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1 Intercollege Graduate Degree Program in Physiology, The Pennsylvania State University, University Park, PA, USA; Noll Laboratory, Department of Kinesiology, The Pennsylvania State University, University Park, PA, USA
2 Noll Laboratory, Department of Kinesiology, The Pennsylvania State University, University Park, PA, USA
* To whom correspondence should be addressed. E-mail: cst116{at}psu.edu.
Cutaneous vasoconstriction (VC) in response to cooling is impaired with human aging. Based on previous findings that older humans rely predominantly on norepinephrine (NE) for reflex VC of skin blood vessels, and that the VC effects of NE are blunted with age in many vascular beds, we tested the hypothesis that cutaneous VC responses to exogenous NE are attenuated in aged skin compared to young skin. In eleven young (18-30 yr) and 11 older (62-76 yr) men and women, skin blood flow was monitored at two forearm sites with laser Doppler (LD) flowmetry while local skin temperature was clamped at 34°C. At one site, five doses of NE (10-10 to 10-2 M) were sequentially infused via intradermal microdialysis while the other site served as control (C; Ringer's). Cutaneous vascular conductance (CVC; LD flux/mean arterial pressure) was expressed as percent change from baseline (% 
CVCbase). At 10-10, 10-8, and 10-6 M NE, older VC responses were attenuated compared to young [10-10: -35 (95%CI: -16,-52) vs. -49 (-40,-58)% CVCbase, P=0.02; 10-8: -38 (-20,-56) vs. -50 (-40,-61)% CVCbase, P=0.03; 10-6: -52 (-35,-70) vs. -67 (-60,-74)% CVCbase, P=0.01]. Older maximal VC responses were also blunted compared to young [-80 (CI: -73,-87) vs. -88 (CI: -87,-90)% CVCbase, P=0.03]. NE-mediated cutaneous VC is blunted at both physiological and super-physiological doses in older subjects compared to young subjects. Considering that NE is the only functional neurotransmitter mediating reflex VC in aged skin, attenuated NE-mediated VC may further predispose older humans to excess heat loss in the cold.
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