Cardiovascular diseases account for more than half of total mortality before the age of 75 in industrialized countries. To develop therapies promoting the compensatory growth of blood vessels could be superior to palliative surgical surgical interventions. Therefore, much effort has been put into investigating underlying mechanisms. Depending on the initial trigger, growth of blood vessels in adult organisms proceeds via two major processes, angiogenesis and arteriogenesis. While angiogenesis is induced by hypoxia and results in new capillaries, arteriogenesis is induced by physical forces, most importantly fluid shear stress. Consequently, chronically elevated fluid shear stress was found to be the strongest trigger under experimental conditions. Arteriogenesis describes the remodelling of pre-existing arterio-arteriolar anastomoses to completely developed and functional arteries. In both growth processes, enlargement of vascular wall structures was proposed to be covered by proliferation of existing wall cells. Recently, increasing evidence emerges, implicating a pivotal role for circulating cells, above all blood monocytes, in vascular growth processes. Since it has been shown that monocytes/macrophage release a cocktail of chemokines, growth factors and proteases involved in vascular growth, their contribution seems to be of a paracrine fashion. A similar role is currently discussed for various populations of bone-marrow derived stem cells and endothelial progenitors. In contrast, the initial hypothesis that these cells -after undergoing a (trans-)differentiation- contribute by a structural integration into the growing vessel wall, is increasingly challenged.

Objective-The role of fluid shear stress (FSS) in collateral vessel growth remains disputed and prospective in vivo experiments to test its morphogenic power are rare. Therefore, we studied the influence of FSS on arteriogenesis in a new model with extremely high levels of collateral flow and FSS in pig and rabbit hind limbs. Methods and Results-A side-to-side anastomosis was created between the distal stump of one of the bilaterally occluded femoral arteries with the accompanying vein. This clamps the collateral reentry pressure at venous levels and increases collateral flow, which is directed to a large part into the venous system. This decreases circumferential wall stress and markedly increases FSS. One week after anastomosis, angiographic number and size of collaterals were significantly increased. Maximal collateral flow exceeded by 2.3-fold that obtained in the ligature-only hind limb. Capillary density increased in lower leg muscles. Immunohistochemistry revealed augmented proliferative activity of endothelial and smooth muscle cells. Intercellular adhesion molecule-1 and vascular cell adhesion molecule (VCAM)-1 were upregulated, and monocyte invasion was markedly increased. In 2-dimensional gels, actin-regulating cofilin1 and cofilin2, destrin, and transgelin2 showed the highest degree of differential regulation. Conclusions-High levels of FSS cause a strong arteriogenic response, reinstate cellular proliferation, stimulate cytoskeletal rearrangement, and normalize maximal conductance. FSS is the initiating molding force in arteriogenesis. Key Words: fluid shear stress Ⅲ shunt Ⅲ arteriogenesis Ⅲ proteomics Ⅲ cytoskeletal proteins N umerous studies have documented the influence of fluid shear stress (FSS) as an arterial molding force, 1-3 but information on the actions of markedly increased in vivo FSS on the development of arterial collateral vessels after occlusion of a conduit artery is lacking. Such studies are needed because attempts at changing FSS often also alter the circumferential wall stress, another acknowledged molding force of growing collateral vessels. The formation of a collateral circulation after an arterial occlusion correlates well with the calculated increase in FSS because of the increased collateral flow caused by the pressure decrease along pre-existent collaterals. However, because of the fast increase in collateral diameter by cellular proliferation, FSS decreases quickly again, and the early termination of the growth process at an incomplete stage of adaptation is believed to be caused by the only transient action of FSS. One of the hypotheses to be tested was, therefore, whether prolonged action of FSS would also improve the final adaptation by continued growth. The present experiments were therefore undertaken to prospectively study the causal relations between arteriogenesis and FSS by a stepwise and lasting increase of collateral flow brought about by the creation of a side-to-side anastomosis between the distal stump of the occluded femoral artery and its accompanying vei...

