Preclinical models of human peripheral arterial occlusive disease: implications for investigation of therapeutic agents

Contrast-Enhanced Ultrasound of Mouse Models of Hindlimb Ischemia Reveals Persistent Perfusion Deficits and Distinctive Muscle Perfusion Patterns

OBJECTIVE

Mouse models of hindlimb ischemia (HLI) are used to study peripheral arterial disease and evaluate novel therapies. Contrast-enhanced ultrasound (CEUS) is a noninvasive perfusion measurement technique that is increasingly being employed in these models. The objective of this study was to evaluate two models of severe HLI by CEUS to characterize perfusion recovery and muscle perfusion patterns.

METHODS

Mice undergoing double femoral artery ligation were measured by CEUS and laser Doppler perfusion imaging (LDPI) at baseline and 1-150 d postsurgery. A second group undergoing femoral artery ligation and excision was measured 1-28 d postsurgery.

RESULTS

By LDPI, both surgeries showed robust perfusion recovery by 14 d postsurgery. However, by CEUS only a ∼40% perfusion recovery plateau was reached in either group. These results are consistent with our previous work, employing a less severe single femoral artery ligation, that showed perfusion in the ischemic limb does not return to normal by 150 d postsurgery. Cluster analysis of muscle perfusion patterns indicated 3-5 different patterns at day 1 postsurgery. The double ligation model yielded significantly less variable perfusion patterns, suggesting that it can provide more reproducible results.

CONCLUSION

Contrary to LDPI, perfusion as measured by CEUS never fully recovers after hindlimb surgery, even when followed 28-150 d postsurgery. Individual mice can manifest different patterns of muscle perfusion to the same surgery, but these patterns are conserved within and between different surgical techniques. These results may have significant implications for the evaluation of novel therapeutics to treat PAD in mice.

Alginate-Encapsulated Mesenchymal Stromal Cells Improve Hind Limb Ischemia in a Translational Swine Model

BACKGROUND

Cellular therapies have been investigated to improve blood flow and prevent amputation in peripheral artery disease with limited efficacy in clinical trials. Alginate-encapsulated mesenchymal stromal cells (eMSCs) demonstrated improved retention and survival and promoted vascular generation in murine hind limb ischemia through their secretome, but large animal evaluation is necessary for human applicability. We sought to determine the efficacy of eMSCs for peripheral artery disease-induced limb ischemia through assessment in our durable swine hind limb ischemia model.

METHODS AND RESULTS

Autologous bone marrow eMSCs or empty alginate capsules were intramuscularly injected 2 weeks post-hind limb ischemia establishment (N=4/group). Improvements were quantified for 4 weeks through walkway gait analysis, contrast angiography, blood pressures, fluorescent microsphere perfusion, and muscle morphology and histology. Capsules remained intact with mesenchymal stromal cells retained for 4 weeks. Adenosine-induced perfusion deficits and muscle atrophy in ischemic limbs were significantly improved by eMSCs versus empty capsules (mean±SD, 1.07±0.19 versus 0.41±0.16, P=0.002 for perfusion ratios and 2.79±0.12 versus 1.90±0.62 g/kg, P=0.029 for ischemic muscle mass). Force- and temporal-associated walkway parameters normalized (ratio, 0.63±0.35 at week 3 versus 1.02±0.19 preligation; P=0.17), and compensatory footfall patterning was diminished in eMSC-administered swine (12.58±8.46% versus 34.85±15.26%; P=0.043). Delivery of eMSCs was associated with trending benefits in collateralization, local neovascularization, and muscle fibrosis. Hypoxia-cultured porcine mesenchymal stromal cells secreted vascular endothelial growth factor and tissue inhibitor of metalloproteinase 2.

CONCLUSIONS

This study demonstrates the promise of the mesenchymal stromal cell secretome at improving peripheral artery disease outcomes and the potential for this novel swine model to serve as a component of the preclinical pipeline for advanced therapies.

Evaluation of a new paclitaxel-coated balloon catheter in an in vivo porcine peripheral venous model: Feasibility, safety, and drug deliverability

PURPOSE

To evaluate in vivo the feasibility, safety, and paclitaxel (PTX) deliverability of a newly developed non-commercially available Paclitaxel-Coated Balloon (PCB) catheter in the swine healthy peripheral vein model.

MATERIALS AND METHODS

In total 12 PCBs were deployed in 12 venous segments. Primary feasibility endpoint was the successful application of the devices to the veins of the animals. Primary efficacy endpoint was the determination of the drug content in the venous tissue at 24 h and 7 days after balloon expansion, as assessed by analysis of the vein tissue with High Performance Liquid Chromatography (HPLC) coupled with tandem mass spectrometry. Primary safety endpoint was freedom from any major adverse event. Secondary endpoint was the investigation of any independent factor affecting the primary endpoints.

RESULTS

Paclitaxel was detected in five out of six tissue samples 24 h post-intervention and five out of six tissues at 7 days following the procedure (10 tissue samples out of 12). The mean weight of tissue that was examined was 0.20604 ± 0.29822 g (range: 1.02823-0.03377 g) and the mean PTX concentration detected was 8.4 ± 13.1 μg/g (range: 0-36.1 μg/g). The average drug content detected at 24 h (17.1 ± 17.1 μg/g) was numerically superior, but non-statistically significant, compared to 7 days (3.1 ± 3.6 μg/g). An average of 33.8% of the drug remained on the balloon after retrieval. According to the multiple linear regression analysis, there was no significant correlation between transition time, PTX remaining on the balloon, time of analysis (24 h/7 days) and PTX tissue concentration. No abnormalities were noted during autopsy.

CONCLUSION

The newly developed PCB successfully delivered within the healthy venous wall a dose of Paclitaxel that inhibits neointimal hyperplasia. No safety issues were raised at short-term follow-up.

Development of a Rabbit Iliac Arterial Stenosis Model Using a Controlled Cholesterol Diet and Pullover Balloon Injury

Purpose

This study aimed to develop a rabbit iliac stenosis model and evaluate the effects of different mechanical injury techniques on the degree of arterial stenosis.

Materials and Methods

Eighteen rabbits were divided into three groups: cholesterol-fed with pullover balloon injury (group A; n = 6), cholesterol-fed with localized balloon dilatation (group B; n = 6), and chow-diet with pullover balloon injury (group C; n = 6). After baseline angiography, the left iliac arteries of all rabbits were injured with a 3 × 10 mm noncompliant balloon using either a wide pullover technique (groups A and C) or a localized balloon dilatation technique (group B). A nine-week follow-up angiography was performed, and the angiographic late lumen loss and percentage of stenosis were compared.

Results

Group A exhibited the most severe late lumen loss (A vs. B, 0.67 ± 0.13 vs. 0.04 ± 0.13 mm, p < 0.0001; A vs. C, 0.67 ± 0.13 vs. 0.26 ± 0.29 mm, p < 0.05; stenosis percentage 32.02% ± 6.54%). In contrast, group B showed a minimal percentage of stenosis (1.75% ± 6.55%).

Conclusion

Pullover-balloon injury can lead to significant iliac artery stenosis in rabbits with controlled hypercholesterolemia. This model may be useful for elucidating the pathogenesis of atherosclerosis and for evaluating the efficacy of novel therapeutic interventions.

Animal experimental models of ischemic limbs - A systematic review

BACKGROUND

The objective of this systematic review is to summarize the available animal models of ischemic limbs, and to provide an overview of the advantages and disadvantages of each animal model and individual method of limb ischemia creation.

METHODS

A review of literature was conducted using the PubMed and Web of Science pages. Various types of experimental animals and surgical approaches used in creating ischemic limbs were evaluated. Other outcomes of interest were the specific characteristics of the individual experimental animals, and duration of tissue ischemia.

RESULTS

The most commonly used experimental animals were mice, followed by rabbits, rats, pigs, miniature pigs, and sheep. Single or double arterial ligation and excision of the entire femoral artery was the most often used method of ischemic limb creation. Other methods comprised single or double arterial electrocoagulation, use of ameroid constrictors, photochemically induced thrombosis, and different types of endovascular methods. The shortest duration of tissue ischemia was 7 days, the longest 90 days.

CONCLUSIONS

This review shows that mice are among the most commonly used animals in limb ischemia research. Simple ligation and excision of the femoral artery is the most common method of creating an ischemic limb; nevertheless, it can result in acute rather than chronic ischemia. A two-stage sequential approach and methods using ameroid constrictors or endovascular blinded stent grafts are more suitable for creating a gradual arterial occlusion typically seen in humans. Selecting the right mouse strain or animal with artificially produced diabetes or hyperlipidaemia is crucial in chronic ischemic limb research. Moreover, the observation period following the onset of ischemia should last at least 14 days, preferably 4 weeks.

Diet-induced obesity augments ischemic myopathy and functional decline in a murine model of peripheral artery disease

Peripheral artery disease (PAD) causes an ischemic myopathy contributing to patient disability and mortality. Most preclinical models to date use young, healthy rodents with limited translatability to human disease. Although PAD incidence increases with age, and obesity is a common comorbidity, the pathophysiologic association between these risk factors and PAD myopathy is unknown. Using our murine model of PAD, we sought to elucidate the combined effect of age, diet-induced obesity and chronic hindlimb ischemia (HLI) on (1) mobility, (2) muscle contractility, and markers of muscle (3) mitochondrial content and function, (4) oxidative stress and inflammation, (5) proteolysis, and (6) cytoskeletal damage and fibrosis. Following 16-weeks of high-fat, high-sucrose, or low-fat, low-sucrose feeding, HLI was induced in 18-month-old C57BL/6J mice via the surgical ligation of the left femoral artery at 2 locations. Animals were euthanized 4-weeks post-ligation. Results indicate mice with and without obesity shared certain myopathic changes in response to chronic HLI, including impaired muscle contractility, altered mitochondrial electron transport chain complex content and function, and compromised antioxidant defense mechanisms. However, the extent of mitochondrial dysfunction and oxidative stress was significantly greater in obese ischemic muscle compared to non-obese ischemic muscle. Moreover, functional impediments, such as delayed post-surgical recovery of limb function and reduced 6-minute walking distance, as well as accelerated intramuscular protein breakdown, inflammation, cytoskeletal damage, and fibrosis were only evident in mice with obesity. As these features are consistent with human PAD myopathy, our model could be a valuable tool to test new therapeutics.

