Cadmium locks A43C unapposed hemichannels in a closed state. (A) Macroscopic currents elicited from an oocyte expressing A43C unapposed hemichannels with the voltage paradigm, steps from −90 to 0 mV, shown at the top of the panel. The central bar indicates the time and duration for which the bath solution containing 100 mM Cs-MES, 1.8 mM CaCl₂, and 10 mM HEPES, pH 7.6, was exchanged with the same bath solution except containing either 20 µM DTT or 10 µM CdCl₂. Currents were decreased to ∼85% of maximum levels after treatment with 10 µM CdCl₂. The reduction in current could only be reversed after a second exposure to 20 µM DTT. (B) 10 µM CdCl₂ was applied to the channel after a long-duration hyperpolarizing step that would favor closure of both loop and V_j gates. After wash with Cs-MES bath solution, the extent of current reduction was assessed by a series of polarizing steps between −90 and 50 mV. Currents were reduced by ∼80% by Cd²⁺ treatment when the channels resided in a closed state. Currents could only be recovered fully after exposure to 20 µM DTT. (C) Macroscopic currents elicited from an oocyte expressing A43C unapposed hemichannels with the voltage paradigm, steps between 0 and 30 mV, which strongly favors population of the open-channel state. Application of 10 µM CdCl₂ had no effect on the level of A43C current, indicating that A43C residues do not coordinate Cd²⁺ when the channel resides in the open state.