A, left panel, zinc concentration-inhibition relationships constructed at external pH 5.4, 7.4 and 9.4, and fitted according to the antagonist inhibition model and IC50 values determined (; n = 9). A, right panel, GABA equilibrium concentration-response curves constructed in the absence and presence of 10 and 100 μm Zn2+ at pH 7.4. B, GABA equilibrium concentration-response curves constructed in the absence and presence of 50 μm zinc (left, pH 5.4) and 10 μm zinc (right, pH 9.4). The EC50 values and Hill coefficients for GABA were determined from the logistic model ( and ; n = 20).