Changes in capillary perfusion induced by different patterns of activity in rat skeletal muscle.

The proportion of "perfused" capillaries was evaluated in rat tibialis anterior at rest and during two different types of contraction after timed injection of thioflavine S. Capillary/fibre (C/F) ratio was estimated for "perfused" capillaries--those filled with fluorochrome--(Cp) from photomicrographs. Sections were subsequently stained for alkaline phosphatase and C/F ratio was estimated for all capillaries (Ct). At 7.5 sec after injection of fluorochrome, Cp:Ct at rest was 0.32 +/- 0.092 in the oxidative core and 0.43 +/- 0.058 in the glycolytic cortex (means +/- SEM). This increased to 0.83 +/- 0.045 and 0.88 +/- 0.026, respectively, during selective activation of glycolytic fibres. Activation of all fibres led to a modest further increase (0.92 +/- 0.040 in the core and 0.91 +/- 0.035 in the cortex). Blood flow (measured by radiolabelled microspheres increased to a similar extent (fivefold) in both regions of the muscle during activation of glycolytic fibres; the further increase during maximal activation was much smaller in the cortex (from 4 to 41 ml/100 g/min) than in the core (from 7 to 196 ml/100 g/min). Increased capillary perfusion during muscle contractions was thus independent of the type of activity, while muscle blood flow increased more in oxidative than in glycolytic regions during maximal activation. Thus the increase in muscle blood flow with maximal activation cannot be accounted for by further recruitment of "unperfused" capillaries and must result from a significant increase in the velocity of capillary blood flow.