Repair in arterial tissue. Demonstration of fibrinogen/fibrin in the normal and healing rabbit thoracic aorta by the indirect immunoperoxidase technique

Experimental thrombosis I: relation with fibrinogen and other haemostatic parameters

Epidemiological studies have shown that the haemostatic parameters Fibrinogen (Fg), Factor VII (F VII), Factor VIII (F VIII), von Willebrand factor (vWF), Tissue Plasminogen Activator (t-PA), Plasminogen Activator Inhibitors (PAI) are risk factors/markers of ischemic cardiovascular disease. Ferritin (sFER) and Leukocytosis have also been implicated. In the present study we have followed the levels of fibrinogen, von Willebrand factor and thrombomodulin in relation to lipids, iron and the appearance of atherosclerotic lesions in New Zealand rabbits fed with a cholesterol enriched diet for a two-month period compared with a group of control rabbits. Hematocrit and white blood cell count (WBC) were measured in parallel. In hyperchlesterolemic rabbits the levels of fibrinogen and von Willebrand factor increased progressively, showing a positive correlation with the increasing cholesterol levels. There was an increase in soluble thrombomodulin beginning at the eighth week of study. In addition, these animals showed gross intimal atherosclerotic lesions in the whole extension of their aortas. Immunohistochemical studies showed the presence of fibrin(ogen) related antigen throughout the arterial wall and in the central portions of the atheromas. In the control group there was no formation of atherosclerotic plaques and all haemostatic, haematological and biochemical parameters were within the normal range. WBC and sFER levels were unaffected in both groups. Our results show that increased levels of fibrinogen and von Willebrand factor, known coronary risk factors, are strongly associated with the formation of atherosclerotic plaques in rabbits. The plaques contain a considerable amount of fibrinogen related antigen.

Lipoproteins and the haemostatic system in atherothrombotic disorders

The remarkable extent to which interactions between the plasma lipoproteins, inflammatory factors and the haemostatic system contribute to the response to injury and growth of the plaque in atherosclerosis is being increasingly documented. High plasma concentrations of very-low density (VLDL) and low-density lipoproteins (LDL), together with oxidatively modified LDL and lipoprotein (a), can induce responses in vascular endothelial cells, smooth muscle cells, monocytes/macrophages, platelets, neutrophils and humoral factors that are in a variety of ways both procoagulant and antifibrinolytic. Plasma high-density lipoproteins appear to promote anticoagulant mechanisms. Post-prandial lipaemia is associated with transient changes in factor VII which may be indicative of temporary hypercoagulability. The cellular and humoral effects of LDL and VLDL on the haemostatic system appear to be largely reversible, which may help to explain the prompt improvement in the atherothrombotic state gained by correction of hyperlipidaemia.

Thrombin inhibition by antithrombin III on the subendothelium is explained by the isoform AT beta

Balloon injury of the rabbit aorta results in thrombin coagulant activity on the injured vessel wall that causes fibrin formation. The anticoagulant activity of both the intact and injured vessel wall has been partly explained by glycosaminoglycans with heparin-like activity that augment that activity of antithrombin III (AT). AT exists in two isoforms, alpha and beta, AT beta, which constitutes only 5% to 10% of AT in plasma, lacks one carbohydrate side chain, has higher affinity for glycosaminoglycans, and associates more readily with the subendothelium. This study evaluated whether AT can inhibit thrombin on the injured vessel wall and, if so, whether one of the isoforms is more effective then the other. The two isoforms were isolated from human plasma by heparin-Sepharose chromatography, and the purity was investigated by isoelectric focusing and crossed immunoelectrophoresis. Rabbits were subjected to balloon injury of the aorta; 3 hours after injury the aorta was excised. Thrombin coagulant activity on the aorta was measured by exposure to fibrinogen and thereafter by measuring the generation of fibrinopeptide A. Injured animals were treated with AT, AT alpha, or AT beta and were compared with control animals. AT was demonstrated on the injured vessel wall by using an immunohistochemical method. Animals receiving crude AT had significantly lower amounts of thrombin coagulant activity on the injured aortic wall than control animals, but AT alpha at a comparable dose had no effect. AT beta was given in the same dose as crude AT and also at a dose (10%) proportional to its presence in plasma. Animals receiving AT beta had significantly lower values of thrombin on the injured aortic wall than control animals. We conclude that the inhibitory effect of AT on thrombin coagulant activity on the injured vessel wall in explained by its AT beta content.

