Control of skin blood flow by whole body and local skin cooling in exercising humans

We examined the independent roles of whole body skin temperature (T-sk) and tissue temperature (local temperature, T-loc) in the control of skin blood flow (SBF) during cooling and the roles of the vasoconstrictor (VC) and active vasodilator (AVD) systems in mediating these effects. SBF was monitored by laser-Doppler flowmetry (LDF) at untreated sites and sites with local VC blockade by pretreat ment with bretylium (BT). Seven subjects underwent four sessions of moderate bicycle exercise (20-30 min duration) at neutral T-sk and T-loc (34 degrees C), neutral T-sk and cool T-loc (27 degrees C), low T-sk (28 degrees C) and neutral T-loc, and low T-sk and T-loc. Cutaneous vascular conductance (CVC; LDF/mean arterial pressure) was expressed relative to the maximum. Cool T-sk increased the threshold level of internal temperature at which CVC began to rise equally at BT-treated and untreated sites (P < 0.05). The rate of increase in CVC relative to internal temperature was reduced by local cooling. BT pretreatment partially reversed this effect (P < 0.05). Thus a cool environment results in reflex inhibition of the onset of AVD activity by cool T-sk and a reduced rate of increase in CVC due, in part, to norepinephrine release stimulated by cool T-loc.