Robust critical limb ischemia porcine model involving skeletal muscle necrosis

This work sought to develop a robust and clinically relevant swine model of critical limb ischemia (CLI) involving the onset of ischemic muscle necrosis. CLI carries about 25-40% risk of major amputation with 20% annual mortality. Currently, there is no specific treatment that targets the ischemic myopathy characteristic of CLI. Current swine models of CLI, with tolerable side-effects, fail to achieve sustained ischemia followed by a necrotic myopathic endpoint. Such limitation in experimental model hinders development of effective interventions. CLI was induced unilaterally by ligation-excision of one inch of the common femoral artery (CFA) via infra-inguinal minimal incision in female Yorkshire pigs (n = 5). X-ray arteriography was done pre- and post-CFA transection to validate successful induction of severe ischemia. Weekly assessment of the sequalae of ischemia on limb perfusion, and degree of ischemic myopathy was conducted for 1 month using X-ray arteriography, laser speckle imaging, CTA angiography, femoral artery duplex, high resolution ultrasound and histopathological analysis. The non-invasive tissue analysis of the elastography images showed specific and characteristic pattern of increased muscle stiffness indicative of the fibrotic and necrotic outcome expected with associated total muscle ischemia. The prominent onset of skeletal muscle necrosis was evident upon direct inspection of the affected tissues. Ischemic myopathic changes associated with inflammatory infiltrates and deficient blood vessels were objectively validated. A translational model of severe hindlimb ischemia causing ischemic myopathy was successfully established adopting an approach that enables long-term survival studies in compliance with regulatory requirements pertaining to animal welfare.

Characteristics of acid-sensing ion channel currents in male rat muscle dorsal root ganglion neurons following ischemia/reperfusion

Peripheral artery diseases (PAD) increases muscle afferent nerve-activated reflex sympathetic nervous and blood pressure responses during exercise (termed as exercise pressor reflex). However, the precise signaling pathways leading to the exaggerated autonomic responses in PAD are undetermined. Considering that limb ischemia/reperfusion (I/R) is a feature of PAD, we determined the characteristics of acid-sensing ion channel (ASIC) currents in muscle dorsal root ganglion (DRG) neurons under the conditions of hindlimb I/R and ischemia of PAD. In particular, we examined ASIC currents in two distinct subpopulations, isolectin B₄ -positive, and B₄ -negative (IB4+ and IB4-) muscle DRG neurons, linking to glial cell line-derived neurotrophic factor and nerve growth factor. In results, ASIC1a- and ASIC3-like currents were observed in IB4- muscle DRG neurons with a greater percentage of ASIC3-like currents. Hindimb I/R and ischemia did not alter the distribution of ASIC1a and ASIC3 currents with activation of pH 6.7 in IB4+ and IB4- muscle DRG neurons; however, I/R altered the distribution of ASIC3 currents in IB4+ muscle DRG neurons with pH 5.5, but not in IB4- neurons. In addition, I/R and ischemia amplified the density of ASIC3-like currents in IB4- muscle DRG neurons. Our results suggest that a selective subpopulation of muscle afferent nerves should be taken into consideration when ASIC signaling pathways are studied to determine the exercise pressor reflex in PAD.

Sympathetic Nerve Activity and Blood Pressure Response to Exercise in Peripheral Artery Disease: From Molecular Mechanisms, Human Studies, to Intervention Strategy Development

Sympathetic nerve activity (SNA) regulates the contraction of vascular smooth muscle and leads to a change in arterial blood pressure (BP). It was observed that SNA, vascular contractility, and BP are heightened in patients with peripheral artery disease (PAD) during exercise. The exercise pressor reflex (EPR), a neural mechanism responsible for BP response to activation of muscle afferent nerve, is a determinant of the exaggerated exercise-induced BP rise in PAD. Based on recent results obtained from a series of studies in PAD patients and a rat model of PAD, this review will shed light on SNA-driven BP response and the underlying mechanisms by which receptors and molecular mediators in muscle afferent nerves mediate the abnormalities in autonomic activities of PAD. Intervention strategies, particularly non-pharmacological strategies, improving the deleterious exercise-induced SNA and BP in PAD, and enhancing tolerance and performance during exercise will also be discussed.

Contrast-Enhanced Ultrasound Reveals Partial Perfusion Recovery After Hindlimb Ischemia as Opposed to Full Recovery by Laser Doppler Perfusion Imaging

Mouse models are critical in developing new therapeutic approaches to treat peripheral arterial disease (PAD). Despite decades of research and numerous clinical trials, the efficacy of available therapies is limited. This may suggest shortcomings in our current animal models and/or methods of assessment. We evaluated perfusion measurement methods in a mouse model of PAD by comparing laser Doppler perfusion imaging (LDPI, the most common technique), contrast-enhanced ultrasound (CEUS, an emerging technique) and fluorescent microspheres (conventional standard). Mice undergoing a femoral artery ligation were assessed by LDPI and CEUS at baseline and 1, 4, 7, 14, 28, 60, 90 and 150 d post-surgery to evaluate perfusion recovery in the ischemic hindlimb. Fourteen days after surgery, additional mice were measured with fluorescent microspheres, LDPI, and CEUS. LDPI and CEUS resulted in broadly similar trends of perfusion recovery until 7 d post-surgery. However, by day 14, LDPI indicated full recovery of perfusion, whereas CEUS indicated ∼50% recovery, which failed to improve even after 5 mo. In agreement with the CEUS results, fluorescent microspheres at day 14 post-surgery confirmed that perfusion recovery was incomplete. Histopathology and photoacoustic microscopy provided further evidence of sustained vascular abnormalities.

Buyang Huanwu Decoction Enhances Revascularization via Akt/GSK3β/NRF2 Pathway in Diabetic Hindlimb Ischemia

Background

Peripheral arterial disease (PAD) is a typical disease of atherosclerosis, most commonly influencing the lower extremities. In patients with PAD, revascularization remains a preferred treatment strategy. Buyang Huanwu decoction (BHD) is a popular Chinese herbal prescription which has showed effects of cardiovascular protection through conducting antioxidant, antiapoptotic, and anti-inflammatory effects. Here, we intend to study the effect of BHD on promoting revascularization via the Akt/GSK3β/NRF2 pathway in diabetic hindlimb ischemia (HLI) model of mice.

Materials and Methods

All db/db mice (n = 60) were randomly divided into 6 groups by table of random number. (1) Sham group (N = 10): 7-0 suture thread passed through the underneath of the femoral artery and vein without occlusion. The remaining 5 groups were treated differently on the basis of the HLI (the femoral artery and vein from the inguinal ligament to the knee joint were transected and the vascular stump was ligated with 7-0 silk sutures) model: (2) HLI+NS group (N = 15): 0.2 ml NS was gavaged daily for 3 days before modeling and 14 days after occlusion; (3) HLI+BHD group (N = 15): 0.2 ml BHD (20 g/kg/day) was gavaged daily for 3 days before modeling and 14 days after occlusion; (4) HLI+BHD+sh-NC group (N = 8): local injection of adenovirus vector carrying the nonsense shRNA (Ad-GFP) in the hindlimbs of mice before treatment; (5) HLI+BHD+sh-NRF2 group (N = 8): knockdown of NRF2 in the hindlimbs of mice by local intramuscular injection of adenovirus vector carrying NRF2 shRNA (Ad-NRF2-shRNA) before treatment; and (6) HLI+BHD+LY294002 group (N = 4): intravenous injection of LY294002 (1.5 mg/kg) once a day for 14 days on the basis of the HLI+BHD group. Laser Doppler examination, vascular cast, and immunofluorescence staining were applied to detect the revascularization of lower limbs in mice. Western blot analysis was used to detect the expression of vascular endothelial growth factor (VEGF), interleukin-1beta (IL-1β), interleukin-6 (IL-6), tumor necrosis factor- (TNF-) α, heme oxygenase-1 (HO-1), NAD(P)H dehydrogenase quinone-1 (NQO-1), catalase (CAT), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), phosphorylated protein kinase B (p-AKT), and phosphorylated glycogen synthase kinase-3 beta (p-GSK3β). HE staining was used to assess the level of muscle tissue damage and inflammation in the lower extremities. Local multipoint injection of Ad-NRF2-shRNA was used to knock down NRF2, and qPCR was applied to detect the mRNA level of NRF2. The blood glucose, triglyceride, cholesterol, MDA, and SOD levels of mice were tested using corresponding kits. The SPSS 20.0 software and GraphPad Prism 6.05 were used to do all statistics. Values of P < 0.05 were considered as statistically significant. Results and Conclusions. BHD could enhance the revascularization of lower limbs in HLI mice, while BHD has no effect on blood glucose and lipid level in db/db mice (P > 0.05). BHD could elevate the protein expression of VEGF, HO-1, NQO-1, and CAT (P < 0.05) and decrease the expression of IL-1β, IL-6, and TNF-α (P < 0.05) in HLI mice. Meanwhile, BHD could activate NRF2 and promote the phosphorylation of AKT/GSK3β during revascularization (P < 0.05). In contrast, knockdown of NRF2 impaired the protective effects of BHD on HLI (P < 0.05). LY294002 inhibited the upregulation of NRF2 activated by BHD through inhibiting the phosphorylation of the AKT/GSK3β pathway (P < 0.05). The present study demonstrated that BHD could promote revascularization on db/db mice with HLI through targeting antioxidation, anti-inflammation, and angiogenesis via the AKT/GSK3β/NRF2 pathway.

