Figure 1. Saturation radioligand binding of³H-QNB to M²(A) or M³(B) muscarinic receptors stably transfected in Chinese hamster ovary cells.³H-QNB (0.05–3 nM) was incubated for 2 h at room temperature with 40 micro gram of membranes prepared from Chinese hamster ovary cells overexpressing the M²(A) or M³(B) muscarinic receptor in the presence or absence of 2 micro Meter atropine. Saturation of specific binding was achieved over the range of sup 3 H-QNB concentrations used (n = 3). Values are mean +/- SEM. The insets show that representative Scatchard transformations of saturation binding allowed determination of receptor affinity for³H-QNB and quantity of total receptors.