Alterations in wall tension and shear stress modulate tyrosine kinase signaling and wall remodeling in experimental vein grafts

Endothelial Response to the Combined Biomechanics of Vessel Stiffness and Shear Stress Is Regulated via Piezo1

How endothelial cells sense and respond to dynamic changes in their biophysical surroundings as we age is not fully understood. Vascular stiffness is clearly a contributing factor not only in several cardiovascular diseases but also in physiological processes such as aging and vascular dementia. To address this gap, we utilized a microfluidic model to explore how substrate stiffness in the presence of shear stress affects endothelial morphology, senescence, proliferation, and inflammation. We also studied the role of mechanosensitive ion channel Piezo1 in endothelial responses under the combined effect of shear stress and substrate stiffness. To do so, we cultured endothelial cells inside microfluidic channels covered with fibronectin-coated elastomer with elastic moduli of 40 and 200 kPa, respectively, mimicking the stiffness of the vessel walls in young and aged arteries. The endothelial cells were exposed to atheroprotective and atherogenic shear stress levels of 10 and 2 dyn/cm2, respectively. Our findings show that substrate stiffness affects senescence under atheroprotective flow conditions and cytoskeleton remodeling, senescence, and inflammation under atherogenic flow conditions. Additionally, we found that the expression of Piezo1 plays a crucial role in endothelial adaptation to flow and regulation of inflammation under both atheroprotective and atherogenic shear stress levels. However, Piezo1 contribution to endothelial senescence was limited to the soft substrate and atheroprotective shear stress level. Overall, our study characterizes the response of endothelial cells to the combined effect of shear stress and substrate stiffness and reveals a previously unidentified role of Piezo1 in endothelial response to vessel stiffening, which potentially can be therapeutically targeted to alleviate endothelial dysfunction in aging adults.

Functional remodeling of an electrospun polydimethylsiloxane-based polyether urethane external vein graft support device in an ovine model

Saphenous vein graft (SVG) failure rates are unacceptably high, and external mechanical support may improve patency. We studied the histologic remodeling of a conformal, electrospun, polydimethylsiloxane-based polyether urethane external support device for SVGs and evaluated graft structural evolution in adult sheep to 2 years. All sheep (N = 19) survived to their intended timepoints, and angiography showed device-treated SVG geometric stability over time (30, 90, 180, 365, or 730 days), with an aggregated graft patency rate of 92%. There was minimal inflammation associated with the device material at all timepoints. By 180 days, treated SVG remodeling was characterized by minimal/nonprogressive intimal hyperplasia; polymer fragmentation and integration; as well as the development of a neointima, and a confluent endothelium. By 1-year, the graft developed a media-like layer by remodeling the neointima, and elastic fibers formed well-defined structures that subtended the neo-medial layer of the remodeled SVG. Immunohistochemistry showed that this neo-media was populated with smooth muscle cells, and the intima was lined with endothelial cells. These data suggest that treated SVGs were structurally remodeled by 180 days, and developed arterial-like features by 1 year, which continued to mature to 2 years. Device-treated SVGs remodeled into arterial-like conduits with stable long-term performance as arterial grafts in adult sheep.

Vascular Adaptation: Pattern Formation and Cross Validation between an Agent Based Model and a Dynamical System

Myocardial infarction is the global leading cause of mortality (Go et al., 2014). Coronary artery occlusion is its main etiology and it is commonly treated by Coronary Artery Bypass Graft (CABG) surgery (Wilson et al, 2007). The long-term outcome remains unsatisfactory (Benedetto, 2016) as the graft faces the phenomenon of restenosis during the post-surgery, which consists of re-occlusion of the lumen and usually requires secondary intervention even within one year after the initial surgery (Harskamp, 2013). In this work, we propose an extensive study of the restenosis phenomenon by implementing two mathematical models previously developed by our group: a heuristic Dynamical System (DS) (Garbey and Berceli, 2013), and a stochastic Agent Based Model (ABM) (Garbey et al., 2015). With an extensive use of the ABM, we retrieved the pattern formations of the cellular events that mainly lead the restenosis, especially focusing on mitosis in intima, caused by alteration in shear stress, and mitosis in media, fostered by alteration in wall tension. A deep understanding of the elements at the base of the restenosis is indeed crucial in order to improve the final outcome of vein graft bypass. We also turned the ABM closer to the physiological reality by abating its original assumption of circumferential symmetry. This allowed us to finely replicate the trigger event of the restenosis, i.e. the loss of the endothelium in the early stage of the post-surgical follow up (Roubos et al., 1995) and to simulate the encroachment of the lumen in a fashion aligned with histological evidences (Owens et al., 2015). Finally, we cross-validated the two models by creating an accurate matching procedure. In this way we added the degree of accuracy given by the ABM to a simplified model (DS) that can serve as powerful predictive tool for the clinic.

