Cutaneous vasoconstrictor response to whole body skin cooling is altered by time of day

To test for a diurnal difference in the vasoconstrictor control of the cutaneous circulation, we performed whole body skin cooling (water-perfused suits) at 0600 (AM) and 1600 (PM). After whole body skin temperature (T-sk) was controlled at 35degreesC for 10 min, it was progressively lowered to 32degreesC over 18-20 min. Skin blood flow (SkBF) was monitored by laser-Doppler flowmetry at three control sites and at a site that had been pretreated with bretylium by iontophoresis to block noradrenergic vasoconstriction. After whole body skin cooling, maximal cutaneous vascular conductance (CVC) was measured by locally warming the sites of SkBF measurement to 42degreesC for 30 min. Before whole body skin cooling, sublingual temperature (Tor) in the PM was significantly higher than that in the AM (P < 0.05), but CVC, expressed as a percentage of maximal CVC (% CVCmax), was not statistically different between AM and PM. During whole body skin cooling, %CVCmax levels at bretylium-treated sites in AM or PM were not significantly reduced from baseline. In the PM, %CVCmax at control sites fell significantly at T-sk of 34.3 ± 0.01°C and lower (P < 0.05). In contrast, in the AM %CVCmax at control sites was not significantly reduced from baseline until T-sk reached 32.3 +/- 0.01degreesC and lower (P < 0.05). Furthermore, the decrease in %CVCmax in the PM was significantly greater than that in AM at T-sk of 33.3 ± 0.01°C and lower (P < 0.05). Integrative analysis of the CVC response with respect to both Tor and T-sk showed that the cutaneous vasoconstrictor response was shifted to higher internal temperatures in the PM. These findings suggest that during whole body skin cooling the reflex control of the cutaneous vasoconstrictor system is shifted to a higher internal temperature in the PM. Furthermore, the slope of the relationship between CVC and T-sk is steeper in the PM compared with that in the AM.