Remodeling in the microcirculation of rat skeletal muscle during chronic ischemia

Objective: To establish the time course and extent of remodeling of terminal microcirculation in ischemic rat skeletal muscle during prolonged low flow that does not lead to inflammation. Methods: One common iliac artery was ligated via laparotomy in adult Sprague-Dawley rats and extensor digitorum longus (EDL) muscles removed at intervals (1, 2, and 5 weeks) postsurgery. Serial frozen EDL sections were stained to show capillaries (alkaline phosphatase), cell proliferation (antibody to proliferating cell nuclear antigen [PCNA]), terminal microvessels (antibodies to a-smooth muscle actin (alpha-SMA) or endothelial nitric oxide synthase [eNOS]), and macrophages (antibodies to infiltrating and resident macrophages). Total muscle eNOS protein was quantified by standard Western blotting techniques. Results: Capillary proliferation was very limited in ischemic EDLs, with a modest 12% increase in the capillary/fiber ratio after 5 weeks, preceded at 2 weeks by increased numbers of PCNA-positive nuclei at capillary sites. There was no muscle necrosis or evidence of inflammation, based on macrophage staining. The number of terminal microvessels that were positive for alpha-SMA and <10 μm in diameter was fewer in ischemic EDLs at all time points, whereas the number of larger positive vessels was unchanged. eNOS-positive vessels <10 mum in diameter were stained similarly throughout ischemic muscles as the controls, and showed a similar increase in vessel/fiber ratio as the capillaries. The total eNOS protein level was similar to that in controls in ischemic EDLs after 1 and 2 weeks, but was 28% lower after 5 weeks. Conclusions: Prolonged, moderate flow reduction to skeletal muscles does not necessarily lead to inflammation or extensive capillary growth. Based on eNOS staining, the terminal microcirculation remains intact, but the loss of alpha-SMA immunoreactivity may indicate remodeling involving the "deinvestment" of microvessels by smooth muscle.