Influence of luminal pore size on the patency rate and endothelialization of polymeric microvenous prostheses

In microvenous prosthetic surgery a continuous search for better patency rates is necessary to enable a clinical application. In this search for better patencies, modifications in the wall structure are being made. Directions found in the literature suggest that pore size plays an important role in achieving better patencies. Thus far, no study has been conducted to evaluate the influence of pore size on the patency rate of polyurethane microvenous prostheses. Since polyurethane is known to yield good patency rates, we conducted this study in which we compared different luminal pore sizes with regard to patency. Pore size varied from 0.6 to 20 microns in microvenous polyurethane-based prostheses (length 5-6 mm, internal diameter 1 mm). The results showed a favorable patency rate in the pore sizes larger than 5.0 microns (patency 75%) when compared to pore sizes smaller than 2.0 microns (patency 50%). This study demonstrates that microvenous polyurethane-based prostheses with a luminal pore size larger than 5.0 microns may yield better patency rates than prostheses with a luminal pore size smaller than 5.0 microns. Further studies are currently being performed to elucidate the very reasons for this effect.

Lipoproteins and the haemostatic system in atherothrombotic disorders

The early belief that the haemostatic system has no active role in the formation of the atheromatous plaque is no longer tenable. Rather, the association between hypercholesterolaemia and atherosclerosis appears to arise in part because of various effects of high concentrations of LDL and VLDL particles on the cellular and humoral components of the system, thereby promoting plaque growth and thrombosis. These may be summarized as follows: 1. High concentrations of native LDL have been reported to promote the adhesion of monocytes to the endothelial cell, suggesting that the latter undergoes a form of activation upon such exposure. Oxidized LDL is more potent in this respect, and persistent exposure of endothelium to such particles can eventually lead to cell injury. 2. Activated endothelial cells acquire characteristics on their luminal surface conducive to thrombin generation and fibrin production. Thrombin has several actions on the endothelial cell, monocyte, smooth muscle cell and platelet which in the presence of hypercholesterolaemia will promote the formation of atheroma. 3. Oxidatively modified LDL can activate circulating monocytes, when they also acquire procoagulant properties which favour thrombin production. 4. Platelets show an increased tendency to aggregate when exposed to hypercholesterolaemic plasma. This effect may arise in part because the platelet of the hypercholesterolaemic patient expresses an increased number of fibrinogen binding sites on its surface following activation by agonists such as ADP. These hyperaggregable platelets adhere to activated endothelial cells which express von Willebrand factor on their surface, and to subendothelial proteins exposed in the gaps that open between injured endothelial cells. Platelets exposed to raised LDL levels also show a reduced sensitivity to prostacyclin, an antiaggregatory agent. Oxidatively modified LDL has been reported to stimulate aggregation of platelets in the absence of other agonists such as ADP or thrombin (spontaneous aggregation). 5. Platelet aggregation and fibrin deposition at sites of endothelial injury will create microthrombi which become incorporated into the lesion by organization, thereby increasing the fibrous and cellular content of the atheromatous plaque. 6. Lipolysis of triglyceride-rich lipoproteins at the endothelial cell surface leads to transient activation of the coagulation mechanism with activation of factor VII. Activated factor VII is a potent procoagulant when it forms a complex with tissue factor in the atheromatous lesion. Persistent hypertriglyceridaemia is accompanied by raised concentrations of factor X, factor IX, factor VII and prothrombin. 7. Hypertriglyceridaemia is associated with an increased plasma concentration of PAI-1 and a reduction in plasma fibrinolytic activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Patency and healing of microvascular prostheses: a review of 10 years of experimental work in Groningen

From 1982 onwards, in Groningen, The Netherlands, we have worked on the experimental evaluation and development of microvascular prostheses in rats and rabbits. In this review article a systematic overview of this experimental work is presented and the results are discussed with regard to the literature to come to a current state of the art on (experimental) microvascular grafting with prosthetic conduits.

On the role of coagulation and fibrinolysis in atherosclerosis

Atherosclerosis is probably caused by multiple interacting factors such as disturbed lipid metabolism; endothelial cell damage, leading to platelet aggregation and monocyte invasion with the release of mitogenic factors; and disorders of fibrin balance, leading to persisting fibrin deposits. Deficient fibrinolysis may (1) predispose to fibrin deposition and contribute to the pathogenesis of atherosclerosis and (2) contribute to occlusive thrombus formation on fissured plaque, provoking atherothrombosis. Prospective epidemiologic studies have so far not provided definitive evidence that deficient fibrinolysis constitutes a significant risk factor for the development of atherosclerosis. Two recent findings, however, strongly suggest a contribution: (1) Increased lipoprotein(a) levels that reduce tissue-type plasminogen activator (t-PA)-mediated clot lysis are a clear risk factor for atherosclerosis; and (2) increased plasminogen activator inhibitor-1 (PAI-1) levels in patients with disturbed glucose tolerance predispose to an accelerated development of atherosclerotic disease. However, deficient fibrinolysis constitutes a risk factor for the development of thrombotic complications (acute myocardial infarction) in patients with coronary artery disease. The potential role of deficient fibrinolysis in the pathogenesis of atherosclerosis and of atherothrombosis suggests that drugs normalizing deficient endogenous fibrinolysis by either reducing PAI-1 synthesis or by stimulating endogenous t-PA synthesis may be of clinical value. Although regulation of the gene expression of PAI-1 and t-PA is presently under active investigation, no potent specific and safe agents to downregulate PAI-1 or to upregulate t-PA have as yet been identified. Retinoic acid appears to be a specific inducer of t-PA synthesis in human endothelial cells in culture and may constitute a model for the development of drugs that stimulate endogenous t-PA synthesis.

