Muscle capillary tortuosity in high altitude mice depends on sarcomere length

At given capillary-to-fiber ratio and sarcomere length, capillary tortuosity determines capillary length and the geometry of blood-tissue exchange in muscles. It is therefore an important determinant of the potential efficiency of O2 supply to the muscle fibers. Reports have indicated that capillary tortuosity increases with adaptation to hypoxia. Unfortunately, sarcomere length was not taken into account. Because capillary tortuosity increases substantially as the muscle shortens, it is important to measure sarcomere length when comparing capillary configuration among muscles. We addressed the question of whether or not capillary tortuosity is greater in skeletal muscles of high altitude mice compared to sea-level, when account is taken of sarcomere length. Calf and thigh muscles of high altitude (3800 m; inspired PO2, 91 mm Hg) and sea-level deer mice, Peromyscus maniculatus, were perfusion-fixed in situ at sarcomere lengths ranging from 1.9 to 2.4 microns. Capillary-to-fiber ratio, capillary length per volume of fiber, and the degree of orientation (anisotropy) of capillaries were estimated by morphometry. In both sea-level and high altitude mice, capillaries were relatively straight in extended muscles, and substantially more tortuous in muscles fixed at shorter sarcomere length. There was no systematic difference in the degree of tortuosity of capillaries in the high altitude compared to sea-level mice, when account was taken of sarcomere length. Capillary length per volume of muscle fiber and capillary-to-fiber ratio were not significantly different between the two groups.