Figure 8

Influence of tBHP on ligand binding to muscarinic receptors in CHO-M3 cells. a The specific binding of [³H]N-methylscopolamine ([³H]MS) was measured at the concentrations indicated on the abscissa. The curve is drawn according to parameters derived from a nonlinear fit of the data to a model for ligand binding to a single population of independent receptors. Each data point reflects the mean from 3 experiments performed in triplicate. Binding parameters and experimental variation are presented in Table 1. Neither receptor concentration nor binding affinity was affected by exposure to tBHP (2 mM for 90 min). b The ability of carbamylcholine to inhibit the specific binding of 0.3 nM [³H]MS was determined in triplicate in control (open symbols) or tBHP-exposed (2 mM for 90 min) (closed symbols) M3-CHO cells in the absence (circles) or presence (triangles) of 10 μM Gpp(NH)p. Each point is the average of 3 measurements from 3 independent experiments. The ability of carbamylcholine to inhibit [³H]MS binding was not affected by a 90 min exposure to 2 mM tBHP (IC50’s = 1.8 ± 0.3 μM in control and 2.4 ± 0.6 μM in exposed cells). Carbamylcholine inhibition of [³H]MS binding was depressed in the presence of 5′-Gpp(NH)p (IC50’s = 5.0 ± 1.2 μM compared to 1.8 ± .03 mM in control cells). Exposure of the cells to tBHP did not affect guanine nucleotide depression of agonist binding affinity (IC50’s = 5.3 ± 0.5 μM and 2.4 ± 0.6 μM in exposed and control cells, respectively). Each point is the average of 3 measurements from 3 independent experiments.