Effects of angiotensin converting enzyme inhibition with cilazapril on intimal hyperplasia in injured arteries and vascular grafts in the baboon

Vascular Graft Implantation Using a Bilateral End-to-Side Aortoiliac Preclinical Model

Arterial bypass grafts are a standard preclinical model for evaluating physiology and pathophysiology at graft-material interfaces. Implantations of vascular grafts are commonly done as end-to-end grafts in small animal models. Here we detail bilateral end-to-side aortoiliac graft implantation, which requires open surgery and the creation of vascular anastomoses between the graft material and the infrarenal aorta and iliac artery in a nonhuman primate model. In this model, the aortoiliac graft configuration is created using two 4 mm inner diameter vascular grafts (e.g., ePTFE). After exposure and control of the infrarenal aorta and bilateral common iliac arteries and heparinization, the proximal aortic-graft anastomosis is sewn on the lateral wall of the aorta, and subsequently the distal graft-common iliac anastomosis is sewn on the anterior wall of the common iliac artery with one tube graft. Another tube graft is sewn on the contralateral side in the same manner.

Improving Surgical Methods for Studying Vascular Grafts in Animal Models

While clinical vascular grafting uses an end-to-side surgical method, researchers primarily use end-to-end implant techniques in preclinical models. This may be due in part to the limitations of using small animal models in research. The work presented here provides support and evidence for the improvement of vascular graft implant techniques by demonstrating the successful implantation of experimental grafts into both large and small animal models. Specifically, models of aortoiliac baboon (Papio anubis) bypass and common carotid rabbit (Oryctolagus cuniculus) bypass were used to test vascular grafts for thrombosis and vascular healing after 1 month using an end-to-side anastomosis grafting procedure. Patency was evaluated with ultrasound or histological techniques, and neointimal growth was quantified with histology. In the development of this procedure for small animals, both an end-to-end/end-to-side and an end-to-side/end-to-side configuration were tested in rabbits. One hundred percent of rabbit implants (2/2) with an end-to-end/end-to-side configuration were patent at explant. However, with the end-to-side/end-to-side configuration, 66% (6/9) of rabbit implants and 93% (13/14) of baboon implants remained patent at 1 month, suggesting the importance of replicating the end-to-side method for testing vascular grafts for clinical use. This study describes feasible preclinical surgical procedures, which simulate clinical vascular bypass grafts even in small animals. Widespread implementation of these end-to-side surgical techniques in these or other animals should improve the quality of experimental, preclinical testing and ultimately increase the likelihood of translating new vascular graft technologies into clinical applications.

Preclinical Restenosis Models: Challenges and Successes

Coronary artery disease remains a major problem for Western societies. The advent of percutaneous interventions, including stents has brought clinical care to a new level of efficacy, yet problems remain. Restenosis following stenting in human coronary arteries appears at last to be yielding to therapeutic strategies, especially drug eluting stents. Because therapeutic percutaneous coronary intervention is widely dominated by the intracoronary stent, restenosis therapies must include the stented coronary artery. Animal models and in particular the porcine coronary model seem to represent the human coronary artery reaction to stenting. It mimics several clinical conditions including thrombosis and neointimal formation. A key question in the era of intravascular technologies is how well this and other models can predict clinical events. This paper discusses the models and their application.

Long-term effects of ACE inhibitor on vascular remodelling

The long-term pathomorphological changes of the injured vessels under angiotensin-converting-enzyme (ACE) inhibitor are still not known. Therefore, we assessed the alternations of vascular architecture after three-month therapy with ACE inhibitor and identified new target cells for this medication. Carotid arteries of spontaneously hypertensive rats underwent balloon angioplasty. 14 days prior intervention, half of the animals was treated with ACE inhibitor. After three months of vascular trauma, the injured vessels were explored by histomorphology and immunohistochemistry for angiotensin-II receptor (AT1R), dendritic and HSP47+ cells. The neointimal growth decreased significantly only up to 28 days under ACE inhibitor. In contrast, the reductive effect of ACE inhibitor on media area persisted up to three months after intervention. A significant fraction of early neointimal cells was of a dendritic cell type. The relevant portion of these cells showed an expression of AT1R and HSP47. AT1R was present in 70% and HSP47 in 18% of all early neointimal cells in both groups. ACE inhibitor may at least temporarily diminish remodelling processes in injured vessels. The detection of AT1R on dendritic cells identifies these cells as important targets for therapeutic strategies involving modulation of the renin-angiotensin system.

Animal Models for Studying Pathophysiology of Hemodialysis Access

Despite extensive efforts, most approaches to reduce arteriovenous (AV) access-related complications did not results in substantial improvement of AV access patency thus far. Part of this disappointing progress relates to incomplete understanding of the underlying pathophysiology of hemodialysis access failure. In order to unravel the pathophysiology of hemodialysis access failure, animal models that closely mimic human pathology are of utmost importance. Indeed, it is impossible to study the extremely complex response of the AV access at a molecular and cellular level in great detail in dialysis patients. Over the past decades, numerous animal models have been developed in an attempt to unravel the vascular pathology of AV access failure and to design new therapeutic strategies aimed to improve durability of these vascular conduits. While large animals such as pigs are suitable for intervention studies, murine models have the greatest potential to gain more insight in the molecular mechanisms underlying AV access failure due to the availability of transgenic mice. In the present review, we describe several existing models of AV access failure and discuss the advantages and limitations of these models.

Role of the renin-angiotensin system in the pathogenesis of intimal hyperplasia: therapeutic potential for prevention of vein graft failure?

The saphenous vein remains the most widely used conduit for peripheral and coronary revascularization despite a high rate of vein graft failure. The most common cause of vein graft failure is intimal hyperplasia. No agents have been proven to be successful for the prevention of intimal hyperplasia in human subjects. The renin-angiotensin system is essential in the regulation of vascular tone and blood pressure in physiologic conditions. However, this system mediates cardiovascular remodeling in pathophysiologic states. Angiotensin II is becoming increasingly recognized as a potential mediator of intimal hyperplasia. Drugs modulating the renin-angiotensin system include angiotensin-converting enzyme inhibitors and angiotensin receptor blockers. These drugs are powerful inhibitors of atherosclerosis and cardiovascular remodeling, and they are first-line agents for management of several medical conditions based on class I evidence that they delay progression of cardiovascular disease and improve survival. Several experimental models have demonstrated that these agents are capable of inhibiting intimal hyperplasia. However, there are no data supporting their role in prevention of intimal hyperplasia in patients with vein grafts. This review summarizes the physiology of the renin-angiotensin system, the role of angiotensin II in the pathogenesis of cardiovascular remodeling, the medical indications for these agents, and the experimental data supporting an important role of the renin-angiotensin system in the pathogenesis of intimal hyperplasia.

Preclinical Models of Restenosis and Their Application in the Evaluation of Drug-Eluting Stent Systems

Coronary arterial disease (CAD) is the leading cause of death in the United States, the European Union, and Canada. Percutaneous coronary intervention (PCI) has revolutionized the treatment of CAD, and it is the advent of drug-eluting stent (DES) systems that has effectively allayed much of the challenge of restenosis that has plagued the success of PCI through its 30-year history. However, DES systems have not been a panacea: There yet remain the challenges associated with interventions involving bare metallic stents as well as newly arisen concerns related to the application of DES systems. To effectively address these novel and ongoing issues, animal models are relied on both to project the safety and efficacy of endovascular devices and to provide insight into the pathophysiology underlying the vascular response to injury and mechanisms of restenosis. In this review, preclinical models of restenosis are presented, and their application and limitation in the evaluation of device-based interventional technologies for the treatment of CAD are discussed.

