Characterization of a New Double-Injury Restenosis Model in the Rat Aorta

Animal Models of Neointimal Hyperplasia and Restenosis: Species-Specific Differences and Implications for Translational Research

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Neointimal hyperplasia is the major factor contributing to restenosis after angioplasty procedures.

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Multiple animal models exist to study basic and translational aspects of restenosis formation.

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Animal models differ substantially, and species-specific differences have major impact on the pathophysiology of the model.

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Genetic, dietary, and mechanical interventions determine the translational potential of the animal model used and have to be considered when choosing the model.

The process of restenosis is based on the interplay of various mechanical and biological processes triggered by angioplasty-induced vascular trauma. Early arterial recoil, negative vascular remodeling, and neointimal formation therefore limit the long-term patency of interventional recanalization procedures. The most serious of these processes is neointimal hyperplasia, which can be traced back to 4 main mechanisms: endothelial damage and activation; monocyte accumulation in the subintimal space; fibroblast migration; and the transformation of vascular smooth muscle cells. A wide variety of animal models exists to investigate the underlying pathophysiology. Although mouse models, with their ease of genetic manipulation, enable cell- and molecular-focused fundamental research, and rats provide the opportunity to use stent and balloon models with high throughput, both rodents lack a lipid metabolism comparable to humans. Rabbits instead build a bridge to close the gap between basic and clinical research due to their human-like lipid metabolism, as well as their size being accessible for clinical angioplasty procedures. Every different combination of animal, dietary, and injury model has various advantages and disadvantages, and the decision for a proper model requires awareness of species-specific biological properties reaching from vessel morphology to distinct cellular and molecular features.

A Rat Carotid Artery Pressure-Controlled Segmental Balloon Injury with Periadventitial Therapeutic Application

Cardiovascular disease remains the leading cause of death and disability worldwide, in part due to atherosclerosis. Atherosclerotic plaque narrows the luminal surface area in arteries thereby reducing adequate blood flow to organs and distal tissues. Clinically, revascularization procedures such as balloon angioplasty with or without stent placement aim to restore blood flow. Although these procedures reestablish blood flow by reducing plaque burden, they damage the vessel wall, which initiates the arterial healing response. The prolonged healing response causes arterial restenosis, or re-narrowing, ultimately limiting the long-term success of these revascularization procedures. Therefore, preclinical animal models are integral for analyzing the pathophysiological mechanisms driving restenosis, and provide the opportunity to test novel therapeutic strategies. Murine models are cheaper and easier to operate on than large animal models. Balloon or wire injury are the two commonly accepted injury modalities used in murine models. Balloon injury models in particular mimic the clinical angioplasty procedure and cause adequate damage to the artery for the development of restenosis. Herein we describe the surgical details for performing and histologically analyzing the modified, pressure-controlled rat carotid artery balloon injury model. Additionally, this protocol highlights how local periadventitial application of therapeutics can be used to inhibit neointimal hyperplasia. Lastly, we present light sheet fluorescence microscopy as a novel approach for imaging and visualizing the arterial injury in three-dimensions.

Quantification of Adventitial Vasa Vasorum Vascularization in Double-injury Restenotic Arteries

BACKGROUND

Accumulating evidence indicates a potential role of adventitial vasa vasorum (VV) dysfunction in the pathophysiology of restenosis. However, characterization of VV vascularization in restenotic arteries with primary lesions is still missing. In this study, we quantitatively evaluated the response of adventitial VV to vascular injury resulting from balloon angioplasty in diseased arteries.

METHODS

Primary atherosclerotic-like lesions were induced by the placement of an absorbable thread surrounding the carotid artery of New Zealand rabbits. Four weeks following double-injury induced that was induced by secondary balloon dilation, three-dimensional patterns of adventitial VV were reconstructed; the number, density, and endothelial surface of VV were quantified using micro-computed tomography. Histology and immunohistochemistry were performed in order to examine the development of intimal hyperplasia.

RESULTS

Results from our study suggest that double injured arteries have a greater number of VV, increased luminal surface, and an elevation in the intima/media ratio (I/M), along with an accumulation of macrophages and smooth muscle cells in the intima, as compared to sham or single injury arteries. I/M and the number of VV were positively correlated (R2 = 0.82, P < 0.001).

CONCLUSIONS

Extensive adventitial VV neovascularization occurs in injured arteries after balloon angioplasty, which is associated with intimal hyperplasia. Quantitative assessment of adventitial VV response may provide insight into the basic biological process of postangioplasty restenosis.

