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During exercise, an increase in blood flow to working skeletal muscle is accomplished by dilation of arteries and arterioles supplying the muscle. Arterioles, located within contracting muscle, are exposed to dilatory metabolites released by the muscle; however, the mechanism by which feed arteries, located external to the muscle, dilate is still unknown.

One potential mechanism for feed artery dilation is flow-induced dilation, occurring when arteries dilate in response to increased vascular wall shear stress. Shear stress is the frictional force between blood and the arterial wall, which increases when blood flow velocity increases. Data from previous in vitro experiments (8) indicate that flow-induced dilation in rat soleus feed arteries occurs at blood flow levels that are far less than normal resting blood flow in conscious rats. This data led to the conclusion that flow-induced dilation was not a plausible mechanism to explain the increase in blood flow during exercise.

Furthermore, the soleus muscle is primarily composed of slow-oxidative fibers and used in maintaining posture; thus, it receives a substantial amount of blood flow at rest. We sought to test whether flow-induced dilation could contribute to exercise hyperemia in rat extensor digitorum longus muscle, primarily composed of fast-glycolytic fibers, and rat gastrocnemius, a muscle of mixed fiber type (4). The differences in fiber type of each muscle may be a factor in how the feed arteries dilate during exercise.

The purpose of this study was to determine if flow-induced dilation potentially contributes to exercise hyperemia in rat extensor digitorum longus and gastrocnemius muscle feed arteries, EDLFA and GFA, respectively. In this study, blood flow was induced through the arteries and corresponding flow measurements (µl/min) were collected. The flow values were used to calculate intraluminal wall shear stress in the arteries and then compared to calculated in vivo shear stress values from previously published studies (1,2,3,7,10,11,12,13,14,15). We hypothesized that flow-induced dilation in GFA and EDLFA occurs at shear stress values lower than the shear stress normally present in non-exercising rats. This would preclude flow-induced dilation from causing the dilation of feed arteries to gastrocnemius and EDL muscles in exercise.

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