Inhibition of injury induced intimal hyperplasia by saralasin in rats

The Neointima Formed in Endothelial Cell Sodded ePTFE Vascular Grafts Results from Both Cellular-Hyperplasia and Extracellular-Hypertrophy

Endothelial cell transplantation onto polymeric vascular grafts results in the formation of a neointima. The formation of this neointima is often suggested to result from a chronic cellular hyperplasia where the terms intimal hyperplasia and intimal thickening are used interchangeably. While the formation of a midgraft neointima in sodded grafts involves a level of cell proliferation, the synthesis and deposition of extracellular matrix proteins is also a ubiquitous observation in these grafts. To assess the composition of midgraft neointima in sodded grafts, a morphometric method was developed to provide a differential quantitation of the cellular-hyperplastic and extracellular-hypertrophic elements of intimal thickening. The formed neointima on microvessel endothelial cell sodded and control (noncell-treated) ePTFE vascular grafts was quantified after 3, 12, and 52 wk of graft implantation in a canine carotid artery model. Midgraft sections of grafts were evaluated for both intimal thickness (IT) and cell density per unit volume and quantified using a PC-based image analysis program. Sodded grafts explanted at 3 wk exhibited an average neointimal cell density (3 × 109 cells/cm3; IT 30 μm) equivalent to cell densities observed in normal arterial media. After 12 wk the mean cell density approached a hyperplastic value (3.7 × 109 cells/cm3; IT 76 μm), while grafts explanted after 52 wk exhibited a mean cell density (2.8 × 109 cells/cm3; IT 30 μm) similar to 3-wk values. Control grafts that received no cells exhibited no midgraft cellular coverage. These results indicate that neointima formation in the midgraft region of sodded grafts occurred via mechanisms involving both a cellular hyperplasia and an extracellular hypertrophy. Differential responses occur presumably due to localized differences in cellular proliferation and cellular biosynthetic activity.

Flow-mediated vascular remodeling in hypertension: relation to hemodyamics

BACKGROUND AND PURPOSE

Changes in shear and medial wall stress induced by blood flow contribute to vascular remodeling, but details of these relations remain undefined. We hypothesized that remodeling has a strong genetic component and that phenotypic responses to hemodynamic stress will differ among rat strains. Here, we characterized phenotypic traits related to carotid remodeling in the 2 rat strains that we previously showed the greatest difference in shear stress regulation: Genetically Hypertensive (GH) and Brown Norway (BN) rat strains.

METHODS

Left internal and external carotid arteries were ligated and blood flow was reduced in the left common (LCA) by 90% and increased in the right common carotid artery (RCA) by 60%. Rats were studied for up to 28 days after flow modification and carotid outer diameters were measured in vivo, and wall and luminal components by histomorphometry, to obtain indices of remodeling. Blood flow and pressure measurements were made at corresponding time points.

RESULTS

By day 28, remodeling in the GH was greater in response to high flow than in BN, and shear stress was normalized. In contrast, remodeling in the BN was greater in the low flow LCA than in GH. Media stress was greater in GH than BN for any value of carotid shear stress and remained relatively unchanged in low flow, but markedly increased in high flow remodeling. Importantly, pressure was not a major determinant of flow remodeling in these conditions.

CONCLUSIONS

There are key differences in the ability of carotids in GH and BN rats to adhere to hemodynamic laws during vascular remodeling. GH rats exhibit intact regulatory mechanisms for increased, but not reduced, shear stress. Moreover, the ability to maintain physiological shear and media stresses during vascular remodeling in response to modified flow appears to be intrinsically "genetically" determined.

Effects of Preinjury Administration of Corticosteroids on Pseudointimal Hyperplasia and Cytokine Response in a Rat Model of Balloon Aortic Injury

