Experimental abdominal aortic aneurysms in mice lacking expression of inducible nitric oxide synthase.

To determine if nitric oxide synthase (NOS) contributes to the pathophysiology of abdominal aortic aneurysms (AAAs), C57BL/6J mice underwent transient aortic injury to induce a chronic inflammatory response. Wild-type mice developed a significant increase in aortic diameter within 14 days of elastase perfusion (115+/-16%, 40% incidence of AAAs), along with intense and widespread staining for nitrotyrosine, mononuclear inflammation, and delayed destruction of the elastic lamellae. Expression of both endothelial and neuronal forms of NOS was substantially decreased within AAAs, whereas inducible NOS (iNOS) mRNA was increased 360%, and the enzyme was localized to infiltrating inflammatory cells. By using mice with targeted deletion of iNOS to evaluate the functional importance of this enzyme, male iNOS(-/-) mice developed the same extent of aneurysmal dilatation as congenic controls (121+/-22%, 40% incidence of AAAs) and exhibited similar structural features except for diminished nitrotyrosine staining. Aneurysmal dilatation was actually enhanced in female iNOS(-/-) mice (141+/-16%, 80% incidence of AAAs; P<0.05), but this effect was reversed by previous oophorectomy. Although extensive protein nitration and increased expression of iNOS accompany the development of elastase-induced experimental AAAs, iNOS is not required in this process and its absence may be deleterious.

Photodynamic therapy. Cytotoxicity of aluminum phthalocyanine on intimal hyperplasia.

OBJECTIVE: To study the cytotoxic effect of photodynamic therapy (PDT) on myointimal hyperplasia (MIH) in 120 New Zealand white rabbits using the chromophore chloroaluminum phthalocyanine tetrasulfonate (APtS). DESIGN: A common carotid artery (CCA) injury model was used to initiate MIH. Photodynamic therapy was administered 1 week after injury (inhibition arm) or 6 weeks after injury (treatment arm). The inhibition arm CCAs were harvested 6 weeks after therapy. The treatment arm CCAs were harvested 1 week or 6 weeks after therapy. Each evaluation included four subgroups (n = 10 each): control, drug only, laser only, and drug plus laser. INTERVENTIONS: An established CCA balloon injury model was used. Photodynamic therapy was administered by exposing CCAs to continuous external laser irradiation 30 minutes after treatment with a 2.5-mg/kg intravenous dose of APtS (fluence = 25 J/cm2, lambda = 672 nm). The control and drug-only subgroups received sham reoperations without laser exposure. MAIN OUTCOME MEASURES: Following harvest, the CCAs were evaluated for area of stenosis and cell density. RESULTS: In the inhibition arm, no PDT effect was seen on intimal cell density or area stenosis. In the treatment arm, intimal cell density was markedly diminished (P < .05) in the rabbits in the drug-laser group that were killed 1 week but not 6 weeks after PDT compared with rabbits in the control, drug-only, and laser-only groups. Area stenosis was not significantly affected by PDT. CONCLUSIONS: Marked acute cytotoxicity of PDT on MIH was verified in vivo in the treatment arm. No sustained benefit of PDT was seen in the inhibition or the treatment arms. Refinements in dosimetry will be necessary to achieve long-term benefit of PDT for MIH.

Histological response to stent graft therapy.

BACKGROUND: The purpose of the present study was to examine the histological consequences of the endoluminal exclusion of blood flow effected with stent graft technology. METHODS AND RESULTS: In 25-kg mongrel dogs, patulous vein patch infrarenal aortoplasty with iliac vein produces a fusiform abdominal aortic dilation (AAD). All aortic tributaries were preserved. Endoluminal exclusion via transfemoral placement of a thin-wall Dacron graft occurred 4 +/- 2 months later (n = 23). Balloon-expandable stents anchored the ends of the graft to the aorta. Hematoxylin and eosin, elastin van Gieson's, and Masson's trichrome staining was performed 6 and 12 months later at death. In control nongrafted AADs, the arterial portion of the AAD was lined by elastin -and collagen-rich intimal hyperplasia, and the venous portion developed medial hyperplasia containing collagen but little elastin. After stent graft placement, the stent struts and the graft were completely incorporated into an elastin-poor, collagen-rich neointima. Fibrosis of the vein patch was observed at 1 year. laminated thrombus did not form in the AAD until immediately after stent graft placement; flow arrest occurred in the space between the graft and the AAD intima despite the patent tributaries. At 6 and 12 months, microscopic recanalization was seen in this thrombus, although macroscopic flow was not discernible by duplex imaging or angiography. No AAD growth was measured. CONCLUSIONS: Aortic dilation was not observed at 1 year after stent graft placement within AADs with patent side branches despite microscopic evidence of thrombus recanalization. A collagen-rich and elastin-poor neointima incorporated the entire stent graft.

Suppression of experimental abdominal aortic aneurysms in the rat by treatment with angiotensin-converting enzyme inhibitors.