Low Intrinsic Aerobic Capacity Limits Recovery Response to Hindlimb Ischemia

Introduction: In this study, we determined the influence of intrinsic exercise capacity on the vascular adaptive responses to hind limb ischemia. High Capacity Running, HCR; Low Capacity Running, LCR, rats were used to assess intrinsic aerobic capacity effects on adaptive responses to ischemia.

Methods: Muscle samples from both ischemic and non-ischemic limb in both strains were compared, histologically for the muscle-capillary relationship, and functionally using microspheres to track blood flow and muscle stimulation to test fatigability. PCR was used to identify the differences in gene expression between the phenotypes following occlusive ischemia.

Results: Prior to ligation, there were not significant differences between the phenotypes in the exhaustion time with high frequency pacing. Following ligation, LCR decreased significantly in the exhaustion time compare with HCRs (437 ± 47 vs. 824 ± 56, p < 0.001). The immediate decrease in flow was significantly more severe in LCRs than HCRs (52.5 vs. 37.8%, p < 0.001). VEGF, eNOS, and ANG2 (but not ANG1) gene expression were decreased in LCRs vs. HCRs before occlusion, and increased significantly in LCRs 14D after occlusion, but not in HCRs. LCR capillary density (CD) was significantly lower at all time points after occlusion (LCR 7D = 564.76 ± 40.5, LCR 14D = 507.48 ± 54.2, both p < 0.05 vs. HCR for respective time point). NCAF increased significantly in HCR and LCR in response to ischemia.

Summary: These results suggest that LCR confers increased risk for ischemic injury and is subject to delayed and less effective adaptive response to ischemic stress.

A Swine Hind Limb Ischemia Model Useful for Testing Peripheral Artery Disease Therapeutics

Currently, there is no large animal model of sustained limb ischemia suitable for testing novel angiogenic therapeutics for peripheral artery disease (PAD) such as drugs, genes, materials, or cells. We created a large animal model suitable for efficacy assessment of these therapies by testing 3 swine hind limb ischemia (HLI) variations and quantifying vascular perfusion, muscle histology, and limb function. Ligation of the ipsilateral external and bilateral internal iliac arteries produced sustained gait dysfunction compared to isolated external iliac or unilateral external and internal iliac artery ligations. Hyperemia-dependent muscle perfusion deficits, depressed limb blood pressure, arteriogenesis, muscle atrophy, and microscopic myopathy were quantifiable in ischemic limbs 6 weeks post-ligation. Porcine mesenchymal stromal cells (MSCs) engineered to express a reporter gene were visualized post-administration via positron emission tomography (PET) in vivo. These results establish a preclinical platform enabling better optimization of PAD therapies, including cellular therapeutics, increasing bench-to-bedside translational success. A preclinical platform for porcine studies of peripheral artery disease therapies including (1) a hind limb ischemia model and (2) non-invasive MSC viability and retention assessment via PET.

Effects of bradykinin on voltage-gated KV 4 channels in muscle dorsal root ganglion neurons of rats with experimental peripheral artery disease

KEY POINTS

During exercise, bradykinin (BK), a muscle metabolite in ischaemic muscles, exaggerates autonomic responses to activation of muscle afferent nerves in peripheral artery disease (PAD). We examined whether BK inhibits activity of KV 4 channels in muscle afferent neurons of PAD rats induced by femoral artery occlusion. We demonstrated that: 1) femoral occlusion attenuates KV 4 currents in dorsal root ganglion (DRG) neurons innervating the hindlimb muscles and decreases the threshold of action potential firing; 2) BK has a greater inhibitory effect on KV 4 currents in muscle DRG neurons of PAD rats; and 3) expression of KV 4.3 is downregulated in DRGs of PAD rats and inhibition of KV 4.3 significantly decreases activity of KV 4 currents in muscle DRG neurons. Femoral artery occlusion-induced limb ischaemia and/or ischaemia-induced metabolites (i.e. BK) inhibit activity of KV 4 channels in muscle afferent neurons and this is likely involved in the exaggerated exercise pressor reflex in PAD.

ABSTRACT

Muscle afferent nerve-activated reflex sympathetic nervous and blood pressure responses are exaggerated during exercise in patients with peripheral artery diseases (PAD) and in PAD rats induced by femoral artery occlusion. However, the precise signalling pathways and molecular mediators responsible for these abnormal autonomic responses in PAD are poorly understood. A-type voltage-gated K+ (KV ) channels are quintessential regulators of cellular excitability in the various tissues. Among KV channels, KV 4 (i.e. KV 4.1 and KV 4.3) in primary sensory neurons mainly participate in physiological functions in regulation of mechanical and chemical sensation. However, little is known about the role of KV 4 in regulating neuronal activity in muscle afferent neurons of PAD. In addition, bradykinin (BK) is considered as a muscle metabolite contributing to the exaggerated exercise pressor reflex in PAD rats with femoral artery occlusion. Our data demonstrated that: 1) KV 4 currents are attenuated in dorsal root ganglion (DRG) neurons innervating the hindlimb muscles of PAD rats, along with a decreasing threshold of action potential firing; 2) KV 4 currents are inhibited by application of BK onto muscle DRG neurons of PAD rats to a greater degree; and 3) expression of KV 4.3 is downregulated in the DRGs of PAD rats and KV 4.3 channel is a major contributor to the activity of KV 4 currents in muscle DRG neurons. In conclusion, data suggest that femoral artery occlusion-induced limb ischaemia and/or ischaemia-induced metabolites (i.e. BK) inhibit the activity of KV 4 channels in muscle afferent neurons likely leading to the exaggerated exercise pressor reflex observed in PAD.

Bach1 plays an important role in angiogenesis through regulation of oxidative stress

Bach1 is a known transcriptional repressor of the heme oxygenase-1 (HO-1) gene. The purpose of this study was to determine whether angiogenesis is accelerated by genetic ablation of Bach1 in a mouse ischemic hindlimb model. Hindlimb ischemia was surgically induced in wild-type (WT) mice, Bach1-deficient (Bach1-/-) mice, apolipoprotein E-deficient (ApoE-/-) mice, and Bach1/ApoE double-knockout (Bach1-/-/ApoE-/-) mice. Blood flow recovery after hindlimb ischemia showed significant improvement in Bach1-/- mice compared with that in WT mice. Bach1-/-/ApoE-/- mice showed significantly improved blood flow recovery compared with that in ApoE-/- mice to the level of that in WT mice. Migration of endothelial cells in ApoE-/- mice was significantly decreased compared with that in WT mice. Migration of endothelial cells significantly increased in Bach1-/-/ApoE-/- mice compared with that in ApoE-/- mice to the level of that in WT mice. The expression levels of HO-1, peroxisome proliferator-activated receptor γ co-activator-1α, angiopoietin 1, and fibroblast growth factor 2 in endothelial cells isolated from Bach1-/-/ApoE-/- mice were significantly higher than those in ApoE-/- mice. Oxidative stress assessed by anti-acrolein antibody staining in ischemic tissues and urinary 8-isoPGF2α excretion were significantly increased in ApoE-/- mice compared with those in WT and Bach1-/- mice. Oxidative stress was reduced in Bach1-/-/ApoE-/- mice compared with that in ApoE-/- mice. These findings suggest that genetic ablation of Bach1 plays an important role in ischemia-induced angiogenesis under the condition of increased oxidative stress. Bach1 could be a potential therapeutic target to reduce oxidative stress and potentially improve angiogenesis for patients with peripheral arterial disease.

HIF-1α inhibition alleviates the exaggerated exercise pressor reflex in rats with peripheral artery disease induced by femoral artery occlusion

Hypoxia-inducible factor 1α (HIF-1α) is a transcription factor mediating adaptive responses to hypoxia and ischemia. Our previous work showed that HIF-1α is increased in muscle sensory nerves of rats with peripheral artery disease (PAD) induced by femoral artery occlusion. The present study was further to examine the role played by HIF-1α in regulating the response of arterial blood pressure (BP) to the activation of muscle afferent nerve during static muscle contraction in rats with femoral artery occlusion. A rat model of femoral artery ligation was used to study PAD in this study. Western blot analysis was employed to examine the protein levels of HIF-1α in the dorsal root ganglion (DRG) tissues. BAY87, a synthesized compound with the characteristics of highly potent and specific suppressive effects on expression and activity of HIF-1α, was given into the arterial blood supply of the ischemic hindlimb muscles before the exercise pressor reflex was evoked by static muscle contraction. First, HIF-1α was increased in the DRG of occluded limbs (optical density: 0.89 ± 0.13 in control versus 1.5 ± 0.05 in occlusion; p < 0.05, n = 6 in each group). Arterial injection of BAY87 (0.2 mg/kg) then inhibited expression of HIF-1α in the DRG of occluded limbs 3 hr following its injection (optical density: 1.02 ± 0.09 in occluded limbs with BAY87 versus 1.06 ± 0.1 in control limbs; p > 0.05, n = 5 in each group). In addition, muscle contraction evoked a greater increase in BP in occluded rats. BAY87 attenuated the enhanced BP response in occluded rats to a greater degree than in control rats. Our data suggest that the inhibition of HIF-1α alleviates the exaggeration of the exercise pressor reflex in rats under ischemic circumstances of the hindlimbs in PAD induced by femoral artery occlusion.

