Fig. 5. The dependence of the channel selectivity on the salt gradient across the membrane. Ion selectivity was quantifies as the zero-current potential (reversal potential) in the presence of a KCl salt gradient. The low-salt side was always at a KCl activity of 0.06 while the high-salt side was varied as indicated. The lines represent the different theoretical predictions of different theories: GHK is Goldman/Hodgkin/Katz; LCT (arithmetic) and LCT (harmonic) are two versions of the large channel theory. Reproduced with permission from Zambrowicz and Colombini (1993).