title: Alpha-Adrenergic receptor responsiveness is preserved during prolonged exercise.
abstract: Our laboratory has previously reported a decline in sympathetic nervous system restraint of skeletal muscle blood flow during prolonged mild-intensity exercise. This decline may be explained by a decrease in alpha(1)- and alpha(2)-adrenergic receptor responsiveness over time. Thus the purpose of the present study was to investigate the effect of exercise duration on alpha(1)- and alpha(2)-adrenergic receptor responsiveness during prolonged constant-load exercise. Mongrel dogs (n = 6) were instrumented chronically with transit-time flow probes on the external iliac arteries and an indwelling catheter in a branch of the femoral artery. On separate days, flow-adjusted doses of selective alpha(1)- (phenylephrine) alpha(2)-adrenergic-receptor (clonidine) agonists, and tyramine (to evoke endogenous norepinephrine release) were infused following 5, 30 and 50 min of mild-intensity treadmill exercise (3 miles/h), with hindlimb blood flow (HBF) and mean arterial pressure (MAP) monitored continuously. Vascular conductance (VC) was calculated as HBF/MAP. While the dogs ran on the treadmill at 3 miles/h, infusion of phenylephrine resulted in similar decreases in VC after 5 [73% (SD 10)], 30 [76% (SD 9)], and 50 [73% (SD 10)] min of exercise. Infusion of the alpha(2)-agonist clonidine also produced similar decreases in VC after 5 [58% (SD 10)], 30 [58% (SD 11)], and 50 [53% (SD 12)] min of exercise. Infusion of tyramine resulted in similar decreases in VC after 5 [55% (SD 15)], 30 [51% (SD 10)], and 50 [50% (SD 7)] min of exercise. These results demonstrate that alpha(1)- and alpha(2)-adrenergic receptor responsiveness to infusion of selective alpha(1)- and alpha(2)-adrenergic-receptor agonists and endogenous norepinephrine release (tyramine) does not decline during prolonged mild-intensity exercise. Thus a decrease in alpha-adrenergic receptor responsiveness over time does not appear to be responsible for the decrease in sympathetic restraint of muscle blood flow during prolonged exercise.
authors.affiliation: Anesthesia Research 151, VA Medical Center, Milwaukee, WI 53295, USA.
authors.forename: Darren S
authors.initials: DS
authors.lastname: DeLorey
authors.forename: Jason J
authors.initials: JJ
authors.lastname: Hamann
authors.forename: Zoran
authors.initials: Z
authors.lastname: Valic
authors.forename: Heidi A
authors.initials: HA
authors.lastname: Kluess
authors.forename: Philip S
authors.initials: PS
authors.lastname: Clifford
authors.forename: John B
authors.initials: JB
authors.lastname: Buckwalter
mesh_terms: D000222:Adaptation, Physiological / Q000502:physiology; D000818:Animals; D001783:Blood Flow Velocity / Q000502:physiology*; D004285:Dogs; D005080:Exercise Test; D018482:Muscle, Skeletal / Q000098:blood supply* / Q000294:innervation / Q000502:physiology*; D010807:Physical Endurance / Q000502:physiology*; D005082:Physical Exertion / Q000502:physiology*; D011942:Receptors, Adrenergic, alpha / Q000378:metabolism*; D013564:Sympathetic Nervous System / Q000502:physiology*
chemical_list: D011942:Receptors, Adrenergic, alpha