Perivascular medical devices and drug delivery systems: Making the right choices

Perivascular medical devices and perivascular drug delivery systems are conceived for local application around a blood vessel during open vascular surgery. These systems provide mechanical support and/or pharmacological activity for the prevention of intimal hyperplasia following vessel injury. Despite abundant reports in the literature and numerous clinical trials, no efficient perivascular treatment is available. In this review, the existing perivascular medical devices and perivascular drug delivery systems, such as polymeric gels, meshes, sheaths, wraps, matrices, and metal meshes, are jointly evaluated. The key criteria for the design of an ideal perivascular system are identified. Perivascular treatments should have mechanical specifications that ensure system localization, prolonged retention and adequate vascular constriction. From the data gathered, it appears that a drug is necessary to increase the efficacy of these systems. As such, the release kinetics of pharmacological agents should match the development of the pathology. A successful perivascular system must combine these optimized pharmacological and mechanical properties to be efficient.

Rationale and practical techniques for mouse models of early vein graft adaptations

Mouse models serve as relatively new yet powerful research tools to study intimal hyperplasia and wall remodeling of vein bypass graft failure. Several model variations have been reported in the past decade. However, the approach demands thoughtful preparation, selected sophisticated equipment, microsurgical technical expertise, advanced tissue processing, and data acquisition. This review compares several described models and aims (building on our personal experiences) to practically aid the investigators who want to utilize mouse models of vein graft failure.

Biomechanics of coronary artery and bypass graft disease: potential new approaches

The contribution of biomechanical factors to the incidence and distribution of coronary artery and bypass graft disease is underrecognized. This review examined the literature to determine which factors were relevant and the evidence for their importance. It identified two primary biomechanical factors that predispose to disease: (1) low-wall shear stress and (2) high-wall mechanical stress or strain. A range of secondary biomechanical factors have also been identified and include: vessel geometry; vessel movement; vessel wall characteristics and the presence of reflection waves. Potential surgical approaches for minimizing these effects are discussed.

Human saphenous vein organ culture under controlled hemodynamic conditions

INTRODUCTION

Saphenous vein grafting is still widely used to revascularize ischemic myocardium. The effectiveness of this procedure is limited by neointima formation and accelerated atherosclerosis, which frequently leads to graft occlusion. A better understanding of this process is important to clarify the mechanisms of vein graft disease and to aid in the formulation of strategies for prevention and/or therapeutics.

OBJECTIVE

To develop an ex vivo flow system that allows for controlled hemodynamics in order to mimic arterial and venous conditions.

METHODS

Human saphenous veins were cultured either under venous (flow: 5 ml/min) or arterial hemodynamic conditions (flow: 50 ml/min, pressure: 80 mmHg) for 1-, 2- and 4-day periods. Cell viability, cell density and apoptosis were compared before and after these intervals using MTT, Hoeschst 33258 stain, and TUNEL assays, respectively.

RESULTS

Fresh excised tissue segments were well preserved prior to the study. Hoechst 33258 and MTT stains showed progressive losses in cell density and cell viability in veins cultured under arterial hemodynamic conditions from 1 to 4 days, while no alterations were observed in veins cultured under venous conditions. Although the cell density from 1-day cultured veins under arterial conditions was similar to that of freshly excised veins, the TUNEL assay indicated that most of these cells were undergoing apoptosis.

CONCLUSION

The results observed resemble the events taking place during early in vivo arterial-vein grafting and provide evidence that an ex vivo perfusion system may be useful for the identification of new therapeutic targets that ameliorate vein graft remodeling and increase graft patency over time.

Transient elastic support for vein grafts using a constricting microfibrillar polymer wrap

Arterial vein grafts (AVGs) often fail due to intimal hyperplasia, thrombosis, or accelerated atherosclerosis. Various approaches have been proposed to address AVG failure, including delivery of temporary mechanical support, many of which could be facilitated by perivascular placement of a biodegradable polymer wrap. The purpose of this work was to demonstrate that a polymer wrap can be applied to vein segments without compromising viability/function, and to demonstrate one potential application, i.e., gradually imposing the mid-wall circumferential wall stress (CWS) in wrapped veins exposed to arterial levels of pressure. Poly(ester urethane)urea, collagen, and elastin were combined in solution, and then electrospun onto freshly-excised porcine internal jugular vein segments. Tissue viability was assessed via Live/Dead staining for necrosis, and vasomotor challenge with epinephrine and sodium nitroprusside for functionality. Wrapped vein segments were also perfused for 24h within an ex vivo vascular perfusion system under arterial conditions (pressure = 120/80 mmHg; flow = 100 mL/min), and CWS was calculated every hour. Our results showed that the electrospinning process had no deleterious effects on tissue viability, and that the mid-wall CWS vs. time profile could be dictated through the composition and degradation of the electrospun wrap. This may have important clinical applications by enabling the engineering of an improved AVG.

