FIGURE 3.

Binding of [³H]quinuclidinylbenzilate to homogenized membranes from CHO cells expressing eGFP-M₂. Total binding was measured at graded concentrations of [³H]quinuclidinylbenzilate alone (upper curve) and in the presence of 1.0 mm N-methylscopolamine (base line). The data were analyzed in terms of the Hill equation, as described previously (i.e. Equation 1 in Ref. 4). The Hill coefficient proved to be indistinguishable from 1 (p = 0.87) and was fixed accordingly in subsequent analyses. The lines represent the best fit of the equation to the data from three experiments (○, □, ▵) taken in concert. A single value of the apparent equilibrium dissociation constant (log K) was common to all of the data; maximal specific binding (B_max) and nonspecific binding were estimated separately for the data from each experiment. The fitted values of log K and B_max are −9.50 ± 0.03 and 95–110 pm, respectively. Values plotted on the ordinate have been normalized to the mean value of B_max (i.e. 103 ± 4 pm).