Concentrative and reversible character of intestinal amino acid transport

Abstract

Either α-aminoisobutyric acid-1-C14 or 1-aminocyclopentanecarboxylic acid-1-C14 was injected into a rat and, after at least 24 hr, a suitable dilution of the same solution of the amino acid was introduced into an intestinal loop in situ and the change in its distribution observed by radioisotope counting. The two amino acids could be concentrated from the small intestine to establish steady-state distribution ratios of from 12 to 100, for the plasma level with respect to the level in the lumen. These values are far higher than the distribution ratios produced across excised intestine. These steady-state values were approached from either above or below, by absorption or release of the amino acid. The release was accelerated by the presence of the same or another amino acid of high transport affinity in fluid perfusing the intestinal loop, an action that tends to identify the outward migration from lumen to plasma with a transport process. The test amino acids were released into the colon also, although at slower rates, the steady-state distribution ratio lying in the neighborhood of unity. This release appears also to be a specific process, and to account for the slow fecal excretion of these difficultly metabolized amino acids.