[Secretion of radioactively labeled amino acids into the digestive tract. 2. 14C labeling and amount of 14C leucine in the intestinal contents after subcutaneous injection of 14C leucine in rats].

Experimental rats received a subcutaneous injection of 14C of varying activity. The rats were then killed in groups within a period of from 2 mins to 30 mins after the injection. A certain amount of 14C activity was detected in the intestinal contents as early as 2 mins after the injection, both as free 14C leucine and in the TCE soluble portion of the intestinal contents (TCE=trichloroacetic acid). A comparison of the degree of labelling in the TCE soluble and the TCE precipitable fractions of the intestinal contents showed that the most likely way for free 14C leucine to get into the lumen of the intestine is via the pancreatic juice while that for protein-bound 14C leucine is through the proteins in the pancreatic gland. The degree of labelling in both fractions decreased very steeply from the first to the third third of the intestine. This may be caused either by absorption of the secreted 14C leucine or may be brought about by a delay in food passage through the intestine.