Heterogeneity of microvascular flow in rat skeletal muscle is reduced by contraction and by hemodilution

The objective was to test the hypothesis that heterogeneity of microvascular perfusion in a mammalian muscle will decrease with increased flow. We used the extensor digitorum longus muscle in rats to quantify, via intravital video microscopy, (1) the spatial distribution of red cell velocity, VRBC, and (2) density of capillaries with moving and stationary red cells (CDPER, CDSTAT) within a large capillary bed (1.05 x 0.78 mm) at the muscle surface. From the VRBC distribution, the coefficient of variation (CV = SD/mean) was used as an index of VRBC heterogeneity. The flow was increased by two procedures: (1) 1 min supramaximal muscle contraction, and (2) hematocrit reduction from 49 to 21% by isovolemic hemodilution with a 6% PVP-40 (polyvinylpyrrolidone) solution. Among 5 rats studied, the post-contraction and post-hemodilution mean VRBC's were significantly larger than the resting VRBC (1.08 and 0.17 vs. 0.11 mm/s). The corresponding CV's were significantly lower (33 and 49 vs. 60%). The percentages of capillaries with low velocity (0-0.1 mm/s) were also significantly lower (0 and 27 vs. 52%). CDPER values did not differ from the resting level (30.8 cap/mm of test line) but CDSTAT was significantly smaller after contraction (0.8 vs. 4.9 cap/mm). The present data demonstrate that heterogeneity of VRBC in rat skeletal muscle decreases after contraction and after hemodilution. The study shows that hemodilution has a major effect on improving flow in a population of poorly perfused capillaries. Since heterogeneity also decreased after contraction in an amphibian muscle (Tyml, Microvasc. Res. 32: 84-98, 1986), the study supports the view that heterogeneity, in general, is a microcirculatory parameter that reflects the vascular network response to a given tissue stimulus.