. The maximum distension is limited to prevent damage to the artery by another major ECM component, collagen. The mix of ECM components in the wall largely determines the passive mechanical behavior of the arteries and the subsequent load on the heart during systole. While much research has focused on initial artery formation, there has been less attention on the continuing development of the artery to produce the mature composite wall complete with endothelial cells (ECs), smooth muscle cells (SMCs), and the necessary mix of ECM components for proper cardiovascular function. This review focuses on the physiology of large artery development, including SMC differentiation and ECM production. The effects of hemodynamic forces and ECM deposition on the evolving arterial structure and function are discussed. Human diseases and mouse models with genetic mutations in ECM proteins that affect large artery development are summarized. A review of constitutive models and growth and remodeling theories is presented, along with future directions to improve understanding of ECM and the mechanics of large artery development.

Pulmonary vascular wall stiffness: An important contributor to the increased right ventricular afterload with pulmonary hypertension

Pulmonary hypertension (PH) is associated with structural and mechanical changes in the pulmonary vascular bed that increase right ventricular (RV) afterload. These changes, characterized by narrowing and stiffening, occur in both proximal and distal pulmonary arteries (PAs). An important consequence of arterial narrowing is increased pulmonary vascular resistance (PVR). Arterial stiffening, which can occur in both the proximal and distal pulmonary arteries, is an important index of disease progression and is a significant contributor to increased RV afterload in PH. In particular, arterial narrowing and stiffening increase the RV afterload by increasing steady and oscillatory RV work, respectively. Here we review the current state of knowledge of the causes and consequences of pulmonary arterial stiffening in PH and its impact on RV function. We review direct and indirect techniques for measuring proximal and distal pulmonary arterial stiffness, measures of arterial stiffness including elastic modulus, incremental elastic modulus, stiffness coefficient β and others, the changes in cellular function and the extracellular matrix proteins that contribute to pulmonary arterial stiffening, the consequences of PA stiffening for RV function and the clinical implications of pulmonary vascular stiffening for PH progression. Future investigation of the relationship between PA stiffening and RV dysfunction may facilitate new therapies aimed at improving RV function and thus ultimately reducing mortality in PH.

A multi-layered computational model of coupled elastin degradation, vasoactive dysfunction, and collagenous stiffening in aortic aging

Arterial responses to diverse pathologies and insults likely occur via similar mechanisms. For example, many studies suggest that the natural process of aging and isolated systolic hypertension share many characteristics in arteries, including loss of functional elastin, decreased smooth muscle tone, and altered rates of deposition, and/or crosslinking of fibrillar collagen. Our aim is to show computationally how these coupled effects can impact evolving aortic geometry and mechanical behavior. Employing a thick-walled, multi-layered constrained mixture model, we suggest that a coupled loss of elastin and vasoactive function are fundamental mechanisms by which aortic aging occurs. Moreover, it is suggested that collagenous stiffening, although itself generally an undesirable process, can play a key role in attenuating excessive dilatation, perhaps including the enlargement of abdominal aortic aneurysms.

The role of collagen in extralobar pulmonary artery stiffening in response to hypoxia-induced pulmonary hypertension

Hypoxic pulmonary hypertension (HPH) causes extralobar pulmonary artery (PA) stiffening, which potentially impairs right ventricular systolic function. Changes in the extracellular matrix proteins collagen and elastin have been suggested to contribute to this arterial stiffening. We hypothesized that vascular collagen accumulation is a major cause of extralobar PA stiffening in HPH and tested our hypothesis with transgenic mice that synthesize collagen type I resistant to collagenase degradation (Col1a1(R/R)). These mice and littermate controls that have normal collagen degradation (Col1a1(+/+)) were exposed to hypoxia for 10 days; some were allowed to recover for 32 days. In vivo PA pressure and isolated PA mechanical properties and collagen and elastin content were measured for all groups. Vasoactive studies were also performed with U-46619, Y-27632, or calcium- and magnesium-free medium. Pulmonary hypertension occurred in both mouse strains due to chronic hypoxia and resolved with recovery. HPH caused significant PA mechanical changes in both mouse strains: circumferential stretch decreased, and mid-to-high-strain circumferential elastic modulus increased (P < 0.05 for both). Impaired collagen type I degradation prevented a return to baseline mechanical properties with recovery and, in fact, led to an increase in the low and mid-to-high-strain moduli compared with hypoxia (P < 0.05 for both). Significant changes in collagen content were found, which tended to follow changes in mid-to-high-strain elastic modulus. No significant changes in elastin content or vasoactivity were observed. Our results demonstrate that collagen content is important to extralobar PA stiffening caused by chronic hypoxia.