Shear stress regulates aquaporin-5 and airway epithelial barrier function

As the interface with the outside world, the airway epithelial barrier is critical to lung defense. Because of respiratory efforts, the airways are exposed to shear stress; however, little is known regarding the effects of shear on epithelial function. We report that low-level shear stress enhances epithelial barrier function, an effect that requires serial activation of the transient receptor potential vanilloid (TRPV) 4 and L-type voltage-gated calcium channel (VGCC) and an increase in intracellular calcium. These changes lead to a selective decrease in aquaporin-5 (AQP5) abundance because of protein internalization and degradation. To determine whether AQP5 plays a role in mediating the shear effects on paracellular permeability, we overexpressed hAQP5 in 16HBE cells, an airway epithelial cell line without endogenous AQP5. We found that AQP5 expression was needed for shear-induced barrier enhancement. These findings have direct relevance to the regulation of epithelial barrier function, membrane permeability, and water homeostasis in the respiratory epithelia.

TGF-beta- and CTGF-mediated fibroblast recruitment influences early outward vein graft remodeling

Luminal shearing forces have been shown to impact both geometric remodeling and the development of intimal hyperplasia. Less well studied is the influence of intramural wall stresses on vessel growth and adaptation. Using a vein graft-fistula configuration to isolate the impact of circumferential wall stress, we identify the reorganization of adventitial myofibroblasts as the dominant histological event that limits early outward remodeling of vein grafts in response to elevated wall stress. We hypothesize that increased production of transforming growth factor-beta (TGF-beta) and connective tissue growth factor (CTGF) induces recruitment of myofibroblasts, promotes adventitial reorganization, and limits early outward remodeling in response to increased intramural wall stress. Vein grafts with a distal arteriovenous fistula in the neck of rabbits were constructed, resulting in a fourfold differential in circumferential wall stress. Using this model, we demonstrate 1) elevated wall stress augments the production of TGF-beta and CTGF, 2) increased TGF-beta expression and CTGF expression are correlated with the enhanced differentiation from fibroblasts to myofibroblasts, as evidenced by the significant increase in the alpha-actin-positive cells in adventitia, and 3) the levels of TGF-beta, CTGF, and alpha-actin are inversely correlated with the magnitude of outward remodeling of the graft wall. Increased wall stress after vein graft implantation appears to induce a TGF-beta- and CTGF-mediated recruitment of adventitial fibroblasts and a conversion to a myofibroblast phenotype. Although important in the maintenance of wall stability in the face of an increased mechanical load, this adventitial adaptation limits early outward remodeling of the vein conduit and may prove deleterious in maintaining long-term vein graft patency.

Phosphatase PTEN is inactivated in bovine aortic endothelial cells exposed to cyclic strain

Hemodynamic forces, including cyclic strain (CS) and shear stress (SS), have been recognized as important modulators of vascular cell morphology and function. PTEN (also known as MMAC1/TEP1) is a lipid phosphatase that leads to a decrease in intracellular phosphatidylinositol 3,4,5 trisphosphate (PIP3) and therefore can modulate the stimulating effect of phosphatidylinositol 3-kinase (PI3K). In this study, we focused on the upstream regulators of the PI3K-Akt pathway by assessing Akt, PTEN, casein kinase 2 (CK2) (a kinase that catalyzes phosphorylation of PTEN), and PI3K activity in endothelial cells (EC) exposed to CS. The activity of phospho-PTEN (n = 4) and phospho-CK2 (n = 4) increased in a time-dependent fashion, reaching maximal activity by 10 min of CS stimulation. The peak of phospho-Akt activity (n = 4) occurred later, at 60 min. Akt activity was altered by transfection of EC with dominant negative PTEN plasmids. Furthermore, CS increased PIP3 immunoreactivity in a time-dependent manner, reaching maximal activity after 60 min of CS stimulation, and these effects were affected by transfection of EC with dominant negative PTEN plasmids. Inhibition of PTEN activity had no effect on CS-mediated cell proliferation but inhibited CS-mediated suppression of apoptosis.

Differential effects of imatinib on PDGF-induced proliferation and PDGF receptor signaling in human arterial and venous smooth muscle cells

