Functional abnormalities of experimental autogenous vein graft neoendothelium

Loss of Tachyphylaxis and Increased Sensitivity to Angiotensin II in Experimental Vein Grants

Intimal hyperplasia and atherosclerosis have been implicated in the pathophysiology of vein graft failure. Several recent studies have also reported alterations in the vasoreactivity of vein grafts. These alterations in vasoreactivity could contribute to vein graft spasm and lead to graft occlusion. This study examined the vasomotor responses of experimental vein grafts to angiotensin II, the most potent natural vasoconstrictor known. The right carotid artery was divided and bypassed in 12 rabbits with use of the right external jugular vein. The left external jugular vein was used as a control.

Eight vein grafts and jugular veins were harvested after fourteen days and 4 vein grafts after twenty-eight days. Segments of vein graft and control vein were mounted under isometric tension in an organ bath, and the dose response curves to angiotensin II obtained. On day 14, the response of the jugular veins was triphasic, while the respnse of the vein grafts was sigmoidal. The vein grafts were hypersensitive to angiotensin II. The ED50 was reduced from 8.4 ± 2.5 x 10-6 M in the jugular veins to 1.62 ± 0.24 x 10-8 M in the vein grafts (p < 0.005). The maximal response to angiotensin II was also increased from 342 ± 24 mg in the jugular veins to 558 ± 108 mg in the vein grafts on day 14 (p < 0.05). There was no significant difference in either the ED50 or the maximal response of the vein grafts on day 14 or 28. Tachyphylaxis (desensitization with repeated doses of agonist) was observed in the jugular veins but not in the vein grafts. The results show that experimental vein grafts are hypersensitive to angiotensin II, with either single or repeated exposure. This increased vasoreactivity to angiotensin II may have important clinical implications, particularly when vein grafts are used in patients with renovascular hypertension.

The Relationship of Human Saphenous Vein Smooth Muscle Contractile Responses to Donor Age and Gender

The smooth muscle contractile properties of 100 human saphenous veins derived from healthy tissue and organ donors were investigated. No significant changes in the contractile responses of saphenous veins to potassium chloride, serotonin, and norepinephrine were associated with age. In contrast, comparison of contractile responses between males and females demonstrated significantly less response to potassium chloride in female veins (P<0.05). Female contractile responses to serotonin (P=0.74) and norepinephrine (P= 0.29) did not differ significantly when compared with male responses. These observations support the conclusion that gender may be an important variable in saphenous vein biology.

The Relationship of Human Saphenous Vein Intimal Area to Age and Gender

The saphenous vein is an essential element in arterial bypass surgery. It is known to develop intimal thickening following transplantation to the arterial circulation. No data are available on the relationship of normal saphenous vein intimal area and the age or the gender of the patient. One hundred sixty-two consecutive, donated, cadaveric, midtibial samples of the long saphenous vein were harvested. The donors ranged in age from twelve to eighty-two years; there were 121 males and 41 females. The areas of the intima and media and the intimal ratio (intimal area/[intimal + medial area]) were calculated by computer videomorphometry. There was no correlation between the three variables and age, but there was a significant correlation between the gender of the donor and the intimal area (r2=0.25, P=0.0012) and the intimal ratio (r2=0.34, p = 0.0001) showing that females have a greater intimal area than males.

These findings suggest that gender should be considered as a variable in the choice of suitable control values in subsequent human studies on the development of intimal hyperplasia in venous bypass grafts.

Gene transfer of COX-1 improves lumen size and blood flow in carotid bypass grafts

BACKGROUND

In autologous saphenous vein grafts, prostacyclin (PGI(1)), a vasoprotective molecule produced by normal endothelial cells, is down-regulated compared with ungrafted saphenous veins and normal carotid arteries. Reduced PGI(2) synthesis may contribute to local platelet deposition, vascular smooth muscle cell (VSMC) accumulation, atherosclerosis, and ultimately failure of venous bypass grafts. We have examined whether gene transfer-mediated overexpression of COX-1 in grafted veins (1) increases PGI(2) and cyclic AMP (cAMP) production, (2) leads to vasodilation and improved local blood flow in the presence of hypercholesterolemia, and (3) reduces neointima formation.

MATERIALS AND METHODS

Jugular veins from New Zealand-White rabbits were incubated for 30 min ex vivo with 1 x 10(10) PFU/mL of an adenoviral vector encoding COX-1 (AdCOX-1; n = 10) or empty control (n = 10) and grafted to the carotid arteries. The rabbits were placed on a high-cholesterol diet for 4 w, and blood flow and histomorphometry of the grafts were assessed.

RESULTS

In the AdCOX-1 group, blood flow was significantly increased (16.0 +/- 3.3 versus 12.5 +/- 3.3 mL/min; P < 0.05) compared with controls, and luminal area (8.9 +/- 1.4 versus 5.3 +/- 1.2 mm(2); P < 0.01) and outer circumference were larger. In six identically treated rabbits, graft PGI(2) and cAMP synthesis was increased at 72 h in AdCOX-1 compared with controls.

CONCLUSION

Our data suggest a 30-min ex vivo exposure of vein grafts to AdCOX-1 increased local synthesis of PGI(2) and cAMP after graft surgery and resulted in better graft lumen and blood flow at 4 w.

The venous graft as an effector of early angiogenesis in a fibrin matrix

The arteriovenous loop (AV loop) model is gaining importance as a means of initiating and sustaining perfusion in tissue engineering constructs in vivo. This study represents an attempt to dissect the morphology of early arterialization and angiogenesis in the AV loop in a fibrin matrix with special focus on the interpositional venous graft (IVG) segment. An AV loop was constructed in 30 rats using the femoral vessels and an IVG. The AV loop was encased in an isolation chamber filled with a fibrin matrix. Evaluation methods included scanning electron microscopy (SEM) of corrosion casts, immune histology and micro magnetic resonance angiography (MRA). Direct luminal neovascular sprouting was evident between day 10 and day 14 from the vein and the IVG but not from the arterial segment. Arterialization of the IVG manifested itself on the corrosion casts as a gradual reduction in luminal caliber with onset after day 7. Microdissection of the microvascular replicas could demonstrate for the first time the presence of direct luminal sprouts from the IVG. MRA was used to display the shunt pattern of perfusion in the patent AV loop. From the three segments of the vascular axis in the AV loop the IVG is the most versatile for applications in the clinical as well as the experimental setting. Kinetics of angiogenesis warrant further investigation in the IVG.

iNOS Gene Transfer for Graft Disease

Vascular bypass surgery involves the use of a vascular conduit to circumvent a site of vascular compromise. Vascular graft failure continues to plague both the patients receiving and the surgeons performing these interventions. Demand for the development of a therapy to reduce intimal hyperplasia--the most common cause of bypass graft failure--is significant and has been the goal of many biotechnology groups. The development of gene therapy as a feasible clinical intervention has allowed for novel methods of inhibiting intimal hyperplasia to be conceived. This review describes the evolution of gene transfer of the inducible nitric oxide synthase (iNOS) gene, one of the most successful preclinical interventions to date for vascular disease.

