Heterogeneity of acetylcholine receptors: Different forms of receptor distinguished by α-bungarotoxin kinetics and by antibody binding properties

The interactions of the nicotinic acetylcholine receptor (AChR), extracted from denervated rat hindlimb muscle, with α-bungarotoxin, Concanavalin-A (Con-A) and immunoglobulins isolated from the sera of specific patients with Myasthenia gravis (MG) have been studied. The association and dissociation of toxin to unfractionated receptors [hydroxylapatite (HTP)-AChR] were best described in terms of two classes of kinetically distinct toxin binding sites present in approximately equal amounts. Incubation of these receptors with soluble Con-A decreased the total toxin-binding capacity by a maximum of 42%. Kinetic studies indicate that the Con-A-induced inhibition of toxin binding is restricted to those toxin binding sites having the fast rate of association with α-bungarotoxin. Incubation of HTP receptors with toxin blocking antibodies (MG-B) from MG patient sera decreased the toxin binding capacity by a maximum of 30%. By contrast, receptors fractionated by elution from Lens-Culinaris agarose affinity columns exhibited only fast rates of association and dissociation with α-bungarotoxin and represented 40% of the total HTP-AChR population. Con-A totally prevented toxin binding whereas MG-B antibodies had no blocking effect on this subpopulation of receptors. Another MG antibody type which interferes with lectin binding to AChR (MG-C antibody) gave a 50% maximum inhibition of both HTP- and Lens culinaris agarose-AChR. This suggests that both types of toxin binding sites (fast and slow) may be further divided in two separable subclasses based on their carbohydrate moieties. These results show that multiple forms of AChR occur in denervated muscle. At least two different forms of receptors are present which show distinctive differences in ligand-binding properties and some antigenic determinants.