Influence of [K ⁺ ] _o on KCNQ1 channels in the absence and presence of KCNE1–3 auxiliary subunits indicated as E1, E2, and E3. (A) Sample current traces of KCNQ1 channels measured at five different [K⁺]_o as indicated. Dashed lines represent 0 current. Oocytes were held at −100 mV and depolarizing pulses of 10 s duration were applied with 120-s interpulse interval. (B) Mean maximal current amplitudes at five different concentrations of K⁺_o normalized to the first point obtained in 0.2 mM K⁺_o (Imax/Imax_0.2); n = 4–9, error bars represent ±SEM. (C) Concentration-dependency of K⁺_o inhibition for heteromeric KCNQ1 channels. Data shown in B were corrected for theoretical (Ith) values [(Imax/Imax_0.2) − Ith] according to the GHK flux equation, assuming oocyte [K⁺]_in of 108 mM (Weber, 1999). Solid lines represent fits of the Hill equation to the data. Mean parameters of the fits are shown in Table 1. (D) Inhibition of peak current of the KCNQ1/KCNE3 heteromeric channels injected at 1:0.5 and 1:1 α to β molecular ratio upon elevation of [K⁺]_o from 0.2 to 100 mM. Corresponding sample traces are shown in A (lower right); n = 4–5, error bars represent ±SEM.