Repair in arterial tissue 2 years after a severe single dilatation injury: the regenerative capacity of the rabbit aortic wall. The importance of endothelium and of the state of subendothelial connective tissue to reconstitution of the intimal barrier

The thoracic aortae from 11 rabbits that survived a single severe dilatation injury for 2 years were studied by vital staining with Evans blue, immunohistochemistry and transmission electron microscopy. Our results have shown almost total restitution of the thoracic aorta. Six of the 11 rabbits submitted to an injury had no blue-stained areas, indicating total reendothelialization. Five rabbits had a few blue areas often on the ventral side of the aorta. The reendothelialization from the first to the seventh pair of intercostal arteries ranged from 82% to 100%. There was intimal thickening inside the original internal elastic lamina in both white and blue areas. All blue areas had a surface composed of smooth muscle cells. Reendothelialized areas consisted of mature endothelium, reticular basal membrane, layered smooth muscle cells and an extracellular matrix consisting of pre-elastin, elastin, collagen and proteoglycans. An effective barrier had apparently been formed against penetration of macromolecules, judged from the absence of fibrinogen/fibrin and unmasked fibronectin. Intimal thickenings without endothelial cover were covered with smooth muscle cells without intercellular junctions. Our results indicate that an extracellular matrix of fibrin and fibronectin plays a role in forming an intimal thickening, and it is suggested that proteoglycans may modulate the biological role of the extracellular matrix in the healing process.

Enhanced healing of 30 microns Gore-Tex PTFE microarterial prostheses by alcohol-pretreatment

Polytetrafluoroethylene (PTFE) microvascular prostheses with a fibril length of 30 microns were pretreated with alcohol (n = 18), implanted into the abdominal aorta of rats and were evaluated at 1 day (n = 3), 1 week (n = 3), 3 weeks (n = 6) and 6 weeks (n = 6) to determine whether alcohol-pretreatment might improve their healing. Untreated PTFE microvascular prostheses (n = 18) functioned as controls (all prostheses: length 10 mm, I.D. 1.5 mm). The alcohol-pretreated PTFE microarterial prostheses were initially completely filled and covered with clot and showed fast and complete healing within 6 weeks: endothelial cells and smooth muscle cells on the luminal surface, and the interstices filled with fibrous-like tissue. In contrast, the untreated PTFE prostheses were initially not filled or covered with clot and showed only healing near the anastomotic sites and scarce tissue ingrowth into the wall. These results demonstrate that alcohol-pretreatment improves the healing characteristics of PTFE microarterial prostheses with a fibril length of 30 microns. The alcohol-pretreatment renders the PTFE material more accessible to clot and subsequently to cells.

Visualization of apo B, fibrinogen/fibrin, and fibronectin in the intima of normal human aorta and large arteries and during atherosclerosis

Apolipoprotein B (apo B), fibrinogen/fibrin, blood platelets, factor VIII-related antigen of the blood coagulation system, and smooth muscle cells (SMC) were identified in the intima of normal and atherosclerotic human aorta and large arteries by the indirect immunofluorescence technique. Fibrinogen/fibrin was revealed by a monoclonal antibody (monAb) against the C-terminal region of human fibrinogen A alpha-chain. Fibronectin was visualized by monAb to the cellular form and against an epitope shared by different fibronectin subunit variants. In normal intima, fatty streaks, small amounts of fibrinogen/fibrin together with large amounts of apo B were observed. Fibronectin detected by two types of monAb was not found in extracellular matrix (ECM), whereas cellular fibronectin encircled SMC. According to the data obtained, fibrinogen/fibrin accumulates in plaques as a result of intramural thrombus incorporation, blood insudation, intramural haemorrhage, and in or around cells, apparently macrophages.

