The skeletal muscle vascular supply closely correlates with the muscle fiber surface area in the rat.

The skeletal muscle capillary supply (capillarity) dynamically changes in response to muscle conditions such as growth, atrophy, and hypertrophy. The capillary number-to-fiber ratio is reported to correlate closely with the muscle fiber cross sectional area. However, little information is available regarding the capillarity of neonatal and very young skeletal muscles. In this study, the vascular endothelium was reliably stained with an anti-PECAM-1 antibody, and relationships between the capillarity and muscle fiber parameters were analyzed. For assessment of the capillarity, we used the capillary length-to-fiber ratio, due to the presence of transversely running vessels. In young and adult rats, the capillary length-to-fiber ratio was proportional to both the muscle fiber cross sectional area and muscle fiber radius. However, when these data were analyzed together with data from neonatal and very young rats, the capillary length-to-fiber ratio correlated more closely with the muscle fiber radius than the muscle fiber cross sectional area in the tibialis anterior muscle. The capillary number-to-fiber ratio demonstrated results very similar to the capillary length-to-fiber ratio. During muscle atrophy after denervation, the number of capillaries was decreased in a non-apoptotic manner as revealed by electron microscopy, maintaining the close relationship between the parameters described above. In conclusion, capillarity was closely correlated with the muscle fiber radius (which represents the perimeter) during growth and atrophy. This indicates that the capillarity is linked to the muscle fiber surface area (which is determined by perimeter and section thickness), in agreement with the essential role of the cell membrane in the transport of materials by simple diffusion or active transport.