Quantification of relative cation/chloride permeability. Current–voltage relationships of MTSES⁻-modified native (A) and F81C (B) currents and unmodified F81E currents (C) in response to different external [CsCl]. Whole cell currents were activated under isosmotic condition (304 mosmol kg⁻¹) with high Ca²⁺ in the pipet and symmetrical CsCl in the bath and pipet (150 mM). The extracellular solution was replaced by solutions containing different [CsCl] as indicated. (D) Changes in E_rev (ΔE_rev) as a function of extracellular salt concentration. ΔE_rev is E_rev at the indicated salt concentration minus the E_rev with 150 mM extracellular salt. Salt is either CsCl or NaCl as indicated. Each data point represents the mean E_rev ± SEM of two to nine cells. Dashed lines were calculated from the GHK equation (ΔE_rev = 25.7 · ln [([X⁺]_o + [Cl⁻]_i · P_Cl/P_X) / ([X⁺]_i + [Cl⁻]_o · P_Cl/P_X)]), assuming that the channel is exclusively permeable to Cl⁻ (P_X/P_Cl = 0) or to the cation X⁺ (P_Cl/P_X = 0). Filled symbols: CsCl solutions; •, MTSES⁻-treated native dBest1 (n = 2–7); ▴, MTSES⁻-modified F81C (n = 4–9); ▪, F81E (n = 2–5). Open symbols: NaCl solutions; ○, MTSES⁻-treated native dBest1 (n = 3–5); ▵, F81C (n = 3–8).