Platelet-derived growth factor (PDGF) has been implicated in smooth muscle cell (SMC) proliferation, a key event in the development of myointimal hyperplasia in vascular grafts. Recent evidence suggests that the PDGF receptor (PDGFR) tyrosine kinase inhibitor, imatinib, can prevent arterial proliferative diseases. Because hyperplasia is far more common at the venous anastomosis than the arterial anastomosis in vascular grafts, we investigated whether imatinib also inhibited venous SMC (VSMC) proliferation, and examined possible differences in its mechanism of action between VSMC and arterial SMC (ASMC). Human ASMC and VSMC were stimulated with PDGF-AB, in the presence or absence of imatinib (0.1-10 microM). Proliferation was assayed using the 5-bromo-2'-deoxyuridine (BrdU) incorporation assay, while PDGFR, Akt and ERK1/2-mitogen activated protein kinase (MAPK) signaling pathways were investigated by immunoblotting. The proliferative response to PDGF at 50 and 100 ng/ml was 32 and 43% greater, respectively, in VSMC than in ASMC. Similarly, PDGF-stimulated proliferation was more sensitive to inhibition by imatinib in VSMC than ASMC (IC(50) = 0.05 microM vs. 0.4 microM; P < 0.01). Imatinib also more effectively inhibited PDGF-induced phosphorylation of PDGFRbeta and Akt in VSMC, compared to ASMC. These data highlight inherent pharmacodynamic differences between VSMC and ASMC in receptor and cell signaling functions and suggest that imatinib therapy may be useful for the prevention of venous stenosis in vascular grafts.

Wall shear modulation of cytokines in early vein grafts

OBJECTIVE

Pro-inflammatory cytokine-driven mechanisms have been implicated in vein graft failure, though little is known about the effect of hemodynamic factors and anti-inflammatory counter-regulatory mechanisms. We hypothesized that early temporal expression of the pro-inflammatory cytokine interleukin (IL)-1 beta and the anti-inflammatory cytokine IL-10 proceeds by way of wall shear stress-dependent pathways in the arterializing vein graft.

METHODS

Rabbits (n = 27) underwent bilateral jugular vein carotid interposition grafts, and simultaneous unilateral distal carotid branch ligation, to produce both low-flow and high-flow grafts in the same animal. Vein grafts were harvested at 1, 3, 7, 14, and 28 days and were assessed for architecture, wall shear stress, and cytokine messenger RNA levels (quantitative real-time two-step reverse transcription polymerase chain reaction).

RESULTS

The model resulted in an immediate 90% flow reduction (P <.001, paired t test) in the vein graft on the ligated side, and a 36% increase (P =.01) in contralateral graft flow. This persisted as approximately 15-fold flow differential throughout the 28-day period. The construction yielded a 15-fold differential in wall shear stress between low-flow and high-flow vein grafts (P <.001, two-way repeated measures analysis of variance). Intimal hyperplasia began by day 3, and was 6-fold more in low wall shear grafts by 28 days (230.6 +/- 35.4 microm intimal thickness vs 36.1 +/- 17.6 microm for low shear versus high shear grafts; P =.001). For both cytokines time independently affected mRNA expression (P <.001, global analysis of variance). Exposure of vein grafts to the arterial circulation markedly up-regulated IL-1 beta at 1 day, with significantly more induction in the low shear setting (P =.002). IL-1 beta protein localized to the developing neointima at days 1 and 3. Conversely, IL-10 slowly increased until day 14, with significantly more expression in the high shear grafts (P <.001).

CONCLUSIONS

Vein graft adaptation induces early pro-inflammatory cytokine IL-1 beta expression and delayed protective IL-10 expression (most notable under high shear conditions), both of which are modulated by wall shear. These differential temporal windows offer strategies for appropriately timed pro-inflammatory or anti-inflammatory therapies to interrupt pathologic vein graft adaptations.

CLINICAL RELEVANCE

Neointimal hyperplasia continues to limit the durability of vein bypass grafts. Emerging evidence suggests that inflammatory mechanisms drive the neointimal hyperplasic response. This study demonstrates that specific hemodynamic forces (altered wall shear stress) differentially affect early pro-inflammatory interleukin (IL)-1 beta and delayed anti-inflammatory IL-10 signaling. These distinct temporal windows for IL-1 beta and IL-10 cytokine expression offer strategies for appropriately timed pro-inflammatory and anti-inflammatory therapies to interrupt pathologic vein graft adaptations.

Altered expression of vascular endothelial growth factor and its receptors in normal saphenous vein and in arterialized and stenotic vein grafts

BACKGROUND

Myointimal thickening is a major cause saphenous vein graft failure. The prominence of medial and adventitial microvessels in stenotic vein grafts and the known angiogenic effects of vascular endothelial growth factor (VEGF) lead us to investigate the expression of VEGF and its receptors in vein graft arterialization and stenosis.

METHODS

Normal and arterialized vein graft segments were evaluated by reverse transcription-polymerase chain reaction (RT-PCR) for expression of VEGF-R1 (flt), VEGF-R2 (KDR), and neuropilin-1. The cells expressing VEGF, VEGF-R1, VEGF-R2, and neuropilin-1 were identified in normal, stenotic, and arterialized vein graft segments by immunohistochemistry.