Novel Strategies for the Prevention of Bypass Graft Failure

Bypass vein graft disease remains a significant limitation to the care of millions of patients with ischemic disease of the heart and lower extremities. The pathogenesis of this rapid, aggressive, occlusive disease lies in the remodeling response of the grafts themselves to the new arterial environment. As such, the molecular and cellular biology of neointimal hyperplasia provides a unique opportunity for cardiovascular researchers to more closely model a human clinical entity from its inception to the development of advanced disease. Recent years have therefore seen a broad new array of possible interventions for vein graft disease based on a sophisticated translation of genetic and molecular science. One of these applications, E2F decoys, has already progressed to phase III clinical studies, and many others will likely follow as the tools for therapeutic translation continue to improve. These include both gene transfer and gene blockade strategies.

Tissue engineering skin flaps: which vascular carrier, arteriovenous shunt loop or arteriovenous bundle, has more potential for angiogenesis and tissue generation?

This study was designed to clarify which vascular carrier, the arteriovenous shunt loop or the arteriovenous bundle, has more potential as a vascular carrier for an artificial skin flap in rats. An arteriovenous shunt loop was constructed between the femoral artery and vein using an interpositional artery (group I) or vein (group II) graft. For arteriovenous bundle groups, the femoral artery and vein were used and subdivided into two groups: distal ligation type (group III) and flow-through type (group IV). The vascular pedicle was wrapped with an artificial dermis and implanted beneath the inguinal skin for 4 weeks. For the control group, a folded sheet of artificial dermis without any vascular carrier was embedded. In experiment 1, the volumes of generated tissue within the artificial dermis were measured in the experimental and control groups (n = 5 in each group). In experiment 2, the origin of new blood vessels sprouting from the arteriovenous shunt loop and arteriovenous bundle were evaluated histologically. The volume of generated tissue in the shunt groups was significantly greater than that in the bundle groups (p < 0.01). However, the bundle groups also showed a great potential for producing new tissue. Serial histological studies showed that new capillaries were derived not only from the vasa vasorum of the femoral vessels but directly from the femoral vein in both the shunt and the bundle groups. This "sprouting" was extensively exhibited in the group III. Although the arteriovenous shunt loop showed a greater potential for producing new tissue and capillaries, the distal ligation type of bundle was thought to be an effective and practical vascular carrier for producing a tissue-engineered skin flap.

Circulating progenitor cells regenerate endothelium of vein graft atherosclerosis, which is diminished in ApoE-deficient mice

Previously we showed that a large number of endothelial cells in vein grafts undergo apoptosis or necrosis during the first few days followed by endothelial regeneration. In the present study, we investigated endothelial cell death and regeneration in vein grafts using transgenic mice carrying LacZ genes driven by an endothelial TIE2 promoter. When a vein fragment from TIE2-LacZ was isografted into the carotid artery of wild-type mice, the number of beta-gal+ cells were reduced at 3 days and disappeared completely by 4 weeks after grafting. Conversely, beta-gal+ cells were observed on the surface of vein segments donated by wild-type mice isografted into TIE2-LacZ mice at 1 week and reached confluence by 4 weeks, suggesting recipient origins of endothelial cells. Interestingly, beta-gal+ cells were evenly distributed on the surface of the whole vein segment grafted into TIE2-LacZ mice, indicating a contribution of circulating progenitor cells. When wild-type veins were grafted into a chimeric mouse carrying TIE2-LacZ genes in bone marrow cells, a proportion of cells displayed a beta-gal+ staining. Furthermore, the number of CD34+ and Flk+ progenitor cells in blood of apoE-deficient mice were significantly lower than those of wild-type controls, which coincided with diminished beta-gal+ endothelial cells on the surface of vein grafts in TIE2-LacZ/apoE-/- mice. Thus, we provide the first evidence that endothelial cells of vein grafts are derived from circulating progenitor cells, of which one-third are derived from bone marrow progenitor cells. Hyperlipidemia due to apoE deficiency results in a lower number of endothelial progenitors in blood and correlated with enhanced atherosclerosis. The full text of this article is available online at http://www.circresaha.org.

Endothelial healing in vein grafts: proliferative burst unimpaired by genetic therapy of neointimal disease

BACKGROUND

Although inhibition of neointimal hyperplasia by cell cycle gene blockade therapy results in improved endothelial cell function in experimental vein grafts, little is known either about endothelial healing immediately after vein grafting or about the effect of this therapy on the healing process.

METHODS AND RESULTS

Scanning electron microscopy demonstrated an immediate decrease in vein graft endothelial cell density associated with vein graft wall stretch, followed by a return to baseline by postoperative day 3. En face detection of bromodeoxyuridine incorporation confirmed a rapid endothelial proliferation by 48 hours. Despite inhibition of underlying vascular smooth muscle cell proliferation, E2F decoy oligonucleotide did not inhibit either endothelial bromodeoxyuridine incorporation or the return to baseline cell density. This differential response to E2F decoy was also observed in human umbilical vein endothelial cell culture, which resisted the E2F decoy inhibition of cell growth that was observed in human umbilical artery smooth muscle cells, despite evidence for nuclear localized delivery of the oligonucleotide into both cell types. Furthermore, the reduction of E2F binding activity seen in a nuclear gel shift assay of cultured smooth muscle cells was not observed in endothelial cells.

CONCLUSIONS

These results suggest a burst of graft endothelial cell proliferation that allows a rapid restoration of cell density in the monolayer. Additionally, there is a selective effect of E2F decoy gene therapy on target smooth muscle cells with sparing of this endothelial healing.

Ex-vivo gene therapy of human vascular bypass grafts with E2F decoy: the PREVENT single-centre, randomised, controlled trial

BACKGROUND

Cell-cycle blockade by ex-vivo gene therapy of experimental vein grafts inhibits the neointimal hyperplasia and subsequent accelerated atherosclerosis that lead to human bypass-graft failure. In a prospective, randomised, controlled trial, we investigated the safety and biological efficacy of intraoperative gene therapy in patients receiving bypass vein grafts.

METHODS

We studied gene therapy that uses decoy oligodeoxynucleotide, which binds and inactivates the pivotal cell-cycle transcription factor E2F. 41 patients were randomly assigned untreated (16), E2F-decoy-treated (17), or scrambled-oligodeoxynucleotide-treated (eight) human infrainguinal vein grafts. Oligonucleotide was delivered to grafts intraoperatively by ex-vivo pressure-mediated transfection. The primary endpoints were safety and inhibition of target cell-cycle regulatory genes and of DNA synthesis in the grafts. Analysis was by intention to treat.

FINDINGS

Mean transfection efficiency was 89.0% (SD 1.9). Proliferating-cell nuclear antigen and c-myc mRNA concentrations and bromodeoxyuridine incorporation were decreased in the EF2-decoy group by medians of 73% [IQR 53-84], 70% [50-79], and 74% [56-83], respectively) but not in the scrambled-oligodeoxynucleotide group (p<0.0001). Groups did not differ for postoperative complication rates. At 12 months, fewer graft occlusions, revisions, or critical stenoses were seen in the E2F-decoy group than in the untreated group (hazard ratio 0.34 [95% CI 0.12-0.99]).

INTERPRETATION

Intraoperative transfection of human bypass vein grafts with E2F-decoy oligodeoxynucleotide is safe, feasible, and can achieve sequence-specific inhibition of cell-cycle gene expression and DNA replication. Application of this genetic-engineering strategy may lower failure rates of human primary bypass vein grafting.

Differential action of angiotensin II and activity of angiotensin-converting enzyme in human bypass grafts

OBJECTIVE

The activity of the renin-angiotensin system may be important in determining the performance of coronary artery bypass grafts. We have examined the activity of tissue angiotensin-converting enzyme and the effects of angiotensin II in vessels used as bypass grafts.