Injury and repair of smaller muscular and elastic arteries. Immunohistochemical demonstration of fibronectin and fibrinogen/fibrin and their degradation products in rabbit femoral and common carotid arteries following a dilatation injury

Indirect immunoperoxidase staining for fibrinogen/fibrin and fibronectin was performed on normal and healing arterial tissue of muscular and smaller elastic arteries. Fibronectin was observed in the wall of the normal arteries, whereas fibrinogen/fibrin could not be demonstrated. Fibronectin was observed in the intima as well as the media deposited in a similar fashion in the femoral and carotid artery during repair. Apart from the early occurrence of fibrin/fibrinogen in the media of both arteries the distribution of fibrinogen/fibrin and degradation products differed. In the femoral artery a progressively weakening positive reaction for fibrinogen/fibrin and degradation products towards the lumen was observed in the intima and the media 7 and 14 days after the lesion. By 28 days the reaction in the media was negative. No thrombus formation was observed. In contrast, all the specimens examined from the common carotid arteries were obliterated by luminal thrombi 28 days after the lesion. The thrombus as well as the damaged intimal thickening and the compressed media were loaded with fibrinogen/fibrin and degradation products. The deposition of fibronectin, fibrinogen, and degradation products in the carotid artery was similar to that previously reported in experimental aortic arteriosclerosis in rabbits as well as in giant cell arteritis.

Giant-cell arteritis. Histological, immunohistochemical and electronmicroscopic studies

Biopsies from the temporal artery of 32 patients suspected of giant-cell arteritis were evaluated retrospectively by light microscopy, histochemical, and immunohistochemical methods, as well as by transmission electron microscopy (TEM). At the clinical follow-up the 32 patients included four clinical groups: temporal arteritis (8 patients), polymyalgia rheumatica (10 patients), rheumatoid arthritis (4 patients), and a group of miscellaneous diseases unrelated to inflammatory rheumatic diseases (10 patients). There were a number of similarities between age-related alterations in the arteries and the changes in giant-cell arteritis. The most important differences were the inflammatory cellular infiltration of the media, the perifocal accumulation of fibronectin, and the occurrence of deposits of fibrin/fibrinogen and fibrin/fibrinogen degradation products. In addition, alpha-2 macroglobulin, lysozyme and factor VIII were also noted in giant-cell arteritis. The alterations in giant-cell arteritis show a number of similarities to the changes following experimental vascular injury of the rabbit aorta. The nature of the findings in human giant-cell arteritis, as well as the similarity to the experimental arteritis, indicate that giant-cell arteritis may reflect a non-specific reaction to injury, independent of the cause of the disease.

Soluble fibrinogen derivatives generated by thrombin: affinity for elastin

When human citrated plasma is dialysed against a phosphate buffer containing Ca++, citrate anions are removed, thrombin is generated and soluble fibrinogen derivatives (fibrin monomers and/or soluble fibrin polymers) are formed. These derivatives are able to combine with human or bovine elastin to form a very stable addition product or adduct. The formation of the adduct is dependent on time, Ca++ and thrombin concentrations.

Regeneration of the arterial wall in microporous, compliant, biodegradable vascular grafts after implantation into the rat abdominal aorta. Ultrastructural observations

The ultrastructure of a new type of vascular graft, prepared from a mixture of polyurethane (95 weight %) and poly-L-lactic acid (5 weight %), was examined six weeks after implantation into the abdominal aorta of rats. These microporous, compliant, biodegradable, vascular grafts function as temporary scaffolds for the regeneration of the arterial wall. Smooth muscle cells, covering the grafts, regenerated a neo-media underneath an almost completely regenerated endothelial layer (neo-intima). These smooth muscle cells varied in morphology from normal smooth muscle cells to myofibroblasts. They were surrounded by elastic laminae and collagen fibers. Macrophages, epithelioid cells, multinucleated giant cells, fibroblasts and capillaries were present in the disintegrating graft lattices. The epithelioid cells and multinucleated giant cells engulfed polymer particles of the disintegrating grafts. The regeneration of the endothelial and smooth muscle cells is similar to the natural response of arterial tissue upon injury. The presence of macrophages, epithelioid cells, multinucleated giant cells, fibroblasts and capillaries in the graft lattices resembles the natural response of tissue against foreign body implants. Both of these responses result in the formation of a neo-artery that possesses sufficient strength, compliance and thromboresistance to function as a small caliber arterial substitute.

Repair in arterial tissue. Demonstration of fibrinogen/fibrin in the normal and healing rabbit thoracic aorta by the indirect immunoperoxidase technique

The results of immunoperoxidase staining for fibrinogen/fibrin in ethanol- and formaldehyde-fixed, normal and healing arterial tissue are presented. Fibrinogen/fibrin was not observed in the normal aortic wall. The thoracic aorta damaged by a balloon catheter contained fibrinogen/fibrin in all layers of the wall. In the healing aortic wall there was a strong positive reaction in neo-intima, whereas the reaction in media was weak or absent. The staining reaction for fibrinogen/fibrin in formaldehyde-fixed neo-intima covered with aortic smooth muscle cells was strong and almost independent of proteolytic digestion, while such treatment increased the staining intensity for fibrinogen/fibrin in neo-intima covered with endothelium. Our results indicate that an extracellular matrix of fibrin and fibronectin may play a role in migration and proliferation of aortic smooth muscle cells.