THE ACETYLCHOLINE RECEPTOR OF THE ADRENAL MEDULLA¹

Abstract

Abstract— The acetylcholine receptor of the bovine adrenal medulla was studied by specific binding of [ 125 1]α‐bungarotoxin to membrane fractions and by perfusion of the isolated gland. The subcellular distribution of the acetylcholine receptor paralleled the distribution of the plasma membrane markers, acetylcholinesterase and calciumstimulated ATPase. The dissociation constant for the binding of α‐bungarotoxin to a purified plasma membrane fraction was calculated from Scatchard plots to be 1.6 nM, with a concentration of 190 fmol of binding sites/mg of membrane protein. Correcting for recovery, this corresponds to 0.9 pmol acetylcholine receptor/g adrenal medulla. In decreasing order of effectiveness, d‐tubocurarine, nicotine, acetylcholine, carbamylcholine, acetate plus choline, decamethonium, atropine and hexamethonium inhibited binding of α‐bungarotoxin. Perfusion experiments showed the acetylcholine receptor to be entirely nicotinic. Stimulation by nicotine was inhibited by atropine and decamethonium, as well as by hexamethonium. Calculated dissociation constants for these antagonist‐receptor interactions were in the range of 1 to 3 × 10 −5 m. α‐Bungarotoxin failed to inhibit nicotine‐stimulated catecholamine release in the perfused adrenal, most likely because of its limited diffusion into the gland.