METHODS

Organ bath studies were used to determine the vasoactive effect of angiotensin II. The activity of the angiotensin-converting enzyme was assessed by metabolism of a specific synthetic substrate.

RESULTS

The saphenous vein produced greater maximum responses to angiotensin II than did the internal thoracic artery. This response was not modified by inhibition of nitric oxide synthase, cyclooxygenase, or by an endothelin receptor antagonist in either vessel. Losartan, an AT1 receptor antagonist, inhibited the vasoconstrictor response in both blood vessels. Homogenates of saphenous vein and internal thoracic artery displayed tissue angiotensin-converting enzyme activity, which was inhibited by captopril. Enzyme activity was threefold greater in the vein. Both the contractile response to angiotensin II and the enzyme activity were retained in venous grafts removed up to 20 years after coronary bypass surgery.

CONCLUSIONS

These data demonstrate that marked differences exist in angiotensin-converting enzyme activity and AT1 receptor responses in the saphenous vein compared with the internal thoracic artery. These findings may have important implications for the performance of the vein when used as a coronary artery bypass graft and may have clinical implications for the use of angiotensin-converting inhibitors and AT1 receptor antagonists in the prevention and treatment of vein graft disease.

The effects of potassium channel openers on saphenous vein exposed to arterial flow

OBJECTIVES

To assess the sensitivity of saphenous vein to potassium channel opening drugs (KCOs).

METHODS

Saphenous vein, harvested at bypass surgery or high ligation for correction of varicose veins, was exposed to an in vitro flow circuit and vasomotor responses assessed by organ bath pharmacology.

OUTCOME MEASURES

Effective drug concentrations for 50% reduction in vein ring tension (IC50).

RESULTS

Vein rings pre-contracted with phenylephrine showed a concentration-dependent relaxation to all the KCOs tested with a potency ranking of HOE 234 > cromakalim > pinacidil > diazoxide. The relaxation to cromakalim was endothelium-independent and was inhibited by glibenclamide (an ATP-sensitive K+ channel blocker). The sensitivity of vein rings to cromakalim increased after exposure to arterial flow conditions for 90 minutes (IC50 before 1.7 +/- 0.25 microM and after 0.25 +/- 0.08 microM, p > 0.001). This effect was not evident after 90 min of venous flow conditions, 2.19 +/- 0.49 microM. When the workload on vein, exposed to arterial flow conditions, was reduced mechanically by external stenting with PTFE the increased sensitivity to cromakalim was abolished.

CONCLUSIONS

Saphenous vein has ATP-sensitive K+ channels responsive to KCOs. The increased sensitivity to cromakalim, induced by arterial flow conditions, may represent an endogenous protective mechanism limiting ischaemic damage resulting from the higher workload imposed on grafted vein.

Reduction of vein graft intimal hyperplasia and preservation of endothelium-dependent relaxation by topical vascular endothelial growth factor

PURPOSE

Recent evidence suggests that vascular endothelial growth factor (VEGF), in addition to stimulating angiogenesis, also serves a repair/maintenance or survival function, modulating various aspects of endothelial cell function. This study was designed to examine the effect of VEGF pretreatment in a model of vein graft intimal hyperplasia.

METHODS

Reversed jugular vein-to-common carotid artery interposition grafts were constructed in New Zealand White rabbits. White rabbits. Vein conduits were immersed in solution containing 500 micrograms rhVEGF165 or saline solution for 20 minutes before implantation. Twenty-eight days later the vein grafts and contralateral control jugular veins were harvested for either histologic or isometric tension studies.

RESULTS

VEGF-treated vein grafts showed a 23% reduction in intimal area (0.76 +/- 0.07 mm2 vs 0.98 +/- 0.06 mm2; p = 0.028) and a 30% reduction in intimal thickness (62 +/- 6 microns vs 89 +/- 5 microns; p = 0.001) when compared with control grafts. After precontraction with norepinephrine, the maximal relaxation to acetylcholine (endothelium-dependent, receptor-mediated agonist) for control vein grafts was 0%, whereas for VEGF-treated vein grafts it was 25% +/- 9% (p < 0.05 vs control grafts). The maximal relaxation to the calcium ionophore A23187 (endothelium-dependent, receptor-independent agonist) was also greater in VEGF-treated grafts than in control grafts (172.3% +/- 19.4% vs 122.5% +/- 13.7%; p < 0.05). There was no difference in the response to sodium nitroprusside (endothelium-independent agonist) between the two groups.

CONCLUSIONS

A single topical application of VEGF before implantation reduces intimal hyperplasia and improves endothelial function in a rabbit vein graft model. Further evaluation of this simple strategy to improve vein graft patency appears warranted.

Hemagglutinating virus of Japan-liposome-mediated gene transfer of endothelial cell nitric oxide synthase inhibits intimal hyperplasia of canine vein grafts under conditions of poor runoff

PURPOSE

Late graft failure is a critical problem, particularly in the presence of poor runoff vessels. Intimal hyperplasia is considered to be the main cause of graft failure. We have already reported that intimal thickening of experimental vein grafts in dogs with poor runoff vessels is more pronounced than that in dogs with normal vessels. We and others also have reported that production of nitric oxide (NO) in the endothelium of canine vein grafts is impaired. In the present study, we asked whether in vivo gene transfer of endothelial cell NO synthase (ecNOS) would inhibit intimal hyperplasia of autogenous vein grafts implanted in limbs with poor distal runoff in dogs.

METHODS

After exposing femoral veins, the nuclear-targeted lac Zgene, bovine ecNOS cDNA, or control vector plasmid encapsulated in the hemagglutinating virus of Japan-liposomes was infused intraluminally, followed by incubation for 10 minutes at room temperature under a distending pressure of 100 mm Hg. Twenty reversed vein grafts were implanted under normal runoff conditions, and 4 days later these were used to confirm gene transfer to the vein grafts. Twelve reversed vein grafts were implanted under conditions of poor runoff, and 4 weeks after the operation intimal thickening was evident.

RESULTS

In vein grafts under normal runoff conditions, lac Z gene transfer exhibited diffuse and frequent X-Gal-positive signals in both medial and adventitial layers 4 days after implantation (n = 3). In case of the ecNOS gene-transferred vein grafts, bovine ecNOS protein was mainly detected in medial smooth muscle cells and adventitial cells 4 days after implantation, determined using immunohistochemical techniques and bovine ecNOS specific antibody (n = 3). In addition, ecNOS-transferred vessels showed intense purple signals by reduced nicotinamide adenine dinucleotide phosphate diaphorase and nitroblue tetrazolium reaction, in both medial and adventitial layers, whereas weak NOS activity was recognized at the adventitial vasa vasorum of the untreated veins or control vector transferred veins (n = 3, respectively). In vein grafts under poor runoff conditions, the intimal thickness at 4 weeks after implantation was significantly reduced by ecNOS gene transfer (n = 4; 90.0 +/- 7.6 microns and 1.18 +/- 0.07 mm2) in comparison with buffer-treated vessels (n = 4; 195.8 +/- 25.7 microns and 2.62 +/- 0.48 mm2) or vector vehicle-treated vessels (n = 4; 193.0 +/- 15.8 microns and 2.65 +/- 0.22 mm2).

CONCLUSIONS

Our findings show that gene transfer of ecNOS inhibited intimal hyperplasia of canine vein grafts caused by poor runoff conditions, as a result of an increased local production of NO. Thus ecNOS gene transfer warrants further study as a possible approach to prevent late graft failure.