Renin-Angiotensin System Intervention to Prevent In-Stent Restenosis

The occurrence of in-stent restenosis is a major drawback of percutaneous transluminal coronary angioplasty with stent placement. Target vessel revascularization is necessary in 15% of patients who receive a stent. Recent advances in the development of drug-eluting stents have reduced these numbers tremendously. However refinement of antirestenotic therapies remains obligatory. The emerging interest in more physiological antirestenotic therapies might unchain an interest in the well-known inhibitors of the rennin-angiotensin system (RAS), the angiotensin-converting enzyme inhibitors, and the angiotensin II type I receptor blockers. Contradictory results overshadow the discussion of whether intervention in the RAS could prevent in-stent restenosis. This review discusses the pathophysiology of in-stent restenosis, the role of the RAS in in-stent restenosis, and the possible role of RAS intervention in the prevention of in-stent restenosis.

Small-caliber heparin-coated ePTFE grafts reduce platelet deposition and neointimal hyperplasia in a baboon model

PURPOSE

Intimal hyperplasia and graft thrombosis are major causes of graft failure. Heparin prolongs graft patency and inhibits neointimal hyperplasia in animal models. The purpose of this study was to evaluate the effect of a heparin-coated expanded polytetrafluoroethylene (ePTFE) graft on platelet deposition and anastomotic neointimal hyperplasia after aortoiliac bypass grafting in a baboon model.

METHODS

Heparin-coated ePTFE grafts (4-mm diameter) were incorporated into exteriorized femoral arteriovenous shunts placed in five baboons. Platelet deposition was analyzed by measuring the accumulation of indium 111-labeled platelets on the grafts, with dynamic scintillation camera imaging. Eight adult male baboons (mean weight, 9.3 kg) underwent bilateral aortoiliac bypass grafting with ePTFE grafts (4-mm internal diameter). In each animal a heparin-coated ePTFE graft was placed in one aortoiliac artery, and an uncoated graft, which served as the control, was placed in the contralateral aortoiliac artery. All grafts were harvested at 4 weeks, and were analyzed quantitatively for neointimal hyperplasia at graft-vessel anastomoses.

RESULTS

Early platelet deposition on heparin-coated grafts after 1 to 4 hours of ex vivo circuitry was significantly reduced. All the harvested aortoiliac grafts were patent at 4 weeks. There was a significant reduction in neointimal area at both proximal (0.26 +/- 0.11 mm(2)) and distal (0.29 +/- 0.14 mm(2)) anastomoses in the heparin-coated grafts, compared with proximal (0.56 +/- 0.18 mm(2)) and distal (0.63 +/- 0.21 mm(2)) anastomoses in the untreated control grafts (P <.05). In addition, neointimal cell proliferation assayed with bromodeoxyuridine (BrdU) incorporation was reduced in the graft neointima (3.47% +/- 0.43%) in heparin-coated grafts compared with the graft neointima (6.21% +/- 0.59%) in untreated control grafts (P <.05).

CONCLUSIONS

Small-caliber heparin-coated ePTFE grafts significantly reduce platelet deposition and anastomotic neointimal hyperplasia and cell proliferation, without measurable side effects, in baboons. Surface coating with heparin in small-caliber ePTFE grafts is useful for improving prosthetic bypass graft patency.

CLINICAL RELEVANCE

An autologous vein graft is the ideal bypass conduit in peripheral arterial reconstruction; however, many patients who undergo bypass grafting do not have adequate or available autologous vein graft. As a result surgeons often must rely on prosthetic grafts as an alternative conduit in arterial bypass procedures. Clinical outcomes with prosthetic grafts in peripheral arterial reconstruction are generally inferior to those with autologous vein bypass grafts, in part because of anastomotic neointimal hyperplasia. This study evaluated the effect of small-caliber heparin-coated expandable polytetrafluoroethylene (ePTFE) grafts in aortoiliac reconstruction in a baboon model. The study found that heparin-coated ePTFE grafts resulted in less intimal hyperplasia and less platelet deposition after implantation, compared with noncoated control ePTFE grafts.

The balloon catheter induces an increase in contralateral carotid artery reactivity to angiotensin II and phenylephrine

1. The effects of balloon injury on the reactivity of ipsilateral and contralateral carotid arteries were compared to those observed in arteries from intact animals (control arteries). 2. Carotid arteries were obtained from Wistar rats 2, 4, 7, 15, 30 or 45 days after injury and mounted in an isolated organ bath. Reactivity to angiotensin II (Ang II), phenylephrine (Phe) and bradykinin (BK) was studied. Curves were constructed in the absence or presence of endothelium or after incubation with 10 microm indomethacin, 500 microm valeryl salicylate or 0.1 microm celecoxib. 3. Phe, Ang II and BK maximum effects (Emax) were decreased in ipsilateral arteries when compared to control arteries. No differences were observed among pD2 or Hill coefficient. 4. Emax to Phe (4 and 7 days) and to Ang II (15 and 30 days) increased in the contralateral artery. In addition, Phe or Ang II reactivity was not significantly different in aorta rings from control or carotid-injured animals. 5. The increased responsiveness of contralateral artery was not due to changes in carotid blood flow or resting membrane potential. The endothelium-dependent inhibitory component is not present in the contraction of contralateral arteries and it is not related to superoxide anion production. 6. Indomethacin decreased contralateral artery responsiveness to Phe and Ang II. Valeryl salicylate reduced the Ang II response in contralateral and control arteries. Celecoxib decreased the Phe Emax of contralateral artery. 7. In conclusion, decreased endothelium-derived factors and increased prostanoids appear to be responsible for the increased reactivity of contralateral arteries after injury.

Dissecting the role of chymase in angiotensin II formation and heart and blood vessel diseases

Inhibition of angiotensin II action or its formation by angiotensin-converting enzyme has been highly successful in the treatment of cardiovascular diseases. Since the identification of chymase as a major angiotensin II-forming enzyme in the human heart and its vessels more than a decade ago, numerous studies have sought to understand the importance of this enzyme in tissue angiotensin II formation and in the pathogenesis of hypertension, congestive heart failure, and vascular disease. Recent studies show that chymase and angiotensin-converting enzyme regulate angiotensin II production in distinct tissue compartments and that, in the pathogenesis of cardiovascular diseases, chymase-dependent effects extend beyond its ability to regulate tissue angiotensin II levels.

The urokinase receptor mediates basic fibroblast growth factor-dependent smooth muscle cell migration through baboon aortic explants

The urokinase receptor is required for vascular smooth muscle cell migration in vitro, but may not be needed in vivo since smooth muscle cell migration and intimal hyperplasia after arterial injury in mice are not affected by urokinase receptor gene deletion. We have used baboon aortic explants as a bridge between cell culture and in vivo experiments to determine if the urokinase receptor is required for smooth muscle cell proliferation and smooth muscle cell migration in primate vessels. Levels of urokinase receptor in explants increased with time after explantation, while blockade of urokinase receptor with an antibody decreased smooth muscle cell proliferation and smooth muscle cell migration from the explants. A blocking antibody to basic fibroblast growth decreased levels of urokinase and urokinase receptor in explants, and it decreased smooth muscle cell migration and mitogenesis. These results suggest that the factor urokinase receptor plays a positive role in smooth muscle cell migration and proliferation in injured primate arterial tissue, in part mediating the pro-migratory and proliferative effects of basic fibroblast growth factor released by damaged smooth muscle cells.

Bradykinin receptor antagonists attenuate neointimal proliferation postangioplasty

Bradykinin has been linked to the development of restenosis in response to vascular injury. We therefore examined the effect of bradykinin on vascular smooth muscle cell growth and neointimal formation in organ culture. Bradykinin stimulated both RNA and DNA synthesis (by 175%) in smooth muscle cells from either porcine or human coronary arteries and increased cell number in a concentration-dependent manner. Both p42/44 mitogen-activated protein kinase (MAPK) and p38 kinase were also activated. Treatment with [Hyp(3),Tyr(Me)(8)]bradykinin, a B(2) receptor agonist, stimulated thymidine incorporation by 146%, whereas B(1)-selective Lys-des-Arg(9)-bradykinin had no effect. Addition of the B(2) antagonist HOE-140 reduced the stimulation by 56%, whereas B(1)-selective des-Arg-HOE-140 had no significant effect. Similarly, HOE-140 attenuated angioplasty-induced neointimal formation in organ culture with an efficacy approaching 100% inhibition. These experiments suggest that bradykinin promotes smooth muscle proliferation after vascular injury, presumably via B(2) receptor-dependent activation of MAPK family pathways, and may explain the negative outcome of angiotensin converting enzyme inhibitor therapy on restenosis in nonrodent models.

