Introduction Long-term femoral artery ligation in the rodent provides a suitable model of peripheral artery disease (PAD). Dramatic recovery of skeletal muscle function following chronic occlusion of the femoral artery is known to occur in these animal models (1). This process is, in part, due to the enormous adaptive potential of the peripheral vasculature. Adequate tissue perfusion can be restored to the ischemic area by adaptive collateralisation and ischemia-driven angiogenesis. The impact of specific cardiovascular risk factors on this process, including atherosclerosis and age, have not been systemically studied. We have previously shown that the assessment of reactive hyperemia, by exploiting the BOLD response, can provide sensitive techniques for the evaluation of arterial insufficiency. We report here on the use of BOLD imaging to assess the effect of atherosclerosis and senescence on tissue remodeling following peripheral ischemia in a mouse model of human familial hypercholesterolemia (Ldlr-/-Apobec1-/-) (2). Methods Genentech´s AAALAC accredited review board approved all animal experimental procedures. Three groups of mice were included in this study: (1) eight week old C57BL6 mice (n=30); (2) Ldlr-/-Apobec1-/-(n=20); (3) 10 month old C57BL6 mice as age-matched controls. All animals were anesthetized with 1.5% isoflurane in a 70/30 mixture of O2/N2O. Peripheral limb ischemia was produced by ligation of the left femoral artery just distal to the inguinal ligament. MRI was used to measure reactive hyperemia 28 days post ligation. A transient ischemic episode, that resulted in reactive hyperemia, was induced remotely by running a nylon filament beneath the descending aorta and vena cava, extending the line through the bore of the magnet, and attaching weights to the end of the line to produce controlled tension. Complete cessation of blood flow to the lower limbs and subsequent reflow was achieved. All imaging was performed at 4.7T (Unity Inova MR system, Varian Inc., Palo Alto, USA) using a 3cm inner diameter RF coil. Reactive hyperemia was monitored using a gradient echo sequence (TR/TE 30/20ms, FOV 30X30mm, 128X64 matrix, NEX 2). B0 optimization was performed on the slice of interest to obtain a T2 relaxation time of 17ms (approx.). The temporal resolution was 7 seconds. Data acquisition consisted of 3 minutes of pre-occlusion imaging, 3 minutes of transient ischemia, and 6 minutes of reflow. Body temperature was maintained at 370C with warm air. Time to the peak of reactive hyperemia response was used as a quantitative metric. Results The ischemia-reactive hyperemia profile for young animals is shown in Figure 1. A significant attenuation is observed following femoral artery ligation that is partially recovered 7 days post ligation but fully recovered by day 28 such that no difference between the sham operated and the ligated animals was observed (0.9±0.2 min, 1.1±0.3 min; n = 10 per group). Figure 1. Reactive hyperemia in young animals (data normalized to pre-occlusion baseline). Conversely, 28 days post femoral artery ligation, the reactive hyperemia response was attenuated in both the Ldlr-/-Apobec1-/-(0.8±0.3 min, 1.3±0.4 min, p<0.01; n = 10 per group; Figure 2) and the aged C57BL6 mice (1.0±0.3 min, 2.0±0.5 min, p<0.01; data not shown). Figure 2. Reactive hyperemia in Ldlr-/-Apobec1-/-(data normalized to pre-occlusion baseline). Discussion The results from this study show that recovery of adequate vascular reserve following an ischemic injury occurs in young healthy animals but is attenuated by age and the presence of atherosclerosis. The combined techniques of BOLD imaging with reactive hyperemia, as described here, provide a valuable method to assess the effects of arterial insufficiency on skeletal muscle function and may be used to evaluate novel therapeutic strategies. References 1) Challiss RA, et al. Biochem J, 236, 2, 461-7, 1986. 2) Powell-Braxton L, et al. Nat Med, 4, 8, 934-8, 1998.