Mechanical and dimensional adaptation of rabbit carotid artery cultured in vitro

Electrospun vascular grafts fabricated from poly(L-lactide-co-ε-caprolactone) used as a bypass for the rabbit carotid artery

The study involved the electrospinning of the copolymer poly(L-lactide-co-ε-caprolactone) (PLCL) into tubular grafts. The subsequent material characterization, including micro-computed tomography analysis, revealed a level of porosity of around 70%, with pore sizes of 9.34 ± 0.19 μm and fiber diameters of 5.58 ± 0.10 μm. Unlike fibrous polycaprolactone, the electrospun PLCL copolymer promoted fibroblast and endothelial cell adhesion and proliferation in vitro. Moreover, the regeneration of the vessel wall was detected following implantation and, after six months, the endothelialization of the lumen and the infiltration of arranged smooth muscle cells producing collagen was observed. However, the degradation rate was found to be accelerated in the rabbit animal model. The study was conducted under conditions that reflected the clinical requirements-the prostheses were sutured in the end-to-side fashion and the long-term end point of prosthesis healing was assessed. The regeneration of the vessel wall in terms of endothelialization, smooth cell infiltration and the presence of collagen fibers was observed after six months in vivo. A part of the grafts failed due to the rapid degradation rate of the PLCL copolymer.

Pulsatile Perfusion Bioreactor for Biomimetic Vascular Impedances

Pulsatile waves of blood pressure and flow are continuously augmented by the resistance, compliance, and inertance properties of the vasculature, resulting in unique wave characteristics at distinct anatomical locations. Hemodynamically generated loads, transduced as physical signals into resident vascular cells, are crucial to the maintenance and preservation of a healthy vascular physiology; thus, failure to recreate biomimetic loading in vitro can lead to pathological gene expression and aberrant remodeling. As a generalized approach to improve native and engineered blood vessels, we have designed, built, and tested a pulsatile perfusion bioreactor based on biomimetic impedances and a novel five-element electrohydraulic analog. Here, the elements of an incubator-based culture system were formulaically designed to match the vascular impedance of a brachial artery by incorporating both the inherent (systemic) and added elements of the physical system into the theoretical approach. Freshly harvested porcine saphenous veins were perfused within a physiological culture chamber for 6 h and the relative expression of seven known mechanically sensitive remodeling genes analyzed using the quantitative polymerase chain reaction (qPCR) method. Of these, we found plasminogen activator inhibitor-1 (SERPINE1) and fibronectin-1 (FN1) to be highly sensitive to differences between arterial- and venous-like culture conditions. The analytical approach and biological confirmation provide a framework toward the general design of long-term hemodynamic-mimetic vascular culture systems.

Aortic pulse pressure homeostasis emerges from physiological adaptation of systemic arteries to local mechanical stresses

Aortic pulse pressure arises from the interaction of the heart, the systemic arterial system, and peripheral microcirculations. The complex interaction between hemodynamics and arterial remodeling precludes the ability to experimentally ascribe changes in aortic pulse pressure to particular adaptive responses. Therefore, the purpose of the present work was to use a human systemic arterial system model to test the hypothesis that pulse pressure homeostasis can emerge from physiological adaptation of systemic arteries to local mechanical stresses. First, we assumed a systemic arterial system that had a realistic topology consisting of 121 arterial segments. Then the relationships of pulsatile blood pressures and flows in arterial segments were characterized by standard pulse transmission equations. Finally, each arterial segment was assumed to remodel to local stresses following three simple rules: 1) increases in endothelial shear stress increases radius, 2) increases in wall circumferential stress increases wall thickness, and 3) increases in wall circumferential stress decreases wall stiffness. Simulation of adaptation by iteratively calculating pulsatile hemodynamics, mechanical stresses, and vascular remodeling led to a general behavior in response to mechanical perturbations: initial increases in pulse pressure led to increased arterial compliances, and decreases in pulse pressure led to decreased compliances. Consequently, vascular adaptation returned pulse pressures back toward baseline conditions. This behavior manifested when modeling physiological adaptive responses to changes in cardiac output, changes in peripheral resistances, and changes in local arterial radii. The present work, thus, revealed that pulse pressure homeostasis emerges from physiological adaptation of systemic arteries to local mechanical stresses.