Chronic blockade of angiotensin II action prevents glomerulosclerosis, but induces graft vasculopathy in experimental kidney transplantation

Long-term renin-angiotensin system blockade is beneficial in a variety of renal diseases. This study examines the long-term (34 weeks) effects of the angiotensin-converting enzyme inhibitor lisinopril and the angiotensin II receptor type I blocker L158,809 in the Fisher to Lewis rat model of chronic renal transplant failure. Treatment in allografted rats with lisinopril or L158,809 was initiated 10 days after transplantation, or at the time when proteinuria exceeded 50 mg/24 h. Untreated allografts and syngrafts served as controls. In contrast to syngrafts, untreated allografts developed proteinuria, hypercholesterolaemia, interstitial damage, and glomerulosclerosis. Lisinopril or L158,809 treatment in allografts starting at day 10 after transplantation completely prevented this, with the exception of interstitial damage, but this treatment also caused a reduction in blood pressure and renal function. Moreover, the intimal surface area of the renal arteries was dramatically increased in allografts treated with either lisinopril or L158,809 compared with untreated allografted rats. Treatment once proteinuria had developed was less effective in preventing glomerulosclerosis, but also caused less intimal expansion. Thus, chronic renin-angiotensin system blockade preserves glomerular morphology in the absence of proteinuria, but enhances intimal hyperplasia and reduces renal function in experimental transplantation. In view of these results, it should be questioned whether such treatment benefits renal transplant patients in the long term.

Structure of carotid artery in baboon and rat and differences in their response to endothelial denudation angioplasty

BACKGROUND

Most studies dealing with vascular response to injury have been conducted using rodent and rabbit models, although it is expected that the response to injury in these species is dissimilar from man.

AIMS

Here we compare the structure of native carotid artery in rat and baboon and the response of these vessels to endothelial denudation angioplasty.

METHODS

In both species, the carotid is a musculoelastic artery. Only baboon carotid has a distinct intima, correlating in size with the weight of male baboons. Complete endothelial denudation of left carotid was performed on eight male baboons and 24 male rats by applying an equivalent pull force with a Fogarthy catheter. The animals were sacrificed prior to and 15 min and 2, 3, 4, 7, 14 and 28 days postinjury, one baboon and three rats per time point.

RESULTS

Re-endothelialization in the baboon was complete already on day 4, whereas in the rat it was still incomplete on day 28. The proliferative response to injury was far smaller in the baboon than in the rat, the intimal area increased only by 5-fold in baboon compared with 25-fold in rat, and the number of intimal nuclei by 4-fold in baboon compared with 12-fold in rat. Complete compensatory remodelling of the lumen size occurred in the baboon, whereas in the rat remodelling remained incomplete. The cell types participating in the response were, however, similar: deposition of thrombocytes on denuded luminal surface, expression of alpha-actin by intimal cells, and lack of any significant white cell infiltration in the denuded intima.

CONCLUSIONS

Baboon carotids are very different from rat carotids both in their native structure and in their response to injury. With the limited amount of information available from human vessels, baboon carotids closely resemble human carotids in both respects.

Nitric oxide and postangioplasty restenosis: pathological correlates and therapeutic potential

Balloon angioplasty revolutionized interventional cardiology as a nonsurgical procedure to clear a diseased artery of atherosclerotic blockage. Despite its procedural reliability, angioplasty's long-term outcome can be compromised by restenosis, the recurrence of arterial blockage in response to balloon-induced vascular trauma. Restenosis constitutes an important unmet medical need whose pathogenesis has yet to be understood fully and remains to be solved therapeutically. The radical biomediator, nitric oxide (NO), is a natural modulator of several processes contributing to postangioplasty restenosis. An arterial NO deficiency has been implicated in the establishment and progression of restenosis. Efforts to address the restenosis problem have included trials evaluating a wide range of NO-based interventions for their potential to inhibit balloon-induced arterial occlusion. All types of NO-based interventions yet investigated benefit at least one aspect of balloon injury to a naive vessel in a laboratory animal without inducing significant side effects. The extent to which this positive, albeit largely descriptive, body of experimental data can be translated into the clinic remains to be determined. Further insight into the pathogenesis of restenosis and the molecular mechanisms by which NO regulates vascular homeostasis would help bridge this gap. At present, NO supplementation represents a unique and potentially powerful approach to help control restenosis, either alone or as a pharmaceutical adjunct to a vascular device.

Action of angiotensin II on DNA synthesis by human saphenous vein in organ culture

Angiotensin II (Ang II), an effector peptide of the renin-angiotensin system, has been reported to stimulate growth of blood vessels in vivo and smooth muscle cells in culture. In this study, the effect of Ang II on DNA synthesis was examined in deendothelialized human saphenous vein in organ culture. After 7 days' exposure to medium containing 0.4% fetal calf serum plus Ang II, there was a marked increase in DNA synthesis. The effect of Ang II was comparable to the response to platelet-derived growth factor. Responses to Ang II were partially inhibited by the AT(1) receptor antagonist candesartan. An AT(2) receptor antagonist, PD123319, had no effect on Ang II-induced DNA synthesis, either alone or in combination with candesartan. The Ang II peptide analogues [Sar(1), Ile(8)]-Ang II (saralasin) and [Sar(1),Thr(8)]-Ang II (sarthran) acted as agonists, increasing DNA synthesis. In the presence of saralasin, responses to Ang II were inhibited. Tyrphostin-23, a tyrosine kinase inhibitor, prevented Ang II-induced DNA synthesis and reduced DNA synthesis in tissues incubated in medium containing only 0.4% fetal calf serum. In conclusion, Ang II stimulates DNA synthesis in human saphenous vein in organ culture. The effect of Ang II was more marked than has been previously reported in isolated cultured saphenous vein smooth muscle cells, and this effect is mediated in part by an angiotensin type 1 receptor. It is possible that an undefined receptor for Ang II may also be involved in the stimulation of DNA synthesis in this preparation.

Secretory production of recombinant human chymase as an active form in Pichia pastoris

We succeeded in expressing in a Pichia pastoris (P. pastoris) host a cDNA encoding a mature human chymase (h-chymase) which was secreted directly into the culture medium. Recombinant human heart chymase (rh-chymase) was purified from the culture medium via a single one-step heparin-agarose column chromatography tracing, using succinyl-Ala-Ala-Pro-Phe-para-nitroanilide (Suc-AAPF-pNA) hydrolysing activity. On SDS-polyacrylamide gel electrophoresis (SDS-PAGE), the rh-chymase showed a diffused protein band with molecular weight of 32-37 kDa. After deglycosylation, however, rh-chymase changed to a sharp protein band with molecular weight 28 kDa, which is equal in size to deglycosylated h-chymase. The rh-chymase had an activity to convert one of the natural substrates, angiotensin I, to angiotensin II. Double reciprocal plot analysis revealed that the K(m) value ofrh-chymase against Suc-AAPF-pNA was approximately 5.1 mM, which is close to that of purified h-chymase.

Inhibition of chymase reduces vascular proliferation in dog grafted veins

We investigated the effect of a chymase inhibitor Suc-Val-Pro-Phe(P)(OPh)(2) on the proliferation of the grafted vein in dog. By 28 days after the operation, the mean intimal area of the grafted vein in the placebo group was 3.24+/-0.32 mm(2). The intimal area of the grafted vein in the chymase inhibitor-treated group was reduced to 63.9%. In the placebo group, the activities of chymase and angiotensin-converting enzyme in grafted vein were significantly increased 15- and 2-fold, respectively. In the chymase inhibitor-treated group, chymase activity in the grafted veins was decreased significantly. These findings suggest that inhibition of chymase appears useful for preventing vascular proliferation.

