Figure 6.

External Ca²⁺ and Mg²⁺ blockage in wild-type and mutant channels. cGMP-gated currents were measured in outside-out patches in the presence of the indicated concentrations of Ca²⁺ (A) or Mg²⁺ (B) in the extracellular solution. Currents were activated by 100 μM cGMP in the patch pipette. Voltage ramps were from −80 to +80 mV. Currents from the experiments shown in A and B in the presence of various Ca²⁺ or Mg²⁺ concentrations were measured at +80 or −80 mV, normalized to the current measured in the absence of divalent cations at the same voltage and plotted as a function of external Ca²⁺ (C–D) or Mg²⁺ (E–F) concentrations. Continuous lines were the best fit of to the data with the following values. (C) At +80 mV, wild type (♦): K _i,Ca = 380 μM, n = 1; E340D (□): K _i,Ca = 16 μM, n = 1; E340G (▴): K _i,Ca = 21 mM, n = 0.8; E340N (○): K _i,Ca = 28 mM, n = 0.9. (D) At −80 mV, wild type (♦): K _i,Ca = 42 μM, n = 0.8; E340D (□): K _i,Ca = 70 μM, n = 0.9; E340G (▴): K _i,Ca = 3 mM, n = 0.6; E340N (○): K _i,Ca = 6.3 mM, n = 0.6. (E) At +80 mV, wild type (♦): K _i,Mg = 650 μM, n = 0.7; E340D (□): K _i,Mg = 112 μM, n = 0.7; E340G (▴): K _i,Mg = 13 mM, n = 0.8; E340N (○): K _i,Mg = 51 mM, n = 0.7. (F) At −80 mV, wild type (♦): K _i,Mg = 90 μM, n = 1.1; E340D (□): K _i,Mg = 48 μM, n = 0.8; E340G (▴): K _i,Mg = 1.4 mM, n = 0.8; E340N (○): K _i,Mg = 26 mM, n = 0.6.