One-Time Acute Heat Treatment Is Effective for Attenuation of the Exaggerated Exercise Pressor Reflex in Rats With Femoral Artery Occlusion

The purpose of this study was to determine the effects of one-time acute heat treatment (HT) on the exaggerated exercise pressor reflex in a model of peripheral arterial insufficiency induced by ligation of the femoral artery and was to further examine the underlying mechanism of ATP-P2X₃ signal activity during this process. The blood pressure (BP) response to static muscle contraction and muscle tendon stretch was recorded to determine the exercise pressor reflex. Also, αβ-methylene ATP (αβ-me ATP) was injected into the arterial blood supply of the hindlimb muscles to stimulate P2X₃ receptors in the muscle afferent nerves. To process one-time acute HT, a heating pad was placed locally on the hindlimb and the muscle temperature (Tm) was increased by ~1.5°C and maintained for 5 min. Compared with control rats, a greater mean arterial pressure (MAP) response to muscle contraction was observed in rats with femoral occlusion in a pre-heat control session (28 ± 2 mmHg in occluded rats/n = 12 vs. 18 ± 2 mmHg in control rats/n = 9; p < 0.05). The one-time acute HT attenuated the amplification of the BP response in rats with femoral artery occlusion (MAP response: 19 ± 8 mmHg in occluded rats + HT/n = 11; p < 0.05 vs. occluded rats). In contrast, HT did not significantly attenuate amplification of MAP response to muscle stretch and αβ-me ATP injection in rats with femoral artery occlusion and controls (all p > 0.05). Our data suggest that one-time acute HT selectively attenuates the amplified pressor response induced by activation of the metabolic and mechanical components of the reflex in rats after femoral artery occlusion. The suppressing effects of acute HT on the exaggerated exercise pressor reflex are likely mediated through a reduction in metabolites (e.g., ATP) stimulating the muscle afferent nerves in contracting muscle, but unlikely through direct alteration of P2X receptors per se.

Effect of intramuscular baculovirus encoding mutant hypoxia-inducible factor 1-alpha on neovasculogenesis and ischemic muscle protection in rabbits with peripheral arterial disease

BACKGROUND AIMS

Peripheral arterial disease (PAD) is a progressive, disabling ailment for which no effective treatment exists. Gene therapy-mediated neovascularization has emerged as a potentially useful strategy. We tested the angiogenic and arteriogenic efficacy and safety of a baculovirus (BV) encoding mutant, oxygen-resistant hypoxia-inducible factor 1-alpha (mHIF-1α), in rabbits with PAD.

METHODS

After assessing the transfection efficiency of the BV.mHIF-1α vector and its tubulogenesis potential in vitro, we randomized rabbits with experimental PAD to receive 1 × 10⁹ copies of BV.mHIF-1α or BV.null (n = 6 per group) 7 days after surgery. Two weeks post-treatment, collateralization (digital angiography) and capillary and arteriolar densities (immunohistochemistry) were measured in the posterior limbs. Ischemic damage was evaluated in adductor and gastrocnemius muscle samples. Tracking of viral DNA in injected zones and remote tissues at different time points was performed in additional rabbits using a BV encoding GFP.

RESULTS

Angiographically visible collaterals were more numerous in BV.mHIF-1α-treated rabbits (8.12 ± 0.42 vs 6.13 ± 1.15 collaterals/cm², P < 0.05). The same occurred with arteriolar (27.9 ± 7.0 vs 15.3 ± 4.0 arterioles/mm²) and capillary (341.8 ± 109.9 vs 208.8 ± 87.7 capillaries/mm², P < 0.05) densities. BV.mHIF-1α-treated rabbits displayed less ischemic muscle damage than BV.null-treated animals. Viral DNA and GFP mRNA were detectable only at 3 and 7 days after injection in hind limbs. Neither the virus nor GFP mRNA was detected in remote tissues.

CONCLUSIONS

In rabbits with PAD, BV.mHIF-1α induced neovascularization and reduced ischemic damage, exhibiting a good safety profile at 14 days post-treatment. Complementary studies to evaluate its potential usefulness in the clinic are needed.

Heat treatment improves the exaggerated exercise pressor reflex in rats with femoral artery occlusion via a reduction in the activity of the P2X receptor pathway

KEY POINTS

During exercise, the blood pressure (BP) response is exaggerated in peripheral artery disease (PAD). We examined whether heat treatment (HT) has beneficial effects on the exaggerated exercise pressor reflex in PAD rats. With HT (increase in basal muscle temperature of ∼1.5°C for 30 min, twice daily for three continuous days), the amplified BP response to muscle contraction is alleviated in PAD. We demonstrated that HT attenuates the enhancement of the BP response induced by stimulation of P2X in muscle afferent nerves of PAD rats. HT also attenuates the upregulation of the P2X₃ and the increase in P2X currents in the muscle afferent neurons of PAD rats. Previous heat exposure plays a beneficial role in modifying the exaggeration of the exercise pressor reflex in PAD and a reduction in the activity of the P2X receptor pathway is probably a part of the mechanism mediating this improvement.

ABSTRACT

The current study was performed to examine if heat treatment (HT) has beneficial effects on the exaggerated exercise pressor reflex in rats with peripheral artery disease (PAD). We further determined if the temperature-sensitive P2X receptor is involved in the effects of HT. The pressor response to static muscle contraction and α,β-methylene ATP (αβ-me ATP, a P2X agonist) was examined. Western blot analysis was used to determine the protein levels of P2X₃ in the dorsal root ganglion (DRG), and the whole cell patch clamp was used to examine the amplitude of P2X currents in the DRG neurons. The basal muscle temperature (T_m ) was lower in PAD rats than in control rats. T_m was increased by ∼1.5°C and this increase was maintained for 30 min. This HT protocol was performed tweice daily for three continuous days. A greater blood pressure (BP) response to contraction was observed in PAD rats. HT attenuated the amplification of the BP response in PAD rats. HT also attenuated the enhancement of the BP response induced by the arterial injection of αβ-me ATP in PAD rats. In addition, HT attenuated the upregulation of the P2X₃ and increased P2X currents in the DRG neurons of PAD rats. In conclusion, previous heat exposure plays an inhibitory role in modifying the exaggeration of the exercise pressor reflex in PAD and a reduction of the activity of the P2X receptor pathway is probably a part of mechanisms leading to the beneficial effects of HT.

Epigenetic regulators of the revascularization response to chronic arterial occlusion

Peripheral arterial disease (PAD) is the leading cause of lower limb amputation and estimated to affect over 202 million people worldwide. PAD is caused by atherosclerotic lesions that occlude large arteries in the lower limbs, leading to insufficient blood perfusion of distal tissues. Given the severity of this clinical problem, there has been long-standing interest in both understanding how chronic arterial occlusions affect muscle tissue and vasculature and identifying therapeutic approaches capable of restoring tissue composition and vascular function to a healthy state. To date, the most widely utilized animal model for performing such studies has been the ischaemic mouse hindlimb. Despite not being a model of PAD per se, the ischaemic hindlimb model does recapitulate several key aspects of PAD. Further, it has served as a valuable platform upon which we have built much of our understanding of how chronic arterial occlusions affect muscle tissue composition, muscle regeneration and angiogenesis, and collateral arteriogenesis. Recently, there has been a global surge in research aimed at understanding how gene expression is regulated by epigenetic factors (i.e. non-coding RNAs, histone post-translational modifications, and DNA methylation). Thus, perhaps not unexpectedly, many recent studies have identified essential roles for epigenetic factors in regulating key responses to chronic arterial occlusion(s). In this review, we summarize the mechanisms of action of these epigenetic regulators and highlight several recent studies investigating the role of said regulators in the context of hindlimb ischaemia. In addition, we focus on how these recent advances in our understanding of the role of epigenetics in regulating responses to chronic arterial occlusion(s) can inform future therapeutic applications to promote revascularization and perfusion recovery in the setting of PAD.

Enhancement by TNF-α of TTX-resistant NaV current in muscle sensory neurons after femoral artery occlusion

Femoral artery occlusion in rats has been used to study human peripheral artery disease (PAD). Using this animal model, a recent study suggests that increases in levels of tumor necrosis factor-α (TNF-α) and its receptor lead to exaggerated responses of sympathetic nervous activity and arterial blood pressure as metabolically sensitive muscle afferents are activated. Note that voltage-dependent Na⁺ subtype Na_V1.8 channels (Na_V1.8) are predominately present in chemically sensitive thin fiber sensory nerves. The purpose of this study was to examine the role played by TNF-α in regulating activity of Na_V1.8 currents in muscle dorsal root ganglion (DRG) neurons of rats with PAD induced by femoral artery occlusion. DRG neurons from control and occluded limbs of rats were labeled by injecting the fluorescent tracer DiI into the hindlimb muscles 5 days before the experiments. A voltage patch-clamp mode was used to examine TTX-resistant (TTX-R) Na_V currents. Results were as follows: 72 h of femoral artery occlusion increased peak amplitude of TTX-R [1,922 ± 139 pA in occlusion (n = 11 DRG neurons) vs. 1,178 ± 39 pA in control (n = 10), means ± SE; P < 0.001 between the 2 groups] and Na_V1.8 currents [1,461 ± 116 pA in occlusion (n = 11) and 766 ± 48 pA in control (n = 10); P < 0.001 between groups] in muscle DRG neurons. TNF-α exposure amplified TTX-R and Na_V1.8 currents in DRG neurons of occluded muscles in a dose-dependent manner. Notably, the amplification of TTX-R and Na_V1.8 currents induced by TNF-α was attenuated in DRG neurons with preincubation with respective inhibitors of the intracellular signaling pathways p38-MAPK, JNK, and ERK. In conclusion, our data suggest that Na_V1.8 is engaged in the role of TNF-α in amplifying muscle afferent inputs as the hindlimb muscles are ischemic; p38-MAPK, JNK, and ERK pathways are likely necessary to mediate the effects of TNF-α.

