Extraction and chromatography of muscarinic receptors derived from pig atrial cell membranes yielded an active preparation that was capable of interacting with other components of the receptor-effector complex. Binding of muscarinic antagonists to human skin fibroblasts was investigated and declining dissociation of the bound ligand in the course of subsequent incubation of 37 degrees was demonstrated. Serum proteins were found to competitively inhibit binding of ligands to whole cells and cell membranes.
