The effect of long-term activity on the microvasculature of rat glycolytic skeletal muscle.

Long-term electrical stimulation of a fast skeletal muscle (rat tibialis anterior, TA), at 10 Hz, 8 hours/day for 7 days, caused an increase in capillarity, determined both as capillary density/mm2 and as capillary to muscle fibre ratio (C/F), and an increase in the weight of corrosion cast of the entire vascular bed. Chronic muscle activity caused a greater linear velocity of RBCs (Vrbc) in the capillaries supplying predominantly glycolytic fibres of the resting TA. During contractions imposed acutely, the absolute value attained was greater than in control TA, but the increase over resting Vrbc was not as great. Factors which did not change with long-term electrical stimulation were the individual capillary dimensions and the intermittency of flow; those remained similar to control values. It was calculated from capillary dimensions and density, and from the increase in corrosion cast/muscle wet weight ratios, that capillary neoformation alone could account for the growth of the microvasculature in the predominantly glycolytic surface layers of TA, but not necessarily in the oxidative core or in other mixed skeletal muscles.