Imaging the Landmarks of Vascular Recovery

Background: Peripheral arterial disease (PAD) is a major worldwide health concern. Since the late 1990s therapeutic angiogenesis has been investigated as an alternative to traditional PAD treatments. Although positive preclinical results abound in the literature, the outcomes of human clinical trials have been discouraging. Among the challenges the field has faced has been a lack of standardization of the timings and measures used to validate new treatment approaches.

Methods: In order to study the spatiotemporal dynamics of both perfusion and neovascularization in mice subjected to surgically-induced hindlimb ischemia (n= 30), we employed three label-free imaging modalities (a novel high-sensitivity ultrasonic Power Doppler methodology, laser speckle contrast, and photoacoustic imaging), as well as a tandem of radio-labeled molecular probes, ^99mTc-NC100692 and ^99mTc-BRU-5921 respectively, designed to detect two key modulators of angiogenic activity, α_Vβ₃ and HIF-1α , via scintigraphic imaging.

Results: The multimodal imaging strategy reveals a set of “landmarks”—key physiological and molecular events in the healing process—that can serve as a standardized framework for describing the impact of emerging PAD treatments. These landmarks span the entire process of neovascularization, beginning with the rapid decreases in perfusion and oxygenation associated with ligation surgery, extending through pro-angiogenic changes in gene expression driven by the master regulator HIF-1α , and ultimately leading to complete functional revascularization of the affected tissues.

Conclusions: This study represents an important step in the development of multimodal non-invasive imaging strategies for vascular research; the combined results offer more insight than can be gleaned through any of the individual imaging methods alone. Researchers adopting similar imaging strategies and will be better able to describe changes in the onset, duration, and strength of each of the landmarks of vascular recovery, yielding greater biological insight, and enabling more comprehensive cross-study comparisons. Perhaps most important, this study paves the road for more efficient translation of PAD research; emerging experimental treatments can be more effectively assessed and refined at the preclinical stage, ultimately leading to better next-generation therapies.

Limb functional recovery is impaired in fibroblast growth factor-2 (FGF2) deficient mice despite chronic ischaemia-induced vascular growth

FGF2 is a potent stimulator of vascular growth; however, even with a deficiency of FGF2 (Fgf2-/-), developmental vessel growth or ischaemia-induced revascularization still transpires. It remains to be elucidated as to what function, if any, FGF2 has during ischaemic injury. Wildtype (WT) or Fgf2-/- mice were subjected to hindlimb ischaemia for up to 42 days. Limb function, vascular growth, inflammatory- and angiogenesis-related proteins, and inflammatory cell infiltration were assessed in sham and ischaemic limbs at various timepoints. Recovery of ischaemic limb function was delayed in Fgf2-/- mice. Yet, vascular growth response to ischaemia was similar between WT and Fgf2-/- hindlimbs. Several angiogenesis- and inflammatory-related proteins (MCP-1, CXCL16, MMPs and PAI-1) were increased in Fgf2-/- ischaemic muscle. Neutrophil or monocyte recruitment/infiltration was elevated in Fgf2-/- ischaemic muscle. In summary, our study indicates that loss of FGF2 induces a pro-inflammatory microenvironment in skeletal muscle which exacerbates ischaemic injury and delays functional limb use.

Reproducible hindlimb ischemia model based on photochemically induced thrombosis to evaluate angiogenic effects

Critical limb ischemia is one of the most common types of peripheral arterial disease. Preclinical development of ischemia therapeutics relies on the availability of a relevant and reproducible in vivo disease model. Thus, establishing appropriate animal disease models is essential for the development of new therapeutic strategies. Currently, the most commonly employed model of hindlimb ischemia is the surgical induction method with ligation of the femoral artery and its branches after skin incision. However, the efficiency of the method is highly variable depending on the availability of skilled technicians. In addition, after surgical procedures, animals can quickly and spontaneously recover from damage, limiting observations of the therapeutic effect of potential agents. The aim of this study was to develop a hindlimb ischemia mouse model with similarities to human ischemic disease. To that end, a photochemical reaction was used to induce thrombosis in the hindlimb. After the photochemical reaction was induced by light irradiation, thrombotic plugs and adjacent red blood cell stasis were observed in hindlimb vessels in the light-irradiated zone. Additionally, the photochemically induced thrombosis maintained the ischemic condition and did not cause notable side effects in mice.

Heat therapy improves soleus muscle force in a model of ischemia-induced muscle damage

Leg muscle ischemia in patients with peripheral artery disease (PAD) leads to alterations in skeletal muscle morphology and reduced leg strength. We tested the hypothesis that exposure to heat therapy (HT) would improve skeletal muscle function in a mouse model of ischemia-induced muscle damage. Male 42-wk-old C57Bl/6 mice underwent ligation of the femoral artery and were randomly assigned to receive HT (immersion in a water bath at 37°C, 39°C, or 41°C for 30 min) or a control intervention for 3 wk. At the end of the treatment, the animals were anesthetized and the soleus and extensor digitorum longus (EDL) muscles were harvested for the assessment of contractile function and examination of muscle morphology. A subset of animals was used to examine the impact of a single HT session on the expression of genes involved in myogenesis and the regulation of muscle mass. Relative soleus muscle mass was significantly higher in animals exposed to HT at 39°C compared with the control group (control: 0.36 ± 0.01 mg/g versus 39°C: 0.40 ± 0.01 mg/g, P = 0.024). Maximal absolute force of the soleus was also significantly higher in animals treated with HT at 37°C and 39°C (control: 274.7 ± 6.6 mN; 37°C: 300.1 ± 7.7 mN; 39°C: 299.5 ± 10 mN, P < 0.05). In the soleus, but not the EDL muscle, a single session of HT enhanced the mRNA expression of myogenic factors as well as of both positive and negative regulators of muscle mass. These findings suggest that the beneficial effects of HT are muscle specific and dependent on the treatment temperature in a model of PAD. NEW & NOTEWORTHY This is the first study to comprehensively examine the impact of temperature and muscle fiber type composition on the adaptations to repeated heat stress in a model of ischemia-induced muscle damage. Exposure to heat therapy (HT) at 37°C and 39°C, but not at 41°C, improved force development of the isolated soleus muscle. These results suggest that HT may be a practical therapeutic tool to restore muscle mass and strength in patients with peripheral artery disease.

Curcumin improves perfusion recovery in experimental peripheral arterial disease by upregulating microRNA-93 expression

In peripheral arterial disease (PAD), angiogenesis is the major process involved in repairing the microvasculature in the ischemic lower limb. Curcumin, a monomer isolated from turmeric roots, has been demonstrated to have pro- and anti-angiogenic effects under different circumstances. Previous studies have indicated that curcumin treatment improves tissue repair and perfusion recovery in a mouse model of diabetic PAD. However, the effects of curcumin on PAD under non-diabetic conditions has remained elusive, In the present study, mice with PAD and a normal glycaemic profile were treated with curcumin, which improved perfusion recovery, increased capillary density and elevated microRNA (miR)-93 expression in ischemic muscle tissue. In cultured endothelial cells under simulated ischemia, curcumin improved endothelial cell viability and enhanced tube formation. However, following miR-93 knockdown using a microRNA inhibitor, endothelial cell tube formation was inhibited. Furthermore, in the presence of the miR-93 inhibitor, curcumin did not alter endothelial cell viability or tube formation. These results demonstrate that curcumin had beneficial effects in non-diabetic PAD by improving angiogenesis, which may have been achieved partially via the promotion of miR-93 expression.

Optimal implantation site of transponders for identification of experimental swine

Use of transponders, small electronic identification devices, in experimental swine is expected to be more reliable than the current common use of ear tags. However, it is necessary to determine the optimal implantation site for transponders with high readability, retentionability (i.e., long-term retention in tissues without detachment or loss), and biocompatibility, as this has not yet been investigated. Thus, we aimed to determine the optimal implantation site. Two types of transponders were subcutaneously implanted into four different sites (ear base, ear auricle, ventral neck, and back) in 3 domestic swine each. The transponders were scanned at 1, 2, 3, and 84 days after implantation. The location of the transponders was examined by X-ray and echography at 84 days. Histopathological examinations were performed at 84 days. The transponders in the back were successfully scanned in a shorter time than those in other implantation sites, without any re-scanning procedures. X-ray examination revealed one transponder in the ventral neck was lost, whereas those in the other sites were retained in their original location for 84 days. Echography indicated that the transponders in the back were retained more deeply than those in other implantation sites, suggesting better retentionability. Acceptable biocompatibility was confirmed in all implantation sites, as evidenced by the finding that all transponders were covered by a connective tissue capsule without severe inflammation. In conclusion, the present results demonstrated that the back is the optimal implantation site for transponders in experimental swine.