Acceleration of impairment of endothelium-dependent responses under poor runoff conditions in canine autogenous vein grafts

OBJECTIVES

To assess the effects of changes in shear stress on endothelium-dependent responses.

MATERIALS AND METHODS

Autologous vein grafts were implanted in poor or normal distal runoff limbs of 10 mongrel dogs. Six weeks after grafting the vein grafts were removed, cut into rings, and suspended in organ chambers for isometric tension recording.

RESULTS

The average value of intimal thickening was 110.7 +/- 45.2 microns in poor runoff limbs and 65.5 +/- 27.9 microns in control limbs, respectively. There was a significant difference between the two groups. Acetylcholine caused comparable endothelium-independent contractions in both groups. In the control group, adenosine diphosphate, thrombin and A23187 caused endothelium-dependent relaxations. In the poor runoff group, the endothelium-dependent relaxations caused by adenosine diphosphate and thrombin were impaired, while A23187 caused comparable endothelium-dependent relaxations. Direct relaxations in response to sodium nitroprusside were comparable between the two groups.

CONCLUSIONS

This dysfunction of the endothelium under conditions of abnormal flow may accelerate intimal thickening of the vein graft and result in late graft failure.

Altered endothelial and smooth muscle cell reactivity caused by University of Wisconsin preservation solution in human saphenous vein

PURPOSE

We have investigated the effect of University of Wisconsin (UW) solution at different temperatures on endothelial and smooth muscle cell function of the human saphenous vein to define the efficacy of UW solution as a preservation solution for saphenous vein conduits.

METHODS

Saphenous vein segments from 38 patients undergoing coronary artery bypass surgery were examined with an isolated organ bath technique to monitor changes in vessel reactivity.

RESULTS

Endothelial-dependent relaxations to acetylcholine were attenuated after incubation in UW solution at both 4 degrees C and 28 degrees C (p < 0.05, n = 10). In contrast, relaxations to sodium nitroprusside were unchanged after incubation in UW solution at both temperatures (n = 8). The responses to 90 mmol/L KCl were increased at both 4 degrees C and 28 degrees C, respectively. Tyrode's: 27.2% +/- 3.1% and 23.8% +/- 3.0%, UW: 64.7% +/- 8.0% and 73.1% +/- 11% (p < 0.001). In addition, the responses to 5-HT were enhanced at 4 degrees C and 28 degrees C (p < 0.05). In contrast, responses to noradrenaline were enhanced only at 28 degrees C compared with the responses after incubations in Tyrode's solution (p < 0.05, n = 6). Furthermore U46619 (0.3 nmol/L to 1 nmol/L) responses were augmented at 4 degrees C (p < 0.05, n = 7). The potency (pD2) values for each agonist were not significantly different after incubations in UW solution.

CONCLUSIONS

We conclude that UW solution produces attenuation of acetylcholine relaxations and temperature-dependent increased reactivity of smooth-muscle cell function in the isolated human saphenous vein. These studies document the complex interactions brought about by UW solution on the different components of the vascular wall that need to be elucidated further if this solution is to attain a place in vascular preservation.

Cell cycle inhibition preserves endothelial function in genetically engineered rabbit vein grafts

We have recently shown that ex vivo gene therapy of rabbit autologous vein grafts with antisense oligodeoxynucleotides (AS ODN) blocking cell cycle regulatory gene expression inhibits not only neointimal hyperplasia, but also diet-induced, accelerated graft atherosclerosis. We observed that these grafts remained free of macrophage invasion and foam cell deposition. Since endothelial dysfunction plays an important role in vascular disease, the current study examined the effect of this genetic engineering strategy on graft endothelial function and its potential relationship to the engineered vessels' resistance to atherosclerosis. Rabbit vein grafts transfected with AS ODN against proliferating cell nuclear antigen (PCNA) and cell division cycle 2 (cdc2) kinase elaborated significantly more nitric oxide and exhibited greater vasorelaxation to both calcium ionophore and acetylcholine than did untreated or control ODN-treated grafts. This preservation of endothelial function was associated with a reduction in superoxide radical generation, vascular cell adhesion molecule-1 (VCAM-1) expression, and monocyte binding activity in grafts in both normal and hypercholesterolemic rabbits. Our data demonstrate that AS ODN arrest of vascular cell cycle progression results in the preservation of normal endothelial phenotype and function, thereby influencing the biology of the vessel wall towards a reduction of its susceptibility to occlusive disease.

Endothelial cell injury in cardiovascular surgery: the intimal hyperplastic response

Arteries and veins respond to injury by a healing process that includes the development of a neointima. This response to injury is implicated as the primary cause of failure after arterial reconstruction. Because it is an integrator and transmitter of blood flow variations, inflammation, and growth stimuli, the endothelium is a potent regulator of long-term arterial wall mass changes. The contribution of the endothelium to intimal development depends on the type of arterial conduit. In arteries, the growth of the intima stops when the endothelium has regrown. In synthetic grafts, the endothelium stabilizes intimal growth. Hence, the mere presence of endothelial cells can influence intimal changes in arterial conduits. Understanding endothelial biology should help us define methods to prevent cell proliferation, extracellular matrix accumulation, intimal hyperplasia, and vessel narrowing.

Short-term preservation of autogenous vein grafts: effectiveness of University of Wisconsin solution

BACKGROUND

Suboptimal preservation of autologous veins in storage solutions causes endothelial cell damage that can contribute to graft failure. The purpose of this study was to compare the effects of short-term storage of veins in autologous whole blood (AWB), 0.9% normal saline solution (NS), and University of Wisconsin solution (UWs) on vein structure and function after grafting.

METHODS

Autogenous jugular and femoral veins were atraumatically harvested from mongrel dogs. One vein segment was immediately implanted to serve as a control, and the other segments were stored for 45 minutes in AWB, NS, or UWs. The veins were implanted as reversed interposition graft in the carotid or femoral arteries. After 6 weeks light and scanning electron microscopy and isometric tension studies were performed on explanted vein grafts.

RESULTS

Morphologic studies revealed an intact endothelium that stained positively for factor VIII. Intimal thickness was similar between controls (48 +/- 12 microns) and veins stored in UWs (53 +/- 8 microns) (p = not significant), but it was significantly increased in veins stored in AWB (151 +/- 29 microns) and NS (149 +/- 18 microns) (p < 0.05). Sensitivity and maximum contraction to norepinephrine were not altered in veins preserved in UWs (6.0 +/- 0.1 mumol/L and 0.19 +/- 0.02 gm/mm2) but were significantly reduced (p < 0.05) in those stored in AWB (7.2 +/- 0.1 mumol/L and 0.08 +/- 0.02 gm/mm2) and NS (7.0 +/- 0.3 mumol/L and 0.09 +/- 0.02 gm/mm2) compared with controls (5.9 +/- 0.2 mumol/L and 0.20 +/- 0.02 gm/mm2). The sensitivity and maximum relaxation to acetylcholine and sodium nitroprusside of veins preserved in AWB, NS, and UWs were similar to controls (p = not significant).

CONCLUSIONS

Vein storage in UWs preserves smooth muscle cell function compared with veins stored in NS or AWB. Therefore UWs is a more suitable medium for short-term preservation of veins in cardiovascular operation.

