FIGURE 2.

Increasing [K⁺]_o changes the concentration dependence of the block of Kv1.5 by Ni²⁺. (A) Representative traces obtained from three different cells showing, superimposed, the currents evoked in the K_o⁺ concentrations indicated either without (−) or with (+) the Ni²⁺ concentration indicated. The voltage protocol consisted of a 300-ms step from −80 mV to 50 mV followed by a step to −40 mV. Increasing [K⁺]_o necessitates a higher concentration of Ni²⁺ to produce roughly the same degree of block. The time calibration is the same for the three sets of traces. (B) The concentration-response relationship for Ni²⁺ in 0, 3.5, and 140 mM K_o⁺ shows that increasing [K⁺]_o from nominally K⁺-free to 3.5 mM shifted the K_D from 0.15 ± 0.01 mM to 0.44 ± 0.02 mM. Increasing K_o⁺ to 140 mM shifted the K_D for the Ni²⁺ block to 3.1 ± 0.3 mM. Each point represents the mean ± SE of measurements from three to seven cells.