Vascular extracellular matrix and arterial mechanics

An important factor in the transition from an open to a closed circulatory system was a change in vessel wall structure and composition that enabled the large arteries to store and release energy during the cardiac cycle. The component of the arterial wall in vertebrates that accounts for these properties is the elastic fiber network organized by medial smooth muscle. Beginning with the onset of pulsatile blood flow in the developing aorta, smooth muscle cells in the vessel wall produce a complex extracellular matrix (ECM) that will ultimately define the mechanical properties that are critical for proper function of the adult vascular system. This review discusses the structural ECM proteins in the vertebrate aortic wall and will explore how the choice of ECM components has changed through evolution as the cardiovascular system became more advanced and pulse pressure increased. By correlating vessel mechanics with physiological blood pressure across animal species and in mice with altered vessel compliance, we show that cardiac and vascular development are physiologically coupled, and we provide evidence for a universal elastic modulus that controls the parameters of ECM deposition in vessel wall development. We also discuss mechanical models that can be used to design better tissue-engineered vessels and to test the efficacy of clinical treatments.

Distinct agonist responsibilities of the first and second branches of mouse mesenteric artery

The mesenteric artery (MA) is suitable for consideration as a typical micro-resistant artery for examination of arteriosclerosis. The MA is comprised of the first (MA1), second (MA2), and additional fine structural branches; however, differences in terms of responsibilities of these branches have not been assessed. The objective of this study was to differentiate contractile responses in the MAs of mice. MA2 rings (100 microm diameter, 1 mm length) displayed maximal force development (846.8 +/- 55.6 microN; n = 5) upon stimulation with 50 mM KCl under 400 microN resting tension. However, both MA1 and aorta required resting tension exceeding 600 microN. Treatment of MA2 with phenylephrine (PE; 10 microM), norepinephrine (NE; 10 microM), thromboxane A(2) (analog U46619; 100 nM), or prostaglandin F(2a) (PG; 10 microM) induced sustained contractions. Responses were 1507.8 +/- 88.8, 1543 + 5 +/- 149.6, 2088.6 +/- 151.6, and 1441.9 +/- 103.6 microN (n = 7), respectively. These values were markedly larger than those of the KCl-induced response. In MA1 and aorta, PE-induced and NE-induced responses were indistinct from the KCl response. This investigation revealed that MA1 exhibits responsibilities similar to those of the aorta, whereas MA2 possesses distinct responsibilities. MA2 might serve as a micro-resistant artery model.

Molecular mechanisms of the vascular responses to haemodynamic forces

Blood vessels are permanently subjected to mechanical forces in the form of stretch, encompassing cyclic mechanical strain due to the pulsatile nature of blood flow and shear stress. Significant variations in mechanical forces, of physiological or physiopathological nature, occur in vivo. These are accompanied by phenotypical modulation of smooth muscle cells and endothelial cells, producing structural modifications of the arterial wall. In all the cases, vascular remodelling can be allotted to a modification of the tensional strain or shear, and underlie a trend to reestablish baseline mechanical conditions. Vascular cells are equipped with numerous receptors that allow them to detect and respond to the mechanical forces generated by pressure and shear stress. The cytoskeleton and other structural components have an established role in mechanotransduction, being able to transmit and modulate tension within the cell via focal adhesion sites, integrins, cellular junctions and the extracellular matrix. Mechanical forces also initiate complex signal transduction cascades, including nuclear factor-kappaB and mitogen-activated protein kinase pathways, leading to functional changes within the cell.

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.

[The vascular smooth muscle of great arteries: local control site of arterial buffering function?]

INTRODUCTION AND OBJECTIVES

To characterize the viscoelastic properties of the aorta and pulmonary arteries and the effects of vascular smooth muscle activation on arterial buffering function.