Acute reductions in blood flow and shear stress induce platelet-derived growth factor-A expression in baboon prosthetic grafts

Abrupt reductions in fluid shear stress induce subendothelial smooth muscle cells (SMCs) to proliferate in experimental prosthetic grafts. Platelet-derived growth factor (PDGF), an important SMC mitogen, is expressed by cultured endothelial cells and modulated by shear stress. We hypothesized that this growth factor would be modulated by changes in shear stress in vivo. Bilateral aortoiliac prosthetic grafts were implanted into five baboons. High flow was generated by construction of femoral arteriovenous fistulas on both sides. Two months later, one of the fistulas was ligated, reducing shear stress in the upstream graft by 78 +/- 6%. Four days after fistula ligation, all grafts were removed and analyzed. As previously reported, SMC proliferation in low-flow grafts exceeded that in high-flow grafts, although the neointimal area was similar. mRNA levels for PDGF-A were significantly increased in low-flow grafts compared with high-flow grafts. In situ hybridization and immunohistochemical studies localized the increased PDGF-A mRNA and protein to the luminal endothelium and subjacent SMCs. Abrupt reductions in blood flow and fluid shear stress may induce accelerated neointimal thickening by a PDGF-A-mediated mechanism, since endothelial expression of this gene is temporally and anatomically associated with neointimal SMC proliferation.

A comparative study of endothelium-derived relaxing factor-mediated relaxation and smooth muscle cell function in arterial and venous vein bypass grafts

The development of intimal hyperplasia in reversed vein grafts is associated with altered endothelial and vasomotor function. This study examines the effect of surgery on the morphology and vasomotor function of experimental arterial and venous vein bypass grafts. Twelve reversed vein grafts, 12 in situ vein grafts and 12 venovenous grafts were placed in 24 New Zealand White rabbits. All grafts remained patent and were harvested after 28 days. Isometric contraction to norepinephrine, histamine, bradykinin, serotonin and relaxation to acetylcholine and sodium nitroprusside following pre-contraction with prostaglandin F(2 alpha) were determined on the grafts and on contralateral jugular veins. Compared to the contralateral jugular veins, norepinephrine supersensitivity was induced in the reversed vein grafts, and venovenous vein grafts but not in the in situ vein grafts. Decrease in histamine sensitivity occurred in all grafted vessels. Bradykinin responses were significantly reduced in the in situ vein grafts and reversed vein grafts. Contractile responses to serotonin developed in the in situ vein grafts and reversed vein grafts only. Acetylcholine-induced endothelium-derived relaxing factor-mediated relaxation of the contralateral jugular veins was preserved in both venovenous grafts and in situ vein grafts but was lost in reversed vein grafts. All tissues relaxed to sodium nitroprusside in dose-dependent manner. The data suggest that norepinephrine supersensitivity in reversed vein grafts results from excision of the vessel. Attenuation of bradykinin responses and the enhanced contractile responses to serotonin appear predominantly to result from arterialization. Decreases in histamine sensitivity appear related both to excision and to arterialization. Neither the excision of the vein nor arterialization individually influences the alterations in endothelium-derived relaxing factor-mediated relaxation. However, a combination of excision and arterialization results in the altered endothelium-derived relaxing factor-mediated relaxation. This study suggests that the surgical preparation of the vein and the surgical procedure used have significantly different effects on endothelium-derived relaxing factor-mediated relaxation and smooth muscle contractility in vein grafts.

Effects of pulsatile perfusion on human saphenous vein vasoreactivity: a preliminary report

This study examined the effects of exposure to arterial blood pressure and flow on human saphenous vein catecholamine sensitivity. Unused portions of saphenous vein from eight patients undergoing peripheral bypass procedures were mounted parallel in a specially designed organ culture apparatus and perfused with tissue culture medium with 95% CO(2) at 37 degrees C. One segment was fixed between two cannulas while the medium was gently agitated (control) and the other was actively perfused via a pulsatile pump system at a rate of 60 beats/min, peak pressure of 100 mmHg and peak flow of 200 ml/min (pulsed; mean pressure 60 mmHg; mean flow 115 ml/min). After 48 h, vein segments were removed and tested for in vitro isometric contraction in response to KCI, norepinephrine and histamine, and relaxation in response to acetylcholine, calcium ionophore A23187, and sodium nitroprusside. There were no differences in mean(s.e.m.) maximal contraction in response to KCI (control 0.61(0.16) g versus pulsed 0.72(0.27)g; P = n.s.), norepinephrine (control 1.00(0.56) g versus pulsed 1.51(0.54) g; P= n.s.), or histamine (control 1.47(0.85) g versus pulsed 1.95(0.64) g; P= n.s.). However, pulsed veins exhibited increased sensitivity to both norepinephrine (control -logED50 6.20(0.23) versus pulsed mean(s.e.m.) 6.60(0.17); P< 0.05) and histamine (control -logED(50) 5.60(0.27) versus pulsed 6.24(0.20); P = 0.05). Pulsed veins exhibited slightly less acetylcholine-induced relaxation although the difference did not reach statistical significance (control mean(s.e.m.) relaxation at 1 x 10(6)M 9.2(14.0)% versus pulsed -13.3(6.4)%; P = n.s.). There were no differences in relaxation in response to either A23187 (control 1 x 10-(4)M 178(19)% versus pulsed 191(68)% or sodium nitroprusside (control 225(15)% versus pulsed 254(17)%; P = n.s.). The data presented herein indicate that exposure of human saphenous vein to the hemodynamics of the arterial environment for 48 h results in catecholamine supersensitivity while contractile and relaxant function are not affected.

Comparison of endothelial function between in situ and reversed vein graft: differences in endothelium-dependent responses

BACKGROUND

The endothelium releases endothelium-derived relaxing factors that are potent vasodilators and inhibitors of platelet aggregation. Experiments were performed to determine whether the endothelium-dependent responses differed between in situ and reversed vein grafts. The influence of valve disruption or of dissection of the adventitia was also examined.

METHODS

Segments of canine jugular veins were grafted into the carotid arteries during procedures such as reversed grafting, in situ grafting with valve disruption, in situ grafting without valve disruption, and in situ grafting with dissection of adventitia. After 4 weeks the endothelium-dependent responses of the grafts were examined by isometric tension recording.

RESULTS

In the reversed and in situ vein graft with valve disruption, acetylcholine caused endothelium-independent contractions, whereas in the in situ vein graft without valve disruption acetylcholine-induced endothelium-dependent relaxations were preserved. Adenosine diphosphate (ADP) caused comparable endothelium-dependent relaxations in the in situ vein graft irrespective of valve disruption. In the reversed vein graft ADP-induced relaxations were significantly impaired. In the in situ vein graft with dissection of the adventitia, relaxations in response to acetylcholine and ADP were significantly reduced.

CONCLUSIONS

These results suggest that endothelial function, in terms of endothelium-derived relaxing factors in the in situ vein graft, can be preserved and that adventitial dissection in the in situ vein graft should be minimized to preserve endothelial function.

