Within-limb variation in skin pigmentation does not influence cutaneous vasodilation.

Ultraviolet radiation (UVR) exposure acutely reduces nitric oxide (NO)-dependent cutaneous vasodilation. In addition, increased constitutive skin melanin is associated with attenuated NO-dependent cutaneous vasodilation. However, the impact of within-limb variation in skin melanization, associated with seasonal UVR exposure, on NO-dependent cutaneous vasodilation is unknown. We investigated the effect of within-limb variation in skin melanin on NO-dependent cutaneous vasodilation. Intradermal microdialysis fibers were placed in the inner-upper arm, ventral forearm, and dorsal forearm of seven adults (33 ± 14 yr; 4 M/3 F) with constitutively light skin pigmentation. Melanin-index (M-index; an index of skin pigmentation), measured via reflectance spectrophotometry, confirmed differences in sun exposure among sites. A standardized local heating (42°C) protocol induced cutaneous vasodilation. After attaining a stable elevated blood flow plateau, 15 mM -nitro-l-arginine methyl ester (l-NAME; NO synthase inhibitor) was infused to quantify the NO contribution. Laser-Doppler flowmetry (LDF) measured red cell flux and cutaneous vascular conductance (CVC = LDF/mean arterial pressure) and was normalized to maximal (%CVC; 28 mM sodium nitroprusside + 43°C local heating). Dorsal forearm M-index was higher [50.5 ± 11.8 au (arbitrary units)] compared with the ventral forearm (37.5 ± 7.4 au; ≤ 0.03) and upper arm (30.0 ± 4.0 au; ≤ 0.001) M-index. Cutaneous vasodilation responses to local heating were not different among sites ( ≥ 0.12). Importantly, neither the magnitude of the local heating plateau (dorsal: 85 ± 21%; ventral: 70 ± 21%; upper: 87 ± 15%; ≥ 0.16) nor the NO-mediated component of that response (dorsal: 59 ± 15%; ventral: 54 ± 13%; upper: 55 ± 11%; ≥ 0.79) was different among sites. These data suggest that within-limb differences in skin pigmentation secondary to seasonal UVR exposure do not alter NO-dependent cutaneous vasodilation. Locally derived endothelial nitric oxide (NO) contributes to the full expression of cutaneous vasodilation responses. Acute ultraviolet radiation (UVR) exposure attenuates NO-mediated vasodilation of the cutaneous microvasculature. Our findings suggest that in constitutively lightly pigmented skin, variation in skin melanin due to seasonal exposure to UVR does not alter the NO contribution to cutaneous vasodilation. Seasonal UVR exposure does not impact the NO-mediated cutaneous microvascular function.