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Daily, rhythmic variation in various biochemical, physiological, and behavioral events is a fundamental property of biological organization. Here, we report analysis of relative levels of gene expression in the liver of 16 Fischer 344 rats as a function of time of day. Expression levels were determined for 3906 genes using high-density oligonucleotide microarrays. Of the 3906 genes, 1171 (30%) were clearly expressed while 2735 (70%) were not expressed or the expression was too low to distinguish from background levels. The maximum estimated changes observed for most genes (1029, 88%) were less than 1.5-fold. Analysis of variance and the Kruskal-Wallis tests were used to identify 67 genes whose expression was significantly altered as a function of time of day. These significantly altered genes were classified according to their functions and fall into key cellular pathways including drug metabolism, ion transport, signal transduction, DNA binding and regulation of transcription, and immune response.

Male C57BL/6N mice were administered a single ip injection of 30 mg of N-methyl-N-nitrosourea (MNU)/kg of body weight. Additional groups were treated similarly every 3 hours for the next 24 hours. Adenocarcinomas of the small intestine were the major treatment-related tumors, with the total incidence being 38% at 250 days after injection. There was a significant circadian variation for tumor induction; the maximum number of intestinal tumors (approximately equal to 55%) tended to occur when the MNU was administered during the middle of the light period (6:00 to 18:00), while the tumor incidence was at a minimum (approximately equal to 10%) when the MNU was given in the middle of the dark phase (18:00 to 6:00). These data are discussed in relation to DNA synthesis and repair and MNU-induced cellular toxicity.

The influence of epidermal growth factor (EGF), 0.75 micrograms g1; insulin, 1.5 micrograms g-1; glucagon, 1.25 micrograms g-1 and their combinations on the activities of hepatic pyruvate kinase (PK) and malic enzymes (ME) was monitored. Male CD2F1 mice were treated toward the end of the light or dark periods, 9 or 23 hours after lights on (9 or 23 HALO), and subgroups of six mice were killed at 4, 8 or 12 hr post-treatment. PK and ME activities from control mice were well characterized by cosine curves. The PK activity was maximal when ME activity was minimal at the transition from light to dark (9 HALO plus 4 hr) and PK was at a minimum when ME was highest (23 HALO plus 4 hr). Both enzymes were influenced by at least one peptide hormone, and the effects were strongly circadian-stage dependent. The only effect attributed to EGF was an increase of PK activity (23%) 12 hr after injection at 23 HALO. PK activity was increased by insulin (23%) at 23 HALO (4 hr after injection), but not at 9 HALO, and decreased (17%) by glucagon 12 hr after injection at 9 HALO. Several reductions in PK activity in response to various combinations of peptides were observed, and appeared to be caused by glucagon but influenced by insulin.(ABSTRACT TRUNCATED AT 250 WORDS)

Mice were standardized to 12 h of light exposure alternating with 12 h of darkness and were fed ad libitum. The mice in the first group were 8 weeks old and in the other 104-116 weeks old. Subgroups of 7 animals from each age group were killed at 6 circadian stages. Enzyme activities of 23 enzymes from liver or brain were measured by an analysis of variance for each age group. All but 1 enzyme from young mice, and all but 9 from older mice showed significant changes over time (p less than 0.01). The data from the 22 enzymes from young mice and the 14 enzymes from old mice were fit to the cosinor regression model to further characterize the rhythm. 15 enzymes from the young and 8 from the aged mice showed a significant regression to a 24-hour cosine curve (p less than 0.01); of the 8, 7 were the same enzymes in both groups. Amplitude changes, where they could be compared from the cosinor data (7 enzymes), were not statistically different. When total variance was compared, 12 enzymes showed unequal variance. Of these, old mice had the larger variance in 9 enzymes. Another difference between the young and the old was changes in mean enzyme activities. 12 enzymes from aged mice had decreased mesors, 2 had increased mesors, and 9 were unchanged. In general, our data suggest that some enzyme activity rhythms were lost, others were altered and a few were not affected by aging. In the case of many enzymes, older mice have a diminished ability to synchronize to the light signals.

The activities of 23 brain or liver enzymes were studied in 5-6 week old C57BL/6JNctr male and female mice that had been fed ad libitum and standardized for 2 weeks to either (1) 12 hr of light (0600-1800) alternating with 12 hr of darkness (1800-0600) (LD 12:12), (2) staggered sequences of 12 hr of light and 12 hr of dark (SLD 12:12) or (3) continuous illumination (LL 12:12) for 2 weeks. Mice in the LD 12:12 and LL 12:12 experiments were killed at 4 hr intervals along a 24-hr span in order to sample at six different circadian stages. Lighting schedules for mice in the SLD 12:12 experiment were organized such that six different circadian stages were sampled when all mice were killed at one time of day. All 23 enzymes demonstrated a prominent circadian rhythm in at least one of the experiments. Moreover, about two-thirds of the enzymes in LD and SLD 12:12 had a statistically significant fit to a 24-hr cosine curve, while only one-third of the enzymes in LL 12:12 had significant fits to cosine curves. Peak activities of enzymes from mice in LD 12:12 were clustered at the time of transition from light to dark. This was also the trend for the activities of enzymes from mice in SLD 12:12, but resynchronization did not appear completed within the 2-week span. This, along with the observation that mesors (mean 24-hr activity) were reduced and amplitudes altered, indicated that the 2-week standardization period was not sufficient for some enzymes. Times of peak activities, mesors and amplitudes were affected for most enzymes from mice in the LL 12:12 environment. This suggests that individual mice became desynchronized from one another with respect to the original light-dark schedule and that rhythms were altered or lost because individual mice were free running with frequencies different from 24 hr.