Ramipril and experimental vein graft intimal hyperplasia

Angiotensin-converting enzyme (ACE) inhibitors have been shown to reduce the intimal proliferation in animal models of arterial angioplasty and vein bypass grafting. This study examines the effect of high-dose ramipril, an ACE inhibitor that does not contain a sulfhydryl group, on the development of intimal hyperplasia in experimental vein bypass grafts. Twenty New Zealand White rabbits underwent common carotid interposition bypass grafting. Twelve were treated with ramipril (2 mg/kg/day; po) five days prior to surgery and thereafter until harvest. The remaining 8 animals were used as controls. Vein grafts were harvested at twenty-eight days by pressure fixation (80 mmHg). The grafts were sectioned into proximal, middle, and distal thirds, and the thickness of the intima and the media and the area of the lumen from each segment were determined by videomorphometry. The effect of ramipril on the [H3]thymidine incorporation into DNA of serum-stimulated smooth muscle cells (culture passage 6 to 12) was also assessed. There was a 50% mortality rate in the rabbits that received ramipril, and this was assumed to be related to the high dose of the drug. Ramipril treatment reduced mean vein graft intimal area by 34% (P > 0.05), but this was accompanied by an increase of 73% in the mean medial area of the vein grafts as compared with controls. These changes resulted in a decrease in the mean intimal ratio (intima/[intima + media]) by 39% in the ramipril group as compared with controls. Ramipril did not inhibit [H3]thymidine incorporation into DNA of serum-stimulated smooth muscle cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic changes in blood flow alter endothelium-dependent responses in autogenous vein grafts in dogs

PURPOSE

Experiments were designed to determine the effects of blood flow on endothelium-dependent relaxations in canine vein grafts.

METHODS

Blood flow through reversed femoral vein grafts was either increased by a distal arteriovenous fistula (increased flow), unmanipulated (normal flow), or reduced by a proximal adjustable clamp (reduced flow). Six weeks after implantation, blood flow through the graft was measured. Rings cut from grafts were suspended for the measurement of isometric force in organ chambers to determine endothelial function.

RESULTS

Blood flow was significantly greater in grafts with a distal fistula compared to grafts with normal or decreased flow. Endothelium-dependent relaxations to acetylcholine were absent in all grafts. Endothelium-dependent relaxations to adenosine diphosphate, thrombin, and the calcium ionophore A23187 were less in grafts with reduced flow compared with grafts with increased flow. Relaxations to these agents in grafts with increased flow were reduced by an analog of L-arginine. Neointimal hyperplasia was increased in grafts with reduced flow.

CONCLUSIONS

These data demonstrate that chronic diminution of blood flow decreases receptor-mediated release of endothelium-derived relaxing factors and increases neointimal hyperplasia in canine vein grafts. The production of endothelium-derived relaxing factors, one of which is nitric oxide, may influence the development of myointimal hyperplasia in vein grafts.

Bradykinin receptor modulation in vein grafts

Following vein grafting, bradykinin B2 receptor-mediated responses in the vein graft are decreased and B1 receptor-mediated responses are increased. It is not known whether these changes in bradykinin receptor responses are due to nonspecific changes, local inflammation, or arterial hemodynamics. Four experimental tissue models were studied to discriminate among these possibilities. Twelve New Zealand White rabbits underwent excision of native vein (n = 4), external jugular veno-venous bypass (n = 4), or common carotid veno-arterial bypass (n = 4). Grafts and unoperated contralateral jugular veins (n = 4) were harvested at 28 days postoperatively. Isometric tension studies were performed on four rings from each vein and vein graft. The responses to bradykinin, a B2 agonist (BK: 10(-12) to 10(-5) M) and des-Arg9-bradykinin, a B1 agonist (DABK; 10(-12) to 10(-5) M) were recorded. Compared to native veins, the results show a progressive attenuation in BK sensitivity in contralateral veins, veno-venous grafts, and veno-arterial grafts, and the development of DABK sensitivity in the contralateral veins and both vein graft models. The alterations in B2 receptor-mediated responses appear therefore to be induced by a combination of the surgical procedure and arterialization, while changes in B1 receptor-mediated responses appear to be related solely to nonspecific sensitization of the cardiovascular tissue following the operative procedure. Thus, it appears from this study that there are two distinct mechanisms involved in the modulation of bradykinin receptors in vein grafts after implantation into the arterial circulation.

The expression and function of G-proteins in experimental intimal hyperplasia

G-proteins are membrane-bound signal transduction proteins which couple extracellular receptor signals to various effectors. This study examines the expression and the function of G-proteins (alpha i, alpha s, alpha q, and alpha o) in experimental intimal hyperplasia. Vein bypass grafts were placed in 30 New Zealand White rabbits and were harvested after 28 d. The contralateral jugular veins served as controls. Isometric tension studies were performed on rings from veins and vein grafts (n = 10), and Western blot and mRNA analyses were performed in another 20 vessels. There was a fivefold increase in alpha q, a 2.7-fold increase in the alpha i2, and a 3.3-fold increase in alpha s expressions in vein grafts compared with veins. Detectable expression of alpha i3 was observed in vein grafts but not in jugular veins. In addition, there was a 3.8-fold increase in beta subunits in the vein grafts compared with the veins. mRNA for alpha s, alpha i3, and alpha i2 were all elevated in the vein grafts. No detectable levels of the alpha i1 protein or its mRNA were present in either veins or vein grafts. Contractile responses in the veins were not inhibited by pertussis toxin. The contractile responses to norepinephrine were enhanced by twofold, and the responses to serotonin developed de novo in vein grafts compared with veins. The contractile responses to both norepinephrine and serotonin were only partially inhibited by pertussis toxin in the vein grafts even though there was 100% ADP ribosylation with pertussis toxin in both veins and vein grafts. These data suggest that intimal hyperplasia is associated with increased or novel expression of G-proteins in vivo which occur simultaneously with the development of pertussis toxin-sensitive contractile responses. Changes in G-proteins at a transcriptional level or at the level of RNA stability may be involved in the response of smooth muscle cells to injury and to intimal hyperplasia formation.

Long-term human vein graft contractility and morphology: a functional and histopathological study of retrieved coronary vein grafts

Vasoreactivity of 11 coronary artery vein bypass grafts and 13 human saphenous veins was examined. Isometric tension studies were performed in response to potassium chloride (110 mmol/l), noradrenaline (10(-9)-10(-4) mol/l), serotonin (10(-9)-10(-4) mol/l) and histamine (10(-8)-10(-2) mol/l). After precontraction with noradrenaline (10(-5) mol/l), the response to acetylcholine (10(-8)-10(-4) mol/l) and the calcium ionophore A23187 (10(-8)-10(-4) mol/l) was also assessed. Results are given as mean(s.e.m.). Compared with saphenous veins, vein grafts showed decreased sensitivity to noradrenaline (1.7(0.5) versus 0.4(0.1) mumol/l, P = 0.01), no change in sensitivity to serotonin (55(18) versus 37(15) mumol/l, P > 0.05) and supersensitivity to histamine (3.2(0.9) versus 30.1(13.2) mumol/l, P = 0.01). Vein grafts had a decreased maximal contraction to potassium chloride (1.1(0.3) versus 5.5(0.8) g, P = 0.0001), noradrenaline (1.2(0.3) versus 4.1(0.8) g, P = 0.005), histamine (1.2(0.3) versus 4.5(0.8) g, P = 0.003) and serotonin (0.7(0.2) versus 5.7(0.6) g, P = 0.0002) compared with saphenous vein. Precontracted vein grafts did not relax in response to acetylcholine; in contrast, saphenous vein relaxed in a dose-dependent manner to a maximal relaxation of 22(3) per cent. Both saphenous vein and vein graft relaxed in response to A23187. Vein graft intimal thickness was approximately fourfold greater than that of saphenous vein (540(110) versus 136(30) microns). Scanning electron microscopy of vein and vein graft revealed an intact endothelium. Coronary artery vein grafts are capable of responding to various contractile agonists; these response are notably different from those of saphenous vein and there is a loss of endothelium-dependent relaxation. Even at a late stage vein grafts are not inert but are functional conduits with an abnormally responsive endothelium and a less potent, but significantly altered, smooth muscle contractile profile.

