Blood flow distribution and its temporal variability in stimulated dog gastrocnemius muscle

The distribution of blood flow in skeletal muscle stimulated to rhythmic isotonic contractions was studied by injections of radioactive micropheres into the arterial supply in 8 gastrocnemius muscles (mean weight (84 g) of 6 anesthetized dogs (20-25 kg body weight). The distribution of 10 μm microspheres in regions of about 0.5 g was very similar to that of the standard 15 μm microspheres, whereas that of 25 μm microspheres was more uneven. The coefficient of variation (CV = SD/mean) of the ratio of simultaneously injected 10 μm and 15 μm microspheres, 0.12, was taken as the inherent scatter of the method. The average spatial distribution inequality of 10-15 μ microspheres corresponded to a CV of 0.45 and the specific local blood flow inhomogeneity to a CV=0.43 (= 0.45²-0.12²), but there were marked differences between muscles. At equal blood flow levels, the inhomogeneity during reactive hyperemia was similar to that observed during stimulation. The temporal variability of blood flow in individual muscle pieces was obtained from th comparison of fractional trapping of 4 to 5 differently labeled microspheres injected at intervals of 2 min into steadily stimulated muscles. The mean CV for the variations in time was 0.23 and that corrected for methodological scatter, 0.19, but the differences in the extent of temporal blood flow changes among muscle pieces within a muscle and between different muscles were large. The presence of considerable spatial and temporal variations of blood flow in exercising muscle during apparent steady state may be important in limiting and/or modulating tissue O₂ supply.