Sequential perfusion of skeletal muscle capillaries

Rats were injected intraarterially with a fluorescent dye that binds to capillary endothelium, thereby labeling any capillary through which it has passed. After 10, 15, or 30 sec of circulation of the dye blood flow was interrupted, the gastrocnemius was frozen, and the density and distribution of labeled capillaries were measured in transverse sections of the central portion of the medial head. These tissue sections were then counterstained by the myosin ATPase method for capillaries to mark all capillaries. After 10 sec, 45% of all capillaries were labeled and after 15 sec, 59% of all capillaries were labeled. Thirty seconds after injection, all capillaries were labeled with the fluorescent dye. In all three time intervals, the distributions of labeled capillaries were ordered, suggesting that there is a tissue-level control mechanism for regulating capillary perfusion to maintain relatively short maximal oxygen diffusion distances.