MATERIAL AND METHOD

Aortic and pulmonary artery pressure and diameter were measured in six anesthetized sheep under baseline conditions, and during arterial hypertension induced by mechanical vascular occlusion (passive), and i.v. phenylephrine (active). Arterial wall elasticity and viscosity were calculated, and buffering function was characterized: a) locally as the viscosity/elasticity ratio, and b) globally for each circuit, as the time-constant of ventricular relaxation.

RESULTS

Viscoelasticity was higher in the aorta than in the pulmonary artery (p < 0.05), however, parietal buffering function was similar in both. Global buffering function was highest in the systemic circuit (p < 0.05). During passive hypertension, elasticity was significantly increased with no change in viscosity; this led to a significant reduction in local buffering function, and in global buffering function in each circuit. During active hypertension, viscosity increased (p < 0.05), while local and global buffering functions returned to baseline values.

CONCLUSIONS

The viscosity/elasticity ratio was higher in the aorta than in the pulmonary artery, and arterial wall buffering function was similar in both vessels. Systemic global buffering function was higher than pulmonary circuit buffering function. Elasticity depends on intravascular pressure, whereas viscosity is a marker of the degree of smooth muscle activation. Smooth muscle activation may benefit the cardiovascular system by maintaining local and global buffering functions.

Developmental adaptation of the mouse cardiovascular system to elastin haploinsufficiency

Supravalvular aortic stenosis is an autosomal-dominant disease of elastin (Eln) insufficiency caused by loss-of-function mutations or gene deletion. Recently, we have modeled this disease in mice (Eln+/-) and found that Eln haploinsufficiency results in unexpected changes in cardiovascular hemodynamics and arterial wall structure. Eln+/- animals were found to be stably hypertensive from birth, with a mean arterial pressure 25-30 mmHg higher than their wild-type counterparts. The animals have only moderate cardiac hypertrophy and live a normal life span with no overt signs of degenerative vascular disease. Examination of arterial mechanical properties showed that the inner diameters of Eln+/- arteries were generally smaller than wild-type arteries at any given intravascular pressure. Because the Eln+/- mouse is hypertensive, however, the effective arterial working diameter is comparable to that of the normotensive wild-type animal. Physiological studies indicate a role for the renin-angiotensin system in maintaining the hypertensive state. The association of hypertension with elastin haploinsufficiency in humans and mice strongly suggests that elastin and other proteins of the elastic fiber should be considered as causal genes for essential hypertension.

Decreased elastin in vessel walls puts the pressure on

Mice haploinsufficient for elastin develop structural changes in vessel walls similar to those seen in patients with mutations in the elastin gene. A new study demonstrates that due to mechanical changes in the vessel wall, these animals exhibit increased mean arterial pressures. The results evoke the possibility that alterations in elastin may contribute to the development of essential hypertension in patients.

Size and blood flow of central and peripheral arteries in highly trained able-bodied and disabled athletes

In a cross-sectional study, central and peripheral arteries were investigated noninvasively in high-performance athletes and in untrained subjects. The diastolic inner vessel diameter (D) of the thoracic and abdominal aorta, the subclavian artery (Sub), and common femoral artery (Fem) were determined by duplex sonography in 18 able-bodied professional tennis players, 34 able-bodied elite road cyclist athletes, 26 athletes with paraplegia, 17 below-knee amputated athletes, and 30 able-bodied, untrained subjects. The vessel cross-sectional areas (CSA) were set in relation to body surface area (BSA), and the cross-section index (CS-index = CSA/BSA) was calculated. Volumetric blood flow was determined in Sub and Fem via a pulsed-wave Doppler system and was set in relation to heart rate to calculate the stroke flow. A significantly increased D of Sub was found in the racket arm of able-bodied tennis players compared with the opposite arm (19%). Fem of able-bodied road cyclist athletes and of the intact limb in below-knee amputated athletes showed similar increases. D of Fem was lower in athletes with paraplegia (37%) and in below-knee amputated athletes proximal to the lesion (21%) compared with able-bodied, untrained subjects; CS-indexes were reduced 57 and 31%, respectively. Athletes with paraplegia demonstrated a larger D (19%) and a larger CS-index in Sub (54%) than able-bodied, untrained subjects. No significant differences in D and CS-indexes of the thoracic and abdominal aorta were found between any of the groups. The changes measured in Sub and Fem were associated with corresponding alterations in blood flow and stroke flow in all groups. The study suggests that the size and blood flow volume of the proximal limb arteries are adjusted to the metabolic needs of the corresponding extremity musculature and underscore the impact of exercise training or disuse on the structure and the function of the arterial system.

