CIRCADIAN VARIATION OF N-NITROSOMETHYLURE A-INDUCED CARCINOGENESIS INFEMALE RATS

One-month-old female LIO rats were blinded and two weeks later N-nitro somethylurea (NMU) was administeres at i.v. single dose of 50 mg/kg at 10.00 hours or at 22.00 hours. At the same time NMU was injected into sighted rats. The incidence of total or malignant tumors only was 1.5 and 2.1 times higher in the rats exposed to NMU in the morning in com parison to the those in the rats exposed toNMU in the evening. The bli ndness increased the survival and decreased the suscpetibility of rats to carcinogenesis induced by morning injection of NMU, whereas it did not influence the tumor incidence or latency in rats exposed to carci nigen at 22.00 hours. The incidence of spontaneous tumors in blind and sighting animals was similar. Our data show the significant circadian variation in the susceptibility of female rats to carcinogenesis indu ced by NMU. Light deprivation inhibited NMU-induced carcinogenesis and abolishes its circadian variations. It was suggested that the effects observed were stimulated by increase of pineal function at night time and in blind animals.