Cross-linking Cd²⁺ by 106C in TM1 and 337C in TM6. (A and B) Reversible inhibition of whole-cell 106C (A) and 337C (B) currents by 1 mM of extracellular Cd²⁺. Note that the effect of CFTR inhibitor 172 on 337C is much weaker than that seen with 106C (A). (C) Inhibition of whole-cell 106C/337C current by external Cd²⁺ and DTT-dependent recovery. The red rectangle encloses a part of the recording where current decays by itself when DTT is not present, suggesting possible spontaneous formation of a disulfide bond between 106C and 337C (see Results for details). (D) Averaged current inhibition ratios by external Cd²⁺ for 106C, 337C, and 106C/337C. Mean ± SEM is shown.