Figure 8. The 2D plot of the ΔGEAAT1(influx) = 0 plane from the 3D plot analogous to Figure 4.
The (in this case) astrocytic glutamate− concentration, [glutamatei−], the membrane potential, Em,oi, and Po were fixed at 1.4 mM (the Km value for glutamine synthetase), −91 mV, and 1 atm, respectively (T = 310 K). The surface is colored green when influx is thermodynamically possible and red when it is impossible. Thus, the intersection of the ΔG surface with the ΔG = 0 plane traces the trajectory of the (Pi/Po)-dependence of the minimum synaptic glutamate− concentration, [glutamateo−]min, value required for astrocyte uptake. The EC50 value of the iGlutR receptor for synaptic glutamateo− (2.3 μM) is indicated with a horizontal dashed line. It is clear glutamateo− is well-cleared from the synapse when Pi is small, and not when Pi is only a few percentage points greater. In the latter case, the receptors would be saturated, and synaptic transmission would be interrupted.