Implications of species difference for clinical investigation: studies on the renin-angiotensin system

The justification for clinical investigation has its roots in the fact that physiological mechanisms and disease pathogenesis in animal models replicate mechanisms and pathogenesis in humans only in part. In the case of the renin-angiotensin system, there is species variation in the anatomic distribution of the renin-angiotensin system, in the active site of the renin enzyme, and in the structure of angiotensin and the AT(1) receptor. The conversion of angiotensin I (Ang I) to angiotensin II (Ang II) may prove to be the most important aspect of species variation. In plasma, all the conversion occurs through a single enzyme, angiotensin-converting enzyme (ACE), and species variation in structure and function have not been reported. Non-ACE-dependent pathways, which occur only at the tissue level, show unambiguous, striking species variation. Specifically, chymase, the most important enzyme responsible for non-ACE conversion of Ang I to Ang II, shows striking species variation. In humans and a number of species, including the hamster, quantitatively important chymase-independent Ang II formation from Ang I occurs in the heart, arteries, and kidney. In rats and rabbits, on the other hand, chymase differs, is not active in the conversion of Ang I to Ang II, and indeed is involved in Ang II degradation. Consequently, one would anticipate that blockade of the system at the ACE step would be equivalent to that at the Ang II receptor in the rat. This has been widely reported. In humans, on the other hand, one would anticipate that the AT(1) receptor blockers will be more effective than ACE inhibitors. Again, preliminary evidence favors this possibility. The implications for therapeutics are clear.

Reduction in vascular lesion formation by hirudin secreted from retrovirus-transduced confluent endothelial cells on vascular grafts in baboons

BACKGROUND

The hypothesis that thrombin mediates the formation of neointimal vascular lesions at sites of mechanical vascular injury has been tested in baboons by measurement of the effects of hirudin delivered by retrovirus-transduced hirudin-secreting vascular endothelial cells (ECs) lining surgically implanted arterial vascular grafts (AVGs).

METHODS AND RESULTS

The antithrombotic efficacy of baboon ECs transduced with cDNA encoding hirudin was assessed in vitro and in vivo on thrombogenic segments in chronically exteriorized femoral arteriovenous (AV) shunts. Bilateral brachial AVGs lined with hirudin-transduced versus nonhirudin control ECs at confluent density were surgically implanted, and vascular lesion formations at distal graft-vessel anastomoses were compared after 30 days. Hirudin-transduced ECs secreted 20+/-6 ng x 10(6) cells(-1) x 24 h(-1) (range, 14 to 24 ng x 10(6) cells(-1) x 24 h(-1)) hirudin in supernatants of static cultures. Hirudin-secreting ECs on segments of collagen-coated graft interposed in chronic AV shunts decreased the accumulation of (111)In-labeled platelets to 0.52+/-0.34 x 10(9) platelets, compared with 0.82+/-0.49 x 10(9) platelets in controls (P = 0.03) and reduced platelet deposition in propagated thrombotic tails extending downstream from segments of vascular graft from 1.38+/-0.41 x 10(9) platelets in controls to 0.59+/-0.22 x 10(9) platelets (P = 0.04). ECs recovered from 30-day AVG implants generated 17+/-9 ng x 10(6) cells(-1) x 24 h(-1) (range, 9 to 25 ng x 10(6) cells(-1) x 24 h(-1)) hirudin. Hirudin-secreting ECs reduced neointimal lesion formation at distal graft-vessel anastomoses, ie, 1.02 mm(2) (range, 0.88 to 1.95 mm(2)) versus 1.82 mm(2) (range, 0.88 to 2.56 mm(2)) in contralateral AVGs bearing nonhirudin control ECs (P<0.01).

CONCLUSIONS

Viral vector-directed secretion of hirudin from ECs lining implanted AVGs significantly reduces the formation of thrombus and neointimal vascular lesions.

A new animal model to study intimal hyperplasia in arteriovenous fistulas

BACKGROUND

Intimal hyperplasia (IH) plays a key role in the failure of arteriovenous (AV) fistulas. The animal models available to study IH in AV fistulas are expensive and do not mimic the development of truly stenotic IH. In this study we examined whether goats are a more suitable model to study IH in AV fistulas.

MATERIALS AND METHODS

Thirteen direct and four bridge graft AV fistulas between the carotid artery and the jugular vein of goats were explanted 10 to 195 days after creation. Immunohistochemical staining and morphometric measurements of intima and media were performed in the artery, the vein, the toe, and the heel of the venous anastomosis. Ratios of intimal to medial thickness (Ith/Mth) and area (Ia/Ma) were calculated.

RESULTS

IH developed in all goats, mainly at the anastomosis (Ia/Ma = 0.17) and the efferent vein (Ia/Ma = 0.31). The artery was almost free of lesions (Ia/Ma = 0.03). In the efferent vein, Ith/Mth varied between 0.59 and 0.68. In the anastomosis the largest value of Ith/Mth was measured at the suture lines (0.88 and 0.91). Absolute intimal area increased with time. IH contained many vascular smooth muscle cells with a patchy display of desmin positivity, an abundance of smooth muscle cell alpha-actin positivity, and almost complete endothelial cell coverage. Occlusion was due to thrombus formation on the IH.

CONCLUSION

A clear intimal hyperplasia developed in AV fistulas in goats at locations comparable to those in humans. Therefore, the AV fistula model in the goat may be seen as an effective model to study IH in hemodialysis AV fistulas.

Vascular injury causes neointimal formation in angiotensin II type 1a receptor knockout mice

Many studies using small-animal models suggest that angiotensin II (Ang II) plays an important role in neointimal formation after vascular injury. In the present study, we examined whether Ang II type 1 receptor (AT1)-mediated Ang II signaling is indispensable for the development of injury-induced neointimal formation using AT1a knockout (KO) mice. Reverse transcriptase-polymerase chain reaction analysis revealed that AT1 mRNA was not detectable in both uninjured and injured carotid arteries of KO mice, whereas the AT1 gene was expressed in uninjured carotid arteries of wild-type (WT) mice. At 14 days after injury, AT1 mRNA levels were increased by 1.5-fold in injured arteries of WT mice. Although AT2 mRNA was not detectable in uninjured arteries, expression of AT2 gene was induced in both animal groups at 2 weeks after injury. Vascular injury induced neointimal formation in KO mice as well as in WT mice. There were no significant differences between WT and KO mice in the extent of histological findings such as increased cross-sectional areas of the neointima and the media, the number of proliferating smooth muscle cells, and the amount of collagen and fibronectin. Treatment with subpressor doses of Ang II after injury enhanced the growth of neointima in WT mice but not in KO mice. Moreover, treatment with the selective AT1 antagonist CV-11974 before injury significantly decreased the formation of neointima in only WT mice, whereas treatment with the selective AT2 antagonist PD-123319 before injury had no effects in both animal groups. These results suggest that AT1-mediated Ang II signaling is not essential for the development of neointimal formation, although it may modify it.

Angiotensin II type 1 receptor-mediated peroxide production in human macrophages

Our previous experiments demonstrated upregulation of the renin-angiotensin system in macrophages, including angiotensin II type 1 (AT1) and type 2 (AT2) receptors, during transformation from monocytes. We investigated the role of angiotensin II in oxidative stress of monocytes/macrophages, which plays a role in the advance of atherosclerosis. THP1, a human monocytic leukemia cell line, was differentiated to macrophages by adding of phorbol 12-myristate 13-acetate for 24 hours. The intracellular production of peroxide was measured by a cytofluorometric assay with 2', 7'-dichlorofluorescein-diacetate with a flow cytometer scan. Peroxide was detected in monocytes and upregulated during the transformation to macrophages by 3.18+/-0.52 times in relative fluorescein of peak value (P<0.01). Angiotensin II (1 micromol/L) induced oxidative stress in macrophages, with the peak at 15 minutes by 451+/-223%, and returned to the control level within 1 hour. EC50 was 5.4x10(-9) mol/L. AT1 antagonist (CV11974, 1 micromol/L) significantly decreased angiotensin II-induced oxidative stress in macrophages, but AT2 antagonist (PD123319, 1 micromol/L) did not. Of interest, AT1 antagonist also decreased basal levels of peroxide production in macrophages in a dose-dependent manner. These results suggest that upregulation of the expression of AT1 receptor in macrophages contributes in part to upregulation of peroxide production. AT1 receptor antagonists may be useful to suppress oxidative stress of macrophages in atherosclerotic lesions.