Role of genetics in peripheral arterial disease outcomes; significance of limb-salvage quantitative locus-1 genes

Peripheral artery disease is a major health care problem with significant morbidity and mortality. Humans with peripheral artery disease exhibit two major and differential clinical manifestations - intermittent claudication and critical limb ischemia. Individuals with intermittent claudication or critical limb ischemia have overlapping risk factors and objective measures of blood flow. Hence, we hypothesized that variation in genetic make-up may be an important determinant in the severity of peripheral artery disease. Previous studies have identified polymorphism in genes, contributing to extent of atherosclerosis but much less is known about polymorphisms associated with genes that can influence peripheral artery disease severity. This review outlines some of the progress made up-to-date to unravel the molecular mechanisms underlining differential peripheral artery disease severity. By exploring the recovery phenotype of different mouse strains following experimental peripheral artery disease, our group identified the limb salvage-associated quantitative trait locus 1 on mouse chromosome 7 as the first genetic modifier of perfusion recovery and tissue necrosis phenotypes. Furthermore, a number of genes within LSq-1, such as ADAM12, IL-21Rα, and BAG3 were identified as genetic modifiers of peripheral artery disease severity that function through preservation of endothelial and skeletal muscle cells during ischemia. Taken together, these studies suggest manipulation of limb salvage-associated quantitative trait locus 1 genes show great promise as therapeutic targets in the management of peripheral artery disease. Impact statement Peripheral artery disease (PAD) is a major health care problem with significant morbidity and mortality. Individuals with similar atherosclerosis burden do display different severity of disease. This review outlines some of the progress made up-to-date in unraveling the molecular mechanisms underlining differential PAD severity with a focus on the role of the Limb Salvage-associated Quantitative trait locus 1 (LSq-1), a key locus in adaptation to ischemia in PAD.

Proteinase-Activated Receptor-2 Sensitivity of Amplified TRPA1 Activity in Skeletal Muscle Afferent Nerves and Exercise Pressor Reflex in Rats with Femoral Artery Occlusion

BACKGROUND/AIMS

Limb ischemia occurs in peripheral artery disease (PAD). Sympathetic nerve activity (SNA) that regulates blood flow directed to the ischemic limb is exaggerated during exercise in this disease, and transient receptor potential channel A1 (TRPA1) in thin-fiber muscle afferents contributes to the amplified sympathetic response. The purpose of the present study was to determine the role of proteinase-activated receptor-2 (PAR2) in regulating abnormal TRPA1 function and the TRPA1-mediated sympathetic component of the exercise pressor reflex.

METHODS

A rat model of femoral artery ligation was employed to study PAD. Dorsal root ganglion (DRG) tissues were obtained to examine the protein levels of PAR2 using western blot analysis. Current responses induced by activation of TRPA1 in skeletal muscle DRG neurons were characterized using whole-cell patch clamp methods. The blood pressure response to static exercise (i.e., muscle contraction) and stimulation of TRPA1 was also examined after a blockade of PAR2.

RESULTS

The expression of PAR2 was amplified in DRG neurons of the occluded limb, and PAR2 activation with SL-NH2 (a PAR2 agonist) increased the amplitude of TRPA1 currents to a greater degree in DRG neurons of the occluded limb. Moreover, FSLLRY-NH2 (a PAR antagonist) injected into the arterial blood supply of the hindlimb muscles significantly attenuated the pressor response to muscle contraction and TRPA1 stimulation in rats with occluded limbs.

CONCLUSIONS

The PAR2 signal in muscle sensory nerves contributes to the amplified exercise pressor reflex via TRPA1 mechanisms in rats with femoral artery ligation. These findings provide a pathophysiological basis for autonomic responses during exercise activity in PAD, which may potentially aid in the development of therapeutic approaches for improvement of blood flow in this disease.

Therapeutic Angiogenesis for Peripheral Artery Disease: Lessons Learned in Translational Science

Peripheral arterial disease (PAD) is a major health care problem. There have been limited advances in medical therapies, and a huge burden of symptomatic patients with intermittent claudication and critical limb ischemia who have limited treatment options. Angiogenesis is the growth and proliferation of blood vessels from existing vasculature. For approximately 2 decades, "therapeutic angiogenesis" has been studied as an investigational approach to treat patients with symptomatic PAD. Despite literally hundreds of positive preclinical studies, results from human clinical studies thus far have been disappointing. Here we present an overview of where the field of therapeutic angiogenesis stands today and examine lessons learned from previously conducted clinical trials. The objective is not to second-guess past efforts but to place the lessons in perspective to allow for trial success in the future to improve agent development, trial design, and ultimately, clinical outcomes for new therapeutics for PAD.

The effects of RSR13 on microvascular Po2 kinetics and muscle contractile performance in the rat arterial ligation model of peripheral arterial disease

Exercise intolerance and claudication are symptomatic of peripheral arterial disease. There is a close relationship between muscle O2 delivery, microvascular oxygen partial pressure (P mvO2), and contractile performance. We therefore hypothesized that a reduction of hemoglobin-oxygen affinity via RSR13 would maintain a higher P mvO2 and enhance blood-muscle O2 transport and contractile function. In male Wistar rats (12 wk of age), we created hindlimb ischemia via right-side iliac artery ligation (AL). The contralateral (left) muscle served as control (CONT). Seven days after AL, phosphorescence-quenching techniques were used to measure P mvO2 at rest and during contractions (electrical stimulation; 1 Hz, 300 s) in tibialis anterior muscle (TA) under saline ( n = 10) or RSR13 ( n = 10) conditions. RSR13 at rest increased TA P mvO2 in CONT (13.9 ± 1.6 to 19.3 ± 1.9 Torr, P < 0.05) and AL (9.0 ± 0.5 to 9.9 ± 0.7 Torr, P < 0.05). Furthermore, RSR13 extended maintenance of the initial TA force (i.e., improved contractile performance) such that force was not decreased significantly until contraction 240 vs. 150 in CONT and 80 vs. 20 in AL. This improved muscle endurance with RSR13 was accompanied by a greater ΔP mvO2 (P mvO2 decrease from baseline) (CONT, 7.4 ± 1.0 to 11.2 ± 1.3; AL, 6.9 ± 0.5 to 8.6 ± 0.6 Torr, both P < 0.05). Whereas RSR13 did not alter the kinetics profile of P mvO2 (i.e., mean response time) substantially during contractions, muscle force was elevated, and the ratio of muscle force to P mvO2 increased. In conclusion, reduction of hemoglobin-oxygen affinity via RSR13 in AL increased P mvO2 and improved muscle contractile performance most likely via enhanced blood-muscle O2 diffusion.

NEW & NOTEWORTHY This is the first investigation to examine the effect of RSR13 (erythrocyte allosteric effector) on skeletal muscle microvascular oxygen partial pressure kinetics and contractile function using an arterial ligation model of peripheral arterial disease in experimental animals. The present results provide strong support for the concept that reducing hemoglobin-O2 affinity via RSR13 improved tibialis anterior muscle contractile performance most likely via enhanced blood-muscle O2 diffusion.

TRPA1 Function in Skeletal Muscle Sensory Neurons Following Femoral Artery Occlusion

BACKGROUND/AIMS

Transient receptor potential channel A1 (TRPA1) is engaged in amplified autonomic responses evoked by stimulation of muscle afferent nerves in rats with experimental peripheral arterial disease. The purposes of this study were to characterize current responses induced by activation of TRPA1 in dorsal root ganglion (DRG) neurons of control limbs and limbs with femoral artery occlusion.

METHODS

DRG neurons from rats were labeled by injecting the fluorescence tracer DiI into the hindlimb muscles and whole-cell patch clamp experiments were performed to determine TRPA1 currents.

RESULTS

Data show that AITC (a TRPA1 agonist) from the concentrations of 50 µM to 200 µM produces a dose-dependent increase of amplitudes of inward current responses. Notably, the peak current amplitude induced by AITC is significantly larger in DRG neurons of ligated limbs than that in control limbs. AITC-induced current responses are observed in small and medium size DRG neurons, and there is no difference in size distribution of DRG neurons between control limbs and ligated limbs. However, femoral occlusion increases the percentage of the AITC-sensitive DRG neurons as compared to control. AITC-induced currents in DRG neurons are significantly attenuated by exposure to 10 µM of HC-030031, a potent and selective inhibitor of TRPA1, in both control and femoral occlusion groups. In addition, capsaicin (a TRPV1 agonist) evokes a greater increase in the amplitude of AITC-currents in DRG neurons of ligated limbs than that in control limbs.

CONCLUSIONS

A greater current response with activation of TRPA1 is developed in muscle afferent nerves when hindlimb arterial blood supply is deficient under ischemic conditions; and TRPV1 is partly responsible for augmented TRPA1 responses induced by arterial occlusion.

3D-printed vascular networks direct therapeutic angiogenesis in ischaemia

Arterial bypass grafts remain the gold standard for the treatment of end-stage ischaemic disease. Yet patients unable to tolerate the cardiovascular stress of arterial surgery or those with unreconstructable disease would benefit from grafts that are able to induce therapeutic angiogenesis. Here, we introduce an approach whereby implantation of 3D-printed grafts containing endothelial-cell-lined lumens induces spontaneous, geometrically guided generation of collateral circulation in ischaemic settings. In rodent models of hind-limb ischaemia and myocardial infarction, we demonstrate that the vascular patches rescue perfusion of distal tissues, preventing capillary loss, muscle atrophy and loss of function. Inhibiting anastomoses between the construct and the host's local capillary beds, or implanting constructs with unpatterned endothelial cells, abrogates reperfusion. Our 3D-printed grafts constitute an efficient and scalable approach to engineer vascular patches able to guide rapid therapeutic angiogenesis and perfusion for the treatment of ischaemic diseases.

Laboratory Models for the Study of Normal and Pathologic Wound Healing

Current knowledge of wound healing is based on studies using various in vitro and in vivo wound models. In vitro models allow for biological examination of specific cell types involved in wound healing. In vivo models generally provide the full spectrum of biological responses required for wound healing, including inflammation and angiogenesis, and provide cell-cell interactions not seen in vitro. In this review, the authors aim to delineate the most relevant wound healing models currently available and to discuss their strengths and limitations in their approximation of the human wound healing processes to aid scientists in choosing the most appropriate wound healing models for designing, testing, and validating their experiments.

