The effects of a myasthenic serum on the acetylcholine receptors of C2 myotubes. II. Functional inactivation of the receptor.

We have investigated the effect of antibodies from a myasthenic serum on the physiological properties of acetylcholine receptors (AChRs) in myotubes of a mouse muscle cell line, C2. The antibodies in this serum blocked the binding of 125I-alpha-bungarotoxin to the myotubes to an extent of about 50%. The antibodies also inhibited the increase in 22Na influx caused by carbamylcholine (CARB). At a concentration of antibody that blocked about 50% of toxin binding, greater than 80% of the AChR-mediated 22Na influx was blocked. The apparent KD for CARB, estimated from the dose-response curve for 22Na influx, was unaffected. The effect of the antibodies was further examined by patch-clamp recording. In greater than 30% of the patches from antibody-treated cells, no channel activity in response to acetylcholine was seen; in contrast, every patch from control cells showed activity. The channels that were seen after antibody treatment were indistinguishable from those seen in normal cells, both in their single-channel conductance and in the kinetic constants used to describe channel opening and closing. We conclude that the antibodies in this serum inhibit the functional response of AChRs in C2 myotubes to acetylcholine and do so by inactivating individual receptors.