[The secretion of radioactively labeled amino acids into the digestive tract. 3. 14C-labeling of the intestinal wall after subcutaneous injection of 14C-L-leucine in rats].

Experimental rats allotted in 4 different groups received 14C leucine injections at 4 different levels of specific 14C activity. Groups A and B were injected a leucine dosis corresponding to the daily requirement, the specific 14C activity being high (A) and low (B). Groups C and D were injected a leucine dosis of one ninth the amount applied to groups A and B, the level of 14C labelling being again high (C) and low (D). The 14C labelling of the TCE-soluble fraction of the 1st third section of the small intestinal wall in all 4 groups proved identical with the course of the curve representing the TCE-soluble fraction of the small intestine contents. The specific 14C leucine activity of the intestinal wall of the 1st third section of the small intestine was found to be 5 to 8 times lower than in the TCE-soluble fraction of the pancreatic gland. Thus a dilution effect has been produced from absorption. Two minutes after injection, the 14C labelling of the intestinal wall proteins had half the level of labelling of the pancreatic proteins. This finding applied to both the 1st and the 3rd sections of the small intestine. The results obtained allow to conclude that 14C labelling of intestinal wall proteins is relatively independent of 14C leucine absorption and is conditioned by the level of labelling of the plasma leucine.