RESULTS

Vascular endothelial growth factor, detected in the wall in endothelial cells and adventitial microvessels in normal vein, localized to smooth muscle cells, endothelial cells and adventitial microvessels in arterialized and stenotic vein. VEGF-R1 and VEGF-R2 were expressed infrequently on endothelial cells, macrophages, and smooth muscle cells in arterialized and stenotic vein. Neuropilin-1 was detected in all specimens. RT-PCR demonstrated significantly greater expression of neuropilin-1 in normal vein compared with arterialized vein (P <0.05).

CONCLUSIONS

The differential expression of VEGF and its receptors in normal, arterialized, and stenotic vein grafts suggests that alterations in VEGF/VEGF-R2/neuropilin-1 interactions may be important determinants of the adaptive response of vein grafts to arterialization.

Oscillatory shear stress increases smooth muscle cell proliferation and Akt phosphorylation

PURPOSE

Hemodynamic forces affect smooth muscle cell (SMC) proliferation and migration both in vitro and in vivo. However, the effects of oscillatory shear stress (SS) on SMC proliferation and signal transduction pathways that control survival are not well described.

METHODS

Bovine aortic SMC were exposed to arterial levels of oscillatory SS (14 dyne/cm(2)) with an orbital shaker; control cells were exposed to static conditions (0 dyne/cm(2)). Cell number and (3)[H]thymidine incorporation were measured after 1, 3, or 5 days of SS. Activation of the Akt pathway was assessed with the Western blot technique. Specificity of the phosphatidylinositol 3-kinase (PI3K) pathway was determined with the Western blot technique with the inhibitors LY294002 (10 micromol/L) or wortmannin (25 nmol/L).

RESULTS

Arterial levels of oscillatory SS increased SMC cell number by 20.1 +/- 3.7% and (3)[H]thymidine incorporation by 33.4% +/- 6.8% at 5 days. To identify whether SS increased activity of the SMC survival pathway, Akt activation was measured. SMC exposed to SS demonstrated increased Akt phosphorylation compared with control cells, with maximal phosphorylation at 60 minutes. Both PI3K inhibitors specifically inhibited the increase in Akt phosphorylation in SMC exposed to oscillatory SS.

CONCLUSION

SMC directly respond to oscillatory SS by increasing DNA synthesis, proliferation, and activation of the PI3K-Akt signal transduction pathway. These results suggest a mechanism of SMC survival and proliferation in response to endothelial-denuding arterial injury.

Shear stress-mediated cytoskeletal remodeling and cortactin translocation in pulmonary endothelial cells

Hemodynamic forces in the form of shear stress (SS) and mechanical strain imposed by circulating blood are recognized factors involved in the control of systemic endothelial cell (EC) cytoskeletal structure and function. However, the effects of acute SS on pulmonary endothelium have not been precisely characterized, nor the mechanism of rapid SS-induced EC cytoskeletal rearrangement understood. We exposed bovine and human pulmonary EC monolayers to laminar SS (10 dynes/cm2) in a parallel plate flow chamber and observed increased actin stress fiber formation 15 min after application of flow. Acute SS-induced pronounced cortical cytoskeletal rearrangement characterized by myosin light chain kinase (MLCK)- and Rho-associated kinase (RhoK)-dependent accumulation of diphosphorylated regulatory myosin light chains (MLC) in the cortical actin ring, junctional protein tyrosine phosphorylation, and transient peripheral translocation of cortactin, an actin-binding protein involved in the regulation of actin polymerization. SS-induced cortactin translocation was independent of Erk-1,2 MAP kinase, p60(Src), MLCK, or RhoK activities, and unaffected by overexpression of a cortactin mutant lacking four major p60(Src) phosphorylation sites. However, both SS-induced transient cortactin translocation and cytoskeletal reorientation in response to sustained (24 h) SS was abolished in cells overexpressing either dominant negative Rac 1 or a dominant negative construct of its downstream target, p21-activated kinase (PAK)-1. Our results suggest a potential role for cortactin in the SS-induced EC cortical cytoskeletal remodeling and demonstrate a novel mechanism of Rac GTPase-dependent regulation of the pulmonary endothelial cytoskeleton by SS.

APOC3, CETP, fibrinogen, and MTHFR are genetic determinants of carotid intima-media thickness in healthy men (the Stanislas Cohort)

The purpose of this study was to examine the relationship between carotid intima-media thickness (CIMT) inter-individual variability and 16 polymorphisms of 11 genes associated with cardiovascular risk factors (genes among lipid and homocysteine metabolisms, blood viscosity, platelet aggregation, leukocyte adhesion and renin-angiotensin system). CIMT was measured by high resolution B-mode ultrasonography in an healthy population of 77 men and 84 women, aged 35-54 years and selected from a French Cohort: the Stanislas Cohort. The polymorphisms studied were genotyped by a multilocus approach. Statistical analyses were carried out by ANOVA, after adjustment of CIMT for age, body mass index, and smoking, and by multiple regression analyses. No association was found with APOB Thr71Ile, APOC3 -482C/T, -455T/C, GpIIIa P1A, AT1R 1166A/C, AGT Met235Thr, CBS Ile278Thr, SELE 98G/T, and SELE Ser128Arg, polymorphisms neither in men nor in women. Although, in women we did not find any association for APOC3 3206T/G, 3175C/G, 1100C/T, CETP Ile405Val, MTHFR 677C/T and fibrinogen -455G/A polymorphisms; in men these polymorphisms were associated with CIMT variability (p< or =0.01; p< or =0.05). The most interesting finding was that altogether these genes in men were able to explain a considerable part, 20.6%, of CIMT variability. Therefore, our study gives a new opportunity to understand CIMT variability.