Restenosis occurs in approximately one-third of patients with coronary or peripheral vascular disease who undergo balloon angioplasty or a surgical bypass procedure primarily because of the development of pseudointimal hyperplasia (PIH). Corticosteroids were effective in suppressing PIH in several experimental studies, but no clinical studies have been reported. To resolve this discrepancy, we studied the effects of preinjury administration of several doses of methylprednisolone (MP) at targeted times in a rat model of balloon aortic injury. Rats were given either no treatment or an intravenous injection of MP (0.5, 5.0, 50, or 500 mg/kg) 2 hours before aortic injury. Four hours later interleukin-6 (IL-6), IL-10, and macrophage migration inhibitory factor (MIF) concentrations in serum and tissue of injured aortas were assessed. Two weeks after injury, damaged aortas were harvested and studied histopathologically. Compared with results in controls, MP at a dose of 5 mg/kg significantly inhibited increases in plasma and tissue levels of IL-6 and significantly reduced the pseudointimal area, pseudointimal/medial area ratio, and proliferating cell nuclear antigen index in injured vessels. Administration of MP had no significant effect on the IL-10 or MIF level. Thus in a rat model of balloon aortic injury, preinjury administration of MP 5 mg/kg mitigated the development of PIH and cell proliferation and suppressed the postinjury increase in serum and tissue IL-6 concentrations. These results suggest that there is an appropriate dosage as well as appropriate timing for MP administration to suppress PIH.

Endovascular photodynamic therapy using mono-L-aspartyl-chlorin e6 to inhibit Intimal hyperplasia in balloon-injured rabbit arteries

BACKGROUND AND OBJECTIVE

Intimal hyperplasia (IH) leading to restenosis is a major complication of arterial revascularization. The purpose of this study was to investigate the effect of photodynamic therapy (PDT) using mono-L-aspartyl chlorin e6 (NPe6) as a photosensitizer and intraluminal radial irradiation for inhibition of IH experimentally.

STUDY DESIGN/MATERIALS AND METHODS

Study of laser transmission through the blood indicated that exclusion of blood is a prerequisite for intraluminal PDT. For homogeneous radial laser irradiation to the vessel wall, we used a newly developed cylindrical diffusing balloon laser fiber. Injuries were induced by pulling a balloon catheter through the right iliac artery of rabbits. One and 6 hours after the NPe6 injection (5mg/kg i.v.), drug distribution was examined by fluorescence microscopy. Nineteen rabbits received NPe6 at the time of injuries and PDT was performed with 664-nm laser at 30 and 10 J/cm(2) (20, 30, 40 mW/cm(2)) 1 hour after the injuries. The arteries were harvested at 2 days. In a second group of rabbits, PDT was given at 30 mW/cm(2) (30 J/cm(2)). Two weeks after treatment, the arteries were removed and examined histologically.

RESULTS

NPe6 was found to be distributed selectively in the injured media. Endovascular NPe6-PDT showed complete depletion of smooth muscle cells even with 10 J/cm(2) at 2 days. IH was significantly inhibited at 14 days after PDT.

CONCLUSIONS

Endovascular PDT of injured artery using NPe6 can prevent IH in this model of arterial wall injury and may become clinically useful for the prophylaxis of IH.

Angiotensin-converting enzyme inhibition suppresses plasminogen activator inhibitor-1 expression in the neointima of balloon-injured rat aorta

BACKGROUND

Plasminogen activator inhibitor-1 (PAI-1), an important regulator of fibrinolysis and extracellular matrix turnover, has been implicated in a number of vascular diseases. Studies demonstrating angiotensin II (Ang II) to be a potent stimulator of PAI-1 expression in cultured vascular cells suggests that the renin-angiotensin system may modulate vascular PAI-1 expression.

METHODS AND RESULTS

We examined the effects of the ACE inhibitor captopril on PAI-1 expression in control and balloon-injured rat aorta. Northern blot analysis demonstrated that aortic PAI-1 mRNA expression was 7.6-fold elevated 3 hours (P<.05) after balloon injury, back to baseline at 2 days, increased again at 4 days, and by 7 days after balloon injury was 3.2-fold elevated (P<.05) when compared with control. In captopril-treated rats, the induction of PAI-1 expression by balloon injury was significantly suppressed by 44% (P<.05) in the 7 day group but was not altered in the 3-hour group. Captopril also reduced baseline aortic PAI-1 mRNA. In situ hybridization and immunohistochemistry revealed dense PAI-1 staining of 7-day neointima in untreated rats and a dramatic decrease in PAI-1 in neointima of captopril-treated rats.

CONCLUSIONS

This report demonstrates that balloon injury results in both a rapid ACE inhibitor-independent induction of aortic PAI-1 expression and a later increase in PAI-1 in the neointima that is significantly suppressed by captopril. This provides the first evidence that the renin-angiotensin system regulates neointimal PAI-1 expression and that ACE inhibitors can reduce PAI-1 in the vessel wall in vivo.

