A chloride requirement for Na+-dependent amino-acid transport by brush border membrane vesicles isolated from the intestine of a Mediterranean teleost (Boops salpa).

The uptake of D-glucose, 2-aminoisobutyric acid and glycine was studied with intestinal brush border membrane vesicles of a marine herbivorous fish: Boops salpa. The uptake of these three substances is stimulated by an Na+ electrochemical gradient (Cout greater than Cin). For glucose, an increase of the electrical membrane potential generated by a concentration gradient of the liposoluble anion, SCN-, increases the Na+-dependent transport. This responsiveness to the membrane potential was confirmed by valinomycin. Differently from glucose, uptake of glycine and 2-aminoisobutyric acid requires, besides the Na+ gradient, the presence of Cl- on the external side of the vesicles. In the absence of Cl-, amino acid uptake is not stimulated by the Na+ gradient and is not influenced by an electrical membrane potential generated by SCN- gradient (Cout greater than Cin) or by a K+ diffusion potential (Cin greater than Cout). This Cl- requirement differs from the Na+ requirement, since a Cl- gradient (Cout greater than Cin) does not result in an accumulation of glycine or 2-aminoisobutyric acid similar to that produced by an Na+ gradient.