Sympathetic restraint of muscle blood flow at the onset of dynamic exercise

Little attention has focused on sympathetic influences on skeletal muscle blood flow at the onset of exercise. We hypothesized that 1) the sympathetic nervous system constrains muscle blood flow and 2) the decline from peak blood flow is mediated by increasing sympathetic vasoconstrictor tone. Mongrel dogs (n = 7) ran on a treadmill after intra-arterial infusion of saline (control) or combined α₁- and α₂-adrenergic blockade (prazosin and rauwolscine). Immediate and rapid increases in hindlimb blood flow occurred at commencement of exercise with peak iliac blood flows averaging 933 ± 79 and 1,227 ± 90 ml/min during control and blockade conditions, respectively. At 1 min of exercise, hindlimb blood flow had decreased to 629 ± 54 and 1,057 ± 89 ml/min. In the absence of sympathetic vasoconstrictor tone, there was an enhanced peak blood flow at the onset of exercise. In addition, α-blockade attenuated the overshoot of hindlimb blood flow compared with the control condition. These data suggest that an immediate and sustained increase in sympathetic outflow restrains hindlimb blood flow at the onset of exercise and is responsible, at least in part, for an overshoot of blood flow to exercising skeletal muscle.