Differences in BQ⁺ action when applied to the cytoplasmic and the extracellular sides. Recordings of a single Cx50 hemichannel with BQ⁺ added to the cytoplasmic side (A) and the extracellular side (B) obtained from separate patches are shown. Membrane potential was held constant at −30 mV in A and −50 mV in B. Regardless of the side of application, BQ⁺ increased loop-gating transitions. All transitions between the open and closed state are slow in kinetics, typical of loop gating, as shown in the expanded views of the current traces recorded at 500 µM BQ⁺ applied to the cytoplasmic and extracellular side (see boxed regions). The solid black line indicates the segments of the traces that were expanded. Notably, the noisy fluctuations from the closed state (dotted line) that characterized BQ⁺ action when applied to the extracellular side were much less prominent when BQ⁺ was applied to the cytoplasmic side. Currents were filtered at 1 kHz, and data were acquired at 5 kHz.