Analysis of two chloride requirements for sodium-dependent amino acid and glucose transport by intestinal brush-border membrane vesicles of fish

Abstract

Intestinal brush border vesicles of a Mediterranean sea fish ( Dicentrarchus labrax) were prepared using the Ca 2+-sedimentation method. The transport of glucose, glycine and 2-aminoisobutyric acid is energized by an Na + gradient ( out > in ). In addition, amino acid uptake requires Cl − in the extravesicular medium (2-aminoisobutyric acid more than glycine). This Na +- and Cl −-dependent uptake is electrogenic, since it can be stimulated by negative charges inside the vesicles. The specific Cl − requirement of glycine and 2-aminoisobutyric acid transport is markedly influenced by pH, a change from 6.5 to 8.4 reducing the role played by Cl −. In the presence of Cl −, the K m of 2-aminoisobutyric acid uptake is reduced and its V max is enhanced. Cl − affects also a non-saturable Na +-dependent component of this amino acid uptake. Amino acid transport is also increased by intravesicular Cl − (2-aminoisobutyric acid less than glycine). This effect is more concerned with glucose uptake, which can be then multiplied by 2.3. A concentration gradient ( in > out ) as well as the presence of Na + in the incubation medium seems to enter into this requirement. This intravesicular Cl − effect is not influenced by pH between 6.5 and 8.4.