The Labelling of Cholinergic Receptors in Smooth Muscle

It was found some years ago (Paton & Rang, 1965) that the longitudinal smooth muscle layer of guinea-pig small intestine binds radioactive atropine. When the uptake of atropine was studied as a function of concentration, three components were discernible; one component, which accounted for most of the binding at low atropine concentrations, appeared to saturate at a level of about 180 pmol/g wet weight, and its equilibrium constant (1.1 × 10 −9 M) was identical with the equilibrium constant for the binding of atropine to muscarinic receptors determined by measurements of pharmacological antagonism. Moreover, the binding of atropine was inhibited by low concentrations of drugs that antagonize the effects of acetylcholine in smooth muscle and unaffected by substances without any muscarinic blocking activity.