Investigating the roles of core and local temperature on forearm skin blood flow.

We sought to isolate the contributions of core and local temperature on forearm skin blood flow (SkBF), and to examine the interaction between local- and reflexive-mechanisms of SkBF control. Forearm SkBF was assessed using laser-Doppler flowmetry in eight males and eight females during normothermia and hyperthermia (+1.2°C rectal temperature). Mean experimental forearm temperature was manipulated in four, 5min blocks between neutral (A: 33.0°C) and warm (B: 38.5°C) in an A-B-A-B fashion during normothermia, and B-A-B-A during hyperthermia. Mean control forearm skin temperature was maintained at ~33°C. Finally, local heating to 44°C was performed on both forearms to elicit maximal SkBF. Data are presented as a percentage of maximal cutaneous vascular conductance (CVC), calculated as laser-Doppler flux divided by mean arterial pressure. No sex differences were observed in any CVC measures (P>0.05). During normothermia, increasing experimental forearm temperature to 38.5°C elevated CVC by 42±8%max (d=3.1, P<0.001). Subsequently decreasing experimental forearm temperature back down to 33.0°C reduced CVC by 36±7%max (d=2.5, P<0.001). Finally, the second increase in experimental forearm temperature to 38.5°C increased CVC by 25±6%max (d=1.9, P<0.0001). During hyperthermia, decreasing experimental forearm temperature to 33.0°C reduced CVC by 6±1%max (d=0.5, P<0.001). Increasing experimental forearm temperature to 38.5°C increased CVC by 4±2%max (d=0.4, P<0.001). Finally, decreasing experimental forearm temperature to 33.0°C reduced CVC by 8±2%max (d=0.7, P<0.001). Compared to normothermia, CVC responses to local temperature changes during hyperthermia were almost abolished (normothermia: d=1.9-3.1; hyperthermia: d=0.4-0.7). These data indicate that local temperature drives SkBF during normothermia, while reflexive mechanisms regulate SkBF during hyperthermia.