Systemic biochemical effects of inhaled NO in rats: increased expressions of NOS III, nitrotyrosine-, and phosphotyrosine-immunoreactive proteins in liver and kidney tissues

Inhaled nitric oxide (iNO) has been shown to reduce pulmonary hypertension associated with several disease states. The effects of iNO are thought to be restricted to the pulmonary vasculature because of its rapid inactivation by hemoglobin. Recent data have suggested, however, that iNO can form nitrosothiols, which can be carried throughout the circulation, thus increasing the half life and bioactivity on NO. Other studies have shown that iNO can affect intestinal ischemia and renal hemodynamics. In this study, rats were exposed to 49 +/- 4 ppm or 107 +/- 13 ppm NO for 4 h and the lung, spleen, liver, and kidney tissues were removed and measured for NOS II and NOS III protein, nitrotyrosine (NT), and phosphotyrosine (PT) immunoreactivity. Following 107 ppm iNO, increases in NOS III protein expression, NT, and PT were observed in the liver and kidney, but not in the lung or spleen. No such increases were noted after the lower dose of iNO. These results paralleled those shown for isobutyl nitrite that we reported earlier and indicated that iNO can cause changes in protein chemistry in organs and tissues beyond the lungs. Since iNO produced little systemic hemodynamic effects, it is unlikely that the observed biochemical alterations were derived secondarily from physiological changes.

The effects of extremely low shear stress on cellular proliferation and neointimal thickening in the failing bypass graft

OBJECTIVE

Previous studies demonstrating a correlation between low shear stress (tau = 5-15 dyne/cm(2)) and experimental vein graft neointimal thickening (NIT) support the role of low tau in vein graft failure. However, a simple linear relationship between low tau and NIT would underestimate the degree of NIT evident in high-grade occlusive lesions of failing human vein grafts. In this study we used a new experimental model that maintains patency at low tau (< 2 dyne/cm(2)), to delineate possible deviations from linearity in the low tau --> NIT hypothesis.

METHODS

Thirty-two New Zealand White rabbits underwent creation of a common carotid vein patch with a segment of ipsilateral external jugular vein. Very low tau was created in 13 patches by ligation of the distal common carotid artery, leaving the only outflow through a small muscular branch. Normal tau was created in 11 patches by leaving the common carotid artery outflow intact. High tau was created in eight patches by ligation of the contralateral common carotid artery. Six patches were harvested after 2 weeks for measurement of cell cycle entry by proliferating cell nuclear antigen (PCNA) immunohistochemistry. The remaining 26 patches were harvested after 4 weeks, perfusion fixed, and excised for morphometric analysis.

RESULTS

Mean blood flow and tau at implantation ranged from 0.5 to 41 mL/min and 0.07 to 15 dyne/cm(2), respectively. At the time of harvest, 30 of 32 patches remained patent, and the artificially created aberrations in blood flow were maintained (range, 0.7-41 mL/min). After 2 weeks PCNA immunohistochemistry showed a significantly higher level of cell cycling in patches exposed to low tau (40 +/- 5 vs 1.6 +/- 0.3 PCNA-positive cells per high-power field; P <.001), which is equivalent to approximately 20% of the total cells present. In patches harvested after 4 weeks, NIT ranged from 42 to 328 microm and significantly correlated with mean tau at implantation. Patches with very low tau exhibited histologic characteristics similar to those of failing human bypass grafts, including laminar thrombus and flow-limiting luminal stenosis. The relationship between tau and NIT was nonlinear in that extremely low tau (< 2 dyne/cm(2)) resulted in NIT beyond that predicted by a simple linear correlation (P =.003).

CONCLUSION

Extremely low tau (< 2 dyne/cm(2)) stimulates high rates of smooth muscle cellular proliferation in arterialized vein patches. NIT is accelerated in these regions of low tau far beyond that predicted by a simple linear model. The nonlinear nature of the cellular proliferative response and NIT at tau less than 2 dyne/cm(2) may explain the rapid progression of neointimal lesions in failing bypass grafts.

