Figure 3. Single-channel currents and the activation properties of αA433 AChR.

A, single-channel currents activated by various concentrations of ACh (filtered at 3 kHz for display). The membrane potential was -70 mV; inward current is down. To the right of the current traces are the corresponding intracluster closed and open duration histograms. Continuous curves are global fits (rate constants shown in Table 1). B, single-channel dose-response curve. The probability of the channel being open within a cluster (P_o) is plotted as a function of agonist concentration. The continuous curve is fitted to data by eqn (2) (K_d= 161 ± 44 μM, θ= 17·5 ± 7, K_g fixed at 0·05). C, effective opening rate (β‘) curve. The continuous curve is fitted to the data by eqn (1). From the high concentration limit, we estimate that β= 58 700 ± 17 420 s⁻¹ (n= 1·51 ± 0·06, EC₅₀= 650 ± 194 μM). The data are from 7 patches (one patch at each concentration), 138 clusters, 46 774 intervals. The inset shows a closed interval duration histogram ((normalized counts per bin)^½vs. time (ms)) from a patch at 1 mM ACh; the arrow indicates the ∼1 ms shut state component that corresponds to state C₆ in Model 1.