Circadian rhythm in the ornithine decarboxylase activity of rat small intestine.

Intestinal ornithine decarboxylase [L-ornithine carboxy-lyase, EC 4.1.1.17] activity was found to show a marked circadian rhythm with a peak 4 h after the start of eating in rats on a diet containing protein. In rats with an intestinal blind loop, the enzyme was induced in the portion of the intestine that came in contact with the protein meal, but not in the blind loop. Injection of tetragastrin or CCK-PZ alone had no effect on enzyme induction, but when a protein suspension was introduced into a tied loop of intestine soon after the injection of tetragastrin or CCK-PZ, the enzyme was induced in the segment to almost the same extent as in the intestines of normal rats eating a protein meal. These results suggest that the circadian rhythm in activity of intestinal ornithine decarboxylase is initiated by release of gastrin, or CCK-PZ, or both, and contact of protein with the small intestine after the intake of food containing protein.