a-Adrenergic vasoconstriction in active skeletal muscles during dynamic exercise

Buckwalter, John B., and Philip S. Clifford. a-Adrenergic vasoconstriction in active skeletal muscles during dynamic exercise. Am. J. Physiol. 277 (Heart Circ. Physiol. 46): H33–H39, 1999.—Sympathetic vasoconstriction in working muscles during dynamic exercise has been demonstrated by intra-arterial administration of a1-adrenergic antagonists. The purpose of this study was to examine the existence of a1and a2-adrenergic receptor-mediated vasoconstriction in active skeletal muscles during exercise. Six mongrel dogs were instrumented chronically with flow probes on the external iliac arteries of both hindlimbs, and a catheter was inserted in one femoral artery. All dogs ran on a motorized treadmill at three exercise intensities (3 miles/h, 6 miles/h, and 6 miles/h at 10% grade) on separate days. After 5 min of exercise, a selective a1(prazosin) or a selective a2-adrenergic antagonist (rauwolscine) was infused as a bolus into the femoral arterial catheter (only one drug per day). The doses of the antagonists were adjusted to maintain the same effective concentration at each exercise intensity. At the mild, moderate, and heavy workloads prazosin infusion produced immediate increases in iliac conductance of 65 6 9, 35 6 6, and 18 6 4% (means 6 SE), respectively, and increases in blood flow of 290 6 24, 216 6 23, and 172 6 18 ml/min, respectively. Rauwolscine infusion produced increases in conductance of 52 6 5%, 36 6 5%, and 26 6 3%, respectively, and blood flow increases of 250 6 34, 244 6 39, and 259 6 35 ml/min at the three workloads. Systemic blood pressure and blood flow in the contralateral iliac artery were unaffected by any of the antagonist infusions. These results demonstrate that there is ongoing a1and a2-adrenergic receptor-mediated vasoconstriction in exercising skeletal muscles even at heavy workloads and that the magnitude of vasoconstriction decreases as exercise intensity increases.