Correlation of circadian changes in tyrosine aminotransferase and tryptophan-2-3-dioxygenase in rat liver to irradiation at different times of the day.

Male SPF Wistar rats adapted to a 12:12 h light: dark regimen were irradiated at 3-hour intervals in the course of 24 h with a dose of 14.35 Gy X-rays; 24 h after irradiation or sham irradiation and starvation for the same length of time, and also in fed intact rats, tyrosine aminotransferase and tryptophan-2-3-dioxygenase activity in the liver and the serum corticosterone level were determined. Although lethal irradiation modified the given enzyme activities, it did not abolish their circadian rhythm, evidently in association with the low sensitivity in association with the low sensitivity of the liver to ionizing radiation. In irradiated animals (compared with sham-irradiated animals), the serum corticosterone concentration fell during the light part of the day and at the beginning of the dark part.