Purinergic P2X Receptors and Heightened Exercise Pressor Reflex in Peripheral Artery Disease

Arterial blood pressure (BP) and vasoconstriction regulated by sympathetic nerve activity (SNA) are heightened during exercise in patients with peripheral artery disease (PAD). The exercise pressor reflex is considered as a neural mechanism responsible for the exaggerated autonomic responses to exercise in PAD. A series of studies have employed a rat model of PAD to examine signal pathways at receptor and cellular levels by which the exercise pressor reflex is amplified. This review will summarize results obtained from recent human and animal studies with respect to contribution of muscle afferents to augmented SNA and BP responses in PAD. The role played by adenosine triphosphate (ATP) and ATP sensitive purinergic P2X receptors will be emphasized.

Extracellular Matrix Hydrogel Promotes Tissue Remodeling, Arteriogenesis, and Perfusion in a Rat Hindlimb Ischemia Model

Although surgical and endovascular revascularization can be performed in peripheral arterial disease (PAD), 40% of patients with critical limb ischemia do not have a revascularization option. This study examines the efficacy and mechanisms of action of acellular extracellular matrix-based hydrogels as a potential novel therapy for treating PAD. We tested the efficacy of using a tissue-specific injectable hydrogel derived from decellularized porcine skeletal muscle (SKM) and compared this to a new human umbilical cord-derived matrix (hUC) hydrogel, which could have greater potential for tissue regeneration because of the younger age of the tissue source. In a rodent hindlimb ischemia model, both hydrogels were injected 1-week post-surgery and perfusion was regularly monitored with laser speckle contrast analysis to 35 days post-injection. There were significant improvements in hindlimb tissue perfusion and perfusion kinetics with both biomaterials. Histologic analysis indicated that the injected hydrogels were biocompatible, and resulted in arteriogenesis, rather than angiogenesis, as well as improved recruitment of skeletal muscle progenitors. Skeletal muscle fiber morphology analysis indicated that the muscle treated with the tissue-specific SKM hydrogel more closely matched healthy tissue morphology. Whole transcriptome analysis indicated that the SKM hydrogel caused a shift in the inflammatory response, decreased cell death, and increased blood vessel and muscle development. These results show the efficacy of an injectable ECM hydrogel alone as a potential therapy for treating patients with PAD. Our results indicate that the SKM hydrogel improved functional outcomes through stimulation of arteriogenesis and muscle progenitor cell recruitment.

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Although surgical and endovascular revascularization can be performed in patients with peripheral arterial disease (PAD), 40% of patients with critical limb ischemia do not have a revascularization option.

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The efficacy of an injectable tissue-specific skeletal muscle extracellular matrix (ECM) hydrogel and a human umbilical cord-derived ECM hydrogel were examined in a rodent hindlimb ischemia model.

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Although both biomaterials increased tissue perfusion 35 days post-injection, likely through arteriogenesis, the skeletal muscle ECM hydrogel more closely matched healthy tissue morphology.

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Transcriptomic analysis indicates the skeletal muscle ECM hydrogel shifted the inflammatory response, decreased necrosis/apoptosis, and increased blood vessel and muscle development.

Endothelial interleukin-21 receptor up-regulation in peripheral artery disease

In most patients with symptomatic peripheral artery disease (PAD), severe stenosis in or occlusion of the major blood vessels that supply the legs make the amount of distal blood flow dependent on the capacity to induce angiogenesis and collateral vessel formation. Currently, there are no medications that improve perfusion to the ischemic limb, and thus directly treat the primary problem of PAD. A recent report from our group in a pre-clinical mouse PAD model showed that interleukin-21 receptor (IL-21R) is up-regulated in the endothelial cells from ischemic hindlimb muscle. We further showed that loss of IL-21R resulted in impaired perfusion recovery in this model. In our study, we sought to determine whether IL-21R is present in the endothelium from ischemic muscle of patients with PAD. Using human gastrocnemius muscle biopsies, we found increased levels of IL-21R in the skeletal muscle endothelial cells of patients with PAD compared to control individuals. Interestingly, PAD patients had approximately 1.7-fold higher levels of circulating IL-21. These data provide direct evidence that the IL-21R pathway is indeed up-regulated in patients with PAD. This pathway may serve as a therapeutic target for modulation.

Evaluation of the clinical relevance and limitations of current pre-clinical models of peripheral artery disease

Peripheral artery disease (PAD) has recognized treatment deficiencies requiring the discovery of novel interventions. This article describes current animal models of PAD and discusses their advantages and disadvantages. There is a need for models which more directly simulate the characteristics of human PAD, such as acute-on-chronic presentation, presence of established risk factors and impairment of physical activity.

The efficacy of extraembryonic stem cells in improving blood flow within animal models of lower limb ischaemia

BACKGROUND

Stem cell (SC) administration is a potential therapeutic strategy to improve blood supply in patients with peripheral artery disease (PAD). The aim of this systematic review and meta-analysis was to investigate the efficacy of extraembryonic tissue-derived SC (ETSC) in improving blood flow within animal models of hindlimb ischaemia (HLI).

METHODS

PubMed, ScienceDirect and Web of Science were searched to identify studies which investigated ETSCs within animal HLI models. A meta-analysis was performed focusing on the effect of ETSCs on limb blood flow assessed by laser Doppler imaging using a random effects model. Methodological quality was assessed using a newly devised quality assessment tool.

RESULTS

Five studies investigating umbilical cord-derived SCs (three studies), placental SCs (one study), amnion and chorionic SCs (one study) were included. A meta-analysis suggested that administration of ETSCs improved the restoration of blood flow within the HLI models used. The methodological quality of the included studies was assessed as poor. Problems identified included lack of randomised design and blinding of outcome assessors; that the animal models did not incorporate recognised risk factors for human PAD or atherosclerosis; the models used did not have established chronic ischaemia as is the cases in most patients presenting with PAD; and the studies lacked a clear rationale for the dosage and frequency of SCs administered.

CONCLUSIONS

The identified studies suggest that ETSCs improve recovery of limb blood supply within current animal HLI models. Improved study quality is, however, needed to provide support for the likelihood of translating these findings to patients with PAD.

Magnetic Resonance Imaging Allows the Evaluation of Tissue Damage and Regeneration in a Mouse Model of Critical Limb Ischemia

Magnetic resonance imaging (MRI) provides non-invasive, repetitive measures in the same individual, allowing the study of a physio-pathological event over time. In this study, we tested the performance of 7 Tesla multi-parametric MRI to monitor the dynamic changes of mouse skeletal muscle injury and regeneration upon acute ischemia induced by femoral artery dissection. T2-mapping (T2 relaxation time), diffusion-tensor imaging (Fractional Anisotropy) and perfusion by Dynamic Contrast-Enhanced MRI (K-trans) were measured and imaging results were correlated with histological morphometric analysis in both Gastrocnemius and Tibialis anterior muscles. We found that tissue damage positively correlated with T2-relaxation time, while myofiber regeneration and capillary density positively correlated with Fractional Anisotropy. Interestingly, K-trans positively correlated with capillary density. Accordingly, repeated MRI measurements between day 1 and day 28 after surgery in ischemic muscles showed that: 1) T2-relaxation time rapidly increased upon ischemia and then gradually declined, returning almost to basal level in the last phases of the regeneration process; 2) Fractional Anisotropy dropped upon ischemic damage induction and then recovered along with muscle regeneration and neoangiogenesis; 3) K-trans reached a minimum upon ischemia, then progressively recovered. Overall, Gastrocnemius and Tibialis anterior muscles displayed similar patterns of MRI parameters dynamic, with more marked responses and less variability in Tibialis anterior. We conclude that MRI provides quantitative information about both tissue damage after ischemia and the subsequent vascular and muscle regeneration, accounting for the differences between subjects and, within the same individual, between different muscles.

Novel method to assess arterial insufficiency in rodent hind limb

BACKGROUND

Lack of techniques to assess maximal blood flow capacity thwarts the use of rodent models of arterial insufficiency to evaluate therapies for intermittent claudication. We evaluated femoral vein outflow (VO) in combination with stimulated muscle contraction as a potential method to assess functional hind limb arterial reserve and therapeutic efficacy in a rodent model of subcritical limb ischemia.

MATERIALS AND METHODS

VO was measured with perivascular flow probes at rest and during stimulated calf muscle contraction in young, healthy rats (Wistar Kyoto, WKY; lean Zucker rats, LZR) and rats with cardiovascular risk factors (spontaneously hypertensive [SHR]; obese Zucker rats [OZR]) with acute and/or chronic femoral arterial occlusion. Therapeutic efficacy was assessed by administration of Ramipril or Losartan to SHR after femoral artery excision.

RESULTS

VO measurement in WKY demonstrated the utility of this method to assess hind limb perfusion at rest and during calf muscle contraction. Although application to diseased models (OZR and SHR) demonstrated normal resting perfusion compared with contralateral limbs, a significant reduction in reserve capacity was uncovered with muscle stimulation. Administration of Ramipril and Losartan demonstrated significant improvement in functional arterial reserve.

CONCLUSIONS

The results demonstrate that this novel method to assess distal limb perfusion in small rodents with subcritical limb ischemia is sufficient to unmask perfusion deficits not apparent at rest, detect impaired compensation in diseased animal models with risk factors, and assess therapeutic efficacy. The approach provides a significant advance in methods to investigate potential mechanisms and novel therapies for subcritical limb ischemia in preclinical rodent models.

