Capillary Density of Skeletal Muscle in Dogs Exposed to Simulated Altitude 1

A number of anatomical and physiological changes have been reported in man and in animals living in chronic hypoxia. It is generally believed that these changes are adaptative in nature, and they help the animal tolerate a low Po₂ environment. Possibly these features or mechanisms are optimally developed in man and in animals native to high altitude. However, the study of the acute situation, when a lowland animal is exposed to a low oxygen environment, offers the opportunity to ascertain how fast these changes occur, and, on occasion, provides insight into the underlying mechanisms.

Valdivia (1) has found that high altitude guinea pigs (at 4540 m) have a greater number of open capillaries in skeletal muscle than sea level controls. He regarded this as an important adaptative mechanism to chronic hypoxia even though muscle fiber size was approximately the same in all animals. More recently Cassin et al. (2) showed that the number of capillaries per unit surface area in skeletal muscle of rats exposed to 6,100 m for 5 wk increased significantly. However, the evidence of an increased capillarity in hypoxic conditions and its role in facilitating O₂ delivery is not conclusive. Furthermore, there is no experimental evidence that exposure of larger mammals to hypoxia results in structural alterations of peripheral tissues.

Materials and Methods. Three adult mongrel dogs (two males and one female) native to Denver (1610 m PB 635 mmHg) were studied before and after a 3-wk exposure to a simulated altitude of about 4880 m (ambient pressure 435 mmHg) during which time they received food and water ad libitum. Because the dogs were trained to run on a treadmill, the sternothyroideous muscle was chosen for study in order to minimize the possible effects of exercise on the skeletal muscle.