Variation in amino acid transport along the rabbit small intestine. Mutual jejunal carriers of leucine and lysine

The jejuno-ileal variation of amino and imino acid transport across the brush-border membrane of intact rabbit small intestine was studied. For the amino acids tested - β-alanine, leucine, lysine, MeAIB, proline - and for d-glucose, the rates of transport at constant concentrations increase form very low values in the proximal to maximum values in the most distal 30 cm of the ileum. The apparent affinity contants for jejunal taurine transport is identical to that of the distal ileum, while the jejunal transport capacity is less than half. In the jejunum, as in the distal ileum, leucine and lysine share both sodium-dependent and sodium-independent carriers. Approx. 50% of the quantitative difference in transport is accounted for by the absence of the β-alanine carrier in the jejunum. These data indicate that the gradients of transport along the small intestine reflect gradients of transport capacities rather than affinities. In comparison with hamster, man and rat, the rabbit seems unique with respect to the location of transport maximum and the steepness of the gradient along the intestine.