Fiber capillary supply related to fiber size and oxidative capacity in human and rat skeletal muscle

The capillary supply of a muscle fiber is thought to be determined by its type, oxidative capacity, size and metabolic surrounding. Size and oxidative capacity, however, differ between fiber types. To investigate which of these factors determines the capillary supply of a myofiber most we analysed in sections from human vastus lateralis (n = 11) and rat plantaris muscle (n = 8) the type, succinate dehydrogenase activity (SDH), reflecting oxidative capacity, and capillary supply of individual fibers. Capillary fiber density differed between fiber types in rat (P < 0.03) but not in human muscle. In human muscle only, the local capillary to fiber ratio (LCFR) correlated with the integrated SDH (fiber cross-sectional area x SDH) of a fiber (R = 0.62; P < 0.001). Backward multiple regression revealed, however, that the LCFR was primarily determined by fiber size, type (R = 0.71, human) and surrounding of the fiber (R = 0.62; rat plantaris muscle), i.e. whether it came from the deep or superficial region of the muscle (all P < 0.001) and not SDH. In conclusion, size, type and metabolic surrounding rather than mitochondrial activity determine the capillary supply to a muscle fiber.