Microdialysis enables in-depth mechanistic study of the cutaneous circulation in humans. However, whether the insertion or presence of the microdialysis fiber (MDF) affects the skin circulation or its responses is unknown. We tested whether the cutaneous vascular response to whole body heating (WBH) was affected by MDF or by pretreatment with ice (part 1) or local anesthesia (LA; part 2). Eleven subjects participated, 9 in part 1 and 8 in part 2 (5 participated in both). In both parts, four sites on the forearm were selected, providing untreated control, MDF only, ice or LA only, and combined MDF plus ice or LA. A tube-lined suit controlled whole body skin temperature, which was raised to ∼38°C for WBH. Skin sites were instrumented with laser-Doppler flow probes. Data were expressed as cutaneous vascular conductance (CVC). Baseline levels were not different among sites (P > 0.05). In part 1, the internal temperature for the onset of vasodilation was higher (P > 0.05) with MDF with or without ice pretreatment than at untreated control sites (control 36.6 ± 0.1°C, Ice 36.5 ± 0.1, MDF 36.8 ± 0.1°C, and Ice+MDF 36.8 ± 0.1°C). Peak CVC during WBH was decreased (P < 0.05) by MDF (control 73 ± 7 vs. MDF 59 ± 6% of maximal CVC). Ice (73 ± 6% of maximal CVC) or Ice+MDF (69 ± 6% of maximal CVC) did not affect (P > 0.05) peak CVC compared with control. In part 2, the temperature threshold for the onset of vasodilation was increased by MDF with or without LA treatment and by LA alone (P < 0.05; control 36.6 ± 0.1°C, MDF 36.7 ± 0.1°C, LA 36.8 ± 0.1°C, and LA+MDF 36.8 ± 0.1°C). Peak CVC was decreased by MDF (control 69 ± 6% of maximal CVC vs. MDF 58 ± 8% of maximal CVC; P < 0.05). LA only (65 ± 10% of maximal CVC) or MDF in the presence of LA (73 ± 12% of maximal CVC) did not affect (P > 0.05) peak CVC compared with control. Thus LA or MDF increases the temperature threshold for the onset of vasodilation. MDF alone decreases the peak vasodilator response in CVC to WBH; however, this attenuation did not occur if ice or LA is used before MDF placement. Ice or LA alone do not affect the peak response in CVC to WBH. How those treatments prevent or reverse the effect of MDF placement is presently unclear
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