Potential blood supply to muscles in horses and dogs and its relation to athletic ability.

A modification of the histochemical reaction to demonstrate myosin adenosine triphosphatase activity in skeletal (striated) muscle was used to define the density of capillaries in transverse sections of 3 muscles (semitendinosus, diaphragm, and pectoralis transversus) of horses and dogs. Thoroughbred horses and Greyhound dogs, breeds noted for their speed in running, were compared with other members of their respective species. Thoroughbred cross horses were grouped with non-Thoroughbreds for comparison with Thoroughbreds. The area of muscle supplied by a capillary was remarkably similar in muscles of all groups of animals, except at early stages of growth. Adult Thoroughbreds had a greater capillary/fiber ratio in semitendinosus and pectoralis transversus than did other adult horses. Intermuscular differences in capillary/fiber ratio and the area per capillary within adults and young of each type of horse were negligible with one exception: the capillary/fiber ratio was less in the pectoralis transversus than in the semitendinosus of young non-Thoroughbreds. There was little between-type difference in capillary density in adult dog muscles, although the capillary/fiber ratio was greater in the semitendinosus of Greyhounds. Although the capillary/fiber ratio increases with growth, the area per capillary did not alter markedly. Cessation of training increased the area per capillary in the pectoralis transversus of Greyhounds, In all adult dogs, capillary/fiber ratios in the diaphragm were less than they were in the semitendinosus and the pectoralis transversus. In adult Greyhounds, this was compensated by a smaller area per capillary in the diaphragm. Seemingly, the ability for high speed running by the Thoroughbred and Greyhound was not associated with an enhanced potential blood supply to skeletal muscle.