Figure 11.

Cs⁺ and NH₄ ⁺ selectivity is not a function of cGMP in recombinant bovine rod α or αβ cGMP-gated channels. Panels illustrate I–V curves measured using cGMP concentration ramps in the presence of biionic solutions of either Cs⁺ (150 mM)/Na⁺ (150 mM) (top) or NH₄ ⁺ (150 mM)/Na⁺ (150 mM) (middle). (Left, top and middle) Currents measured in α channels. (Right, top and middle) Currents measured in αβ channels. Reversal potentials as a function of [cGMP] for Cs⁺ (bottom left) or NH₄ ⁺ (bottom right) measured in α (▴) and αβ (•) channels. Open symbols are data measured with steps of cGMP, while filled symbols are data measured with a cGMP concentration ramp in the same membrane. The average reversal potentials are, for α channels: Cs⁺ = 28.3 mV; NH₄ ⁺ = −31.8 mV, which correspond to PCs/PNa = 0.32 and PNH₄/ PNa = 3.59. For αβ channels: Cs⁺ = 22.5 mV and NH₄ ⁺ = −24.5 mV, which correspond to PCs/PNa = 0.40 and PNH₄/ PNa = 2.68.