The effect of proximal artery flow on the hemodynamics at the distal anastomosis of a vascular bypass graft: computational study

The formation of distal anastomotic intimal hyperplasia (IH), one common mode of bypass graft failure, has been shown to occur in the areas of disturbed flow particular to this site. The nature of theflow in the segment of artery proximal to the distal anastomosis varies from case to case depending on the clinical situation presented. A partial stenosis of a bypassed arterial segment may allow residual prograde flow through the proximal artery entering the distal anastomosis of the graft. A complete stenosis may allow for zero flow in the proximal artery segment or retrograde flow due to the presence of small collateral vessels upstream. Although a number of investigations on the hemodynamics at the distal anastomosis of an end-to-side bypass graft have been conducted, there has not been a uniform treatment of the proximal artery flow condition. As a result, direct comparison of results from study to study may not be appropriate. The purpose of this work was to perform a three-dimensional computational investigation to study the effect of the proximal artery flow condition (i.e., prograde, zero, and retrograde flow) on the hemodynamics at the distal end-to-side anastomosis. We used the finite volume method to solve the full Navier-Stokes equations for steady flow through an idealized geometry of the distal anastomosis. We calculated the flow field and local wall shear stress (WSS) and WSS gradient (WSSG) everywhere in the domain. We also calculated the severity parameter (SP), a quantification of hemodynamic variation, at the anastomosis. Our model showed a marked difference in both the magnitude and spatial distribution of WSS and WSSG. For example, the maximum WSS magnitude on the floor of the artery proximal to the anastomosis for the prograde and zero flow cases is 1.8 and 3.9 dynes/cm2, respectively, while it is increased to 10.3 dynes/cm2 in the retrograde flow case. Similarly, the maximum value of WSSG magnitude on thefloor of the artery proximal to the anastomosis for the prograde flow case is 4.9 dynes/cm3, while it is increased to 13.6 and 24.2 dynes/cm3, respectively, in the zero and retrograde flow cases. The value of SP is highest for the retrograde flow case (13.7 dynes/cm3) and 8.1 and 12.1 percent lower than this for the prograde (12.6 dynes/cm3) and zero (12.0 dynes/cm3) flow cases, respectively. Our model results suggest that the flow condition in the proximal artery is an important determinant of the hemodynamics at the distal anastomosis of end-to-side vascular bypass grafts. Because hemodynamic forces affect the response of vascular endothelial cells, the flow situation in the proximal artery may affect IH formation and, therefore, long-term graft patency. Since surgeons have some control over the flow condition in the proximal artery, results from this study could help determine which flow condition is clinically optimal.

Compliance in Anastomoses With and Without Vein Cuff Interposition

OBJECTIVE

to compare anastomotic compliance in end-to-side anastomoses with and without vein cuff interposition. Materials polytetrafluoroethylene graft to bovine carotid artery without (standard) and with vein interposition (Linton-patch and Miller-cuff).

METHODS

zonewise compliance measurement of end-to-side anastomoses in an in-vitro circulation system. The zone most distal to the suture-line served as reference compliance.

RESULTS

directly distal to the suture-line the compliance of the Linton-patch (5.6+/-1.6%/100 mmHg) and Miller-cuff anastomosis (5.2+/-1.1%/100 mmHg) more closely approached reference compliance (standard: 5.0+/-1.2, Linton-patch: 4.5+/-1.5, Miller-cuff: 4.9+/-1.0%/100 mmHg) than that of the standard anastomosis (7.9+/-3.0%/100 mmHg). The maximal compliance values of the Linton-patch (9.5+/-2.3%/100 mmHg) and Miller-cuff anastomoses (9.8+/-2.7%/100 mmHg) were significantly higher than that of the standard end-to-side anastomosis (7.9+/-3.0%/100 mmHg). However, maximal compliance was shifted from the zone directly distal to the suture line in the standard end-to-side anastomosis, to the vein cuff interposition in the Linton-patch and Miller-cuff anastomoses.

CONCLUSION

the shift in maximal compliance to the wider portion of the anastomosis in the Miller-cuff and Linton-patch anastomoses may obviate reocclusion.

Vein adaptation to arterialization in an experimental model

PURPOSE

The events preceding myointimal thickening in vein grafts after vascular reconstructions are not well characterized. Indeed, the injury response associated with vein graft arterialization may be different than that observed in the balloon angioplasty model. Therefore, we used a rat model to study the early cellular response after arterialization of vein grafts.

METHODS

Epigastric veins were placed as femoral artery interposition grafts in 37 male Lewis rats (weight range, 350-400 g). Vein grafts and contralateral epigastric veins were harvested at different time points (6 hours, 1 day, 2 days, 3 days, 7 days, 14 days, 21 days, 30 days, and 70 days). Tissue specimens were processed for histology and immunohistochemistry with antibodies for the proliferating cell nuclear antigen (PCNA) and for different cell types. Terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling (TUNEL) assay was used as a means of determining the presence of apoptosis. Electron microscopy was used as means of assessing the integrity of the endothelial cell surface (SEM) and confirming the presence of apoptosis (TEM). Specimens were also snap frozen in liquid nitrogen for RNA isolation and molecular analysis.

