Block of the outward NMDG⁺ current by internal Mg²⁺. (A) A current recording from a cell stably expressing Kv3.1 channels with symmetrical 140 mM NMDG⁺, 1 mM Mg²⁺ solutions. The double-pulse protocol used to elicit the currents in A and B is shown at the top of A. The cells were held at −100 mV. A depolarizing pulse to +60 mV was interrupted by a short repolarizing pulse to −60 mV to assess the inward currents. (B) A current recording obtained with similar experimental conditions as in A except for the absence of internal Mg²⁺. (C) A current recording obtained with similar experimental conditions as in A. The double-pulse protocol used to elicit these currents is shown at the top of C. The cells were held at −100 mV, and then depolarized to +150 mV, interrupted by a short repolarizing pulse to −150 mV to assess the inward currents. (D) The averaged ratio of the outward current over the inward current in the absence or presence of internal Mg²⁺. The amplitude of the outward and inward currents was measured at the end of the prepulse and the beginning of the repolarizing pulse as indicated by arrows in A. The data are from four to five cells. *, P < 0.01; **, P < 0.001.