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Am J Physiol Heart Circ Physiol 292: H392-H398, 2007. First published September 1, 2006; doi:10.1152/ajpheart.00787.2006
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{alpha}-Adrenergic receptor responsiveness is preserved during prolonged exercise

Darren S. DeLorey, Jason J. Hamann, Zoran Valic, Heidi A. Kluess, Philip S. Clifford, and John B. Buckwalter

Departments of Anesthesiology and Physiology, Medical College of Wisconsin and Veterans Affairs Medical Center, Milwaukee, Wisconsin

Submitted 23 July 2006 ; accepted in final form 29 August 2006

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.

vascular conductance; sympathetic nervous system; skeletal muscle; blood flow; functional sympatholysis


Address for reprint requests and other correspondence: J. B. Buckwalter, Anesthesia Research 151, VA Medical Center, Milwaukee, WI 53295 (e-mail: jbuckwal{at}mcw.edu)






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