1. Biological clocks and the digestive system
Lawrence A. Scheving 
Gastroenterology 
Volume 119, Issue 2, Pages (August 2000)
DOI: /gast
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2. Fig. 1
Prominent circadian fluctuation of the principal serum steroids in rats and humans. The rats were standardized to a light-dark cycle (14 hours of light alternating with 10 hours of darkness) and were fed ad libitum for 2 weeks before the study. Meal times for humans were 7 AM, 12:45 PM, and 4:45 PM; rest or sleep was between 9 PM and 6 AM. Subjects were awakened for sampling at midnight and 3 AM. The peak levels of steroids for diurnally active humans occurred 12 hours out of phase with the nocturnally active rodents. (Reprinted from Endeavour, vol. 35, Scheving LA, “The dimension of time,” pp. 66–72, © 1976, with permission from Elsevier Science.)
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3. Fig. 2
Circadian variation of the mitotic index in the rat corneal epithelium. Summary of 8 studies for animals standardized to a light-dark (12:12) photoperiod. The mitotic index increases during the dark phase and decreases during the light phase. CST, central standard time. (Reprinted with permission.16)
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4. Fig. 3
Circadian variation in [3H]thymidine incorporation into DNA in 5 different regions of the digestive tract in BDF1 mice over a 48-hour span. The amplitude, phasing, and average level of DNA synthesis vary from one tissue to the next; however, for all tissues, the lowest level occurs at or near the light-to-dark transition. (Reprinted with permission from “Circadian Variation in Cell Division of the Mouse Alimentary Tract, Bone Marrow, and Corneal Epithelium” Scheving et al., Anatomical Record, Copyright© 1978, Wiley-Liss, Inc. a subsidiary of John Wiley & Sons, Inc.4)
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5. Fig. 4
Circadian variation in DNA bromodeoxyuridine (BRDU) labeling (top) and cyclin A immunohistochemical labeling (bottom) of the esophagus of BDF1 mice at two times of day: 5 PM (left) and 5 AM (right). These two times are shown because they represent the times that DNA synthesis in the mouse esophagus is at its lowest (5 PM) or highest (5 AM) point. Bromodeoxyuridine labeling and cyclin A immunolabeling in the basal layer of the esophageal epithelium show striking circadian variation (L. A. Scheving, unpublished data).
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6. Fig. 5
Influence of fasting or exogenous EGF in circadian rhythmicity in DNA synthesis between 20 and 60 hours after food removal. (A) One group of CD2F1 mice were allowed to eat ad libitum; (B) the other group were fasted for up to 60 hours. Half of the ad libitum–fed or fasted animals were injected with EGF (10 μg intraperitoneal injection) during the light phase 16, 20, or 24 hours after food removal. The light-dark cycle is shown at the base of the x-axis. DNA synthesis rhythmicity persists during fasting and can be phase-shifted in both ad libitum–fed and fasted animals by injecting EGF into animals during the light phase when DNA synthesis is normally lowest. (Reprinted from Peptides, Vol. 8, Scheving LA, et al., “Effect of epidermal growth,” p. 349, © 1987, with permission from Elsevier Science.12)
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7. Fig. 6
Circadian variation in (A) sucrase activity and (B) immunoreactivity. (A) Effect of fasting on the circadian rhythms of sucrase and maltase in the small intestine of rats fed from 9 AM to 3 PM for 2 weeks. Activities are expressed as micromoles of product formed per minute per milligram of homogenate protein, with each point representing a mean value for 5 rats. (Reprinted and adapted from Biochemica Biophysica Acta, Vol. 421, Saito M, et al., Circadian Rhythms in Disaccharidases of Rat Small Intestines and Its Relation to Food Intake, 1976, with permission from Elsevier Science.36) (B) Expression of sucrase in the jejunum and ileum by immunoblotting. Equal amounts of membrane protein from different adult rats were resolved by SDS–polyacrylamide gel electrophoresis, and then subjected to an immunoblot using a rabbit antisucrase antibody (courtesy of Dr. Gary M. Gray; Stanford University; Stanford, CA). Note that the major immunoreactivity is greatest at night, when activity is normally highest (L. A. Scheving, unpublished data).
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8. Fig. 7
Circadian rhythm in the degradation of sucrase. The degradation of sucrase-isomaltase between 5 AM and 5 PM. Values are the mean (±SE) for 4 rats. Note that the rate of degradation of sucrase-isomaltase increases after feeding begins. (Reprinted with permission.40)
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9. Fig. 8
Circadian variation in the expression of rat uroguanylin mRNA as revealed by Northern blot. Equal amounts of total RNA were resolved on agarose gel, transferred to nylon membranes, and then probed with a uroguanylin (UGN) cDNA probe. The ileum blot was exposed to autoradiographic film for a longer period of time because of reduced uroguanylin expression at this site. (Data from Scheving and Jin.43)
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10. Fig. 9
Circadian changes in the ultrastructure of periportal hepatocytes in rats (fed ad libitum and standardized to a light-dark cycle, light from 6 AM to 6 PM). (A) The end of the light phase when almost no glycogen is detected; the rER is more evenly dispersed in the cytoplasm surrounding individual mitochondria. (B) Glycogen pattern at the end of the dark period when there is abundant glycogen; rER is arranged in stacks and is associated with mitochondria (original magnification 20,000×). (Reprinted with permission.63)
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11. Fig. 10
(A) Possible mechanism to account for rhythmic variation in per expression. According to this model, the interaction of CLOCK and BMAL1 drives the transcription of per, which is translated into PER protein. The PER protein negatively feeds back on the CLOCK–BMAL1 dimer to inhibit per transcription. PER phosphorylation leads to its degradation, with a resumption in per transcription. Any gene containing the E-box is capable of being directly regulated by clock genes. (B) Schematic summarizing the potential interactions between the brain and digestive system in the control of digestive circadian rhythms. The inputs, clock location, and outputs involved in the generation and coordination of digestive rhythms are largely unknown.
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12. Fig. 11
In vitro circadian variation of several transcription factors. Rat 1 fibroblasts were shocked with 50% serum for 2 hours and then exposed to 0% serum for the next 72 hours. RNase protection assays were done to assess the level of the different mRNAs for Rev-Erba, rPER1, DBP, TEF, rPER2, RDRa, TBP, and actin. Rhythmic variation is observed for a number of the transcription factors. (Adapted and reprinted with permission.76)
Gastroenterology  , DOI: ( /gast )
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