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Received February 16, 2007
Revised March 15, 2007
Accepted after revision April 24, 2007
-Adrenergic Vasoconstrictor Responsiveness in Healthy Men
1 Colorado State University
2 Heart Center of the Rockies
* To whom correspondence should be addressed. E-mail: fdinenno{at}cahs.colostate.edu.
Muscle sympathetic vasoconstrictor nerve activity increases with advancing age, but does not result in elevated forearm vasoconstrictor tone due to a selective reduction in 
1-adrenoceptor responsiveness. In contrast, the leg circulation of older adults is under greater tonic sympathetic vasoconstriction, but it is unclear whether -adrenoceptor responsiveness is altered with age. In the present study, we tested the hypothesis that postjunctional -adrenergic vasoconstrictor responsiveness is reduced in the leg circulation with age. We measured femoral blood flow (Doppler ultrasound) and calculated the vascular conductance (FVC) responses to -adrenoceptor stimulation during local blockade of 
-adrenoceptors in 12 young (24±1 yrs) and 7 healthy older men (62±2 yrs). Whole-leg vasoconstrictor responses to local intra-femoral artery infusions of tyramine (evokes noradrenaline [NA] release), phenylephrine (1-agonist) and dexmedetomidine (2-agonist) were assessed. Consistent with previous data, resting femoral blood flow and vascular conductance were ~30% lower in older compared with young men (P<0.05). Maximal vasoconstrictor responses (%
FVC) to tyramine (-30±3 vs -41±3%), phenylephrine (-25±4 vs -45±5%), and dexmedetomidine (-22±4 vs -44±3%) were all significantly lower in older compared with young men (all P<0.05). Our results indicate that human ageing is associated with a reduction in leg postjunctional -adrenoceptor responsiveness to endogenous NA release, and this reduction is evident for both 1- and 2-adrenoceptors. However, given that basal leg vascular conductance is reduced with age and is primarily mediated by sympathetic vasoconstriction, impaired -adrenoceptor responsiveness does not negate the ability of the sympathetic nervous system to evoke greater tonic vasoconstriction in the leg vasculature of older men.
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