Ligand interactions with the solubilized porcine atrial muscarinic receptor

Ligand interactions with porcine atrial muscarinic receptor solubilized in a mixed-detergent system (0.4% w/v digitonin and 0.08% w/v cholate) are described. The solubilized receptor interacts with ligands in a stereospecific manner, showing about the same affinity for local anesthetics and antagonists as was found for the membrane-bound protein [Schimerlik, M. I., & Searles, R. P. (1980) Biochemistry 19, 3407-3413]. Agonists appear to interact with a single class of noninteracting sites that correspond to the low-affinity agonist sites in the membrane-bound preparation. Kinetic studies of L-[3H]quinuclidinyl benzilate binding to the receptor indicated a two-step mechanism. The first step, in rapid preequilibrium (K = 5.7 x 10(-9) M), was followed by a slow conformational change (k1 = 4 x 10(-3) s-1; k-1 = 1.7 x 10(-4) s-1) in the receptor-ligand complex. The overall dissociation constant calculated from the association kinetics (2.3 x 10(-10) M) agreed well with the thermodynamic value for Kov (2.5 x 10(-10) M); however, direct determination of K-1 gave a value about 4-fold lower (4.0 x 10(-5) s-1) than predicted. Possible reasons for this discrepancy are discussed.