Neural reorganisation in porcine vein grafts: a potential role for endothelin-1

Large animal model of vein grafts intimal hyperplasia: A systematic review

Coronary artery bypass grafting remains the treatment of choice for a large cohort of patients with significant coronary disease. Despite the increased use of arterial grafts, the long saphenous vein remains the most commonly used conduit. Long-term graft patency continues to be the Achilles heel of saphenous vein grafts. This is due to the development of intimal hyperplasia, a chronic inflammatory disease that results in the narrowing and occlusion of a significant number of vein grafts. Research models for intimal hyperplasia are essential for a better understanding of pathophysiological processes of this condition. Large animal models resemble human anatomical structures and have been used as a surrogate to study disease development and prevention over the years. In this paper, we systematically review all published studies that utilized large animal models of vein graft disease with a focus on the type of model and any therapeutic intervention, specifically the use of external stents/mesh.

Endothelin-1, endothelin receptor antagonists, and vein graft occlusion in coronary artery bypass surgery: 20 years on and still no journey from bench to bedside

The saphenous vein is the most commonly used bypass graft in patients with coronary artery disease. During routine coronary artery bypass, grafting the vascular damage inflicted on the vein is likely to stimulate the release of endothelin-1, a potent endothelium-derived vasoconstrictor that also possesses cell proliferation and inflammatory properties, conditions associated with vein graft failure. In both in vitro and in vivo studies, endothelin receptor antagonists reduce neointimal thickening. The mechanisms underlying these observations are multifactorial and include an effect on cell proliferation and cell/tissue damage. Much of the data supporting the beneficial action of endothelin-1 receptor antagonism at reducing intimal thickening and occlusion in experimental vein grafts were published over 20 years ago. The theme of the recent ET-16 conference in Kobe was "Visiting Old and Learning New". This short review article provides an overview of studies showing the potential of endothelin receptor antagonists to offer an adjuvant therapeutic approach for reducing saphenous vein graft failure and poses the question why this important area of research has not been translated from bench to bedside given the potential benefit for coronary artery bypass patients.

Role of Perivascular Adipose Tissue in Health and Disease

Perivascular adipose tissue (PVAT) is cushion of fat tissue surrounding blood vessels, which is phenotypically different from other adipose tissue depots. PVAT is composed of adipocytes and stromal vascular fraction, constituted by different populations of immune cells, endothelial cells, and adipose-derived stromal cells. It expresses and releases an important number of vasoactive factors with paracrine effects on vascular structure and function. In healthy individuals, these factors elicit a net anticontractile and anti-inflammatory paracrine effect aimed at meeting hemodynamic and metabolic demands of specific organs and regions of the body. Pathophysiological situations, such as obesity, diabetes or hypertension, induce changes in its amount and in the expression pattern of vasoactive factors leading to a PVAT dysfunction in which the beneficial paracrine influence of PVAT is shifted to a pro-oxidant, proinflammatory, contractile, and trophic environment leading to functional and structural cardiovascular alterations and cardiovascular disease. Many different PVATs surrounding a variety of blood vessels have been described and exhibit regional differences. Both protective and deleterious influence of PVAT differs regionally depending on the specific vascular bed contributing to variations in the susceptibility of arteries and veins to vascular disease. PVAT therefore, might represent a novel target for pharmacological intervention in cardiovascular disease. © 2018 American Physiological Society. Compr Physiol 8:23-59, 2018.

Nerve-perivascular fat communication as a potential influence on the performance of blood vessels used as coronary artery bypass grafts

Perivascular fat, the cushion of adipose tissue surrounding blood vessels, possesses dilator, anti-contractile and constrictor actions. The majority of these effects have been demonstrated in vitro and may depend on the vessel and/or the experimental method or species used. In general, the relaxant effect of perivascular adipose tissue is local and may be either endothelium-dependent or endothelium-independent. However, nerve stimulation studies show that, in general, perivascular adipose tissue (PVAT) has an anti-contractile vascular effect likely to involve an action of the autonomic vascular nerves. Apart from a direct effect of perivascular fat-derived factors on bypass conduits, an interaction with a number of neurotransmitters and other agents may play an important role in graft performance. Although the vascular effects of PVAT are now well-established there is a lack of information regarding the role and/or involvement of peripheral nerves including autonomic nerves. For example, are perivascular adipocytes innervated and does PVAT affect neuronal control of vessels used as grafts? To date there is a paucity of electrophysiological studies into nerve-perivascular fat control. This review provides an overview of the vascular actions of PVAT, focussing on its potential relevance on blood vessels used as bypass grafts. In particular, the anatomical relationship between the perivascular nerves and fat are considered and the role of the perivascular-nerve/fat axis in the performance of bypass grafts is also discussed.

