Effects of Thr¹⁵ substitution on the pattern of current and permeability to NMDG⁺ schematic models. (A) Two patterns of currents: biphasic (left) and high amplitude monophasic (right). The biphasic response reflects three steps in gating: closed (c), opened (o), and dilated (d), and is observed in cells expressing WT receptors and T15A, T15V, and T15S mutants. The high amplitude monophasic response reflects instantaneous transition from the closed to the dilated stage, and is observed in cells expressing T15W, T15E, and T15K mutants. (B) A shift in the reversal potential (RP) for WT and mutant receptors during sustained agonist application. Mean values for the reversal potential at the end of a 40-s recording for WT and six T15 mutant receptors were subtracted from the initial reversal potential value for the WT receptor (−36.5 mV; Table I, NMDG⁺-KR column). The side chains of amino acid residues used in substitution studies are shown at the bottom.