Figure 2

Schild analysis of the concentration response data for the muscarinic antagonists atropine, 4-DAMP, and methoctramine. The Log (DR-1) versus log inhibitor concentration is plotted in this figure. DR is the dose ratio, the ratio of concentrations of ACh giving equal responses (EC50 for this plot) in the absence and presence of test compound. The slope of the lines drawn through the data shown is 1. The X-intercepts of the lines equal pA₂values, which are equal to the -log K_i values, where K_i is the affinity of the antagonist. Schild plots are shown for atropine (red) a nonselective muscarinic antagonist (n = 9), 4-DAMP (green, n = 12), M3 selective, and methoctramine (black), m2 selective (n = 11).