FIGURE 4.

Dilution potentials in NaNO₃ solutions and biionic NaCl:NaNO₃ solutions to determine relative P_NO3/P_Na and P_NO3/P_Cl values, respectively, for the WT α1 GlyR channel. (A) Sample whole cell measurements of I/V curves in different dilutions of NaNO₃ solutions to determine P_NO3/P_Na. The experimental procedures are identical to those described for Fig. 1 and the solution composition is given in Materials and Methods. As in Figs. 1 and 2, the ΔV_rev values for the 0.5 and 0.25 dilution solutions are both positive, indicating that the WT GlyR channel is still anion-selective when replaces Cl⁻. (B) Semilogarithmic plots of the averaged shifts in reversal potential versus external activity (on a log scale). The solid lines show the fits of the GHK equation (Eq. 3) to these ΔV_rev values, as used to determine P_NO3/P_Na, averaged from individual experiments and shown in panel A. Again, as in Fig. 3, the dashed line indicates the ΔV_rev values expected if the counterion permeability had been zero. (C and D) Sample I/V curves in symmetrical NaCl solutions (C), before (solid circles) and after (open circles) measurements of biionic potentials in NaCl:NaNO₃ solutions (D). Both panels A and B are for the same cell, and panels C and D are for the same cell (though for a different one from that in panels A and B). The insets with the I/V curves show representative current traces in response to voltage steps (from 0 mV to −30 mV) (lower traces) through to +30 mV (top traces) in increments of 10 mV. The lower sets of current traces in each panel show control responses in the absence of glycine.