Figure 10. Prepulse-dependent shift of G(V). (A and B) Ionic currents through spHCN channels in response to voltage steps in 10-mV increments between +50 mV and −100 mV, from V_{H} = −10 mV with (A) or without (B) a 50-ms prepulse to −100 mV. Tail currents at +50 mV. 100-K bath solution. The relatively large “leak” currents seen in A and B are a property of spHCN channels (compare recording in Fig. 4, which is the result of a subtraction of identical recordings executed on the same cell before and after an application of the HCN-channel blocker ZD7288; even in these ZD7288-subtracted currents, there was an instantaneous current component when the voltage was stepped from a 0-mV holding potential). (C) G(V) curves for spHCN after prepulses with durations of 0 (□), 10 (○), 25 (▵), 50 (▿), and 100 (⋄) ms, measured from an instantaneous tail current at +50 mV. Fitted to G(V) = A + B/(1 + exp(−ze_{0}(V − V_{1/2})/kT). V_{1/2} = −43, −30, −12, −3, +2. z = 1.59, 1.33, 1.37, 1.87, and 2.10. 100-K bath solution. (D) V_{1/2} versus prepulse duration from C, fitted with a single exponential with τ = 24 ms. (E–H) Computer simulations of G(V) shifts for the models in Fig. 5. Holding potential is 0 mV followed by prepulses to −100 mV for 0, 75, 150, 225, 300, and 375 ms (from left to right), followed by equilibrium steps of 200 ms to the voltages indicated on the x axis. Compared with the other models, the four-state model exhibits the largest and the slowest shifts. (I) G(V) shifts for the four-state (open symbols) and the two-state (closed symbols) models for different prepulse potentials (circle, −180 mV; triangle, −140 mV; square, −100 mV; diamond, −60 mV). The continuous curves are best fits of single exponentials through origin. (J) Time constants from I. Note that the time constants from the four-state model, in contrast to the two-state model, levels out as was found in the experiments.