Capillary development during exposure to chronic hypoxia.

CASSIN, S.,R. D. GILBERT, C. E. BUNNELL, AND E. M. JOHNSON. Capillary development during exposure to chronic hypoxia. Am. J. Physiol, 220(Z) : 448-451. 1971.-Thirty male rats were exposed to an altitude of 6,150 m for 36 days in a decompression chamber while 26 similar animals were maintained at sea level for the same time. Capillary counts in gracilis and plantaris muscles in some experimental and control animals were compared by utilizing a specific stain for endothelial cells. A significant increase in muscle capillary counts was found in experimental animals if the counts were expressed per unit area. However, the number of muscle fibers per square millimeter of tissue also increased. Because there were probably no new muscle fibers in these animals, the relative increase in fibers per unit area is thought to be due to the smaller size of the muscle fibers in the experimental animals. When the counts were compared in experimental and control animals on the basis of capillaries per muscle fiber, a difference between the two groups was not observed. On the basis of these data it appears to us that exposure to hypoxia results in an opening of preexisting capillaries rather than an increase in vascularity. both groups were housed at sea level in a room kept between 22 and 23 C for the duration of the experiment. The experimental animals for both groups were exposed to simulated altitude at the same ambient temperature in a low-pressure chamber. The altitude animals were taken from sea level to 6,150 m (307 m/ min) in 4 days at daily increments in altitude of 1,538 m. They were maintained at 6,150 m for 36 days, and were then returned to sea level. Animals were, however, returned to sea level for approximately 1 hr daily to permit us to clean their cages and replenish their food and water. Both groups of animals received tap water and Purina Laboratory Chow ad libitum.