Arterial repair after stenting and the effects of GM6001, a matrix metalloproteinase inhibitor

OBJECTIVES

This study compared the extracellular matrix (ECM) and cellular responses after stenting to balloon angioplasty (BA) and to determine the late effects of matrix metalloproteinase (MMP) inhibition on arterial repair after stenting.

BACKGROUND

Although stenting is the predominant form of coronary intervention, there is limited understanding of the early and late arterial response.

METHODS

In a double-injury rabbit model, adjacent iliac arteries in 87 animals received BA (3.0 mm diameter) or stenting (3.0 mm NIR). Rabbits were treated for 1 week postprocedure with either GM6001 (100 mg/kg per day), an MMP inhibitor or placebo and sacrificed at 1 week or at 10 weeks' postprocedure. Arteries were analyzed for morphometry, collagen content, gelatinase activity, cell proliferation and DNA content.

RESULTS

Stented arteries had significant increases in collagen content (2-fold) at 10 weeks compared to BA-treated arteries. At one week, overall gelatinase activity was increased >2-fold in stented arteries, with both 72 kD and 92 kD gelatinase activity. Stented arteries also had increases in both intimal DNA content (1.5-fold) and absolute cell proliferation (4-fold). Compared to placebo, GM6001 significantly inhibited intimal hyperplasia and intimal collagen content, and it increased lumen area in stented arteries without effects on proliferation rates.

CONCLUSIONS

Stenting causes a more vigorous ECM and MMP response than BA, which involves all layers of the vessel wall. Inhibition by MMP blocks in-stent intimal hyperplasia and offers a novel approach to prevent in-stent restenosis.

Gene expression of tropoelastin is enhanced in the aorta proximal to the coarctation in rabbits

To assess elastin biosynthesis in the aortic wall in response to acute elevation of blood pressure, we studied the aortic gene expression of tropoelastin in a rabbit midthoracic aortic coarctation model. The time points of the study were 1, 3, and 7 days and 2, 4, and 8 weeks after coarctation. Additional animals were subjected to hypercholesterolemia for analysis of tropoelastin expression in the intimal lesion. mRNA for tropoelastin was quantitated by Northern blot analysis and its distribution was revealed by in situ hybridization. The 65-kDa tropoelastin was analyzed by Western blotting and immunohistochemistry. Tropoelastin mRNA proximal to the coarctation was increased at 2 weeks and returned to baseline by 8 weeks (P < 0.05 versus control). Changes in 65-kDa tropoelastin corresponded to those of mRNA. Tropoelastin gene was expressed mainly in the intima and in the outer media at the proximal region to the stenoses, which was particularly remarkable in the intimal lesion. The results indicate that tropoelastin gene expression was enhanced in the early remodeling response to elevated blood pressure. The distribution of newly synthesized tropoelastin in the outer media suggests a reenforcement role of tropoelastin, which preserves mechanical resiliency in response to changes in tensile stress.

A hypothesis about a mechanism for the programming of blood pressure and vascular disease in early life

1. There is now a great deal of evidence that people whose weight at birth was low tend to have higher blood pressure and increased risk of death from cardiovascular disease as adults. 2. We argue that, in fetuses whose growth is impaired, synthesis of elastin in the walls of the aorta and large arteries is deficient and that this deficiency leads to permanent changes in the mechanical properties of these vessels. 3. Over a lifetime, such changes could predispose an individual to higher blood pressure and cardiovascular disease.

