Chloride-dependence of amino acid transport in rabbit ileum

Abstract

Chloride-dependence of influx across the brush-border membrane of distal rabbit ileum was examined for β-alanine, 2-methylaminoisobutyric acid (MeAIB), leucine, lysine, proline and d-glucose. Influx of leucine at 2 mM and of d-glucose at 0.5 mM was chloride-independent indicating that substitution of isethionate for chloride has no unspecific effect on sodium gradient driven transport processes. In contrast influx of β-alanine and MeAIB was totally dependent on the presence of chloride ions. In the absence of chloride, proline transport was reduced to 20% of its control level. This remaining transport can be accounted for by the function of the carrier of α-amino-monocarboxylic acids. Transport of leucine at 0.1 mM was reduced by absence of chloride. This is in accordance with the observation of leucine transport by the β-alanine carrier. The kinetics of chloride and sodium activation of transport of MeAIB were examined at 1 mM MeAIB. Chloride activation was characterized by a Hill coefficient of 1 and a K 1 2 of 23.5 mM, and sodium activation by a Hill coefficient of 2 and a K 1 2 of 51 mM. Thus cotransport of chloride with an imino acid would be compatible with the known rheogenic nature of this transport. This study adds the imino acid carrier and the β-alanine carrier to the group of chloride-dependent, epithelial amino acid transport systems.