The External K + Concentration and Mutations in the Outer Pore Mouth Affect the Inhibition of Kv1.5 Current by Ni 2+

Abstract

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 Ni 2+ is similar to that for proton block. Ni 2+ 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, Ni 2+ had no effect on gating currents. We propose that, as with H o +, Ni 2+ 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, Ni 2+ is much less potent, indicating incomplete overlap of the profiles of these two metal ions. Analyses with Co 2+ and Mn 2+, 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 Zn 2+ ( K D ∼ 0.07 mM) ≥ Ni 2+ ( K D ∼ 0.15 mM) > Co 2+ ( K D ∼ 1.4 mM) > Mn 2+ ( K D > 10 mM).