Determinants of mechanical properties in the developing ovine thoracic aorta

We previously reported changes in mechanical properties and collagen cross-linking of the ovine thoracic aorta during perinatal development and postnatal maturation, and we now report changes in biochemical composition (elastin, collagen, and DNA contents per mg wet wt) over the same developmental intervals. A comparison of results from the present and previous studies has yielded novel and important observations concerning the relationship between aortic mechanics and composition during maturation. Developmental changes in aortic incremental elastic modulus at low tensile stress (E(low)) closely followed changes in relative elastin content (i.e., per mg wet wt). An 89% increase in E(low) during the perinatal period was associated with a 69% increase in relative elastin content, whereas neither variable changed during postnatal life. Incremental elastic modulus at high tensile stress (E(high)) did not change during the perinatal period but increased 88% during postnatal life. This pattern closely paralleled changes in collagen cross-linking index, which did not change perinatally but almost doubled postnatally. In contrast, relative collagen content (per mg wet wt) increased only slightly from fetal to adult life, a trend that was unrelated to aortic mechanics. Substantial, progressive decreases in measures of wall viscosity (pressure wave attenuation coefficient and viscoelastic phase angle) from fetal to adult life followed the pattern observed for relative DNA (smooth muscle cell) content (per mg wet wt). Our findings suggest that accumulation of elastin per milligram wet weight contributes most to developmental changes in E(low), change in collagen cross-linking is the primary determinant of developmental changes in E(high), and cell accumulation contributes most to developmental changes in wall viscosity.

In vivo and in vitro mechanical properties of the sheep thoracic aorta in the perinatal period and adulthood

The mammalian aorta undergoes rapid remodeling during the perinatal period and more gradual remodeling during subsequent development, but the implications of this remodeling for arterial mechanics are poorly understood. In this study in vivo and in vitro techniques were used to determine the static and viscoelastic properties of the thoracic aortas of 119-day-gestation fetal sheep (full term = 145 days), 21-day-old lambs, and adult sheep at control distending pressures and after 70% increases or 30% decreases in pressure. In the weeks surrounding birth, aortic wall tissue became substantially stiffer (static elastic modulus in vitro increased by 28%, and pressure wave velocity in vivo increased by 61%) but less viscous (pressure wave attenuation in vivo decreased by 46%, and viscoelastic phase angle in vitro decreased by 15%), whereas the wall thickness-to-radius ratio was unchanged. By contrast, modest changes in tissue viscoelasticity from neonatal to adult life were accompanied by a halving of the wall thickness-to-radius ratio from 0.19 +/- 0.01 to 0.10 +/- 0.01. The relative thinning of the vessel wall, combined with a doubling of blood pressure after birth, resulted in a 265% increase in aortic wall tensile stress over the period of study. We concluded that rapid remodeling in the perinatal period primarily alters the viscoelastic properties of aortic wall tissues, whereas more gradual postnatal remodeling largely affects vessel geometry.

Impaired synthesis of elastin in walls of aorta and large conduit arteries during early development as an initiating event in pathogenesis of systemic hypertension

There is much evidence that people who had low birthweight tend to have higher blood pressure in later life. However, the mechanisms that mediate this relation are unknown. We argue that, in fetuses whose growth is impaired, synthesis of elastin in the walls of the aorta and large arteries may be deficient, and that this deficiency would lead to permanent changes in the mechanical properties of these vessels. Over a lifetime, such changes could predispose an individual to higher blood pressure, increased left-ventricular mass, and cardiovascular disease.

Cellular and molecular mechanisms of pulmonary vascular remodeling

In many organs and tissues, the cellular response to injury is associated with a reiteration of specific developmental processes. Studies have shown that, in response to injury, vascular wall cells in adult organisms express genes or gene products characteristic of earlier developmental states. Other genes, expressed preferentially in adult cells in vivo, are down-regulated following injurious stimuli. Complicating matters, however, are recent observations demonstrating that the vascular wall is comprised of phenotypically heterogeneous subpopulations of endothelial cells, smooth muscle cells, and fibroblasts. It is unclear how specific subsets of cells respond to injury and thus contribute to the vascular remodeling that characterizes chronic pulmonary hypertension. This review discusses vascular development in the lung and the cellular responses occurring in pulmonary hypertension; special attention is given to heterogeneity of responses within cell populations and reiteration of developmental processes.