Endothelialization of anastomoses in the aorta and inferior vena cava of the rat

Denuding endothelial damage at anastomoses may lead to thrombotic complications and failure of microvascular reconstructions. Confluent endothelial healing at anastomoses may reduce thrombotic and other complications. The progress of endothelial coverage of anastomotic structures in the aorta and inferior vena cava (IVC) of the rat was evaluated at 3, 5, 8, 11, and 14 days after completion of the anastomoses, by means of scanning electron microscopy. Arterial anastomotic patency was 100%; venous patency was 84%. It was found that endothelial healing progressed towards thin confluent coverage first, thereafter thickening. Coverage was first achieved at the needle defect, then the anastomotic junction, and lastly the suture. The same stage of endothelial coverage of anastomotic structures was reached 5 to 8 days sooner at the IVC anastomosis than in the aorta.

Infrainguinal vein graft stenosis

Although knowledge of the biological processes involved in the development of intimal hyperplasia has increased markedly in recent years, the precise aetiology of infrainguinal vein graft stenosis remains undetermined. Current therapy is therefore directed at treatment of the established lesion rather than its prevention. There seems little doubt, however, that recent advances in understanding of the vascular biology of normal and pathological saphenous vein will eventually lead to specific targeted therapy that will allow the prevention of vein graft stenosis.

Human saphenous vein grafts explanted from the arterial circulation demonstrate altered smooth-muscle and endothelial responses

PURPOSE

Animal models have been used to assess the function of vascular smooth muscle and endothelium of veins grafted into arterial circulation. The primary model consists of grafting the external jugular vein into the carotid artery of the rabbit. These studies suggest a selective increase in the responsiveness of the grafted veins to serotonin. However, in both human cardiac and peripheral vascular operations, the saphenous, not the jugular, is the vein most frequently used. Thus the propriety of the rabbit model is unknown.

METHODS

Human saphenous veins and vein grafts were obtained from patients undergoing leg vein bypass graft revisions (n = 8). The reversed vein grafts were placed into arterial circulation for periods ranging from 4 to 26 months before removal (mean 16 months). All vessels were immediately cut into rings and suspended in organ chambers for recording isometric contractions to norepinephrine and serotonin.

RESULTS

The maximal contractions elicited by both norepinephrine and serotonin were reduced in human vein grafts in comparison to the results in human saphenous vein (maximal response to norepinephrine 1.42 +/- 0.34 gm [vein graft] vs 4.59 +/- 1.13 gm [saphenous vein], p = 0.031; maximal response to serotonin 2.68 +/- 0.58 gm [vein graft] vs 4.72 +/- 1.11 gm [saphenous vein], p = 0.042). Human vein grafts were less responsive to norepinephrine than was saphenous vein (negative log of concentration that caused 50% of the maximal response -5.91 +/- 0.10 and -6.84 +/- 0.22, respectively; p < 0.009). After precontraction with norepinephrine (to 30% of the maximal response), saphenous vein, but not vein grafts, demonstrated endothelium-dependent relaxation to acetylcholine (maximum relaxation 27.4% +/- 6.8%; p = 0.001).

CONCLUSIONS

Human saphenous veins grafted into arterial circulation exhibit loss of endothelium-dependent relaxation to acetylcholine and diminished contractions to agonists (norepinephrine and serotonin). In contrast to rabbit data, serotonin elicits dose-dependent contractions in both human saphenous vein and human vein grafts. Since the vascular wall contractility varies widely across species, the relevance of rabbit vein graft data to human bypass grafts is uncertain.

Natural course of endothelium-dependent and -independent responses in autogenous femoral veins grafted into the arterial circulation of the dog

We examined the natural course of endothelium-dependent and -independent responses in reversed autogenous vein grafts during regeneration and tissue repair processes after vein grafting in dogs. Vein grafts implanted in the canine femoral artery were removed, cut into rings, and suspended in organ chambers for isometric tension recording at 3 days and 1, 2, 4, and 6 weeks after implantation. Endothelial cells were denuded from some rings. Control veins were taken from nonsurgically treated femoral veins. Acetylcholine caused endothelium-dependent relaxations in the control veins, whereas in the vein grafts there was no evidence of endothelium-dependent relaxations to acetylcholine 3 days after the operation. Acetylcholine caused endothelium-independent contractions throughout the study. The endothelium-dependent responses to ADP and calcium ionophore A23187 were constantly maintained. Three days after the operation, the amplitude of norepinephrine-induced contractions of the vein grafts was impaired, and at 1 week the amplitude was recovered, although it was significantly smaller than the amplitude of contractions of the control veins at any postoperative period. Endothelium-independent relaxations to sodium nitroprusside were maintained throughout the study. Thus, there was a selective loss of acetylcholine-mediated relaxation in vein grafts in the early postoperative stage, a time when the intima is not thickened. These altered responses in vein grafts in the early postoperative period may have a role in graft failure.

Endothelium derived relaxing factor is absent in experimental in situ vein grafts

Endothelium derived relaxing factor (EDRF) is a local modulator of vasomotor tone in most blood vessels. Reversed vein grafts in several animal models do not produce EDRF when stimulated, and have been shown to have altered responses to several vasoconstrictors. The preservation of EDRF production and vasomotor function in in situ vein grafts was examined in this study. In situ vein grafts of the right carotid artery were performed in five mongrel dogs using external jugular vein. The contralateral jugular vein was used as a control. The vessels were harvested after 6 weeks and rings from the central portion of each vessel studied in vitro in an organ bath. Following cumulative dose response curves to noradrenaline, the rings were precontracted with the estimated ED50 (50% effective dose) dose of noradrenaline, and cumulative doses of acetylcholine added to induce EDRF release. The vein grafts were hypersensitive to noradrenaline, the ED50 being reduced from 8.7 +/- 2.7 x 10(-7) M in the jugular veins to 1.7 +/- 0.7 x 10(-7) M in the vein grafts (p < 0.05). All of the jugular vein segments relaxed in response to acetylcholine with a mean maximal relaxation of 42 +/- 6% of precontraction. None of the vein graft segments relaxed in response to acetylcholine despite the presence of an intact endothelium on scanning electron microscopy, but did relax in response to calcium ionophore. The results indicate that in situ vein grafts, like reversed grafts, fail to produce EDRF in response to acetylcholine. The grafts are also hypersensitive to noradrenaline.(ABSTRACT TRUNCATED AT 250 WORDS)

Alteration of reactivity of native arteries induced by venous graft placement

Occlusion of aortocoronary venous grafts can be due to thrombosis, atherosclerosis, or vasospasm. Investigations have focused on properties of the graft itself, and little is known about the vascular reactivity and function of the native arteries proximal and distal to the vein graft, although spasm of the native artery distal to the graft site has been observed in patients. We hypothesized that the function of the endothelium of the native arteries may be altered after surgery. Autogenous venous grafts were placed in femoral arteries of rabbits to study the reactivity of the native arteries after grafting. Four weeks after graft implantation, the vein graft, ipsilateral vein, and native artery proximal and distal to the graft were removed for in vitro studies. Morphological evaluation by scanning electron microscopy and fluorescence microscopy after labeling with acetylated low density lipoprotein labeled with 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate indicated the presence of an intact, metabolically active endothelial layer. There was no alteration in the contractile responses to phenylephrine of the arteries, vein grafts, or veins. Precontracted vein grafts, veins, and arterial segments proximal to the grafts relaxed when exposed to endothelium-dependent vasodilators (acetylcholine, arachidonic acid, and substance P), but the native arteries distal to the grafts did not. In bioassay cascade experiments, the distal artery did not release any measurable relaxing factor when exposed to acetylcholine. We conclude that the endothelium of the distal artery did not function normally. The extent and reversibility of altered endothelial function remain to be determined. This observation may help to explain the occurrence of myocardial infarction after aortocoronary bypass grafting in some patients.

