Changes in responses of UVB irradiated skin of brownish guinea pigs with aging.

It is known that skin often shows irregular pigmentation during aging, which is frequently associated with hyperpigmentation. Many studies have utilized brownish A1 guinea pigs to investigate the pathogenesis of ultraviolet B (UVB)-induced skin pigmentation, however, responses associated with aging following UVB irradiation have not been elucidated. To characterize those responses, dorsal skin of A1 guinea pigs from 14-weeks to 5-yr old were investigated. The minimal erythema dose was found to increase with aging. Further, in pigmentation induced by UVB radiation, skin brightness (DeltaL*-value) decreased equally in both the 14-week old (young) group and in the 3-yr old (old) group of guinea pigs. The DeltaL*-value recovered in the young group from 21 d after UVB irradiation, whereas no such recovery was seen in the old group. In addition, the amount of melanin and the number of melanocytes returned near pre-irradiation levels in the young group, while they remained high in the old group. Our results therefore demonstrate for the first time that skin responses following UVB irradiation change with aging in A1 guinea pigs.