Abstract: Objective To investigate the extent and features of adaptation reaction of melanocytes induced by low dose UVA radiation.Methods Cultured skin melanocytes were irradiated by a lethal dose of 86.4 J/cm² UVA with or without pre-irradiation of single or repetitive low dose 7.2 J/cm² UVA.Alterations of cellular morphology were observed by light and electron microscopy.Proportions of apoptosis were assayed by flow cytometry.The extent of DNA damage was determined by single cell gel electrophoresis (SCGE).Results In cultured skin melanocytes,pretreatment with single or repetitive irradiation of 7.2 J/cm² UVA could alleviate toxic reaction of cellular morphology,decrease apoptosis ratio,reduce DNA chain breakages and enhance repair of DNA damage caused by subsequent 86.4 J/cm² UVA irradiations,compared to those without pretreatment (P<0.01 or P<0.05).The adaptation reaction induced by a single pretreatment of 7.2 J/cm² UVA disappeared 12 h after irradiation,however,the accumulated dosage of 28.8 J/cm² by repeated low dose UVA pretreatments could prevent damage by 86.4 J/cm² UVA irradiation even after 14 d.Conclusions Low dose U-VA irradiation could induce adaptation reaction which protects cultured skin melanocytes from damages by subsequent challenged dose of UVA.The period the adaptation reaction lasts and protective extents are clearly related to the accumulated dosage of repeated low dose UVA irradiations and intervals.

Key words: Ultraviolet rays, Melanocytes, Cells,cultured, Adaptation,biological