Design and fabrication of a mechanically matched vascular graft

The study provides a pathway to design a mechanics-matching vascular graft for an end-to-end anastomosis to a host artery. For functional equivalence, we submit that the graft and a host artery should have equal inner deformed diameters, equal pressure-radius module, and experience equal axial forces when subjected to mean arterial pressure. These criteria for mechanical equivalence are valid for a large class of materials that can be considered as elastic incompressible and orthotropic solids. As an example, specific known artery properties were used to design or select a graft made from a new synthetic biomaterial to demonstrate that reliable and robust technology for graft fabrication is possible.

A novel cylindrical biaxial computer-controlled bioreactor and biomechanical testing device for vascular tissue engineering

It is becoming evident that tissue-engineered constructs adapt to altered mechanical loading, and that specific combinations of multidirectional loads appear to have a synergistic effect on the remodeling. However, most studies of mechanical stimulation of engineered vascular tissue engineering employ only uniaxial stimulation. Here we present a novel computer-controlled bioreactor and biomechanical testing device designed to precisely and simultaneously control mean and cyclic values of transmural pressure (at rates up to 1 Hz and ranges of 40 mmHg), luminal flow rate, and axial length (or load) applied to gel-derived, scaffold-derived, and self-assembly-derived tissue-engineered blood vessels during culture, while monitoring vessel geometry with a resolution of 6.6 mum. Intermittent monitoring of the extracellular matrix and cells is accomplished on live tissues using multi-photon confocal microscopy under unloaded and loaded conditions at multiple time-points in culture (on the same vessel) to quantify changes in cell and extracellular matrix content and organization. This same device is capable of performing intermittent cylindrical biaxial biomechanical testing at multiple time-points in culture (on the same vessel) to quantify changes in the mechanical behavior during culture. Here we demonstrate the capabilities of this new device on self-assembly-derived and collagen-gel-derived tissue-engineered blood vessels.

Differences in transmural pressure and axial loading ex vivo affect arterial remodeling and material properties

Arterial axial strains, present in the in vivo environment, often become reduced due to either bypass grafting or the normal aging process. Since the prevalence of hypertension increases with aging, arteries are often exposed to both decreased axial stretch and increased transmural pressure. The combined effects of these mechanical stimuli on the mechanical properties of vessels have not previously been determined. Porcine carotid arteries were cultured for 9 days at normal and reduced axial stretch ratios in the presence of normotensive and hypertensive transmural pressures using ex vivo perfusion techniques. Measurements of the amount of axial stress were obtained through longitudinal tension tests while inflation-deflation test results were used to determine circumferential stresses and incremental moduli. Macroscopic changes in artery geometry and zero-stress state opening angles were measured. Arteries cultured ex vivo remodeled in response to the mechanical environment, resulting in changes in arterial dimensions of up to approximately 25% and changes in zero-stress opening angles of up to approximately 55 degrees . While pressure primarily affected circumferential remodeling and axial stretch primarily affected axial remodeling, there were clear examples of interactions between these mechanical stimuli. Culture with hypertensive pressure, especially when coupled with reduced axial loading, resulted in a rightward shift in the pressure-diameter relationship relative to arteries cultured with normotensive pressure. The observed differences in the pressure-diameter curves for cultured arteries were due to changes in artery geometry and, in some cases, changes in the arteries' intrinsic mechanical properties. Relative to freshly isolated arteries, arteries cultured under mechanical conditions similar to in vivo conditions were stiffer, suggesting that aspects of the ex vivo culture other than the mechanical environment also influenced changes in the arteries' mechanical properties. These results confirm the well-known importance of transmural pressure with regard to arterial wall mechanics while highlighting additional roles for axial stretch in determining mechanical behavior.

