The external K+ concentration and mutations in the outer pore mouth affect the inhibition of Kv1.5 current by Ni2+

By examining the consequences both of changes of [K+](o) and of point mutations in the outer pore mouth, our goal was to determine if the mechanism of the block of Kv1.5 ionic currents by external Ni2+ is similar to that for proton block. Ni2+ block is inhibited by increasing [K+](o), by mutating a histidine residue in the pore turret (H463Q) or by mutating a residue near the pore mouth (R487V) that is the homolog of Shaker T449. Aside from a slight rightward shift of the Q-V curve, Ni2+ had no effect on gating currents. We propose that, as with H-o(+), Ni2+ binding to H463 facilitates an outer pore inactivation process that is antagonized by K-o(+) and that requires R487. However, whereas H-o(+) substantially accelerates inactivation of residual currents, Ni2+ is much less potent, indicating incomplete overlap of the profiles of these two metal ions. Analyses with Co2+ and Mn2+, together with previous results, indicate that for the first-row transition metals the rank order for the inhibition of Kv1.5 in 0 mM K-o(+) is Zn2+ (K-D similar to 0.07 mM) greater than or equal to Ni2+ (K-D similar to 0.15 mM) > Co2+ (K-D similar to 1.4 mM) > Mn2+ (K-D > 10 mM).