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 (Tsk) and tissue temperature (local temperature, Tloc) 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 pretreatment with bretylium (BT). Seven subjects underwent four sessions of moderate bicycle exercise (20-30 min duration) at neutral Tsk and Tloc (34 degrees C), neutral Tsk and cool Tloc (27 degrees C), low Tsk (28 degrees C) and neutral Tloc, and low Tsk and Tloc. Cutaneous vascular conductance (CVC; LDF/mean arterial pressure) was expressed relative to the maximum. Cool Tsk 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 Tsk and a reduced rate of increase in CVC due, in part, to norepinephrine release stimulated by cool Tloc.