MTSES⁻ increases the cation permeability of F81C currents. Current–voltage relationships from a typical native S2 cell (A) and a S2 cell rescued with F81C (B) before and after MTSES⁻ modification. Currents were activated with Δ40 mosmol kg⁻¹ hyposmotic solutions. The arrow in B points out the reversal potential of F81C current after MTSES⁻ treatment. (C) Changes in E_rev of F81C currents under different ionic conditions. The record begins after the dBest1 current had stabilized under hyposmotic solutions (I340/E300, Δ40 mosmol kg⁻¹). MTSES⁻ was then applied in the bath. Extracellular solution containing symmetrical Cl⁻ and Na⁺ as the major cation (E300) was then replaced by solutions with Cs⁺ or NMDG⁺ as the major cations as indicated. The osmolality of all the extracellular solutions was 300 mosmol kg⁻¹.