DECREASED ACTIVE VASODILATOR SENSITIVITY IN AGED SKIN

Older men and women respond to local and reflex-mediated heat stress w ith an attenuated increase in cutaneous vascular conductance (CVC). Th is study was performed to test the hypothesis that an augmented or sus tained noradrenergic vasoconstriction (VC) may play a role in this age -related difference. Fifteen young (22 +/- 1 yr) and 15 older (66 +/- 1 yr) men exercised at 50% peak oxygen uptake in a 36 degrees C enviro nment. Skin perfusion was monitored at two sites on the right forearm by laser-Doppler flowmetry: one site pretreated with bretylium tosylat e (BT) to block the local release of norepinephrine and thus VC and an adjacent control site. Blockade of reflex VC was verified during whol e body cooling using a water-perfused suit. CVC (perfusion divided by mean arterial pressure) at each site was reported as a percentage of t he maximal CVC (%CVCmax) induced at the end of each experiment by prol onged local heating at 42 degrees C. Neither age nor BT affected the % CVCmax (75-86%) attained at high core temperatures. During the early r ise phase of CVC, the %CVCmax-change in esophageal temperature (Delta T-es) curve was shifted to the right in the older men (effective Delta T-es associated with 50% CVC response for young, 0.22 +/- 0.04 and 0. 39 +/- 0.04 degrees C and for older, 0.73 +/- 0.04 and 0.85 +/- 0.04 d egrees C at control and BT sites, respectively). BT had no interactive effect on this age difference, suggesting a lack of involvement of th e VC system in the attenuated CVC response of individuals over the age of 60 yr. Additionally, increases in skin vascular conductance were q uantitatively compared by measuring increases in total forearm vascula r conductance (FVC, restricted to the forearm skin under these conditi ons). After the initial similar to 0.2 degrees C increase in T-es, FVC was 40-50% lower in the older men (P < 0.01) for the remainder of the exercise. Decreased active vasodilator sensitivity to increasing core temperature, coupled with structural Limitations to vasodilation, app ears to limit the cutaneous vascular response to exertional heat stres s in older subjects.