Figure 7.

Cs⁺ and NH₄ ⁺ selectivity of cGMP-gated channels in rod photoreceptors is independent of cGMP concentration. (Top left) I–V curves activated with a cGMP concentration ramp in the presence of biionic solutions: Cs⁺ (150)/Na⁺ (150). The cGMP concentration at the moment each I–V curve was measured is given next to the curve. Also shown is the I–V curve activated by 300 μM cGMP in the same patch in the presence of symmetric Na⁺ solutions. (Top right) I–V curves measured with a cGMP concentration ramp in the presence of biionic NH₄ ⁺ (150)/Na⁺ (150) solutions. The cGMP concentration at the moment each I–V curve was measured is given next to the curve. Also shown is the I–V curve activated by 300 μM cGMP in the same patch in the presence of symmetric Na⁺ solutions. (Bottom left) Reversal potential as a function of [cGMP] measured in the biionic Cs⁺/Na⁺ solutions shown on top. The dashed line is the mean of these values, 18.6 mV, which yields PCs/PNa = 0.49. (Bottom right) Reversal potential as a function of [cGMP] measured in the biionic NH₄ ⁺/Na⁺ solutions shown on top. The dashed line is the mean of these values, −20.8 mV, which yields PNH₄/PNa = 2.31.