Pathways for angiotensin II generation in intact human tissue: evidence from comparative pharmacological interruption of the renin system

Multiple lines of evidence have suggested that alternative pathways to the angiotensin-converting enzyme (ACE) exists for angiotensin II (Ang II) generation in the heart, large arteries, and the kidney. In vitro studies in intact tissues, homogenates, or membrane isolates from the heart and large arteries have repeatedly demonstrated such pathways, but the issue remains unresolved because the approaches used have not made it possible to extrapolate from the in vitro to the in vivo situation. For our in vivo model, we studied young and healthy human volunteers, for the most part white and male; when these subjects achieved balance on a low salt diet to activate the renin system, the response of renal perfusion to pharmacological interruption of the renin system was studied. With this approach, we studied the renal vasodilator response to 3 ACE inhibitors, 2 renin inhibitors, and 2 Ang II antagonists at the top of their respective dose-response relationships. When these studies were initiated, our premise was that a kinin-dependent mechanism contributed to the renal hemodynamic response to ACE inhibition; therefore, the renal vasodilator response to ACE inhibition would exceed the alternatives. To our surprise, both renin inhibitors and both Ang II antagonists that were studied induced a renal vasodilator response of 140 to 150 mL/min/1.73 m2, approximately 50% larger than the maximal renal hemodynamic response to ACE inhibition, which was 90 to 100 mL/min/1.73 m2. In light of the data from in vitro systems, our findings indicate that in the intact human kidney, virtually all Ang II generation is renin-dependent but at least 40% of Ang I is converted to Ang II by pathways other than ACE, presumably a chymase, although other enzyme pathways exist. Preliminary data indicate that the non-ACE pathway may be substantially larger in disease states such as diabetes mellitus. One implication of the studies is that at the tissue level, Ang II antagonists have much greater potential for blocking the renin-angiotensin system than does ACE inhibition-with implications for therapeutics.

Mechanisms of neointima formation--lessons from experimental models

The lamina intima of an artery is the region between the endothelial cell surface and the internal elastic lamina, which forms the luminal margin of the media. In humans the intima of atherosclerosis-prone arteries becomes thicker due to accumulation of smooth muscle cells, which originate from the media. The introduction of percutaneous transluminal coronary angioplasty (PTCA) boosted scientific interest in intimal thickening, because restenosis remains an unresolved problem of this intervention. In order to unravel the mechanisms of intimal thickening there is a need for appropriate animal models. A brief overview of these models is given together with factors that control proliferation and/or migration. Despite intensive research on neointima formation, an effective therapy for restenosis has not emerged to date. This may be due to the fact that other processes, such as acute elastic recoil and chronic constrictive remodeling may contribute to lumen narrowing as well. Other limitations of neointima models are related to species and anatomical differences. Most studies are performed in arteries that are either lesion-free, or contain relatively mild plaques, in contrast to the complicated, stenotic lesions that are the substrate for human PTCA. Other differences are the severity of the injury and incorporation of a mural fibrin-rich thrombus. Nevertheless, studies based on superficial injury, like the frequently used balloon denudation model, are useful. There are similarities with angioplasty, such as endothelial cell damage and proliferation of medial and intimal smooth muscle cells. The use of techniques such as differential display, gene transfer and application of antisense oligonucleotides may provide new therapeutic approaches to reduce neointima formation.

Bradykinin-mediated cardiovascular protective actions of ACE inhibitors. A new dimension in anti-ischaemic therapy?

In addition to being accepted therapy in hypertension and heart failure, ACE inhibitors may well offer a new dimension in anti-ischaemic therapy. Currently, anti-ischaemic properties have been demonstrated by ACE inhibitors in selected patient groups, including patients with left ventricular dysfunction with or without a direct temporal relationship with myocardial infarction. Anti-ischaemic effects of ACE inhibitors become apparent late after initiation of treatment and suggest a structural rather than a functional effect. Underlying mechanisms may include a reduction in ventricular dilatation and (abnormal) cardiac hypertrophy, leading to less myocardial oxygen demand and, possibly, improved subendocardial blood supply, and vasculoprotective effects, i.e. anti-atherosclerotic and antiremodelling properties, a beneficial effect on the fibrinolytic system and an improvement in abnormal endothelial vasodilator function. The latter aspect is most probably the pivotal mode of action where the anti-ischaemic profile of ACE inhibition is concerned. An improvement in endothelial dysfunction has been shown in patients with mild to moderate coronary artery disease [Trial on Reversing ENdothelial Dysfunction (TREND)]. It is of importance that, in both clinical experiments and human studies, the role of bradykinin appears central in the structural and functional cardiovascular effects of ACE inhibition. This is particularly true for the improvement of impaired endothelial function. Myocardial ischaemia evokes vasoconstrictor neurohormonal activation, which may lead to coronary vasoconstriction in diseased coronary segments. The subsequent abnormal endothelial function leads to diminished coronary flow and also increases systemic vasotone and afterload, thus unfavourably altering the myocardial oxygen supply/demand ratio. Under laboratory conditions, acute ACE inhibition counteracts this activation in humans. However, it is speculated that this anti-ischaemic mechanism may become operative and clinically important during long term oral ACE inhibitor therapy when endothelial function improves, and may subsequently protect against the vasoconstrictor effect of neurohormonal activation. As it is unlikely that the mechanisms mentioned above are only relevant in patients with ventricular dysfunction or heart failure, several large controlled trials are currently examining the long term anti-ischaemic and cardiovascular protective effects of ACE inhibition in patients at risk of a cardiovascular event [Heart Outcomes Prevention Evaluation study (HOPE)], with a normal cardiac function [Prevention of Events with ACE inhibition study (PEACE)] or in all patients with coronary artery disease irrespective of cardiac function [EUropean trial of Reduction Of cardiac events with Perindopril in stable coronary Artery disease (EUROPA)].

Primate smooth muscle cell migration from aortic explants is mediated by endogenous platelet-derived growth factor and basic fibroblast growth factor acting through matrix metalloproteinases 2 and 9

BACKGROUND

Migration of arterial smooth muscle cells (SMCs) is regulated by basic fibroblast growth factor (bFGF), platelet-derived growth factor (PDGF), and matrix metalloproteinases (MMPs) in the injured rat carotid artery. We have recently shown that migration of SMCs from baboon aortic explants depends on the activity of MMPs, but the identity of the stimulatory MMPs and the role of bFGF and PDGF in this primate system are not known.

METHODS AND RESULTS

These experiments were designed to determine whether MMP2, MMP9, bFGF, or PDGF plays a role in SMC migration from medial explants of baboon aorta. Explants were cultured in serum-free medium with insulin, transferrin, and ovalbumin. Neutralizing antibodies to MMP2 and antibodies that inhibit activation of proMMP9 decreased SMC migration from the aortic explants. Antibodies to bFGF and to the alpha- and beta-subunits of the PDGF receptor also inhibited migration from the explants. Addition of bFGF and PDGF-BB but not PDGF-AA increased migration. The antibodies to bFGF but not the antibodies to the PDGF receptor subunits decreased the levels of MMP9, whereas all the antibodies decreased activated MMP2.

CONCLUSIONS

These data demonstrate that SMC migration from primate aortic explants is dependent on endogenous MMP2, MMP9, PDGF, and bFGF. The data also suggest that PDGF-induced (PDGF-BB or possibly PDGF-AB) migration is dependent on MMP2, whereas bFGF-induced migration depends on both MMP2 and MMP9.

