alpha-adrenergic vasoconstriction is active skeletal muscles during dynamic exercise

Sympathetic vasoconstriction in working muscles during dynamic exercise has been demonstrated by intra-arterial administration of alpha(1)-adrenergic antagonists. The purpose of this study was to examine the existence of alan d alpha(2)-adrenergic receptor-mediated vasoconstriction in active skeletal muscles during exercise. Six mongrel dogs were instrumented chronically wi th flow probes on the external iliac arteries of both hindlimbs, and a cath eter was inserted in one femoral artery. All dogs ran on a motorized treadm ill at three exercise intensities (3 miles/h, 6 miles/h, and 6 miles/h at 1 0% grade) on separate days. After 5 min of exercise, a selective alpha(1)- (prazosin) or a selective alpha(2)-adrenergic antagonist (rauwolscine) was infused as a bolus into the femoral arterial catheter (only one drug per da y). The doses of the antagonists were adjusted to maintain the same effecti ve concentration at each exercise intensity. At the mad, moderate, and heav y workloads prazosin infusion produced immediate increases in iliac conduct ance of 65 +/- 9, 35 +/- 6, and 18 +/- 4% (means +/- SE), respectively, and increases in blood flow of 290 +/- 24, 216 +/- 23, and 172 +/- 18 ml/min, respectively. Rauwolscine infusion produced increases in conductance of 52 +/- 5%, 36 +/- 5%, and 26 +/- 3%, respectively, and blood flow increases of 250 +/- 34, 244 +/- 39, and 259 +/- 35 ml/min at the three workloads. Syst emic blood pressure and blood flow in the contralateral iliac artery were u naffected by any of the antagonist infusions. These results demonstrate tha t there is ongoing alpha(1)- and alpha(2)-adrenergic receptor-mediated vaso constriction in exercising skeletal muscles even at heavy workloads and tha t the magnitude of vasoconstriction decreases as exercise intensity increas es.