Figure 9.

Permeant ion dependence of constitutive conduction for P475Q. (A) Families of ionic currents for Shaker P475Q with either 50 mM Rb⁺ or 50 mM K⁺ as the external permeant ion. The recordings for external Rb⁺ and K⁺ are from different cells. In each case the membrane was held at −100 mV and stepped to −150 mV for 200 ms before the traces shown and tail voltage was −150 mV. Depolarizations were from −150 to −20 mV in 10-mV steps for both left and right families. For these families both linear capacity and background conductances were identified and subtracted by blocking the Shaker channel with Agitoxin-2. Horizontal dashed lines correspond to the zero current level. Middle two traces are uncorrected currents elicited by depolarization to −50 mV in either control or Agitoxin-2 with K⁺ as the external permeant ion. (B) G-V relations obtained by plotting normalized tail current amplitudes against test voltage for Wt and P475Q in external Rb⁺ and K⁺. Smooth curves are single Boltzmann fits to the data with parameters as follows: Wt (Rb⁺ external), V₅₀ = −27.3 mV, z = 3.6; P475Q (Rb⁺ external), V₅₀ = −96.7 mV, z = 2.6; P475Q (K⁺ external), V₅₀ = −80.5 mV, z = 2.4.