Effect of colostomy on the circadian rhythm in DNA synthesis in the rat colon

Synthesis of DNA and mitosis in gut epithelium are not constant or random events but rather are characterized by circadian rhythmicity, which we reported persists even in fasted rats. Others suggest that rhythms persist because rats anticipate food, causing nerve impulses to propagate caudally in the gut at usual meal times, or that digestive products from previous feedings cause rhythms in the lower tract. We studied colonic DNA synthesis in rats that had been given colostomies. In one study, the distal colon was isolated neurally from proximal gut by means of an end colostomy. In a second study, rats were subjected to loop colostomy; some intrinsic innervation of the gut wall remained intact. Sprague-Dawley male rats, 8 weeks old, were acclimated to a 12:12 light-dark cycle. Colostomies were performed after a 48-h fast. The rats were fed ad libitum for 4 weeks after surgery. Operated rats and an equal number (n = 30) of control rats (unoper-ated) were divided into four subgroups that were killed at 07:00,13:00,19:00, and 01:00 h. Each rat was injected with tritiated thymidine 30 min before it was killed. Proximal and distal colon were analyzed for incorporation of radioactivity (DNA synthesis). Results are reported as counts per minute per microgram of DNA and were analyzed using analysis of variance and the t test. Significant daily variation was found in proximal colon, both from control and operated rats. Rhythms were still present in colon distal to loop colostomy but were lost in the distal stump in rats that received an end colostomy. The mammalian "biological clock" regulates most circadian rhythms by neurohumoral mechanisms; however, our results suggest that the intrinsic enteric nervous system is important in the propagation of a signal that causes rhythmic cell proliferation in the gut in the intact animal.