Differential time courses of skin blood flow and hyperalgesia in the human sunburn reaction following ultraviolet irradiation of the skin.

The response of skin to ultraviolet (UV) irradiation is an inflammation with pronounced vasodilation and hyperalgesia. Volunteers underwent UV irradiation of patches of forearm skin 3 cm in diameter. The intensity of the UV irradiation (290-320 nm) ranged between 133 mJ/cm2 and 400 mJ/cm2. Changes in skin blood flow were measured with laser Doppler technique at 3, 6, 9, 12, 24, 30, 36, 48, 60, 72, 96 and 216 h post UV irradiation. The alteration in pain threshold was measured for heat with a Peltier thermode and for pressure with a metal stylus. The effect of repeated topical pre-treatment 4 days prior to UV irradiation with 1.5 g of 0.04% capsaicin containing ointment was also investigated.UV irradiation resulted in a dose-dependent increase of skin blood flow for more than 96 h post-irradiation. There were two peaks of blood flow at 12 h and 36 h. Topical application of capsaicin prior to UV irradiation attenuated blood flow from 30 h to 45 h post UV irradiation. Enhanced blood flow was also present 5 mm outside the irradiated area (flare reaction). The control level of heat pain threshold was 44.5+/-0.7;C in normal skin. Heat pain thresholds were lowered by up to 7.761.2;C in UV-irradiated areas in a dose-dependent fashion. The control level of pressure pain threshold was 15.260.3N. Pressure pain thresholds were lowered by up to 6 N in irradiated areas. Maximal hyperalgesia coincided with the second peak of skin blood flow between 30 h and 60 h post UV irradiation. The effects of topical application of capsaicin suggests an involvement of neuropeptide mechanisms in the late phase of the human sunburn reaction.