Voluntary running induces fiber type-specific angiogenesis in mouse skeletal muscle

Adult skeletal muscle undergoes adaptation in response to endurance exercise, including fast-to-slow fiber type transformation and enhanced angiogenesis. The purpose of this study was to determine the temporal and spatial changes in fiber type composition and capillary density in a mouse model of endurance training. Long-term voluntary running (4 wk) in C57BL/6 mice resulted in an approximately twofold increase in capillary density and capillary-to-fiber ratio in plantaris muscle as measured by indirect immunofluorescence with an antibody against the endothelial cell marker CD31 (466 ± 16 capillaries/mm² and 0.95 ± 0.04 capillaries/fiber in sedentary control mice vs. 909 ± 55 capillaries/mm² and 1.70 ± 0.04 capillaries/fiber in trained mice, respectively; P < 0.001). A significant increase in capillary-to-fiber ratio was present at day 7 with increased concentration of vascular endothelial growth factor (VEGF) in the muscle, before a significant increase in percentage of type IIa myofibers, suggesting that exercise-induced angiogenesis occurs first, followed by fiber type transformation. Further analysis with simultaneous staining of endothelial cells and isoforms of myosin heavy chains (MHCs) showed that the increase in capillary contact manifested transiently in type IIb + IId/x fibers at the time (day 7) of significant increase in total capillary density. These findings suggest that endurance training induces angiogenesis in a subpopulation of type IIb + IId/x fibers before switching to type IIa fibers.