Role of PVAT in coronary atherosclerosis and vein graft patency: friend or foe?

Perivascular adipose tissue (PVAT) releases numerous factors and adipokines with paracrine effects on both vascular structure and function. These effects are variable as they depend on regional differences in PVAT among blood vessels and vary with changes in adiposity. There is considerable evidence demonstrating an association between coronary PVAT and the development and progression of coronary artery disease, which is associated with inflammation, oxidative stress, angiogenesis, vascular remodelling and blood clotting. However, PVAT also has a protective role in vascular grafts, especially the no-touch saphenous vein, in patients undergoing coronary artery bypass. This beneficial influence of PVAT involves factors such as adipocyte-derived relaxing factor, nitric oxide (NO), leptin, adiponectin, prostanoids, hydrogen sulphide and neurotransmitters, as well as mechanical protection. This article aims to highlight and compare the dual role of PVAT in the development and progression of coronary atherosclerosis, as well as in increased graft patency. Different deleterious and protective mechanisms of PVAT are also discussed and the inside-outside signalling paradigm of atherosclerosis development re-evaluated. The bidirectional communication between the arterial and venous wall and their surrounding PVAT, where signals originating from the vascular wall or lumen can affect PVAT phenotype, has been shown to be very complex. Moreover, signals from PVAT also influence the structure and function of the vascular wall in a paracrine manner.

LINKED ARTICLES

This article is part of a themed section on Molecular Mechanisms Regulating Perivascular Adipose Tissue - Potential Pharmacological Targets? To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.20/issuetoc.

Endoscopic Versus “No-Touch” Saphenous Vein Harvesting for Coronary Artery Bypass Grafting

The advantage in terms of wound infection, wound healing, and scarring has resulted in the recent adoption of endoscopic vein harvesting (EVH) as a standard of care for coronary artery bypass grafting in some centers. However, concerns regarding the quality of these grafts have been raised after recent evidence of decreased graft patency, increased reoperation rate, and myocardial infarct, problems that are associated with vascular trauma caused when using this technique. Simultaneously, an atraumatic, “no-touch” technique for harvesting the saphenous vein was developed producing grafts with improved patency comparable to the internal thoracic artery. However, wound complications remain a problem using this technique. This review outlines the need to consider the poor graft quality that may result from EVH and raises the question what is likely to be the “best practice principle” in saphenous vein harvesting?

External support in preventing vein graft failure

Saphenous vein remains a widely used conduit in coronary surgery. However, the long-term success of surgical myocardial revascularization is largely limited by the development of neointimal hyperplasia and superimposed atherosclerosis in vein grafts. Although strategies for preventing vein graft failure have been constantly explored, few therapeutic interventions to date have shown sustained benefits in the clinical setting. The application of external support has emerged as a promising strategy for modulating the overall biomechanical responses in venous wall. Nonetheless, clinical translation of this intervention has been formerly challenged, primarily due to several technique limitations. The purpose of the current review is to summarize the possible mechanisms involved in the external support strategy for preventing vein graft failure. Furthermore, several previously tested biomaterials and delivery techniques are also highlighted.

'No-touch' saphenous vein harvesting improves graft performance in patients undergoing coronary artery bypass surgery: a journey from bedside to bench

The saphenous vein is the most commonly used conduit in patients undergoing coronary artery bypass surgery yet its patency is inferior to the internal thoracic artery. Vascular damage inflicted to the vein when using conventional harvesting techniques affects its structure. Endothelial denudation is associated with early vein graft failure while damage of the outermost vessel layers has adverse long-term effects on graft performance. While many in vitro and in vivo experimental studies aimed at improving vein graft patency have been performed to date no significant 'bench to bedside' advances have been made. Among experimental strategies employed is the use of pharmacological agents, gene targeting and external stents. A 'no-touch' technique, where the saphenous vein is removed with minimal trauma and normal architecture preserved, produces a superior graft with long term patency comparable to the internal thoracic artery. Interestingly, many experimental studies are aimed at repairing or replacing those regions of the saphenous vein damaged when harvesting conventionally. 'No-touch' harvesting is superior in coronary artery bypass patients with long-term data published 5years ago. Here we describe a 'bedside to bench' situation where the mechanisms underlying the improved performance of 'no touch' saphenous vein grafts in patients have been studied in the laboratory.

