The complete rate equation, including the explicit dependence on Na+ ions, for the influx of α-aminoisobutyric acid into mouse brain slices

Abstract

The rate equation, including dependence on Na⁺-ion concentration for the influx of α-aminoisobutyric acid into mouse brain slices incubated in isotonic glucose medium at 37°C, isv=0.402S/{1.02(1+788/[Na⁺]²)+S}+0.0477S, wherev=influx in μmol/min, g final wet wt of slices; [Na⁺]=concentration of Na⁺ ions in medium, inmm; andS=concentration of α-aminoisobutyric acid in medium, inmm. This equation shows two kinetically independent, parallel pathways of concentrative uptake: one, saturable and dependent on Na⁺; the other, unsaturable and independent of Na⁺. Influx is independent of ionic strength, Cl⁻ ionper se, and a moderate increase in tonicity. The binding of substrate to the saturable carrier depends on the Na⁺ concentration; the maximum capacity of this carrier does not. For transport, 2 Na⁺ ions must interact with each saturable transport site. This does not imply coupling between the flux of Na⁺ and the flux of α-aminoisobutyric acid.