Capillary recruitment and heterogeneity of microvascular flow in skeletal muscle before and after contraction

The two objectives of this study were to evaluate the spatial distribution of velocities measured simultaneously in a population of capillaries at the surface of a sartorius muscle in anesthetized frogs, and to estimate the cross-sectional density of capillaries perfused with red cells in this muscle at rest and after supramaximal stimulation. In each of 10 muscles studied, the mean velocity associated with this distribution increased significantly after the stimulation (overall increase from 0.12 mm/sec in control to 0.46 mm/sec at the peak of hyperemia). The coefficient of variation (i.e., ratio of standard deviation to mean velocity), however, decreased, indicating that the individual velocities became relatively more homogeneous and that microvascular adjustments occurred at the capillary level. Additionally, the proportion of capillaries with zero or very low velocities (0-0.1 mm/sec) was reduced practically in all muscles suggesting an improved oxygen exchange after every stimulation. In contrast to these consistent velocity responses in all muscles, capillary recruitment was detected only in 5 out of 10 muscles (overall density increase from 104 to 134 cap/mm2). This partial occurrence of recruitment (range, 23-149%) was associated with control mean velocities less than 0.1 mm/sec. Since, in light of the velocity data, the absence of recruitment in the 5 remaining muscles did not necessarily signify the lack of microvascular adjustments at the capillary level, it is proposed that more sensitive indexes, such as the reduction in the proportion of low flow capillaries and the increase in relative homogeneity, be used in addition to or instead of recruitment, to describe more completely the microvascular response to stimulation.