Distribution of transported amino acid within rabbit ileal mucosa.

1. Pieces of rabbit distal ileum, incubated for short periods of time in solutions containing tritiated amino acids, have been processed for autoradiography and the profiles of amino acid concentration across the villi determined by microdensitometry. 2. The concentration profiles of a series of amino acids could be described in terms of two descending exponentials, one extending from the brush border to the basal membrane of the enterocyte and the other from the base of the epithelial layer to the centre of the villus. 3. Exponential coefficients describing the steepness of these gradients were highest for basic amino acids. Coefficients for short-chain amino acids were greater than for long-chain neutral amino acids. None of these values changed for times of incubation varying from 5 to 180 sec. 4. Enterocytes accumulated amino acids in the apical cytoplasm, against a concentration gradient, within the first few seconds of incubation. This step-up in concentration decreased as the external concentration was increased, in a manner dependent on the amino acid used. 5. It is suggested that amino acid concentration gradients within enterocytes arise by diffusion and that the amino acid specificity of this process originates from an ability of the more lipophilic amino acids to permeate structures acting as barriers to the more hydrophilic molecules.