Catheterization of the hepatic artery via the left common carotid artery in rats

The commonly used approach for rat hepatic artery catheterization is via the gastroduodenal artery, which is ligated after the procedure. A new method of rat hepatic artery catheterization via the left common carotid artery (LCCA) is described. The LCCA is repaired after catheterization. The catheterization procedures included the following: (1) opening the rat's abdominal cavity and exposing the portion of abdominal aorta at the level of the celiac trunk; (2) separating and exposing the LCCA; inserting a microguidewire and microcatheter set into the LCCA via an incision; after placement into the descending aorta, the microguidewire and microcatheter are maneuvered into the hepatic artery under direct vision; (3) after transcatheter therapy, the catheter is withdrawn and the incision at the LCCA is repaired. This technique was employed on 60 male Sprague-Dawley rats with diethylnitrosamine-induced liver cancer, using a 3F microguidewire and microcatheter set. Selective hepatic artery catheterization was successfully performed in 57 rats. One rat died during the operation and five rats died within 7 days after the procedure. It is envisaged that as experience increases, the catheterization success rate will increase and the death rate will decrease. A new approach for selective hepatic artery catheterization via the LCCA in rats is introduced, which makes repeat catheterization of this artery possible and allows large embolization particles to be delivered by using a 3F catheter.

2005 Dr. Gary J. Becker Young Investigator Award: Periprocedural Oral Administration of the Leflunomide Analogue FK778 Inhibits Neointima Formation in a Double-injury Rat Model of Restenosis

PURPOSE

To test the efficacy of limited oral administration of the new leflunomide analogue FK778 for suppression of neointima proliferation in a double-injury restenosis model in the rat.

MATERIALS AND METHODS

For induction of aortic lesions, silicon cuffs were placed operatively around the infrarenal aortas of Lewis rats. After 21 days, the aortic cuffs were removed and the lesions were dilated with 2-F Fogarty catheters inserted via the left common carotid artery. The novel immunosuppressant FK778 was administered at a dose of 5 mg/kg body weight (group 1) or 15 mg/kg body weight (group 2) in a total of 38 animals. For both doses, three different periinterventional time periods, each with a 5-day course of oral FK778, were defined as follows: (i) days -2 to 2, (ii) days 1-5, and (iii) days 7-11, with six or seven rats in each group. After 3 weeks, intima/media ratios were assessed morphometrically and immunohistochemistry for quantification of intimal alpha-actin expression was performed.

RESULTS

In both dose groups, there was a trend toward inhibition of neointima formation when the 5-day course of FK778 was started before or 1 day after the intervention. However, in the lower-dose group, inhibition of neointima was not statistically significant regardless of the time frame of treatment (groups 1a-c). With the higher dose, suppression of intimal hyperplasia was significant when FK778 was administered between days 1 and 5 after angioplasty (group 2b; P<.01). Expression of alpha-actin in the intima of FK778-treated rats was significantly reduced when the drug was started 2 days before angioplasty in group 1a (P<.05) or 1 day after angioplasty in both dosage groups (group 1b, P<.01; group 2b, P<.05).

CONCLUSION

In the double-injury rat model presented, balloon-mediated proliferation of smooth muscle cells in the intima with consecutive intimal thickening was influenced by FK778 in a dose-dependent manner. However, long-term studies are needed to exclude a delay of vascular healing in this particular model.

Short-term rapamycin for inhibition of neointima formation after balloon-mediated aortic injury in rats: is there a window of opportunity for systemic prophylaxis of restenosis?

PURPOSE

To evaluate the efficacy of limited short-term systemic administration of rapamycin to prevent neointimal intimal hyperplasia (NIH) in a double-injury rat model of restenosis.

METHODS

Aortic lesions were induced by perivascular placement of silicone cuffs around the aorta of 36 Lewis rats. After 3 weeks, the cuffs were removed, and the vessels were subjected to secondary balloon injury. Rapamycin (sirolimus) was intravenously administered for 5 days in dosages of 0.5 or 2 mg/kg/d beginning at various time points relative to the balloon injury: (1) days -2 to +2, (2) days 1 to 5, or (3) days 7 to 11. For each treatment period, 6 rats received the 5-day course of the lower or higher dose of rapamycin. Eight rats served as controls undergoing 2-stage injury without rapamycin treatment. Morphometry and immunohistochemistry were performed at 21 days after angioplasty.

RESULTS

NIH and intimal alpha-actin expression were inhibited by both dosages when treatment started 2 days before or 1 day after angioplasty. Results were statistically significant for the lower dose when started 1 day after angioplasty (p < 0.01) and for the higher dose when initiated 2 days before the intervention (p < 0.05). Treatment commencing at 7 days did not reduce NIH in either dosage group.

CONCLUSIONS

In a double-injury rat model, NIH can be inhibited by short-term systemic rapamycin, but suppression of early cell migration and proliferation is pivotal. A limited peri-interventional antiproliferative therapy may be of value as an adjunct to control restenosis after balloon angioplasty and/or stenting.