Circadian rhythms of resistance to UV-C and UV-B radiation in Euglena as related to "escape from light" and "resistance to light".
Radiation-induced stress, either from visible or UV light, is strongest at midday. We found that, in the absence of stress or time cues, Euglena gracilis Z was the most resistant to UV-C and UV-B at subjective midday, whether judged from immediate or reproductive survival. The circadian UV-resistance rhythms were free-running in stationary cultures under 1-h light/1-h dark cycles or continuous darkness, indicating that cell-cycle dependent DNA susceptibility to UV was not involved. We moreover examined what was the primary cause of the circadian UV resistance, estimated as the immediate cell survival. The half-maximal lethal dose (LD(50)) of UV-C at subjective midday (the most resistant phase) was 156 J/m(2), which is approximately 3-fold that at subjective midnight. The same was true for UV-B, except the LD(50) was approximately 13-fold that of UV-C. Temperature during UV irradiation had little effect, indicating that survival was not mediated via enzymatic reactions. Non-enzymatic antioxidants were added 5 min before UV irradiation. Dimethylsulfoxide (a hydroxyl radical scavenger) increased survival after UV-B, but had little effect after UV-C; conversely, sodium ascorbate increased survival after UV-C, but not after UV-B. These findings suggest that circadian rhythms of resistance to UVs involve a common mechanism for maximizing non-enzymatic antioxidative capacity at subjective midday, but the specific antioxidants differ.
Bolige
Aoen
A
Laboratory of Biological Rhythms, Obihiro University of Agriculture and Veterinary Medicine, Obihiro 080-8555, Japan.
Kiyota
Maki
M
Goto
Ken
K
D000818:Animals; D001205:Ascorbic Acid / Q000494:pharmacology; D002453:Cell Cycle / Q000528:radiation effects; D002940:Circadian Rhythm; D004260:DNA Repair; D048028:DNA, Algal / Q000528:radiation effects; D003624:Darkness; D004121:Dimethyl Sulfoxide / Q000494:pharmacology; D005055:Euglena / Q000187:drug effects / Q000528:radiation effects*; D007928:Lethal Dose 50; D008027:Light; D009499:Neutral Red; D011836:Radiation Tolerance / Q000502:physiology*; D014466:Ultraviolet Rays
D048028:DNA, Algal; D009499:Neutral Red; D001205:Ascorbic Acid; D004121:Dimethyl Sulfoxide