Circadian changes in serum and liver metabolites and liver lipogenic enzymes in ad libitum- and meal-fed, lean and obese Zucker rats

Circadian rhythms were studied in ad libitum- and meal-fed Zucker rats. Lean and obese female rats were fed ad libitum or in a single meal from 1400-1800 for 12 d. Both groups of ad libitum-fed rats exhibited a nocturnal feeding pattern; obese rats ate more than the lean rats during the light and dark parts of the cycle. After 12 d, groups of rats were killed at 4-h intervals over a 24-h period, inclusively, and the presence of circadian rhythms was evaluated for the serum metabolites, liver constituents and the liver lipogenic enzymes. The obese ad libitum-fed rats demonstrated rhythms for serum triglycerides, serum cholesterol, liver protein and glycogen, and the activities of fatty acid synthetase, citrate cleavage enzyme (EC 4.1.3.8), and glucose-6-P dehydrogenase (EC 1.1.1.49). Meal feeding the obese rats modified the rhythms for serum cholesterol, liver glycogen and the activities of glucose-6-P dehydrogenase, and citrate cleavage enzyme and a rhythm was established for serum glucose. The feeding schedule did not cue a rhythm for serum triglycerides and phosphogluconate dehydrogenase (EC 1.1.1.44) or modify the rhythm for liver protein. The lean ad libitum-fed rats displayed rhythms for all the measured parameters except serum glucose. Feeding-cued rhythms for all metabolites except serum cholesterol were observed when the lean rats were meal-fed. These data suggest that the rhythms of the ad libitum-fed obese rats may not be similar to those observed in the lean rat. Yet, changing the feeding schedule may be important in establishing rhythm patterns that will alter the anabolic profile characteristic of the Zucker obese rat.