Kinetics of L-glutamate influx in single barnacle muscle fibers under 0-trans conditions

The dependence of L-glutamate influx on extracellular Na and L-glutamate concentrations was determined using internally dialyzed single muscle fibers of Balanus nubilus. Internal Na and glutamate concentrations were held at zero, and the cell membrane potential was constant. Flux activation curves for external glutamate were measured for five different external Na concentrations, and flux activation curves for external Na were measured independently for three different external glutamate concentrations. An analysis of alternative kinetic models for the transporter mechanism was made and led to the conclusions that under 0-trans conditions the Na:glutamate stoichiometry is 1:1, that glutamate first binding to the external transporter binding site is the preferred order under most extracellular conditions, and that the Na:glutamate coupling is too tight to permit measurable Na-independent glutamate uptake by the transporter.