Figure 6.

Sr²⁺ selectivity of the cGMP-gated channels in rods is a function of cGMP concentration. (Top) I–V curves measured with a cGMP concentration ramp in the presence of symmetric Na⁺ (150 mM) with 20 mM Sr²⁺ added to the cytoplasmic surface of the membrane. The cGMP concentration at the moment each I–V curve was measured is given next to the curve. (Bottom left) Reversal potential and membrane conductance as a function of [cGMP]. (•) Reversal potentials measured in data in top, (○) normalized current amplitude (plotted as scaled, 1 − I/I_max) measured in the same patch at +15 mV with steps of cGMP concentration (5, 10, 20, 40, 300 μM). The continuous line is the modified Hill equation (Eq. 4) optimally fit to the data. The values of the adjustable parameters are K _1/2 = 11.0 μM, n = 2.1, min = −1.38 mV, and max = −5.33 mV. (Bottom right) PSr/ PNa calculated from the reversal potentials on left as a function of [cGMP]. The continuous line is the same Hill function as on the left, but with PSr/PNa_min = 0.47 and PSr/PNa_max = 2.71.