Acetylcholine receptor in myasthenia gravis: Increased affinity for α‐bungarotoxin

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Abstract
Studies of the binding of 125I‐labeled α‐bungarotoxin to myasthenic motor end‐plates have been interpreted as showing a decrease in the number of acetylcholine (ACh) receptors at these end‐plates. Equilibrium binding studies of 125I‐tagged α‐bungarotoxin to detergent‐extracted ACh receptors from normal and myasthenic intercostal muscle were carried out to determine whether the reduced toxin binding previously reported could be due to a reduced affinity of myasthenic receptors for α‐bungarotoxin rather than to a decreased number of receptors. Our results show increased rather than decreased affinity of myasthenic receptors for α‐bungarotoxin and also suggest that the number of ACh receptors is indeed reduced. The presence of a change in binding affinity, in addition to the reduced number of ACh receptors, suggests the presence of membrane changes that may contribute to the pathogenesis of myasthenia gravis.