Angiotensin-converting enzyme inhibition abolishes medial smooth muscle PDGF-AB biosynthesis and attenuates cell proliferation in injured carotid arteries: relationships to neointima formation

BACKGROUND

ACE inhibitors can attenuate the development of intimal fibrocellular lesions after balloon catheter vessel injury, but the mechanisms responsible are unknown.

METHODS AND RESULTS

To evaluate how basic fibroblast growth factor (FGF-2) and the platelet-derived growth factor (PDGF) isoforms are affected by ACE inhibition in injured rat carotid arteries in relation to smooth muscle cell (SMC) proliferation, we examined the effects of oral perindopril on FGF-2 and PDGF isoform levels in carotid arteries 2 days after balloon catheter injury. [3H]Thymidine incorporation into medial and intimal SMCs was also assessed. Uninjured vessels contained two forms of FGF-2, with molecular weights of 18 and 22 kD, and PDGF-AA. Two days after injury, FGF-2 and PDGF-AA levels were markedly reduced, but high levels of PDGF-AB became apparent when the SMCs were proliferating. Perindopril completely abolished the biosynthesis of PDGF-AB but had little effect on residual FGF-2. This was accompanied by a 25% reduction in medial SMC proliferation. Neointimal cell proliferation 10 days after injury was unaffected by perindopril, although neointima size was reduced by 30%. Commencing perindopril treatment 4 days after the injury confirmed that early events associated with effects on medial SMCs were the major contributors to the attenuated neointimal lesions.

CONCLUSIONS

The ability of ACE inhibitors such as perindopril to attenuate neointima formation and growth in balloon catheter-injured rat carotid arteries is dependent on early events in the media, the inhibition of SMC PDGF-AB biosynthesis and attenuation of proliferation. Neointima formation in similarly injured vessels containing SMCs that are either unresponsive to PDGF-AB or exhibit an ACE-independent profile of growth factor biosynthesis responses may account for the ineffectiveness of ACE inhibition in some species.

Reduction in stent and vascular graft thrombosis and enhancement of thrombolysis by recombinant Lys-plasminogen in nonhuman primates

BACKGROUND

To enhance thrombolytic responses without increasing hemorrhagic risks, the antithrombotic effects of recombinant Lys-plasminogen (r-LysPgn), a prothrombolytic plasminogen intermediate, were examined in baboon models of thrombus formation and dissolution.

METHODS AND RESULTS

The dose-response effects of r-LysPgn, alone or in combination with subthreshold dosing of tissue plasminogen activator (TPA), were measured with respect to the accumulation of (111)In-labeled platelets and (125)I-fibrin in thrombus forming on endovascular metallic stents or thrombogenic segments of vascular graft interposed in exteriorized long-term arteriovenous (AV) femoral shunts. Thrombolytic losses have also been determined for preformed, stable, (111)In-platelet- and (125)I-fibrin-labeled graft thrombus and corresponding propagated thrombotic tails, together with changes in blood tests of thrombosis, thrombolysis, and hemostasis. Bolus intravenous r-LysPgn in escalating doses (2, 4, or 8 mg/kg) increased circulating plasminogen levels in a dose-dependent manner, was removed by log-linear clearance with a T50 of 120 minutes, and reciprocally decreased the accumulating thrombus on metallic stents and segments of vascular graft (P<.001 in all cases for 8-mg/kg doses). r-LysPgn also impaired platelet aggregatory responses to physiological agonists in vitro but not ex vivo. Prethrombosis administration of low-dose r-LysPgn (2 mg/kg) greatly enhanced the lysis of radiolabeled nonoccluding thrombus by a subthreshold dose of TPA (0.1 mg/kg) compared with TPA-only controls (P=.03).

CONCLUSIONS

Elective bolus injections of r-LysPgn before stent deployment decrease the amount of thrombus formed without compromising hemostasis by facilitating endogenous TPA thrombolysis. r-LysPgn may provide effective and safe antithrombotic therapy for interventional vascular procedures.

Early induction of angiotensin I-converting enzyme in rat carotid artery after balloon injury

Several studies have demonstrated the effectiveness of angiotensin I-converting enzyme (ACE) inhibitors in preventing the neointima formation found after denudation of the rat carotid artery by balloon injury. The aim of the present study was to determine the role of ACE in this model and to compare the treatment with the ACE inhibitor ramiprilat with that with the angiotensin II antagonist HR 720. The endothelial layer of the left carotid artery was removed using an inflated balloon catheter. Injured and control vessels were both submitted to histomorphological analysis and DNA content quantification at 2, 4, 6, 8, 12, and 14 days after injury. Evaluation of neointima thickening demonstrated a slow but steady increase of neointima that was significant after day 6 and reached 30% of the lumen in 2 weeks. This was paralleled by an increase in DNA content, which was significant 4 days after injury. ACE mRNA levels were quantified by polymerase chain reaction after reverse transcription. Measurement of ACE mRNA levels revealed a significant upregulation 2 and 8 days after injury, with no significant difference when compared with control tissue at later time points. ACE activity was also significantly enhanced at 2 and 8 days after injury, with no significant difference when compared with control tissue at later time points. In addition, the treatment with ramiprilat was more efficient in reducing neointima formation than that with HR 720. These data underlie the role of ACE in this model of restenosis. The early induction of ACE expression after endothelial injury but before significant changes in the vessel structure suggests that ACE activity might be one of the mechanisms that trigger neointima formation in the rat.

Localization of lipoprotein(a) in a monkey model of rapid neointimal growth

Lipoprotein(a) [Lp(a)] has been proposed as a restenosis risk factor, but it is not known if Lp(a) is present in the injured arterial wall during the initial neointimal growth. The purpose of this study was to determine if Lp(a) is incorporated into the vessel wall during rapid neointimal formation after arterial injury in primates. In this model, distention of the iliac artery with an angioplasty catheter caused focal breaks in the internal elastic lamina (IEL) in 80% of the vessels and extensive IEL fragmentation with medial disruption in 20% of the vessels. Neointimal growth was noted in all injured arteries; thrombus formation was noted in 40% of the vessels. Based on morphometric measurements, injured arteries had neointimal areas of 0.41 +/- 0.05 (n = 4) and 0.83 +/- 0.23 (n = 6) mm2 at 14 and 28 days after injury, respectively. Control arteries had an intact IEL and a monolayer of intimal cells. Lp(a) localization was examined histologically by using a mouse monoclonal anti-Lp(a) antibody. Lp(a), found in all injured arteries, was localized primarily in the neointima in 50% of the vessels. In the subset of vessels with evidence of thrombus formation, intense Lp(a) immunostaining was associated with the thrombus. Lp(a) was specific to injured arteries as uninjured vessels did not stain. In addition, staining was not seen with a negative control, a nonspecific mouse IgG1 antibody. The presence of Lp(a) at the site of rapid neointimal growth supports a role for this lipoprotein in the response to vascular injury after balloon angioplasty.

The role of plasminogen, plasminogen activators, and matrix metalloproteinases in primate arterial smooth muscle cell migration

The migration of arterial smooth muscle cells (SMCs) plays an important role in normal vessel development as well as the pathobiology of blood vessels. Because it is difficult to study cell migration in primates, we used ex vivo explants. The response of baboon aortic medial explants incubated in vitro in a serum-free medium with insulin and transferrin was compared with the response of whole artery injured in vivo by a balloon catheter to establish the validity of the explant model. Both the time course of entry of SMCs into the S phase and the changes in matrix metalloproteinase 9 were similar in the artery and the explants. SMCs began migrating from explants after a lag of 3 days. By day 11, > 90% of the explants exhibited SMC migration from the tissue (percent of explants with > or = 1 migrating cell). Basal migration was inhibited by antibodies to urokinase and tissue-type plasminogen activator, whereas addition of plasminogen to the explants increased migration. An inhibitor of matrix metalloproteinases. BB-94 (Batimistat), decreased migration, as did alpha 2-macroglobulin. These data demonstrate that proteinases of the matrix metalloproteinase and plasminogen/plasminogen activator families play an important role in the migration of primate arterial SMCs through the extracellular matrix.

