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

The rate equation, including dependence on Na+-ion concentration for the influx of alpha-aminoisobutyric acid into mouse brain slices incubated in isotonic glucose medium at 37 degrees C, is v = 0.402 S/(1.02(1 + 788/[Na+]2)+S)+0.0477S, where v = influx in mu mol/min, g final wet wt of slices; [Na+] = concentbutyric acid in medium, in mM. 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- ion per 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 alpha-aminoisobutyric acid.