Endothelin-1 and vein graft occlusion in patients undergoing bypass surgery

The saphenous vein is the most commonly used graft for revascularization procedures in patients with coronary artery disease and critical limb ischaemia. However, the patency rate of this vessel is poor, with a high proportion of patients requiring further surgery. Early graft occlusion is caused by vasoconstriction or thrombus formation, with later stages of graft failure being due to neointimal formation or atherosclerosis. Apart from its potent constrictor action, endothelin-1 is also a potent proliferative and proinflammatory peptide that is implicated in a number of vascular diseases. The surgical trauma caused during preparation of the saphenous vein as a bypass graft stimulates the release of a number of factors affecting vascular reactivity and structure, including endothelin-1. Endothelin-1 not only constricts animal and human isolated saphenous vein segments but also causes vascular smooth muscle proliferation and neointimal thickening in vitro, actions that are mediated via endothelin (A and B) receptors. Experimentally, the effects of subtype-selective and dual receptor antagonists have been shown to inhibit endothelin-1-mediated constriction and cell proliferation of the saphenous vein. In this review, data supporting a role of endothelin-1 in vein graft occlusion are presented, and the therapeutic potential of endothelin receptor antagonists in improving graft performance is discussed.

Endothelin-1 and endothelin receptors in the basilar artery of the capybara

Little is known about cerebral vasculature of capybara, which seems may serve as a natural model of studying changes in cerebral circulation due to internal carotid artery atrophy at animal sexual maturation. This is the first study of the light- and electron-immunocytochemical localisation of endothelin-1 (ET-1) and ETA and ETB endothelin receptors in the basilar artery of capybaras (6 to 12-month-old females and males) using an ExtrAvidin detection method. All animals examined showed similar patterns of immunoreactivity. Immunoreactivity for ET-1 was detected in the endothelium and adventitial fibroblasts, whilst immunoreactivity for ETA and ETB receptors was present in the endothelium, vascular smooth muscle, perivascular nerves and fibroblasts. In endothelial cells immunoreactivity to ET-1 was pronounced in the cytoplasm or on the granular endoplasmic reticulum. Similar patterns of immunolabelling were observed for ETA and ETB receptors, though cytoplasmic location of clusters of immunoprecipitate seems dominant. These results suggest that the endothelin system is present throughout the wall of the basilar artery of capybara.

Does external stenting reduce porcine vein-graft occlusion via an action on vascular nerves?

BACKGROUND

Neural reorganization occurs in porcine vein grafts and placement of an external stent reduces graft occlusion.

AIM OF THE STUDY

To determine the effect of external stenting on the innervation of porcine vein grafts.

METHODS

Saphenous vein into carotid artery grafting (with and without external stents) was performed in 16 pigs. After one and six months, grafts were removed, nerves were counted, and neointima was assessed.

RESULTS

In vein graft compared to ungrafted vein, there was a significant (p < 0.05) decrease in medial perivascular nerves, but a dramatic increase in paravascular nerves in the adventitia (p < 0.05). In stented vein grafts there was also a reduction of perivascular nerves and the paravascular nerve proliferation observed in vein grafts at one month was inhibited (p < 0.05). Neointima formation and the appearance of large paravascular nerve bundles in the adventitia of vein grafts were abolished by external stenting.

CONCLUSIONS

Neural reorganization plays a role in vein-graft failure, possibly through the local release of mitogens; the prevention of this reorganization contributes to the inhibitory effect of the external stent on neointima formation.

External supports and the prevention of neointima formation in vein grafts

AIMS AND METHODOLOGY

the aim of this review is to provide an overview of the aetiology of neointima formation in vein grafts and to highlight the use of an external support to modulate this phenomenon. A systematic literature review was performed via computerised search on MEDLINE, OVID and the Cochrane Library. The search terms initially employed were broad-based; "vein graft", "neointima" and "external stent". Subsequently, more specific search terms were utilised; "perivenous mesh", "external prosthesis" and "varicose vein". Articles from indexed journals relevant to the objective, external venous supports, from the earliest reports in the 1960's to the latest in 2001 were included to obtain an exhaustive list. Reviews, abstracts and proceedings of scientific meetings, case reports and the results of both animal model investigations and human clinical trials in all languages were included. Articles describing an external support employed in both peripheral and aortocoronary bypass investigations were included.

Recent strategies to reduce vein graft occlusion: a need to limit the effect of vascular damage

Despite early identification and aggressive modification of atherosclerotic risk factors, many patients still require surgical revascularisation for established atherosclerotic vascular disease. However, bypass surgery is hampered by a high incidence of vein graft failure. New strategies are being introduced to improve these results, with early data suggesting that improved patency rates are possible. These vary from the use of adjuvant pharmacological agents and local gene transfer strategies to the modification of vein harvesting techniques in order to reduce vascular damage to all layers of the graft. Advances in vascular biology have resulted in new insights into the role of the endothelium and adventitia in vein graft remodelling. Although recent pharmacological adjuvant therapy and molecular techniques have been described that may be used to reduce the incidence of vein graft occlusion a more desirable approach for improved graft patency rates may be achieved simply by using atraumatic surgical techniques aimed at minimising vascular damage during vessel harvesting and subsequent anastamoses during bypass surgery.