PURPOSE: Pathologic remodeling of the extracellular matrix is a critical mechanism in the development and progression of abdominal aortic aneurysms (AAAs). Although angiotensin-converting enzyme (ACE) inhibitors are known to alter vascular wall remodeling in other conditions, their effects on AAAs are unknown. In this study we assessed the effect of ACE inhibitors in a rodent model of aneurysm development. METHODS: Male Wistar rats underwent transient aortic perfusion with porcine pancreatic elastase, followed by treatment with one of three ACE inhibitors (captopril [CP], lisinopril [LP], or enalapril [EP]), an angiotensin (AT)1 receptor antagonist (losartan [LOS]), or water alone (9 rats in each group). Blood pressure and aortic diameter (AD) were measured before elastase perfusion and on day 14, with an AAA defined as an increase in AD (DeltaAD) of more than 100%. The structural features of the aortic wall were examined by means of light microscopy. RESULTS: Aneurysmal dilatation consistently developed within 14 days of elastase perfusion in untreated rats, coinciding with the development of a transmural inflammatory response and destruction of the elastic media (mean DeltaAD, 223% +/- 28%). All three ACE inhibitors prevented AAA development (mean DeltaAD: CP, 67% +/- 4%; LP, 18% +/- 12%; and EP, 14% +/- 3%; each P <.05 vs controls). ACE inhibitors also attenuated the degradation of medial elastin without diminishing the inflammatory response. Surprisingly, the aneurysm-suppressing effects of ACE inhibitors were dissociated from their effects on systemic hemodynamics, and LOS had no significant effect on aneurysm development compared with untreated controls (mean DeltaAD, 186% +/- 19%). CONCLUSION: Treatment with ACE inhibitors suppresses the development of elastase-induced AAAs in the rat. Although this is associated with the preservation of medial elastin, the mechanisms underlying these effects appear to be distinct from hemodynamic alterations alone or events mediated solely by AT1 receptors. Further studies are needed to elucidate how ACE inhibitors influence aortic wall matrix remodeling during aneurysmal degeneration.

Results of endoluminal grafting in an experimental aortic aneurysm model.

We studied the impact of an endoluminally placed stented aortic graft on the geometry of a surgically created abdominal aortic dilation (AAD) in nonatherosclerotic mongrel dogs. Patulous iliac vein patch infrarenal aortoplasty produced a fusiform AAD, doubling the aorta diameter. Lumbar and mesenteric aortic tributaries were preserved and no mural thrombus formed. AADs created in 23 dogs were endoluminally excluded through transfemoral placement of a thin-wall Dacron graft 4 +/- 2 months later. Balloon-expandable stents were used to anchor each end of the graft to the aorta. The graft was crimped radially in its body and longitudinally at its ends to provide longitudinal and radial expandability in these respective zones. Serial color duplex, angiography, and direct caliper measurements were made. Before graft placement, a 19% +/- 11% diameter growth was observed. At graft placement, flow arrest immediately occurred in the space between the graft and the AAD intima in all cases. Although microscopic recanalization of the thrombus in this space was seen at sacrifice 6 and 12 months later, no macroscopic duplex flow was imaged. A 10% +/- 11% reduction in AAD diameter was measured at 6 months (p < 0.001), with no further reduction at 12 months. Graft dimensions remained stable. No anastomotic leaks developed. AAD growth stopped during the first year after effective endoluminal exclusion in normotensive dogs despite patent side branches (< 1.5 mm internal diameter) and no mural thrombus at the time of graft placement. Whether microscopic recanalization of the thrombus that forms outside the graft has an impact after 1 year remains to be seen.

Cytotoxic effect of photodynamic therapy with Photofrin II on intimal hyperplasia.

This study evaluates the effect of photodynamic therapy using Photofrin II on prevention and treatment of intimal hyperplasia in a rabbit model of common carotid artery balloon injury. An established model was used. One week after injury (inhibition arm) or 6 weeks after injury (treatment arm), each common carotid artery was exposed to continuous external laser irradiation 48 hours after a 5 mg/kg intravenous dose of Photofrin II (fluency = 7.6 joules/cm2, lambda = 630 nm). Histologic evaluation was performed 6 weeks following therapy in the inhibition arm and 1 day, 1 week, and 6 weeks following therapy in the treatment arm. Each arm included four subgroups (N = 10/subgroup): control, drug only, laser only, and drug plus laser. The first two subgroups underwent sham reoperations without laser exposure. In the inhibition arm no effect was seen on intimal cell density or area stenosis 6 weeks after photodynamic therapy. In the treatment arm intimal cell density was markedly diminished in the drug plus laser subgroup sacrificed 1 day and 1 week (but not 6 weeks) after treatment as compared to the remaining subgroups. There was no significant impact on area of stenosis. A marked acute cytotoxic effect of photodynamic therapy on intimal hyperplasia was verified in vivo in the treatment arm. The extracellular matrix was not affected. Cellular repopulation of the treatment zone was observed. No sustained benefit was seen in either the inhibition or the treatment arm. Refinements in dosimetry will be necessary to achieve long-term benefits.