The effects of fasting were examined on the rhythmic changes in the activities of maltase [EC 3.2.1.20] and leucine aminopeptidase [EC 3.4.11.1] in the small intestine of rats which has been kept under scheduled feeding conditions. Irrespective of whether the rats had been kept on a daytime or nighttime feeding schedule, the rhythms of maltase and leucine aminopeptidase persisted when the animals were starved. However, the amplitude of the leucine aminopeptidase rhythm began to decrease from the first day of fasting, while that of maltase did not. Conspicuous rhythms persisted for at least 2 days during fasting, but they gradually became vague and disappeared after 5 days. When rats were refed after fasting, the leucine aminopeptidase activity increased within a few hours, but the maltose activity did not. It is suggested that the rhythms of the digestive enzymes in the small intestine of rats are not a direct consequence of food intake, but are triggered off by the anticipatory mechanism which operates when rats expect to be fed. The rhythmic change of leucine aminopeptidase seemed to be intensified by food intake.
The circadian rhythm of blood corticosterone was examined in rats whose feeding was restricted to a specific time of day. The peak of the corticosterone level was found just before feeding time regardless of whether, the rats were kept on a 12-h light, 12-h dark cycle or in constant light or were blinded. It took at least 10 days to establish the corticosterone rhythm corresponding to the feeding time, but once it had been established, it persisted for at least 2 days of stravation regardless of the lighting condition. It is concluded that the corticosterone rhythm entrained by food is endogenous.
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