Reflex control of active cutaneous vasodilation by skin temperature in humans

The purpose of this study was to examine whether reflex effects of changes in whole body skin temperature (T(sk)) on cutaneous vasculature are mediated through the vasoconstrictor or the active vasodilator arm of the sympathetic nervous system. In six subjects, reflex responses in forearm skin blood flow (SkBF) to changes in T(sk) were monitored by laser-Doppler flowmetry. SkBF was monitored at a control site and at a 0.6-cm² site where bretylium (BT) had been iontophoretically applied to abolish sympathetic vasoconstrictor control. Reflex control of SkBF at BT-treated sites is solely through active vasodilator activity. An index of cutaneous vascular conductance (CVC) was calculated from the blood flow signal and mean arterial pressure, measured noninvasively. Data are expressed relative to maximum CVC (CVC(max)) achieved by local warming of measurement sites to 42°C at the end of each study. T(sk) was controlled with a water-perfused suit covering the entire body except for the head and arms. Esophageal temperature (T(es)) was measured as an index of internal temperature. In part A (rest), raising T(sk) at rest from 31.9 ± 0.3 to 36.7 ± 0.2°C increased CVC at control sites from 3 ± 0.2 to 5 ± 0.6% of CVC(max). CVC did not change at BT-treated sites, suggesting that at rest, with a normal internal temperature, reflex effects of raising T(sk) on SkBF are mediated through vasoconstrictor withdrawal. In part B (exercise), exercise at a low T(sk) increased T(es) to 37.49 ± 0.1°C. Raising T(sk) during exercise from 31.4 ± 0.3 to 36.7 ± 0.2°C produced large simultaneous increases in CVC at control (6 ± 2 to 33 ± 3% of CVC(max)) and BT-treated sites (10 ± 3 to 40 ± 5% of CVC(max)). There was no net change in T(es) during this period. Thus, at the elevated internal temperature of exercise, reflex cutaneous vascular effects of raising T(sk) are largely mediated through the active vasodilator system. In conclusion, reflex effects of whole body T(sk) can be mediated through either of the efferent systems controlling SkBF, depending on background internal temperature.