Reduced vascular responsiveness to α₁ - Adrenergic agonist during dynamic exercise

The existence of reduced responsiveness of the skeletal muscle vasculature to sympathetic agonists during exercise (sympatholysis) is controversial. The purpose of this study was to examine α₁-adrenergic mediated vasoconstriction to the skeletal muscle vasculature during exercise. Six mongrel dogs (16-24 kg) were instrumented chronically with a transit time ultrasound flow probe on each external iliac artery. A catheter was also implanted chronically for drug infusion into one femoral artery. All dogs ran on a motorized treadmill at 2 intensities (3 mph & 6 mph). An intraarterial bolus of 25 μg phenylephrine (PE, selective α₁ agonist) was given at rest; at 5, 10, and 15 min of exercise; and post-exercise at 5 and 10 min. At rest there was a 93±5% (mean±sem) reduction in iliac vascular conductance. During exercise at 3 mph there were 68±3%, 68±5%, and 66±4% reductions in iliac conductance at 5, 10, and 15 min of exercise, respectively. PE at 6 mph reduced iliac conductance 51±7%, 47±5%, and 44±5% at the same time points. Postexercise injections of PE at 5 and 10 min were similar to those at rest (3 mph=91±7% and 89±7%, 6 mph=89±10% and 88±14%). Blood flow in the contralateral iliac artery, systemic blood pressure and heart rate were unaffected by drug infusion. These results support the existence of sympatholysis during, but not following exercise, and demonstrate an intensity-dependent reduction in responsiveness to an α₁ agonist during submaximal exercise in dogs.