Mechanical and contractile properties of in situ localized mesenteric arteries in normotensive and spontaneously hypertensive rats

An in situ model was developed for studying mechanical properties of mesenteric arteries in rats. A branch of the mesenteric artery was exposed and dissected in normotensive (WKY) and spontaneously hypertensive rats (SHR). A catheter was introduced into the larger branch of the mesenteric artery and connected to a pressure chamber. The artery was submitted to transmural pressures ranging from 0 to 200 mmHg per steps of 25 mmHg and observed using a microscope-video-camera system. The diameter-pressure relations were established under basal conditions, under contraction (phenylephrine 10(-6) M), and after abolition of the smooth muscle tone by potassium cyanide (KCN, 0.1 mg/mL). The arterial segment was then fixed (glutaraldehyde 2.5%), and the wall cross-sectional areas were measured in transverse sections. Compliances, distensibility, wall tensions, and wall stresses were calculated from diameter, pressure, and media thickness values under three conditions. Active tension and active stress were defined as differences in wall stresses and wall tensions calculated under passive and active conditions. Comparison of WKY and SHR when arteries were studied at the respective operating pressure indicates (1) thicker and stiffer mesenteric arteries in SHRs than in WKY rats, (2) similar wall stresses in mesenteric arteries from WKY and SHRs despite larger wall tensions in the hypertensive group, and (3) larger contractility to phenylephrine in SHRs than in WKY mesenteric arteries.

Pulmonary artery size after bidirectional cavopulmonary connection

Pulmonary arterial size was measured during cineangiography in 23 patients, 1.9 months before, and 14 months after bidirectional cavopulmonary connection (BCPC). Measurements were standardized for body surface area using the method of Nakata and co-workers (pulmonary artery index). There was a significant reduction in pulmonary artery index after BCPC. These data suggest that pulmonary arterial growth is impaired after the creation of a BCPC. This may be related to an absolute reduction in pulmonary arterial flow, and/or the loss of systolic expansion of the pulmonary artery.

A compliant tubular device to study the influences of wall strain and fluid shear stress on cells of the vascular wall

PURPOSE

Cellular constituents of the blood vessel wall are continuously subjected, in vivo, to both mechanical and hemodynamic forces, which elicit structural and biologic responses. We have developed a compliant tubular system, the vascular simulating device (VSD), that reproduces these forces, while supporting the attachment and the experimental manipulation of endothelial and smooth muscle cells.

METHODS

The VSD consists of a compliant silicone rubber tube coupled to a pump system, which permits the simultaneous application of known levels of pressure and flow, to vascular wall cells cultured on the inner surface of the tube. Seeded cells can be monitored visually under phase contrast or fluorescent optics, as well as harvested and analyzed for biologic responses.

RESULTS

The elastic modulus and compliance of the silicone rubber tube are similar to those of canine and human arteries. Endothelial and smooth muscle cells cultured on the lumenal surface of the tubes remain attached and viable after subjecting them to physiologic pulsatile flow and cyclic strain.

CONCLUSION

The VSD makes it possible to approximate, in vitro, those forces encountered by vascular wall cells, in vivo and therefore may make it possible to determine whether specific combinations of mechanical and hemodynamic forces are causally associated with specific vascular diseases.

Effect of wall tension on DNA and protein synthesis in bovine mesenteric arteries in vitro

The aim of this investigation was to study the effect of wall tension and calcium antagonists on DNA and protein synthesis in bovine mesenteric arteries in vitro. The wall tension of the bovine mesenteric arteries was raised by stretching the vessel wall perpendicular to the length axis of the vessel. DNA and protein synthesis were determined by measuring incorporation of 3H-thymidine into DNA and incorporation of 14C-leucine into protein respectively. Elevating the wall tension from 0.05 N to 0.5 N significantly increased 3H-thymidine incorporation and 14C-leucine incorporation after an incubation period of 3 hr. Stretch had no effect on the distribution of 3H-thymidine. The distribution of 14C-leucine was increased by stretch in regular medium and to a less extent in calcium-free medium, which suggest that stretch stimulates the membrane transport of 14C-leucine. When the tension was increased from 0.05 N to 0.5 N for 10 min. before the incubation with 3H-thymidine, no effect was found. One microM nifedipine or felodipine inhibited the increase in 3H-thymidine incorporation caused by stretching, while no effect was found on 14C-leucine incorporation. In calcium-free medium, stretch-induced DNA synthesis was completely abolished. 14C-Leucine incorporation was impaired in calcium-free medium but the stretch-induced increase still remained. The results suggest that mechanical force may play an important role in DNA synthesis and protein metabolism of vascular smooth muscle.