Glutamate substitutions do not alter the apparent pore size. (A–F) Reversal potentials were measured after substitution with anions (open symbols) and cations (closed symbols) of different diameters. The dashed and dotted lines indicate the calculated reversal potential shift for an impermeant anion or cation, respectively, and were based on the determined relative permeabilities for that particular mutant. For the wild-type receptor, the reversal potential shift approaches its maximum value for anions between 4 and 6 Å. Using this approximation, the data indicate that for all cases, the pore size was between 4 and 6 Å. The cations and their diameters (in Å) were as follows: cations, sodium, 1.9; rubidium, 3.0; cesium, 3.4; imadazole, 4.8; choline, 5.6; triethanolamine, 7.0; N-methyl-d-glucamine, 9.0; anions, bicarbonate, 4.1; acetate, 4.5; propionate, 5.1; gluconate, 6.9; isethionate, 7.0. Under the particular mutation, we provide the difference in side chain volume imparted by the substitution.