Reduced vascular responsiveness to α₂-adrenergic agonist during dynamic exercise

The existence of reduced responsiveness of the skeletal muscle arterial vasculature to sympathetic agonists during exercise (sympatholysis) is controversial. The purpose of this study was to examine α₂-adrenergic mediated vasoconstriction in the skeletal muscle vasculature during exercise. Five mongrel dogs (20-24 kg) were instrumented chronically with transit time ultrasound flowprobes on each external iliac artery. A catheter was also implanted chronically for drug infusion into one femoral artery. An intraarterial bolus of 5 μg clonidine (selective α₂ agonist) was given at rest and after 5 min of exercise at a mild intensity (3 mph/0% grade) and a heavy intensity (6 mph/10% grade). During exercise the dose of clonidine was increased in proportion to the increase in iliac blood flow. At rest there was a reduction of 80±6% (mean±sem) in iliac vascular conductance in the limb of infusion. During exercise at 3 mph/0% grade and 6 mph/10% grade there were 50±4% and 14±3% reductions in iliac conductance, respectively. Blood flow in the contralateral iliac artery, systemic blood pressure, and heart rate were unaffected by drug infusion. These results support the existence of α₂-adrenergic mediated sympatholysis during exercise. Moreover, this reduction in responsiveness to an α₂ agonist is intensity-related.