RESULTS

At 1 day, endothelial denudation with platelet deposition on the surface was shown by means of SEM. Both apoptosis and necrosis of smooth muscle cells (SMCs) were present in the media, along with monocyte infiltration. Cellular proliferation and apoptosis were most intense within the first week of implantation. PCNA staining was first seen in the adventitial fibroblasts and microvessels, then in the medial SMCs at 3 days. With reverse transcriptase polymerase chain reaction, upregulation of vascular endothelial growth factor (VEGF) messenger RNA (mRNA) was noted at 1 day. Myointimal thickening progressively developed, with no apparent diminution of the luminal area as long as 70 days after implantation. By means of the analysis of the transforming growth factor beta1, mRNA showed expression during intimal thickening and accumulation of extracellular matrix. Reendothelialization was complete at 30 days.

CONCLUSIONS

These observations indicate that the cellular composition in our vein graft model is similar to human stenotic explants. Endothelial denudation is observed in rat vein grafts with complete regeneration by 30 days. VEGF mRNA is upregulated at 1 day, followed by proliferation of microvessel endothelial cells in the adventitia. Cellular proliferation and apoptosis are minimal after 21 days, with progressive intimal thickening likely to be the result of matrix accumulation.

Endothelial cell response to different mechanical forces

PURPOSE

Endothelial cells (ECs) are subjected to the physical forces induced by blood flow. The aim of this study was to directly compare the EC signaling pathway in response to cyclic strain and shear stress in cultured bovine aortic ECs.

MATERIALS AND METHODS

The ECs were seeded on flexible collagen I-coated silicone membranes to examine the effect of cyclic strain. The membranes were deformed with a 150-mm Hg vacuum at a rate of 60 cycle/min for up to 120 minutes. For a comparison of the effect of shear stress, ECs from the same batch as used in the strain experiments were seeded on collagen I-coated silicone sheets. The ECs were then subjected to 10 dyne/cm(2) shear with the use of a parallel flow chamber for up to 120 minutes. Activation of the mitogen- activated protein kinases was assessed by determining phosphorylation of extracellular signal-regulated kinase (ERK), c-jun N-terminal kinase (JNK), and p38 with immunoblotting.

RESULTS

ERK, JNK, and p38 were activated by both cyclic strain and shear stress. Both cyclic strain and shear stress activated JNK with a similar temporal pattern and magnitude and a peak at 30 minutes. However, shear stress induced a more robust and rapid activation of ERK and p38, compared with cyclic strain.

CONCLUSIONS

Our results indicate that different mechanical forces induced differential activation of mitogen-activated protein kinases. This suggests that there may be different mechanoreceptors in ECs to detect the different forces or alternative coupling pathways from a single receptor.

Reduction of aortic wall motion inhibits hypertension-mediated experimental atherosclerosis

Hypertension is a well-known risk factor for coronary artery disease and carotid and lower extremity occlusive disease. Surgically induced hypertension in hypercholesterolemic animals results in increased aortic wall motion and increased plaque formation. We tested the hypothesis that reduction in aortic wall motion, despite continued hypertension, could reduce plaque formation. New Zealand White rabbits (n=26) underwent thoracic aortic banding to induce hypertension and were fed an atherogenic diet for 3 weeks. In 13 rabbits, a segment of aorta proximal to an aortic band was externally wrapped to reduce wall motion. All animals were fed an atherogenic diet for 3 weeks. Four groups were studied: 1, coarctation control (no wrap, n=7); 2, coarctation with loose wrap (n=6); 3, coarctation with firm wrap (n=7); and 4, control (noncoarcted, n=6). Wall motion, blood pressure, and pulse pressure were measured at standard reference sites proximal and distal to the coarctation by use of intravascular ultrasound. Quantitative morphometry was used to measure intimal plaque. Mean arterial pressure and cyclic aortic wall motion were equally increased proximal to the aortic coarctation in all 3 coarcted rabbit groups compared with the control group (P:<0.001). Wall motion in the segment of aorta under the loose and firm wraps was no different from the control value. The external wrap significantly reduced intimal thickening in the 4 groups by the following amounts: group 1, 0.30+/-0.03 mm(2); group 2, 0.06+/-0.02 mm(2); group 3, 0. 04+/-0.02 mm(2); and group 4, 0.01+/-0.01 mm(2) (P:<0.001). Localized inhibition of aortic wall motion in the lesion-prone hypertensive aorta resulted in significant reduction in intimal plaque formation. These data suggest that arterial wall cyclic motion may stimulate cellular proliferation and lipid uptake in experimental atherosclerosis.