Capillary configuration in contracted muscles: comparative aspects.

We compared the degree of orientation (anisotropy) of capillaries in skeletal muscles of animals with large differences in oxygen needs and/or tolerance to hypoxia (mammals of different size; reptiles; birds; mammals native to high altitude; diving mammals). In terrestrial mammals, we found a substantial increase in capillary tortuosity with fiber shortening, in muscles with large differences in capillary density (capillary counts/fiber mm2 in transverse sections ranging 450-4350). There was no systematic difference in muscle capillary tortuosity with body size (mouse to pony), or with adaptation to high altitude (deer mice) or to prolonged periods of anoxia (Harbor seals), when account was taken of sarcomere length. A substantial increase in capillary tortuosity was also found in contracted skeletal muscles of the alligator with remarkably low capillary density (capillary counts/fiber mm2 in transverse sections, 120-280). On the contrary, we found that in pigeon pectoralis, a highly aerobic muscle with large capillary density and a large number of capillary anastomoses running perpendicular to the muscle fiber axis, the decrease in capillary anisotropy with decreasing sarcomere length was smaller than in other muscles. Our results indicate that 1) sarcomere length at which samples are fixed needs to be taken into account when capillary counts in transverse sections are compared between muscles and/or after different experimental conditions, and 2) muscle capillary tortuosity is a consequence of fiber shortening, rather than an indicator of the O2 requirements of the tissue.