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

Objective: We recently provided evidence for contraction-induced attenuatio n of reflex sympathetic vasoconstriction in human skeletal muscle microcirc ulation. We now asked whether contraction-induced modulation of alpha-adren oceptor 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 ques tions in humans, we used phase-contrast magnetic resonance imaging to measu re blood flow velocity and cross-sectional area of the brachial artery duri ng brachial-artery infusion of the alpha-adrenoceptor agonist norepinephrin e (NE) (1.1 g/min for 5 min) at rest and during mild ipsilateral rhythmic h andgrip (20% of maximum). At rest, brachial artery conductance decreased pr ogressively during the entire 5 min period of infusion (baseline to first h alf 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, conductanc e 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 no t 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). Conc lusion: These data provide new evidence in humans that alpha-adrenoceptor m ediated vasoconstriction is sensitive to modulation by skeletal muscle cont raction. Such modulation is evident at the level of a large conduit artery and it involves a post-junctional mechanism of action. (C) 1999 Elsevier Sc ience B.V. All rights reserved.