Fig. 2. The voltage dependent behavior of a single VDAC channel in the presence of a salt gradient. The conditions were as in Fig. 1 except that one side of the membrane contained 10 times less KCl. The indicated voltage refers to the value on the high-salt side. The recorded current was converted to a net ion flux. The arrow indicates the point in time at which a single channel inserted. When the voltage was raised to 40 mV, channel closure caused the current to decline. The different current levels indicate different conducting states of this one channel. Four different low-conductance (closed) states are visible. These states may vary in either conductance and/or selectivity. Reproduced with permission from Colombini (1980b).