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.

Pathophysiology of smooth muscle in hypertension

Structural changes of the arteries in hypertension are determined by the unique genetics of the animals and by various growth promoters and growth inhibitors. Vascular smooth muscle cell growth promoting factors include fibroblast growth factor, platelet-derived growth factor, and vasoactive peptides such as norepinephrine, angiotensin II, and endothelin. Endothelial cells secrete three types of growth inhibiting factors. These are heparin--heparan sulfate, transforming growth factor beta, and nitric oxide. The effect of sympathetic innervation on vascular growth is probably dependent on its interaction with the renin-angiotensin system. In the mesenteric vascular bed, the elevated resistance in the arterial system is present in both the macroarteries and in the more distal microarteries and veins. Changes in resistance arteries include hypertrophy and reduction in outer diameter (remodelling). In the resistance arteries from human essential hypertensives, remodelling is the predominant finding. Long-term treatment with an angiotensin I converting enzyme inhibitor but not with a beta-blocker was effective in reversing this type of vascular change. Studies have suggested that in addition to angiotensin II, endothelin may play a role in vascular remodelling of resistance arteries.

Acute phase vascular endothelial injury: a comparison of temporary arterial occlusion using an endovascular occlusive balloon catheter versus a temporary aneurysm clip in a pig model

We compared the degree of acute endothelial injury after temporary vessel occlusion using two different occlusion modalities--external clipping and endovascular balloon occlusion. The common carotid and subclavian arteries in eight weanling pigs were temporarily occluded with either a 5 Fr occlusion balloon catheter or a temporary microvascular clip for 0 (control), 5, 10, and 30 minutes. Two animals (eight vessels; four clip and four balloon occluded) were used at each time interval. Segments of each experimental vessel were harvested and analyzed by scanning electron microscopy. Each vessel specimen was graded on the following scale: Grade 1 showed no evidence of injury; Grade 2 showed minimal evidence of endothelial cell injury; Grade 3 showed moderate evidence of endothelial cell injury; Grade 4 showed marked evidence of endothelial cell injury; Grade 5 showed severe endothelial and subendothelial injury. Control vessels showed no evidence of injury. Grade 2 injuries were seen in both clip- and balloon-occluded vessels at 5 minutes. At 10 minutes, focal Grade 2 and 3 injuries were appreciated in the clip group, with diffuse Grade 2 and 3 injuries in the balloon group. After the 30-minute occlusion, the balloon group showed Grade 2, 3, and 4 injuries, whereas the clip group showed entirely Grade 2 injuries. The degree of injury with either occlusion modality was equivalent and worsened with time. However, clip-occluded vessels displayed injury that was confined to an area closely adjacent to the clip site, whereas balloon-occluded vessels demonstrated a more widespread injury centered around the balloon site.

Regulation of cell proliferation and growth by angiotensin II

The peptide hormone angiotensin II (AngII) has clearly defined physiologic roles as a regulator of vasomotor tone and fluid homeostasis. In addition AngII has trophic or mitogenic effects on a variety of target tissues, including vascular smooth muscle and adrenal cells. More recent data indicate that AngII exhibits many characteristics of the 'classical' peptide growth factors such as EGF/TGF alpha, PDGF and IGF-1. These include the capacity for local generation ('autocrine or paracrine' action) and the ability to stimulate tyrosine phosphorylation, to activate MAP kinases and to increase expression of nuclear proto-oncogenes. The type 1 AngII receptor, which is responsible for all known physiologic actions of AngII, has been cloned. Activation of this receptor leads to elevated phosphoinositide hydrolysis, mobilization of intracellular Ca2+ and diacylglycerol, and activation of Ca2+/calmodulin and Ca2+/phospholipid-dependent Ser/Thr kinases, as well as Ca2+ regulated tyrosine kinases. The existence of other AngII receptor subtypes has been postulated, but the function(s) of these sites remains unclear. In vascular smooth muscle, AngII can promote cellular hypertrophy and/or hyperplasia, depending in part on the patterns of induction of secondary factors that are known to stimulate (PDGF, IGF-1, basic FGF) or inhibit (TGF-beta) mitosis. Together, these findings have suggested that AngII plays important roles in both the normal development and pathophysiology of vascular, cardiac, renal and central nervous system tissues.