Figure 3.

A representation and review of P2X2 data from Clyne et al. (2003). I_max and pEC₅₀ from (a) native P2X2 receptors shown in Fig. 3B of Clyne et al.¹² and (b) C-terminal 76 amino acid deleted P2X2 receptors shown in Fig. 4A of Clyne et al.¹². Values were estimated by visual inspection and represented in a similar fashion to Fig. 1a in this report. (c) A simulation of the predicted response (fitted to the 3PL equation) resulting from two distinct populations of channels each with similar expression levels, Hill coefficients of 2.4, but with different EC₅₀ values equal to 6.6 µM (red) and 37 µM (blue). The mixed population of channels has an intermediate EC₅₀ of 15.6 µM and an apparently lower Hill coefficient of approximately 1.4. (d) Predicted change in apparent Hill coefficient (n_H) and EC₅₀ for a mixed population of the two channel subtypes defined in Panel (c), as the proportion of those subtypes changes from 0 to 100%. (e) A representation of Fig. 3B from Clyne et al.¹² with the x axis shown on a log scale. The line is the hyperbolic model fitted by Clyne et al.¹² indicating a high EC₅₀ of 37 µM, a low EC₅₀ of 6.6 µM and a mid-point I_max of 6.108 µA, as described in their manuscript. (f) An equivalent representation of Fig. 4A from Clyne et al.¹². The line is the fitted hyperbolic model indicating a high EC₅₀ of 45.1 µM, a low EC₅₀ of 4.8 µM and a mid-point I_max of 0.637 µA. Panels (e,f) illustrate how the parameter estimates lie in relation to the data. Note that Clyne et al.¹² constrained the lower EC₅₀ values to be no smaller than the lowest measured EC₅₀ and consequently the low EC₅₀ limits (dashed lines) pass through the lowest value data points.