Recombinant mitotoxin basic fibroblast growth factor-saporin reduces venous anastomotic intimal hyperplasia in the arteriovenous graft

BACKGROUND

The plant cytotoxin saporin (SAP) is a potent ribosome-inactivating protein. When conjugated to basic fibroblast growth factor (FGF2), it selectively kills proliferating cells that have upregulated FGF receptors. In this study, we evaluated the effect of the recombinant chimeric mitotoxin rFGF2-SAP on venous anastomotic intimal hyperplasia, a major cause of failure of arteriovenous (AV) grafts.

METHODS AND RESULTS

Recently designed expanded polytet-rafluoroethylene-based local infusion devices were implanted bilaterally as femoral AV conduits in six dogs. The venous anastomoses were the sites of continuous delivery of rFGF2-SAP (2.7 micrograms.kg-1.d-1) to one side and vehicle (4.6 microL.kg-1.d-1) as control to the contralateral side for 14 days. All animals survived, and all grafts were patent. Liver enzyme levels and histological analyses of liver, kidneys, and brain were normal, indicating the absence of systemic toxicity. Morphometric measurements and measurements of cell proliferation by bromodeoxyuridine index analysis were performed at both arterial and venous anastomoses. There were no significant differences between the treated grafts and the control grafts in intimal hyperplasia and intimal cell proliferation at the arterial anastomoses. In contrast, rFGF2-SAP reduced intimal thickness by 32%, intimal area by 40%, and cell proliferation index by 33% at the treated venous anastomoses compared with the control venous anastomoses (P < .05).

CONCLUSIONS

These data demonstrate that local infusion of rFGF2-SAP significantly reduces venous anastomotic intimal hyperplasia and cell proliferation without systemic toxicity. This study suggests a new strategy for reducing intimal hyperplasia by the selective killing of proliferating smooth muscle cells with a potent chimeric mitotoxin through a novel local infusion device.

Effects of angiotensin I and angiotensin II in blood vessels: greater influence of converting enzyme activity in the rabbit basilar artery

We investigated the constrictor effects of Angiotensin I (Ang I) and Angiotensin II (Ang II) on rabbit peripheral (aorta, carotid artery, mesenteric artery, saphenous artery) and cerebral (basilar artery) vessels and in rat aorta in functional organ bath studies. The effect of angiotensin converting enzyme (ACE) inhibition by captopril was also assessed in these preparations. Ang II elicited concentration-dependent contractions with comparable potency in rabbit and rat endothelium-free vascular rings (pD2 about 8.5) which indicates a lack of species and regional variation in the contractile responses to Ang II. The responses to Ang II were reduced by the presence of a functional endothelium in rabbit mesenteric artery and in rat aorta. Since ACE determines the plasma and tissue conversion of Ang I to active Ang II, we calculated the ratio R (EC50 Ang I-induced contraction: EC50 Ang II-induced contraction) as an indicator of the tissue ACE effectiveness. In the aorta without endothelium, Ang I was found to be much less potent than Ang II in the rabbit (R = 44) compared with the rat (R = 3.5). This species difference in the aortic conversion of Ang I to Ang II was confirmed by the use of captopril. Captopril (10(-6) M) shifted the Ang I concentration/ response curve by 2- and 14-fold to the right in rabbit and rat respectively. In other rabbit blood vessels, the rank order of potency to Ang I in endothelium denuded rings was basilar artery > > carotid artery > or = aorta > or = saphenous artery. In addition, the R value was the lowest for the basilar artery (R = 2.5). This is in agreement with the highest rightward shift (78-fold) of the Ang I concentration/response curve by captopril for basilar artery in comparison with only 3-, 8- and 3-fold shifts observed in carotid artery, saphenous artery and aorta respectively. In conclusion, our data provide evidence for a greater influence of ACE in rabbit basilar artery than in peripheral vessels.

Inhibitor of angiotensin-converting enzyme modifies myointimal origin in an arterial autograft model

Pharmacologic modulation by an inhibitor of angiotensin-converting enzyme (IACE: cilazapril) of vascular proliferative response to a full-thickness arterial injury (autograft) was studied in rats. An arterial autograft 5 mm long was made in the right common iliac artery of 50 female Sprague-Dawley rats (weight 250-300 g) by microsurgical techniques. The animals were divided into two study groups: group I (controls), 20 animals that underwent arterial autograft but received no other treatment; and group II (cilazapril-treated), 20 rats that underwent arterial autograft and received cilazapril (Roche), 10 mg/day orally (p.o.) in an excipient of 2% arabic gum, for 4 days before operation. Animals were killed on postoperative days 7, 14, 21, 30, and 50, and grafts were studied by light microscopy, scanning and transmission electron microscopy, and morphometry. In the control group, the hyperplasic response had begun by postoperative day 14 and was established by postoperative day 50. In the medial layer, the muscle cells changed in phenotype from contractile to secretory cells. The adventitia had a highly proliferative appearance. In the cilazapril-treated group, fibrin deposits and platelets formed a layer on the internal elastic lamina. This layer appeared to evolve toward an intimal hyperplasia that became quantifiable by postoperative day 21. The medial layer was clearly thinned and showed intense accumulation of lipid microvacuoles, elastic degeneration, and vacuolized cells. Our results suggest that the use of an inhibitor of ACE modified the origin of the intimal hyperplasia in the arterial autograft model. Enhancement of the thrombogenicity of the luminal surface favors myointimal development by thrombus reorganization.

Effects of subtype-selective and balanced angiotensin II receptor antagonists in a porcine coronary artery model of vascular restenosis

BACKGROUND

Numerous studies have demonstrated the ability of angiotensin II (Ang II) receptor antagonists and angiotensin-converting enzyme (ACE) inhibitors to inhibit intimal hyperplasia after balloon dilation of noncoronary arteries in small-animal models, suggesting an important role for Ang II in the response to injury. Although ACE inhibitors have not been similarly effective in nonhuman coronary models or in human restenosis trials, questions remain regarding the efficacy ACE inhibitors against tissue ACE and the contributions of ACE-independent pathways of Ang II generation. Unlike ACE inhibitors, Ang II receptor antagonists have the potential to inhibit responses to Ang II independent of its biosynthetic origin.

METHODS AND RESULTS

In separate studies, three Ang II receptor antagonists, including AT1 selective (L-158,809), balanced AT1/AT2 (L-163,082), and AT2 selective (L-164,282) agents, were evaluated for their ability to inhibit vascular intimal thickening in a porcine coronary artery model of vascular injury. Preliminary studies in a rat carotid artery model revealed that constant infusion of L-158,809 (0.3 or 1.0 mg X kg-1 X d-1) reduced the neointimal cross-sectional area by up to 37% measured 14 days after balloon dilatation. In the porcine studies, animals were treated with vehicle or test compound beginning 2 days before and extending 28 days after experimental angioplasty. Left anterior descending, left circumflex, and/or right coronary arteries were injured by inflation of commercially available angioplasty balloons with placement of coiled metallic stents. Infusion of L-158,809 (1 mg X kg-1 X d-1), L-163,082 (1 mg X kg-1 X d-1), or L-164,282 (1.5 mg X kg-1 X d-1) in the study animals yielded plasma drug levels sufficient either to chronically block or, for L-164,282, to spare pressor responses to exogenous Ang II. Neither L-158,809, L-163,082, nor L-164,282 had statistically significant effects (P=.12, P=.75, and P=.48, respectively, compared with vehicle-treated controls) on neointimal thickness (normalized for degree of injury) measured by morphometric analysis at day 28 after angioplasty.

CONCLUSIONS

These findings indicate that chronic blockade of Ang II receptors by either site-selective or balanced AT1/AT2 antagonists is insufficient to inhibit intimal hyperplasia after experimental coronary vascular injury in the pig. The results further suggest that, unlike in the rat carotid artery, Ang II is not a major mediator of intimal thickening in the pig coronary artery.