The endothelium: a key to the future

The vascular endothelium is a complex modulator of a variety of biological systems and may well be the key to definitive success in the treatment of cardiovascular disorders. Surgically-induced endothelial injury may occur preoperatively during cardiac catheterization and intraoperatively from mechanical manipulation, ischemia, hypothermia, and exposure to cardioplegic solutions. The normal endothelium is antithrombogenic and yet promotes platelet aggregation and coagulation if injured. Vasospasm, occlusive intimal hyperplasia, and accelerated arteriosclerosis can also all occur as a result of endothelial injury. Furthermore, endothelial injury is harmful even in the absence of disruption of its monolayer integrity. Thus, preservation of the endothelium should be an additional objective for all cardiovascular surgeons. Synthetic vascular grafts, cardiac valves, and artificial ventricles do not spontaneously endothelialize and thus usually require some form of anticoagulation to maintain patency. Hence, endothelialization of prosthetic implants became an attractive concept. A number of different methods of obtaining an endothelial lining of prosthetic material has since been developed; these include facilitated endothelial cell migration, and endothelial cell seeding by using either venous or microvascular endothelial cells. Manipulating the endothelium might well provide the next major advancement for therapeutic and preventive measures for cardiovascular disease.

Saphenous vein endothelium-dependent relaxation in patients with peripheral vascular disease

In vitro vasomotor responses of saphenous veins of 15 patients undergoing peripheral vascular bypass procedures were studied. Vessels were harvested by standard techniques, sectioned into 4 mm rings, and suspended in organ baths under isometric tension. Stimulation with cumulative doses of norepinephrine revealed a -logED50 of 6.85 +/- 0.12 M and maximal tension of 8.64 +/- 1.77 g. Patient characteristics suggesting high maximal response (by univariate analysis) included male sex (male 11.69 +/- 2.49 g versus female 5.08 +/- 1.69 g; p = 0.058). Intact and denuded rings were additionally tested for endothelium-dependent relaxation following submaximal norepinephrine precontraction. The vessels relaxed in response to acetylcholine (maximal relaxation 31.1 +/- 10.7% at 1 x 10(-6) M), calcium ionophore A23187 (85.3 +/- 11.8% at 1 x 10(-5) M), and sodium nitroprusside (150.8 +/- 15.2% at 1 x 10(-5) M), but only acetylcholine relaxation was completely endothelium-dependent. Calcium ionophore A23187 relaxation was partially dependent on the endothelium while sodium nitroprusside relaxation was entirely endothelium-independent. Negligible relaxation was observed in response to adenosine diphosphate (ADP) (12.1 +/- 12.8% at 1 x 10(-5) M) while histamine and serotonin caused additional contraction only. We concluded that, in patients undergoing vascular surgical procedures, the saphenous vein (1) demonstrates variable contractile function which appears to be greater in males following spinal anesthesia, and (2) exhibits moderate endothelium-dependent relaxation in response to acetylcholine and calcium ionophore A23187 but not to ADP, histamine, or serotonin.

Endothelium-derived relaxing factor and vein grafts

Platelet thrombosis, intimal hyperplasia and progression of atherosclerosis are the most important factors determining the patency of vein grafts for arterial occlusive disease. Interactions between aggregating platelets and the vessel wall play an important role in all of these processes. Recently, it has become evident that the endothelium modulates the underlying vascular smooth muscle by releasing an endothelium-derived relaxing factor, a potent vasodilator and anti-aggregating substance. Little information is available, however, on such endothelial functions of autogenous vein grafts. This review focuses on vascular modulation by endothelium-derived relaxing factor in vein grafts.

Endothelium-dependent responses in long-term human coronary artery bypass grafts

In the present study, responses of long-term human coronary artery bypass grafts (CABGs) to known endothelium-dependent vasodilators, acetylcholine, calcium ionophore A23187, thrombin, and histamine, as well as authentic nitric oxide, the putative endothelium-derived relaxing factor, were studied. Sixteen CAGBs were isolated within 1-2 hours from hearts of 14 patients receiving a cardiac transplant. A total of 109 ring segments were prepared from these CABGs and studied in vitro. The duration of the CABGs ranged from 7 months to 12 years. Addition of acetylcholine (0.01-10 microM), calcium ionophore A23187 (0.01-1.0 microM), thrombin (0.01-1.0 unit/ml), and histamine (0.01-1.0 microM) consistently produced a dose- and endothelium-dependent relaxation, reaching a maximum of -35.3 +/- 3.3%, -45.3 +/- 5.5%, -26.9 +/- 4.8%, and -17.8 +/- 2.5% (mean +/- SEM), respectively. No significant difference was observed among the CABGs with different duration of transplantation, whereas the relaxant responses of different segments along the entire length of a CABG were markedly different. These latter differences in the endothelium-dependent responses appear to correlate inversely with the development of intimal proliferative lesions in these CABGs. Addition of nitric oxide (0.01-10 microM) produced a potent dose- and endothelium-independent relaxation, which was also slightly depressed in CABGs with severe intimal proliferation. These results demonstrate that long-term transplanted human saphenous vein grafts retain their endothelium-dependent responses and that development of severe intimal proliferative lesions, rather than the duration of the grafts, result in marked alterations in the reactivity of these transplanted CABGs.

[Neoangiogenesis in aneurysmatic autologous venous transplants]

In two patients with femoro-popliteal bypass using autogenous saphenous vein, aneurysmatic dilatation of the transplants occurred after a follow-up of 4 and 20 years. In the inner thrombus layer of the aneurysms side-branches could be shown by transfemoral angiography. A special microcorrosion cast technique for SEM-analysis was able to make a vascular network visible, which spread within the inner thrombus layer of the aneurysms. Venules were found predominantly. According to clinical and experimental results the potential of the phylogenetic older venous system is greater than the ability of the arterial system.

Pulmonary vascular steal in chronic thromboembolic pulmonary hypertension

After pulmonary thromboendarterectomy, performed for relief of chronic thromboembolic pulmonary hypertension, perfusion lung scans have frequently disclosed new perfusion defects in segments served by undissected pulmonary arteries. Our hypotheses were that these new postoperative defects occurred with great frequency and did not represent postoperative vessel occlusion. We retrospectively reviewed the preoperative and postoperative perfusion scans of 33 consecutive patients undergoing pulmonary thromboendarterectomy. New postoperative perfusion defects were noted in 23 of 33 patients. The incidence of new defects was increased tenfold in segments that had (1) normal preoperative angiographic findings, (2) normal preoperative radionuclide perfusion, and (3) not been entered at the time of surgery. Postoperative angiograms, available in 15 of 33 patients, documented the nonembolic, nonocclusive nature of the new perfusion scan defects. The most plausible alternate explanation for this previously undescribed finding is a redistribution of pulmonary arterial resistance induced by the thromboendarterectomy, namely, a pulmonary vascular "steal."