Dissipation of amino acid concentration gradients within rabbit ileal enterocytes

The distribution of alanine, lysine and methionine within the cytoplasm of functionally mature enterocytes in rabbit ileum was measured by autoradiography after a short period of contact with tritiated substrate. Pronounced intracellular concentration gradients were noted for alanine and lysine, the concentration of these amino acids in the apical part of the enterocyte being 2- to 3-times that found near the base of the cell. No such concentration gradient was seen for methionine. Subsequent superfusion of the mucosal surface of the tissue with substrate-free medium caused intracellular concentration gradients for alanine and lysine to disappear. There was also a decrease in the enterocyte content of all three amino acids unassociated with backflux into the intestinal lumen. The ease with which intracellular concentration gradients for alanine and lysine can be manipulated is used as an argument against the possibility that their creation results from selective attachment to cytoplasmic structures in the apical part of the enterocyte.