Comparison of the Portal Absorption of Protein and Small Peptides in the Fed and Fasted Rat

Experiments were designed to determine the effect of fasting on the intestinal absorption of proteins and small molecular weight peptides. Studies were performed to test the effects of fasting on the uptake of radioactivity by the tissues of the rat. Techniques were developed for the in vivo catheterization of the rat's portal vein. Fed animals were allowed stock diet ad libitum until just before administration of a test suspension. Fasting involved the withdrawal of feed for 15 hours. In the first group of experiments, animals received, by stomach intubation, 10 µCi of 14C-Chlorella protein and 250 mg of Chlorella carrier protein. The portal blood was collected continuously for 60 minutes and then analyzed for radioactivity. In a second group of experiments, rats were administered a suspension containing 7.5 to 60 µCi of 14C-Chlorella protein and 0 to 1,000 mg of recrystallized (5×) egg albumin. The animals were killed at 10, 20, 30, 60, 120, 240 minutes or 24 hours after administration of the test suspension. Radioactivity was measured in portal blood, liver, systemic blood, muscle, adrenal and spleen. In the last group of experiments, 2 ml of saline containing 2 µCi of 14C-glycylglycine·HCl and 2 mg of carrier glycylglycine were infused via duodenum to fed and fasted animals. The appearance of radioactivity in the portal vein blood was determined. The data indicate that there is a significant increase in the absorptive capacity of the small intestine following an overnight fast and an enhancement of tissue uptake of amino acids. The increased concentration of protein in the oral suspension did not reduce the percentage absorption of amino acids.