Time course of cellular proliferation, intimal hyperplasia, and remodeling following angioplasty in monkeys with established atherosclerosis. A nonhuman primate model of restenosis

Animal models of arterial injury have failed to predict effective therapy to prevent restenosis in humans. While this may relate to species differences in the control of smooth muscle cell growth, many studies have used nonatherosclerotic animals, thereby failing to consider the importance of atherosclerosis in the response to injury. In an attempt to model human restenosis more accurately, we characterized the response to angioplasty in atherosclerotic monkeys. Twenty-one cynomolgus monkeys were fed an atherogenic diet for 36 months (plasma cholesterol, 12 +/- 1 mmol/L [470 +/- 23 mg/dL]). Angioplasty was then performed in the left iliac artery. After 4, 7, 14, or 28 days, bromodeoxyuridine was given to label proliferating cells, and iliac arteries were fixed in situ at physiological pressure (5 or 6 animals at each time point). Comparisons were made between injured and uninjured iliac arteries within each animal. Angioplasty often fractured the intimal plaque and media, transiently increasing lumen caliber (4 days: lumen area, 232.5 +/- 80.3% of control) and artery size as reflected by external elastic lamina area (EEL). EEL and lumen caliber returned to baseline by 7 days. Proliferation was increased throughout the artery wall at 4 and 7 days and later declined to control rates (4 days, injured versus uninjured: adventitia, 45.0 +/- 6.2% versus 16.3 +/- 7.2%; media, 8.6 +/- 2.6% versus 0.6 +/- 0.1%; intima, 16.0 +/- 5.6% versus 7.8 +/- 3.1%). The intima thickened markedly from 14 to 28 days, but an increase in EEL generally prevented further loss of the short-term gain in lumen caliber (28 days, percent of control: intimal area, 342.8 +/- 88.9%; EEL area, 150.2 +/- 28.9%; lumen area, 119.3 +/- 21.3%). The response to angioplasty in atherosclerotic monkeys appears to closely resemble that in humans. Plaque fracture, delayed recoil, intimal hyperplasia, and remodeling may each be important in determining late lumen caliber. This primate model should prove valuable in defining cellular and biochemical mediators of human restenosis.

Mechanism of inhibition of neointimal formation by the angiotensin-converting enzyme inhibitor cilazapril. A study in balloon catheter-injured rat carotid arteries

We investigated the mechanism of inhibition of neointima formation by the angiotensin-covering enzyme the carotid artery. We looked for the effects of cilazapril on all phases of the response to injury, ie, on proliferation of smooth muscle cells (SMCs) in the media, their migration, their proliferation in the neointima, and their disposition of extracellular matrix in the neointima. Although treatment was discontinued after 2 weeks, the inhibitory effect of cilazapril on neointimal formation was evident even 52 weeks after injury. The amount of extracellular matrix deposited in the intima during cilazapril treatment was decreased by 20% 2 weeks after injury, but no effect was seen if tissues were analyzed at 4 or 52 weeks. [3H]Thymidine-labeled cells (pulse labeling as well as 14-day continuous labeling) showed a decrease in SMC labeling in the tunica medica by 50%, but no inhibition in the labeling indices was seen in the neointima. The fraction of unlabeled neointimal cells in the cilazapril-treated rats as judged from continuous labeling experiments was inhibited by 86%. Taken together, these data suggest an antiproliferative effect on medial SMCs and an inhibition of SMC migration into the intima by cilazapril. Since intimal extracellular matrix deposition was only delayed, the decrease in medial SMC proliferation and subsequent migration seems to be the main reason for the reduction of neointima formation.

Pharmacologic intervention to prevent graft failure

Lower extremity vascular grafts, either vein or synthetic, fail for diverse reasons. Technical defects or poor surgical judgment doom a graft beyond any benefit pharmacotherapy can offer. Graft failure due to spontaneous thrombosis particularly affects prosthetic conduits, and use of antiplatelet agents (dextran, ASA) or anticoagulants (heparin, warfarin) is probably useful in this setting. An effective way to inhibit vein graft or anastomotic intimal hyperplasia remains elusive. Perhaps the most permanent and longstanding influence on lower extremity graft survival can be made through risk factor intervention aimed at arresting the progression of atherosclerosis. Aggressive treatment of hyperlipidemia, hypertension, smoking, and other known risk factors should be routinely and aggressively pursued in patients with lower extremity grafts, either autogenous or prosthetic. Lower extremity graft patency is optimally ensured by technically adept insertion of a proper autologous conduit in a well-selected patient. Pharmacotherapy may have a significant adjunctive role in the maintenance of graft patency, especially in high-risk settings such as limb salvage with associated poor outflow, a marginal vein graft, or the obligatory use of prosthetic material.

Failure of heparin to inhibit intimal hyperplasia in injured baboon arteries. The role of heparin-sensitive and -insensitive pathways in the stimulation of smooth muscle cell migration and proliferation

BACKGROUND

Heparin is a potent inhibitor of smooth muscle cell (SMC) growth and intimal hyperplasia in animal models but has been ineffective in inhibiting restenosis in humans. This difference may relate to flaws in clinical study design or, alternatively, to interspecies differences in SMC response to heparin. To determine whether heparin could inhibit intimal hyperplasia in a species more closely related to humans, we studied the effect of a low-molecular-weight heparin (LMWH) on baboon SMC proliferation and migration in culture and in arteries subjected to experimental angioplasty.

METHODS AND RESULTS

LMWH or saline was infused continuously after experimental angioplasty of baboon peripheral arteries (six animals per group). After 28 days, bromodeoxyuridine (BrdU) was given to label proliferating cells, and balloon-injured arteries were perfusion-fixed in situ and removed for analysis. All arteries had reendothelialized (Evans blue dye exclusion). LMWH increased partial thromboplastin time (LMWH, 81.7 +/- 8.4 seconds versus saline, 34.7 +/- 0.8 seconds [mean +/- SEM]; P = .004) but failed to inhibit intimal thickening or SMC proliferation (intimal area: LMWH, 0.19 +/- 0.03 mm2 versus saline, 0.21 +/- 0.03 mm2; BrdU labeling: LMWH, 2.9 +/- 0.6% versus saline, 2.4 +/- 0.4%; P = NS). In culture, LMWH and standard heparin (100 micrograms/mL) significantly inhibited serum-induced but not platelet-derived growth factor (PDGF-BB)-induced SMC proliferation (% control, serum: LMWH, 60.5 +/- 4.0%, P = .0002; standard heparin, 29.4 +/- 8.2%, P = .0001; % control, PDGF-BB: LMWH, 117.7 +/- 11.3%, P = NS; standard heparin, 90.9 +/- 14.4%, P = NS) and SMC migration (% control, serum: LMWH, 15.3 +/- 1.9%, P = .0198; standard heparin, 26.4 +/- 13.8%, P = .0032; % control, PDGF-BB: LMWH, 98.5 +/- 14.3%, P = NS; standard heparin, 100.0 +/- 13.5%, P = NS).

CONCLUSIONS

LMWH failed to inhibit intimal hyperplasia in a baboon angioplasty model. Furthermore, LMWH blocked serum-induced but not PDGF-BB-induced SMC proliferation and migration in culture. Thus, heparin-sensitive and -insensitive pathways exist for SMC activation. The relative importance of each pathway induced by injury may vary between species and thus account for different responses to heparin.

Regional heterogeneity of arterial structural changes

Arterial structural changes in experimental models of hypertension and restenosis differ between vessel types and within vessels. Inspired by the diversity of short-term functional responses to vasoactive agents, hypotheses are presented with respect to the heterogeneity of structural alterations. Considered are the multifactorial nature of smooth muscle cell growth control and the possibility that vascular smooth muscle is not homogeneous but composed of different smooth muscle cell populations. These hypotheses may help explain the origin of both intervascular and intravascular heterogeneity of vascular structural responses.