System and method for investigating arterial remodeling

Organ culture systems are used to study remodeling of arteries and to fabricate tissue engineered vascular grafts. Investigations to date focused on changes in geometry and mechanical response of arteries or constructs associated with controlled sustained alterations in the global load parameters such as the arterial pressure, flow, or axial stretch. A new experimental paradigm is proposed, which is based on the simultaneous independent control of local mechanical parameters such as mean strain or stress in the arterial wall and flow-induced shear at the intima. An organ culture system and methodology were developed, which controls pressure, flow, and axial length of a specimen in order to maintain the local mechanical parameters at prescribed values. The operation of the system is illustrated by maintenance of elevated axial medial stress in porcine carotid artery, while keeping the mean circumferential stress and flow-induced shear stress at baseline values. Previously unknown aspects of remodeling that might be revealed by the novel approach are discussed.

Alterations of pulse pressure stimulate arterial wall matrix remodeling

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

Transmural pressure and axial loading interactively regulate arterial remodeling ex vivo

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

Arteries respond to independent control of circumferential and shear stress in organ culture

Arteries respond to changes in global mechanical parameters (pressure, flow rate, and longitudinal stretching) by remodeling to restore local parameters (circumferential stress, shear stress, and axial strain) to baseline levels. Because a change in a single global parameter results in changes of multiple local parameters, the effects of individual local parameters on remodeling remain unknown. This study uses a novel approach to study remodeling in organ culture based on independent control of local mechanical parameters. The approach is illustrated by studying the short term effects of circumferential and shear stress on remodeling-related biological markers. Porcine carotid arteries were cultured for 3 days at a circumferential stress of 50 or 150 kPa or, in separate experiments, a shear stress of 0.75 or 2.25 Pa. At high circumferential stress, matrix synthesis, smooth muscle cell proliferation, and cell death are significantly greater, but matrix metalloproteinase-2 (MMP-2) and pro-MMP-2 activity are significantly less. In contrast, biological markers measured were unaffected by shear stress. Applications of the proposed approach for improved understanding of remodeling, optimizing mechanical conditioning of tissue engineered arteries, and selection of experimentally motivated growth laws are discussed.

Sustained axial loading lengthens arteries in organ culture

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

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

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

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

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

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

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

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

Stretch-dependent modulation of contractility and growth in smooth muscle of rat portal vein

Increased intraluminal pressure of the rat portal vein in vivo causes hypertrophy and altered contractility in 1 to 7 days. We have used organ cultures to investigate mechanisms involved in this adaptation to mechanical load. Strips of rat portal vein were cultured for 3 days, either undistended or loaded by a weight. Length-force relations were shifted toward longer length in stretched cultured veins compared with freshly dissected veins, whereas the length-force relations of unstretched cultured veins were shifted in the opposite direction. This occurred after culture either with or without 10% FCS to promote growth. The wet weight of loaded veins increased by 56% in the presence of FCS, whereas that of undistended control veins increased by 24%. No weight increase was seen in serum-free culture. The dry/wet weight ratio decreased during culture with FCS but was not affected by stretch. Electron microscopy revealed increased cell cross-sectional area in stretched relative to unstretched veins, and protein contents were greater, as were [(3)H]thymidine and [(3)H]leucine incorporation rates. Growth responses were associated with the activation of stretch-sensitive extracellular signal-regulated kinases 1 and 2 and were inhibited by herbimycin A and PD 98059, inhibitors of extracellular signal-regulated kinases 1 and 2. The results demonstrate that by culture of whole vascular tissue, smooth muscle cells are maintained in the contractile phenotype and respond to stretch with a physiological adaptation involving hypertrophy/hyperplasia and remodeling of the contractile system, similar to that in vivo. Mechanical stimulation and growth factors are both required for functionally significant growth.