TY - JOUR T1 - Multiple Affinities for Binding of Cholinergic Ligands to a Particulate Fraction of <em>Torpedo</em> Electroplax JF - Molecular Pharmacology JO - Mol Pharmacol SP - 420 LP - 428 VL - 7 IS - 4 AU - M. E. ELDEFRAWI AU - A. T. ELDEFRAWI AU - L. P. GILMOUR AU - R. D. O'BRIEN Y1 - 1971/07/01 UR - http://molpharm.aspetjournals.org/content/7/4/420.abstract N2 - The binding of four radioactive cholinergic ligands to a particulate fraction of Torpedo electroplax was measured by equilibrium dialysis at ligand concentrations ranging from l0-9 to l0-4 M. Multiple affinities for the four ligands were found, two each for muscarone, nicotine, and dimethyl-d-tubocurarine, and three for decamethonium. Binding was reversible in every case. The two agonists, muscarone and nicotine, each showed two kinds of binding, a low-affinity type of approximately 0.5 nmole/g, and a high-affinity type of approximately 0.1 nmole/g. Binding occurred to Phospholipoproteins or to a phospholipid-protein complex, as judged from sensitivity to hydrolases. The antagonist dimethyl-d-tubocurarine also showed two kinds of binding. The low-affinity type was insensitive to hydrolases, however, and therefore probably was to a different macromolecule; it was almost 10 times more extensive: 3.6 nmoles/g of electroplax. Decamethonium showed three kinds of binding, all sensitive to the hydrolases, amounting to 0.6, 1.4, and 3.2 nmoles/g. It is suggested that muscarone and nicotine bind to two diffenennt sites, which exhibit binding properties similar to the acetylcholine receptor, and that decamethonium and dimethyl-d-tubocurarine bind to other distinct sites and perhaps also to the sites binding muscarone and nicotine. ACKNOWLEDGMENT The authors are grateful to Professor H. Howland for devising the program for computer analysis, according to the Scatchard equation. ER -