Altered neurotransmitter control of reflex vasoconstriction in aged human skin

Cutaneous vasoconstriction (VC) in response to cooling is attenuated in older humans; however, mechanisms underlying this functional decline remain unclear. The present study tested the hypothesis that the contributions of noradrenaline (NA) and sympathetic cotransmitters to reflex-mediated cutaneous VC are altered with age. In 11 young (18-26 years) and 11 older (61-77 years) men and women, forearm skin blood flow was monitored at three sites using laser Doppler flowmetry (LDF) while mean skin temperature was lowered from 34 to 30.5°C using a water-perfused suit. Cutaneous vascular conductance (CVC; LDF/mean arterial pressure) was expressed as percentage change from baseline (% ΔCVC_base). Solutions of yohimbine + propranolol (Y + P), bretylium tosylate (BT), and lactated Ringer solution were infused via intradermal microdialysis at each LDF site to antagonize α- and β-adrenoceptors, block sympathetic release of NA and cotransmitters, and act as control, respectively. During cooling, VC was attenuated at the control site in older subjects compared to young subjects (-16 ± 3 versus -34 ± 4% ΔCVC_base, P < 0.001). Y + P attenuated VC in young subjects (-13 ± 8% ΔCVC_base, P < 0.001 versus control) and abolished VC in older subjects (0 ± 3% ΔCVC_base, P > 0.9 versus baseline). BT completely blocked VC in both age groups. Cutaneous VC in young subjects is mediated by both NA and sympathetic cotransmitter(s); however, reflex VC in aged skin is attenuated compared to young and appears to be mediated solely by NA.