ALTERATIONS IN THE CIRCADIAN RHYTHMICITY OF RAT SMALL INTESTINAL FUNCTIONS

When rats were fed a complete diet for 3 hours out of every 12 hours (0600-0900 and 1800-2100, EST), feeding cued 12-hour cycle rhythms were found in the activity rhythms were found in the activity patterns of monosaccharide transport, and mucosal sucrase, isomaltase, maltase, trehalase, alkaline phosphatase, γ-glutamyltransferase, leucylnaphthylamide-hydrolyzing activity, protein and DNA, and perhaps in lactase. These same patterns i.e., higher levels of activity at the end of each meal, were also observed in the specific activities of sucrase, isomaltase and alkaline phosphatase but not in leucylnaphthylamide-hydrolyzing activity in the brush border membrane fractions of these mucosal homogenates. Thus, one complete meal every 12 hours is capable of cuing these small intestinal rhythms to a 12-hour cycle. The association of the functional activities and the DNA patterns, and the rhythms in the specific activities found in the brush border membranes is suggestive of a role for both fluctuations in the absorptive cell number and enrichment of the brush border membranes in the control of these functional rhythms. When the diet was modified so that all the protein was fed at one meal and all the starch at the other meal, 24-hour cycles were found in the activity patterns of the above mentioned functions. Monosaccharide transport was the only function cued and maintained at a high level by the starch meal. Most of the enzymes, irrespective of substrate, DNA and protein were cued and maintained by the protein meal. It is suggested that while there is a feeding schedule cue for these rhythms there is another cue which is related to the protein and carbohydrate composition of the diet.