Summary of the effects of Cd²⁺ on spHCN channels containing introduced cysteine residues. (A) The lock-open effects of 1 µM Cd²⁺, as indicated by the proportion of the nondecaying tail current, in the single C-linker controls (left-most panel) and the double mutants containing one of the F359C, V361C, or A364C mutations are shown. Gray bars indicate values for control conditions, and green bars indicate values after the addition of Cd²⁺. The mutants giving rise to significant lock-open effects are clustered in two separate regions. (B) The lock-closed effects of 1 µM Cd²⁺, as indicated by the prolongation of the rise time between 10 and 50% of the maximum current in a 500-ms pulse, in the single C-linker controls (left-most panel) and the double mutants containing F359C, V361C, or A364C mutations are shown. Gray bars indicate values for control conditions, and red bars indicate values after the addition of Cd²⁺. (C) A summary of the effects of 1 µM Cd²⁺ on the double mutants tested in this study.