Abstract-Natural adaptation to femoral artery occlusion in animals by collateral artery growth restores only Ϸ35% of adenosine-recruitable maximal conductance (C max ) probably because initially elevated fluid shear stress (FSS) quickly normalizes. We tested the hypothesis whether this deficit can be mended by artificially increasing FSS or whether anatomical restraints prevent complete restitution. We chronically increased FSS by draining the collateral flow directly into the venous system by a side-to-side anastomosis between the distal stump of the occluded femoral artery and the accompanying vein. After reclosure of the shunt collateral flow was measured at maximal vasodilatation. C max reached 100% already at day 7 and had, after 4 weeks, surpassed (2-fold) the C max of the normal vasculature before occlusion. Expression profiling showed upregulation of members of the Rho-pathway (RhoA, cofilin, focal adhesion kinase, vimentin) and the Rho-antagonist Fasudil markedly inhibited arteriogenesis. The activities of Ras and ERK-1,-2 were markedly increased in collateral vessels of the shunt experiment, and infusions of L-NAME and L-NNA strongly inhibited MAPK activity as well as shunt-induced arteriogenesis. Infusions of the peroxinitrite donor Sin-1 inhibited arteriogenesis. The radical scavengers urate, ebselen, SOD, and catalase had no effect. We conclude that increased FSS can overcome the anatomical restrictions of collateral arteries and is potentially able to completely restore maximal collateral conductance. Increased FSS activates the Ras-ERK-, the Rho-, and the NO-(but not the Akt-) pathway enabling collateral artery growth. Key Words: arteriogenesis Ⅲ fluid shear stress Ⅲ shunt Ⅲ growth factors Ⅲ microarrays T he restoration of maximal conductance (C max ) in animals after arterial occlusion remains defective (35% in the canine coronary circulation 1 and 40% in the rabbit hind limb 2 ) in spite of the fact that normal resting blood flow is reached early. As a consequence exercise testing in experimental animals reveals defects similar to those in human patients. 3 It was not known until now whether collateral vessels are potentially able to restore the full dilatory reserve of a normal vascular bed. Many observations would predict that this is not the case: the multitude of small vessels that replace an occluded artery is inefficient according to Poissieulle's Law, and the tortuosity of collateral vessels offers finite resistance because of curvature flow and increased collateral length. 4 One reason for the defective adaptation may lie in the fact that fluid shear stress normalizes prematurely: FSS falls by the third power of the growing radius. We tested the hypothesis whether a sustained increase of FSS is able to prolong the growth process and to restore normal maximal conductance. The method to achieve this was the creation of a shunt between the distal stump of the occluded femoral artery and the accompanying vein. 5 The novelty of the present findings is the demonstration that C max can be reach...

The multiple demands placed on small calibre cardiovascular bypass grafts have meant that a synthetic prosthesis with good long-term patency has not been developed. A tissue-engineered graft could fulfil the ideal characteristics present in an artery. However, the great disadvantage of such a conduit is the time necessary for maturation leading to unacceptable delays once the decision to intervene surgically has been made. This maturation process is essential to produce a graft which can withstand haemodynamic stress. Once implanted, the tissue-engineered graft can contract in response to immediate haemodynamic conditions and remodel in the long term. We review the latest tissue engineering approaches used to give the favourable properties of mechanical strength, arterial compliance, low thrombogenicity, long-term resistance towards biodegradation as well as technological advances which shorten the time required for production of an implantable graft.

Introduction It is known that in patients with ischaemic vascular diseases col- Abstract The development of a collateral circulation (arteriogenesis), bypassing an arterial occlusion, is important for tissue survival, but it remains functionally defective. Micro array data of growing collateral vessels, exposed to chronically elevated fluid shear stress (FSS), showed increased transcription of the transient receptor potential cation channel, subfamily V, member 4 (Trpv4). Thus, the aim of this study was to investigate the role of the shear stress sensitive Trpv4 in transmitting this physical stimulus into an active growth response. qRT-PCR at different time points during the growth of collateral vessels after femoral artery ligature (FAL) in rats showed a strong positive correlation of Trpv4 transcription and the intensity of FSS. An increased protein expression of Trpv4 was localized in the FSS-sensing endothelium by means of confocal immunohistochemistry. Cultured porcine endothelial cells showed a dose-dependent expression of Trpv4 and an increased level of

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Previous studies showed that targeted endothelial nitric oxide synthase (eNOS) disruption in mice with femoral artery occlusion does not impede and transgenic eNOS overexpression does not stimulate collateral artery growth after femoral artery occlusion, suggesting that nitric oxide from eNOS does not play a role in arteriogenesis. However, pharmacologic nitric oxide synthase inhibition with L-NAME markedly blocks arteriogenesis, suggestive of an important role of nitric oxide. To solve the paradox, we studied targeted deletion of eNOS and of inducible nitric oxide synthase (iNOS) in mice and found that only iNOS knockout could partially inhibit arteriogenesis. However, the combination of eNOS knockout and treatment with the iNOS inhibitor L-NIL completely abolished arteriogenesis. mRNA transcription studies (reverse transcriptase-polymerase chain reaction) performed on collateral arteries of rats showed that eNOS and especially iNOS (but not neural nitric oxide synthase) become upregulated in shear stress-stimulated collateral vessels, which supports the hypothesis that nitric oxide is necessary for arteriogenesis but that iNOS plays an important part. This was strengthened by the observation that the nitric oxide donor DETA NONOate strongly stimulated collateral artery growth, activated perivascular monocytes, and increased proliferation markers. Shear stress-induced nitric oxide may activate the innate immune system and activate iNOS. In conclusion, arteriogenesis is completely dependent on the presence of nitric oxide, a large part of it coming from mononuclear cells.