title: Exercise attenuates alpha-adrenergic-receptor responsiveness in skeletal muscle vasculature.
abstract: Attenuation of sympathetic vasoconstriction (sympatholysis) in working muscles during dynamic exercise is controversial. A potential mechanism is a reduction in alpha-adrenergic-receptor responsiveness. The purpose of this study was to examine alpha(1)- and alpha(2)-adrenergic-receptor-mediated vasoconstriction in resting and exercising skeletal muscle using intra-arterial infusions of selective agonists. Thirteen mongrel dogs were instrumented chronically with flow probes on the external iliac arteries of both hindlimbs and a catheter in one femoral artery. The selective alpha(1)-adrenergic agonist (phenylephrine) or the selective alpha(2)-adrenergic agonist (clonidine) was infused as a bolus into the femoral artery catheter at rest and during mild and heavy exercise. Intra-arterial infusions of phenylephrine elicited reductions in vascular conductance of 76 +/- 4, 71 +/- 5, and 31 +/- 2% at rest, 3 miles/h, and 6 miles/h and 10% grade, respectively. Intra-arterial clonidine reduced vascular conductance by 81 +/- 5, 49 +/- 4, and 14 +/- 2%, respectively. The response to intra-arterial infusion of clonidine was unaffected by surgical sympathetic denervation. Agonist infusion did not affect either systemic blood pressure, heart rate, or blood flow in the contralateral iliac artery. alpha(1)-Adrenergic-receptor responsiveness was attenuated during heavy exercise. In contrast, alpha(2)-adrenergic-receptor responsiveness was attenuated even at a mild exercise intensity. These results suggest that the mechanism of exercise sympatholysis may involve reductions in postsynaptic alpha-adrenergic-receptor responsiveness.
authors.affiliation: Departments of Anesthesiology and Physiology, Medical College of Wisconsin and Veterans Affairs Medical Center, Milwaukee, Wisconsin 53295, USA. jbuckwal@mcw.edu
authors.forename: J B
authors.initials: JB
authors.lastname: Buckwalter
authors.forename: J S
authors.initials: JS
authors.lastname: Naik
authors.forename: Z
authors.initials: Z
authors.lastname: Valic
authors.forename: P S
authors.initials: PS
authors.lastname: Clifford
mesh_terms: D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000818:Animals; D003000:Clonidine / Q000494:pharmacology; D004285:Dogs; D005263:Femoral Artery / Q000187:drug effects / Q000502:physiology*; D006439:Hemodynamics / Q000502:physiology; D006614:Hindlimb; D007083:Iliac Artery / Q000187:drug effects / Q000502:physiology*; D007269:Injections, Intra-Arterial; D009043:Motor Activity / Q000502:physiology*; D018482:Muscle, Skeletal / Q000098:blood supply* / Q000294:innervation; D010656:Phenylephrine / Q000494:pharmacology; D011942:Receptors, Adrenergic, alpha / Q000502:physiology*; D012039:Regional Blood Flow / Q000187:drug effects; D013562:Sympathectomy
chemical_list: D000316:Adrenergic alpha-Agonists; D011942:Receptors, Adrenergic, alpha; D010656:Phenylephrine; D003000:Clonidine