Abstract

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.