Figure 6. Displacement of [sup 3 Hydrogen]R(-)-quinuclidinyl benzilate ([sup 3 Hydrogen]QNB) and [sup 3 Hydrogen]8-cyclopentyl-1,3-dipropylxanthine ([sup 3 Hydrogen]CPX) binding to guinea pig ventricular membranes by propofol. (A) Radiolabeled ligand ([sup 3 Hydrogen]QNB 0.1 nM or [sup 3 Hydrogen]CPX 2.0 nM), crude ventricular membranes and competing unlabeled drug (propofol) were incubated together for 2 h at room temperature. The dissociation constants (Kⁱ) for propofol displacement of [sub 3 Hydrogen]CPX and [sup 3 Hydrogen]QNB binding to guinea pig ventricular membranes were 198 and 63.8 micro Meter, respectively. (B) [sup 3 Hydrogen]QNB (0.1 nM), crude ventricular membranes, and competing unlabeled drugs (propofol, atropine, and 8-cyclopentyl-1,3-dipropylxanthine [CPX]) were incubated together as described above. The Kⁱvalues for propofol and atropine displacement of [sup 3 Hydrogen]QNB binding were 52.6 micro Meter and 1.7 nM, respectively. CPX did not displace [sup 3 Hydrogen]QNB binding. Each symbol = the mean of triplicate determinations.