Mechanism underlying the attenuated cutaneous vasodllation of older subjects exercising in the heat

We have previously demonstrated that older men and women icspond to local and reflex-mediated thermal challenges with an attenuated increase in cutaneous blood flow and cutaneous vascular conductance (CVC = hlood flow/arterial pressure). The purpose of this study was to test the hypothesis that an augmented noradrenergic vasoconstriction (VC) may play a role in this age-related difference. Fifteen young (Y; 22±1 yr) and 15 older <O; 66±1 yr) men exercised at 50% VO2peak in a 36°C environment. Changes in CVC were monitored at 2 sites on the forearm by laser-Doppler flowmetry. one site prctreated with bretylium (BT) to block local release of norepinephrine and an adjacent control site. Blockade of the VC system was verified during whole-body cooling using a water-perfused suit. CVC was reported as a percent of the maximal local CVC (CVCmai) induced by prolonged local heating at 42°C. Neither age nor BT affected the final CVC achieved, i.e , the CVC plateau at high core temperatures (Y: 76±3 and 74+3 %CVCmax, O: K(>+3 and 78+4 %CVCmax at control and BT sites, respectively). However, during the early rapid rise phase, the slope of the CVC:esophageal temperature response was significantly lower in the O men (6.3 vs 10.5 % per 0.1°C). BT had no effect on this age difference. These findings suggest a lack of involvement of (he VC system in the lower cutaneous blood flow response of individuals over the age of 65 years. Based on the present and other studies, structural changes in the skin vasculaturc which limit maximal CVC arc a more tenable mechanism for this difference.