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Cardiovascular Research 1999 41(1):220-228; doi:10.1016/S0008-6363(98)00226-0
© 1999 by European Society of Cardiology
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Copyright © 1999, European Society of Cardiology

Exercise-induced attenuation of alpha-adrenoceptor mediated vasoconstriction in humans: evidence from phase-contrast MRI

Jim Hansena,*, Dany Sayadb, Gail D. Thomasb, Geoffrey D. Clarkeb, Ronald M. Peshockb and Ronald G. Victorb

aCopenhagen Muscle Research Center, Rigshospitalet, 20 Tagensvej, DK-2200 Copenhagen, Denmark
bDivision of Cardiology, UT Southwestern Medical Center, 5323 Harry Hines Blv, Dallas, TX 75235, USA

* Corresponding author. Tel.: +45-35-457-610; fax: +45-35-457-634; e-mail: j.hansen@post2.tele.dk

Objective: We recently provided evidence for contraction-induced attenuation of reflex sympathetic vasoconstriction in human skeletal muscle microcirculation. We now asked whether contraction-induced modulation of alpha-adrenoceptor mediated vasoconstriction in the human forearm (a) is evident in a large artery supplying the contracting skeletal muscle and (b) implicates a post-junctional site of action. Methods and Results: To address these questions in humans, we used phase-contrast magnetic resonance imaging to measure blood flow velocity and cross-sectional area of the brachial artery during brachial-artery infusion of the alpha-adrenoceptor agonist norepinephrine (NE) (1.1 g/min for 5 min) at rest and during mild ipsilateral rhythmic handgrip (20% of maximum). At rest, brachial artery conductance decreased progressively during the entire 5 min period of infusion (baseline to first half to second half of infusion: 0.421±0.157 to 0.255±0.187 to 0.012±0.014 ml/min/mmHg, P<0.05). When NE was superimposed on handgrip, conductance at first decreased sharply (1.205±0.127 to 0.330±0.097 ml/min/mmHg, P<0.05). However, during the second half of the infusion, conductance did not decrease further but rather returned progressively toward baseline (0.476±0.199 ml/min/mmHg at the end of the exercise, P<0.05 vs. NE alone). Conclusion: These data provide new evidence in humans that alpha-adrenoceptor mediated vasoconstriction is sensitive to modulation by skeletal muscle contraction. Such modulation is evident at the level of a large conduit artery and it involves a post-junctional mechanism of action.

KEYWORDS Regional blood flow; Vasoconstriction/dilation; Adrenoceptor agonists; NMR; Exercise


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