TRPA1 mediates amplified sympathetic responsiveness to activation of metabolically sensitive muscle afferents in rats with femoral artery occlusion

Autonomic responses to activation of mechanically and metabolically sensitive muscle afferent nerves during static contraction are augmented in rats with femoral artery occlusion. Moreover, metabolically sensitive transient receptor potential cation channel subfamily A, member 1 (TRPA1) has been reported to contribute to sympathetic nerve activity (SNA) and arterial blood pressure (BP) responses evoked by static muscle contraction. Thus, in the present study, we examined the mechanisms by which afferent nerves' TRPA1 plays a role in regulating amplified sympathetic responsiveness due to a restriction of blood flow directed to the hindlimb muscles. Our data show that 24-72 h of femoral artery occlusion (1) upregulates the protein levels of TRPA1 in dorsal root ganglion (DRG) tissues; (2) selectively increases expression of TRPA1 in DRG neurons supplying metabolically sensitive afferent nerves of C-fiber (group IV); and (3) enhances renal SNA and BP responses to AITC (a TRPA1 agonist) injected into the hindlimb muscles. In addition, our data demonstrate that blocking TRPA1 attenuates SNA and BP responses during muscle contraction to a greater degree in ligated rats than those responses in control rats. In contrast, blocking TRPA1 fails to attenuate SNA and BP responses during passive tendon stretch in both groups. Overall, results of this study indicate that alternations in muscle afferent nerves' TRPA1 likely contribute to enhanced sympathetically mediated autonomic responses via the metabolic component of the muscle reflex under circumstances of chronic muscle ischemia.

Impaired Coronary Endothelial Vasorelaxation in a Preclinical Model of Peripheral Arterial Insufficiency

The present study was designed to determine whether adult swine with peripheral artery insufficiency (PAI) would exhibit vascular dysfunction in vessels distinct from the affected distal limbs, the coronary conduit arteries. Moreover, we sought to evaluate the effect of exercise training on coronary vasomotor function in PAI. Eighteen female healthy young Yucatan miniature swine were randomly assigned to either occluded exercise trained (Occl-Ex, n=7), or occluded-sedentary (Occl-Sed, n=5), or non-occluded, non-exercised control (Non-Occl-Con, n=6) groups. Occl-Ex pigs were progressively trained by running on a treadmill (5days/week, 12 weeks). The left descending artery (LAD) and left circumflex (LCX) coronary arteries were harvested. Vasorelaxation to adenosine diphosphate (ADP), bradykinin (BK), and sodium nitro-prusside (SNP) were assessed in LAD’s; while constrictor responses to phenylephrine (PE), angiotensin II (Ang II), and endothelin-1 (ET-1) were assessed in LCX’s. Vasorelaxation to ADP was reduced in LADs from Occl-Sed and Occl-Ex pigs (P<0.001) as compared to Non-Occl-Con pigs; however, Occl-Ex pigs exhibited partial recovery (P<0.001) intermediate to the other two groups. BK induced relaxation was reduced in LADs from Occl-Ex and Occl-Sed pigs (P<0.001), compared to Non-Occl-Con, and exercise modestly increased responses to BK (P<0.05). In addition, SNP, PE, Ang II, and ET-1 responses were not significantly different among the groups. Our results indicate that ‘simple’ occlusion of the femoral arteries induces vascular dysfunction in conduit vessels distinct from the affected hindlimbs, as evident in blunted coronary vasorelaxation responses to ADP and BK. These findings imply that PAI, even in the absence of frank atherogenic vascular disease, contributes to vascular dysfunction in the coronary arteries that could exacerbate disease outcome in patients with peripheral artery disease. Further, regular daily physical activity partially recovered the deficit observed in the coronary arteries.

A new murine model of sustainable and durable chronic critical limb ischemia fairly mimicking human pathology

OBJECTIVE

To establish a chronic mouse model of critical limb ischemia (CLI) with in vivo and ex vivo validation, closely mimicking human pathology.

METHODS

Swiss mice (n = 28) were submitted to sequential unilateral femoral (day 0) and iliac (day 4) ligatures. Ischemia was confirmed by clinical scores (tissue and functional damages) and methoxyisobutylisonitrile (MIBI) scintigraphies at days 0, 4, 6, 10, 20, and 30. At days 10, 20, and 30, muscle mitochondrial respiration, calcium retention capacity (CRC), and production of reactive oxygen species (ROS) were investigated, together with transcripts of mitochondrial biogenesis and antioxidant enzymes. Histological analysis was also performed.

RESULTS

Clinical and functional damage confirmed CLI. MIBI scintigraphies showed hypoperfusion of the ischemic limb, which remained stable until day 30. Mitochondrial respiration was impaired in ischemic muscles compared with controls (Vmax = 7.93 ± 0.99 vs. 10.09 ± 2.87 mmol/L O2/minute/mg dry weight [dw]; p = .01), together with impaired CRC (7.4 ± 1.6 mmol/L minute/mg dw vs. 11.9 ± 0.9 mmol/L minute/mg dw; p < .001) and biogenesis (41% decrease in peroxisome proliferator-activated receptor gamma coactivator [PGC]-1α, 49% decrease in PGC-1β, and 41% decrease in nuclear respiratory factor-1). Ischemic muscles also demonstrated increased production of ROS under electron paramagnetic resonance (0.084 ± 0.029 vs. 0.051 ± 0.031 mmol/L minute/mg dw; p = .03) and with dihydroethidium staining (3622 ± 604 arbitrary units of fluorescence vs. 1224 ± 324; p < .01), decreased antioxidant enzymes (32% decrease in superoxide dismutase [SOD]1, 41% decrease in SOD2, and 49% decrease in catalase), and myopathic features (wider range in fiber size, rounded shape, centrally located nuclei, and smaller cross-sectional areas). All defects were stable over time.

CONCLUSION

Sequential femoral and iliac ligatures closely mimic human functional, clinical, scintigraphic, and skeletal muscle mitochondrial characteristics, and could prove useful for testing therapeutic approaches.

The exercise pressor reflex and peripheral artery disease

The exercise pressor reflex contributes to increases in cardiovascular and ventilatory function during exercise. These reflexive increases are caused by both mechanical stimulation and metabolic stimulation of group III and IV afferents with endings in contracting skeletal muscle. Patients with peripheral artery disease (PAD) have an augmented exercise pressor reflex. Recently, an animal model of PAD was established which allows further investigation of possible mechanisms involved in this augmented reflex. Earlier studies have identified ASIC3 channels, bradykinin receptors, P2X receptors, endoperoxide receptors, and thromboxane receptors as playing a role in evoking the exercise pressor reflex in healthy rats. This review focuses on recent studies using a rat model of PAD in order to determine possible mechanisms contributing to the exaggerated exercise pressor reflex seen in patients with this disease.

Alteration of the mu opioid receptor: Ca2+ channel signaling pathway in a subset of rat sensory neurons following chronic femoral artery occlusion

The exercise pressor reflex, a crucial component of the cardiovascular response under physiological and pathophysiological states, is activated via metabolic and mechanical mediators that originate from contracting muscles and stimulate group III and IV afferents. We reported previously that stimulation of mu opioid receptors (MOR), expressed in both afferents, led to a significant attenuation of the reflex in rats whose femoral arteries had been occluded for 72 h. The present study examined the effect of arterial occlusion on the signaling components involved in the opioid-mediated modulation of Ca(2+) channels in rat dorsal root ganglion neurons innervating the triceps surae muscles. We focused on neurons that were transfected with cDNA coding for enhanced green fluorescent protein whose expression is driven by the voltage-gated Na(+) channel 1.8 (Na(V)1.8) promoter region, a channel expressed primarily in nociceptive neurons. With the use of a small interference RNA approach, our results show that the pertussis toxin-sensitive Gα(i3) subunit couples MOR with Ca(2+) channels. We observed a significant leftward shift of the MOR agonist [D-Ala2-N-Me-Phe4-Glycol5]-enkephalin concentration-response relationship in neurons isolated from rats with occluded arteries compared with those that were perfused freely. Femoral occlusion did not affect Ca(2+) channel density or the fraction of the main Ca(2+) channel subtype. Furthermore, Western blotting analysis indicated that the leftward shift did not result from either increased Gα(i3) or MOR expression. Finally, all neurons from both groups exhibited an inward current following exposure of the transient potential receptor vanilloid 1 (TRPV1) agonist, 8-methyl-N-vanillyl-6-nonenamide. These findings suggest that sensory neurons mediating the exercise pressor reflex express Na(V)1.8 and TRPV1 channels, and femoral occlusion alters the MOR pharmacological profile.

Nerve Growth Factor, Muscle Afferent Receptors and Autonomic Responsiveness with Femoral Artery Occlusion

The exercise pressor reflex is a neural control mechanism responsible for the cardiovascular responses to exercise. As exercise is initiated, thin fiber muscle afferent nerves are activated by mechanical and metabolic stimuli arising in the contracting muscles. This leads to reflex increases in arterial blood pressure and heart rate primarily through activation of sympathetic nerve activity (SNA). Studies of humans and animals have indicated that the exercise pressor reflex is exaggerated in a number of cardiovascular diseases. For the last several years, a series of studies have employed a rodent model to examine the mechanisms at receptor and cellular levels by which responses of SNA and blood pressure to static exercise are heightened in peripheral artery disease (PAD), one of the most common cardiovascular disorders. Specifically, femoral artery occlusion is used to study intermittent claudication that is observed in human PAD. Our studies have demonstrated that the receptors on thin fiber muscle afferents including transient receptor potential vanilloid type 1 (TRPV1), purinergic P2X3 and acid sensing ion channel subtype 3 (ASIC3) are engaged in augmented autonomic responses this disease. This review will present some of recent results in regard with several receptors in muscle sensory neurons in contribution to augmented autonomic responses in PAD. We will emphasize the role played by nerve growth factor (NGF) in regulating those sensory receptors in the processing of amplified exercise pressor reflex. Also, we will discuss the role played by hypoxia-inducible facor-1α regarding the enhanced autonomic reflex with femoral artery occlusion. The purpose of this review is to focus on a theme namely that PAD accentuates reflexively autonomic responses to exercise and further address regulatory mechanisms leading to abnormal autonomic responsiveness.